Basic Knowledge of Centrifugal Compressors.doc

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Chapter 6 Basics of Centrifugal Compressors Section 1 Overview 1. Applications of Centrifugal Compressors A centrifugal compressor is a type of rotary vane compressor (i.e., a turbine compressor). In a centrifugal compressor, the high-speed rotating impeller exerts a centrifugal force on the gas, and the diffuser section exerts a diffusing effect on the gas, thereby increasing the gas pressure. In the early days, this type of compressor went unnoticed because it was suitable only for applications involving low to medium pressures and high flow rates. However, recently, due to the development of the chemical industry and the construction of various large-scale chemical plants and refineries, centrifugal compressors have become key machines for compressing and transporting various gases used in chemical production, thus holding an extremely important position. As advances in gas dynamics have led to improved efficiency in centrifugal compressors, and thanks to the development of key technologies such as high-pressure sealing, manufacturing of small-diameter impellers for low-flow applications, and multi-oil- wedge bearings, a range of issues related to the operation of centrifugal compressors at higher pressures and wider flow rates have been resolved. This has greatly expanded the scope of application for centrifugal compressors, enabling them to replace reciprocating compressors in many situations and thus **significantly broadening their utility. The pressure of industrial high-pressure centrifugal compressors ranges from (150~350)×105 Pa, while those used for gas injection in offshore oil fields can reach pressures as high as 700×105 Pa. The flow rate of centrifugal blowers used for blast furnace ventilation is around 7000 m3/min, the power of some of them reaches 52900 KW, and their rotational speed is generally above 10000 r/min. Some basic chemical raw materials, such as propylene, ethylene, butadiene, and benzene, can be processed into important chemical products like plastics, fibers, and rubber. In petrochemical plants that produce such basic raw materials, centrifugal compressors also play an important role and are among the key equipment. In addition, in other industries such as oil refining and refrigeration, centrifugal compressors are also extremely important equipment. The reason why centrifugal compressors are so widely used is mainly due to the following advantages over piston compressors. 1. Centrifugal compressors have a large gas handling capacity, a simple and compact structure, low weight, small unit size, and require less floor space. 2. It features balanced operation, reliable performance, high efficiency, and few friction components; as a result, there is a low demand for spare parts, along with reduced maintenance costs and the need for fewer maintenance personnel. 3. In chemical processes, centrifugal compressors enable the compression of chemical media in a completely oil-free manner. 4. The centrifugal compressor is a type of rotating machine that is suitable for being directly driven by industrial steam turbines or gas-fired turbines. In typical large-scale chemical plants, waste steam is commonly used to drive industrial steam turbines for power generation, thereby enabling the comprehensive utilization of thermal energy. However, centrifugal compressors also have some disadvantages. 1. Centrifugal compressors are not currently suitable for applications with too low gas flow rates or excessively high pressure ratios. 2. The stable operating range of centrifugal compressors is narrow; although air volume regulation is relatively easy, their economic efficiency is poor. 3. Currently, the efficiency of centrifugal compressors is generally lower than that of piston compressors. As early as the 1950s, our country was able to manufacture centrifugal compressors. Starting from the early 1970s, focusing on petroleum chemical plants and large-scale fertilizer plants, a series of high-performance centrifugal compressors for medium and high pressures were introduced, enabling extensive practical experience to be gained. On the basis of assimilating and integrating these imported technologies, our country has enhanced its own capabilities in research, design, and manufacturing. II. Types of centrifugal compressors There are a wide variety of centrifugal compressors, and they can be classified into several categories based on their performance and structural characteristics. Classification, Name, Description: By exhaust pressure – Low-pressure compressors: exhaust pressure between 3–10 Kg/cm2; Medium-pressure compressors: exhaust pressure between 10–100 Kg/cm2; High-pressure compressors: exhaust pressure between 100–1000 Kg/cm2; Ultra-high-pressure compressors: exhaust pressure > 1000 Kg/cm2. By power – Micro-compressors: shaft power less than 10 KW; Small compressors: shaft power between 10–100 KW; Medium-sized compressors: shaft power between 100–1000 KW; Large compressors: shaft power above 1000 KW. By the flow rate of the inlet gas – Low-flow compressors: flow rate less than 100 Nm3/min; Medium-flow compressors: flow rate between 100–1000 Nm3/min; High-flow compressors: flow rate greater than 1000 Nm3/min. By structural characteristics – Horizontal split type; Vertical split type. Section 2: Working principle and structure of centrifugal compressors. I. Working principle: A turbine (or electric motor) drives the impeller on the compressor’s main shaft to rotate; under the effect of centrifugal force, the gas is thrown into the diffuser located behind the working wheel. A thin zone is formed in the middle of the working wheel, and the gas from ahead enters the impeller through the inlet section at the center of the working wheel. As the working wheel keeps rotating, the gas is continuously thrown out, thereby maintaining a continuous flow of gas within the compressor. The gas experiences increased pressure due to centrifugal force, and it can leave the working wheel at high speeds. As the gas passes through the diffuser, its speed decreases gradually, with kinetic energy being converted into static pressure energy, thereby further increasing the pressure. If the pressure generated by a single working impeller is not sufficient, the required outlet pressure can be achieved by operating multiple impellers in series. The series connection between stages is achieved through bends and return lines. This is the working principle of a centrifugal compressor. II. Basic Structure A centrifugal compressor consists of two main parts: the rotor and the stator, as shown in Figure 6-1. The rotor includes a shaft, as well as components such as an impeller fixed to the shaft, shaft sleeves, balance discs, thrust discs, and couplings. The stator consists of cylinders, various partitions positioned on the cylinder block, as well as components such as bearings. Sealing elements are also provided at the areas where gas needs to be sealed between the rotor and the stator. The functions of each component are described as follows. 1. Impeller: The impeller is the most important component in a centrifugal compressor. It is through this rapidly rotating impeller that the mechanical power from the drive unit is applied to the gas, thereby imparting energy to it. It is the only component in the compressor that performs work; it is also known as the working wheel. An impeller generally consists of a shroud, a disk, and blades, forming a closed impeller; there are also semi-open impellers without a shroud. 2. Spindle: The spindle serves to support rotating components and transmit torque. Based on its structural form. There are two types: stepped shaft and optical axis. The optical axis features a simple shape and is easy to process. 3. Balance disk: In multi-stage centrifugal compressors, due to the unequal gas forces acting on both sides of each impeller stage, a resultant force acts on the rotor in the direction of the low-pressure side; this resultant force is known as the axial force. Axial force is harmful to the proper operation of compressors; it can easily cause damage to the thrust bearings, leading to the rotor shifting toward one end. This results in the moving parts losing their correct relative position with respect to the fixed components. In severe cases, the rotor may collide with the fixed parts, resulting in an accident. A balance disk is a component that uses the pressure difference between the gases on its two sides to balance axial forces. The pressure on one side of it is the pressure in the clearance on the side of the final impeller disk, while the other side leads to the atmosphere or the intake pipe. Typically, the balance disk only balances a portion of the axial force; the remaining axial force is borne by the thrust bearings. An air seal must be installed at the outer edge of the balance disk to prevent gas leakage and maintain the pressure difference between the two sides. The balance of axial force can also be achieved through air intake on both sides of the impeller or by installing the impeller in reverse. 4. Thrust disc: Since the balance disc only balances part of the axial force, the remaining axial force is transmitted to the thrust blocks on the thrust bearings through the thrust disc, thereby achieving force equilibrium. The contact surface between the thrust disc and the thrust blocks must be very smooth, and the gap between them should be filled with appropriate lubricating oil, so that the thrust blocks do not wear out under normal operation. When a centrifugal compressor starts up, the rotor tends to move towards the other end; to ensure that the rotor remains in its proper position, it needs to be held in place by thrust forces on both sides. This is because, at the time of startup, the gas pressure on either side of the balance disc has not yet been established, and as long as there is gas flow, the rotor will move in a direction opposite to the normal axial force. Therefore, it is necessary to have thrust forces on both sides of the rotor to prevent accidents. 5. Couplings: Due to the fact that centrifugal compressors operate at high speeds, with high power levels, and inevitably experience some vibration during operation, the couplings used must be able to transmit large torques while also allowing for slight radial and axial displacement. There are two types of couplings: toothed couplings and diaphragm couplings; currently, diaphragm couplings are the most commonly used. These couplings do not require lubricants and are easy to manufacture. 6. Shell: The shell, also known as the cylinder, is used in medium and low-pressure centrifugal compressors; a horizontally split-shell design is generally adopted to facilitate assembly. The upper and lower shells are positioned using dowel pins, and they are connected together with bolts. For high-pressure centrifugal compressors, a cylindrical forged steel casing is used to withstand high pressures. The end cover of this structure is connected to the cylindrical casing using bolts. 7. Diffuser: When the gas flows out of the impeller, it still has a high flow velocity. To make full use of this portion of the kinetic energy in order to increase the gas pressure, a diffuser with a gradually increasing flow area is installed behind the impeller. Diffusers generally come in various forms such as bladeless, bladed, and straight-wall diffusers. 8. Curves: In multi-stage centrifugal compressors, gas must change direction between stages; to achieve this, curves are used. These curves are annular spaces formed by the casing and partition plates. 9. Return duct: The channel connected behind the curve is the return duct. Its function is to allow the airflow to enter the next stage in a uniform manner in the desired direction; it consists of partitions and guide vanes. The guide vanes are usually arc-shaped; they can be cast as one piece with the cylinder or manufactured separately and then connected together with bolts. 10. Volute: The main purpose of the volute is to collect the gas that flows out after the diffuser or the impeller, and guide it out of the machine. The cross-sectional shapes of the volute include circular, plow-shaped, trapezoidal, and rectangular. 11. Sealing: To reduce the amount of air leakage through the gap between the rotor and the fixed components, sealing is often employed. Sealing is divided into internal sealing and external sealing. The function of internal seals is to prevent gas from flowing back between stages, such as the wheel cover seal at the wheel housing, and the partition seal between the partitions and the rotor. The external seal is designed to reduce and prevent gas from inside the machine from leaking out, as well as to stop outside air from entering the machine, such as the seals at the machine’s ends. There are many types of seals used in centrifugal compressors, and the commonly used ones include the following: 1) Labyrinth seal. The labyrinth seal is currently a widely used sealing device in centrifugal compressors, employed for both external and internal sealing. The gas flow in a labyrinth seal (see Figure 6-2): as the gas flows through the gaps between the comb-shaped labyrinth seal elements, it undergoes an expansion process, with pressure dropping from P1 to P2 at the right end. This expansion occurs gradually; when the gas enters the sealing chamber through these gaps, the sudden increase in cross-sectional area causes the gas flow to form strong vortices, resulting in almost complete loss of velocity. There is a pressure difference between the gases on either side of the sealing surface. The pressure inside the sealing chamber is equal to the pressure in the gaps. According to the laws of gas expansion, as pressure decreases, velocity should increase and temperature should decrease. However, since the flow of gas through the narrow gaps is of a throttling nature, the kinetic energy obtained by the gas due to the pressure drop is completely lost in the sealing chamber and converted into useless heat energy. This heat energy then warms up the gas again, causing the temperature to rise back to its level before the pressure dropped. This process repeats itself each time the gas passes through another seal element and cavity, until the pressure reaches P2. It can be seen that a labyrinth seal utilizes the principle of throttling: each time gas passes through a tooth, its pressure decreases, and after passing through a certain number of teeth, a significant pressure drop occurs. In essence, a labyrinth seal creates a pressure difference resistance to the flow of gas, thereby reducing the amount of gas that can pass through. The commonly used types of labyrinth seals include the following. The smooth type is shown in Figure 6-3; the shaft serves as the optical axis, and the seal body is equipped with comb-like teeth or embedded tooth pieces, resulting in a simple structure. Figure 6-3: Smooth labyrinth seal; the twisted type is shown in Figure 6-4. To enhance the throttling and pressure-reduction effect of each tooth, a twisted labyrinth seal was developed, which provides a better sealing performance than the smooth type. Figure 6-4: Curved labyrinth seal, step-type – see Figure 6-5. The sealing performance of this type is also better than that of the smooth-type; it is commonly used for sealing impeller covers, and usually has 3 to 5 sealing teeth. 2) Oil film sealing, also known as floating ring sealing. The principle of floating ring sealing relies on the film formed by high pressure between the floating ring and the shaft sleeve, which creates throttling and pressure reduction to prevent gas from flowing from the high-pressure side to the low-pressure side. Floating ring sealing enables an oil film to be formed in the gap between the ring and the shaft, while allowing the ring itself to move freely radially. The ring on the high-pressure side is called the high-pressure ring, while the ring on the low-pressure side is called the low-pressure ring. These rings can move freely in the radial direction but cannot rotate. The pressure of the sealing oil is usually about 0.5 Kg/cm2 higher than that of the process gas; this oil enters the sealing chamber and flows toward the high-pressure side through the gap between the high-pressure ring and the shaft, where it forms an oil film that seals off the high-pressure gas. The other stream of oil flows out through the gap between the low-pressure ring and the shaft, returning to the oil tank. Usually, there are several low-pressure rings in order to achieve proper sealing. The floating ring seal is made of steel, with its end faces coated in tin bronze; the inner side of the ring is filled with babbitt alloy to prevent short-term contact between the shaft and the oil ring, as babbitt alloy serves as an anti-wear material. Floating ring seals can achieve complete leaklessness, and are widely used as shaft sealing devices for compressors. 3) Mechanical seal: Mechanical seal units are sometimes used for shaft sealing in small compressors. The differences between mechanical seals used in compressors and those used in ordinary pumps lie mainly in the high rotational speed, high linear velocity, high PV value, significant frictional heat generation, and high requirements regarding dynamic balance. Therefore, structurally, the spring and its loading mechanism are generally designed to be stationary, and the geometric shape of the rotating parts is kept as symmetrical as possible. Pins, chains, and similar elements are not used in the transmission mechanism in order to reduce the effects of centrifugal force caused by unbalanced masses. Additionally, from the perspective of frictional components and surface pressure, a double-face partial-balancing design is preferred, with narrow surface widths; the friction coefficients of the materials used in the friction pairs should be low. Cooling and lubrication must also be enhanced to allow for rapid removal of the heat generated at the sealing surfaces. 4) Dry gas seal: With the continuous improvement and development of hydrodynamic mechanical seal technology, one important type of such seal, namely the helical groove hydrodynamic gas seal or dry gas seal, has been widely used in the petrochemical industry. Compared to oil-sealed floating ring seals, dry gas seals offer numerous advantages: they operate stably and reliably and are easy to maintain; they require fewer auxiliary systems, which reduces the workload for operators. Additionally, only a small amount of nitrogen is used for sealing, making them both energy-efficient and environmentally friendly. Figure 6-6 shows a schematic diagram of the dry gas seal with spiral grooves. It consists of a moving ring 1, a stationary ring 2, springs 4, O-rings 3, 5, 8, an assembly sleeve 7, and a shaft 6. Figures 6-7 show the sealing surface, which was precision-machined with threaded grooves on its moving ring surface and then ground and polished. Generally, the depth of the helical groove is around 2.5 to 10 μm. The surface parallelism of the sealing ring must be very high, being less than 1 μm, and the shape of the helical groove is approximately that of a logarithmic spiral. As shown in Figures 6-7, when the moving ring rotates, nitrogen used for sealing is drawn circumferentially into the spiral grooves; it flows from the outer diameter toward the center in a radial direction. The sealing baffle acts to prevent the gas from flowing toward the center, thereby compressing the gas and causing a rise in pressure. This pressure of the gas film attempts to push against the seal, thus creating the desired gas film. The typical value of this equilibrium gap or film thickness h is 3 μm. In this way, the pressure of the sealed gas, the spring force, and the pressure of the gas film work together to ensure that the gas film has good elasticity; it possesses high stiffness, which enables stable operation and prevents the sealing surfaces from coming into contact with each other. Meanwhile, the nitrogen gas film with its high stiffness can effectively prevent leakage of the medium. The force acting on the dry gas seal is shown in Figure 6-8. Under normal operating conditions, the closing force of this seal (caused by the spring and gas forces) is equal to the opening force (caused by the gas film force). When external forces interfere and the gap decreases, the gas shear rate increases, which enhances the effectiveness of the helical groove in allowing the seal to open; as a result, the opening force becomes greater than the closing force, and the gap returns to its original size. If external disturbances cause the gap to increase, the pressure inside the gap decreases, the opening force becomes less than the closing force, and the sealing surfaces come together again, restoring the gap to its original size. 12. Bearings: Centrifugal compressors are equipped with radial bearings and thrust bearings. The radial bearings are sliding bearings; their function is to support the rotor so that it can rotate at high speeds. The thrust bearings, on the other hand, bear the remaining axial forces on the rotor, preventing axial movement of the rotor and maintaining its axial position within the cylinder. (1) Radial bearings: Radial bearings are mainly composed of a bearing housing, a bearing cover, and upper and lower half-shaft bushings. Bearing housing: It is used to hold the bearing shells; it can be cast together with the cylinder, or it can be cast separately and then supported on the machine base. The forces exerted by the rotor on the bearings are ultimately transmitted, directly or indirectly, to the machine base and the foundation through this housing. Bearing cover: It is placed over the bearing bush and maintains a certain degree of tension with the bush in order to prevent the bearing from shifting. The bearing cover is secured to the bearing housing using bolts. Bearing shells: Used to directly support the shaft journals. The circular surfaces of these bearing shells are coated with babbitt alloy; due to its good friction-reducing properties, high plasticity, and ease of casting and running-in, it is widely used in centrifugal compressors. In practice, for ease of loading and unloading, bearing shells are usually made in upper and lower halves and fastened together with bolts; currently, the thickness of babbitt used is typically between 1 and 2 mm. There are two ways in which bearing shells can be positioned within the bearing housing: one is where the bearing shell remains fixed, and the other is where it is movable – that is, there is a spherical surface on the back side of the bearing shell, which allows it to adjust its position automatically as the main shaft bends during operation, thereby ensuring even stress distribution along the entire length of the bearing shell. Lubricating oil enters the bearing through the oil holes on the side surface of the bearing. On the path of the oil as it enters the bearing, a throttle orifice plate is installed; by changing the diameter of this orifice plate, it is possible to regulate the amount of oil that enters the bearing. An annular oil groove is present in the upper part of the bearing bush, which facilitates better circulation of the lubricating oil and helps to cool the shaft journal. (2) Thrust bearings: Like radial bearings, thrust bearings are also divided into upper and lower halves; a positioning pin is present on the mid-surface, and they are connected together with bolts. A positioning sleeve is used between the spherical housing and the spherical seat to prevent relative rotation. Due to the spherical support, these bearings can adjust automatically according to the degree of shaft deflection. Thrust bearings work together with thrust discs; the thrust disc mounted on the shaft rotates along with the shaft, and the thrust transmitted by the shaft is applied to several stationary thrust blocks. A layer of babbitt is also cast on the working surface of these thrust blocks, and the thickness tolerance of these thrust blocks is less than 0.01–0.02 mm. Mitchell thrust bearings and Kingsbury bearings are widely used in centrifugal compressors. During normal operation, the axial force in a centrifugal compressor always acts in the direction of the low-pressure side; the thrust block that bears this axial force is known as the main thrust block. When the compressor starts, the direction of the airflow force is toward the high-pressure side, and this force causes axial movement of that high-pressure side. To prevent such axial movement, another thrust block is installed; this block is located opposite to the main thrust block and is referred to as the auxiliary thrust block. A certain gap is left between the thrust disc and the thrust block to facilitate the formation of an oil film; this gap is generally within the range of 0.25 to 0.35 mm. Most importantly, the maximum value of this gap should be less than the minimum axial gap between the fixed and rotating components, so as to prevent contact between them. The lubricating oil enters the spherical housing through the inlet at the bottom of the sphere, and then splits into two paths: one path goes to the radial bearings via the mid-surface, while the other path leads to the thrust bearings through two sets of inclined holes. Part of the oil that reaches the thrust bearings flows into the main thrust block, while the other part goes into the secondary thrust block. Section 3: Regulation of Centrifugal Compressors The operating points of centrifugal compressors are indicated on their characteristic curves, and pressure and flow rate are in a one-to-one relationship. However, the specific operating point at which stability will be achieved must be determined in conjunction with the compressor’s piping system. The compressor has a certain stable operating point under specific pipeline network conditions; however, when the conditions of the pipeline network change, the compressor’s operating conditions will also change accordingly. I. Pipeline system characteristic curve: By pipeline system, it is generally referred to the intake pipelines and exhaust pipelines connected to the compressor, as well as the accessories and equipment installed on these pipelines. But for centrifugal compressors, the piping network refers only to the pipelines behind the compressor and all associated equipment. By stipulating this in this way, when studying the relationship between the compressor and its piping network, the issue of the compressor’s intake conditions changing with operating conditions can be avoided, thus simplifying the problem. Figure 5-6-8 shows a schematic diagram of the compressor connected to the first device in the exhaust system, with an adjustment valve on the exhaust pipe. In order to deliver the gas into a device with an internal pressure of Pr, the pressure at the beginning of the pipeline network (referred to as the back pressure at the compressor outlet) is given by: Pe = Pr + △P = Pr + AQ2 (1) Where △P includes the frictional losses and local resistance losses in the pipeline network, and A is the coefficient used to calculate the total resistance loss. Q Figure 6-9: Pipeline system performance curve. Equation (1) is plotted in Figure 6-9 as a quadratic curve; it represents the relationship between the terminal pressure of the pipeline system and the air intake volume, and is known as the pipeline system performance curve. The pipeline performance curve is essentially equivalent to the pipeline resistance curve, and the shape of this curve is related to the pressure in the container as well as the resistance encountered as fluid flows through the pipeline. When the pipeline from the compressor to the container is short and the valves are fully open, resulting in minimal resistance loss, the pipeline characteristic curve is almost a horizontal line, as shown by line 1. When the pipeline is very long or the valve is closed, the pressure loss increases, and the slope of the pipe network performance curve rises, resulting in the pattern shown in line 2. The smaller the valve opening, the steeper the curve becomes, as shown by line 3. If the pressure in the container drops, the pipeline network performance curve will shift downward ； When Pr is at atmospheric pressure, the performance curve of the piping network is line 4; it can be seen that this curve changes as the pressure and resistance in the piping network vary. II. Operating point of the centrifugal compressor: When a centrifugal compressor delivers gas to the piping network, if both the gas flow rate and the discharge pressure remain quite stable (i.e., with minimal fluctuations), this indicates that the performance of the compressor and the piping network is well coordinated, and the system is operating in a stable state. This stable operating point is subject to two conditions: one is that the exhaust volume of the compressor is equal to the intake volume of the pipeline network ； Second, the discharge pressure provided by the compressor is equal to the terminal pressure required by the piping network. Therefore, this stable operating point must be the intersection of the compressor performance curve and the piping network performance curve, as this intersection meets the above two relevant conditions. For ease of explanation, the volumetric flow rate is converted to a mass flow rate G. In Figures 6-10, line 1 represents the compressor performance curve, while line 2 represents the pipeline network performance curve; their intersection point is point A. Assume that the compressor operates not at point A but under conditions at point A1. In this case, the flow rate of the compressor, G1, is greater than G0 at point A. With a flow rate of G1, the pipeline system requires an end pressure of PB1, which is ΔP higher than the pressure PA1 that the compressor can provide. As a result, the compressor must automatically reduce its output (by reducing the kinetic energy of the gas) in order to make up for the insufficient pressure energy ； As the gas volume decreases, its exhaust pressure gradually rises until it returns to condition A. Assume that instead of returning to operating point A, the system reaches operating point A2; in this case, the exhaust pressure provided by the compressor is higher than the pressure required by the piping network. As a result, the compressor’s flow rate will increase automatically, while the exhaust pressure will decrease until it equals the pressure in the piping network, at which point stability is achieved. This shows that only the intersection point A of the two curves represents the stable operating point for the compressor. Figures 6-10 Stable operating points of centrifugal compressors. III. Maximum flow rate condition and surge condition. 1. Maximum flow rate condition: The condition in which the compressor’s flow rate reaches its maximum value is the maximum flow rate condition. There are two possible reasons for this condition: first, the airflow at a certain throat in the flow channel of the stage reaches a critical state; at this point, the volumetric flow rate of the gas is already at its maximum value, and no matter how much the back pressure of the compressor decreases, the flow rate cannot increase any further. This condition is known as a \"blocking\" condition. Another scenario is one in which the flow channel has not reached a critical state; in other words, a \"blocking\" condition has not yet occurred. However, at such high flow rates, the flow losses inside the compressor are very large, resulting in a very low exhaust pressure – almost zero head. This pressure is just sufficient to overcome the flow resistance in the exhaust pipe and maintain such a high flow rate, and this represents the compressor’s maximum flow rate condition. 2. Surge condition: The condition of the centrifugal compressor at its minimum flow rate is the surge condition. As shown in Figures 6-10, line 1 represents the P-G characteristic curve of the centrifugal compressor with a hump shape; point A3 is the peak point. When the flow rate of the centrifugal compressor decreases to the level at which it operates at point A3 on this curve, if the flow rate further drops for some reason, the outlet pressure of the compressor will decrease. However, due to the large volume of the pipelines and system, as well as the compressibility of the gas, the pressure in the piping network does not drop immediately and remains higher than the discharge pressure of the compressor, resulting in gas flowing back into the compressor. The compressor increases the flow rate, which in turn raises the outlet pressure; once the outlet pressure exceeds that of the pipeline network, gas is discharged back into the system. In this way, when the compressor operates to the left of point A3, it causes the gas to flow back and forth within the compressor, resulting in severe fluctuations in flow rate and outlet pressure – a phenomenon known as surge. When the compressor experiences surge, the discharge pressure fluctuates significantly, with gas flowing in and out intermittently, resulting in periodic roaring noises and intense vibration of the machine. If no measures are taken to address this issue promptly, the bearings and seals of the compressor will be damaged first; in severe cases, the rotor may even come into contact with the fixed components, leading to serious accidents. The operating condition corresponding to point A3 is the compressor’s minimum flow condition. Surge occurs because the flow rate of the compressor is too low, below its minimum flow rate, and the pressure in the piping system is higher than the discharge pressure provided by the compressor, resulting in gas backflow and significant airflow fluctuations. The principle of anti-surge is to address the causes of surge, and immediately increase the compressor flow rate when surge is about to occur. 3. Analysis of surge cases: When the performance curve of the compressor and/or the performance curve of the piping network change, the intersection point changes as well; in other words, the operating conditions of the compressor change, resulting in operation under varying conditions. The characteristic curve (ε-Q) of a centrifugal compressor is related to the compressor’s speed, the properties of the medium, and the inlet conditions. The changes in the performance curve are shown in Figure 5-6-11. Figure 6-11 Changes in performance curves: The variation in operating conditions of centrifugal compressors sometimes does not occur under direct, intentional control by humans (such as by adjusting valves), but rather as a result of indirect disturbances from the production system or even the drive mechanism. Centrifugal compressors in chemical plants often experience unexpected surging. Examples are as follows. Figure 6-12: Conditions of surge caused by changes in the performance of a centrifugal compressor. a) Initially, the inlet temperature of a certain compressor was 20°C, and its operating point was at point A (see Figure 6-12a). Due to a malfunction in the cooler during operation, the temperature of the incoming air increased sharply to 60°C, resulting in surge in the compressor. The reason for this is that an increase in intake temperature causes the compressor’s performance curve to shift downward, from line 1 to 1’, while the pipeline network’s performance curve remains unchanged. As a result, the compressor’s operating point moves to point A’, and if this point falls on the surge limit, surge will occur. b. A certain compressor was originally operating normally at point A as shown in Figure 6-12b, but later, for some reason, surging occurred due to the intake pipe being blocked by foreign objects. The reason for this is that the intake pipe is blocked, causing the compressor’s intake pressure to drop from Pj to Pj’, which results in the machine’s performance curve dropping to line 1’. The performance curve of the piping system remains unchanged; as a result, the operating point shifts to A’, falling within the surge limit. c. A certain compressor originally operated normally at a speed of n1, with the operating condition at point A (see Figure 6-12C). Later, due to an insufficient supply of high-pressure steam during production, the speed of the steam turbine acting as the drive mechanism dropped to n2. At this point, the operating point A’ of the compressor moved into the surge zone, resulting in surge. In addition, there are also cases of surge caused by changes in the molecular weight of the gas. In all the above cases, surge occurs due to a downward shift in the compressor performance curve, with no change in the pipeline network performance. Sometimes, surge is caused by changes in the performance curve of the piping network (such as the curve moving upward or becoming steeper). Figure 6-13: Situation of surge caused by changes in pipeline network performance. A compressor was originally operating at point A’ (see Figure 6-13); later, due to instability in the production system, the pressure in the pipeline network increased significantly. The performance curve of the pipeline network shifted from line 2 to line 2’ (while the compressor’s performance curve remained unchanged), resulting in surge in the compressor. Another similar situation occurs when the exhaust valve is closed too tightly, causing the performance curve of the piping system to become steeper; once the compressor’s operating point falls into the surge zone, surge occurs suddenly. When certain factors cause changes in the performance of both the compressor and the piping network, surge will occur suddenly as long as the intersection point of the two curves ends up within the surge region. For example, during the startup process of a centrifugal compressor (speed and pressure increase) and its shutdown process (speed and pressure decrease), both types of performance curves change gradually. Changing the speed alters the compressor’s performance curve, while changing the pressure in the system alters the performance curve of the piping network. During operation, it is necessary to constantly pay attention to ensuring that the two change in coordination, so as to keep the compressor operating within its stable operating range at all times. IV. Adjustment of the operating conditions of centrifugal compressors The essence of adjusting a compressor lies in changing its operating point; in principle, the methods used involve either altering the compressor’s performance curve or the performance curve of the piping system. Specifically, there are the following adjustment methods: a) Outlet throttling control, which involves installing a control valve at the compressor outlet; by adjusting the opening degree of this valve, the performance curve of the pipeline is altered, the operating point of the compressor is changed, and thus flow rate can be regulated. The method of adjusting outlet throttling involves artificially increasing the outlet resistance to regulate flow, which is an uneconomical approach. Its disadvantages become even more pronounced when the compressor’s performance curve is steep and the amount of flow (or pressure) that needs to be adjusted is large. Nowadays, aside from its use in fans and small blowers, this adjustment method is rarely employed in compressors. b. Import throttling control: a control valve is installed on the compressor inlet pipe, and the inlet pressure is adjusted through this valve. The decrease in inlet pressure directly affects the compressor’s exhaust pressure, causing the compressor’s performance curve to shift downward; therefore, the effect of adjusting the inlet pressure is actually to alter the compressor’s performance curve, thereby achieving the goal of regulating flow rate. Compared with outlet throttling, inlet throttling offers better economic efficiency. According to relevant data, tests on a certain compressor showed that within a flow rate variation range of 60–80%, inlet throttling saves approximately 4–5% more power than outlet throttling. So this is a relatively simple and commonly used adjustment method. However, there are still certain throttling losses, and changes in operating conditions can have an impact on the efficiency of the compressor itself. Another advantage of the inlet throttling method is that closing the inlet valve shifts the compressor’s performance curve toward the low-flow region, allowing the compressor to operate at lower flow rates and reducing the risk of surge. c. Change the speed regulation. When the compressor speed changes, its performance curve also changes accordingly; therefore, this method can be used to adjust the operating point in order to meet the requirements of production adjustments. The energy head of a centrifugal compressor is approximately proportional to n2, so a considerable range of adjustment can be achieved using speed control methods. Variable speed regulation does not cause any additional losses; it is merely the case that the new operating point resulting from this adjustment is not necessarily the point of highest efficiency, which leads to a slight decrease in efficiency. Therefore, from an energy-saving perspective, this is an economical adjustment method. The method of changing the rotation speed does not require altering the structure of the compressor itself; it is only necessary to take into account issues such as the strength of the rotor at increased speeds, the critical speed, and the lifespan of the bearings. However, this method requires the drive motor to be variable-speed. Section 4: Startup and Shutdown of Centrifugal Compressor Units I. Preparations and Inspections Before Compressor Unit Operation 1. The drive motor and gear transmission should undergo separate tests as well as combined tests, and after passing the inspections, they must be in good condition and ready for use. Install the coupling between the drive motor, the gear transmission, and the compressor, and recheck the alignment of the rotors to ensure it meets all requirements. 2. The cleaning and adjustment of the unit’s oil system have been completed successfully; the oil quality meets the required standards, and the oil storage level is appropriate. Inspect the main oil tank, oil filter, and oil cooler; add oil if the oil level in the tank is low. Check the oil temperature; if it is below 24°C, a heater should be used to raise the oil temperature above 24°C. The oil cooler and oil filter should also be filled with oil and of any air removed, and the switching position between the oil cooler and the filter should be set to the side that needs to be in use. Check the main oil pump and the auxiliary oil pump to ensure they are operating properly and in the correct direction. Oil thermometers and pressure gauges should be complete, have appropriate ranges, and function properly. Dry nitrogen is filled into the accumulator to keep the gas pressure inside it within the specified range. Adjust the oil pressure throughout the oil circuit system to meet the design requirements. Check that all interlock devices in the oil system are operating properly to ensure the safety of the unit. 3. The inlet filters of the compressor should be clean and undamaged; the filter elements in the inlet filters have been replaced, and the filters are in good condition. 4. The drain valves of the compressor cylinder and pipelines have been opened to drain the condensate; after that, they should be closed partially, and finally closed once inflation is complete. 5. Establish a cooling water circulation by introducing water to the intercoolers at various stages of the compressor, remove any air from the system, and bring it into operation. 6. The process piping system must be in good condition; all blind flanges should have been removed and restored to their original positions. It is not allowed for the expansion, contraction, or vibrations of the pipes to exert stress on the cylinder body. 7. Adjust the valves on the process gas pipelines to the appropriate positions as required for startup. Generally, the inlet and outlet valves of the compressor should be closed, the reflux valve or vent valve used to prevent surge should be fully open, and the outlet valve of the process system should also be fully closed. The opening and closing of various valves should be smooth and accurate, without any sticking. 8. Verify that safety valves and explosion-proof plates on the compressor pipelines and associated equipment are properly installed; the safety valves have been adjusted to meet the required specifications, and the explosion-proof plates also meet the specified standards. 9. The compressors and their associated machinery are equipped with complete instrumentation; the ranges, temperatures, pressures, and accuracy levels of these instruments meet the required standards, and important instruments should come with certification of calibration compliance. Check that the electrical wiring and instrument air system are in good condition. The control valves must be flexible and accurate, and the automatic control safety system must pass inspections to ensure precise operation. 10. All interlocks of the unit have been tested and adjusted, and all set values meet the requirements. The anti-surge protection control system has passed the calibration tests; all vent valves and anti-surge return valves should operate quickly without any sticking. 11. According to the analysis, it has been confirmed that the gas composition within the process system before and after the compressor inlet and outlet valves meets the design requirements, or the system has been properly purged with nitrogen. 12. Perform a turning test to check whether the unit rotor can rotate smoothly, with no friction or sticking. II. Startup and shutdown of turbine-driven units The system structure of turbine-driven centrifugal compressor units is relatively complex. Turbines are thermal machines that operate at high temperatures and speeds, making their startup, shutdown, and operation processes complex and slow. After the installation and maintenance of such units, trial operations are required; in accordance with professional regulations, a separate trial run of the turbine is carried out first, during which necessary adjustments and tests are performed. After passing the acceptance test, it is connected to the gear transmission for series no-load operation. Only after completing the trial operation projects and passing the acceptance tests can it be connected in series with the compressor for trial operation as well as normal start-up and shutdown operations. The key points for the start-up and shutdown of such units are as follows. 1. Startup of the oil system: The startup of the compressor is similar to that of other power units; the auxiliary equipment is started first before the main unit. After connecting various external energy sources (such as electricity, instrument air, cooling water, and steam), the oil system is brought online first. —The fuel system is fully ready and in a state where it can be started at any time. If the oil temperature is low, it should be heated until it reaches the appropriate level. After the oil system is put into operation, adjust the oil pressure in each section to the specified values, and then carry out the following steps: check the automatic start-up of the auxiliary oil pump ； Check the oil return condition of the bearing to see if the oil flow is normal ； Check the oil pressure drop across the oil filter, and fill the lubricant tank ； Check the oil level in the high-level tank; it should be between the highest and lowest levels controlled by the level controller. 2. Gas displacement: When the compressed medium is a flammable or explosive gas, gas displacement must be carried out before starting up the system after it is operating normally. First, nitrogen is used to displace the air from the pipes and equipment in the compressor system. Then, a compressible medium is used to completely displace the nitrogen, so that the gas composition meets the requirements specified in the design. The main steps of this two-stage displacement process are as follows: ① Close the inlet and outlet valves of the compressor, and fill it with nitrogen at a pressure of generally 0.3–0.6 MPa (gauge) through the compressor’s pipelines, liquid separation tank, buffer tank, and the discharge connections on the compressor cylinder. If conditions permit, the compressor’s inlet valve can be opened to enable simultaneous displacement of both the compressor and the process system. ②Once the compressor system is filled with nitrogen and at a certain pressure, open the compressor piping and cylinder discharge valves to release the nitrogen and reduce the pressure. It is essential to ensure that the pressure within the system remains higher than atmospheric pressure at all times, in order to prevent air from entering the system. Then close the discharge valve and fill the system with nitrogen, repeating this process until the oxygen content in the gas samples taken from various parts of the system is less than 0.5%. ③After the nitrogen pressure stabilizes, the sealing system should be activated in a timely manner before introducing the compressed medium, and it must operate properly. ④Check the replacement status of the process system, and conduct acceptance upon qualification. When performing gas displacement, the following points must be noted: ① Before introducing the process gas officially, all interlock testing for the compressor oil system must be completed, and the results of all tests must meet the design requirements. ②For compressors with high inlet gas pressure, the inlet valve should be opened slowly when performing replacement, and it is strictly prohibited to allow gas flow to cause the rotor to rotate. ③The compressor’s dry gas seal must not leak, nor should any of the system pipes leak; if a leak is detected, the cause must be identified promptly and steps taken to eliminate it. 3. Compressor startup: The centrifugal compressor unit can be started in accordance with the prescribed procedures only after all preparations have been completed and it has passed the inspection and acceptance. For turbine-driven centrifugal compressors, the speed increases gradually from low to high after startup; there is no issue of overload caused by too rapid acceleration as in motor-driven compressors. Therefore, it is common to keep the inlet valve fully open, as well as the reflux valve or vent valve used for anti-surge protection. After preparation in accordance with the requirements of the relevant process, all instruments and interlocks were put into use, and water flow through the intermediate cooler was smooth. Once everything is ready, the pipe warming, rotor turning, impulse starting of the rotor, and machine warming are carried out first, in accordance with the regulations outlined in the turbine operation procedures. Run the unit at 500–1000 r/min for half an hour to reach stable operating conditions, then conduct a thorough inspection of the unit, including the oil temperature and pressure in the lubrication system, with particular attention to the bearing oil temperature ； Check the temperatures and pressures of the gases at the inlet and outlet of each section of the power oil system, vacuum system, turbine seal system, steam system, and compressor, as well as look for any abnormal noises. If everything is normal, the turbine has been warmed up to the required level and the oil temperature in the lubricating oil tank is above 32°C, then speed increase can begin. When the oil temperature reaches 40°C, heating with oil supply can be stopped, and cooling water can be passed through the oil cooler. The unit accelerates according to the specified acceleration curve. During the speed-up process, care must be taken not to remain within a speed range of ±10% of the critical speed of either rotor. When passing through the critical speed, the speed should be increased rapidly; generally, an increase of about 20% of the design speed per minute is appropriate. When passing through the critical speed, close attention must be paid to the vibration of the unit. After leaving the critical speed range, it can be increased by 7% of the designed speed per minute. From the low speed of 500–1000 r/min to the normal operating speed, appropriate pauses should be made at various stages to prevent fluctuations in the pressure of the steam pipeline system due to too rapid changes in steam load. This also facilitates monitoring of the unit’s operation; only when everything is normal can the speed be increased further, until reaching the lowest speed at which the governor takes effect (usually around 85% of the design speed). 4. Pressure increase of the compressor: After the compressor starts operating, its exhaust gas is vented or returned back; at this point the exhaust pressure is very low, no gas is being delivered to the process piping network, and the rotational speed is also low. At this time, the compressor is operating under no load, or more precisely, it is operating at a low load. Operating at light load for extended periods is detrimental to both turbines and compressors. For steam turbine units, operating at low load for extended periods accelerates the wear of the turbine’s control valves ； At low speeds, the turbine can achieve very high torque. If the weight flow rate passing through the compressor is very high, the shaft of the unit may experience excessive stress ； Furthermore, operating at low pressure for extended periods also affects the efficiency of the compressor and has an adverse effect on the sealing system as well. Therefore, it is very necessary to increase the voltage and load at an appropriate time after the unit has stabilized and is operating normally. The voltage increase should generally begin after the turbine governor has come online and the normal speed has been reached. Increasing the compressor pressure (imposing more load) can be achieved by raising the rotational speed and closing the vent valve or bypass return valve until it is fully closed; however, such operations must be carried out with care, without rushing, to avoid surge. Several points need to be considered when increasing the pressure of a compressor: ① To increase the compressor’s pressure, some methods involve closing the vent valve, while others involve closing the bypass valve; some units even have more than one vent valve. During startup, these vent valves or bypass valves are open; to increase the outlet pressure, they can be gradually closed. During the process of closing the valve to increase pressure, close attention must be paid to surge phenomena. Once signs of surge are detected, the valve should be opened promptly. After the outlet vent valve is fully closed, the flow control valve should be gradually opened; at this point, the flow rate is primarily controlled by the flow control valve. After the relief valve is fully closed, put the anti-stall flow control valve under automatic control. Gradually close the flow control valve until the compressor outlet pressure reaches the specified value. During the valve closing process, it is also necessary to avoid surge. If the pressure cannot be brought to the desired level by adjusting the valves, it is necessary to increase the speed of the turbine; however, this increase should not be too rapid in order to prevent surge in the compressor. ②The general principle of the pressure-boosting operating procedure is to ensure that, in each compressor stage, the outlet pressure does not fall below the inlet pressure, and to prevent the operating point from entering the surge zone. For each unit, the correct sequence for closing the various vent valves and bypass valves, as well as the appropriate rate of operation, must be determined. The outlet valve of the compressor will open only at normal operating speed, when the pressure in the compressor’s pipeline is equal to or slightly higher than the pressure in the piping system, in order to deliver gas to the piping system. ③When boosting pressure, it is necessary to control the water flow in the intercooler to keep the inlet temperature of each section at the specified value. ④After boosting the pressure, set the anti-surge automatic control valve to the “automatic” position. Special attention must be paid to the fact that compressors must under no circumstances operate in a surge condition. Signs of compressor surge can be observed in severe vibrations of the compressor, a roaring sound, as well as significant fluctuations in pressure and flow at the outlet. If signs of surge are detected, the relief valve or bypass valve should be opened until the pressure and flow rate stabilize. 5. Compressor anti-surge test: For safety reasons, the anti-surge automatic device should be tested before the compressor is connected to the process pipeline network, to verify its reliable operation; such a test is particularly necessary during the first start-up. Before the test, the characteristic curve of the compressor should be studied to determine what the surge flow rate of the compressor is at the operating speed it is currently running at, as well as what the current flow rate is. The compressor did not experience surge, and of course the flow rate delivered was greater than the surge flow rate. Then, change the setting value of the anti-surge flow control valve and adjust the flow control setting to the current operating flow; at this point, the anti-surge automatic vent valve or return valve should open automatically. If it fails to open, it indicates a malfunction in the automatic anti-stall system, which should be checked and repaired promptly. Be sure to be very careful during testing so as not to cause the compressor to surge. 6. Pressure holding and gas supply to the grid by the compressor: Once the turbine reaches the speed at which the governor operates, the compressor increases the pressure to bring the outlet pressure to the specified level. After verifying that everything is functioning properly and smoothly, the compressor unit can be used to supply gas to the system; in other words, the high-pressure gas from the compressor’s outlet line is directed to various areas that require gas supply. Only when the compressor outlet pressure is higher than the pressure in the process system and a gas supply command is received, can the compressor outlet valve be gradually opened to supply gas to the system, in order to avoid sudden changes in the compressor’s operating conditions due to either a lack of pressure in the system or excessive pressure. When various gas-using sections draw gas from the compressor outlet pipeline into their respective process systems, an increase in the amount of gas drawn will inevitably lead to a decrease in the compressor outlet pressure. Therefore, while allowing gas to flow, the compressor must maintain pressure, that is, it must regulate the flow rate to keep the outlet pressure stable. When adjusting the flow rate for gas guidance and pressure holding, care must be taken to prevent surging. Before making adjustments, the surge flow rate should be kept in mind to ensure that the adjusted flow rate does not approach it ； During the adjustment process, attention should be paid to the dynamic and static conditions of the unit; when signs of surge are detected, the vent flow rate or return flow rate should be increased promptly to prevent surge. If the specified outlet pressure cannot be achieved even through flow regulation, the turbine must then be accelerated. Under normal operating conditions with proper air supply to the process system, all anti-surge return valves or vent valves should be fully closed. Only when production needs to be reduced while maintaining the original pressure is it permissible, as a last resort, to slightly open the reflux valve or vent valve in order to keep the compressor’s power consumption at the lowest level. Once normal production is underway, all manual operations should be switched to automatic control. At the same time, the operation of various components of the unit should be checked regularly, with particular attention paid to the temperature of the bearings or the temperature of the oil returning to the bearings; any abnormalities should be addressed promptly. It is necessary to regularly monitor changes in the gas parameters at the compressor’s inlet and outlet, and make corresponding adjustments to the unit in order to prevent surging. 7. Routine inspections during operation: When the unit is operating normally, regular inspections of the machine are necessary. For data that cannot be automatically recorded by instruments, operators should note it down on the machine’s data sheet, so as to keep track of all aspects of the machine’s operation, enable comparative analysis, help understand its performance, and address any issues that arise promptly. When the compressor unit is operating at normal speed, the following checks are generally required: ① Turbine inlet pressure and temperature ； ②Extract steam flow, temperature, and pressure ； ③Condenser vacuum level ； ④Fuel tank level ； ⑤The oil temperature is within the specified range ； ⑥Oil pressure (including oil pressure at the oil pump outlet, oil pressure drop across the filter, oil pressure in the main lubricating oil pipeline, bearing oil pressure, and nitrogen pressure for the dry gas seal) ； ⑦Oil flow conditions in the return oil pipe (samples are taken periodically from the main oil tank for analysis) ； ⑧Axial thrust of the compressor, axial displacement of the rotor, and vibration level of the unit ； ⑨The temperature and pressure of the gases at the inlet and outlet of each stage of the compressor, as well as the water temperature at the inlet and outlet of the cooler. 8. Shutdown of the compressor: There are two types of shutdowns for compressor units – one is a planned shutdown, that is, a normal shutdown carried out manually ； Another type is an emergency shutdown, also known as an accident shutdown – it occurs automatically as a result of the activation of the safety system, or through manual \"shutting down\" to effect an emergency stop. The key points and procedures for planned shutdown are as follows: ① Upon receiving the shutdown notification, switch the flow automatic control valve to the “manual” position; use the control system in the main control room or on-site to open the bypass valves or vent valves for each section, close the outlet valve, thereby disconnecting the compressor from the process system and allowing everything to operate in a self-circulating mode. ②Slow down the turbine from the main control room or on-site until the lowest speed set by the governor. Slow down the speed while reducing the load to avoid compressor surge. ③Shut down the turbine in accordance with the turbine shutdown requirements and procedures. ④The lubricating oil pump and the seal oil pump should be shut down after the unit has been completely stopped and cooled down. ⑤According to the regulations, if it is possible to close the inlet valve of the compressor, then it should be closed ； If the valve needs to remain open under pressure, the sealing system must stay operational. ⑥The lubricating oil pump and the seal oil pump must remain in operation until the temperature at the outlet of the compressor casing drops below 20°C. Check the lubricating oil temperature and adjust the water flow in the oil cooler to keep the outlet oil temperature at around 50°C. ⑦After parking, open the discharge valves of the compressor casing and the intercooler, and close the inlet valve of the intercooler. All drain valves or plugs on the compressor casing should be opened after shutdown to allow condensate to drain, and they should be closed again before the next startup. ⑧If some residual pressure remains in the compressor after it stops operating, the sealing system must continue to function; the heating coils in the seal oil tank should keep heating the oil, and the high-level oil reservoirs and seal oil collectors must remain stable. When the ambient temperature drops below 5°C, for certain piping systems, it is necessary to provide heating and insulation for the accompanying pipes of the system. III. Prevention of compressor reverse rotation: Reverse rotation must be strictly prevented after the compressor is stopped. When the compressor rotor comes to a stop, there is still a large amount of process gas remaining in the pipelines, along with a certain pressure; at this point the compressor rotor has ceased rotating, and the pressure inside the compressor is lower than that in the pipelines. At this point, if no check valve is installed on the compressor outlet pipeline or if the check valve is located far away from the compressor outlet, the gas in the pipeline will flow back, causing the compressor to rotate in the opposite direction. This, in turn, causes the rotors of the turbine, electric motor, and gear transmission to also rotate in the opposite direction. Reverse rotation of the compressor unit rotor disrupts the proper lubrication of the bearings, alters the stress conditions on the thrust bearings, and may even lead to the failure of those bearings. The dry gas seal is also damaged as a result of the reverse rotation of the compressor. To prevent the compressor from reversing, several points need to be taken into account: ① A check valve must be installed on the compressor’s outlet pipeline, and it should be placed as close as possible to the outlet flange, thereby minimizing the distance between the check valve and the compressor outlet and reducing the gas volume in that section of the pipeline to the lowest possible level, thus preventing reversal. ②Depending on the conditions of each unit, vent valves, exhaust valves, or recirculation lines are installed; these valves must be opened promptly when the unit is shut down to release the high-pressure gas at the compressor outlet, thereby reducing the amount of gas stored in the pipelines. ③The gas within the system may backflow when the compressor stops operating; high-pressure, high-temperature gas flowing back into the compressor can not only cause the compressor to reverse direction but also damage the bearings and seals. Since gas backflow causes many accidents in domestic settings, it is highly noteworthy! To effectively prevent the occurrence of the aforementioned accidents, the following actions must be carried out before reducing speed or shutting down the machine: ① Open the vent valve or return valve to allow the gas to be vented or returned. ②Make sure to properly close the check valves on the system pipelines. After completing the above tasks, gradually reduce the speed and shut down the machine. IV. Operation of the compressor in a closed loop: Due to certain specific requirements of the compressor, it may need to operate in a closed loop. Operating with air, oxygen, and oxygen-containing gases in a closed loop is dangerous and can easily lead to explosions. Therefore, it is not allowed to use these gases as a medium in closed-loop operations. For gas combustion and explosion, three conditions are generally required, namely fuel, an oxidizer, and heat. Heat is generated when a gas is compressed, causing its temperature to rise significantly as pressure increases ； It is inevitable that the compression work applied to the gas is converted into heat and stored within the gas. Heat alone, without fuel and an oxidizer, will not cause combustion or explosion. If the compressing medium is air, oxygen, or an oxygen-containing gas, this provides conditions that facilitate combustion. The fuel is generally oil, namely the lubricating oil that leaks into the cylinder and comes into contact with the medium, the sealing oil, or the oils remaining from installation or maintenance. When these factors come together, they can easily cause combustion and explosions. To prevent combustion and explosion, it is necessary to eliminate one of the three elements that cause them: oxygen, fuel, and heat. Since heat cannot be eliminated, it is necessary to get rid of either fuel or oxygen. To prevent explosions, it is absolutely forbidden to use air or other oxygen-containing gases in the closed compressor system. If it is indeed necessary to operate in a closed loop for some reason (such as inspection, testing, etc.), an inert gas such as helium, nitrogen, or carbon dioxide should be used, depending on the desired molecular weight. Preventing oil from coming into contact with the gas inside the compressor is also an important measure to prevent explosions. To ensure the cleanliness of the internal components of the compressor and the connecting pipelines, it is important to keep them free of oil. This is particularly important for compressing oxygen-containing gas media. Before the compressor sealing system is put into operation, lubricating oil should not pass through the bearings ； Before shutting down the sealing system, the lubrication pump should be stopped first ； The compressor should stop automatically when the pressure of the sealing system is insufficient. The above is only a brief overview; specific precautions should be followed in accordance with the relevant regulations. V. Surge in Compressors and Its Prevention A special phenomenon that occurs during the operation of centrifugal compressors is surge, and preventing surge is an extremely important issue in compressor operation. Many cases have shown that a large number of compressor failures are related to surge. Surge can cause severe damage because, during surge, the airflow generates strong reciprocating pulses that strike the compressor rotor and other components back and forth ； Intense and irregular oscillations of the air flow cause severe vibrations in the unit, leading to various serious consequences. Surge once caused the rotor main shaft to bend ； The seal is damaged, resulting in severe air and oil leakage ； Surge increases axial thrust, causing the thrust bearing to burn out ； It damages alignment and installation quality, thereby exacerbating vibrations ； Intense vibrations can cause instruments to malfunction ； Severe and prolonged surge can cause the rotor to collide with the stationary parts, leading to the breakage of the main shaft and partitions, and even rendering the entire compressor unusable; such incidents have already occurred abroad. Surge is a problem that must be constantly monitored during operation. 1. Signs of surge: During operation, when a compressor is experiencing surge, the first signs are usually a significant drop in flow rate, a marked reduction in the compressor’s discharge volume, fluctuations in outlet pressure with the gauge pointer moving back and forth. The unit also experiences intense vibration, accompanied by intermittent low-frequency roaring sounds, similar to someone coughing. In addition to relying on human perception, detecting surge can also be done by using instruments and operating parameters in conjunction with performance curves. 2. Conditions for surge occurrence: According to the principle of surge, it occurs under the following conditions: ① It happens when the flow rate decreases to the surge flow rate at that speed. The compressor characteristics determine that, at a constant speed, a certain flow rate corresponds to a specific outlet pressure or pressure rise ratio; moreover, at a given speed, there exists a maximum flow rate – the surge flow rate. When the actual flow rate during compressor operation is below this surge flow rate, the compressor cannot operate stably and surging occurs. The combined relationship between these flow rates, discharge pressure, speed, and surge flow rate constitutes the characteristic line of the compressor, also known as the performance curve. If the flow rate is made greater than the surge flow rate at a certain speed, surge will not occur. ②Surge occurs when the pressure of the gas in the piping system exceeds the maximum pressure corresponding to a certain speed. If the compressor operates in conjunction with the system piping network, when the system pressure exceeds the maximum pressure allowed for the compressor to operate at that speed, the high-pressure gas within the system creates a high \"back pressure\" at the compressor outlet, which causes obstruction at the outlet, a reduction in flow rate, and even backflow of gas within the piping network ； The incoming air supply decreases or is cut off, such as when the compressor does not receive enough air or has no source of air to replenish it. If all these conditions are not detected and adjusted in a timely manner, the compressor may experience surge. ③Surge can occur when mechanical components are damaged and fall off. Incomplete installation of components such as mechanical seals, balance disk seals, and O-rings, incorrect installation positions, or these components falling off can lead to air leakage between different stages or sections, which may cause surging ； Excessive filter resistance, as well as a failed or damaged check valve, can both cause surging. ④During operation, accelerating too quickly and increasing voltage too rapidly, without first reducing voltage before decelerating, can lead to surge. The speed and pressure should be increased slowly and evenly; before reducing the speed, pressure-relief measures such as venting or backflow should be taken to prevent backflow of airflow once the speed decreases. ⑤As the operating conditions change, the operating point enters the surge zone. Changes in operating conditions, such as altering speed, flow rate, or pressure, are made without checking the characteristic curves, resulting in the compressor’s operating point falling into the surge zone. ⑥During normal operation, the anti-surge system is not set to automatic mode. When external factors change, such as a drop in steam pressure or fluctuations in steam volume ； The turbine speed drops while the anti-surge system is not fast enough to make manual adjustments ； Or interruption due to anger, etc ； Failure to use an automatic anti-surge device may cause surging. ⑦Change in medium state. The occurrence of surge is closely related to the state of the gas medium, as the state of the gas affects the flow rate, which in turn influences the surge flow rate. Of course, various factors such as inlet temperature, inlet pressure, and the composition of the gas, including its molecular weight, also have an impact on surge. When the rotational speed and outlet pressure remain constant, an increase in the gas inlet temperature can easily lead to surge ； When the rotational speed is constant, the higher the intake pressure, the greater the surge flow rate. When the intake pressure and outlet pressure are constant and the rotational speed remains unchanged, surge is likely to occur if the molecular weight of the gas decreases significantly. 3. Causes of surge during operation: ① Excessively high system pressure. The reasons for this situation include an emergency shutdown of the compressor, with no venting or backflow of gas ； The one-way check valve on the outlet pipeline is not functioning properly or does not close tightly ； Or the check valve is too far from the compressor outlet, resulting in a large volume of gas in front of the valve; when the system volume suddenly decreases, the compressor does not have time to adjust, and the anti-surge system is not activated automatically. ②Insufficient inhalation flow. External factors cause the intake volume to drop below the surge flow rate. With the rotational speed remaining unchanged, the compressor entered the surge zone, resulting in surging; the filter at the compressor inlet became clogged, creating excessive resistance, and the compressor’s rotational speed could not be adjusted ； This can happen when the filter element is too dirty, or during freezing in winter. 4. Methods to prevent and eliminate surge: The fundamental measure to prevent and eliminate surge is to increase the gas flow rate entering the compressor; for generally non-toxic and non-hazardous gases such as air and carbon dioxide, venting can be used ； Recirculation can be employed for gases such as natural gas, syngas, and ammonia. By using the above method, the gas flow rate passing through the compressor can be increased, thereby eliminating surge ； But as the pressure decreases, it leads to wasted efficiency and a decline in economic viability. If the system needs to maintain a constant pressure, the speed should be increased after venting or backflowing to restore the discharge pressure to its original level. Before boosting pressure, as well as before reducing speed or shutting down, the vent valve or return valve should be opened in advance to reduce backpressure, increase flow rate, and prevent surging. The anti-surge margin should also be controlled based on the compressor performance curve, and the anti-surge system should operate automatically under normal conditions. Before increasing the speed or pressure, it is essential to first examine the performance curve and select the appropriate operating point for the next stage; the increase in pressure or speed should then be controlled based on the anti-stall safety margin. The anti-surge safety margin is the ratio of the normal operating flow rate at a certain operating speed to the surge flow rate at that same speed; generally, the normal operating flow rate should be 1.05 to 1.3 times greater than the surge flow rate. The margin is too large; although surge is less likely, the pressure drops significantly, resulting in considerable waste and reduced economic efficiency. In actual operation, it is best to set the value of the anti-surge valve (backflow control valve) based on the anti-surge margin; too large a value is uneconomical, while too small a value is unsafe. Once the anti-surge system is set according to the safety margin, during normal operation the anti-surge valve should be closed and placed in automatic mode, which is both safe and economical. In some units, the anti-surge device is not set to automatic mode but is operated manually; as a result, surge may occur and operators are reluctant to close the anti-surge valves fully. During normal operation, a large amount of gas flows back or is vented, which is neither economical nor safe. This is because manual operation is not fast enough when surge occurs, and thus surge cannot be prevented. When increasing voltage or changing speed, it is important to adhere to the principle that \"increasing voltage requires first increasing speed, and reducing speed requires first reducing voltage.\" The compressor should be pressurized after the turbine governor has come online ； Check the performance curve before increasing the pressure, determine the desired speed, and raise the pressure only after reaching that speed ； The compressor speed reduction should be initiated only after the anti-surge valve has been properly installed ； The increase in speed and voltage should not be too rapid or excessive ； The reduction in speed and pressure should also be gradual and uniform. The opening and closing of the anti-surge valve must be done slowly and alternately; the operation should not be too forceful to avoid excessive shaft displacement as well as increased axial thrust and vibration. If a compressor unit has more than two anti-surge valves, they should be opened and closed alternately to ensure a uniform pressure change in each cylinder, which is beneficial for the stress on each cylinder, anti-surge performance, and the coordination of the sealing system. Section 5: Accident Handling of Centrifugal Compressors. The performance of centrifugal compressors is influenced by the suction pressure, suction temperature, suction flow rate, molecular weight composition of the inlet gas, as well as the speed and control characteristics of the driving motor. Generally, failures or accidents occur most often due to the interaction of various factors. The common possible causes of failures and the corresponding corrective measures are listed in the table below. 1. The compressor’s performance does not meet the requirements. Possible causes and corrective measures:
① Design errors: Review the original design to check whether the technical parameters meet the requirements. If issues are found, negotiate with the seller and manufacturer to take corrective actions.
② Manufacturing errors: Examine the original design and manufacturing processes, as well as the material quality and processing precision. Address any issues promptly by contacting the seller and manufacturer.
③ Differences in gas properties: Check various properties of the gas. If these differ significantly from those specified in the original design, it will inevitably affect the compressor’s performance.
④ Changes in operating conditions: Identify the reasons for these changes.
⑤ Deposited impurities: Check for impurities in the gas flow channels, impellers, and cylinders; remove them if present.
⑥ Excessive gaps: Inspect the gaps between various components. Those that do not meet the requirements must be adjusted.

2. Insufficient compressor flow rate and discharge pressure. Possible causes and corrective measures:
① Issues with flow rate: Compare the discharge pressure and flow rate with the compressor’s performance curve to identify any problems.
② Compressor running in reverse: Check the direction of rotation, which should match the direction indicated by the arrow on the compressor housing.
③ Low suction pressure: Refer to the manual to determine the cause.
④ Mismatched molecular weight: Check the actual molecular weight and chemical composition of the gas, and compare them with the values specified in the manual. If the actual molecular weight is lower than the specified value, the discharge pressure will be insufficient.
⑤ Low operating speed: Check the operating speed and compare it with the values specified in the manual. If the speed is low, increase the speed of the prime mover. ⑥ The volume of air circulating from the exhaust side to the intake side increases; check the volume of circulating air, inspect the external piping, and check the opening degree of the circulation valves. If the circulation volume is too high, make adjustments accordingly. ⑦ Faulty pressure gauges or flow meters: Check all measuring instruments; if problems are found, calibrate, repair, or replace them. 3. Fluctuations in discharge pressure: Possible causes and corrective measures: ① Too low flow rate: Increase the flow rate; if necessary, install a bypass pipe in the discharge line to supplement the flow. ② Faulty flow control valve: Check the flow control valve and address any issues found promptly. 4. Zero flow and pressure when the compressor starts: Possible causes and corrective measures: ① Problems with the rotating system, such as incorrect installation or absence of keys or connecting shafts; disassemble and inspect, then repair the relevant components. ② Intake and exhaust valves are closed: Check the valves and ensure they are opened to the correct position. 5. Decreased flow rate: Possible causes and corrective measures: ① Improper position of the inlet guide vanes: Check whether the inlet guide vanes and their positioners are functioning properly, especially whether the actual position of the inlet guide vanes matches the readings on the indicator. If not, adjust the inlet guide vanes and positioners accordingly. ② Faulty anti-surge valves and relief valves: Check whether the sensors for anti-surge control and the relief valves are working properly; if there are issues, make adjustments to ensure stable operation without vibrations or leaks. ③ Compressor surge: Check whether the compressor is experiencing surge, and ensure that the flow rate is sufficient to keep it out of the surge zone; also ensure that the inlet temperature at each stage is normal. ④ Excessively large sealing gaps: Adjust the sealing gaps according to specifications or replace the seals. ⑤ Clogged inlet filter: Check the inlet pressure and verify whether the gas filter is clogged; clean the filter. 6. High gas temperature: Possible causes and corrective measures: ① Insufficient cooling water volume: Check the flow rate, pressure, and temperature of the cooling water; adjust the water pressure and temperature as needed. ② Reduced cooling capacity of the cooler: Check the amount of cooling water used; the flow rate within the cooler tubes should be less than 2 m/s. ③ Dirt accumulation on the cooling tube surfaces: Check the temperature difference across the cooler to determine if dirt accumulation is reducing the cooling efficiency; clean the cooler tubes. ④ Broken cooling tubes or loose connections between tubes and the tube sheet: Seal both ends of the damaged tubes or use an expander to tighten the loose tube ends. ⑤ Bubbles in the water side channels of the cooler: Check for bubbles in the water side channels of the cooler; open the vent valve to release the gas. ⑥ Operating point deviating too much from the design point: Check whether the actual operating point is too far from the specified operating range; adjust the operating conditions. 7. Abnormal vibration and noise in the compressor: Possible causes and corrective measures: ① Loss of alignment accuracy of the unit: Check the vibration levels of the unit; if the axial vibration amplitude is high and the vibration frequency matches the rotational speed, or is twice, three times, etc., of it, remove the coupling and let the prime mover rotate alone. If there is no abnormal vibration in the prime mover, then misalignment is likely; re-align the unit. ② Unbalanced rotor: Check the vibration levels; if the radial vibration amplitude is high and the vibration frequency is n, then the amplitude is proportional to the amount of imbalance and n² ； At this point, the rotor should be inspected to check for any dirt or damage; if necessary, the rotor should be rebalanced. ③ Friction and damage to the rotor impeller: Examine the rotor impeller for signs of friction or damage, and repair or replace it as needed. ④ Bending of the main shaft: Check whether the main shaft is bent; if necessary, straighten it. ⑤ Faults or imbalance in the coupling: Inspect the coupling and remove it to check its balance, then repair any issues. ⑥ Abnormal bearings: Check the radial clearance of the bearings and make adjustments accordingly; also check the interference between the bearing cover and the bearing shells – if it is too small, increase it ； If the bearing alloy is damaged, replace the bearing shell. ⑦ Poor sealing: friction between the sealing elements results in irregular vibration patterns, and a metallic grinding sound can be heard during startup or shutdown. Repair or replace the sealing ring. ⑧ Poor gear meshing in the gear speed increaser: Check the gear meshing condition of the gear speed increaser. If the vibration is low but the frequency is high and it corresponds to a multiple of the number of teeth, with the noise changing rhythmically, then the misalignment between the meshing gears should be corrected. ⑨ Loose foundation bolts or an unstable foundation: Repair the foundation and tighten the foundation bolts. ⑩ Abnormal oil pressure or oil temperature: Check the oil pressure, oil temperature, and operation status of each oil system; make adjustments if any abnormalities are found ； If the oil temperature is low, heat the lubricating oil. ⑾ If there is dirt in the oil, which prevents it from being clean, it can cause wear on the bearings. Check the quality of the oil, improve filtration, and change the oil regularly. Inspect the bearings and replace them if necessary. ⑿ Contaminants have entered or accumulated inside the machine – Check the rotor and the air flow channels in the cylinders, and remove any contaminants. ⒀ Condensate water has accumulated inside the machine – Inspect the interior of the compressor and remove the condensate water. ⒁ Compressor surging – Check whether the compressor is operating away from the surging point; ensure that there is sufficient surge margin. Adjust the operating conditions according to the specified performance curves and increase the suction volume. Also check whether the anti-surge device is functioning properly. ⒂ The gas pipes exert additional stress on the casing – The gas pipelines must be properly secured to prevent excessive stress from being applied to the compressor cylinders ； The piping must have sufficient elastic compensation to accommodate thermal expansion. ⒃ Machines operating near the compressor should have their foundations and bases separated from each other; additionally, the connecting pipes should be made more flexible. ⒄ In cases where the compressor load changes abruptly, adjust the throttle valve opening accordingly. ⒅ If any components become loose, tighten them and add anti-loosening measures.

8. Compressor surging
Possible causes and corrective measures:
① The operating point falls within the surging region or is too close to its boundary. Check the position of the compressor’s operating point on its characteristic curve; if it is too close to or within the surging region, take steps to move it away from that area promptly.
② The anti-surge margin is set too low. The pre-set anti-surge margins under various operating conditions should be maintained at around 1.03–1.50; they must not be too small.
③ Insufficient intake flow rate. This may be caused by insufficient opening of the intake valve, dirty or frozen filter elements, blocked intake passages, or reduced/interrupted supply of inlet gas. Identify the cause and take appropriate measures.
④ Excessive pressure in the compressor discharge system. When the compressor slows down or shuts down, gas may not be properly vented or returned; a malfunctioning or leaky check valve can also lead to backflow of gas. Determine the cause and implement necessary corrective actions.
⑤ Vent or return valves fail to open in time when operating conditions change. When intake flow decreases, rotational speed drops, or increases rapidly, consult the characteristic curve and promptly open the anti-surge vent or return valve.
⑥ Anti-surge device not set to automatic mode. During normal operation, the anti-surge device must be set to automatic mode.
⑦ Malfunction or improper functioning of the anti-surge device/mechanism. Regularly inspect the anti-surge device; if any malfunctions, inaccuracies, or sticking occur, repair or adjust it immediately.
⑧ Incorrectly set anti-surge values. Carefully calibrate the anti-surge values and test them periodically; correct any inaccuracies without delay.
⑨ Excessive acceleration or pressure increase. Changes in operating conditions should be gradual; abrupt increases in speed or pressure must be avoided.
⑩ Pressure not reduced prior to slowing down. Pressure must be lowered before reducing the compressor speed; proper procedures must be followed to prevent surging.
⑪ Alteration in gas properties or state. Before any changes occur, recalculate the characteristic curve and adjust the anti-surge settings accordingly.
⑫ Damage or detachment of compressor components. Damage or detachment of inter-stage seals, balance disc seals, or O-rings can trigger surging; regularly inspect these parts to ensure they remain intact.
⑬ Malfunctioning check valve on compressor discharge line. Frequently inspect the check valve on the discharge line to ensure it operates reliably and prevents backflow of gas when speed drops or the compressor stops.

9. Abnormal machine noises
Possible causes and corrective measures:
① Machine damage. Shut down the machine for inspection and repairs.
② Unstable machine operation. Adjust process parameters; if adjustments cannot be made immediately, request a shutdown for inspection.
③ Friction between bearings and seals. Inspect bearings and seals; repair or replace them as needed.
④ Foreign objects entering the machine. Shut down and remove any foreign objects.

10. Compressor air leakage
Possible causes and corrective measures:
① Poor performance of the sealing system. Inspect all components of the sealing system and repair any issues immediately.
② Defective O-ring seals. Examine all O-rings; replace any that are damaged or degraded.
③ Leakage at cylinder joints or pipe connections. Inspect cylinder mating surfaces and flange connections; address any leaks promptly.
④ Degraded sealant. Check sealants and packing materials at cylinder joints and other areas; replace those that have degraded.
⑤ Soft sealing seats that cannot move. Replace corroded components; analyze gas composition if solid substances are found inside seals or springs.
⑥ Abnormal operation. Verify whether operating procedures are correct; resolve any issues immediately.
⑦ Damaged, broken, corroded, or worn seals. Inspect all seals; identify causes of damage and take corrective actions.

11. Bearing failures
Possible causes and corrective measures:
① Improper lubrication. Ensure use of suitable lubricating oil; regularly inspect to prevent water or dirt contamination.
② Misalignment. Check alignment; make corrections if necessary.
③ Bearing clearance not meeting specifications. Measure clearance; adjust or replace bearings as required.
④ Imbalance in compressor or coupling. Inspect compressor and coupling for dirt buildup or missing parts; rebalance if necessary.

12. Thrust bearing failures
Possible causes and corrective measures:
① Excessive axial thrust. Ensure the coupling remains clean; avoid transferring excessive axial thrust from the driver to the compressor during assembly.
② Improper lubrication. Inspect oil pump, filters, and coolers; monitor oil temperature, pressure, and volume; replace oil if quality does not meet requirements.

13. Rising bearing temperatures
Possible causes and corrective measures:
① Blocked oil lines, clogged filters, or insufficient oil supply. Clean oil lines and filters; increase oil flow rate.
② High oil inlet temperature. Increase water flow through the oil cooler.
③ Too small or uneven bearing clearance. Re-machine bearing surfaces and adjust clearances.
④ Water-contaminated or degraded lubricating oil. Analyze oil quality and replace it with fresh oil.
⑤ Dust or impurities entering bearings. Clean bearings thoroughly.
⑥ Clogged or inefficient oil cooler. Clean the oil cooler.
⑦ Severe vibration of the unit. Identify and eliminate sources of vibration.
⑧ Incorrect or undersized oil wedge in thrust bearings. Replace bearing pads.
⑨ Too small an opening in the bearing oil inlet throttle; insufficient oil flow. Enlarge the throttle diameter appropriately.
⑩ Insufficient cooling water flow through the oil cooler; excessively high oil temperature. Increase cooling water flow.
⑪ Incorrect Babbitt alloy grade or defective casting of bearing linings. Recast using the specified Babbitt alloy grade per drawings.
⑫ Too shallow or narrow oil reservoirs in bearing linings. Deepen and widen oil reservoirs as needed.

14. Alarm indicating increased shaft displacement
Possible causes and corrective measures:
① Malfunctioning axial displacement sensor. Diagnose and repair sensor faults.
② Damaged thrust bearings. Repair or replace bearing pads.
③ Unstable machine operation. Identify and eliminate underlying causes.
④ Improper installation. Inspect and adjust the axial displacement monitoring system.
⑤ Blocked oil lines; insufficient oil supply to bearings. Clean oil lines thoroughly.
⑥ Machine vibration accompanied by rising bearing temperatures. Perform emergency shutdown and conduct inspections/repairs.

15. Oil seal ring and seal ring failures
Possible causes and corrective measures:
① Misalignment and vibration. Refer to sections dealing with vibration issues.
② Contaminants in oil. Inspect oil filters; replace filters containing contaminants; verify overall pipeline cleanliness.
③ Incorrect seal ring clearance. Measure clearance; adjust or replace seal rings if necessary.
④ Insufficient oil pressure. Verify reference gas pressure; it must not fall below minimum limits.

16. Unstable or abnormal sealing system operation
Possible causes and corrective measures:
① Inadequate precision of seal rings. Inspect seal rings; repair or replace them if necessary.
② Poor quality or incorrect temperature of sealing oil. Evaluate oil quality; replace it if parameters do not meet requirements ； Check the temperature of the seal oil and make adjustments accordingly. ③ The pressure difference system for oil and air is not functioning properly; check the pressure of the reference gas as well as the related circuits, and adjust them to the specified values ； Check the operation of all components in the pressure difference system. ④ If the sealing parts are worn or damaged, remove the seals and reassemble them; carry out repairs or replacements as specified. ⑤ If the sealing rings are unevenly worn, gently grind the contact surfaces between the shaft sleeves, impeller hubs, etc., and the seals, and adjust them to be at a right angle. ⑥ If there are gaps on the end face of the floating seat or if the sealing surface is worn, eliminate the damage caused by suction and reduce wear; replace it with a new one if necessary. ⑦ If the contact surfaces of the floating seat are not worn evenly, grind or adjust these surfaces or replace them with new ones. ⑧ If the sealing rings are broken or damaged, be careful not to cause further damage during assembly; minimize idling operation. Replace them if they cannot be repaired. ⑨ If the sealing surfaces, seals, and “O”-rings are corroded, analyze the properties of the gas and replace the materials or components. ⑩ If freezing occurs in the sealing parts due to low-temperature operation, try to eliminate the freezing or purify the atmosphere surrounding the seals using dry nitrogen. ⑾ If there are errors in the readings of the measuring instruments, check these instruments; repair or replace them if they are inaccurate. 17. Damaged compressor impeller: Possible causes and corrective measures. ① Poor quality material with insufficient strength: Re-examine the materials used in the original design and manufacturing; if the material is unsuitable, replace the impeller. ② Reduced strength due to unfavorable operating conditions: If the operating conditions are not meeting the requirements and this leads to reduced strength, improve the conditions so that they meet the design specifications. ③ Excessive load resulting in reduced strength: High rotation speeds, high flow rates, or high pressure ratios can reduce the strength of the impeller and cause damage ； Operation under severe overload or at excessive speeds is prohibited. ④ Abnormal vibrations, as well as collisions between moving and stationary parts, are also not allowed; excessive vibration can cause the moving parts to come into contact with the stationary parts, leading to damage. It is strictly forbidden to operate the equipment when the vibration levels are too high ； Eliminate abnormal vibrations ⑤ Debris entering the compressor can damage the impeller or other components ； It is strictly prohibited for impurities to enter the compressor; the intake air must be filtered. ⑥ Condensate water – If condensate water enters or if the gas contains moisture that condenses inside the machine, this can lead to water hammer and corrosion. It is necessary to prevent water from entering and accumulating. ⑦ Deposition of impurities – It is important to maintain the purity of the gas; any deposits in the flow channels and cylinders should be removed promptly. ⑧ Stress corrosion and chemical corrosion – Measures must be taken to prevent stress concentration ； Prevent harmful substances from entering the compressor ； Take proper anti-corrosion measures for the compressor. 18. Abnormal noise from the gear speed increaser: Possible causes and corrective actions: ① The gears break suddenly due to overload or shock loads (fatigue fracture or load concentration fracture); repair or replace the gears ； Start the machine smoothly and slowly, and ensure stable operation. ② Fatigue pitting, adhesion damage, or plastic deformation on the gear surfaces: Repair or adjust the gears; replace them if the damage is severe. ③ Poor meshing between the gear working surfaces: Reinstall and adjust the gear meshing. ④ Inappropriate gear clearance: Readjust the clearance. 19. Increased gear vibration – Possible causes and corrective measures: ① Gear wear or damage: Adjust the meshing clearance or replace the gear. ② Poor contact accuracy of the gear surfaces: Improve machining precision and trim the gear surfaces. ③ Poor alignment of the centerlines: Reinstall and align the gears properly. ④ Too small clearance in the bearing shells: Adjust the bearing shells accordingly. ⑤ Poor lubrication: Identify the cause and eliminate it. ⑥ Caused by vibration from the drive motor or compressor: Identify the cause and remove the source of vibration. 20. Poor lubrication of the gears – Possible causes and corrective measures: ① Oil has deteriorated, contains water, or has impurities: Conduct a chemical analysis of the oil, identify the cause, and change the oil. ② Blockage in the oil supply system: Inspect the oil circuit system and clean it. 21. Drop in lubricating oil pressure – Possible causes and corrective measures: ① Faulty main oil pump: Switch to another pump, inspect, and repair the oil pump. ② Broken oil pipes or leaks at connections: Inspect, repair, or replace the affected pipe sections. ③ Blockage in the oil circuit or oil filter: Switch to another filter, clean it. ④ Low oil level in the oil tank: Top up the oil. ⑤ Malfunction in the oil circuit control mechanism: Inspect and adjust it. ⑥ Failure of the oil pressure control system or pressure gauge: Inspect, repair, or replace the pressure gauge. ⑦ Sudden increase in bearing temperature: Stop the machine and check the surface of the babbitt material. 22. Severe fluctuations in oil pressure – Possible causes and corrective measures: ① Air or other impurities mixed in the oil circuit: Open the vent valve and remove the impurities. ② Faulty oil pressure control valve: Adjust or replace the valve. ③ Defective oil pressure gauge: Inspect, repair, or replace it. ④ Severe vibration in the oil pump or pipelines: Identify the cause and eliminate the source of vibration. 23. High oil temperature after the oil cooler – Possible causes and corrective measures: ① Insufficient cooling water volume: Increase the amount of cooling water circulating. ② Scaling in the cooler, reducing efficiency: Remove the scale. ③ Deteriorated lubricating oil: Change the oil. ④ Low cooling water pressure, resulting in high water temperature: Increase the cooling water pressure and volume. ⑤ Pipeline faults causing interruption in cooling water supply: Inspect the pipelines and resolve the faults. 24. Vibration, heating, or noise from the main oil pump – Possible causes and corrective measures: ① Improper assembly of the oil pump: Reassemble it according to the instructions. ② Misalignment between the oil pump shaft and the motor shaft: Realign them properly. ③ Loose foundation bolts: Tighten the foundation bolts. ④ Excessive clearance in the bearing shells: Adjust the bearing shell clearance. ⑤ Pulsations in the pipelines: Tighten the connections or add pipe clamps. ⑥ Wear or damage to components: Repair or replace the damaged components. ⑦ Unstable relief valve or safety valve: Adjust or replace the valve. 25. Increase in oil temperature – Possible causes and corrective measures: ① High temperature of the oil at the outlet: Increase the amount of cooling water circulating. ② Insufficient cooling water volume: Increase the flow rate of cooling water. ③ Bubbles or deterioration in the lubricating oil system: Release the gas from the oil system and change the oil. ④ Scaling in the oil cooler, reducing its cooling efficiency: Inspect the oil cooler and remove the scale. 26. Deterioration of the lubricating oil – Possible causes and corrective measures: ① Water and gases from the compressor mixing with the lubricating oil, causing it to become cloudy or change color: Check the mechanical seal of the compressor to determine if there is any increasing leakage ； Check the “O”-ring of the shaft sleeve; address any issues found promptly. ② If the oil level is too high or the oil is foamy, stop the machine to check the oil level; replace the oil if its quality is poor. 27. Sudden decrease in the amount of lubricating oil: Possible causes and corrective measures ① Fault in the oil pump: Check whether the main oil pump is operating ； When the main oil pump is switched, is the auxiliary oil pump running? ② Oil leakage at the input shaft of the oil pump: Check the amount of leakage at the input shaft; replace the oil seal if necessary. ③ Oil leakage at the mechanical seal of the gearbox: Inspect the mechanical seal and address any issues promptly. 28. Overload of the prime mover: Possible causes and corrective measures ① The molecular weight of the gas is higher than the specified value: Check the actual molecular weight and compare it with the values given in the manual. ② Electrical problems with the prime mover: Check the thermal capacity and operating condition of the circuit breaker, verify whether the voltage has decreased, and ensure that the current difference between phases is within 3%; address any issues found promptly. ③ Mechanical defects in the prime mover, gearbox, compressor, etc., or component collisions: Disassemble the prime mover and check whether the shafts of the prime mover and gearbox are free to rotate smoothly ； Study the discharge condition of the lubricating oil to check for metal wear particles ； Disassemble the compressor body to check for any signs of contact or scraping. ④ The surface of the diffuser adjacent to the impeller is corroded, resulting in a reduced diffusion effect; disassemble the machine for inspection. Examine each flow channel in the diffuser; if corrosion is present, the material should be improved or the surface hardness increased ； Clean the surface (by rubbing it with emery cloth) to make it smooth ； If the impeller comes into contact with the diffuser, or if the diffuser is deformed, it should be replaced. ⑤ If the impeller or diffuser is deformed, it must be repaired or replaced. ⑥ If the rotating parts come into contact with the stationary parts, disassemble the prime mover, compressor, and gearbox, check the gaps between various components and compare them with the specifications in the manual; address any issues that are found promptly. ⑦ High suction pressure: A high suction pressure results in a higher mass flow rate and greater power consumption. Compare this situation with the specifications in the manual to identify the cause and take corrective action