Principles, structure, inspection, and maintenance of centrifugal pumps

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Principles, structure, inspection, and maintenance of centrifugal pumps. Principles, structure, inspection, and maintenance of centrifugal pumps. Classification and applications of pumps: 1. Classification of pumps: 1.1 Centrifugal pumps: 1.1.1 Vane pumps; 1.1.2 Axial flow pumps; 1.1.3 Vortex pumps. 1.2 Reciprocating pumps: 1.2.1 Piston pumps; 1.2.2 Plunger pumps; 1.2.3 Diaphragm pumps (positive displacement pumps). 1.3 Rotating pumps: 1.3.1 Gear pumps; 1.3.2 Screw pumps; 1.3.3 Vane pumps. Other types of pumps: Jet pumps, vacuum pumps. 1) Positive displacement pumps: These pumps increase the pressure of the liquid by continuously changing the volume of the working chamber, thereby enabling the transport of the liquid (e.g., reciprocating pumps, gear pumps, etc.). 2) Vane pump: It relies on the rotating impeller; during rotation, due to the interaction between the vanes and the liquid, the vanes transfer mechanical energy to the liquid, thereby increasing its pressure energy and enabling the transport of the liquid (such as centrifugal pumps). 3) Hydrodynamic pump: It utilizes a pressurized working fluid flowing through the pump in order to transport another liquid (e.g., jet pump). 2. Uses of pumps: Pumps are general-purpose machinery used for transporting fluids. In petroleum and chemical plants, pumps are needed to transport, circulate, and pressurize liquids and gases such as raw materials, reflux streams, intermediate products, and solvents. Due to the differences in the processes of various installations, the type and quantity of pumps required also vary; each installation needs between 10 and 20 pumps, with some requiring as many as 100 to 150 pumps. Without a pump, processes such as heating, reacting, and separating the materials in the device cannot take place. It can be seen that pumps play an important role in petrochemical plants. Among various types of pumps, centrifugal pumps are the most widely used. Thanks to their large flow rates, head capacities, and performance ranges, as well as advantages such as simple structure, small size, light weight, smooth operation, and easy maintenance, they account for over 80% of the pumps used in chemical manufacturing. II. Working Principle of Centrifugal Pumps 1. Working Principle of Centrifugal Pumps: The impeller is installed inside the pump casing 2 and fixed to the pump shaft 3, which is driven directly by the motor. In the center of the pump casing, there is a liquid suction port 4 that is connected to the suction pipe 5. The liquid enters the pump through the bottom valve 6 and the suction pipe. The liquid outlet 8 on the pump casing is connected to the discharge pipe 9. When the speed of the pump remains constant, its head and flow rate are related to the diameter of the impeller. The effect of the impeller diameter on these parameters is such that this formula is only valid when the change in impeller diameter does not exceed 10%. 1.3 Pump operation in series or parallel: When a single centrifugal pump is not sufficient to meet the transportation requirements, it is possible to use centrifugal pumps in parallel or in series. By connecting two centrifugal pumps of the same model in parallel, the head change is minimal, but the total flow rate increases. The overall efficiency of the pumps in parallel is the same as that of a single pump ； When centrifugal pumps are connected in series, the total head increases while the flow rate remains roughly the same; the overall efficiency of the series-connected pumps is equal to that of a single pump. When two pumps are connected in parallel, both the flow rate and head increase. However, due to the constraints imposed by the pipeline characteristic curve, the pipeline resistance rises, and the total flow rate delivered by the two pumps in parallel is less than twice the flow rate delivered by a single pump. When two pumps are connected in series, both the head and flow rate increase, but the total head obtained by connecting the two pumps in series is still less than twice the head of a single pump. When the pressure P1 at the pump inlet is equal to or less than the saturated vapor pressure PV of the liquid at the same temperature, the liquid vaporizes. Under high pressure, these bubbles rapidly coalesce or burst, generating shocks with extremely high pressure and frequency. This causes the pump body to vibrate violently and produce noise, while the liquid flow rate, head (exit pressure), and efficiency all decrease significantly. This phenomenon is known as cavitation in centrifugal pumps. Before starting the pump, the pump casing is filled with the liquid to be transported ； Once it is started, the impeller rotates at high speed driven by the shaft, and the liquid between the blades must also rotate along with it. Under the effect of centrifugal force, the liquid is thrown from the center of the impeller toward the outer edge, gaining energy as it moves at high speed away from the impeller’s outer edge and into the volute pump casing. In the volute, the liquid slows down as the flow channel widens, converting some of its kinetic energy into static pressure energy; it then flows into the discharge pipe at a higher pressure and is sent to the desired location. As the liquid flows from the center of the impeller toward the outer edge, a vacuum is created at the center of the impeller. Since the pressure above the liquid level in the reservoir is greater than the pressure at the pump inlet, the liquid is continuously forced into the impeller. It can be seen that as long as the impeller keeps rotating, the liquid will continue to be drawn in and discharged. 2. Air entrapment phenomenon: When there is air inside the pump casing, the lower density of air compared to that of the liquid results in a smaller centrifugal force. As a result, the pressure difference between the surface of the liquid in the tank and the pump’s suction inlet is not sufficient to force the liquid in the tank into the pump; in other words, the centrifugal pump does not have self-priming capability, which prevents it from transporting liquid. This phenomenon is known as the \"suction lock\" phenomenon. To fill the pump with liquid, a bottom valve equipped with a filter is usually installed at the bottom of the suction pipe. This bottom valve serves as a check valve, and the filter prevents solid substances from entering the pump and damaging the impeller or interfering with its normal operation. III. Main components of centrifugal pumps The main components include the impeller, pump casing, and shaft sealing device. 1. Impeller: The function of the impeller is to transfer the mechanical energy of the prime mover directly to the liquid, thereby increasing the liquid’s static pressure energy and kinetic energy (with an emphasis on increasing the static pressure energy). An impeller generally has 6 to 12 backward-curving blades. Impellers come in three types: open, semi-closed, and closed, as shown in Figure 2-2. Open impellers have no covers on either side of the blades; they are simple to manufacture and easy to clean. They are suitable for transporting materials containing a large amount of suspended solids, but their efficiency is low and they cannot handle liquids at high pressures ； The semi-closed impeller has no cover on the suction side but one on the other side; it is suitable for transporting materials that tend to settle or contain particles, and its efficiency is relatively low ； A closed impeller has front and rear cover plates on both sides of the blades; it offers high efficiency and is suitable for transporting clean liquids free from impurities. Most centrifugal pump impellers are of this type. Impellers come in two types of liquid suction methods: single-suction and double-suction. 2. Pump casing: Its function is to enclose the impeller within a certain space, so that the impeller can draw in and discharge liquid. The pump casing is often designed in a volute shape, which is why it is also called a volute. As the cross-sectional area of the flow channel gradually increases, the high-speed fluid ejected from around the impeller sees its velocity decrease, allowing part of its kinetic energy to be effectively converted into static pressure energy. The pump casing not only collects the liquid thrown out by the impeller but also serves as an energy conversion device. 3. The shaft seal device serves to prevent the liquid inside the pump casing from leaking out along the shaft, or to prevent outside air from entering the pump casing. The common shaft sealing devices are packing seals and mechanical seals. The filler is usually oil-impregnated or graphite-coated asbestos rope. Mechanical seals achieve sealing by means of the relative motion between the rotating ring mounted on the shaft and the stationary ring fixed to the pump casing. IV. Classification of centrifugal pumps: There are many ways to classify centrifugal pumps. (1) Classification by the way in which fluid is drawn into the impeller: single-suction pumps and double-suction pumps. (2) Classification by the number of stages: single-stage pumps and multi-stage pumps. (3) Classification based on the orientation of the main shaft: horizontal pumps, vertical pumps, and inclined pumps. V. Performance parameters and working principle of centrifugal pumps: (1) Performance parameters of centrifugal pumps. 1) Flow rate: The flow rate is the amount of liquid that the pump can deliver per unit of time. Let qv represent the volumetric flow rate, in units of m3/s, and let qm represent the mass flow rate, in units of kg/s. Where: qm=ρqv. ②Head: Head is the increase in energy per unit weight of liquid from the pump inlet to the pump outlet; it represents the effective energy gained by 1 N of liquid as it passes through the pump. Its unit is N*m/N = m, which represents the height of the liquid column pumped by the pump. Head is primarily reflected in the increase in liquid pressure. ③Rotational speed: It refers to the number of revolutions per unit time of the pump shaft, usually denoted by n, with the unit being r/min. ④NPSH: Also known as net positive suction head, NPSH is a key parameter that indicates the cavitation resistance of a system; it is generally denoted by NPSH and is measured in meters. ⑤The power of a pump usually refers to the input power, that is, the shaft power transmitted from the prime mover to the pump shaft, denoted by N, with units of W or KW. ⑥Efficiency: The effective power of a pump is denoted by Ne; it represents the effective energy acquired by the liquid pumped out per unit of time, and Ne = gρqvH. The efficiency of a pump is the ratio of the useful power to the shaft power, that is, η=Ne/N. It reflects the degree of energy loss in the pump. The losses in a pump can generally be divided into: volumetric losses (energy loss caused by flow leakage), flow losses, and mechanical losses (friction losses in bearings, sealing devices, and the impeller). ⑵Working principle of centrifugal pumps: Before starting a centrifugal pump, the outlet valve must be closed, and the pump should be filled with liquid; this process is known as priming the pump. During operation, the prime mover is started to rotate the impeller; the blades in the impeller cause the liquid to rotate as well, generating centrifugal force that pushes the liquid along the blade channels toward the outlet of the impeller. From there, it is sent through the volute into the discharge pipe with the outlet valve open. The liquid gains mechanical energy from the impeller, which increases its pressure energy and kinetic energy; it is this energy that enables the liquid to reach the working location. As the liquid is continuously ejected, a low pressure is created at the inlet of the impeller. A pressure difference is created between the liquid in the suction chamber and the liquid on the centerline at the inlet of the impeller. Under this pressure difference, the liquid in the suction chamber continuously flows through the suction pipe and the pump’s suction chamber into the impeller, thereby enabling the centrifugal pump to operate continuously. VI. Cavitation in Centrifugal Pumps 1. Mechanism of cavitation: When a centrifugal pump is in operation, the pressure of the fluid decreases as it moves from the pump inlet to the inlet of the impeller; the liquid pressure is lowest near the blades. Thereafter, as the impeller does work on the liquid, the pressure rises rapidly. When the pressure near the inlet of the impeller blades is less than or equal to the saturated vapor pressure at the liquid transport temperature, the liquid vaporizes. At the same time, gases dissolved in the liquid may also escape, forming many bubbles. When the bubble moves with the liquid to a region of higher pressure within the flow channel, the external liquid pressure is higher than the vaporization pressure inside the bubble, causing the bubble to collapse and form a cavity. In an instant, the liquid around it rushed toward the cavity at extremely high speeds, causing the liquids to collide with each other and leading to a sudden surge in local pressure (which can reach several hundred atmospheres). This not only hinders the normal flow of the fluid, but more seriously, if these bubbles collapse near the wall of the impeller, the liquid acts like countless small projectiles, continuously striking the metal surface at a high frequency (ranging from 2000 to 3000 Hz), causing the metal surface to crack due to impact fatigue. If the bubbles contain certain reactive gases (such as oxygen), they utilize the energy released during bubble condensation (with local temperatures reaching 200–300°C) to form thermocouples and induce electrolysis, thereby causing electrochemical corrosion of the metal and accelerating the rate of its degradation. The combined phenomenon of vaporization, condensation, and shock of such liquids, which results in high-pressure, high-temperature, high-frequency shock loads that cause mechanical spalling and electrochemical corrosion damage to metal materials, is known as cavitation. 2. Severe consequences of cavitation: Cavitation is a phenomenon specific to hydraulic machinery, and it brings about many serious consequences. ①Cavitation causes erosion and damage to the components through which fluid flows. In centrifugal pumps, the areas that are damaged by cavitation first appear near the inlet of the blades, and then extend to the outlet of the impeller. Initially, pitting appears on the metal surface; subsequently, the surface develops grooves, honeycomb-like patterns, and fish-scale-like cracks. In severe cases, this leads to perforations in the front and rear covers of the blade or impeller, or even to the rupture of the impeller itself, resulting in serious accidents. Therefore, cavitation severely affects the safe operation and service life of the pump. ②Cavitation reduces the performance of the pump. It severely disrupts the energy conversion between the impeller and the fluid, leading to a decline in the pump’s performance; in severe cases, it can even cause the flow of liquid to be interrupted and the pump to stop working. ③Cavitation causes noise and vibration in the pump. When bubbles collapse, the liquid particles collide with each other as well as with the wall surfaces, generating noises of various frequencies. In severe cases, a \"cracking\" explosive sound can be heard inside the pump, simultaneously causing vibration in the unit. The vibration of the unit further causes more bubbles to form and collapse; this mutual interaction leads to intense cavitation resonance, forcing the unit to shut down, otherwise it will be damaged. ④Cavitation is also a major obstacle to the development of hydraulic machinery toward higher flow rates; as the fluid flow rate increases, the pressure decreases, making it easier for vaporization to occur and leading to cavitation. 3. Reasons for cavitation in centrifugal pumps: 1. The temperature of the medium being transported is too high ； 2. The water level in the tank is too low, allowing air to be drawn in ； 3. The pump is installed at too high a height ； 4. The flow rate and the resistance in the suction pipeline are too high ； 5. The intake pipeline and pressure gland (referring to those without a liquid seal) have poor sealing, allowing air to enter. VII. Common causes of failures in centrifugal pumps and methods for troubleshooting Fault symptoms Possible causes Troubleshooting methods 1. No water output from the pump a. Inlet and outlet valves not open, blockages in the inlet and outlet pipelines, blockages in the impeller’s flow channels. b. The motor is running in the wrong direction, and its speed is very low due to a missing phase. c. Air leakage in the suction tube. d. The pump is not filled with liquid; there is air in the pump chamber. e. Insufficient imported water supply, excessive suction lift, and leakage in the bottom valve. f. Excessive pipeline resistance and improper pump selection. a. Check and remove obstructions. b. Adjust the motor direction and tighten the motor connections. c. Tighten all sealing surfaces to remove air. d. Open the pump cover or the exhaust valve to discharge the air. e. Stop the machine for inspection and adjustment (this phenomenon often occurs with water supply pipes connected to the grid and those used at a suction lift). f. Reduce the number of bends in the pipelines and select a different pump. 2. Insufficient flow rate of the water pump: a. First, check according to the reasons listed in 1. b. Blockages in the pipelines or the flow channels of the pump impeller, as well as scale deposition. c. The voltage is too low. d. Impeller wear a. First, follow step 1 to eliminate the issue. b. Remove obstructions and readjust the valve opening. c. Voltage stabilization d. Replace the impeller. 3. Excessive power: a. Use beyond the rated flow rate. b. Excess suction lift c. Worn pump bearings. a. Adjust the flow by closing the outlet valve. b. Reduce the suction lift. c. Replace the bearings. 4. Noise vibration a. Unstable pipeline support b. Gas mixed in the liquid. c. Cavitation occurs. d. Bearing damage. e. The motor runs hot due to overload. a. Secure the pipelines. b. Increase the suction pressure and exhaust pressure. c. Reduce the vacuum level. d. Replace the bearing. e. Adjustment: 5. Motor heating: a. Excessive flow rate, operating under overload conditions. b. Collision. c. Damaged motor bearings. d. Insufficient voltage. a. Close the outlet valve. b. Check and rule out. c. Replace the bearings. d. Voltage stabilization. 6. Water pump leakage a. Wear of mechanical seal. b. The pump body has sand holes or is cracked. c. The sealing surface is flat. d. Loose mounting bolts. a. Replace. b. Weld repair or replacement. c. Trimming. d. Durable. 7. Bearing overheating: Lack of oil in the bearing or too high oil viscosity affects lubrication. The bearing wear gap is too large. The pump and motor are not aligned; refuel. Replace it with oil or an oil of lower viscosity. Replace the bearing. Adjust the pump and motor to ensure they are concentric. 8. The outlet pressure gauge shows pressure, but the pump outputs very little water or no water at all. The outlet pipe has high resistance. The direction is wrong. Impeller clogged. The rotation speed is insufficient; check the flexibility of the bottom valve flap and remove any blockages. Keep the suction pipeline as simple as possible. Reduce the water absorption height. 9. The outlet pressure gauge indicates pressure, but the pump outputs very little water or no water at all. The outlet pipe has high resistance. The direction is wrong. Impeller clogged. The rotation speed is not high enough. Reduce pipe resistance. Check the motor direction. Remove impurities from the impeller. Increase the speed. 10. Excessive power consumption by the pump: The packing is compressed too tightly, causing heating in the packing chamber. Impeller wear. The pump is delivering too much water – loosen the packing gland. Replace the impeller. Close the gate valve to reduce the flow rate. 11. Abnormal noise inside the pump; no water is pumped out due to excessive resistance in the suction pipe. The water absorption height is too high. Air has entered the suction pipe. The temperature of the liquid being pumped is too high. Check whether the suction pipe is blocked. Clean the bottom valve, lower the liquid temperature, or reduce the suction height. VIII. Starting, Operation, and Maintenance of Centrifugal Pumps (I) Starting and Operation 1) Open the inlet valve fully and close the outlet pipeline valve. 2). Connect the power supply; once the pump reaches its normal operating speed, gradually open the valve on the discharge pipeline and adjust it to the desired operating conditions. 3). Pay attention to the instrument readings and check for leaks in the shaft seals; under normal conditions, the mechanical seal should leak at a rate of less than 3 drops per minute. Check that the temperature rise at the motor and bearings is ≤70 degrees. If any abnormalities are detected, they must be addressed promptly. (II) Shutdown: 1. Gradually close the valve on the discharge pipeline and cut off the power supply ； 2. Close the inlet valve ； 3. If the ambient temperature is below 0 degrees, the liquid inside the pump should be drained to prevent it from freezing and cracking ； 4. If it is to be out of use for an extended period, the pump should be disassembled, cleaned, and stored in packaging. (III) Maintenance and Care A. Maintenance during operation 1. The water pipelines must be highly sealed ； 2. It is prohibited to operate the pump for extended periods in a cavitation state ； 3. It is prohibited to allow the motor to operate with excessive current for an extended period while the pump is running at its flow rate ； 4. During regular inspections of the pump’s operation, the motor experiences excessive current over a prolonged period ； 5. A dedicated person should monitor the pump during operation to prevent accidents ； 6. The bearings should be lubricated every 500 hours of pump operation. Motors with a power of over 11 KW are equipped with a filling system that allows oil to be injected directly using a high-pressure gun, ensuring optimal lubrication of the bearings ； 7. After the pump has been in operation for an extended period, mechanical wear can lead to increased noise and vibration in the unit; in such cases, it should be stopped for inspection. If necessary, the vulnerable parts and bearings can be replaced. The general maintenance cycle for the unit is one year. B. Mechanical seal maintenance 1. The lubricant for mechanical seals should be clean and free of solid particles ； 2. It is strictly prohibited for mechanical seals to operate under dry grinding conditions ； 3. Before starting, the pump (motor) should be rotated a few times to prevent the seal rings from breaking or being damaged due to sudden startup.
Fault symptoms, causes, and corrective measures:
Vibration, heating, smoking, leakage, wear, and formation of deposits in the mechanical seal; excessive end face width – reduce the end face width and lower the spring pressure; excessive end face specific pressure – reduce the end face specific pressure; rough surfaces of the rotating and stationary rings – improve the surface finish; improper pairing of the friction pairs – replace the rotating and stationary rings and pair them properly; poor cooling effects and deteriorating lubrication – enhance cooling measures and improve lubrication conditions; poor corrosion and heat resistance of the end faces – replace the rotating and stationary rings with those that offer better corrosion and heat resistance; intermittent leakage – excessive axial movement of the rotor, preventing the rotating ring from compensating for this movement – adjust the axial movement; unstable operation of the pump and pressure fluctuations – stabilize the operating pressure of the pump; constant leakage – severe vibration of the pump shaft – stop the pump for maintenance to address the issue of shaft movement; inaccurate sealing positioning and poor fit between the friction pairs – adjust the positioning; damaged or uneven friction surfaces – replace or grind the friction surfaces; the seal ring not fitting tightly against the rotating ring – inspect or replace the seal surface; insufficient or eccentric spring force – adjust or replace the spring; improper fixation of the end cover, resulting in misalignment – adjust the fastening screws of the end cover so they are perpendicular to the shaft; severe leakage – damaged or broken friction pairs – inspect and replace the rotating and stationary rings; the fixing ring rotating – replace the seal ring to secure the stationary ring; the rotating ring unable to move axially – check the spring force and whether the thrust ring is stuck; broken spring – replace the spring; broken or ineffective anti-rotation pin – replace the anti-rotation pin; leakage when restarting after shutdown – carbon buildup or scale on the friction surfaces – clean the seals; crystals or solid particles between the springs – the rotating ring or thrust ring is stuck – excessive wear on the friction surface surfaces; excessive spring force and high end face specific pressure – replace the spring; unclean sealing medium – use a filtering device; excessive compression of the spring – adjust the spring.

IX. Principles and Steps for Selecting Centrifugal Pumps
(A) Selection principles: Pumps are versatile mechanical devices that are widely used in various industries such as petroleum, chemicals, power and metallurgy, mining, shipbuilding, light industry, agriculture, civil applications, and defense. They play an important role in the national economy. According to statistics from 1979, China’s pump production reached 1.256 million units. The electrical energy consumption of pumps accounts for over 21% of the country’s total electrical energy consumption. Therefore, significantly reducing the energy consumption of pumps is of great importance for saving energy. The so-called rational selection of pumps involves taking into account a range of technical and economic factors, such as the investment and operating costs associated with the pump units and pumping stations, in order to ensure that the choice meets the principles of economy, safety, and suitability. Specifically, there are the following aspects: 1. It is necessary to meet the requirements regarding flow rate and head; in other words, the operating point of the pump (the intersection of the device’s performance curve and the pump’s performance curve) must remain within the high-efficiency range as much as possible. This not only helps save energy but also reduces the risk of damaging the pump’s components. 2. The pump selected should be small in size, light in weight, and inexpensive, while also possessing good performance characteristics and high efficiency. 3. It has excellent cavitation resistance, operates smoothly, and has a long service life. 4. The selected pump requires less capital investment and has lower operating costs. (II) Selection steps: 1. List the basic data: 1) Properties of the medium: name of the medium, specific gravity, viscosity, corrosivity, toxicity, etc. 2) The particle diameter and concentration of the impurities contained in the medium. 3) Medium temperature: (°C) 4) Required flow rate. For general industrial pumps, the leakage in the piping system can be ignored in the process flow, but the impact of process changes on the flow rate must be taken into account. If agricultural pumps are used for water conveyance through open channels, leakage and evaporation rates must also be taken into account. 5) Pressure: pressure in the water intake tank, pressure in the water discharge tank, and pressure drop (head loss) in the piping system. 6) Pipeline system data (pipe diameter, length, types and quantities of pipeline accessories, geometric parameters from the suction tank to the pressure tank, etc.). If necessary, the device characteristic curve should also be prepared. When designing and arranging pipes, the following points should be taken into account: A. Select the pipe diameter appropriately. A larger pipe diameter results in a lower flow velocity at the same flow rate, thereby reducing frictional losses; however, it increases costs. A smaller pipe diameter leads to a sharp increase in frictional losses, which in turn requires a pump with higher head pressure, greater power consumption, and higher costs as well as operating expenses. Therefore, it should be considered from both technical and economic perspectives. B. The discharge pipe and its fittings should take into account the maximum pressure they can withstand. C. The piping layout should be arranged as straight as possible; accessories within the pipes should be minimized, and the length of the pipes should also be reduced as much as feasible. When bends are necessary, the radius of curvature of those bends should be 3 to 5 times the diameter of the pipes, with the angle being as large as possible, ideally greater than 90°. D. A valve (such as a ball valve or globe valve) and a check valve must be installed on the discharge side of the pump. Valves are used to adjust the operating point of the pump, while check valves prevent the pump from rotating in reverse when fluid flows backward, thus protecting the pump from water hammer effects. (When the liquid flows back, a huge reverse pressure is generated, which can damage the pump.) II. Determining flow rate and head 1. Determination of flow rate a. If the minimum, normal, and maximum flow rates are specified in the production process, the maximum flow rate should be taken into consideration. b. If only the normal flow rate is specified in the production process, a certain margin should be considered. For high-flow, low-head pumps with ns>100, the flow margin is set at 5%, for ns