[Weekly Topic] (Main components of equipment and their functions) 2011.08.22–08.28

A centrifugal compressor consists of a rotor, stator, bearings, and other components. Components such as impellers are fitted on the main shaft to form the rotor, which is supported by bearings and rotated at high speed by a power engine. The stator includes components such as the casing, baffles, seals, intake chamber, and volute chamber. Fixed components such as diffusers, bends, and return channels are formed between the partitions. A centrifugal compressor with only one impeller is called a single-stage centrifugal compressor, while those with two or more impellers are called multi-stage centrifugal compressors. A stage consists of channels such as the impeller and the diffuser located behind it. The impeller is a key component of centrifugal compressors, and there are two types: closed and semi-open. A closed impeller consists of blades, an impeller cover, and a disk, while a semi-open impeller lacks an impeller cover. When the impeller rotates at high speed, due to the mutual interaction of forces between the blades and the gas—primarily the effect of centrifugal force—the gas is drawn in from the center of the impeller and flows along the blade passages (the spaces between the blades) toward the outer edge of the impeller. The impeller does work on the gas; the gas gains energy, and its pressure and velocity increase. Then, as the gas flows through passages such as diffusers, its velocity decreases while the pressure increases further, that is, kinetic energy is converted into pressure energy. The gas exiting the diffuser enters the volute for delivery, or passes through a bend and a return stream to proceed to the next stage for further compression. Throughout the compression process, the specific volume of the gas decreases while its temperature increases. As the temperature increases, compressed gas requires more energy. To save power, multi-stage centrifugal compressors often employ intercooling when the pressure ratio is greater than 3. The groups of stages separated by intercooling are called sections. The gas enters the intermediate cooler from the previous stage; after being cooled to lower its temperature, it proceeds to the next stage for further compression. Intercoolers are generally cooled by water. The parts contained in each casing are called cylinders. Centrifugal blowers have a low exhaust pressure, so they generally feature a single-cylinder design without intermediate cooling.

Centrifugal compressor unit: 1. Motor, gearbox, compressor. Function: 1. The motor serves as the driving device; alternatively, a turbine can be used as the driving device. The motor is 10KV in voltage and 418A in current, and it is started using a state-of-the-art soft-start high-voltage cabinet. There are no manufacturers of high-capacity frequency conversion cabinets. 2. The gearbox is a closed-type variable-speed gearbox, which is determined based on the direction and speed of the main equipment. Center distance: 650 mm, rotational speed: 6320 revolutions per minute, herringbone gears; both the input and output sides are equipped with diaphragm couplings. 3. As a key piece of equipment in the gas, chemical, metallurgical, and air separation industries, the stable operation of compressors depends on whether these industries can operate normally. The structure is relatively complex; the rotor, support bearings, thrust bearings, radial bearings, impeller, instrumentation and control system, water system, and oil system all comply with API standards, while the mechanical manufacturing system meets API standards as well as relevant mechanical standards. Due to the high rotational speed, lower vibration requirements are needed. The current operation of the compressor industry is primarily controlled by PLCs.

Roots vacuum pump: The pump contains two lobed rotors that rotate synchronously in opposite directions. Pumping is achieved by the pushing action of a pair of leaf-shaped rotors rotating synchronously in opposite directions within the pump chamber to move the gas.

This post was last edited by Waiter on 2011-8-22 at 16:58. The Roots vacuum pump (simply referred to as Roots pump) is a rotary positive displacement vacuum pump. Rotary vane pumps have the following characteristics: · They exhibit a high pumping speed over a wide pressure range ； · Starts quickly and can begin working immediately ； · Not sensitive to dust and water vapor contained in the gas being drawn in ； · The rotor does not require lubrication, and there is no oil in the pump chamber ； · Low vibration, good dynamic balance conditions of the rotor, no exhaust valve ； · Low driving power and low mechanical friction losses ； · Compact structure, small footprint ； · Operating and maintenance costs are low. Therefore, Roots pumps are widely used in the metallurgy, petrochemical, papermaking, food, and electronics industries. file:///C:/DOCUME~1/lenovo/LOCALS~1/Temp/ksohtml/wps_clip_image-11534.png Working principle of the Roots pump: The structure of the Roots pump is shown in the figure. Inside the pump chamber, there are two \"8\"-shaped rotors that are mounted perpendicularly on a pair of parallel shafts, and they are driven by a pair of gears with a gear ratio of 1 to rotate synchronously in opposite directions. A certain gap is maintained between the rotors and between each rotor and the inner wall of the pump casing, enabling high-speed operation. Since a Roots pump is a vacuum pump without internal compression and typically has a very low compression ratio, high- and medium-vacuum pumps require a pre-pump. The ultimate vacuum of a Roots pump depends not only on the pump’s own structure and manufacturing precision but also on the ultimate vacuum of the pre-pump. To increase the ultimate vacuum of the pump, Roots pumps can be used in series. The working principle of a Roots pump is similar to that of a Roots blower. Due to the continuous rotation of the rotor, the gas to be pumped is drawn in through the inlet into the space v0 between the rotor and the pump casing, and then discharged through the outlet. Since the v0 volume is completely sealed after inhalation, the gas in the pump chamber does not compress or expand. But when the top of the rotor passes over the edge of the exhaust port and space v0 becomes connected to the exhaust side, the higher gas pressure on the exhaust side causes some gas to flow back into space v0, resulting in a sudden increase in gas pressure. As the rotor continues to rotate, the gas is discharged outside the pump. The figure shows the pumping process of the Roots pump rotor as it moves from 0° to 180°. At the 0° position (figure a), the lower rotor seals in a volume of gas v0 from the pump inlet. When rotated to the 45° position (point b in the figure), this cavity communicates with the exhaust port. Due to the higher pressure on the exhaust side, some of the gas is pushed back. When turned to the 90° position (shown as c in the diagram), the gas enclosed in the lower rotor, along with the backflowing gas, is discharged outside the pump. At this time, the upper rotor also seals in a volume v0 of gas at the pump inlet. When the rotor continues to rotate to 135° (shown as d in the diagram), the gas enclosed in the upper rotor comes into contact with the exhaust port, and the above process repeats. The 180° position (Figure e) is the same as the 0° position. One full rotation of the rotor shaft displaces a total of four volumes of gas at v0. file:///C:/DOCUME~1/lenovo/LOCALS~1/Temp/ksohtml/wps_clip_image-2341.pngfile:///C:/DOCUME~1/lenovo/LOCALS~1/Temp/ksohtml/wps_clip_image-20713.png file:///C:/DOCUME~1/lenovo/LOCALS~1/Temp/ksohtml/wps_clip_image-22190.pngfile:///C:/DOCUME~1/lenovo/LOCALS~1/Temp/ksohtml/wps_clip_image-6815.png file:///C:/DOCUME~1/lenovo/LOCALS~1/Temp/ksohtml/wps_clip_image-18879.png

Turbine diaphragms: The specific structure of a diaphragm is determined by its operating temperature and the steam pressure difference acting on both sides. There are mainly three types: (1) Welded diaphragms: These have high strength and stiffness, good airtightness, and are easy to manufacture; they are widely used in the high and medium pressure sections of turbines with medium to high parameters. ⑵ Narrow-nozzle welded diaphragms: In the high-pressure sections of high-parameter, high-power steam turbines, where the steam pressure difference across each stage is large, the diaphragms are made thick, while the static blade height is short; therefore, narrow-nozzle welded diaphragms with a smaller width are used. The advantage is low nozzle loss, but the presence of a considerable number of guide ribs will increase the resistance of the steam flow. Domestically produced 125MW and 300MW turbines both feature useful narrow-nozzle welded diaphragms. ⑶ Cast diaphragms: These are easy to manufacture and have low costs, but the surface finish of the static blades is poor, and they cannot be used at very high temperatures – generally, the temperature should be below 300°C. Therefore, they are used in the low-pressure sections of steam turbines.

The task of the condensing equipment is to establish and maintain a specified vacuum level at the exhaust port of the turbine. The steam turbine exhaust is condensed into water and sent back to the boiler for reuse. The functions of the various components are as follows: The condenser’s role is to transfer the heat from the exhaust steam (vaporization latent heat) to the cooling water, creating a high vacuum at the exhaust port to cause the steam to condense into water, and to remove some of the oxygen from the condensed water. The function of the circulating water pump is to continuously supply cooling water to the condenser, and to discharge or cool down the water that has absorbed heat and thus increased in temperature, so that it can be reused. The function of the condensate pump is to continuously transfer the condensate from the hot water wells of the condenser to the deaerator under vacuum conditions, while maintaining an appropriate water level in the condenser. The function of the air extraction equipment (air ejector or vacuum pump) is to continuously extract non-condensable gases from the condenser in order to maintain a vacuum within it.

Structure, principle, and function of the gas ballast valve in vacuum pumps. The gas ballast valve accelerates the evacuation of vapors without contaminating the oil. If a vacuum pump is used to extract only inert gases, and those gases do not liquefy as pressure increases, then there is no issue with the degradation of the pump oil. However, if the pump is used for vacuum drying or to extract humid air, the gas mixture contains not only inert gases but also water vapor. When a pump without a gas trap is used to remove this water vapor, the vapor will liquefy and dissolve in the oil, thereby deteriorating the oil’s vacuum properties and reducing the pump’s pumping speed and vacuum level. The compression process of water vapor is further explained as follows: The water vapor pumped out must be compressed within the compression chamber until the exhaust valve opens. Assuming that the temperature inside the pump is 60°C, the saturated vapor pressure of water at this temperature is 20,000 pascals. Once the water vapor reaches this pressure during compression, it begins to condense into water. However, a pressure of 20,000 pascals is not sufficient to open the exhaust valve, as the valve leads to the atmosphere and is also held closed by a spring; therefore, the internal pressure in the pump chamber must reach over 120,000 pascals in order to open the exhaust valve. By the end of the compression process, all the water vapor has condensed into water, which then mixes with the oil. To meet the requirement of extracting moist air without allowing water vapor to contaminate the oil, this pump is equipped with a gas ballast device. The principle behind this device is that a certain amount of air is introduced during the compression process to increase the pressure of the gas mixture. The pressure of this mixture is equal to the sum of the partial pressure of air and the partial pressure of water vapor. Before the partial pressure of water vapor reaches its saturation pressure at the pump temperature, the pressure of the gas mixture exceeds the pressure at which the exhaust valve opens; as a result, the vapor is expelled from the pump before it has time to condense within it. Another use of the gas-ballast valve is to restore the ultimate pressure of a vacuum pump. Although it is sometimes used to extract ordinary air that contains little condensable gas, the gas-ballast valve is usually kept closed. Over time, however, the oil can become contaminated by the small amount of condensable gas present in the air. For pumps without a gas-ballast mechanism, the only way to restore the pump’s original ultimate pressure is to replace the oil or heat the pump oil to allow the liquid vapor to evaporate. In the case of pumps equipped with a gas-ballast mechanism, it is sufficient to open it for 1 to 2 hours to restore the vacuum pump’s ultimate pressure. When the pressure inside the gas ballast valve is lower than the external pressure, the valve opens. Gas then enters the pump chamber; prior to the partial pressure of condensable gases within the pump chamber reaching the saturation vapor pressure at the pump temperature, the pressure of the compressed gas has already reached the exhaust pressure, thereby expelling the condensable gases ； When the pressure inside the gas-actuated valve is higher than the external pressure, the valve closes

Reply to 1# hannianlong: Reciprocating compressor components: The casing consists of two parts – the cylinder block and the crankcase – and is usually cast as a single unit from high-strength gray cast iron (HT20-40). It is the structure that supports the weight of the cylinder liners, the crankshaft connecting rod mechanism, and all other components, while ensuring that these components maintain the correct relative positions among themselves. The cylinder features a cylinder liner structure, which is mounted in the liner seat holes on the cylinder block, allowing for easy maintenance or replacement when the liner wears out. Crankshaft: The crankshaft is one of the main components of a reciprocating compressor, transmitting all the power of the compressor. Its main function is to convert the rotational motion of the motor into the reciprocating linear motion of the piston via a connecting rod. As it moves, the crankshaft is subjected to alternating combined loads of tension, compression, shear, bending, and torsion; it operates under harsh conditions, which requires it to have sufficient strength and stiffness, as well as wear resistance in its main journal and crankpin areas. Therefore, crankshafts are generally forged from high-quality carbon steel grades 40, 45, or 50. Connecting rod: The connecting rod is the component that connects the crankshaft to the piston; it converts the rotational motion of the crankshaft into the back-and-forth motion of the piston, and transfers power to the piston so that it can do work on the gas. The connecting rod includes a rod body, a rod small-end bushing, a rod large-end bearing shell, and rod bolts. Piston set: The piston set refers to the piston, piston pin, and piston rings together. Driven by the connecting rod, the piston assembly moves back and forth in a straight line within the cylinder; together with the cylinder, this forms a variable working volume that enables processes such as intake, compression, and exhaust. Pistons—Pistons can be divided into two main categories: cylindrical and disc-shaped. The material of the piston is usually aluminum alloy, or cast iron. The piston pin – which is the component used to connect the piston to the small end of the connecting rod – is subjected to complex alternating loads during operation. Piston rings—Piston rings include gas rings and oil rings. The main function of the piston ring is to create a seal between the piston and the cylinder wall, preventing the compressed gas from leaking through the gap between them ； The function of the oil ring is to distribute oil and scrape off excess lubricating oil from the cylinder walls. Air valve and shaft seal: The air valve is an important component of the compressor and is considered a wear part. Its quality and performance directly affect the gas delivery volume of the compressor, power loss, and operational reliability. The valve assembly includes a suction valve and an exhaust valve; each time the piston moves up and down once, both the suction valve and the exhaust valve open and close respectively, thereby controlling the compressor and enabling it to carry out the four working processes of suction, compression, and exhaust. Shaft seal — The function of the shaft seal is to prevent compressed gas from leaking outward along the extension end of the crankshaft, or to prevent outside air from entering when the pressure inside the crankcase is lower than atmospheric pressure. Lubrication system: The lubrication methods for compressors can be divided into two types: splash lubrication and pressure lubrication. Splash lubrication makes use of the mechanical action of moving parts to deliver lubricating oil to the friction surfaces that need it, and many semi-hermetic compressors employ this type of lubrication. On one hand, an oil flinging spoon is installed at the lower end of the connecting rod’s large end, which flings the oil from the crankcase onto the cylinder surface in order to lubricate the friction surfaces between the piston and the cylinder wall ； On the other hand, an oil flinger is mounted on the shaft at one end of the motor; it flings the oil and collects it in the oil collection chamber on the end cover on the motor side, which then uses the oil channels in the crankshaft to lubricate the main bearings and the rod bearings. In some small vertical open-type compressors, splash lubrication is achieved solely through the movement of the crank-slider mechanism.

Dry gas seal: The dry gas seal was developed on the basis of gas dynamic pressure bearings. Structurally, compared to ordinary mechanical seals, the rotating ring and stationary ring sealing surfaces of a dry gas seal are wider ； Special-shaped hydrodynamic pressure grooves, such as spiral grooves, arc grooves, T-shaped grooves, etc., are machined on the end faces of the rotating or stationary ring; the groove depth is generally in the order of 10^-9 m. Rings with dynamic pressure grooves are usually made of SiC, while rings without dynamic pressure grooves are made of C-graphite. The operating principle of dry gas seals is illustrated using a helical groove dry gas seal. When the rotating ring spins at high speed, the helical grooves on the end faces of either the rotating or stationary ring pump the high-pressure gas at the outer diameter downward into the space between the sealing end faces. The gas flows from the outer diameter toward the center, while the sealing barrier controls the flow of gas toward the center; as a result, the gas is compressed, causing the pressure to rise and a high-pressure zone to form at the root of the grooves. The pressure of the gas film on the end face generates an opening force; when the seal is operating stably, this opening force is balanced by the closing force produced by the gas pressure acting on the back side of the compensating ring and the spring force, allowing the seal to operate in a contact-free and wear-free manner. If certain disturbances cause the sealing gap to decrease, the air film pressure generated by the helical grooves will increase, resulting in an increased opening force; meanwhile, the closing force remains unchanged, and the sealing gap will expand until equilibrium is restored ； Conversely, if certain disturbances cause the sealing gap to increase, the air film pressure generated by the helical grooves will decrease, resulting in a reduced opening force; while the closing force remains unchanged. The sealing gap will then decrease, and the seal will quickly return to equilibrium. The ability of a dry gas seal to resist changes in the gas film gap is known as gas film stiffness. Dry gas seals for compressors require a seal gas treatment system for pressure stabilization and filtration; nitrogen is generally used for this purpose. Once the compressor is operating properly, the treated process gas can also be used as a sealing gas.

Balance discs, balance drums, and balance pistons are common components in multi-stage centrifugal pumps and multi-stage centrifugal compressors. Function: To balance the axial force of the equipment. Method: A pipe connected to the equipment’s inlet is attached to the back side of these components; the axial seal is achieved through clearance, while the radial seal is accomplished via clearance as well as a labyrinth seal. Principle: After multi-stage compression, the pressure difference between the outlet and inlet of the equipment becomes very large. Relying solely on thrust bearings to balance this force can easily lead to fatigue and damage of those bearings. By connecting the back side of these components to the equipment’s inlet, a force that opposes the previous force is generated.