Have you mastered the 94 questions on the basics of chemical engineering equipment?

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1. What is the definition of a chemical process pump? Answer: A pump is a device that converts the mechanical energy of a prime mover (internal combustion engine, electric motor, turbine, etc.) into static pressure energy and kinetic energy, thereby enabling the transport of fluids. 2. How are chemical pumps classified? Answer: Based on their working principle, they can be classified into the following categories: Positive displacement pumps: These pumps draw in or discharge liquid by means of continuous changes in their working volume. Common types include reciprocating pumps, piston pumps, etc. Velocity pump: It transfers energy to the liquid through the high-speed rotation of the impeller, thereby generating pressure and flow in the liquid. Based on the flow pattern of the liquid, they are further classified into centrifugal pumps, axial flow pumps, vortex pumps, etc. Other types of pumps: jet pumps, air lifters, etc. 3. What is the working principle of a centrifugal pump? Answer: When the prime mover drives the impeller to rotate at high speed, the liquid contained within the pump is forced, by centrifugal force, from the center of the impeller toward its outer edges. In this process, the liquid gains energy, resulting in an increase in its static pressure energy; at the same time, as the flow velocity increases, its kinetic energy also increases. The liquid leaves the impeller and enters the pump casing; as the flow channel widens, the fluid velocity decreases, and part of the kinetic energy is converted into static pressure energy. The liquid then enters the discharge duct at a higher pressure. When the liquid is ejected from the center of the impeller, a low pressure is created at the center of the impeller, while the pressure at the liquid surface is higher than that value. Driven by this pressure difference, the liquid enters the pump through the suction pipe. As the impeller keeps rotating, liquid is continuously drawn in from the center of the impeller and discharged at a constant pressure. 4. What are the main components of a centrifugal pump? Answer: A centrifugal pump consists of a pump casing, pump body, impeller, suction chamber, and discharge chamber. 5. What are the main performance parameters of centrifugal pumps? Answer: (1) Flow rate: The amount of liquid discharged by the pump per unit time, also known as the pumping capacity. The unit is m3/s. In engineering, m3/h or L/s are commonly used. The flow rate depends on the structure, size, and rotational speed. ⑵ Head, also known as pump head, is the energy actually obtained per unit weight of liquid by the pump. Its unit can be expressed as. ⑶ Power is the work done by a pump per unit of time. The work actually obtained by the pump per unit of time is also known as the useful power. The power that the pump receives from the prime mover is called shaft power. ⑷ Efficiency is the ratio of effective power to shaft power. The efficiency is always less than 1. 6. How does cavitation in centrifugal pumps occur, and what are its hazards? How to prevent cavitation? Answer: When the pressure at the pump’s suction inlet drops below the saturated vapor pressure of the liquid inside the pump at that temperature, the liquid boils, resulting in the formation of numerous bubbles. At the same time, certain gases dissolved in the liquid will also escape to form bubbles as the pressure decreases. As the bubbles enter the high-pressure area along with the liquid, they burst rapidly, creating a local vacuum. The surrounding liquid then rushes into the space occupied by the bubbles at high speed, colliding with each other and converting its kinetic energy into pressure energy, thereby generating a very high local impact force in an instant. This impact force causes significant damage to the impeller, and can result in a honeycombed or spongy structure forming on its surface. When cavitation occurs, the pump body is subjected to shocks that cause vibration and noise; the pump’s performance deteriorates sharply, and in severe cases flow stops and the pump can no longer function properly. Means to prevent cavitation are: (1) the installation height of the pump body must be lower than the allowable suction vacuum level. ⑵ The effective NPSH provided by the pump installation is greater than the NPSH required by the pump. ⑶ As the flow rate increases, the net positive suction head also increases, and this should be taken into account during operation. ⑷ Be careful that the operating temperature of the liquid being transported is not too high. 7. Why must the outlet valve be closed first when starting and stopping a centrifugal pump? Answer: If the outlet valve is not closed when starting a centrifugal pump, the motor may be damaged due to excessive starting power. By closing the outlet valve, Q = 0, and the required power is minimized, thereby preventing such accidents from occurring. When stopping the pump, if the outlet valve is not closed first and a sudden stop occurs, the high-pressure liquid in the discharge pipe may flow back into the pump, causing the impeller to rotate at high speed and resulting in damage. To protect the equipment, it is necessary to close the outlet valve first, and then shut down the motor. 8. Why is the pump filled with liquid before it is started? Answer: If the pump is not filled with liquid before it is started, air will be present inside the centrifugal pump. The density of air is much lower than that of liquid, so the centrifugal force generated by the rotating impeller is very small, resulting in a low level of vacuum. Consequently, the pressure difference between the liquid level in the tank and the inlet of the pump is also small, which is not sufficient to push liquid into the pump. This leads to the impeller spinning idly without being able to draw in any liquid – this phenomenon is known as \"air locking\". 9. What are the main components of a reciprocating pump? How does its flow regulation method differ from that of centrifugal pumps? Answer: A reciprocating pump is mainly composed of a pump cylinder, plunger, piston rod, suction valve, and discharge valve. The flow rate of centrifugal pumps is generally adjusted by controlling the degree of opening of the outlet valve, whereas reciprocating pumps usually regulate their flow rate by changing the stroke of the plunger and by installing a return circuit. 10. What are the startup and shutdown procedures for ordinary centrifugal pumps? Answer: (1) Driving (with power supplied to the motor beforehand): 1) Rotate the shaft for more than one full turn to ensure that the rotor is not stuck ； 2) Check that the oil level in the oil cup is normal (above 1/2) ； 3) Confirm that the outlet valve is closed and open the inlet valve ； 4) Vent the pump body; close it once it meets the requirements ； 5) Open the minimum flow line valve ； If seal water is to be used, apply the seal water ； 6) Start the motor; once the outlet pressure is normal, open the outlet valve to confirm that the motor pump is operating properly. 7) If the centrifugal pump is equipped with an auxiliary oil pump, establish oil circulation first before starting the pump ； Those that require a heat pump must have a qualified heat pump. ⑵ Parking: 1) Close the pump outlet valve and stop the motor ； If there is a auxiliary oil pump, confirm that it starts automatically; otherwise, start it manually. 2) For maintenance, the centrifugal pump should be powered off, the oil drained, and after proper replacement it can be handed over for maintenance. 11. What are the advantages and disadvantages of centrifugal pumps, reciprocating pumps, and rotary pumps? Answer: Centrifugal pump: It has a simple and compact structure, provides uniform flow rates that are easy to adjust, generates little vibration, requires minimal foundation support, has a wide range of applications. Its head is not high, its efficiency is low for liquids with high viscosity, it handles only small flow rates, and its head is limited ； Reciprocating pump: high head, fixed flow rate, capable of dry suction, high efficiency, can be used as a metering pump ； But the flow is uneven, the structure is complex, and it has a transmission mechanism ； Rotary pump: Suitable for liquids with low flow rates, high head, and high viscosity. 12. What are the differences in starting a centrifugal pump and a gear pump? Answer: To reduce the starting load and avoid overloading, the outlet valve of a centrifugal pump is generally closed at start-up, while for a gear pump, the outlet valve must be opened to prevent pressure buildup. 13. In which applications within petrochemical production are cast iron pipes, water-gas pipes, and seamless steel pipes used respectively? Answer: Cast iron pipes are used for underground water mains, gas pipes, sewage pipes, etc. Water-gas pipes are commonly used in water pipes, heating systems, gas pipelines, compressed air lines, and vacuum systems with relatively low pressures; they can also be used in steam branches and condensate pipelines with modest pressure levels. Seamless steel pipes are widely used in the transportation of materials under high hydraulic pressure and high temperatures. 14. What are the main components of commonly used petrochemical pipelines? Answer: Commonly used petrochemical pipelines mainly consist of pipes, pipe fittings, and valves. 15. Into which categories can pipe fittings be divided based on their functions? Give one or two examples to illustrate. Answer: (1) To change the direction of the pipeline: such as elbows. (2) To connect branches of a pipeline: such as tees and crosses. (3) To connect two sections of pipe: such as pipe clamps and couplings. (4) To modify pipeline branches: such as reducers. (5) To block a pipeline: such as plug valves. 16. Why are safety accessories installed on pressure vessels? Answer: To ensure the safe operation of pressure vessels and prevent accidents caused by overpressure, and in order to take measures to eliminate or reduce various factors that lead to overpressure in the vessels, safety pressure relief devices must be installed on them. 17. Explain the applications of safety valves? Answer: 1) Vessels in which pressure may increase during the production process due to chemical reactions or other reasons. 2) Containers for storing liquefied gas. 3) There is no safety valve installed at the pressure source of the container, or although a safety valve is installed, the permitted operating pressure of the container is lower than the pressure at the source. 4) Pressure vessels with a maximum operating pressure lower than that of the pressure source. 18. What are the basic conditions for the proper operation of pressure vessels? Answer: 1) Do not exceed temperature, pressure, or load limits. 2) The pressure addition and removal speeds should not be too fast. 3) The temperature increase and decrease should be carried out at the specified rate. 4) The number of starts and stops must be controlled as specified, and pressure and temperature must not experience significant fluctuations. 5) Safety facilities must be complete, sensitive, and reliable. 6) Regular inspections at fixed locations, at fixed times, and along fixed routes. 7) Immediate and proper countermeasures must be taken in case of abnormalities in the container. 19. Why is the outlet valve closed when starting a centrifugal pump, and why cannot it remain closed for too long? Answer: Close the outlet valve to temporarily reduce the outlet flow to zero; at this point, the power required by the pump is at its minimum. This allows the motor load to be kept at the lowest level, thereby preventing the motor from being damaged due to excessive starting current. After the motor is running properly, the outlet valve should be gradually opened; otherwise, it will cause the pump casing to heat up and damage the components. 20. What are the hazards of cavitation? Answer: When cavitation occurs, the pump generates noise and vibration, and its head, flow rate, and efficiency all decrease. Cavitation accelerates material degradation and shortens the pump’s service life. Severe cavitation can disrupt the normal operation of the pump and pose a threat to safe production. 21. Why can’t the compression ratio of a reciprocating compressor be too high? Answer: The temperature of a gas rises during compression; therefore, it needs to be cooled while being compressed. Otherwise, this will consume energy, undermine the lubricating effect of the oil, and may even cause the oil to catch fire. Cylinders and gases are generally cooled using water jackets; however, when the compression ratio is too high (e.g., above 6), this cooling method alone is not effective, so the compression ratio cannot be too high. 22. What are the reasons why pressure vessels experience operational issues or have their existing internal defects exacerbated during use? Answer: Corrosion of the medium, abrasive wear from high-speed airflow, pressure fluctuations, temperature fluctuations, and prolonged stress. 23. What are the common failure modes of pressure vessels? Answer: Plastic failure, brittle failure, fatigue failure, creep failure, corrosion failure, etc. 24. Why do some centrifugal pumps require a minimum flow rate? Answer: Centrifugal pumps transport liquids by imparting kinetic energy to them. When a centrifugal pump with high power operates with the outlet valve closed, the fluid becomes increasingly hot due to the increase in kinetic energy; in severe cases, this can damage the bearings and cause cavitation. The minimum flow rate of some pumps is intended for the pump’s hot standby. 25. Why do some standby pumps need to be in hot standby, and how many types of hot standby methods are there? Answer: When the medium temperature exceeds 150°C, the standby pump should be placed in thermal standby mode. The main purpose is to prevent sudden changes in fluid temperature, which could cause thermal stress within the pump. There are generally two methods for hot standby: 1) The hot fluid is directed from the outlet of the operating pump to the outlet and then to the inlet of the standby pump, with a small amount of flow returning inside the pump. 2) Jacketed steam heating. 26. Function of valves: Answer: Valves are components used to control the flow rate and pressure of fluids in pipelines; operators use various valves to achieve control and regulation during the production process. (1) Opening and closing function – shutting off or allowing the flow of fluid within the pipe. (2) Regulating function – Regulates the flow rate of the fluid inside the pipe. (3) Throttling effect – Adjusting the valve opening to change the pressure drop, thereby regulating the pressure. 27. Globe valve: Answer: A globe valve is composed of components such as a valve core, valve seat, valve body, valve stem, packing, packing cover, and handwheel. Based on the direction of fluid flow, stop valves can be divided into straight-through type, straight-flow type, and angle type. Based on the position of the valve stem thread, they can be divided into open-ended and concealed stem types. Small-diameter stop valves are generally of the dark-threaded stem type ； Stop valves with larger diameters, higher operating temperatures, and those used in corrosive media are generally of the exposed-thread rod type. When installing a stop valve, the medium must flow from bottom to top through the gap between the valve core and the valve seat. The advantages of globe valves are their simple structure, good sealing performance, and ease of manufacturing and maintenance. Its disadvantages are high fluid resistance, a long valve body, and large space requirements. Stop valves are widely used to shut off fluids and regulate flow rates. 28. Gate Valve: Answer: A gate valve, also known as a gate-type valve, has a gate plate installed inside the valve body that is perpendicular to the direction of fluid flow; the valve opens when the gate plate rises and closes when it descends. A gate valve is composed of components such as a handwheel, valve stem nut, gland, valve stem, valve body, gate plate, and sealing surface. Valve stems can be divided into exposed stems and concealed stems. Straight-stem gate valves are generally used for corrosive media and in indoor environments ； Dark stem gate valves are used for non-corrosive media and in locations where operational space is limited. Based on the structure of the gate plate, they can be divided into wedge-type and parallel-type categories. Wedge-type valves are usually designed with a single gate, whose sealing surfaces on both sides are wedge-shaped. Parallel types are mostly designed with double gate plates, and the sealing surfaces on both sides are parallel to each other. Parallel types are easier to manufacture and repair than wedge types, but they are not suitable for transporting fluids containing impurities; they can only handle clean fluids. Gate valves are commonly used to shut off materials, oils, gases, and other fluids, and are not suitable for regulating flow rates. Since a gate valve in a partially open position can cause the portion of the gate that remains below to be worn by the medium, this over time leads to poor sealing at the contact surfaces and leakage; therefore, the gate valve should be kept either fully closed or fully open. The advantages of gate valves are good sealing performance, low flow resistance when fully open, and a short valve body. The disadvantages are its complex structure, prone-to-wear sealing surfaces, and difficulty in maintenance. Gate valves are commonly used as discharge valves for containers, inlet valves for centrifugal pumps, in pipelines where the medium flows in both directions, in pipelines where low resistance to medium flow is required, and in locations where the installation length of the valve body is limited. 29. Check valve: A check valve, also known as a non-return valve or one-way valve, operates automatically based on the pressure difference between the fluid on the upstream and downstream sides of the valve, thereby preventing the medium from flowing in reverse. When the fluid flows in the correct direction, the valve core rises or lifts up ； When the fluid flows in reverse, the valve core closes automatically, allowing the fluid to flow in only one direction. The commonly used check valves fall into two main categories: lift-type and swing-type. (1) Lift-type check valve. It is mainly composed of components such as the valve body, the valve cover (with guide grooves), and the valve seat. In this type of valve, the valve core is connected to the valve stem; there is a guide groove on the valve cover that allows the valve stem to slide up and down, enabling the valve core to move vertically relative to the valve body. It is commonly installed in horizontal pipelines. (2) Swing check valve. This type of valve is mainly composed of components such as a valve body, valve cover, valve core, and valve seat, and it features a simple structure and low flow resistance. The upper end of its valve core is connected to the valve body via a pin, and opening or closing is achieved through the free swinging of the valve core; such valves are commonly used in vertical or large-diameter pipelines. The disadvantages are high noise and poor sealing. 30. Ball valve: The disc of a ball valve is spherical in shape; by rotating this disc by a certain angle, the flow of fluid within the pipe can be allowed to pass or blocked. It is mainly used in pipelines for transporting fluids at low temperatures with high viscosity. 31. Safety valve: Answer: A safety valve is a device for overpressure protection. Its function is to automatically open when the pressure inside the container exceeds a specified value, thereby rapidly releasing the excess pressure gas from the container and emitting a sound to warn the operator to take measures to reduce the pressure. When the pressure returns to the allowable level, the safety valve closes automatically again, keeping the pressure inside the container below the upper limit of the allowable range and preventing explosions caused by overpressure. Based on their overall structure and the type of loading mechanism, safety valves can be divided into lever-type and spring-type. This device is commonly used as a spring-loaded safety valve. A spring-loaded safety valve is primarily composed of components such as a valve body, valve core, valve seat, valve stem, spring, spring gland, adjustment screw, pin, cover, and lifting handle. It utilizes the elastic force generated by the compressed spring to counteract the force exerted by gas on the valve core. 32. Butterfly valve: It achieves opening and closing by the rotation of the valve disc; in use, it is generally required that the valve disc be fully open or fully closed to operate the valve. It is commonly used for inlet and return water valves in circulating cooling water pipelines. 33. Operation and maintenance of valves: Answer: (1) It is strictly prohibited to use large wrenches to operate valves with a small diameter. (2) Valves that can be operated by hand must never be operated with an F wrench when force is applied. (3) After the valve is fully opened, it should be closed a notch to prevent sticking. (4) When closing the valve, if sticking occurs, turn it back a few turns before closing. Forcible closure is strictly prohibited. 34. What is the structure of a tubular heat exchanger? Answer: Tubular heat exchangers are the most widely used type of heat exchanger in chemical production. They have a simple structure and are easy to manufacture, making them the primary heat exchange equipment in chemical plants today. (1) A fixed-tube-sheet heat exchanger is mainly composed of components such as a shell, tube sheet, and tube bundle end caps. A balance tube bundle is placed inside the shell, and the tube bundle is connected to the tube sheet by welding or expansion welding. The tube sheet is welded directly to the shell, while the head is connected to the shell flange. Fixed-tube-sheet heat exchangers have thin tube sheets and low manufacturing costs, but it is difficult to clean the shell side, and temperature difference stress exists. (2) A U-tube heat exchanger features heat exchange tubes that are bent into a U shape, with both ends fixed to the same tube sheet. The tube bundle can expand and contract freely, so no thermal stress is generated due to temperature differences in the fluid. Since this type of heat exchanger has only one tube sheet, the tube bundle can be removed for cleaning between the tubes; however, the U-shaped bends inside the tubes are difficult to clean. When the diameter of the heat exchanger is large, it is difficult to support the U-shaped sections, and the tube bundle has poor vibration resistance. Such heat exchangers are suitable for situations where there is a large temperature difference between the two media, or where the fluid in the shell side tends to form scale and requires cleaning ； The pipeline medium is impurity-free and does not tend to clog. (3) In a floating-head heat exchanger, one end of the tube sheet is fixed between the shell side and the tube box, while the other end of the tube sheet can move freely within the shell. The tube bundle in such a heat exchanger can expand freely and can be removed for easy cleaning of the spaces between and inside the tubes. Its disadvantages include a complex structure and high cost; leaks at the floating head area are difficult to detect and repair during operation. Floating-head heat exchangers are suitable for situations where there is a large temperature difference between the shell and the tube bundle, or where the medium on the shell side tends to form scale. 35. Maintenance of heat exchangers: Answer: Regular inspections are an important means of detecting and addressing sudden failures early; the inspection items include temperature, pressure, appearance, etc. (1) Temperature is an important parameter for the operation of heat exchangers. By measuring the temperatures of the fluids entering and leaving the heat exchanger, it is possible to determine the flow rate of the medium and the efficiency of heat transfer. The quality of heat transfer is reflected in the heat transfer coefficient, which generally remains relatively stable over short periods; should it change, it will decrease steadily. Regularly measuring the temperatures of the fluids entering and leaving the heat exchanger allows one to determine whether there is scaling or blockage in the heat exchanger, and whether cleaning is necessary. (2) By measuring and checking the fluid pressure as well as the pressure difference between the inlet and outlet, it is possible to determine the degree of scaling and blockage inside the heat exchanger, as well as the fluid flow rate or any leaks. High-pressure fluid leaks into the low-pressure fluid, causing its pressure to rise rapidly, even to excessive levels, which can lead to various adverse consequences. (3) The visual inspection of the heat exchanger includes checking for vibration, external leaks, and any damage to the insulation layer. (4) For the circulating water heat exchanger, maintain a log of the inlet and outlet water temperatures. (5) Regular calibration and inspection of the main safety valve and explosion-proof plate. 36. What is a pump? Answer: A machine that transports fluids and increases their pressure is called a pump. The pump itself is a machine that consumes energy. 37. How to detect cavitation in a pump. Answer: The usual signs of trouble are noise and vibration in the pump, a decrease in pressure and flow rate, as well as a reduction in efficiency. 38. How to prevent cavitation? Answer: (1) The inlet valve should be fully open to avoid excessive negative pressure at the inlet. (2) The pump’s installation height must not exceed the specified limits. (3) Pumps designed for high flow rates should not be used in applications requiring low flow rates, and the outlet valve should not be set to a too high opening degree. (4) The medium temperature must not exceed the specified limit: (5) High-head pumps cannot be used in applications requiring low head ； (6) The speed of the adjustable motor must not exceed the specified value. 39. It is known that the pump’s flow rate is 2000 m3/h and the diameter of the suction pipe is 700 mm. What is the flow velocity? Answer: Q = V·S = V·(1/4)·πD². Therefore, V = Q·(4/πD²) = 2000×4/(3.14×0.72)÷3600 = 1.44 m/s.
40. What is the function of the pump casing? What is the function of the impeller? Answer: Functions of the pump casing: (1) To collect the water that is ejected from the impeller’s flow channels and direct it toward the pump’s outlet ； (2) Slow down the flow rate of the water that is ejected, and increase its pressure: (3) Serve to connect other components and provide support. The function of the impeller is to force water to flow out of the pump through its centrifugal force or power. 41. What are the power and efficiency of a pump? What is the allowable suction vacuum level? What are the units? Answer: Power N: The power transmitted to the pump shaft by the prime mover per unit of time, under certain flow rates and head conditions, is called shaft power; its unit is kilowatts or horsepower ； Efficiency ŋ: During the process of pumping liquids, various energy losses occur; as a result, not all of the power supplied to the pump shaft is transferred to the liquid, and this value reflects the relative degree of those energy losses. The suction vacuum height Hs represents the maximum height at which a water pump can draw water, measured in meters. 42. What is the working principle of a centrifugal pump? What are the volumetric losses in a centrifugal pump? Answer: (1) When a centrifugal pump is filled with liquid, the rotation of the impeller generates centrifugal force. As a result, the liquid in the impeller’s flow channels is pushed toward the edges of the impeller and flows into the pump casing. At this point, the pressure at the center of the impeller decreases; this lower pressure compared to that in the inlet pipe causes the fluid to flow toward the center of the impeller. Due to the continuous rotation of the impeller, the liquid is continuously drawn in and discharged. (2) Sealing ring leakage loss, balance mechanism leakage loss, and inter-stage leakage loss. 43. What are the reasons for overload of the centrifugal pump motor? Answer: (1) The pressure at the discharge end of the pump exceeds the designed pressure, resulting in excessive back pressure ； (2) Due to the decrease in liquid temperature causing an increase in viscosity, or the viscosity exceeding that specified during design ； (3) The opening of the control valve is too small, or the check valve is malfunctioning, or the pipeline is blocked, etc., resulting in an increased resistance in the pump’s discharge pipeline ； (4) The impeller is clogged by sticky debris, or the intermediate stages of a multi-stage pump are blocked ； (5) The pump shaft or motor shaft is not aligned, and the coupling is not properly aligned ； (6) The mouth ring of the impeller is severely damaged (worn), with an excessive gap ； (7) The stuffing box is too tight, the sealing of the stuffing box is damaged, or the shaft sleeve is worn out. 44. What are the disadvantages of running a centrifugal pump at idle for a long time? Answer: When a pump runs at idle for an extended period, no liquid is being pumped through it. As a result, the sealed parts within the pump, such as the mouth ring, intermediate bushings, and shaft seals, have small gaps. Lack of lubrication leads to friction, causing wear or even shaft seizure ； In the case of mechanical seals, rotating contact surfaces such as balance discs suffer from overheating due to friction, and may even be damaged, as there is no liquid lubrication or cooling available. 45. What are the mechanical losses of centrifugal pumps? What are the hazards of cavitation in centrifugal pumps? Answer: The mechanical losses of centrifugal pumps include friction losses in the bearings and shaft seals, as well as losses in the impeller. Its harms include: (1) damaging the material ； Impeller, guide vanes, pump casing, etc. (2) It affects the performance of the pump: its output and head decrease, while power consumption increases. (3) The pump vibrates and generates loud noise, preventing it from functioning properly. 46. Describe the four practical performance curves of centrifugal pumps. What is pump shutdown water hammer? Answer: (1) Flow–head curve (Q–H curve), Flow–power curve (Q–N curve), Flow–suction vacuum height curve (Q–Hs curve), Flow–efficiency curve (Q–η curve). (2) Pump shutdown water hammer refers to the pressure variation that occurs in the pump and its piping systems when the pump stops operating due to a sudden power outage or other reasons, as a result of changes in the flow rate of water. 47. What are the operational characteristics of a centrifugal pump? Answer: The pump and the piping system form an inseparable whole; the head generated by the pump must be equal to the head required in the piping system. The flow rate discharged by the pump should be equal to the flow rate required within the pipeline; at this point, the device is in a stable operating state. This operating state is achieved automatically through balance. Therefore, plotting the characteristics of the pump along with those of the pipeline on a single diagram is referred to as the performance characteristics of the centrifugal pump. 48. How is the suction performance of a centrifugal pump measured? What are the characteristics of submersible pumps? Answer: (1) The suction performance of a centrifugal pump is measured by the allowable suction vacuum height Hs; the higher the value of Hs, the better the pump’s suction performance or its NPSH capacity ； (2) Integrated pump and motor; no long rotating shaft required, light weight ； Both the motor and the water pump are submerged underwater, eliminating the need to build a pump house ； The maintenance costs are low. 49. What should be checked during the operation of a centrifugal pump? Answer: (1) Check whether all operating parameters (pressure, flow rate, oil temperature, etc.) meet the specified values ； (2) Check whether the lubricating oil is of qualified quality and whether the oil level is within the required range ； (3) Check the condition of the cooling water. (4) Check whether there are any abnormalities such as vibration in the pumps ； (5) Check the shaft temperature. (6) Check for leaks in all areas. 50. What are the causes of seal leakage in centrifugal pumps? Answer: (1) The water seal tube or valve is blocked ； (2) The filler is pressed too tightly ； (3) Crushing of the water gland and friction with the shaft ； (4) Shaft bending or broken shaft; (5) The water seal ring and the water seal pipe opening do not align, preventing the sealing water from reaching inside the water seal ring ； 51. What are the causes of seal leakage in centrifugal pumps? Answer: (1) The packing is compressed too tightly, the packing seal is damaged, or the shaft sleeve is worn out; (2) The packing material was not selected appropriately, or the packing or water seal ring was not installed correctly; (3) The mechanical seal was not selected or installed properly, resulting in its damage. 52. What are the reasons for a centrifugal pump failing to discharge liquid? Answer: (1) The pump’s rotation speed is insufficient or the motor is running in reverse; (2) The impeller is loose or installed incorrectly (in the case of double-suction pumps), or the impeller is severely corroded; (3) The pressure at the pump’s discharge end exceeds the designed pressure, resulting in excessive backpressure ； (4) The viscosity increases as the liquid temperature drops, or it exceeds the viscosity specified during design. (5) The resistance in the pump’s discharge pipeline increases due to reasons such as too small an opening of the control valve, malfunctioning check valves, or blockages in the pipeline. (6) There are air bubbles in the discharge pipeline. (7) The impeller becomes blocked as it sucks in sticky debris, or the intermediate stages of a multi-stage pump become blocked. 53. What is cavitation in centrifugal pumps? Answer: According to the working principle of centrifugal pumps, the rotation of the impeller generates centrifugal force, which creates a local vacuum at the pump inlet. If the pressure at this point is lower than the saturated vapor pressure of the liquid being pumped at that temperature, the liquid begins to vaporize and bubbles are formed. When these bubbles reach areas within the impeller where the pressure is higher, they collapse. Due to the very fast rate at which these bubbles burst, the surrounding liquid rushes into the space once occupied by the bubbles at extremely high speeds. This phenomenon is known as cavitation. 54. Why cannot a centrifugal pump operate for an extended period with the outlet valve closed? Explain the purpose of connecting centrifugal pumps in series. Answer: (1) If the outlet valve remains closed for too long, the rotating impeller continues to stir the fluid inside the pump, and friction within the pump generates high heat, which can cause the components to deform and get damaged. (2) I. Increase the pump’s head ； II. The flow rate is equal to the flow rate of a single pump. 55. What are the reasons for insufficient flow rate in centrifugal pumps? How to address it? Answer: (1) There is air inside the pump or pipelines; simply remove the air. (2) There are leaks in the pump body or inlet pipelines; fix these leaks. (3) The inlet and outlet pipelines are blocked; clean them. (4) The viscosity of the liquid exceeds the specified values; adjust the liquid’s viscosity. (5) There are foreign objects in the impeller; inspect and clean them ； (6) If the mouth ring is worn or the clearances between various parts inside the pump are too large, replace the mouth ring or adjust the clearances. 56. What are the reasons for the overheating of centrifugal pump shafts? Answer: (1) The pump and motor are not on the same axis, and the coupling is not aligned ； (2) Damage to the bearing shells or rolling bearings, with the bearing shells being either too tight or having too large a clearance ； (3) Scaling and blockage in the cooling system, insufficient or interrupted supply of cooling water ； (4) Dust, dirt, or corrosive liquids enter the bearing shells or bearings. 57. What are the causes of vibration and noise in centrifugal pumps? Answer: (1) Air leakage at the suction line flange ； (2) Pump not primed or gas not fully discharged ； (3) Clogging due to the intake of sticky debris by the impeller, or clogging in the intermediate stages of multi-pump systems ； (4) Rotor imbalance or shaft bending deformation ； (5) The pump and motor are not coaxial, and the coupling is not aligned. (6) The foundation bolts are loose, or the foundation is weak ； (7) Wear of the impeller mouth ring; excessive clearance ； (8) The stuffing box is too tight, the packing seal is damaged, or the shaft sleeve is worn out ； (9) The material of the filler was not selected properly, or the filler or water seal ring was not installed correctly. 58. What are the reasons for a decrease in the pressure value of a centrifugal pump? Answer: (1) The bottom valve of the suction pipe is not open or is malfunctioning; the suction bottom valve or filter screen is clogged ； (2) The liquid level drops, the submersion depth of the suction pipe opening is insufficient, and the installation height exceeds the allowable suction height ； (3) The liquid level at the suction side drops, or the liquid temperature rises ； (4) Insufficient pump speed or motor running in reverse ； (5) The impeller is loose or installed in the wrong direction, and the impeller is severely corroded ； (6) The pressure at the pump’s discharge end exceeds the design pressure, resulting in excessive backpressure ； (7) An increase in viscosity due to a decrease in liquid temperature, or a viscosity that exceeds the design value ； (8) The resistance faced by the pump increases (pipeline resistance) due to reasons such as too small an opening of the control valve, a failed check valve, or blockages in the pipeline ； (9) The impeller gets clogged by sticky debris drawn in, or the intermediate stages of a multi-stage pump become blocked. 59. What are the reasons for a centrifugal pump to lose suction (fail to draw in liquid)? Answer: (1) Air enters the suction side of the pump due to a poor seal at the suction flange ； (2) The pump has not been filled, or air was not removed after filling; gas remains inside the pipes or within the pump casing ； (3) The bottom valve of the suction pipe is not open or is malfunctioning; the suction bottom valve or filter screen is clogged ； (4) The liquid level drops, the submersion depth of the suction pipe is insufficient, and the installation height exceeds the pump’s allowable suction vacuum level. (5) The liquid level at the suction side drops, or the liquid temperature rises ； (6) Insufficient pump speed or motor running in reverse ； (7) The impeller is loose or installed in the wrong direction (double-suction pump), or the impeller is severely corroded ； (8) Wear of the impeller mouth ring; excessive clearance ； (9) The packing gland is tightened too much, the packing seal is damaged, or the shaft sleeve is worn out ； (10) Improper selection of filler material, or incorrect installation of the filler or water seal ring. (11) Inappropriate selection or installation of the mechanical seal leads to its damage. 60. Why is it necessary to fill the pump with water before starting a centrifugal pump? Why must the outlet valve of a centrifugal pump be closed before it can be started? Answer: (1) The density of air is much lower than that of water; the centrifugal force generated by the rotating impeller is not sufficient to create a sufficient vacuum inside the pump. As a result, there is no pumping capacity. Only when the pump is filled with liquid can the centrifugal force of the impeller create enough vacuum to expel the liquid, which is why it is necessary to fill the pump with water. (2) Since starting without closing the outlet valve means starting under load, the current is too high during such startup, which can easily damage the motor or other electrical equipment. 61. Why open impellers and semi-open impellers? How many are the basic performance characteristics of a water pump? What are they each? Answer: (1) An open impeller refers to an impeller that has only blades without a complete cover ； A semi-open impeller is an impeller that has only a rear cover but no front cover ； (2) The basic performance parameters of a water pump are six: a, flow rate Q ； b、Head H ； c、Shaft power N ； d. Efficiency η ； e. Rotational speed n ； f. Allowable suction vacuum level: 62. What are the regulations regarding emergency pump shutdown? Answer: Whenever possible, the pump in use should be turned off first; however, the pump can be stopped immediately in the following emergency situations: (1) Smoke or fire emerging from the cable connector ； (2) Smoking from pumps, motors, and bearings ； (3) The current excess is too high; even after reducing the outlet valve, the load remains above the limit ； (4) Pump shaft breakage ； (5) Pump casing and pipes are cracked, with excessive leakage or no connection to the motor ； (6) Personal safety is at risk. 63. Explain the reasons why water does not flow out after the water pump is started Answer: (1) The motor wiring is reversed, causing the impeller to rotate in the opposite direction ； (2) The impeller is installed in the wrong direction; the orientation of the impeller blades is incorrect ； (3) The water absorption surface is too low ； (4) Evacuation caused by cavitation ； (5) Air leaks into the inlet pipe, causing evacuation ； (6) The impeller or inlet pipe is blocked by debris. 64. What is the working principle of a reciprocating pump? Answer: Reciprocating pumps belong to the category of positive displacement pumps. As the piston moves from the left end to the right end of the pump cylinder, the volume of the working chamber within the cylinder gradually increases, causing the pressure to drop. The liquid drawn in rises along the suction pipe under the effect of atmospheric pressure, pushing open the suction valve and entering the pump cylinder. Then, under the action of an external force, the piston moves to the left, compressing the liquid; as a result, the pressure increases, and the liquid is pushed out through the discharge pipe into the discharge container. When the piston moves to this left dead center, it drains out the liquid that has been drawn in, completing one reciprocating cycle; as the piston continues to move, the pump keeps delivering liquid continuously. 65. Under what circumstances should an electric pump be stopped urgently? Answer: (1) The motor is overheating or smoking ； (2) Bearing overheating or smoking ； (3) Sudden intense vibration of the equipment ； (4) There is an obvious metallic sound inside the motor or pump ； (5) The bearing packing has burned out, resulting in severe leakage. 66. What are the reasons why a water pump stops delivering water suddenly during operation? How to handle it? Answer: (1) Inlet valve and flanged air intake. Identify the cause and stop the vehicle for repair ； (2) The water level is above the surface or too low; stop the pump and wait for the water level to rise before restarting it ； (3) If there is a blockage in the head or in the impeller flow channels, remove the debris after shutting down the machine ； (4) If the pump shaft breaks, major repairs are required ； (5) If the impeller is damaged or completely dislodged, stop the pump for maintenance. 67. How are single-stage and multi-stage pumps, as well as single-suction and double-suction pumps, distinguished from each other? What causes noise inside the pump? Answer: (1) The number of stages refers to the number of impellers; in other words, the count of impellers equals the number of stages. Single-suction and double-suction refer to whether the impellers come into contact with water from one side or both sides. (2) a. Air gets into the inlet pipe or cavitation occurs; b. Local damage to the rotor or stator; c. Foreign objects are drawn in; d. Poor lubrication and cooling, leading to wear. 68. Why is it that the water pump is usually installed first in a unit? What is the reason for installing the pump on the same shaft as the motor? Answer: (1) The water pump is generally installed first, followed by the motor, because the motor is only related to the location of the water pump, whereas the location of the water pump involves the connections with other equipment and pipelines. (2)a. Unbalanced water inflow on both sides, resulting in axial thrust; b. Bearing wear; c. Excessively large clearance in the bearings; d. Air entering the pump; e. Damaged impeller or blockage at the impeller’s inlet. 69. What are the factors that affect changes in the characteristics of a pipeline? Answer: (1) Changes in the opening degree of the control valves on the pipeline, as well as changes in the length of the pipeline, can all cause changes in the K value, thereby making the pipeline’s characteristic curve steeper or flatter ； (2) The static head H_static changes only when the pressure difference and liquid level difference between the suction area and the discharge area change, at which point the pipeline characteristic curve shifts downward. 70. What are the reasons for motor overload? How to deal with it? Answer: (1) Incorrect rotation direction; correct the rotation direction. (2) There are foreign objects in the impeller; inspect and clean them. (3) The specific gravity or viscosity of the liquid exceeds the specified values; adjust the specific gravity or viscosity. (4) The couplings are not aligned; realign them. (5) Friction between the impeller and the pump casing; make adjustments. (6) The outlet valve is open too much. 71. What is the function of a stuffing box? Answer: The stuffing box, also known as a packing box, consists of a packing seat, packing, a packing gland, a water seal ring, etc. It is used to seal the gap where the pump shaft exits the pump casing, preventing water and air from leaking in. It also serves to support the pump shaft to some extent, as well as to provide lubrication and cooling for the pump shaft. The function of the packing is that, once it is placed in the packing box and compressed, it can provide a sealing effect. 72. What are the reasons for high bearing temperatures? Answer: The reasons include: (1) Low oil level, which reduces the amount of oil available to lubricate the bearings; (2) Poor quality of the oil, with impurities entering it or the oil becoming emulsified and degraded; (3) Insufficient or interrupted cooling water supply to the bearings; (4) Poor quality of the bearings or improper assembly, such as them being too tight or too loose ； (5) Bearing damage, caused by severe vibration or poor concentricity of the pump. 73. What is the function of a bearing? How many types of bearings are commonly used? Answer: A bearing is a component that supports the shaft journal; it is sometimes also used to support rotating parts on the shaft. It is an important part of machinery. Based on the direction of the load they bear, bearings can be divided into two categories: radial bearings and thrust bearings. Based on the nature of friction during bearing operation, they can also be divided into two categories: sliding friction bearings and rolling friction bearings. 74. What are the causes of vibration in water pump bearings? Answer: (1) Air entering the inlet pipe of the pump body ； (2) Cavitation phenomenon occurs ； (3) The pump flow rate is much lower than the designed flow rate, and the outlet valve is opened too wide ； (4) Loose foundation bolts ； (5) Shaft bending or poor rotor dynamic balance ； (6) Foreign objects or wear in the impeller ； (7) Poor lubrication, bearing wear ； (8) Excessive out-of-roundness of the wheels. 75. What is the function of lubricating oil? How should overheating bearings be dealt with? Answer: I. Functions: (1) To provide lubrication and prevent dry friction ； (2) Acts as a cooling and rinsing agent ； (3) Vibration damping and impact load mitigation ； II. (1) If the lubricating oil has deteriorated, it should be replaced ； (2) Overfilling; adjust to the specified oil level ； (3) Wheel alignment ； (4) Bearing damage, correction ； 76. What is an axis seal? What are the two most common types of axis seal structures used in centrifugal pumps? Answer: An axis seal refers to the seal between the rotating shaft of a centrifugal pump and the stationary pump casing. The two common shaft seal structures used in centrifugal pumps are as follows: (1) Packing seal ； (2) Mechanical seal ； 77. What is the working principle of a screw pump? Answer: The working principle of a screw pump is that the screw rotates eccentrically within a pump casing with internal helices, which pushes the liquid in an axial direction and forces it out through the discharge port. 78. Why are safety inspections conducted on chemical enterprises? Answer: The production efficiency of any chemical enterprise is closely related to the good condition of its equipment. Well-maintained equipment is the most fundamental guarantee for safe production and increased efficiency in enterprises. If equipment is used and maintained improperly, it will inevitably lead to various production accidents; in mild cases, this results in leaks that cause raw material losses, reduced output, and environmental pollution, while in severe cases it can lead to equipment failure, explosions, casualties, and property damage. Therefore, it is particularly important for chemical enterprises to conduct regular inspections and daily maintenance of production equipment, to ensure its good condition and safe operation, and to prevent sudden shutdowns. 79. What are the categories of chemical industry maintenance? Answer: Chemical plant maintenance can be divided into two categories, namely planned maintenance and unplanned maintenance. (1) Planned maintenance: In accordance with the provisions of the \"Regulations for the Maintenance of Chemical Equipment,\" a maintenance plan for the equipment is formulated based on information regarding the technical condition of the existing equipment in the enterprise as well as factors such as the production cycle. Maintenance carried out in accordance with this plan is referred to as planned maintenance. Planned maintenance can be further divided into minor repairs, medium repairs, and major repairs, depending on their scale, the amount of time required, and the number of maintenance tasks involved. Typically, chemical companies carry out regular major shutdown repairs once a year; however, it can also be every two years or every few years. It can be determined based on the operation status of the enterprise’s equipment. (II) Unplanned maintenance: When a failure or accident occurs suddenly during the operation of equipment, maintenance or emergency repairs that must be carried out without stopping operations or with temporary suspension of operations are referred to as unplanned maintenance. Unplanned maintenance refers to unplanned repairs carried out without any prior preparation for unexpected shutdowns; it falls under the category of reactive maintenance. Due to its lack of planning, the quality of such repairs cannot be guaranteed, and sometimes repairs are done hastily in an effort to resume production as soon as possible. Repetitive failures will occur again soon. Due to the complexity of production in chemical systems, it is inevitable for shutdowns to occur as a result of unexpected factors; issues such as equipment leaks, blockages, or malfunctions in control or transmission devices are all difficult to predict in advance. 80. What are the characteristics of chemical plant maintenance? Answer: The maintenance of chemical enterprises is characterized by frequent and complex operations that require high technical skills, as well as significant risks during the maintenance process. There are a wide variety of chemical processing equipment, such as furnaces, towers, kettles, heat exchangers, reactors, compressors, centrifuges, pumps, storage tanks, vessels, as well as pipelines, valves, instruments, and meters. These devices have complex structures, varying pressure resistance capacities, and different types of failures can occur; the causes of such failures are complex. 81. What are the general requirements for chemical plant maintenance? Answer: It is necessary to strengthen the scientific management of equipment maintenance, improve the predictability of such maintenance, plan maintenance tasks in an organized manner, ensure maintenance quality, reduce maintenance costs, and prevent and minimize the occurrence of serious accidents. 82. What is the importance of equipment lubrication? How can operators perform lubrication work properly? Answer: Lubrication is an effective measure to reduce friction, minimize wear, and lower energy consumption. Proper lubrication is an essential condition for the proper operation of machinery and equipment. Strict implementation of the “five fixations” and “three-level filtration” is required. “The “five fixes” refer to: fixing the quality, quantity, timing, location, and frequency of cleaning ； “\"Three-stage filtration\" refers to three stages of filtration: the oil tank, the oil bottle, and the oil filling point. 83. What should be noted when operating switches and valves? Answer: When opening and closing valves, do not go too far or use excessive force. The screw of the valve should be cleaned regularly and lubricated during use. If it is found that the valve does not operate properly, the cause should be identified and the fault fixed; it is absolutely not permissible to force it to open or close, as this may damage the valve and lead to accidents. If there is leakage at the packing gland of the valve stem, the bolts at the packing gland can be tightened. If the packing is worn out, it should be replaced promptly whenever production permits. 84. Important indicators of lubricating oil: Answer: (1) Viscosity. It represents the magnitude of the internal friction between various thin layers of the liquid as it flows. The pressure that an oil film can withstand is proportional to the product of its viscosity and the relative velocity of motion; the higher the operating speed or lubrication speed of a mechanism, the lower the viscosity of the lubricating oil should be. At high speeds, the relative displacement of the oil layer increases, and the heat generated by internal friction within the oil molecules also increases. Therefore, lubricant serves to reduce both frictional losses and energy losses. Oils with higher viscosity generate more heat during operation; however, as the heat increases and the oil temperature rises, its viscosity decreases, thereby providing a form of automatic regulation – that is, the lubricant can carry away the energy generated by friction. (2) Freezing point. The temperature at which oil loses its fluidity is called the freezing point. Since oil is a complex mixture of hydrocarbon molecules, as the temperature drops, these molecules can precipitate out and turn into a solid state, causing the oil to become cloudy. Therefore, oil should be used at a temperature several degrees higher than its freezing point. (3) Residual carbon. It refers to the percentage of coke formed by the thermal pyrolysis and condensation of lubricating oil relative to the weight of the lubricating oil; it is an indicator used to control the degree of refinement of the lubricating oil. (4) Flash point. Under certain heating conditions, as the temperature rises, the amount of lubricant that evaporates increases, and the vapor pressure above the oil surface also rises. When the mixture of oil vapor and the surrounding air comes into contact with fire and produces a flame that extinguishes quickly, the temperature at that point is known as the flash point. The flash point is an important safety parameter for lubricants; it is generally required that the operating temperature of the oil be 20–30°C below its flash point. (5) Anti-emulsifying type. Under specified conditions, the time it takes for lubricating oil to separate from water is known as the anti-emulsification capacity. The shorter this time, the better the oil’s anti-emulsification properties; it indicates that the oil and water cannot form a stable emulsion, and they will separate quickly even when mixed together. (6) Antioxidant safety. The ability of a lubricant to resist oxidation under heating conditions and in the presence of metals is known as the antioxidant safety of the lubricant, and it reflects the chemical properties of the oil. (7) Viscosity ratio. The ratio of the dynamic viscosity of the same oil at 50°C to its dynamic viscosity at 100°C; the lower this ratio, the better the viscosity characteristics of the oil, and the wider the range of temperatures for which it can be used. 85. Steps for the normal start-up and shutdown of a centrifugal pump: Answer: (1) Start-up: ① Carry out the preparations before starting the pump: a. The liquid level in the tower or tank should be above 50% of the normal level. b. Confirm that the inlet valve is open, the outlet valve is closed, the return valve is open, and the pump discharge drain is closed. c. Open the vent valve to exhaust air from the pump, and close it once liquid begins to flow out. d. Check that the oil levels in the bearing housing and oil cups are between 1/2 and 3/2. e. Check whether the cooling water in the bearing housing is flowing smoothly. f. Rotate the disk 2/3 turns without any jamming. ②During the first start-up, operate it in jog mode to confirm that the pump motor is rotating in the correct direction. ③Start the pump; after it begins operating, pay attention to the readings on the ammeter, the pressures at the inlet and outlet, and whether there is any leakage from the mechanical seal. Once everything is normal, slowly open the outlet valve. ④Check that the bearing temperature of the pump does not exceed 65°C, and the motor temperature does not exceed 70°C. Observe the fluctuations in the outlet pressure gauge and ammeter, check for vibration and leakage during pump operation, as well as any changes in the levels of cooling water and lubricating oil. ⑤Under normal conditions, the outlet valve of the pump cannot be used to regulate flow. ⑥For pumps that operate for extended periods, it is necessary to replace the lubricating oil or grease regularly to ensure that the pump functions under optimal lubrication conditions. (2) Steps to stop the centrifugal pump: ① Gradually close the pump outlet valve. ②Stop the pump when the pump outlet valve is fully closed. ③After the pump stops, open the warm-up valve (cool-down valve) of the pump to maintain a standby state. ④Conduct regular inspections and turn the shaft. ⑤For maintenance, completely isolate the pump, relieve pressure, and drain the liquid. ⑥During winter maintenance, the cooling water must be drained intermittently to prevent freezing. 86. Function and application scope of the explosion-proof plate in heat exchangers: Answer: An explosion-proof plate, also known as a rupture disc, is a type of failure-based overpressure protection device used in pressure vessels where it is not appropriate to install safety valves. It ruptures automatically when the pressure inside the vessel exceeds the normal operating pressure and reaches the design pressure, thereby preventing the vessel itself from exploding. Its installation should generally meet the following conditions. (1) When the medium inside the container tends to crystallize or has high viscosity, impurities or crystals can easily accumulate on the valve element when a safety valve is used, preventing the valve from opening at the specified pressure and rendering it ineffective. (2) The pressure inside the container rises rapidly due to chemical reactions or other reasons, and the safety valve is unable to relieve the excessive pressure in a timely manner. (3) When the medium in the container is a highly toxic gas or minor leaks are not permissible, and safety valves cannot ensure that such gases do not leak, explosion-proof discs should be used. 87. What is a stage in a steam turbine? Answer: In a steam turbine, the working unit composed of nozzles and the moving blades that work in conjunction with them is called a stage. The nozzles are fixed on the partition of the cylinder and do not move, while the moving blades are mounted on the rotor and rotate along with it. The stage is the most fundamental working unit in a steam turbine; the process of converting the thermal energy of steam into mechanical energy takes place within the stage. 88. What are the main components of a steam turbine? What are the features? Answer: A steam turbine consists mainly of stationary and rotating parts. The stationary parts include the main steam valve, control valves, cylinders, front and rear bearing housings, the machine base, and the slide pin system, etc. The rotating parts refer primarily to the rotor assembly, which includes the steam turbine shaft, impeller, drum, moving blades, crisis safety devices, etc. Main steam valve: Also known as the emergency shutdown valve, it automatically closes quickly during an emergency shutdown, cutting off the gas supply and enabling the unit to stop safely. This steam engine uses hydraulic cylinders to control the opening and closing of the main valve. Control valve: It consists of five valve heads; during startup and normal operation, by controlling the opening degree of this control valve, the amount of steam entering the steam turbine can be adjusted, thereby controlling the speed and power output of the steam turbine. Bearing housing: Used to install radial bearings and thrust bearings; the radial bearings are inclined pad bearings with five oil pockets, while the thrust bearings are Kingsbury bearings. Cylinder: The cylinder has a horizontal split structure, consisting of an upper cylinder and a lower cylinder, which are connected to each other by bolts. The front part of the cylinder is supported by the upper cylinder section, with the cat’s paw-like structure of the upper cylinder resting on the front cylinder; the rear part of the cylinder is supported by the lower cylinder section, which sits on the base through support feet on both sides. 89. How is the axial force of a steam turbine rotor balanced? Answer: When a steam turbine is operating normally, the axial force acting on the rotor is quite large; in some high-pressure reaction steam turbines, this force can reach 2–3 MN. Such a large axial force cannot be handled by thrust bearings alone, and measures must be taken to balance most of it, with the remaining portion being borne by the thrust bearings. In multi-stage steam turbines, the common methods for balancing axial forces are as follows: (1) Balance piston – A balance piston is installed at the front end of the rotor. Low-pressure steam is present in front of the piston, while high-pressure steam is located behind its protrusion. This creates an axial thrust in the forward direction on the balance piston, thereby counteracting the backward-directed axial force exerted on the rotor. (2) Opening balance holes: Several holes are made in the impeller of an impulse steam turbine, allowing the steam on both sides of the disk to flow towards each other, thereby reducing the pressure difference and thus the axial thrust on the disk. The number of balance holes is usually odd, often 5 or 7. (3) Adopting a counter-flow arrangement: By arranging the flow of steam within the steam turbine in opposite directions, the axial forces generated are directed in opposite ways, thereby canceling each other out and achieving equilibrium. The steam can also be directed in opposite directions under high and low pressure to automatically balance the axial force. 90. What are the causes of scaling on the nozzles and rotor blades of steam turbines? What are the harms? Answer: Due to reasons such as poor boiler water quality, the steam quality is poor and contains certain impurities. As these vapors pass through the steam turbine, different impurities precipitate at various stages and locations, depositing on the flow-through parts of the steam turbine and forming scale. Scaling occurs in the flow-through section; generally, there is less scaling in the first and second stages, with it increasing gradually in subsequent stages. Most of the scaling substances deposit in areas with lower airflow speeds, or they may form at areas where the flow is interrupted and on surfaces with a rough texture. When the nozzle and moving blades are excessively fouled, it leads to a reduction in the flow area and an increase in resistance. Due to the reduced flow area, with the total enthalpy drop of the unit remaining unchanged and the pressures at each stage staying constant, a decrease in steam flow rate will result in a reduction in output. Sometimes, to ensure output, it is necessary to increase the initial steam pressure, thereby increasing the power output; this reduces the safety of the unit’s operation. Furthermore, scaling distorts the shape of the nozzles and rotor blade passages, roughening the blade surfaces and increasing frictional losses, which severely affects the efficiency of the steam turbine. 91. What is the critical speed of a rotor? What are the factors that affect the critical speed? Answer: When a steam turbine is in operation, the rotor vibrates. During operation, the rotor begins to vibrate slightly; when the speed reaches certain values, the vibration of the rotor increases significantly. Once the speed exceeds these values, the vibration of the rotor decreases again. During the deceleration of the rotor, similar phenomena occur at similar speeds as well. These rotational speeds are referred to as the rotor’s critical speeds, and they are classified in order of increasing speed as the first critical speed, the second critical speed, and so on. In practical manufacturing applications, the first and second critical speeds are those that are most commonly encountered. The critical speed of a rotor depends primarily on its mass and stiffness; the greater the mass and the better the stiffness of the rotor, the higher its critical speed. In addition, it is also related to factors such as the influence of support elasticity, the influence of blade rotation torque, the influence of the extended portion of the rotor, the influence of the shafting system, and the influence of damping. 92. Why must a rotor be dynamically balanced after major repair? What are the standards? Answer: Due to factors such as machining errors, assembly errors, and uneven material distribution, the center of mass of various rotating parts of a rotor after major repair is not always located on the axis of rotation. When the rotor rotates, unbalanced forces and moments are generated, causing severe vibrations in the rotor and threatening the safe and stable operation of the unit. Therefore, it is necessary to perform strict dynamic balancing on the rotor after major repair. The dynamic balance accuracy standard for rotors is that the vibration intensity should not exceed 1g·mm/s. 93. What materials are commonly used to manufacture compressor rotors? Answer: The main shaft of the compressor rotor is made of nickel-chromium-molybdenum alloy structural steel, such as 40NiCrMo7; the impeller is generally made of stainless steel, X15CrMo13. 94. How to adjust the clearance of the thrust bearing? Answer: Adjusting the clearance of the thrust bearing can be achieved by thickening or thinning the shims on the non-working side of the thrust bearing; this adjusts the thrust clearance without changing the axial position of the rotor. When measuring the clearance of the thrust bearing, make sure to fasten the upper cover of the bearing housing and tighten the bolts on the mid-surface; otherwise, the measurement error will be large. Additionally, only one adjusting shim may be used on the working and non-working sides of the thrust bearing, and its thickness must be uniform around the entire circumference without any deviation.