What a pump operator should know and be able to do

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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 transportation 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 ones include reciprocating pumps, piston pumps, etc. Positive-displacement 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, under the effect of centrifugal force, from the center of the impeller toward its outer edge. 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 effective 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 honeycomb or sponge-like 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 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 machine, 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 it necessary to fill the centrifugal pump with liquid before starting it? 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 prevents the liquid from flowing 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 a centrifugal pump? 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 use methods such as changing the plunger stroke and installing a return circuit to regulate the flow rate. 10. What are the startup and shutdown procedures for general centrifugal pumps? Answer: (1) Driving (with power supplied to the motor beforehand): 1) Rotate the rotor for more than one full turn to ensure there is no jamming ； 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, then use 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 compliant 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 replacement is verified to be satisfactory, 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 startup, while for a gear pump, the outlet valve must be opened at startup 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 main water supply pipes, gas pipes, and sewage pipes that are buried underground. 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 union fittings. (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 whose 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 allowable 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 proper operation of pressure vessels? Answer: 1) Do not exceed temperature, pressure, or load limits. 2) The pressure addition and release rates should not be too fast. 3) The temperature increase and decrease should be carried out at the specified speed. 4) The number of starts and stops must be controlled as per regulations, 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 starts operating normally, the outlet valve should be gradually opened; otherwise, it may cause the pump casing temperature to rise, damaging its 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 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. 22. 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. 23. 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. 24. 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. 25. 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 drop in efficiency. 26. 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. 27. 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. 28. 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 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. 29. 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 delivered 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 kilowatt-horsepower ； Efficiency ŋ: During the process of pumping liquid, 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 vacuum lift height Hs indicates the maximum height at which a water pump can draw water, measured in meters. 30. 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 forced toward the edges of the impeller and enters 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. 31. 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 and motor shaft are not coaxial; the couplings are misaligned ； (6) The mouth ring of the impeller is severely damaged (worn), with an excessive gap ； (7) The stuffing box is tightened too much, the sealing of the stuffing box is damaged, or the shaft sleeve is worn out. 32. 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 and even shaft seizure ； For mechanical seals, rotating contact surfaces such as balance discs can experience overheating due to friction, and may even be damaged, as there is no lubrication or cooling provided by liquid. 33. What are the mechanical losses in centrifugal pumps? What are the hazards of cavitation in centrifugal pumps? Answer: The mechanical losses in centrifugal pumps include bearing and shaft seal friction losses, as well as impeller losses. Its harms include: (1) damaging the material ； Impellers, guide vanes, pump casings, 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. 34. 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 level 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. 35. What are the installation characteristics of centrifugal pumps? Answer: The pump and the piping system form an inseparable whole; the head generated by the pump must be equal to the head required by 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 and those of the pipeline on a single diagram is referred to as the performance characteristics of the centrifugal pump. 36. 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 capability ； (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. 37. 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. 38. What are the causes of seal leakage in centrifugal pumps? Answer: (1) Water seal pipe or valve is clogged ； (2) The filler is pressed too tightly ； (3) Compression 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 entering the water seal ring ； 39. What are the causes of seal leakage in centrifugal pumps? Answer: (l) The packing is compressed too tightly, the packing seal is damaged, or the shaft sleeve is worn out; (2) The packing material was not chosen appropriately, or the packing or water seal ring was not installed correctly; (3) The mechanical seal was selected or installed improperly, resulting in its damage. 40. What are the reasons for a centrifugal pump failing to discharge liquid (i.e., unable to pump out fluid)? Answer: (1) The pump’s rotational speed is insufficient or the motor is rotating in reverse; (2) The impeller is loose or installed incorrectly (in double-suction pumps), or it 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 value 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 pipelines. (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. 41. 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 rapid collapse of these bubbles, the surrounding liquid rushes into the space once occupied by the bubbles at extremely high speeds. This phenomenon is known as cavitation. 42. Why cannot a centrifugal pump operate for an extended period of time 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 the friction caused by pumping generates high heat, which can lead to deformation and damage of the pump’s components. (2) I. Increase the pump’s head ； II. The flow rate is equal to the flow rate of a single pump. 43. What are the reasons for insufficient flow rate in a centrifugal pump? How to deal with it? Answer: (1) There is air inside the pump or pipes; remove the air. (2) There are leaks in the pump body or inlet pipes; fix these leaks. (3) The inlet and outlet pipes 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 gaps between various parts inside the pump are too large, replace the mouth ring or adjust the gaps. 44. What are the reasons for overheating of the centrifugal pump shaft? 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 gap ； (3) Scaling, blockage in the cooling system, insufficient or interrupted supply of cooling water ； (4) Dust, dirt, or corrosive liquids enter the bearing bush or bearings. 45. What are the causes of vibration and noise in centrifugal pumps? Answer: (1) Air leakage at the inhalation pipeline flange ； (2) Pump not primed or gases not completely expelled ； (3) Impeller gets clogged with entrained impurities, or intermediate stages of multi-stage pumps become blocked ； (4) Rotor imbalance or shaft bending deformation ； (5) The pump and motor are not aligned coaxially, and the coupling is not properly 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 ； (9) The material of the filler was not selected properly, or the filler or water seal ring was not installed correctly. 46. What are the reasons for a decrease in the pressure value of a centrifugal pump? Answer: (1) The suction pipe foot valve is not open or is malfunctioning; the suction foot valve or strainer 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 surface drops, or the liquid temperature rises ； (4) Insufficient pump speed or motor running in reverse ； (5) Impeller is loose or installed backwards; impeller is severely corroded ； (6) The pressure at the pump’s discharge end exceeds the design pressure, resulting in excessive backpressure ； (7) Increase in viscosity due to a decrease in liquid temperature, or viscosity exceeding the design value ； (8) Due to reasons such as the regulating valve being opened too little, a malfunctioning check valve, or clogged pipelines, the resistance encountered by the pump increases (pipeline resistance) ； (9) The impeller gets blocked by sticky debris drawn in, or the intermediate stages of a multi-stage pump become blocked. 47. What are the causes of cavitation in centrifugal pumps (inability to draw in liquid)? Answer: (1) Air enters the suction side of the pump due to a loose connection at the suction flange ； (2) The pump was not primed, or air was not expelled during priming; air remains inside the pipes or 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 decreases, 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. 48. 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. 49. 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. What is the allowable suction vacuum level? 50. 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 appears at 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. 51. 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. 52. 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 reaches this left dead center, it expels the liquid that has been drawn in, completing one reciprocating cycle; as the piston continues to move, the pump keeps delivering liquid continuously. 53. Under what circumstances must 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. 54. What are the reasons why water flow from a pump stops 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. 55. What is the difference between single-stage and multi-stage pumps, as well as between single-suction and double-suction pumps? What causes noise inside a pump? Answer: (1) The number of stages refers to the number of impellers; the number 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 or cooling, leading to wear. 56. Why is it that water pumps are generally installed first in a unit? What is the reason for installing the pump on the same shaft as the motor? Answer: (1) Water pumps are usually installed first, followed by the motor, because the motor is only related to the location of the water pump, while the location of the water pump involves the connections to 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 due to dirt. 57. 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 the level difference between the suction area and the discharge area change; in such cases, the pipeline characteristic curve shifts downward. 58. 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 opening degree of the outlet valve is too large. 59. 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 extends out of 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. 60. 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. 61. 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 neck; it is sometimes also used to support rotating parts on the shaft. It is an important part of machinery. Depending 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. 62. 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 water discharge volume is much lower than the pump’s 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. 63. 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 ； 64. 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 ； 65. 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. 66. What is the importance of equipment lubrication? How can operators perform lubrication tasks 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: oil tank, oil pot, and oil filling point. 67. 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 movement; the higher the operating speed or lubrication speed of the 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 goes out immediately, 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 called the anti-emulsification capacity; the shorter this time, the better the anti-emulsification property, as it indicates that the oil and water cannot form a stable emulsion, and they will separate quickly even when mixed. (6) Antioxidant safety. The ability of a lubricant to resist oxidation under heating conditions in the presence of metals is referred to 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. 68. 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 cup are between 1/2 and 3/2. e. Check whether the cooling water in the bearing housing is flowing properly. f. Spin the wheel 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, gradually 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 over long periods of time, it is necessary to replace the lubricating oil or grease regularly to ensure that the pump functions in a properly lubricated condition. (2) Steps to shut down 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 keep it in standby mode. 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 discharged intermittently to prevent freezing.