1. What is the definition of a chemical 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 to achieve the function of conveying media.
2. How are chemical pumps classified?
Answer: According to the working principle, it can be divided into the following categories:
Positive displacement pump: relies on the continuous change of working volume to suction or discharge liquids. Common types include reciprocating pumps, plunger pumps, etc.
Speed pump: relies on the high-speed rotation of the impeller to transfer energy to the liquid, thereby generating pressure and flow in the liquid. According to the liquid flow situation, it can be divided into centrifugal pumps, axial flow pumps, vortex pumps, etc.
Other types of pumps: jet pumps, air boosters, 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 filled in the pump body is thrown from the center of the impeller to the outer edge of the impeller under the action of centrifugal force. In this process, the liquid gains energy, increases static pressure energy, and due to the increase in flow velocity, kinetic energy also increases. The liquid leaves the impeller and enters the pump casing. As the flow channel gradually increases, the fluid velocity decreases, and some kinetic energy is converted into static pressure energy. The liquid enters the extrusion duct at a higher pressure. When the liquid is ejected from the center of the blade, a low pressure is created at the center of the impeller, and the pressure at the liquid level is higher than here. Under the pressure difference, the liquid enters the pump through the suction pipe. When the impeller rotates continuously, the liquid is constantly sucked in from the center of the impeller and discharged continuously at a certain 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: ⑴ Flow rate: The amount of liquid discharged by a pump per unit time, also known as the liquid delivery capacity. The unit is m3/s. Commonly used in engineering are m3/h or L/s. The size of the flow depends on the structure, size, and rotational speed.
⑵ Head, also known as pump pressure head, refers to the actual energy obtained by a unit weight of liquid through a pump. Its unit can be expressed in [m liquid column].
⑶ Power, the work done by the pump per unit time. The actual power obtained by the pump in a unit of time is also called effective power. The power obtained by the pump from the prime mover is called shaft power.
⑷ Efficiency, the ratio of effective power to shaft power. Efficiency is always less than 1.
6. How does cavitation occur in centrifugal pumps and what are the hazards? How to prevent cavitation?
Answer: When the pressure at the suction port of the pump drops below the saturated vapor pressure of the liquid inside the pump at that temperature, the liquid will boil, forming a large number of bubbles. At the same time, certain gases dissolved in the liquid will also escape and form bubbles due to the decrease in pressure. Bubbles enter the high-pressure zone with the liquid, rapidly bursting and creating a local vacuum. As a result, the surrounding liquid rushes towards the space occupied by the bubbles at great speed, colliding with each other and immediately converting their kinetic energy into pressure energy, producing a high local impact force in an instant. This impact force can cause significant damage to the impeller, leading to the formation of honeycomb or sponge like surfaces on the impeller. When cavitation occurs, the pump body is impacted and vibrates, producing noise. The performance of the pump deteriorates sharply, and in severe cases, it may experience flow interruption and malfunction.
The means to prevent cavitation are:
⑴ The installation height of the pump body must be less than the allowable vacuum suction height.
⑵ The effective cavitation allowance provided by the installation of the pump body is greater than the required cavitation allowance of the pump body.
⑶ When the flow rate increases, the cavitation margin also increases, and attention should be paid during operation.
⑷ Note that the operating temperature for transporting liquids should not be too high.
7. Why do centrifugal pumps need to close the outlet valve before starting and stopping?
Answer: If the outlet valve is not closed when starting the centrifugal pump, it may burn out the motor due to excessive starting power. Closing the outlet valve, Q=0, requires the minimum power and can avoid the above accidents. When parking, if the outlet valve is not closed first and the vehicle suddenly stops, the high-pressure liquid in the discharge pipe may backflow into the pump, causing high-speed reversal of the impeller and resulting in damage. To protect the equipment, it is necessary to first close the outlet valve and then stop the motor.
8. Why is the centrifugal pump filled with liquid before starting?
Answer: If the pump is not filled with liquid before starting, there will be air in the centrifugal pump, and the density of air is much lower than that of liquid. The centrifugal force generated by the impeller rotation on the pump is very small, and the vacuum degree formed is also very low. The static pressure head difference between the liquid level in the storage tank and the suction port of the pump is also very small, which cannot push the liquid into the pump, causing the impeller to idle and unable to suck up the liquid. This is the so-called "air binding" phenomenon.
9. What are the main components of a reciprocating pump? What is the difference between its flow regulation method and centrifugal pumps?
Answer: The reciprocating pump is mainly composed of a pump cylinder, a plug, a piston rod, a suction valve, and a discharge valve. The flow rate of centrifugal pumps is generally adjusted by controlling the opening degree of the outlet valve, while reciprocating pumps usually adjust the flow rate by changing the stroke of the plug and installing a reflux branch.
10. What are the general steps for starting and stopping a centrifugal pump?
Answer: ⑴ Driving (previously powered by the motor):
1) Drive for more than one week and confirm that the rotor is not jammed;
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) Exhaust the pump body and close it after it is qualified;
5) Open the minimum flow line valve; If there is sealing water, use sealing water;
6) Start the motor, open the outlet valve after the outlet pressure is normal, and confirm that the motor pump is running normally.
7) If the centrifugal pump is equipped with an auxiliary oil pump, establish oil circulation first and then start the pump; The pump that needs to be warmed up must be qualified.
⑵ Parking:
1) Close the pump outlet valve and stop the motor; If there is an auxiliary oil pump, confirm that it starts automatically, otherwise start manually.
2) If maintenance is required, the centrifugal pump should be powered off, the oil should be drained, and the replacement should be qualified before 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: simple structure, compact, uniform flow rate, easy adjustment, low vibration, low foundation requirements, wide application range, low head, low efficiency for liquids with high viscosity, limited head for small flow rates;
Reciprocating pump: high head, fixed flow rate, dry suction capacity, 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 rate, high head, and high viscosity.
12. What is the difference between starting a centrifugal pump and a gear pump?
Answer: In order to reduce the starting load and avoid overload, centrifugal pumps generally need to close the outlet valve when starting, while gear pumps need to open the outlet valve when starting to avoid pressure build-up.
13. Which occasions are cast iron pipes, water and gas pipes, and seamless steel pipes suitable for in petrochemical production?
Answer: Cast iron pipes are used for buried water supply pipes, gas pipes, sewage pipes, etc.
Water gas pipes are commonly used in low-pressure water pipes, heating, gas, compressed air, and vacuum pipelines, as well as in low-pressure steam branch pipes and condensate pipelines.
Seamless steel pipes are widely used in the transportation of materials with high water pressure and temperature.
14. What are the main components of commonly used petrochemical pipelines?
Answer: The commonly used petrochemical pipelines mainly consist of three parts: pipes, fittings, and valves.
15. What types of pipe fittings can be classified according to their different functions? Give one or two examples each to illustrate.
Answer: ⑴ Used to change the direction of pipelines, such as bending pipes
⑵ Connecting pipeline branches: such as three-way and four-way
⑶ Connect two sections of pipes: such as pipe clamps and flexible joints
⑷ Change pipeline branches: such as large and small heads
⑸ Blockage of pipelines: such as screw plugs
16. Why do pressure vessels need to be equipped with safety accessories?
Answer: In order to ensure the safe operation of pressure vessels and prevent accidents caused by overpressure, in addition to taking measures to eliminate or reduce various factors that cause overpressure in containers, safety relief devices must be installed on pressure vessels.
17. Can you explain the usage scenarios of safety valves?
Answer: (1) Containers that may experience increased pressure due to chemical reactions or other reasons during the production process.
(2) A container for holding liquefied gases.
(3) There is no safety valve installed at the pressure source of the container, or although a safety valve is installed, the allowable working pressure of the container is lower than the pressure at the pressure source.
(4) Pressure vessels with a maximum working pressure lower than the pressure source.
18. What are the basic conditions for correctly operating pressure vessels?
Answer: (1) Do not exceed temperature, pressure, or load.
(2) The speed of adding and releasing pressure should not be too fast.
(3) The temperature rise and fall should be carried out at the specified speed.
(4) The frequency of driving and parking should be controlled according to regulations, and the pressure and temperature should not fluctuate significantly.
(5) Safety facilities must be complete, sensitive, and reliable.
(6) Fixed point, scheduled, and fixed route patrol inspections.
(7) Immediate and correct measures should be taken in case of container abnormalities.
19. Why is the outlet valve of the centrifugal pump closed and cannot be closed for too long when driving?
Answer: Close the outlet valve and temporarily reduce the outlet flow to zero. At this time, the pump requires the minimum power, which can limit the motor load to the lowest level and prevent the motor from burning out due to excessive starting current. After the motor is running normally, the outlet valve should be gradually opened, otherwise it may cause the temperature of the pump casing to rise and damage the components.
20. What are the hazards of cavitation phenomenon?
Answer: When cavitation occurs, the pump will produce noise and vibration, and its head, flow rate, and efficiency will all decrease. Cavitation accelerates material damage and shortens the service life of the pump. Severe cavitation can disrupt the normal operation of the pump and threaten safety production.
21. Why can't the compression ratio of reciprocating compressors be too high?
Answer: During the compression process, the temperature of the gas needs to increase, so it needs to be cooled while the gas is compressed. Otherwise, it will consume power and damage the lubrication effect of the lubricating oil, and even cause the lubricating oil to catch fire. Cylinder and gas cooling generally use water jacket. When the compression ratio is too high (such as 6 or above), this method alone cannot be effective for cooling, so the compression ratio cannot be too high.
22. What are the reasons for pressure vessels being operated or causing internal defects to expand during use?
Answer: Corrosion of media, erosion and wear caused by high-speed airflow, pressure fluctuations, temperature fluctuations, long-term compression, etc.
23. What are the common forms of damage to pressure vessels?
Answer: Plastic failure, brittle failure, fatigue failure, creep failure, corrosion failure, etc.
24. Why do some centrifugal pumps need to set a minimum flow rate?
Answer: Centrifugal pumps rely on applying kinetic energy to liquids to achieve liquid transportation. When high-power centrifugal pumps operate with the outlet valve closed, the fluid will become increasingly hot due to the increase in kinetic energy, and in severe cases, it may burn out bearings and cause cavitation. Some pumps have a minimum flow rate for pump thermal backup.
25. Why do some backup pumps need to be hot standby, and how many hot standby methods are there?
Answer: When the temperature of the medium exceeds 150 ℃, the standby pump should adopt hot standby. The main purpose is to prevent sudden changes in fluid temperature and thermal stress in the pump body. There are generally two types of hot standby methods:
1) The heat medium flows from the outlet of the running pump to the outlet of the standby pump and then to the inlet, with a small amount of backflow in the pump body.
2) Jacket steam heating.
26. Function of valves:
Answer: Valves are components used to control fluid flow and pressure in pipelines. Operators can control and regulate the production process by operating various valves.
(1) Opening and closing function - cutting off or communicating the flow of fluid inside the pipe.
(2) Regulating function - regulating the flow rate of fluid inside the pipe.
(3) Throttling effect - adjusting the valve opening to change the pressure drop and achieve the function of regulating pressure.
27. Stop valve
Answer: A globe valve consists of components such as valve core, valve seat, valve body, valve stem, packing, packing cover, and handwheel.
Globe valves can be divided into three types based on the direction of medium flow: straight through, straight through, and angled. According to the position of the valve stem thread, it can be divided into two types: visible stem and hidden stem. Small diameter globe valves are generally of the concealed threaded rod type; Globe valves with larger diameters, higher operating temperatures, and used for corrosive media are generally of the open threaded rod type. When installing a globe valve, the medium should flow from bottom to top through the gap between the valve core and the valve seat.
The advantages of globe valves are simple structure, good sealing performance, and easy manufacturing and maintenance. Its disadvantages are high fluid resistance, long valve body, and large footprint. Globe valves are widely used to cut off fluids and regulate flow rates.
28. Gate valve
Answer: Gate valve, also known as gate valve, is equipped with a gate plate perpendicular to the direction of medium flow inside the valve body. When the gate plate is raised, the gate valve opens, and when it is lowered, it closes. A gate valve consists of components such as a handwheel, stem nut, cover, stem, valve body, gate, sealing surface, etc.
There are two types of valve stems: visible stem and hidden stem. Bright stem gate valves are generally used for corrosive media and indoor environments; Concealed stem gate valves are used in non corrosive media and restricted operating positions. According to the different types of gate structures, they can be divided into two categories: wedge type and parallel type. Wedge type is mostly made as a single gate, with wedge-shaped sealing surfaces on both sides. Parallel type is mostly made with double gate plates, and the sealing surfaces on both sides are parallel. Parallel type is easier to manufacture and repair than wedge type, but it is not suitable for transporting fluids containing impurities and can only transport clean fluids.
Gate valves are commonly used to cut off materials, oil, gas and other media, and are not suitable for regulating flow. Because when the gate valve is partially open, it is easy for the part of the gate valve that has not been lifted to be worn by the medium, and over time, the contact surface may not be tight and leak. Therefore, the gate valve should be fully closed or fully opened. The advantages of gate valves are good sealing performance, low medium flow resistance when fully opened, and a shorter valve body. The disadvantage is that the structure is complex, the sealing surface is prone to wear, and maintenance is difficult.
Gate valves are commonly used for discharge valves in containers, inlet valves in centrifugal pumps, pipelines with bidirectional medium flow, pipelines with low medium transport resistance requirements, and places where the installation length of valve bodies is limited.
29. Check valve
Answer: A check valve, also known as a check valve or one-way valve, relies on the pressure difference between the fluid before and after the valve core to automatically open and close to prevent medium backflow. When the fluid flows downstream, the valve core rises or lifts; When the fluid flows back, the valve core automatically closes, so the fluid can only flow in one direction. The commonly used check valves are divided into two categories: lift and swing.
(1) Lift check valve. It is mainly composed of valve body, valve cover (with guide groove), valve seat and other components. The valve core of this type of valve is connected to the valve stem, and there is a guide groove on the valve cover that allows the valve stem to slide up and down, allowing the valve core to move vertically to the valve body. It is often installed on horizontal pipelines.
(2) Swing check valve. This type of valve is mainly composed of components such as valve body, valve cover, valve core, and valve seat, and has the advantages of simple structure and low flow resistance. The upper end of the valve core is connected to the valve body with a pin, and can be opened or closed by the free swing of the valve core. This type of valve is mostly used in vertical or larger diameter pipelines. The disadvantages are high noise and poor sealing.
30. Ball valve
Answer: The valve disc of the ball valve is spherical, which uses the valve disc to rotate a certain angle to communicate or cut off the flow of fluid in the pipeline. It is mainly used for transporting fluids with low temperature and high viscosity in pipelines.
31. Safety valve
Answer: A safety valve is an overpressure protection device. Its function is to automatically open and quickly discharge the overpressure gas inside the container when the pressure inside the container exceeds a certain specified value, and make a sound to warn the operator to take pressure reduction measures. When the pressure returns to the allowable value, the safety valve automatically closes again, keeping the pressure inside the container below the upper limit of the allowable range, so as not to cause explosion accidents due to overpressure.
Safety valves can be divided into two types based on their overall structure and loading mechanism: lever type and spring type. This device is commonly used as a spring-loaded safety valve. Spring type safety valves are mainly composed of valve body, valve core, valve seat, valve stem, spring, spring cover, adjusting screw, pin, outer cover, lifting handle and other components. They use the compressed elastic force of the spring to balance the force of gas acting on the valve core.
32. Butterfly valve
Answer: The rotation of the valve plate is used to achieve the purpose of opening and closing. In use, it is generally required that the valve plate be fully open or fully closed as an opening and closing valve. Commonly used for water supply and return valves in circulating cooling water pipelines.
33. Operation and maintenance of valves
Answer: (1) It is strictly prohibited to use a large F-wrench to open or close valves with small diameters during operation.
(2) Valves that can be operated by hand are strictly prohibited from being operated with an F wrench when applying force.
(3) After the valve is fully opened, it should be closed by one turn to prevent it from getting stuck.
(4) When the valve is closed, if it gets stuck, it should be returned a few turns before closing. It is strictly prohibited to forcefully close it.
34. What is the structure of a tubular heat exchanger?
Answer: Tube heat exchanger is the most widely used heat exchanger in chemical production. Its structure is simple and easy to manufacture, and it is currently the main heat exchange equipment in chemical plants.
(1) The fixed tube plate heat exchanger is mainly composed of components such as the outer shell, tube plate, and tube bundle head. The balanced tube bundle is installed inside the outer shell, and the tube bundle and tube plate are connected by welding or expansion welding. The tube plate is directly welded to the shell, and the head is connected to the shell flange. The fixed tube plate heat exchanger has thin tube plates and low cost, but it is difficult to clean the shell side and there is temperature difference stress.
(2) A U-shaped tube heat exchanger is a type of heat exchanger where the heat exchange tubes are bent into a U-shape and fixed at both ends on the same tube plate. The tube bundle can expand and contract freely without generating temperature stress due to medium temperature differences. This type of heat exchanger only has one tube plate, and the tube bundle can be pulled out for cleaning between the tubes, but the U-shaped bend inside the tube is not easy to clean. When the diameter of the heat exchanger is large, it is difficult to support the U-shaped part and the vibration resistance of the tube bundle is poor. This type of heat exchanger is suitable for two media with large temperature differences or shell side media that are prone to scaling and need to be cleaned; The pipeline medium is free of impurities and is not easily clumped.
(3) One end of the tube plate of the floating head heat exchanger is fixed between the shell side and the tube box, and the other end of the tube plate can move freely inside the shell. The thermal expansion of the tube bundle of this heat exchanger is free, and the tube bundle can be extracted for easy cleaning between and inside the tubes. Its disadvantages are complex structure, high cost, and difficult to inspect and deal with leakage at the floating head during operation. The floating head heat exchanger is suitable for conditions where the temperature difference between the shell and the tube bundle is large or the medium on the shell side is prone to scaling.
35. Maintenance of heat exchanger
Answer: Daily inspection is an important means of early detection and handling of sudden faults, including temperature, pressure, appearance, etc.
(1) Temperature is an important indicator for the operation of a heat exchanger. Measuring the temperature of the inlet and outlet fluids can determine the flow rate of the medium and the heat transfer situation. The heat transfer efficiency is mainly reflected in the heat transfer coefficient, which usually changes little in the short term and continuously decreases when it changes. Regularly measuring the temperature of the two media entering and leaving the heat exchanger can determine whether the heat exchanger has scaling and blockage, and whether it needs to be cleaned.
(2) By measuring and inspecting the fluid pressure and the pressure difference between the inlet and outlet, the internal scaling, blockage, fluid flow rate or leakage of the heat exchanger can be determined. High pressure fluid leaks into low pressure fluid, causing the pressure of the low pressure fluid to quickly rise and even overpressure, and may result in various adverse consequences.
(3) The appearance inspection of the heat exchanger includes whether there is vibration, external leakage, and whether the insulation layer is damaged.
(4) Circulating water heat exchanger, establish water and return water temperature records
(5) Regularly calibrate and inspect the safety valve and explosion-proof plate of the body.
36. What is a pump?
Answer: A machine that transports fluid and increases its pressure is called a pump. The pump itself is an energy consuming machine.
37. How to know if cavitation occurs in the pump.
Answer: The usual symptoms are noise and vibration in the pump, a decrease in pressure and flow rate, and a simultaneous decrease in efficiency.
38. How to prevent cavitation?
Answer: (1) The inlet valve should be fully opened to avoid excessive negative pressure at the inlet;
(2) The installation height of the pump should not exceed the specified limit;
(3) High flow pumps cannot be used in areas with low flow rates, and the outlet valve cannot be controlled too large;
(4) The temperature of the medium cannot exceed the specified limit;
(5) High lift pumps should not be used in places with low lift;
(6) The speed of the adjustable motor cannot exceed the specified limit.
39. Given a pump flow rate of 2000m3/h and a suction pipe diameter of 700mm, what is the flow rate?
Answer: Q=V · S=V · 1/4 ·π D2
V=Q·4/πD2=2000×4/(3.14×0.72)÷3600=1.44m/s
40. What is the function of the pump casing? What is the function of the impeller?
Answer: The function of the pump casing:
(1) Collect the water thrown out from the impeller channel and guide it to the outlet of the water pump;
(2) Slow down the flow rate of the water being thrown out and increase its pressure:
(3) Plays a role in connecting other components and supporting.
The function of the impeller is to make the water pump discharge water by the centrifugal force or power of the impeller.
41. What are the power and efficiency of a pump? Is it allowed to draw up the vacuum height? What is the unit?
Answer: Power N: The power transmitted by the prime mover to the pump shaft per unit time at a certain flow rate and head is called shaft power, which is measured in kilowatts per horsepower;
Efficiency ŋ: During the process of conveying liquid, due to various energy losses, the pump shaft power is not fully transmitted to the liquid, reflecting the relative degree of energy loss.
The allowable vacuum suction height Hs represents the height at which the water pump can suction water, in meters.
42. What is the working principle of a centrifugal pump? What are the volume losses of centrifugation?
Answer: (1) When the centrifugal pump is filled with liquid, the impeller rotates to generate centrifugal force. The liquid in the impeller channel is thrown towards the edge of the impeller under the action of centrifugal force and flows into the pump casing. At this time, the pressure at the center of the impeller decreases, which is lower than the pressure inside the inlet pipe. The fluid flows towards the center of the impeller under the action of this pressure difference. Due to the continuous rotation of the impeller, the liquid is continuously sucked in and discharged. (2) Sealing ring leakage loss, balance mechanism leakage loss, and inter stage leakage loss.
43. What are the reasons for overloading the centrifugal pump motor?
Answer (1) The pressure at the pump discharge end exceeds the design pressure, causing excessive back pressure;
(2) Due to the decrease in liquid temperature and the increase in viscosity, or exceeding the designed viscosity;
(3) The opening of the regulating valve is too small, or the one-way valve fails, or the pipeline is blocked, which increases the resistance of the pump's discharge pipeline;
(4) The impeller is blocked by the suction of sticky debris, or the middle stage of the multi-stage pump is blocked;
(5) The pump shaft or motor shaft has different shafts and the wheels are not aligned correctly;
(6) The impeller's mouth ring is severely damaged (worn) and the clearance is too large;
(7) The stuffing box is compressed too tightly, the stuffing box seal is damaged, or the shaft sleeve is worn.
44. What are the harms of long-term idling of centrifugal pumps?
Answer: Long term idling of the pump means that there is no liquid being transported inside the pump. As a result, the sealing parts inside the pump, such as the mouth ring, intermediate lining, and shaft seal, have small clearances. Lack of lubrication leads to friction, causing wear and even shaft hugging; For rotating contact surfaces such as mechanical seals and balance disks, due to the lack of liquid lubrication and cooling, overheating and even burning can occur due to friction.
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 bearing, shaft seal friction losses, and impeller losses.
Its hazards include: (1) damaging materials; Impeller, guide vanes, pump casing, etc.
(2) Affects the performance of the pump, reducing its output and head, and increasing power consumption:
(3) The pump vibrates and emits strong noise, causing it to malfunction.
46. Describe the four actual performance curves of centrifugal pumps. What is pump stop water hammer?
Answer: (1) Flow head curve (Q-H curve)
Flow power curve (Q-N curve)
Flow rate - suction vacuum height curve (Q-Hs curve)
Flow efficiency curve (Q - ŋ curve)
(2) Pump shutdown water hammer refers to the pressure gradient phenomenon caused by the change in water flow velocity in the pump and pipeline when the pump unit is shut down due to sudden power outage or other reasons.
47. What are the device characteristics of a centrifugal pump?
Answer: The pump and pipeline are inseparable entities, and the head generated by the pump should be equal to the required head value on the pipeline. The flow rate discharged by the pump should be equal to the required flow rate inside the pipe. At this time, the device is in a stable working state, which automatically reaches equilibrium. Therefore, drawing the characteristics of the pump and the pipeline characteristics on one drawing is called the device characteristics of the centrifugal pump.
48. What is the water absorption performance of a centrifugal pump measured by? What are the characteristics of submersible pumps?
Answer: (1) The water absorption performance of a centrifugal pump is measured by the allowable vacuum suction height Hs. The larger the Hs value, the better the water absorption performance or cavitation margin performance of the pump; (2) Integrated pump, without the need for a long rotating shaft, lightweight; Both the electric motor and water pump are submerged in water, so there is no need to build a pump room; The maintenance cost of IIl is relatively small.
49. What should be checked during the operation of a centrifugal pump?
Answer: (1) Check whether each operating parameter (pressure, flow rate, oil temperature, etc.) meets the specified indicators;
(2) Check whether the lubricating oil is qualified and whether the oil level meets the requirements;
(3) Check the condition of the cooling water;
(4) Check for any abnormal conditions such as vibration in the pump;
(5) Check the axle temperature condition;
(6) Check for leaks in all parts.
50. What are the reasons for seal leakage in centrifugal pumps?
Answer: (1) Water seal pipe or valve blockage;
(2) The packing is pressed too tightly;
(3) Flattening of water pressure cover and friction with shaft;
(4) Bending or breaking of the shaft;
(5) The water seal ring and the water seal pipe mouth do not match, and the sealing water cannot enter the water seal ring.
51. What are the reasons for seal leakage in centrifugal pumps?
Answer: (l) The packing pressure is too tight, the packing seal is damaged, or the shaft sleeve is worn;
(2) Improper selection of packing materials, improper installation of packing or water seal rings;
(3) Improper selection or installation of mechanical seals can cause damage to the seals.
52. What are the reasons for centrifugal pump emptying (unable to deliver liquid)?
Answer: (1) Insufficient pump speed or motor reversal;
(2) Loose or reversed impeller (double suction pump), severe corrosion of impeller;
(3) The discharge pressure of the pump exceeds the design pressure, causing excessive back pressure;
(4) Due to the decrease in liquid temperature and the increase in viscosity, or exceeding the designed viscosity;
(5) Due to the small opening of the regulating valve or the malfunction of the one-way valve, as well as pipeline blockage, the resistance of the pump's discharge pipeline increases;
(6) There are airbags in the discharge pipeline;
(7) Due to the suction of sticky debris by the impeller, the impeller may become blocked, or the middle stage of the multi-stage pump may become blocked.
53. What is the cavitation phenomenon of centrifugal pumps?
Answer: From the working principle of centrifugal pumps, it is known that centrifugal force is generated by the rotation of the impeller, which creates a local vacuum at the pump inlet. If the minimum pressure at this time is lower than the saturated vapor pressure of the liquid being sucked in at that temperature, the liquid begins to vaporize and produce bubbles. When the bubbles are brought into the high-pressure area inside the impeller, they condense and burst. Due to the very fast speed of bubble rupture, the surrounding liquid impacts the original space of the bubbles at a very high speed, which is called cavitation phenomenon.
54. Why can't centrifugal pumps run for too long with the outlet valve closed? What is the function of centrifugal pump series connection?
Answer: (1) Due to the prolonged closure of the outlet valve, the impeller continuously rotates, causing the fluid inside the pump to be constantly stirred and rubbed by the pump, resulting in high heat and causing deformation and damage to the parts.
(2) I. Increase the head of the pump; II、 The flow rate is equal to the flow rate of a single pump.
55. What is the reason for insufficient flow of centrifugal pump? How to handle it?
Answer: (l) If there is air in the pump or pipeline, the gas can be discharged;
(2) Eliminate any leaks in the pump body or inlet pipeline;
(3) There is a blockage in the inlet and outlet pipelines, please clean them up;
(4) Adjust the viscosity of the liquid if it exceeds the design specifications;
(5) There are foreign objects in the impeller, check and clean them;
(6) Replace the ring or adjust the clearance if the ring is worn or the clearance between various parts of the pump body is too large.
56. What are the reasons for the heating of the centrifugal pump shaft?
Answer: (1) The pump and motor have different shafts, and the wheels are not aligned correctly;
(2) Damaged bearing or rolling bearing, too tight bearing or too large clearance;
(3) Scaling, blockage, insufficient or interrupted cooling water supply in the cooling system;
(4) Dust, dirt, or corrosive liquids enter the bearing shell or bearing.
57. What are the causes of vibration and noise in centrifugal pumps?
Answer: (1) Leakage of suction pipeline flange;
(2) Failure to pump or exhaust gas completely;
(3) Impeller suction blockage of sticky debris, or intermediate stage blockage of multiple pumps;
(4) Unbalanced rotor or bent shaft deformation;
(5) The pump and motor have different shafts, and the wheels are not aligned correctly
(6) Loose anchor bolts or weak foundation;
(7) The impeller mouth ring is worn and the clearance is too large;
(8) The packing box is compressed too tightly, the packing seal is damaged, or the shaft sleeve is worn;
(9) Improper selection of filler material, improper installation of filler or water seal ring.
58. What are the reasons for the decrease in pressure values of centrifugal pumps?
Answer: (1) The bottom valve of the suction pipe is not open or the bottom valve fails, and the suction bottom valve or filter screen is blocked;
(2) The liquid level drops, the depth of submergence of the suction pipe is insufficient, and the installation height exceeds the allowable suction height;
(3) The liquid level on the suction surface decreases, or the liquid temperature increases;
(4) Insufficient pump speed or motor reversal;
(5) Loose or reversed impeller, severe corrosion of impeller;
(6) The pressure at the pump discharge end exceeds the design pressure, causing excessive back pressure;
(7) Due to the decrease in liquid temperature causing an increase in viscosity, or viscosity exceeding the design value;
(8) Due to the small opening of the regulating valve, malfunction of the one-way valve, blockage of the pipeline, etc., the resistance of the pump increases (pipeline resistance);
(9) Due to the blockage of the impeller caused by the suction of sticky debris, or the blockage of the middle stage of the multi-stage pump.
59. What are the reasons for centrifugal pump vacuum (unable to suck up liquid)?
Answer: (1) Due to the loose connection of the suction flange, air enters the suction end of the pump;
(2) The pump has not been filled or the pump has not been emptied of gas, and there is gas inside the pipe or pump casing;
(3) The bottom valve of the suction pipe is not open or the bottom valve is malfunctioning, and the suction bottom valve or filter screen is blocked;
(4) The liquid level drops, the depth of submergence of the suction pipe is insufficient, and the installation height exceeds the allowable vacuum suction height of the pump
(5) The liquid level on the suction surface decreases, or the liquid temperature increases;
(6) Insufficient pump speed or motor reversal;
(7) Loose or reversed impeller (double suction pump), severe corrosion of impeller;
(8) The impeller mouth ring is worn and the clearance is too large;
(9) The packing box is compressed too tightly, the packing seal is damaged, or the shaft sleeve is worn;
(10) Improper selection of filler material, improper installation of filler or water seal ring;
(11) Improper selection or installation of mechanical seals can cause damage to the seals.
60. Why is it necessary to fill the centrifugal pump with water before starting it? Why do centrifugal pumps need to close the outlet valve before they can start?
Answer: (1) The density of air is much lower than that of water, and the centrifugal force generated by the rotation of the impeller is not enough to create sufficient vacuum inside the pump, so there is no suction ability. Only when the pump is filled with liquid can the centrifugal force of the impeller create enough vacuum inside the pump to discharge the liquid, so water needs to be filled.
(2) Because starting without closing the outlet valve means starting with load, excessive current during load starting can easily burn out the motor or other electrical equipment.
61. Why is it an open impeller or a semi open impeller? What are the basic performance characteristics of a water pump? What are they each?
Answer: (1) Open impeller refers to an impeller with only blades and no complete cover plate; A semi open impeller is an impeller with only a rear cover plate and no front cover plate; (2) The basic performance of a water pump includes six aspects: a. Flow rate Q; b、 Head H; c、 Axis power N; d、 Efficiency η; e、 Speed n; f、 Allow suction vacuum degree.
62. What are the regulations for emergency pump stop?
Answer: If allowed, the used pump should be started first, but the pump can be stopped immediately in the following emergency situations:
(1) The cable head is emitting smoke and fire;
(2) Pumps, motors, and bearings are emitting smoke;
(3) The current exceeds the limit too much, and even after the outlet valve is reduced, the load still exceeds the limit;
(4) Pump shaft breakage;
(5) The pump casing and orchestra have ruptured, resulting in excessive water leakage or failure to reach the motor;
(6) Personal safety is endangered.
63. Can you explain the reason why the water pump cannot produce water after starting?
Answer: (1) The motor circuit is reversed, and the impeller is reversed;
(2) The impeller is installed in reverse, and the direction of the impeller blades is incorrect;
(3) The suction 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: A reciprocating pump belongs to a positive displacement pump. When the piston moves from the left end of the pump cylinder to the right, the working chamber volume of the pump cylinder gradually increases, the pressure decreases, and the liquid sucked in rises along the suction pipe under the action of atmospheric pressure, and opens the suction valve to enter the pump cylinder. Then, under external force, the piston moves to the left, squeezing the liquid and increasing the pressure. The liquid opens the discharge pipe and flows into the discharge container. When the piston moves to the left dead center, it exhausts the liquid sucked in, completing a reciprocating process. The piston continues to move, and the pump continues to deliver liquid continuously.
65. Under what circumstances does an electric pump need to be stopped urgently?
Answer: (1) Motor overheating or smoking;
(2) Bearing overheating or smoking;
(3) The device suddenly experiences strong vibrations;
(4) There is a noticeable metallic sound inside the motor or pump;
(5) The bearing disc root is burnt and leaking severely.
66. What is the reason for the sudden interruption of water discharge during the operation of the water pump? How to handle it?
Answer: (1) Inlet valve and flange intake. Identify the cause and park for repair;
(2) If the water head is above or too shallow from the water surface, stop and wait for the water surface to rise before turning on the pump;
(3) Water head blockage or impeller channel blockage, remove debris after parking;
(4) The pump shaft is broken and requires major repairs;
(5) If the impeller is damaged or completely detached, stop the pump for maintenance.
67. What is the difference between single-stage and multi-stage, single suction and double suction pumps? What is the reason for the noise inside the pump?
Answer: (1) The number of stages refers to the number of impellers, that is, the number of impellers is the stage. Single suction and double suction refer to whether the impeller enters the water from one side or both sides;
(2) A. Air leakage or cavitation occurs in the inlet pipe; b、 Local damage to the rotor and stator; c、 Inhalation of foreign objects; d、 Poor lubrication and cooling can cause wear and tear.
68. Why is the water pump usually installed first when installing the unit? What is the reason for pump shaft coupling?
Answer: (1) Generally, the water pump is installed first, and then the electric motor is installed, because the electric motor is only related to the position of the water pump, while the position of the water pump involves the connection of other equipment and pipelines.
(2) A. Unbalanced water inflow on both sides, causing axial pushing; b、 Bearing wear; c、 The clearance left by the bearing is too large; d、 Air enters the pump; e、 The impeller is damaged or the impeller suction port is blocked by dirt.
69. What are the factors that affect changes in pipeline characteristics?
Answer: (1) Changes in the factors that adjust the valve opening on the pipeline, as well as changes in the length of the pipeline, can cause changes in the K value, making the pipeline characteristic curve steeper or flatter;
(2) Only when the pressure difference and liquid level difference between the suction and discharge zones change, will the static head H-static change, and the pipeline characteristic curve will shift downward.
70. What are the reasons for motor overload? handle?
Answer: (l) Incorrect rotation direction, correct the rotation direction;
(2) There are foreign objects in the impeller, check and clean them;
(3) Adjust the specific gravity or viscosity of the liquid if it exceeds the specified limit;
(4) The coupling is not concentric, please realign it;
(5) Friction between impeller and pump casing, adjust.
(6) The outlet valve is too wide open.
71. What is the function of the stuffing box?
Answer: The packing box, commonly known as a packing box, consists of a packing seat, packing, packing gland, water seal ring, etc. It is used to close the gap between the pump shaft and the pump casing, prevent water leakage and intake, and partially support the pump shaft and guide water for lubrication and cooling. The function of packing is to provide a sealing effect after being compressed inside the packing box.
72. What are the reasons for high bearing temperature?
Answer: The reasons are:
(l) The low oil level reduces the amount of oil in the inlet bearing;
(2) Unqualified oil quality, water ingress, impurities ingress, or emulsification deterioration;
(3) Insufficient or interrupted cooling water for bearings;
(4) Poor bearing quality or poor assembly, such as being too tight or too loose;
(5) Bearing damage, such as severe vibration or poor concentricity of the pump.
73. What is the function of bearings? How many types of bearings are commonly used?
Answer: Bearings are components that support shaft necks and are sometimes used to support rotating parts on shafts. They are important components in machinery. According to the direction of load bearing, bearings can be divided into two types: radial bearings and thrust bearings. According to the frictional properties of bearings, they can be divided into two types: sliding friction bearings and rolling friction bearings.
74. What are the causes of vibration in the water pump bearings?
Answer: (1) Air enters the inlet pipe of the pump body;
(2) There is cavitation phenomenon;
(3) The water output of the pump is much lower than the designed flow rate, and the opening of the outlet valve is too large;
(4) Loose foundation bolts;
(5) Bending of the shaft or poor dynamic balance of the rotor;
(6) There are foreign objects or wear in the impeller;
(7) Poor lubrication and bearing wear;
(8) The concentricity of the wheels exceeds the tolerance.
75. What is the function of lubricating oil? How to deal with bearing overheating?
Answer: 1. Function: (1) It has a lubricating effect and prevents dry friction; (2) Plays a cooling and flushing function; (3) Vibration reduction and shock load resistance; 2、 (1) Lubricating oil has deteriorated and should be replaced; (2) Add too much fuel and reach the specified oil level; (3) Align the wheels; (4) Bearing damage, correction;
76. What is a shaft seal? What are the two commonly used shaft seal structures in centrifugal pumps?
Answer: The so-called shaft seal refers to the seal between the rotating centrifugal pump shaft and the fixed pump body. There are two commonly used shaft seal structures in centrifugal pumps: (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 it relies on the eccentric rotation of the screw in the pump casing with an internal spiral, pushing the liquid in the axial direction, squeezing it to the discharge port, and then discharging it.
78. Why conduct safety inspections on chemical enterprises?
Answer: The production efficiency of any chemical enterprise is closely related to the condition of its equipment. Well functioning equipment is the fundamental guarantee for enterprise safety production and increased efficiency. If the equipment is not used and maintained properly, it will inevitably lead to various production accidents. Mild cases may cause material loss, production decline, and environmental pollution due to leakage, while severe cases may cause equipment rupture, explosion, personnel injury, and property damage. Therefore, regular inspections and daily maintenance of chemical production equipment are particularly important for chemical enterprises to ensure the integrity and safe operation of the equipment, and to prevent sudden shutdowns and production stoppages.
79. What are the classifications of chemical maintenance?
Answer: Chemical maintenance can be divided into two categories, namely planned maintenance and unplanned maintenance.
（1） Planned Maintenance: According to the "Regulations for Maintenance and Repair of Chemical Equipment", an equipment maintenance plan is developed based on the technical status information and production cycle of the existing equipment in the enterprise. The maintenance carried out according to the plan is called planned maintenance. Planned maintenance can be divided into minor, medium, and major repairs based on their scale, required time, and number of maintenance items. General chemical enterprises usually conduct regular major repairs once a year, but there are also cases where they are carried out every two years or several years. It can be determined based on the operation of enterprise equipment.
（2） Unplanned maintenance: In the event of a sudden malfunction or accident during equipment operation and installation, maintenance or repair that must be carried out without stopping or temporarily stopping is called unplanned maintenance. Unplanned maintenance is an unplanned repair that is carried out without prior preparation for a malfunction or shutdown. It belongs to post maintenance, and due to its poor planning, the quality of maintenance cannot be guaranteed. Sometimes, it is forced to repair in a hurry to restore production. Soon, repeated failures will occur again. Due to the complexity of chemical system production, it is inevitable for unexpected factors to cause failures and shutdowns, such as equipment leaks, blockages, control or transmission device failures, which are all unpredictable in advance.
80. What are the characteristics of chemical maintenance?
Answer: The characteristics of maintenance in chemical enterprises are frequent, complex, highly technical, and high-risk maintenance processes. There are various types of chemical equipment, such as furnaces, towers, kettles, heat exchangers, reactors, compressors, centrifuges, pumps, storage tanks, tanks, as well as pipelines, valves, instruments, meters, etc. The structures are complex and have different pressure bearing capacities. The types of production failures are different and the causes are complex.
81. What are the general requirements for chemical maintenance?
Answer: Strengthen the scientific management of equipment maintenance, improve the predictability of maintenance, coordinate and arrange maintenance tasks, ensure maintenance quality, reduce maintenance costs, and must prevent and reduce the occurrence of malignant accidents.
82. What is the importance of equipment lubrication? How do operators do lubrication work well?
Answer: Lubrication is an effective measure to reduce friction, wear, and energy consumption. Good lubrication is an essential condition for the normal operation of machinery and equipment. Strictly implement the "five determinations" and "three-level filtering" requirements. The "Five Determinations" are: quality determination, quantity determination, timing, fixed-point, and regular cleaning; The "three-level filtration" refers to the three-stage filtration of oil drums, oil pots, and oil injection points.
83. What should be paid attention to when opening and closing valves?
Answer: When opening and closing the valve, do not open it too much or apply too much force. The screw of the valve should be cleaned regularly and lubricated during use. If it is found that the valve switch is not working properly, the cause should be identified and the fault should be eliminated. It must not be forcefully opened or closed, otherwise it will damage the valve and cause accidents. If leakage is found at the packing box of the valve stem, the bolts at the packing gland can be tightened. If the packing is aging, it should be replaced in a timely manner when production permits.
84. Important indicators of lubricating oil:
Answer: (1) Viscosity. It represents the magnitude of the internal friction between the thin layers of liquid during its flow. The pressure that the oil film can withstand is proportional to the product of viscosity and relative motion speed. The higher the operating or lubrication speed of the mechanism, the lower the viscosity of the lubricating oil should be. When traveling at high speeds, the relative displacement of the oil layer increases, and the heat generated by the internal friction of oil molecules also increases accordingly. Therefore, lubricating oil has the effect of reducing both friction loss and energy loss. Oil with higher viscosity will increase its heat generation during operation, but when the heat generation increases, the oil temperature will increase and the viscosity will decrease, thus playing a certain automatic adjustment role, that is, lubrication can take away the energy generated by friction.
(2) Freezing point. The temperature at which oil loses its fluidity is called the freezing point. As oil is a complex mixture of hydrocarbon molecules, when the temperature decreases, the lipids in the oil may precipitate and become solid, causing the oil to become cloudy. Therefore, when using the oil, it should be several degrees higher than the freezing point.
(3) Residual carbon. The percentage of coke formed by thermal cracking and condensation of lubricating oil to the weight of lubricating oil is an indicator that controls the degree of refinement of lubricating oil.
(4) Flash point. Under certain heating conditions, as the temperature increases, the evaporation of lubricating oil increases, and the pressure of oil vapor formed on the surface of the oil also increases. When the mixture of oil vapor and surrounding air meets fire and produces a flash flame, the temperature at this point is called the flash point. Flash point is a safety indicator for lubricating oil operation, and generally requires the working temperature of the oil to be 20-30 ℃ lower than the flash point.
(5) Anti emulsification type. Under specified conditions, the time for lubricating oil to separate from water is called emulsification resistance. The shorter the time, the better its emulsification resistance. It indicates that oil and water cannot form a stable emulsion, and even if mixed with water, they will quickly separate.
(6) Antioxidant safety. The ability of lubricating oil to resist oxidation under heating and the presence of metals is called the antioxidant safety of lubricating oil, which reflects the chemical properties of the oil.
(7) Viscosity ratio. The ratio of the kinematic viscosity of the same oil at 50 ℃ to that at 100 ℃, the smaller the ratio, the better the viscosity characteristics of the oil and the wider the allowable range of use.
85. Steps for normal start-up and shutdown of centrifugal pump:
Answer: (1) Start up: ① Prepare well before starting the pump: a. The liquid level of the tower and tank should be above 50% of normal. b. Confirm that the inlet valve is open, the outlet valve is closed, the reflux valve is open, and the pump discharge drain is closed. c. Open the exhaust valve and exhaust the pump body until liquid flows out, then close it. d. Check that the oil level in the bearing box and oil cup is between 1/2 and 3/2. e. Check if the cooling water in the bearing box is unobstructed. f. Drive 2/3 turns without any jamming.
② When starting for the first time, jog to confirm that the pump motor is rotating in the correct direction.
③ Start the pump and pay attention to the ammeter indication, inlet and outlet pressure indication, and whether the mechanical seal leaks after starting. After everything is normal, slowly open the outlet valve Check that the bearing temperature of the pump should not exceed 65 ℃, and the motor temperature should not exceed 70 ℃. Observe the fluctuation of the outlet pressure gauge and ammeter, check the operation vibration and leakage of the pump, as well as the changes in the cooling water and lubricating oil levels Under normal circumstances, the outlet valve of the pump cannot be used to adjust the flow rate.
⑥ For pumps that operate for a long time, it is necessary to regularly replace lubricating oil or grease to ensure that the pump operates in good lubrication condition.
(2) Steps for stopping the centrifugal pump: ① Gradually close the pump outlet valve. ② When the outlet valve of the pump is fully closed, stop the pump After the pump stops, open the warm valve (cold valve) of the pump to ensure standby status Regular inspection and turning. ⑤ If maintenance is required, the pump should be thoroughly isolated, depressurized, and drained During winter maintenance, it is necessary to intermittently discharge the cooling water to prevent freezing.
86. The function and scope of use of anti riot plates in heat exchangers:
Answer: Explosion proof disc, also known as bursting disc, is a type of rupture type overpressure protection device used to be installed on pressure vessels that are not suitable for installing safety valves. When the pressure inside the vessel exceeds the normal working pressure and reaches the design pressure, it will automatically explode to prevent the vessel from exploding. Its installation should generally meet the following conditions.
(1) When the medium inside the container is prone to crystallization or has high viscosity, if a safety valve is used, impurities or crystallization may accumulate on the valve core, causing the safety valve to fail to open at the specified pressure and lose its function.
(2) The pressure inside the container rises rapidly due to chemical reactions or other reasons, making it difficult for the safety valve to promptly eliminate the excessive pressure.
(3) When the medium inside the container is highly toxic gas or there is no trace leakage allowed, and it is difficult to ensure that these gases do not leak with a safety valve, explosion-proof discs should be used.
87. What is a stage in a steam turbine?
Answer: In a steam turbine, a power unit consisting of nozzles and their corresponding moving blades forms a stage. The nozzles are mounted on the partition of the cylinder and remain stationary, while the moving blades are mounted on the impeller and rotate together with it. The stage is the most basic unit of work in a steam turbine, and the process of converting the thermal energy of steam into mechanical energy occurs within the stage.
88. What are the main components of a steam turbine? What are the characteristics?
Answer: A steam turbine mainly consists of a stationary part and a rotating part. The stationary part includes the main air valve, regulating valve, cylinder, front and rear bearing seats, machine base, sliding pin system, etc. The rotating part mainly refers to the rotor components, including the steam turbine main shaft, impeller, drum, moving blades, crisis protector, etc.
Main steam valve: also known as emergency stop valve, when emergency stop occurs again, the main steam valve automatically and quickly closes, cutting off the gas source and ensuring the safe shutdown of the unit. The steam engine uses a hydraulic oil cylinder to control the opening and closing of the main air valve.
Regulating valve: divided into five valve heads, during start-up and normal operation, the opening of the regulating valve is controlled to regulate the amount of steam entering the steam turbine, thereby controlling the speed and power of the steam turbine.
Bearing seat: used to install radial bearings and thrust bearings. The radial bearings use five oil groove tilting pad bearings, and the thrust bearings use Kingsbury bearings.
Cylinder: The cylinder is a horizontal split structure, divided into an upper half cylinder and a lower half cylinder, which are connected by bolts. The front part of the cylinder is supported by the upper half of the cylinder, with the cat claws of the upper half resting on the front cylinder. The rear part of the cylinder is supported by the lower half of the cylinder, which is seated on the base through two side legs.
89. How to balance the axial force of steam turbine rotor?
Answer: During normal operation, the axial force on the rotor of a steam turbine is quite large, and some high-pressure reaction steam turbines can reach 2-3MN. Such a large axial force cannot be achieved solely by thrust bearings. Often measures need to be taken to balance most of it, and a small part is shared by thrust bearings. In multi-stage steam turbines, the commonly used methods for balancing axial force are as follows:
(1) Balance piston. At the front end of the rotor, a balancing piston is machined, with low-pressure steam in front of the piston and high-pressure steam behind the piston boss. This generates a forward axial thrust on the balancing piston to counteract the axial force of steam in the opposite direction on the rotor.
(2) Open a balance hole. Drill several holes on the impeller of an impulse steam turbine to allow steam to flow between the two sides of the disc, reducing the pressure difference and thus reducing the axial thrust on the disc. The balance holes are generally odd, usually 5 or 7.
(3) Adopting a reverse flow arrangement method. Arrange the flow of steam in the steam turbine in opposite directions to generate axial forces in opposite directions, which cancel each other out to achieve equilibrium. Steam can also be directed in opposite directions at high and low pressures to automatically balance axial forces.
90. What are the reasons for scaling on the nozzles and moving blades of steam turbines? What are the hazards?
Answer: Due to poor boiler water quality and other reasons, the steam quality is poor and contains certain impurities. When these steam pass through the steam turbine, different impurities precipitate at different stages and deposit on the flow passage of the steam turbine, forming scale. The scaling of the flow passage is generally less in the first and second stages, and gradually increases in the later stages. Most of the scaling substances deposit in areas with low airflow velocity, or may occur in areas with interrupted flow and rough surfaces.
When there is excessive scaling on the nozzle and moving blades, it will cause a decrease in the flow area and an increase in resistance. Due to the decrease in circulation area, under the condition of constant total enthalpy drop and constant pressure at all levels of the unit, the output will decrease due to the decrease in steam flow rate. Sometimes, in order to ensure output, it is necessary to increase the initial steam pressure and increase the force, which reduces the safety of the unit operation and lowers the safety of the unit operation. In addition, scaling damages the shape of the nozzle and moving blade channels, making the blade surface rough and increasing friction losses, seriously affecting the efficiency of the steam turbine.
91. What is the critical speed of the rotor? What are the factors that affect the critical speed?
Answer: During the operation of a steam turbine, the rotor will vibrate. During the driving process, the rotor starts to vibrate slightly. When the speed reaches certain values, the rotor vibration increases significantly. After the rotor exceeds these values, the rotor vibration will decrease again. During the deceleration process of the rotor, similar phenomena may also occur near the same rotational speeds. These speeds are referred to as the critical speeds of the rotor, which are divided into the first critical speed, the second critical speed, etc. in ascending order of speed. In practical production, the first and second critical speeds are commonly encountered.
The critical speed of the rotor mainly depends on its mass and rigidity. The larger the mass and rigidity of the rotor, the higher the critical speed of the rotor. In addition, it is also related to the following factors: the influence of support elasticity, the influence of impeller rotational torque, the influence of rotor extension section, the influence of shaft system, and the influence of damping.
92. Why is it necessary to perform dynamic balancing on the rotor after major repairs? What is the standard?
Answer: Due to machining errors, assembly errors, and uneven material distribution, the mass centers of each rotating part of the rotor after major repairs are not all on the rotating shaft. When the rotor rotates, unbalanced forces and moments are generated, causing strong vibration of the rotor and threatening the safe and stable operation of the unit. Therefore, the rotor after major repairs must undergo strict dynamic balancing. The standard for the dynamic balance accuracy of the rotor 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 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 reducing the non working side adjustment gasket of the thrust bearing. This not only adjusts the thrust clearance, but also does not change the axial working position of the rotor. When measuring the clearance of the thrust bearing, it is important to fasten the upper cover of the bearing housing and tighten the bolts on the split surface, otherwise the measurement error will be significant. In addition, only one piece of adjusting gasket is allowed for the working and non working sides of the thrust bearing, and the thickness should be uniform along the entire circumference without deviation.
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