30 Common Questions About Centrifugal Pumps

1. What is the main working principle of a centrifugal pump?

The motor drives the impeller to rotate at high speed, causing the liquid to generate centrifugal force. Due to the action of centrifugal force, the liquid is thrown into the side channel and discharged out of the pump, or enters the next stage impeller, thereby reducing the pressure at the impeller inlet, forming a pressure difference with the pressure acting on the suction liquid. The pressure difference acts on the liquid suction pump. Due to the continuous rotation of the centrifugal pump, the liquid is continuously sucked in or discharged.

2. What are the functions of lubricating oil (grease)?

Lubrication and cooling, flushing, sealing, vibration reduction, protection and unloading functions.

3. What are the hazards of water in pump lubricating oil?

1.Water can reduce the viscosity of lubricating oil, weaken the strength of the oil film, and reduce the lubrication effect.

2.Water will freeze when the temperature is below 0℃, which will seriously affect the low-temperature fluidity of lubricating oil.

3.Water can accelerate the oxidation of lubricating oil and promote the corrosion of metals by low molecular organic acids.

4.Water increases the foaming properties of the lubricant, making it easier for the lubricant to foam.

5.Moisture can cause metal parts to rust.

4. What is the function of sealing oil ?

Cooling sealing parts; lubricating friction parts; preventing vacuum damage.

5. What are the common standards for shaft seal leakage?

Packing seal: Light oil less than 20 drops/min Heavy oil less than 10 drops/min

Mechanical seal: less than 10 drops/min for light oil and less than 5 drops/min for heavy oil

6. How to switch the centrifugal pump?

First, all preparations should be made before starting the pump, such as preheating the pump. Switch between the two pumps according to the relevant parameters such as the pump's outlet flow, current, pressure, and liquid level. The principle is to start the standby pump first. After all parts are normal and the pressure comes up, slowly open the outlet valve and slowly close the outlet valve of the switched pump until the outlet valve of the switched pump is completely closed. Stop the switched pump, but try to minimize the fluctuation of parameters such as flow caused by the switch.

7. Why can’t the centrifugal pump be started when the disc does not move?

If the centrifugal pump disc does not move, it indicates an internal pump failure. This could be due to the impeller being stuck, the pump shaft being excessively bent, rust on the pump's moving and static parts, or excessive pressure within the pump. If the pump disc does not move and is forced to start, the powerful motor force driving the pump shaft can cause damage to internal components, such as shaft breakage and distortion, impeller shattering, motor coil burnout, and possibly motor tripping, resulting in a startup failure.

8. Why does the standby pump need to be rotated regularly?

There are three functions of regular cranking: preventing scale from forming inside the pump and causing it to get stuck; preventing the pump shaft from deforming; and cranking can also bring lubricating oil to various lubrication points to prevent the shaft from rusting. Lubricated bearings facilitate immediate start-up in an emergency.

9. When the machine pump is operating normally, what is the appropriate cooling water volume?

When the pump is operating normally, the optimal cooling water flow rate is when the discharge water temperature is around 40°C. At this point, the discharged water is slightly warm to the touch, achieving optimal cooling. In particular, the cooling water flow rate for the end seals should not be set too high in winter to prevent the dynamic and static seals from hardening due to the low water temperature, losing their elasticity and causing seal failure.

10. What happens when a centrifugal pump is evacuated?

When a running pump begins to pump out liquid, it will suddenly make noise and vibrate, accompanied by a decrease in pressure, flow rate and current. When the pumping is serious, the pump will vibrate violently, the pressure will return to zero, and no liquid will be pumped out of the pump.

11. What are the reasons for bearing box heating?

1.The pump shaft and motor shaft are not concentric.

2.Too much or too little lubricating oil (grease), or the oil (grease) deteriorates.

3.The oil ring is broken or jumps out of the fixed position.

4.The axial thrust increases.

5.Insufficient or interrupted cooling water supply.

6.Bearing wear.

7.The flow rate is too small, causing the pump to be almost empty.

8.Pump vibration is caused by reasons such as unstable foundation, unbalanced rotor, loose bolts, etc.

12. Why does the pump need to be protected from freezing in winter?

1.Water expands in volume when it is below zero degrees. If the water left in the pump body is not cleaned out, the force generated by the volume expansion at low temperatures will cause the pump body to burst, causing unnecessary damage.

There are several main methods of antifreeze:

2.Drain any water from the idle pump.

3.Keep the cooling water flowing steadily.

4.Insulate the pump or heat it with steam or hot water.

5.A backup pump maintains inlet and outlet flow.

13. What to do if the pump is frozen?

1.After the pump is frozen, never blow it directly with steam to prevent it from bursting due to uneven thermal expansion of the pump body.

2.When the pump is frozen, pour cold water on it first, and then when the car is turned on, you can pour steam or hot water on it.

14. Why does the pump body need to be cooled?

When the pump is running, the flow friction between the liquid and the pump body, the rotating parts and the fixed parts such as the ball and inner and outer rings of the bearing, the sleeve and the packing will produce friction, which will generate heat. At the same time, the high temperature of the medium is transmitted to the pump, making the pump body hot. The purpose of cooling is to reduce the temperature of the pump body, pump seat, bearing box and shaft seal to prevent these parts from deformation, aging and damage due to temperature rise.

15. Under what circumstances does the motor need to be stopped urgently?

1.When personal safety is endangered.

2.When the motor emits smoke, odor, or catches fire.

3.When large vibration or axial vibration occurs.

4.When the fuselage or bearings heat up to the limit.

5.The motor speed is slow and accompanied by abnormal noise.

1. Explain the model of domestic centrifugal pumps. For example, what does IH80-65J-125A mean?

80 --Pump suction port diameter; IH --Single-stage centrifugal mechanical pump; J-Use at reduced speed; 125 --Nominal diameter of the impeller; A --the first cutting of the impeller.

17. How are pumps classified?

Pumps are generally classified into two categories based on how they act on liquids: vane pumps and positive displacement pumps. Vane pumps transport liquids via a rotating impeller within the pump. Depending on the structure of the vanes within the pump, vane pumps are further categorized as centrifugal pumps, axial flow pumps, and vortex pumps. Positive displacement pumps transport liquids by periodically changing the volume of the pump's working chamber. These pumps are divided into reciprocating pumps (piston pumps, plunger pumps, diaphragm pumps, etc.) and rotary pumps (gear pumps, screw pumps, etc.).

Pumps are often named according to their purpose, such as: water pump, oil pump, ammonia pump, liquid hydrocarbon pump, mud pump, corrosion-resistant pump, condensate pump, etc.

18. How is the axial force of a centrifugal pump eliminated?

1) Measures for balancing the axial force of a single-stage pump are:

①Use double suction impeller; ②Open balance hole; ③Balance pipe

2) The axial force balancing measures for multi-stage centrifugal pumps are:

The impellers are arranged symmetrically; ② Using a balancing drum plus a balancing pipe; ③ Using a balancing disc plus a balancing pipe; ④ Using a combined balancing measure of a balancing drum, a balancing disc and a balancing pipe.

19. Under what circumstances does the pump need to be cooled? What is the function of cooling water?

1.When the temperature of the pump conveying medium is greater than 100℃, the bearings need to be cooled. When it is greater than 150℃, the sealing chamber generally needs to be cooled. When it is greater than 200℃, the pump supports generally need to be cooled.

2.The functions of cooling water are:

3.Reduce bearing temperature;

4.Take away a small amount of liquid leaking from the shaft seal and conduct friction heat;

5.Reduce the temperature of the stuffing box, improve the working conditions of the mechanical seal and extend its service life;

6.Cool the pump support to prevent the concentricity of the pump and motor from shifting due to thermal expansion;

7.Cooling water liquid should be recycled water or fresh water as much as possible. Only when their total hardness is greater than 4.5 mm g equivalent/L, softened water can be used and recycled.

20. What is cavitation?

When the pressure at the impeller inlet falls below the saturation pressure of the working medium, some of the liquid evaporates (i.e., vaporizes). When these bubbles enter a higher-pressure area, they suddenly condense under pressure, forcing surrounding liquid to flow in, causing hydraulic shock. This phenomenon is known as cavitation. This continuous, localized impact load gradually fatigues the material surface, causing erosion of the metal surface and the appearance of large and small honeycomb-like cavities.

The instability of the cavitation process causes the pump to vibrate and make noise. At the same time, since bubbles block the impeller grooves during cavitation, the flow rate and head are reduced, and the efficiency is reduced. Therefore, cavitation should be prevented.

21. How to identify cavitation ?

1.There is a loud noise inside the pump.

2.The outlet pressure is unstable and in severe cases, the output cannot be measured.

3.The pump body vibrates.

4.The medium temperature rises.

22. What are the causes of cavitation?

1.The causes of cavitation are:

2.The suction liquid level drops or the filling head height is insufficient.

3.The atmospheric pressure decreases.

4.The pressure in the system decreases.

5.As the medium temperature rises, the saturated vapor pressure increases (referring to the medium that is easy to vaporize).

6.As the liquid flow rate increases, the resistance loss increases.

7.The suction pipe has high resistance. (This depends on the pump structure and whether the suction pipe is properly installed.)

8.Air leak in the suction line.

23. How many types of transmission connections are there between pumps and motors?

There are three types: belt drive, gear box drive, and motor direct linkage.

24. What is pump flow?

The amount of liquid discharged from the pump outlet pipe per unit time is called flow rate, which is represented by Q. Flow rate can be divided into weight flow rate and volume flow rate, with volume flow rate being the main type.

Weight flow rate refers to the weight of fluid passing through a device per unit time. Units: kg/s, T/h, etc. Volume flow rate refers to the volume of fluid passing through a device per unit time. Units: M³/h.

25. What should be done to prevent cavitation before starting the pump?

1.First, exhaust the residual gas in the pump;

2.When installing the pump, the suction vacuum height should be reduced as much as possible;

3.The temperature of liquid heating should be kept as low as possible during operation;

4.Reduce resistance loss in pipelines;

5.Minimize hydraulic loss in the suction pipe so that the pressure of the liquid at the inlet of the pump is much greater than the saturated vapor pressure of the liquid.

6.Reducing the speed at which the liquid enters the impeller, that is, reducing the number of revolutions and flow of the pump, can also prevent cavitation.

7.The use of an inducer can also improve pump cavitation.

8.The impeller is made of materials with strong cavitation resistance.

26. Why is the inlet pipeline of the pump thick and the outlet pipeline thin?

Because a pump draws in liquid through its pressure differential, its suction capacity is less than its discharge capacity when the diameters are the same. When the amount of liquid drawn in is less than the amount discharged, the pump will experience vacuum. A slightly thicker inlet pipeline can reduce suction resistance and increase the pump's suction capacity. Therefore, the pump's inlet pipeline should be thicker than the outlet pipeline.

27. Why should a one-way valve be installed on the outlet pipeline of a centrifugal pump?

A one-way valve, also known as a return valve, allows liquid to flow in one direction only and not in the opposite direction. Installing a one-way valve at the pump outlet can prevent reverse rotation of the pump due to backflow of liquid when the outlet valve is not closed for some reason, causing the nuts on the rotor to loosen or fall off.

28. What to do when the pump temperature is too high?

If a pump overheats during operation, the cause of the temperature rise should be identified and addressed promptly. If the pump itself is faulty, it should be stopped for inspection and corrective action. If the pump is operating improperly, adjust operating parameters and increase flow. If the cause is excessively high temperatures or the medium being pumped, cool it with water or air (use water to cool it gradually, not suddenly). However, water should not be used to cool overheated motors and cables, as it is not insulated. If the motor leaks electricity, the leaked electricity could be transferred to the human body through the water, causing an electric shock. Furthermore, if water enters areas such as junction boxes, it can damage insulation, directly impacting the normal operation and safe production of the equipment.

29. Why can’t a centrifugal pump be reversed or idle?

Both reverse rotation and idling can cause unnecessary damage to centrifugal pumps. Reverse rotation can loosen or even cause pump fixing screws, such as the shaft sleeve and impeller cap, to fall off, potentially causing accidents. During idling, no liquid enters or exits the pump, causing friction within the pump to generate heat and vibration, potentially damaging parts. In severe cases, this can cause the shaft to seize and other accidents.

30. What does the normal and good operation of a centrifugal pump include?

1.The pressure and flow are stable, and the outlet pressure can meet normal production needs or reach more than 90% of the nameplate capacity.

2.The lubrication and cooling systems are unobstructed, and the oil rings, bearing boxes, liquid level pipes, etc. are complete and in good working order. The lubricating oil is selected in accordance with regulations, and the bearing temperature meets the standards.

3.The operation is smooth and noiseless, and the shaft stringing and vibration meet the standards.

4.The shaft seal leakage meets the standards.