Frequently Asked Questions and Answers About Centrifugal Pumps

Frequently Asked Questions and Answers About Centrifugal Pumps

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Classification of centrifugal pumps?

What preparations need to be done before starting the centrifugal pump?

What are the things to pay attention to?

What is the content of vibration analysis of centrifugal pump?

What is a centrifugal pump?

Centrifugal pump refers to a pump that transports liquid by the centrifugal force generated by the rotation of the impeller. Centrifugal pumps work by rotating the impeller to cause centrifugal motion of water. Before the pump is started, the pump casing and the suction pipe must be filled with water, and then the motor is started, so that the pump shaft drives the impeller and the water to rotate at a high speed. into the pressurized water line of the pump.

How many parts does a centrifugal pump consist of?

The basic structure of centrifugal pump is composed of six parts, namely impeller, pump body, pump shaft, bearing, sealing ring, stuffing box.

1. Impeller

It is the core part of the centrifugal pump. It has a high speed and a large output. The blades on the impeller play a major role. The impeller must pass a static balance test before assembly. The inner and outer surfaces on the impeller are required to be smooth to reduce the frictional loss of the water flow.

2. The pump body is also called the pump casing

It is the main body of the water pump. It plays the role of supporting and fixing, and is connected with the bracket on which the bearing is installed.

3. Pump shaft

The function is to connect the motor with the coupling and transmit the torque of the motor to the impeller, so it is the main component for transmitting mechanical energy.

4. Bearing

Sliding bearings use transparent oil as lubricant, add oil to the oil level line. Too much oil will seep along the pump shaft, and too few bearings will overheat and burn out, causing an accident! During the operation of the pump, the temperature of the bearing is as high as 85 degrees, and generally runs at about 60 degrees.

5. The sealing ring is also called the leakage reducing ring.

6. Stuffing box

It is mainly composed of packing, water sealing ring, packing cylinder, packing gland and water sealing tube. The function of the stuffing box is mainly to close the gap between the pump casing and the pump shaft, so that the water flow in the pump does not flow to the outside and the outside air does not enter the pump.

Classification of centrifugal pumps?

(1) Classification according to the number of impellers

1. Single-stage pump: that is, there is only one impeller on the pump shaft.

2. Multistage pump: that is, there are two or more impellers on the pump shaft. At this time, the total head of the pump is the sum of the heads generated by n impellers.

(2) Classification according to work pressure

1. Low pressure pump: the pressure is lower than 100 meters of water column;

2. Medium pressure pump: the pressure is between 100 and 650 meters of water column;

3. High pressure pump: the pressure is higher than 650 meters of water column.

(3) Classification by impeller suction method

1. Single-side inlet pump: also known as single-suction pump, that is, there is only one water inlet on the impeller;

2. Double-side inlet pump: also known as double-suction pump, that is, there is a water inlet on both sides of the impeller. Its flow is twice as large as that of a single-suction pump, which can be approximated as two single-suction pump impellers placed back-to-back.

(4) Classification according to the combination of pump casings

1. Horizontal split pump: that is, there is a joint seam on the horizontal plane passing through the axis line.

2. Vertical joint surface pump: that is, the joint surface is perpendicular to the axis line.

(5) Classification according to the position of the pump shaft

1. Horizontal pump: the pump shaft is in a horizontal position.

2. Vertical pump: the pump shaft is in a vertical position.

(6) Classification according to the way of the impeller

1. Volute casing pump: After the water comes out of the impeller, it directly enters the pump casing with a spiral shape.

2. Guide vane pump: After the water comes out of the impeller, it enters the guide vane set outside it, and then enters the next stage or flows into the outlet pipe.

(7) Classification by installation height

1. Self-priming centrifugal pump: the pump shaft is lower than the surface of the suction pool, and it does not need to be filled with water when starting, and it can be started automatically.

2. Suction centrifugal pump (non-self-priming centrifugal pump): the pump shaft is higher than the surface of the suction pool. Before starting, it is necessary to fill the pump casing and the suction pipe with water, and then drive the motor to make the impeller rotate at a high speed. The water is thrown out of the impeller under the action of centrifugal force, and the center of the impeller forms a negative pressure. The water in the suction pool enters the impeller under the action of atmospheric pressure. Under the action of the high-speed rotating impeller, it is thrown out of the impeller and enters the pressurized water pipeline.

In addition, it can also be classified according to the application, such as oil pump, water pump, condensate pump, ash discharge pump, circulating water pump, etc.

What preparations need to be done before starting the centrifugal pump?

1. Check before starting the centrifugal pump

Whether the name, model, main performance and filling quantity of lubricating oil meet the requirements of technical documents;

Whether the bearing lubrication system, sealing system and cooling system are in good condition, and whether the oil circuit and water circuit of the bearing are unblocked;

Turn the rotor of the disc pump 1 to 2 turns to check whether the rotor is rubbed or stuck;

Is there any debris that hinders rotation near the coupling or belt guards, etc.;

Whether the foundation anchor bolts of the pump, bearing seat and motor are loose;

The valves or auxiliary devices of the pump working system should be in the position with the least load when the pump is running, and the outlet regulating valve should be closed;

Jog the pump to see if the impeller steering is consistent with the design steering. If it is inconsistent, the impeller needs to stop rotating completely, and the motor wiring can be adjusted before restarting.

2. Centrifugal pump filling

Before the pump is started, the pump casing and suction pipe must be filled with water, because in the presence of air, the vacuum at the suction port of the pump cannot be formed and maintained.

3. Centrifugal pump warmer

Multistage centrifugal pumps that transport high-temperature liquids, such as boiler feed pumps in power plants, must warm up the pump before starting. This is because when the feed pump is started, the high-temperature feed water flows through the pump, so that the temperature of the pump body quickly rises from normal temperature to 100-200 ° C, which will cause the temperature difference between the inside and outside of the pump and between the components. Heat transfer time and measures to properly control the temperature rise will cause uneven expansion of the pump, resulting in deformation, wear, vibration and bearing shaft holding accidents of various parts of the pump body.

What are the things to pay attention to?

Centrifugal pump is a kind of vane pump. During the rotation process of the rotating impeller, due to the interaction between the vane and the liquid, the vane transmits the mechanical energy to the liquid, which increases the pressure energy of the liquid and achieves the purpose of conveying the liquid. There are four points to pay attention to when starting the centrifugal pump:

①The lift generated by the centrifugal pump at a certain speed has a limited value. The operating point flow and shaft power depend on the conditions of the plant system (head difference, pressure difference and line losses) to which the pump is connected. Head varies with flow.

② Stable work, continuous delivery, no pulsation in flow and pressure.

③ Generally, there is no self-priming ability, and the pump needs to be filled with liquid or the pipeline can be evacuated before starting to work.

④The centrifugal pump is started when the valve of the discharge pipeline is closed, and the vortex pump and the axial flow pump are started when the valve is fully opened to reduce the starting power.

Because the centrifugal pump relies on the suction of the impeller to form a vacuum to lift the water, when the centrifugal pump is started, the gate valve must be closed first and water is filled. When the water level exceeds the impeller, the air in the centrifugal pump can be discharged before starting. After starting, a vacuum is formed around the impeller, which sucks the water upward, and its gate valve can be automatically opened to lift the water. Therefore, the gate valve must be closed first.

What is the content of vibration analysis of centrifugal pump?

1. The rotor of the centrifugal pump is unbalanced and misaligned. This problem accounts for a large proportion of the vibration problems of centrifugal pumps, about 80%. Factors causing the imbalance of the rotor of the centrifugal pump: uneven material resistance, unqualified parts structure, resulting in the imbalance between the center line of the rotor mass and the center line of the rotating shaft, resulting in eccentricity. Correction of rotor imbalance of centrifugal pump can be divided into two categories. Static balance and dynamic balance: also known as single-sided balance and double-sided balance. The difference is: single-sided balancing is performed on one correction plane, while double-sided balancing is performed on two correction planes.

2. Installation reasons: the foundation bolts are loose, and the level of the adjustment is not adjusted properly. Before the centrifugal pump works, it is necessary to check whether the foundation bolts are loose and whether the installation of the centrifugal pump is level. These will also cause the centrifugal pump to vibrate during operation.

3. There is a foreign body in the centrifugal pump. Before the centrifugal pump works, check the inside of the lower pump. Due to long-term use, there may be some foreign objects such as weeds in the water.

4. Cavitation perforation inside the centrifugal pump due to long-term use.

5. There are unreasonable situations in the design of the centrifugal pump, such as the size of the parts. However, this is relatively rare. Before the centrifugal pump leaves the factory, it will be tested many times in the workshop to ensure the pass rate of the centrifugal pump.