What are the basic working principles of centrifugal pumps?

1. centrifugal pump

1 The working principle of centrifugal pump

There are many types of centrifugal pumps, but the working principle is the same, and the structure is similar. The main working parts are the rotating impeller and the fixed pump casing (Figure 2-1). Impeller is a centrifugal pump to do work directly on the liquid components, which has a number of backward curved blades, generally 4 to 8 pieces. When the centrifugal pump works, the impeller is driven by the motor for high-speed rotation (1000 ~ 3000r/min), forcing the liquid between the blades to rotate. At the same time, due to the centrifugal force, the liquid from the center of the impeller to the outer edge of the radial movement. The liquid obtains energy during the movement of the impeller and leaves the outer edge of the impeller at high speed into the volute pump housing. In the volute, due to the gradual expansion of the flow channel and deceleration, but also part of the kinetic energy into static pressure energy, to reach a higher pressure, then along the tangential flow into the pressure pipe.

While the liquid is forced from the center of the impeller to the outer edge, a vacuum is created at the center of the impeller. One end of the suction pipeline of the pump is connected to the center of the impeller, and the other end is immersed in the transported liquid. Under the action of the pressure difference between the liquid level pressure (usually atmospheric pressure) and the pressure in the pump (negative pressure), the liquid enters the pump through the suction pipeline. As long as the impeller rotates, the centrifugal pump will continuously suck in and discharge the liquid. It can be seen that the centrifugal pump mainly relies on the centrifugal force generated by the high-speed rotating impeller to transport the liquid, hence the name of the centrifugal pump.

If the centrifugal pump is not filled with liquid before starting, there is air in the pump. Because the air density is very small, the centrifugal force generated is also very small. The vacuum formed at the suction port is not enough to suck the liquid into the pump. Although the centrifugal pump is started, it cannot transport the liquid. This phenomenon is called "gas binding". Therefore, the centrifugal pump must be filled with liquid before starting the shell, and a bottom valve with a filter screen is installed at the bottom of the suction pipe. The bottom valve is a check valve to prevent the liquid from leaking from the pump before starting. The filter screen prevents solid matter from entering the pump. The pressure-out pipeline near the pump outlet is equipped with a regulating valve for regulating the flow rate.

1-impeller; 2-pump casing; 3-pump shaft; 4-suction pipe;

5-bottom valve; 6-pressure-out pipe; 7-outlet valve

2 Structure of centrifugal pump

There are many kinds of centrifugal pump, we commonly used irrigation area single-stage single suction centrifugal pump, single-stage double suction centrifugal pump, vertical single-stage centrifugal pump, vertical multistage centrifugal pump three.

single-stage single-suction centrifugal pump

1. Pump body 2. Impeller 3. Pump cover 4. Mechanical seal 5. Suspension part 6. Pump shaft

IS type horizontal centrifugal pump is mainly composed of pump body, pump cover, impeller, shaft, seal ring, shaft sleeve and suspension bearing. IS type of pump body and pump cover part, is split from the back of the impeller, which is commonly referred to as the rear door structure. Its advantage is that it is convenient for maintenance. The pump body, suction pipeline, discharge pipeline and motor are not moved during maintenance. It only needs to remove the intermediate coupling of the extension coupling to exit the rotor part for maintenance.

The housing of the pump (I. e. the pump body and the pump cover) constitutes the working chamber of the pump. The impeller, shaft and rolling bearing are the rotor of the pump. The suspension bearing part supports the rotor part of the pump, and the rolling shaft bears the radial and axial forces of the pump. In order to balance the axial force of the pump, most of the pump impeller before and after are equipped with a sealing ring, and in the impeller back cover plate with a balance hole, because some pump axial force is not large, the impeller back without a sealing ring and balance hole.

The axial seal ring of the pump is composed of packing gland, packing ring and packing to prevent air intake or a large amount of water leakage. If the impeller of the pump is balanced, the cavity equipped with soft packing is connected with the suction port of the impeller. If the liquid at the inlet of the impeller is in a vacuum state, it is easy to enter the air along the surface of the shaft sleeve. Therefore, the packing cavity is equipped with a packing ring. Through the small hole on the pump cover, the pressure water in the pump chamber is led to the packing ring for sealing. If the impeller of the pump does not have a balance hole, the liquid pressure on the back of the impeller is greater than atmospheric pressure, so there is no air leakage problem, so the packing ring may not be installed. In order to avoid shaft wear, the shaft through the packing cavity is equipped with shaft sleeve protection. O-ring between the shaft sleeve and the shaft to prevent air or water leakage along the mating surface.

The transmission mode of the pump is connected with the motor by extending the elastic coupling. The rotation direction of the pump is clockwise from the driving end.

single-stage double-suction centrifugal pump

1. Pump body 2, pump cover 3, impeller 4, shaft 5, double suction sealing ring 6, shaft sleeve 7, shaft coupler

8. Bearing body 9. Sealing parts

The suction inlet and discharge outlet of S, SH single-stage double-suction pump are both below the axis of the pump, and the horizontal direction is vertical to the axis. In the middle, there is no need to disassemble and feed water during maintenance. When the discharge pipeline and motor (or other prime mover) are viewed from the coupling to the pump, the water pump rotates in the opposite direction of the inch needle. Such as according to the user's special order needs can also be changed to the inch needle rotation.

The main parts of S, SH single-stage double-suction pump are: pump body, pump cover, impeller, shaft, double-suction sealing ring, shaft sleeve, bearing, etc. In addition to the shaft material for high-quality carbon steel, the rest are made of cast iron.

The pump body and the pump cover form the working chamber of the impeller. The inlet and outlet flanges are provided with pipe screw holes for installing vacuum gauge and pressure gauge, and the lower part of the inlet and outlet flanges is provided with pipe screw holes for discharging water.

After static balance verification, the impeller is fixed with a shaft sleeve and a shaft sleeve nut on both sides. Its axial position can be adjusted by the shaft sleeve nut. The axial force of the impeller is balanced by the symmetrical arrangement of its blades, and there may be some remaining The axial force is borne by the bearing at the coaxial end.

The S-type pump is supported by rolling bearings for bearings with a shaft diameter less than 60mm, which is called a-type SH-type pump; the bearing is larger than 70mm and is supported by babbit alloy sliding bearings, which is called B- type SH-type pump. Many pumps larger than 70mm now also use rolling bearings. Type A pump is supported by two single-row radial ball bearings. The bearings are installed in the bearing bodies at both ends of the pump body and lubricated with butter. The double-suction sealing ring is used to reduce the water leakage from the water pump pressure water chamber back to the suction chamber.

The water pump is directly driven by the motor through the coupling.

The shaft seal is a soft packing seal. In order to cool and lubricate the sealing cavity and prevent air from leaking into the pump, there is a water seal ring between the fillers. When the pump is working, a small amount of high-pressure water flows into the filling cavity through the water seal pipe to act as a water seal.