Cavitation and selection of pumps

First, the pump type principle

1. Definition of pump: The machine that usually raises liquid, transports liquid or increases the pressure of the liquid, that is, the machine that converts the mechanical energy of the prime mover into liquid energy to achieve the purpose of pumping liquid is collectively referred to as a pump.

2, the working principle of the pump:

(1) Positive displacement pump ----Use the volume change of the working chamber to transport liquid.

(2) Vane pump ----using blade and liquid interaction to transport liquid

3, the specific use of the pump: the different uses of the pump, different transport liquid medium, different flow rates, the range of different lifts, the structure of the pump is of course different, the materials are also different, summed up, can be roughly divided into:

(1) Urban water supply ( 2) Sewage system (3) Civil engineering, building system (4) Agricultural water conservancy system (5) Power station system (6) Chemical system (7) Petroleum industry system (8) Mining metallurgical system (9) Light industrial system (10) Ship system

Second, cavitation

When the liquid is at a certain temperature and the pressure is reduced to the vaporization pressure at the temperature, the liquid generates bubbles. This phenomenon of generating bubbles is called cavitation. When the bubble generated by cavitation flows to a high pressure, its volume is reduced to be shattered. This phenomenon in which the bubble disappears in the liquid due to the pressure rise is called cavitation collapse.

When the pump is in operation, if a partial area of ​​the overcurrent portion (usually somewhere later in the impeller blade inlet), for some reason, the absolute pressure of the pumped liquid is reduced to the liquid vaporization pressure at that temperature, the liquid is here. At the vaporization, a large amount of vapor is generated to form bubbles. When the liquid containing a large amount of bubbles passes forward through the high pressure region in the impeller, the high pressure liquid around the bubbles causes the bubbles to sharply shrink to cause cracking. At the same time that the bubble condenses and ruptures, the liquid particle fills the cavity at a high speed, which generates a strong water hammer at this moment, and strikes the metal surface with a high punching frequency, and the impact stress can reach several hundred to several thousand. Atmospheric pressure, in severe cases, will break the wall thickness.

The process of generating bubbles in the water pump and rupturing the air bubbles to damage the overcurrent portion is the cavitation process in the water pump. In addition to the damaging effect on the overcurrent components, the pump generates vibration and noise, and causes the original performance of the pump to drop. In severe cases, the liquid in the pump is interrupted and cannot work normally.

Third, the basic relationship of pump cavitation

The conditions under which the pump cavitation are determined by both the pump itself and the inhalation device. Therefore, the study of the conditions of cavitation should be considered from both the pump itself and the inhalation device. The basic relationship of the pump is:

NPSHc≤ NPSHr≤[NPSH]≤NPSHa

NPSHa=NPSH(NPSHc)-------- pump starts cavitation

NPSHa> NPSHr (NPSHc) ------ pump without cavitation

In the formula, the NPSH---device NPSH is also called the effective NPSH.

NPSHr---- pump NPSH, also known as NPSH or pump inlet pressure drop, the smaller the cavitation resistance, the better;

NPSHc----- critical NPSH is the NPSH corresponding to a certain value of pump performance degradation;

[NPSH]-----The allowable NPSH is to determine the NPSH of the pump. Generally, take [NPSH]=(1.1~1.5) NPSHc.

Fourth, to prevent cavitation measures

To prevent cavitation from occurring, it is necessary to increase NPSHa and make NPSHa>NPSHr. The measures to prevent cavitation are as follows:

1. Reduce the geometric suction height hg (or increase the geometric backflow height);

2, reduce the inhalation loss hc, for this purpose, you can try to increase the pipe diameter, try to reduce the length of the pipe, elbows and accessories;

3. Prevent long-term work under large flow;

4. Under the same speed and flow rate, the double suction pump is adopted, and the pump is not easy to cavitation due to the reduction of the inlet flow rate;

5. When the pump cavitation occurs, the flow should be reduced as much as possible;

6. The situation of the pump suction basin has an important influence on the pump cavitation;

7. For pumps operating under severe conditions, to avoid cavitation damage, cavitation resistant materials may be used.

Fifth, the pump selection

When designing equipment, the design institute should determine the purpose and performance of the pump and select the pump model. This choice begins with choosing the type and form of the pump. So what is the principle to choose the pump? What is the basis?

1, pump selection principle

(1) Make the form and performance of the selected pump meet the requirements of process flow parameters such as device flow, head, pressure, temperature, NPSH, and suction stroke.

(2) The requirements of the dielectric characteristics must be met. For pumps that transport inflammable, explosive, toxic or valuable media, shaft seals are required to be reliable or leak-free pumps, such as magnetically driven pumps, diaphragm pumps, and shielded pumps. For pumps that transport corrosive media, corrosion-resistant materials such as AFB stainless steel corrosion -resistant pumps and CQF engineering plastic magnetic drive pumps are required for overcurrent components . For pumps containing solid particulate media, it is required to use wear-resistant materials for the flow-through components, and if necessary, the shaft seals are rinsed with a clean liquid.

(3) High mechanical reliability, low noise and low vibration.

(4) The total cost of equipment, operating, maintenance and management fees should be considered economically.

(5) Centrifugal pump has the characteristics of high speed, small volume, light weight, high efficiency, large flow, simple structure, no pulsation of liquid, stable performance, easy operation and convenient maintenance. Therefore, in addition to the following conditions, the centrifugal pump should be selected as much as possible: when there is a metering requirement, the metering pump is selected; the head is very high, the flow rate is very small, and there is no suitable small-flow high-lift centrifugal pump. If optional, a repetitive pump can be used; When the requirements are not high, the vortex pump can also be selected; when the head is very low, the axial flow pump and the mixed flow pump can be selected when the flow rate is large; when the medium viscosity is large (more than 650~1000mm ² /s), the rotor pump or repeat can be considered. Pump (gear pump, screw pump); medium gas content of 75%, flow rate is small and viscosity is less than 37.4mm ² / s, vortex pump can be selected; for frequent start-up or inconvenient pumping, it should be self-priming Pumps such as self-priming centrifugal pumps, self-priming vortex pumps, pneumatic (electric) diaphragm pumps.