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What is the pump cavitation

Author : Xiong Date : 9/4/2012 7:37:57 AM
What is the pump cavitation

First, cavitation phenomenon

Liquid in a certain temperature, reduce the pressure to the temperature of evaporation pressure, liquid and produce steam bubble. The produce bubble phenomenon called cavitation. Cavitation is generated when bubbles, flow to the high place, its volume reduced so that the burst. This due to the pressure rise bubble disappear in the liquid phenomenon called cavitation crush.

Pump in motion, if its flow part of the local area (usually impeller inlet later somewhere) for some reason, pumping liquid absolute pressure reduced to at that time under the temperature of liquid pressure, liquid was it began to evaporation, produce a large number of steam, form bubbles, when contains a lot of bubble liquid by impeller forward within the high, bubble around the high pressure liquid the bubble sharply reduced and fracture. In the bubble condensation broken at the same time, the liquid particle at a high speed filling hole, in this moment produce very strong water hammer, and at a high impact frequency strike metal surface, impact stress can reach hundreds to thousands of atmospheric pressure, impact frequency can reach tens of times per second, when serious will wall thickness breakdown.
In the water pump to have a bubble and bubbles make flow components suffered damage process is the water pump cavitation process. Water pump cavitation in addition to produce after about flow components will be damage effect beyond, still can produce noise and vibration, and lead to pump performance decline, serious worsened pump the liquid interrupt, can't work normally.

Second, the pump cavitation basic relation

Happened pump cavitation condition is by the pump itself and suction device two aspects of the decision. Therefore, research the condition of cavitation happened, we should, according to the pump itself and suction device both sides to consider, pump cavitation for the basic relation
NPSHc acuities NPSHr acuities [NPSH] NPSHa or less
NPSHa = NPSHr (NPSHc) - start pump cavitation
NPSHa NPSHa > NPSHr (NPSHc) - pump no cavitation
The type NPSHa - device NPSH and that effective NPSH, the bigger the not easy cavitation;
NPSHr - pump NPSH, also called the necessary NPSH or pump inlet dynamic pressure drop, the small resistance of cavitation performance is good;
NPSHc - critical cavitation allowance, it is to point to the corresponding drop in pump performance must value the cavitation allowance;
[NPSH] - allowable NPSH, is to determine the pump use condition of NPSH, usually take [NPSH] = (1.1 ~ 1.5) NPSHc.

Third, device NPSH calculation

NPSHa = Ps/ρ g + Vs / 2 g - Pc/ρ g = Pc/ρ g ± hg - hc - Ps/ρ g

Fouth, prevent the occurrence of cavitation measures

To prevent the occurrence of cavitation must improve NPSHa, make NPSHa > NPSHr can prevent the occurrence of cavitation measures are as follows:
1. Reduce suction loss hc, therefore can try to increase the pipe diameter, minimize line length, elbow and accessories, etc.;
2. Minimize geometrical height on absorption hg (or increase geometric flow backward height);
3. In the same speed and flow, the double suction pump, for reducing import velocity, which seldom happened pump cavitation;
4. to prevent the long hours in the large flow run;
5. Pump cavitation occurred, should put flow down or drop speed operation;
6. Pump suction pool on pump cavitation has important influence;
7. In the ambient conditions operation of the pump, to avoid cavitation damage, can use resistant to cavitation material.

The above: Pressure switch's structure and working principle