The cavitation allowance is a very important basic parameter of cavitation for pump design, test and use. When designing the pump, design the pump according to the requirements of cavitation performance. If the user gives specific conditions of use, the cavitation allowance NPSHr of the design pump must be smaller than the device's cavitation allowance NPSHa determined according to the use conditions. To improve the cavitation performance of the pump, the NPSHr should be minimized. During pump test, NPSHr is verified by cavitation test. This is the only reliable method to determine NPSHr. It can verify whether the pump reaches the designed NPSHr value. [NPSH], as the basis for the user to determine the geometric installation height. Obviously, it is very important to correctly understand and determine the cavitation margin.
In order to deeply understand the concept of cavitation, the following types of cavitation allowances should be distinguished.
1) NPSHa——Apparatus cavitation margin, also known as effective cavitation margin. It is provided by a suction device. The larger the NPSHa, the less likely the pump is to cause cavitation;
2) NPSHr——Pump cavitation margin, also called the necessary cavitation margin. It is a parameter that specifies the cavitation performance of the pump. The smaller the NPSHr, the better the cavitation resistance of the pump;
3) NPSHc-Critical cavitation allowance, which is a value determined by cavitation tests. There are different cavitation allowances for different inlet pressures in a cavitation test, but test cavitation allowances corresponding to a decrease in pump performance. There is only one, called the critical cavitation margin, which is expressed by NPSHc;
4) [NPSH] —— Allowable cavitation allowance. This is the cavitation margin used to determine the pump's operating conditions (such as installation height). It should be greater than the critical cavitation margin to ensure that cavitation does not occur when the pump is running. Usually take [NPSH] = (1.1 ~ 1.5) NPSHc or [NPSH] = NPSHc + K, K is the safe value.
These cavitation margins have the following relationship:
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