1. NPSH NPSH pump suction inlet per unit mass of liquid excess liquid vaporization pressure surplus energy (in meters of liquid column), known as the NPSH, the value equal to the pump inlet from the reference surface of the total suction head ( Absolute pressure in meters of liquid) minus the vaporization pressure of the liquid (absolute pressure in meters of liquid). The reference plane is fixed according to the following two principles: ①IS0 standard and GB standard The reference plane is the horizontal plane of the center of the circle drawn through the outer end of the impeller blade inlet side. For multistage pumps, use the first stage impeller as a reference and the vertical double suction pump as a reference for the upper section (see Figure 2-89). ② APl 610 provides for horizontal pumps, the reference plane is the pump shaft center line; vertical pipeline pump, the reference plane is the pump suction inlet centerline; for other vertical pumps, the reference plane is based on the top surface. 2. NPSH NPSH. The NPSH, as determined by the pump system (at rated flow rate and normal pumping temperature), is the NPSH, also known as the effective NPSH or available NPSH (in meters Liquid column meter) whose size is determined by the parameters of the pipette system and the flow in the pipe, irrespective of the structure of the pump. 3. Pump NPSH, the NPSH, as determined by the pump plant based on the test (usually at 20 ° C water at nominal flow rate) and the pump's required NPSH (expressed in meters of liquid column). For centrifugal pump, the pump plant gives NPSH, the value generally left 0.3m margin; for the metering pump, reciprocating pump, NN with the same population valve and local flow is very complicated, the pump plant is generally only to test the flow rate of the inlet pressure to change Determine the NPSH, value, does not consider the partial flow within the valve, in order to ensure that no local cavitation occurs, the pump is usually given NPSH, leaving a value of 0.2bar margin. It should be emphasized that the margin mentioned here is the margin left by the pump factory to calculate NPSH. Safety margin S generally should not be taken into account. Necessary NPSH measured at suction flange and converted to datum. In comparing the NPSHa and NPSHr values ​​should pay attention to whether the datum is consistent, such as inconsistencies should be converted to the same datum. 4. Suction on the degree of vacuum H. Suction on the degree of vacuum H. Pump suction from the pump reference plane from the vacuum of El (in meters of liquid column), also known as vacuum on the suction height. The domestic old-fashioned samples to pump the vacuum reflects the cavitation performance of the pump, now the indicator has been eliminated. Hs and NPSHr value conversion according to the following formula: HPSHr = l0-Hs (2-35) 5. Pump installation height h Pump installation height h, also known as pump suction height, refers to the level difference between the pump reference surface and the suction liquid level. 6. Cavitation curves NPSHa and NPSHr are changes with the flow rate changes. The general NPSH increases with the increase of traffic, while NPSHa decreases with the increase of traffic (see Figure 2-90). The pump performance curve provided by the pump plant should generally have NPSH, a Q curve. 7. Centrifugal pump NPSHa safety margin S To ensure that cavitation does not occur, the centrifugal pump NPSH. There must be a safety margin S that satisfies NPSHa - NPSHr ≧ S. For the general centrifugal pump, S take 0.6 ~ 1.0m. However, for some special purposes or conditions of the centrifugal pump, S value to be determined according to Table 2-19.
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