What does it mean NPSH

NPSH on the main pumping capacity is a measure of a parameter. We all know that a standard atmospheric pressure equivalent to 10m water column, meaning that if the pump into a deep pool sub above the surface and the atmosphere is the same, then let out the water discharge pump, the pump is the most likely below the surface down to the axis of the vertical distance of 10m with the pump where If the pump continue to run, when the water can not be dropped. Can not continue to pump out water to, the discharge will be the gas, this state, we call it cavitation. But in fact the pump is not completely perpendicular to the axis of 10m below the surface down to its distance, how many of the remaining part. The rest of this part of the water is also calculated in units of m, then, is this pump cavitation margin, also called the pump NPSH NPSHr, usually this value is 20 ℃ water pump works to pump rated at flow rate measured, the unit is meters.
NPSHr suction pump on the smaller shows better performance.
But in the real conditions, the pumps are not all placed vertically on the surface, the resistance of the pump inlet pipe entrance is usually due to friction, inlet elbows, valves, caused by the resistance, not by the pump suction pipe liquid caused by the vertical gravity, from a device other than the pump system determined. The device NPSH NPSHa, also known as effective or available NPSH NPSH, the unit is meters. That its value is set, that is, determine the pipe device, the NPSHa also determined.
Well, since the device NPSH NPSHa determine how to ensure pumps work, cavitation does not occur? It necessary to pump NPSH NPSH NPSHr and devices have a safety margin between NPSHa S, which satisfy the NPSHa-NPSHr ≥ S.
Centrifugal pumps for general, S is usually taken 0.6 ~ 1.0m.