Cavitation occurs in a fluid when the absolute pressure drops below the vapor pressure of the fluid, resulting in cavitation (formation and collapse of small vapor pockets). The most common place where this occurs in water and wastewater systems is at the suction side of a pump. The head needed to prevent cavitation is called the Net Positive Suction Head required (NPSHr). This value is compared with the Net Positive Suction Head Available (NPSHa) at the pump. If NPSHa > NPSHr, then cavitation damage is unlikely to occur.
The software calculates the NPSHa and can compare it with the NPSHr which the user obtains from the pump specifications. Available NPSH is calculated at every time step for every pump using the equation below:
NPSHa = Hb + HGL - Elev - Hvap - Loss
Where: Hb = barometric pressure, ft
HGL = hydraulic grade at pump suction, ft
Elev = elevation of pump impeller
Hvap = vapor pressure (absolute) of water at temperature, ft
Loss = head loss between inlet of pump and impeller (Usually negligible), ft
Barometric pressure and vapor pressure can be specified in the calculation options.
Starting with CONNECT Edition Update 1, a NPSH Required field is available in the Pump Definition for a pump. To include NPSH analysis in the calculation, check the box beside “Define NPSH Required Curve for pump cavitation check?” You can then include flow and head values that represent the required head and flow to satisfy this.