NPSH of pump: When the liquid passes from the pump section to the eye of the impeller, the velocity increases and the pressure decreases. There are also pressure losses due to shock and turbulence as the liquid strikes the impeller.
The centrifugal force of the impeller vanes further increases the velocity and decreases the pressure of the liquid. Thus, the Net positive suction Head required (NPSHr) or sometimes in short as NPSH is the total head at the pump section to overcome these pressure drops in the pump and maintain the majority of the liquid above its vapor pressure.
The term NET refers to the actual pressure head at the pump section flange and not the static section head. NPSH required is a function of the pump design and is determined based on actual pump test by vendor.
NPSH of pump
NPSHa Net positive suction head available
Net Positive Suction Head Available is a function of the system in which the pump operates. It is the excess pressure of the liquid in feet absolute over its vapor pressure as it arrives at the pump suction, to be sure that the pump selected does not cavitate. It is calculated based on system or process conditions.
NPSHa calculation is stated below:
NPSHaS = hps + hs – hvps – hfs
Hps = Pressure Head i.e. Barometric pressure of the suction vessel converted to head.
hs = Static Suction Head i.e. the vertical distance between the eye of first stage impeller centerline and suction liquid level.
hvps = Vapor pressure Head i.e. vapor pressure of liquid at its maximum pumping temperature converted to Head.
hfs = Frication Head i.e. friction and entrance pressure losses on suction side converted to Head.
- It is important to correct for the specific gravity of the liquid and to convert all terms to units of “feet absolute” in using the formula.
- Any discussion of pump NPSH or cavitation is only concerned about the suction side of the pump. There is almost always plenty of pressure on the discharge side of the pump to prevent the fluid from vaporizing.
NPSHa in a nutshell
NPSHa = Pressure head + Static head – Vapor pressure head of the product – Friction head loss in the piping, valves and fittings. “All terms in feet absolute”
In an existing system, the NPSHa can also be approximated by a gauge on the pump suction using the formula:
NPSHa = hpS – hvpS ? hgS + hvS
hpS = Barometric pressure in feet absolute.
hvpS = Vapor pressure of the liquid at maximum pumping temperature, in feet absolute.
hgS = Gauge reading at the pump suction expressed in feet (plus if above atmospheric, minus if below atmospheric) corrected to the pump centerline.
hvS = Velocity head in the suction pipe at the gauge connection, expressed in feet.
Importance of NPSHr and NPSHa
The NPSH of the pump available must always be greater than the NPSH required for the pump to operate properly. It is normal practice to have at least 2 to 3 feet of extra pump NPSH available at the suction flange to avoid any problems at the duty point.