NET POSITIVE SUCTION HEAD

Net Positive Suction Head (NPSH)

• Power supplied to the pump depend on the difference in the pressure between discharge and suction and is independent of the pressure level.
• There is no effect on pump when suction pressure is below atmospheric pressure or well above it, as long as the fluid remains liquid.
• But if suction pressure is only slightly greater than the vapor pressure, some liquid may flash to vapor inside to pump, a process called cavitation, which greatly reduces the pump capacity.
• If the suction pressure is actually less than the vapor pressure, there will be vaporization in the suction line, and no liquid can be drawn into the pump.
• To avoid cavitation, the pressure at the pump inlet must exceed the vapor pressure by certain value called net positive suction head (NPSH)

Hence, Net Positive Suction Head or NPSH for pumps can be defined as the difference between liquid pressure at pump suction and liquid vapor pressure, expressed in terms of height of liquid column.

• Net positive suction head (NPSH) may refer to one of two quantities in the analysis of cavitation.

1. The Available NPSH (NPSHA): A measure of how close the fluid at a given point is to flashing, and so to cavitation.
2. The Required NPSH (NPSHR): The head value at a specific point (e.g. the inlet of a pump) required to keep the fluid from cavitating.

Net Positive Suction Head Required, NPSHr

• Pumps can pump only liquids, not vapors
• The satisfactory operation of a pump requires that vaporization of the liquid being pumped does not occur at any condition of operation.
• This is so desired because when a liquid vaporizes its volume increases very much.
• For example, 1 ft3 of water at room temperature becomes 1700 ft3 of vapor at the same temperature.
• This makes it clear that if we are to pump a fluid effectively, it must be kept always in the liquid form.
• Rise in temperature and fall in pressure induces vaporization
• The vaporization begins when the vapor pressure of the liquid at the operating temperature equals the external system pressure, which, in an open system is always equal to atmospheric pressure.
• Any decrease in external pressure or rise in operating temperature can induce vaporization and the pump stops pumping.
• Thus, the pump always needs to have a sufficient amount of suction head present to prevent this vaporization at the lowest pressure point in the pump.
• NPSH as a measure to prevent liquid vaporization
• NPSH required is a function of the pump design and is determined based on actual pump test by the vendor.
• NPSHr increases as capacity increases
• The NPSH required varies with speed and capacity within any particular pump.
• The NPSH required increase as the capacity is increasing because the velocity of the liquid is increasing, and as anytime the velocity of a liquid goes up, the pressure or head comes down.
• The NPSH is independent of the fluid density as are all head terms.

Net Positive Suction Head available, NPSHa

• 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.
• The formula for calculating the NPSHa is as below

where

hps= pressure head i. e. absolute pressure at surface of reservoir converted into head
hs= static suction head
hvps= vapor pressure head
hfs= friction head 

Significance of NPSHr and NPSHa

• The NPSH available must always be greater than the NPSH required for the pump to operate properly.