What is the Net Positive Suction Head for Skid Mounted Pumps?

Net positive suction head (NPSH) is the difference between a pump’s inlet pressure and the lowest pressure generated inside it, particularly the eye or the center. Technically, it refers to the Absolute pressure head as decreased by the liquid’s vapour. If the inlet pressure in the equipment is too low, NPSH causes the lowest pressure inside a skid-mounted pump to reduce below the vapour pressure of the pumped fluid. Consequently, cavitation occurs in the system, creating noise and causing breakdowns.

Calculating NPSH for a Skid-Mounted Pump

Determining the Value of NPSH

NPSH = P_s – P_vap

Here,

NPSH = From the system at the inlet when the pump is running

P_vap = Vapour pressure of the liquid at the inlet temperature

P_s = Suction pressure at the pump’s inlet when it is running

Vapour pressure is stated in absolute terms. Thus, suction pressure should also be expressed in absolute terms or kPaa or psia. To convert Gauge pressure to absolute pressure, you have to add atmospheric pressure to it.

This can be represented in the following equation:

Absolute Pressure = Gauge Pressure + Atmospheric Pressure

This equation helps to determine the answer in units of pressure (psi). It can be converted into units of the head (feet) using the following:

h = 2.31p/SG

Determining the Value of NPSHa

Net positive suction head available refers to the amount of head pressure available from the system. It is decided by:

It can be represented in the following equation:

NPSHa = P_z + P_sg + P_atm + P_vel – P_vap

Here,

NPSHa = Net Positive Suction Head available to the skid-mounted pump (psi)

P_z = Elevation or height of the gauge above the pump’s center in pressure units or psi

P_sg = Gauge pressure as determined at the suction nozzle (psig)

P_atm = Atmospheric pressure (psia)

P_vel = Velocity head in pressure units or psi

P_vap = Vapor pressure of the pump as noted at the pump suction nozzle (psia)

Determining NPSHa When the System is Not Built

If you do not have the system already built, you may calculate the NPSHa by beginning with the operating pressure in the vessel or suction tank.

This can be represented in the following equation:

NPSHa = P_t + P_atm + P_zt – P_f – P_vap

Where:

P_t = Tank pressure (psig)

P_zt = Elevation of fluid in the suction tank (As converted into pressure units or psi)

P_f = Friction losses in the suction line and at tank exit (As converted into pressure units or psi)

Avoiding Skid Pump Cavitation

Plugged inlet strainers or partially closed valves are often the cause of restricting the flow to a pump. The pump manufacturer provides the net positive suction head required (NPSHR) for all pumps (pump curve). NPSHR refers to the lowest inlet pressure in the pump required at a given flow to avoid cavitation. It is recommended to keep the NPSHa higher than the NPSHr to avoid pump cavitation.

We Have Expert Water Skid Pump Designers

Aspire’s designers understand the significance of an accurately designed piping system upstream of a pump to its operation. This is why we pay special attention to the pump inlet configuration on all process skids with a pump. For more information about our skid mounted pumps, produced water injection pumps, or dehydration units, please call us at 403-314-5422 or toll-free at 1-800-993-9958.

You might also want to check our related post:

Positive Displacement & Centrifugal Pumps