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 = Ps – Pvap
Here,
NPSH = From the system at the inlet when the pump is running
Pvap = Vapour pressure of the liquid at the inlet temperature
Ps = 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:
- Determining the suction pressure at the pump’s suction nozzle
- Correcting the datum
- Combining atmospheric pressure to it
- Adding velocity head
- Deducting the vapour pressure
It can be represented in the following equation:
NPSHa = Pz + Psg + Patm + Pvel – Pvap
Here,
NPSHa = Net Positive Suction Head available to the skid-mounted pump (psi)
Pz = Elevation or height of the gauge above the pump’s center in pressure units or psi
Psg = Gauge pressure as determined at the suction nozzle (psig)
Patm = Atmospheric pressure (psia)
Pvel = Velocity head in pressure units or psi
Pvap = 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 = Pt + Patm + Pzt – Pf – Pvap
Where:
Pt = Tank pressure (psig)
Pzt = Elevation of fluid in the suction tank (As converted into pressure units or psi)
Pf = 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.
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