Explanation: To calculate NPSH Available, take the source pressure , add the atmospheric pressure , subtract the losses from friction within the pipeline and subtract the vapor pressure of the fluid . The result equals the NPSHA (or Net Positive Suction Head Available) of your system.

Net Positive Suction Head Available is calculated from the suction-side system configuration. It is essentially the suction-side pressure less the vapour pressure of the pumped fluid at that point. NPSH-A must exceed the pump’s NPSH-R rating for the chosen operating conditions to ensure that cavitation is avoided.

NPSH can be defined in two parts: NPSH Available (NPSHA): The absolute pressure at the suction port of the pump. NPSH Required (NPSHR): The minimum pressure required at the suction port of the pump to keep the pump from cavitating. NPSH or Net Positive Suction Head is a very important part of a pumping system.

NPSHA is a function of your system and must be calculated, whereas NPSHR is a function of the pump and must be provided by the pump manufacturer. NPSHA MUST be greater than NPSHR for the pump system to operate without cavitating.

You can improve the NPSHa by either increasing the elements that add energy to the liquid (Hs, Hp), or by reducing the elements (Hvp, Hf, Hi) that subtract energy from the liquid. Hs is suction static head. It is the elevation of the liquid in the suction vessel referenced above (or below) the pump centerline.

The pump head H=z+hw z is the height difference of the pumping height, that is, the water level from the inlet to the water surface at the exit. Hw is the head loss, including the Darcy formula or Xie Cai formula for calculating the head loss hf and the local head loss hw hf along the path.

NPSH (A) = (Pressure on the liquid surface) (ha) ± (suction head/lift) (friction loss) (vapour pressure) (hvp) NPSH (A) = 9.02 + 4-1.2 – 0.25 = 11.57 Mts.

Can You Provide Too Much NPSH? Unfortunately, the facts indicate that if the pump is operating near the 3 percent head‑drop NPSHA, an increase in NPSHA will result in a noisier pump with a higher rate of damage. Grist (3) reported that maximum cavitation‑erosion rate corresponded with maximum cavitation noise.

When the pump is running, the reading from this gauge will be equal to your NPSHA, less vapor pressure. If after subtracting vapor pressure this value is less than the pump’s NPSHR, you have confirmed that this is a cavitation problem.

Quick Formula

1. Water horsepower = minimum power required to run water pump.
2. TDH = Total Dynamic Head = Vertical distance liquid travels (in feet) + friction loss from pipe.
3. Q = flow rate of liquid in gallons per minute.
4. SG = specific gravity of liquid (this equals 1 if you are pumping water)
5. Water horsepower =

Net Positive Suction Head Available formula is defined as (P a-P v+P n)/ρ. Add the atmospheric pressure and gage pressure, then subtract the obtained value with vapor pressure.

Hydraulics. NPSH is the absolute suction head minus the vapor pressure of the liquid being pumped.

Suction head is the term used to describe liquid pressure at pump suction in terms of height of liquid column. When vapour pressure is also expressed in terms of equivalent height of liquid column, and subtracted from the suction head, the difference is npsh available at the pump suction.