What is net positive suction head?
NPSH is the absolute pressure above the liquid's vapor pressure at the pump suction flange, expressed as head. It is an energy check at that flange to see whether the liquid will vaporize at the lowest pressure point in the pump. It is not just the static height of liquid above the pump, as the name suggests.
There are two values to compare:
- NPSHA (available): what the system supplies. It is unique to each system and flow rate.
- NPSHR (required): what the pump needs, published by the manufacturer on the performance curve.
Most pumps draw from open sumps with the liquid level above the suction inlet. Atmospheric pressure on the surface plus the static head usually prevents cavitation, unless the suction piping is unusually long or complex or the liquid is hot. When that arrangement is not possible, suction conditions need a proper NPSH evaluation.
What is NPSH required?
NPSHR is a characteristic of the pump's inlet geometry, published by the manufacturer on its performance curves. It increases with speed and with flow, so read it at your actual operating point, not at the design point alone. Slurry pumps range from 1 to 12 m NPSHR, with 2 to 4 m typical.
How do you calculate NPSH available?
Add the absolute pressure head on the liquid surface and the static suction head, then subtract suction line friction losses and the vapor pressure head. Every term is in feet or meters of the liquid being pumped. Static head is positive for a flooded suction and negative for a suction lift.
NPSHA = Hatm + Hstatic − Hfriction − Hvapor
To convert pressure to head:
H (ft) = 2.31 × P (psi) / SG H (m) = 10.2 × P (bar) / SG
The inputs you need:
- Vapor pressure of the liquid at operating temperature
- Static elevation change between the liquid surface and the pump suction
- Flow rate
- Suction piping details, for friction losses
- Pressure on the supply surface: atmospheric if the tank is open, the absolute tank pressure if it is closed
Sign conventions: surface pressure is always absolute and positive, vapor pressure is always negative, suction friction losses are negative, and static suction head is positive for a flooded suction and negative for a lift.
What are the common suction arrangements?
There are four basic arrangements: a suction lift or a flooded suction, each from either an open tank at atmospheric pressure or a closed tank at some other pressure. The same NPSHA formula applies to all four. Only the surface pressure and the sign of the static head change from one to the next.
- Suction lift: the liquid surface is below the pump. Static head is negative, and the pump needs some means of priming.
- Flooded suction: the liquid surface is above the pump. Static head is positive.
What does a worked NPSHA example look like?
Take a flooded suction from an open sump at sea level, pumping dirty water at 20 °C. Atmospheric pressure supplies 10.35 m, the liquid level adds 3 m, friction takes away 2.4 m and vapor pressure takes away 0.24 m. The result is about 10.7 m of NPSH available.
| Term | Value |
|---|---|
| Surface pressure, pa = 1.013 bar | +10.35 m |
| Static suction head, Hs | +3.0 m |
| Suction line losses at design flow | −2.4 m |
| Vapor pressure, 0.023 bar at 20 °C | −0.24 m |
| NPSHA | 10.71 m |
How much NPSH margin do you need?
If NPSHA exceeds NPSHR, the pump will not cavitate. For a good installation, NPSHA should be 15 to 30 percent above NPSHR to allow for normal process variation, such as changes in level, temperature or flow. In the example above, a pump with an NPSHR of up to about 8 to 9 m at the operating point would meet that margin.
NPSHA ≥ 1.15 to 1.30 × NPSHR
How does specific gravity affect NPSH available?
It depends on whether the suction is flooded or a lift. For a flooded suction, ignore specific gravity and calculate NPSHA as if pumping water. For a suction lift, specific gravity must be included, because a heavier liquid uses up more of the available surface pressure just to rise to the pump.
Flooded suction
NPSH is expressed as head but is really based on pressure. Slurry NPSHR equals clear water NPSHR, so with a slurry, cavitation starts at a higher absolute pressure. Do not add the extra pressure from a specific gravity above 1.0 to NPSHA. That keeps the system safe when it runs on water or a dilute slurry.
Suction lift
A centrifugal pump lowers pressure at the impeller eye, so the only energy available to raise the liquid is the pressure difference between the supply surface and the pump inlet. With a specific gravity above 1.0, part of that energy goes to lifting the extra weight, which reduces NPSHA.
At sea level, the atmosphere supports a 10.35 m column of cold, pure water against a perfect vacuum. With a specific gravity of 1.5, that column drops to 6.9 m. The simplest way to account for this on a lift is to divide the surface pressure head by the specific gravity.
Suction lift comparison
Consider clear water at 30 °C, a 1 m lift, 0.5 m of suction friction losses and 1 bar surface pressure (about 10.2 m of water). Vapor pressure of water at 30 °C is about 0.042 bar (about 0.43 m), so NPSHA is roughly 8.3 m. With the same conditions and a specific gravity of 1.5, the surface pressure head drops to about 6.8 m and NPSHA falls to roughly 5 m. A pump with an NPSHR of 6 m would run without cavitation on water but cavitate on the heavier slurry.
Can you fix low NPSH after installation?
Usually only with difficulty and expense. Raising the tank, lowering the pump, enlarging suction piping or cooling the liquid all add NPSHA, but each is costly once the system is built. Careful system design and pump selection at the start is the best way to avoid cavitation.
PCT can check your suction conditions and select a pump with the right NPSHR. Request a quote or see our process pumps.