NPSH Available (NPSHa) Calculator: Will Your Pump Cavitate?
- Sullair dealer since 1984
- Serving industry since 1978
- No form fill, nothing gated
What this calculator tells you
Every pump needs enough suction head to keep the liquid from flashing to vapor at the impeller. Check what your installation actually provides, then compare it against the pump curve.
Results
NPSH available:
0 feet
Enter your pump's NPSH required to check the cavitation margin.
Cavitation sounds like pumping gravel and eats impellers. If your margin is thin, call the Pump Team at 920-393-4958 ext 2401 before it becomes a rebuild.
How this is calculated
- Atmospheric pressure from elevation: Pa = 14.696 x (1 - 6.8754e-6 x elevation)^5.2559 PSIA.
- NPSHa (feet) = (Pa + tank PSIG - vapor pressure) x 2.31 / specific gravity + static head - friction loss. Static head is positive for a flooded suction and negative for a suction lift.
- Vapor pressures used: water 0.256 PSIA at 60°F, 2.89 PSIA at 140°F, 7.51 PSIA at 180°F. Custom liquids use your entered value.
- Margin = NPSHa - NPSHr. A common rule is at least 3 feet of margin, verified at maximum flow and maximum liquid temperature.
Estimates only. Actual results depend on your equipment and operating conditions. Call 800.825.3222 for exact sizing.
Common questions
What is NPSH available and how is it calculated?
NPSH available is the total suction head pushing liquid into the pump above the liquid's vapor pressure, in feet. NPSHa = (atmospheric pressure + tank gauge pressure - vapor pressure) x 2.31 / specific gravity, plus static head if the liquid sits above the pump or minus it for a suction lift, minus suction line friction loss. It must exceed the pump's NPSH required, which comes from the manufacturer's curve.
How much NPSH margin does a pump need?
A common rule is at least 3 feet of margin, or 10 to 25 percent above NPSH required, whichever is greater, checked at maximum flow and maximum liquid temperature. Margin shrinks fast with hot liquids because vapor pressure climbs steeply with temperature, and at higher flows because both friction loss and NPSH required increase.
What does pump cavitation sound like and what does it damage?
Cavitation sounds like the pump is passing gravel or marbles. Vapor bubbles form in the low pressure zone at the impeller eye and collapse violently against the metal, pitting and eroding the impeller, dropping flow and head, and shaking bearings and seals loose. Left alone it turns into a rebuild; fixing the suction conditions is far cheaper.
Want a tech to verify these numbers at your facility?
Request received. If you need someone now, call 800.825.3222.
Please fill in all required fields (name, company, email, phone, preferred contact method, ZIP, and city), or call 800.825.3222.
These are estimates. Your pump curve has the real answer.
Our pump team checks flow, head, and NPSH against the manufacturer curve before anything is quoted. Since 1978, across 14 Midwest locations.