Field calculator
NPSH available calculator (pump cavitation)
A pump cavitates when the suction pressure drops to the liquid's vapor pressure, flashing it to vapor that collapses on the impeller, eroding metal and killing flow. The way to prevent it is to confirm the net positive suction head available (NPSHa) beats what the pump requires. NPSHa equals atmospheric pressure head plus static suction head minus vapor pressure head minus suction friction loss, all in feet of the liquid. Enter the four heads. Atmospheric head is about 33.9 feet for water at sea level and drops with altitude and temperature. Static suction head is positive when the liquid level sits above the pump (a flooded suction) and negative when the pump has to lift liquid from below. Vapor pressure head is small for cool water, roughly 0.6 feet at 60 degrees, and climbs steeply as the liquid gets hotter, which is why hot-water and condensate pumps cavitate so easily. Friction loss is the loss through the suction pipe, fittings, and any strainer at the operating flow. The available NPSH must exceed the pump's required NPSH from the manufacturer curve with a safety margin, commonly 2 to 5 feet or more. Confirm the required NPSH and the margin with the pump manufacturer and the engineer.
Result
NPSH available: NPSHa = atmospheric pressure head + static suction head - vapor pressure head - suction friction loss, all in feet of the liquid. Enter the four heads. Atmospheric head is about 33.9 ft for water at sea level (less at altitude and at higher temperature). Static suction head is positive when the liquid level is above the pump (a flooded suction) and NEGATIVE when the pump lifts liquid from below. Vapor pressure head is small for cool water (about 0.6 ft at 60 F) and rises steeply with temperature, which is why hot liquids cavitate easily. Friction loss is the loss through the suction pipe, fittings, and strainer at the flow rate. The available NPSH must exceed the pump NPSH required from the manufacturer curve, with a safety margin (commonly 2 to 5 ft or more), or the pump cavitates, eroding the impeller and losing flow. Confirm the required NPSH and the margin with the pump manufacturer and the engineer.
anvilfield.com/calculators/npsh-available-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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NPSH available FAQ
What is pump cavitation?
Pump cavitation is vapor bubbles forming and collapsing inside a running pump. It happens when the suction pressure falls below the water's vapor pressure, so the water flashes to vapor at the impeller and the bubbles then implode against the vanes, eroding them. It means the pump is not getting enough suction pressure.
What is NPSH?
NPSH is net positive suction head, the suction-side pressure margin that keeps water from flashing to vapor at the pump. NPSH available, what the system delivers, must exceed NPSH required, what the pump needs off its curve, with margin. Fall below it and the pump cavitates. NPSHr rises as the flow rises.
Why does my pump sound like gravel?
A pump that sounds like gravel or marbles is cavitating. The noise is thousands of vapor bubbles collapsing against the impeller every second, with vibration and falling flow alongside it. The cause is on the suction side: a clogged strainer, a throttled valve, low fill pressure, or water too hot for the margin. Check the suction first.
How do you fix a cavitating pump?
Raise the suction pressure above the water's vapor pressure by fixing the suction-side cause. Clean the strainer, open the suction valve, restore the fill pressure, pipe the pump away from the expansion tank, lower the water temperature where you can, and shorten or upsize the suction line. Fix the cause before replacing any pitted impeller.
How do you calculate NPSH available?
NPSHa equals the absolute pressure at the suction, plus the static height of water above the pump, minus the suction friction loss, minus the water's vapor pressure at its temperature, all in feet. In the field, read suction pressure and water temperature, convert to absolute, subtract the vapor pressure, and compare the result to the pump's NPSHr.