Pumps don't actually suck fluid. They create a low-pressure zone at the inlet, and atmospheric pressure (plus any elevation head) pushes fluid in. If the inlet pressure drops below the fluid's vapor pressure, the liquid flashes to vapor inside the pump — the bubbles collapse violently on the impeller, and you get cavitation damage, noise, and lost flow. NPSH (Net Positive Suction Head) is the bookkeeping engineers use to make sure this doesn't happen.

Two values matter:

• NPSH_available (NPSHa): a property of your system. How much pressure margin above vapor pressure the fluid actually has at the pump inlet.
• NPSH_required (NPSHr): a property of the pump. Published on the pump curve. The minimum margin the impeller needs to avoid cavitation at a given flow.

The rule: NPSHa > NPSHr, with a safety margin of typically 0.6 m (2 ft) or 10–20%, whichever is larger.

The NPSHa equation, all in head units (meters or feet of fluid):

NPSHa = H_atm + H_static − H_friction − H_vapor

• H_atm: atmospheric pressure as head (~10.3 m of water at sea level)
• H_static: elevation of fluid surface relative to pump centerline (negative if pump is above the source — a "suction lift")
• H_friction: pipe and fitting losses in the suction line
• H_vapor: vapor pressure of the fluid at operating temperature (~0.24 m for water at 20°C, but ~10.3 m at 100°C — boiling water cannot be lifted at all)

Worked example: pumping 60°C water from an open tank 3 m below the pump, with 1.5 m of friction loss in the suction line. Vapor pressure of water at 60°C is ~2.0 m.

NPSHa = 10.3 − 3.0 − 1.5 − 2.0 = 3.8 m

If the pump curve says NPSHr = 3.5 m at your flow, you have only 0.3 m of margin — below the typical safety buffer. Options: lower the pump, fatten the suction pipe (less friction), cool the fluid, or pick a pump with lower NPSHr.

Practical rules of thumb:

• Suction pipe should be one size larger than the pump inlet flange.
• Keep suction velocity under 1.5 m/s (5 ft/s) for cold water.
• Never put elbows, reducers, or valves right at the pump inlet — give it 5–10 pipe diameters of straight run.
• Hot fluids and high altitude both eat your margin fast.