Cavitation and NPSH centrifugal pumps: how to calculate and fix

Invasion occurs when the local pressure at the pump entrance door drops below the slightly saturated pressure of the fluid. The foam forms and breaks when it enters the higher pressure zone, generating small but repeated shock pulses thousands of times per second, causing noise such as stones, vibrations, reduced flow and worn wings.

NPSH Available and NPSH Required

System Availability NPSH (NPSHa) = (pressure at the air surface − slightly saturated pressure)/(ρ·g) ± high difference between the air surface and the pump center − suction pipe loss. NPSH requirement (NPSHr) provided by the manufacturer in the curve. The safety condition is that NPSHa is larger than NPSHr a sufficient amplitude, usually from 0.5 m or above or 10–30% depending on standard and application.

For example, fast

Open tank at 1 atm (about 10.3 mH2O), water 30 °C (damp pressure about 0.43 mH2O), water level lower than the pump center 2 m, suction pipe loss 0.7 m: NPSHa ≈ 10.3 − 0.43 − 2 − 0.7 ≈ 7.2 m. If the pump needs NPSHr 6 m, there is about 1.2 m left.

Signs to recognize.

• Noise like stones or whistling in the pump.
• The flow and pressure column decreases even if the suction valve is fully opened.
• Spots like bees nest on the wings when removing the pump.

How to handle.

Lower the pump setting or lift the suction ink, increase the diameter of the suction tube and remove the shrinkage, valve to reduce losses, reduce the temperature of the liquid, reduce the speed of the pump, choose a pump with lower NPSHr or add a pressure pump at the suction end. Absolutely do not use the suction valve to save flow.

Invasion also occurs in water-free foot pumps.

Non-separate centrifugal pump: in the water-ringed vacuum pump, when the suction pressure drops tightly down to the saturated vapor pressure of the ring liquid, the foam forms and breaks in the compression chamber, causing noise and wing corrosion.Other treatment: install additional gas-grade valve (bleed) on the suction path to keep the pump outside the invasive area, lower the water temperature of the sealing and do not place the vacuum target deeper than the pump’s capabilities.

Select a Kenflo centrifugal pump according to NPSH

Each model inThe Kenflo pump.There is a NPSHr curve in the technical profile. With a system with hot water or low suction ink, choose a standing pump in the submersible tank, a pump with two-sided suction doors such as KPS, KHP, or put a pressure pump in front. Two-door suction pumps usually have NPSHr lower than one-door suction pumps with the same flow. We are ready to help test NPSH when youContact us .and .

Quick checklist before running the pump

• The vacuum valve is completely open, the filter net is clean.
• The suction pipe slopes up towards the pump, without air bags.
• Liquid intensity is higher than the minimum.
• Exhausted air in the pump.

Frequently asked questions.

How big should NPSHa be than NPSHr?

Usually a minimum of 0.5 m or 10–30% depending on the standard and application; near-boiling medium or high-power pump should choose a wider edge.

Does decreased speed reduce NPSHr?

The NPSHr ratio is close to the equal speed, so deceleration is an effective way to reduce the risk of aggression.