English is currently available.
Chemical Engineering DISCUSSION
A centrifugal pump draws water from an open tank whose surface is 3 m below the pump centreline. I estimate 1 m of friction loss in the suction line, and the pump curve gives NPSH required as 4 m at the duty flow. It runs quietly on water at 25 °C.
The process will later run at 80 °C, and I have been warned the pump will cavitate. How is NPSH available calculated for both temperatures, and what can be changed if it turns out to be too low?
NPSH available is the absolute pressure head at the pump inlet in excess of the liquid's vapour pressure head. For a tank open to atmosphere:
NPSHa = (P_atm - P_vap) / (ρ × g) + z - h_f
where z is the height of the liquid surface above the pump (negative for a suction lift) and h_f is the suction line friction loss. At 25 °C water has P_vap = 3.2 kPa and ρ = 997 kg/m³, so the first term is (101.3 - 3.2) × 1000 / (997 × 9.81) = 10.0 m. Then NPSHa = 10.0 - 3 - 1 = 6.0 m, which is 2 m above the 4 m required.
At 80 °C the vapour pressure of water is 47.4 kPa and the density 972 kg/m³. The first term drops to (101.3 - 47.4) × 1000 / (972 × 9.81) = 5.65 m, and NPSHa = 5.65 - 3 - 1 = 1.65 m, far below the 4 m required. The warning is correct.
What happens physically: the pressure falls further between the suction flange and the impeller eye. If it reaches the vapour pressure, the liquid boils locally, and the bubbles collapse violently as they move into higher pressure in the impeller. The signs are a noise like gravel in the casing, vibration, loss of head and flow, and pitting of the impeller over time.
Every remedy acts on the suction side. The strongest is elevation: put the tank above the pump. With the liquid surface 2 m above the centreline, NPSHa at 80 °C becomes 5.65 + 2 - 1 = 6.65 m.
Next, cut the suction friction with a short, straight, larger pipe and no unnecessary fittings. For a given flow, friction varies with roughly the fifth power of diameter, so going from 50 mm to 80 mm bore reduces a 1 m loss to about 0.1 m. Other options are cooling the liquid before the pump, pressurising the supply vessel, or selecting a pump with a lower NPSH requirement, which usually means a lower speed or a larger inlet. Never control flow with a valve in the suction line; throttle on the discharge.
Leave a margin above the curve value. The published NPSH required is conventionally the point at which cavitation has already reduced the pump head by 3 percent, so a pump running exactly at that value is cavitating. A margin of at least 0.5 to 1 m, or a ratio of available to required of 1.1 or more depending on the service and the applicable pump standard, is normal practice.
Check at the highest flow the pump can reach, not only the duty point: NPSH required rises steeply with flow while NPSH available falls because suction friction rises. Allow for site altitude as well. At 1500 m the atmospheric pressure is about 84.5 kPa, which removes 1.7 m from the first term.
A special case worth knowing is a liquid stored at its boiling point, such as condensate in a receiver, a deaerator or the bottom of a distillation column. There the pressure on the surface equals the vapour pressure, the first term is zero whatever the vessel pressure, and NPSHa = z - h_f. Static height is all you have, which is why such vessels are mounted high above their pumps.
To confirm cavitation on an installed pump, fit a gauge at the suction flange and compute NPSHa from the measured absolute pressure. A quick test is to throttle the discharge slightly: if the noise stops as the flow falls, it is cavitation. If it does not, suspect air being drawn in, for example through a vortex at low tank level, which sounds similar.
Back to Chemical Engineering community
