Cavitation in pumps and NPSH calculation

(a) Cavitation

Cavitation is the formation of vapour bubbles in a flowing liquid where the local absolute pressure falls to the vapour pressure, and the sudden collapse of these bubbles when they move into a higher-pressure region.

Causes: high suction lift, low atmospheric pressure (high altitude), high liquid temperature (high vapour pressure), high pump speed, large losses in the suction pipe and an insufficient NPSH. The pressure is lowest at the impeller eye, on the suction side of the vane.

Effects:

• Noise and vibration.
• Pitting and erosion of the impeller and casing by the shock of collapsing bubbles.
• Fall in head, discharge and efficiency.

Prevention: lower the pump (reduce suction lift) or use a flooded suction, use a larger suction pipe with fewer bends and valves, reduce the speed, cool the liquid, use an inducer or a pump with a lower required NPSH, use cavitation-resistant materials (stainless steel, bronze).

(b) Numerical

Available NPSH (net head above vapour pressure at the pump suction):

$$\text{NPSH}_a=\frac{p_a-p_v}{\rho g}-H_s-h_{fs}$$ $$\frac{p_a-p_v}{\rho g}=\frac{(101.3-4.25)\times10^3}{996\times9.81}=9.933\ \text{m}$$ $$\text{NPSH}_a=9.933-4.0-0.8=5.13\ \text{m}$$

Since $\text{NPSH}_a=5.13\ \text{m}>\text{NPSH}_r=4.2$ m, there is a margin of $5.13-4.2=0.93$ m and the pump will not cavitate.

Maximum suction lift (taking $\text{NPSH}_a=\text{NPSH}_r$):

$$H_{s,max}=9.933-0.8-4.2=4.93\ \text{m}$$

(A safety margin of at least 0.5 m is kept in practice.)

At 79.5 kPa:

$$\frac{p_a-p_v}{\rho g}=\frac{(79.5-4.25)\times10^3}{996\times9.81}=7.702\ \text{m}$$ $$\text{NPSH}_a=7.702-4.0-0.8=2.90\ \text{m}<4.2\ \text{m}$$

The available NPSH is now short by about 1.30 m, so the pump will cavitate. The suction lift must be reduced (or a flooded suction used) to restore the margin.

Answer: $\text{NPSH}_a=5.13$ m (no cavitation); $H_{s,max}=4.93$ m; at 79.5 kPa $\text{NPSH}_a=2.90$ m, so cavitation occurs.