NPSH available

What is NPSH_a

As described in the previous topic, pumps need a certain pressure at the inlet to avoid cavitation.

Such pressure is made available by the system, is expressed in meters of water column, and is therefore called Net Positive Suction Head Available (NPSH_a).

NPSH_a must be compared with the NPSH_r (the pressure needed by the pump). There is an extra safety margin to be added, to guarantee the safest possible conditions. The safety margin can be calculated in 2 ways: as a percentage of the NPSH_r (add 25% margin), or as a fixed quantity (add 0.6m of water column), whatever is larger should be used.

NPSH_a depends on four main factors:

• The pressure P_a acting on the free surface of the upstream tank
• The vertical distance Z₁ between free surface and pump inlet
• Pressure losses DeltaP_loss
• Vapor pressure of the fluid P_vap

The formula that describes their relationship is

with

How to maximize NPSH_a

As clarified at the beginning of this topic, NPSH_a should be as large as possible, so that it is larger than the NPSH_r over the working flow-rate range of interest. To maximize NPSH_a, we can can observe in the equation above how increasing the pressure P_a above the free surface (i.e. by pressurizing the vessel) has a positive effect. Increasing the height difference between pump inlet and free surface level also helps, as the Z₁ term will increase. The pressure losses P_loss term must be reduced as much as possible, as it increases with the square of the flow rate. To do so, the tubes at the inlet side must be as large as reasonably possible, as short as possible, and the components that cause resistance to the flow (e.g. elbows, fittings, filters, heat exchangers, regulation valves, etc) must be moved downstream of the pump. Lastly, the fluid itself should be as far as possible from the boiling point (P_vap as low as possible), meaning that the temperature should be kept as low as possible, compatibly with the process (no unnecessary heating upstream of the pump).

Comparison between NPSH_r and NPSH_a

Graphically, the NPSH_r appears as a set of curves (as shown in the previous topic) increasing with the flow rate. At the same time, the NPSH_a has the shape of an upside-down parabola (due to the -P_loss term, offset by the P_a–P_vap and Z₁ terms) as shown in Figure 2.

As shown in Figure 2, there is no risk of cavitation if the flow-rate is below 32 lpm, because the NPSH_a is larger than the NPSH_r + safety margin. Between 32 lpm and 35 lpm there a chance of cavitation, because the safety margin is violated. For flow-rates larger than 35 lpm, cavitation will occur.

If the system that produced the plot in Figure 2 gets improved (upstream vessel pressurized, elbows removed, larger tubes), it could produce a new combination of curves, such as in Figure 3. It is clear that now the cavitation will be very unlikely over most of the operative range.