When considering Net Positive Suction Head, it is useful to differentiate between available net positive suction head (NPSHA) and required net positive suction head (NPSHR).

NPSHA is a characteristic of the system in which a centrifugal pump operates and represents the difference between the absolute suction head and the fluid vapor pressure at the prevailing temperature.

NPSHR is a function of the pump design and represents the minimum required margin between the suction head and fluid vapor pressure.

The way NPSHA is calculated depends on the system configuration. The following Figures help illustrate this point.

Figure 1 outlines how NPSHA is calculated for a given capacity of water at 80 F based on a system with suction lift (15ft). Figure 1: NPSHA Calculation with Suction Lift.

The hand calculation of NPSHA for this scenario (at sea level) would be:

NPSHA = 2.31 (Ps – Pv) / SG + Z – Hf

where;

Ps         is the pressure above the liquid surface (psia).

Pv         is the vapor pressure of the liquid (psia).

Z          is the static head (ft)

Hf         is the friction losses (ft). The friction losses total 2.38 + 0.46 + 0.16 = 3 ft.

Solution:

NPSHA = 2.31 (Ps – Pv) / SG + Z – Hf

NPSHA = 2.31 (14.7 – 0.5) / 1.0 – 15 – 3

NPSHA = 14.8 ft.

The hand calculation produces a NPSHA of 14.8 ft which match the modeled result.

Let’s consider a scenario whereby the system inlet features a pump taking its suction from a pressurized tank at an elevation of 10ft. The fluid, in this case, is once again water at 80 F. Figure 2: NPSHA Calculation with Suction from a pressurized tank.

The hand calculation of NPSHA for this scenario would be:

NPSHA = 2.31 (Ps – Pv) / SG + Z – Hf

NPSHA = 2.31 (14.7 + 5 – 0.5) / 1.0 + 10 – 4.001

NPSHA = 2.31 (14.7 + 5 – 0.5) / 1.0 + 10 – 4.001

NPSHA = 50.4 ft

The hand calculation produces a NPSHA of 50.4 ft which matches the modeled result.

Note, both NPSHA and NPSHR vary with capacity. With a given static pressure or elevation difference at the suction side of a centrifugal pump, NPSHA is reduced at larger flow rates by the friction losses in the suction piping and fittings. On the other hand, NPSHR being a function of the velocities in the pump suction passages and at the inlet of the impeller, increases basically as the square of the capacity. The NPSHR for a given pump is provided by a pump vendor. Many factors affect the estimation of NPSHR such as eye diameter, number of impeller vanes, suction area of the impeller, shape of the vanes, shaft and impeller hub diameter, impeller specific speed, shape of suction passages, etc. it is therefore not recommended that a designer estimates a value of NPSHR for a pump but obtain this specific data from the pump vendor.

The NPSHR describes the amount of pressure required at the inlet of a pump to prevent air bubbles from forming inside the pump unit. If a scenario arises whereby the NPSHA is lower than the NPSHR by the pump, air bubbles are allowed to form. These bubbles can implode violently inside the pump causing significant damage. This effect is known as cavitation.

Pump cavitation becomes evident when there is one or more of the following conditions present in a system; noise, vibration, drop in pump head-capacity and efficiency curves and with time, damage to the impeller by pitting and erosion.

A useful point to note is that NPSHR curves provided by pump manufacturer’s are usually based on using cold water for the pump test conditions. Thus, it might be assumed that the NPSHR by a centrifugal pump for satisfactory operation is independent of liquid vapor pressure at the pumping temperature and of course, this is not true. The NPSHR for a given capacity can vary appreciably for different fluids over a range of temperatures. In its simplest form, even when pumping water the NPSHR decreases when the water temperature increases.

What options are available to me to increase the NPSHA:

1. Reduce the resistance in the suction side of the system and hence the associated pressure losses.
2. Raise the height of the supply tank in open systems.
3. Raise the liquid level in the supply tank.
4. Lower the pump installation height.
5. Increase the surface pressure of the liquid in a closed system.
6. Monitor and control the fluid temperature.
7. Lower the pump operating speed.
8. Use a larger impeller eye area.
9. Use several smaller pumps in parallel.

Item 9 above may initially appear to be a costly solution however, in many cases, three half capacity pumps of which one may be a spare, are often no more expensive than one full capacity pump plus its spare. In many cases, just two half-capacity pumps can be installed without a spare since part-load can still be carried if one pump has failed and is temporarily out of service. In addition, if the demand varies widely, operating a single pump during light-load conditions will save energy.

This technical paper attempts to briefly describe NPSH, outline some of problems which can arise when the NPSHA is lower than the NPSHR and what measures can be considered to remedy such conditions. In the end, careful design together with liaising with the pump vendor regarding the NPSH requirements for a proposed pump model will go a long way to eliminating any potential operational problems.

References:

1. CIBSE Guide B1 2016.
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