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Useful information on NPSH

What is NPSH?

NPSH stands for Net Positive Suction Head and is a measure of the pressure experienced by a fluid on the suction side of a centrifugal pump. It is quoted as a Head (in feet or metres) rather than as an actual pressure (lb/in2 , psi or Pa) because ‘head’ is a fluid-independent property: a pump will lift different fluids to the same height irrespective of their densities.

NPSH is defined as the total head of fluid at the centre line of the impeller less the fluid’s vapour pressure. The purpose of NPSH is to identify and avoid the operating conditions which lead to vaporisation of the fluid as it enters the pump – a condition known as flashing. In a centrifugal pump, the fluid’s pressure is at a minimum at the eye of the impeller. If the pressure here is below the vapour pressure of the fluid, bubbles are formed which pass on through the impeller vanes towards the discharge port. As the bubbles of vapour are transported into this higher pressure region, they can spontaneously collapse in a damaging process called cavitation (Figure 1). The repeated shock waves produced by this process can be a significant cause of wear and metal fatigue on impellers and pump cases. 

Cavitation also results in vibration and noise in the pump, placing greater strain on the drive shaft and other components, and also in downstream pipework. This can lead to greater maintenance costs and a higher incidence of pump failures.

NPSH is normally considered in two forms: NPSH-R (NPSH Required) and NPSH-A (NPSH Available).

What is NPSH-R?

NPSH-R is a pump property. Net Positive Suction Head Required is quoted by pump manufacturers as a result of extensive testing under controlled conditions. NPSH-R is a minimum suction pressure that must be exceeded for the pump to operate correctly and minimise flashing and cavitation.

How is NPSH-R measured?

Manufacturers test pumps under conditions of constant flow and observe the discharge pressure (differential head) as NPSH (the suction pressure) is gradually reduced. Tests are usually performed with water at 20°C. NPSH-R is defined as the value at which the discharge pressure is reduced by 3% because of the onset of cavitation (Figure 2). NPSH-R is sometimes shown as NPSH3 or NPSH3% to highlight this fact. For multistage pumps, only the first stage is taken into consideration for determining the 3% pressure drop. 

What is NPSH-A?

NPSH-A is a system property. Net Positive Suction Head Available is calculated from the suction-side system configuration. It is essentially the suction-side pressure less the vapour pressure of the pumped fluid at that point. NPSH-A must exceed the pump’s NPSH-R rating for the chosen operating conditions to ensure that cavitation is avoided. Normally, a safety margin of 0.5 to 1m is required to take account of this and other factors such as:

  • The pump’s operating environment – is the temperature constant?
  • Changes in the weather (temperature and atmospheric pressure).
  • Any increases in friction losses that may occur occasionally or gradually during the lifetime of the system.

A greater margin (see What is a safe NPSH margin?) may be necessary with some systems. For example, sealless pumps can become unbalanced even with minor cavitation and this may cause bearings to fail. In these cases, a higher margin is necessary.

How is NPSH-A calculated?

NPSH-A is calculated from the suction side configuration taking into account friction losses and the vapour pressure of the pumped fluid:

NPSH-A = (Pe – Pv).10.2 + Hz – Hf + V².

                       ρ                                   2g

where:

Pe=  Absolute pressure in pumped vessel (bar)

Pv=  Vapour pressure of fluid (bar)

ρe=  Density of fluid (kg/dm³)

Hz=  Minimum fluid level above pump (m)
         (negative term if below pump)

Hf  =  Friction losses in suction side pipework (m)

Ve=  Fluid velocity in pump flange (m/s)

g    =  Acceleration due to gravity (9.81m/s²)

 NPSH-A can also be determined experimentally by measurements on the suction-side system.

What is a safe NPSH margin?

The NPSH margin describes the safety factor by which NPSH-A must exceed NPSH-R to avoid cavitation. It can be quoted in two ways:

  • As a ratio of NPSH-A to NPSH-R. For example, an NPSH margin ratio of 1.1 indicates that NPSH-A is 10% greater than NPSH-R
  • As the difference between NPSH-A and NPSH-R. As a rule of thumb, it is necessary to ensure that the NPSH margin is 0.5m or higher (that is: NPSH-A ³ NPSH-R + 0.5m)

Both methods are in common use but, for higher energy pumps where NPSH-R may be large, a ratio is more commonly used. The Hydraulic Institute’s publication ANSI/HI 9.6.1-2012 Guideline for NPSH Margin provides specific values and margin ratios for common pump applications.

Selected NPSH Margins from ANSI/HI 9.6.1-2012 Guideline for NPSH Margin

Industry Application NPSH Margin
(use whichever is the greater value)
Petroleum / hydrocarbon process Typical, except vertical canned pumps 1.1 ratio or 1.0 m (3.3 ft)
Chemical process Typical 1.1 to 1.2 ratio or 0.6 m (2.0 ft) to 1.0 m (3.3 ft)
Electric power generation Circulating / cooling water 1.0 m (3.3 ft)
Boiler feed < 250 kW/stage 1.3 ratio
Water Typical, stainless steel or aluminum-bronze impeller, < 75 kW/stage 1.1 ratio or 1.5 m (4.9 ft) minimum
Building services Typical for pumps in open systems (not pressurized) 1.0 ratio up to a 1.1 ratio or
0.6 m (2.0 ft)
General Often a standard catalog pump 1.1 ratio or 1.0 m (3.3 ft)

 

NPSH-R and NPSH-A are not constants – they are both functions of flow (Figure 3) and also temperature. If there are any changes in capacity, it is important to reassess the NPSH margin. NPSH-A reduces at high flows, as frictional losses and the head requirement increase. Conversely, NPSH-R increases with flow rate. 

What is the pump curve?

The documentation supplied with a pump may contain charts showing how the head pressure varies with flow. This is called the pump curve (or pump performance curve). It may also show how power consumption and efficiency vary with flow. In some cases, an NPSH-R chart may be incorporated into the pump curves for an entire family of pumps with a range of impeller sizes (see Figure 4). Although pump performance is different for each impeller size, requiring separate pump curves, NPSH-R may be essentially the same for all the pumps in the range so only one NPSH-R chart is shown. When this is not the case, the manufacturer will provide separate NPSH-R charts for each model.

In Figure 4, the pump curve shows that, for a model with a 4.5” impeller, operated with a head of 7m and at a flow of 12m3/hr, the NPSH-R is 5m. In designing the suction-side pipework for such a system it must be ensured that NPSH-A exceeds this value by an approved safety margin.

What is NPSH40,000?

NPSH-R represents a point at which cavitation has already had an impact on pump performance with the discharge pressure reduced by 3%. Cavitation actually begins at a higher pressure than the published NPSHR and is essentially already occurring when the inlet pressure equals the NPSH-R. However this may not necessarily result in any damage to the pump as the bubbles may be imploding further towards the discharge, away from the impeller eye, and be at such a low level that the energy released can be dissipated in the pumped liquid. For damage to occur, cavitation must occur at or near the impeller surface and be sufficiently energetic to cause erosion.

To take account of the potential for low level cavitation damage and to offer a form of guarantee of performance and pump lifetime, an additional definition, NPSH40,000, is sometimes used. If NPSH-A exceeds this value (which tends to be higher than the standard NPSH-R) the manufacturer effectively guarantees no loss of performance or parts replacement for 40,000 hours (about 4.5 years) of continuous operation. 

Summary

NPSH (Net Positive Suction Head) is a measure of the pressure experienced by a fluid on the suction side of a centrifugal pump. It is used to avoid running a pump under conditions which favour cavitation. NPSH-R (NPSH Required) and NPSH-A (NPSH Available) are two key NPSH values:

  • NPSH-R is a pump property quoted by pump manufacturers as the suction pressure at which cavitation has already reduced pump performance by 3%.
  • NPSH-A is a system property calculated from the suction-side system configuration. It is essentially the suction-side pressure less the vapour pressure of the pumped fluid at that point.

To avoid cavitation, it is necessary to ensure that NPSH-A exceeds NPSH-R by a sufficient safety margin, for example: NPSH-A ³ NPSH-R + 0.5m. This margin depends on the type of pump and application and may be quoted as a ratio or a head difference.

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