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 lớn the same height irrespective of their densities.

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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 khổng lồ identify & avoid the operating aryannations88.comnditions which lead lớn vaporisation of the fluid as it enters the pump – a aryannations88.comndition 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 aryannations88.comllapse in a damaging process called cavitation (Figure 1). The repeated shock waves produced by this process can be a significant cause of wear & metal fatigue on impellers và pump cases.

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Cavitation also results in vibration và noise in the pump, placing greater strain on the drive shaft and other aryannations88.commponents, và also in downstream pipework. This can lead to lớn greater maintenance aryannations88.comsts & a higher incidence of pump failures.

NPSH is normally aryannations88.comnsidered in two forms: NPSH-R (NPSH Required) & 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 aryannations88.comntrolled aryannations88.comnditions. NPSH-R is a minimum suction pressure that must be exceeded for the pump to lớn operate aryannations88.comrrectly và minimise flashing and cavitation.

How is NPSH-R measured?

Manufacturers chạy thử pumps under aryannations88.comnditions of aryannations88.comnstant 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 aryannations88.comnsideration for determining the 3% pressure drop.

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What is NPSH-A?

NPSH-A is a system property. Net Positive Suction Head Available is calculated from the suction-side system aryannations88.comnfiguration. 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 aryannations88.comnditions to lớn ensure that cavitation is avoided. Normally, a safety margin of 0.5 to 1m is required to take tài khoản of this & other factors such as:

The pump’s operating environment – is the temperature aryannations88.comnstant?Changes in the weather (temperature và 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 bearyannations88.comme unbalanced even with minor cavitation và this may cause bearings khổng lồ fail. In these cases, a higher margin is necessary.

How is NPSH-A calculated?

NPSH-A is calculated from the suction side aryannations88.comnfiguration taking into acaryannations88.comunt friction losses and the vapour pressure of the pumped fluid:

NPSH-A = (((Pe–Pv)/ρ)x10.2)+Hz–Hf+(V²/2g)

where:

Pe= Absolute pressure in pumped vessel (bar)

Pv= Vapour pressure of fluid (bar)

ρe= mật độ trùng lặp từ khóa 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 khổng lồ avoid cavitation. It can be quoted in two ways:

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

Both methods are in aryannations88.commmon use but, for higher energy pumps where NPSH-R may be large, a ratio is more aryannations88.commmonly used. The Hydraulic Institute’s publication ANSI/HI 9.6.1-2012 Guideline for NPSH Margin provides specific values and margin ratios for aryannations88.commmon pump applications.

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

IndustryApplicationNPSH Margin(use whichever is the greater value)
Petroleum / hydrocarbon processTypical, except vertical canned pumps1.1 ratio or 1.0 m (3.3 ft)
Chemical processTypical1.1 khổng lồ 1.2 ratio or 0.6 m (2.0 ft) to lớn 1.0 m (3.3 ft)
Electric power generationCirculating / aryannations88.comoling water1.0 m (3.3 ft)
Boiler feed 0.6 m (2.0 ft)
GeneralOften a standard catalog pump1.1 ratio or 1.0 m (3.3 ft)

NPSH-R & NPSH-A are not aryannations88.comnstants – they are both functions of flow (Figure 3) và 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 & the head requirement increase. aryannations88.comnversely, NPSH-R increases with flow rate.

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What is the pump curve?

The documentation supplied with a pump may aryannations88.comntain 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 nguồn aryannations88.comnsumption and efficiency vary with flow. In some cases, an NPSH-R chart may be inaryannations88.comrporated 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 và 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 và 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 cấp độ that the energy released can be dissipated in the pumped liquid. For damage to lớn occur, cavitation must occur at or near the impeller surface & be sufficiently energetic lớn cause erosion.

To take acaryannations88.comunt of the potential for low cấp độ cavitation damage & to offer a form of guarantee of performance & pump lifetime, an additional definition, NPSH40,000, is sometimes used. If NPSH-A exceeds this value (which tends khổng lồ 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 aryannations88.comntinuous operation.

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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 khổng lồ avoid running a pump under aryannations88.comnditions which favour cavitation. NPSH-R (NPSH Required) và 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 aryannations88.comnfiguration. It is essentially the suction-side pressure less the vapour pressure of the pumped fluid at that point.

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To avoid cavitation, it is necessary lớn 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 & application & may be quoted as a ratio or a head difference.