NPSH – How much is enough?

A general introduction about NPSH in CENTRIFUGAL PUMP (Advanced) article.

But the confusion raised that you still not understand how it works and how to evaluate the NPSH of our current pump is enough or Not ?

Confusion will occur if someone does not know that:

There are two basic NPSH parameters to consider (for the inspector)

  • First, the NPSH of the system – as called  NSPH MARGIN required by recommendation or code.
  • Second, NPSH Available – NPSHA and can be calculated!

The other is the NPSH Required – NPSHR by the pump experiment carried out by Manufacturer and we can see it in Pump characteristic chart.

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How do we define ‘NPSH Margin’?

There are two ways. One way is to express NPSH Margin as the ratio of [NPSHA divided by NPSHR]. Another way is to express NPSH Margin in absolute terms as the difference of [NPSHA minus NPSHR]. For instance, an NPSH Margin ratio of 1.1 indicates the NPSHA is 10% above the NPSHR.

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

Industry Application NPSH Margin for the AOR; select 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)
Electric power generation 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)

As above table, the NPSHA should be bigger than NPSHR about 1m-1.5m in general.

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Beside figure illustrates the component factors of NPSHA.

In this chart, atmospheric pressure is dominant but in other systems the other factors may have relatively greater or lesser importance. In a system open to atmosphere, the liquid surface elevation above or below the first stage impeller and estimated intake friction losses may be used in lieu of a pressure reading at the pump intake. Convert pressures readings to head, in units of meters or feet, by dividing by the specific weight. Pump texts, and other publications show how and where the readings are taken and how to perform the calculations.

It is important to understand how the system’s NPSHA varies across the range of pump operation. This may influence the pump selection, the intake piping and the necessary NPSH Margin. Intake friction losses vary with the rate of flow, and can be significant with long lengths of piping and a number of fittings. Changes in overall system pressure or changes in the liquid elevation above the pump intake impeller directly add or subtract from the NPSHA.

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TDSL: Total dynamic suction lift.

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Through above example, now we can optimize the pump suction ability by follow solutions.,

1st, building in additional NPSHA into a system:

It may require larger piping to reduce intake friction losses, elevating tank or pressure vessel liquid levels or placing pumps at a lower elevation setting. These design measures add to the capital cost of the system.

2nd, lower NPSHR

It could be achieved by selecting a pump with a slower rotational speed. This pump option will cost more, but it may be a lot less expensive than attempting to alter the intake piping design.

For the last but not least,

Does the appropriate NPSH Margin discard cavitation?

An NPSH Margin ratio of 2 to 10 – often greater than a ratio of 4 – would be required to suppress all cavitation. It depends mainly upon the inlet geometry of the pump and the range of operation. For most applications, the pump will tolerate some cavitation without compromising reliability. For most applications it is impractical and unnecessary to completely suppress all cavitation.

NPSH Margin can only be determined by understanding the performance of both the pump and system. An appropriate NPSH Margin will allow the pump to operate over the range of flows required by the process, will keep the pump from cavitating excessively or losing suction prime and it should not add excessively to the capital cost of the system.

 

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