by Terry Henshaw, P.E.

Exercise care in applying any correction, though, because hydrocarbons that have their vapor pressures below atmospheric pressure, and are therefore generally stored in atmospheric vessels, will absorb air. The dissolved air will increase the vapor pressure of the solution and will flash out of solution at a pressure above the vapor pressure of the air-­free liquid. Although this flashing does not result in cavitation damage to pump components, it can cause a drop in head and erratic pump performance.

API 610 does not allow use of any "hydrocarbon correction" for NPSH, letting any lower degree of cavitation result in less than the 3 percent head drop.

Stepping NPSHR to Different Speeds

Although much has been written stating that it does, the NPSH requirement of a centrifugal pump, based on a 3 percent head drop, does not vary as the square of the speed as the head does. At the BEP (best efficiency point), the NPSHR varies as the speed-ratio to approximately the 1.5 power for the conventional 3 percent head‑drop criterion.

 

NPSH2 = NPSH1 (N2/N1)1.5

This relationship normally exists only at the BEP. At half the BEP, the exponent is usually about one. At 1.3 x Qbep, the exponent is usually about two.

 

Problem No. 1: Stepping NPSHR to a Different Speed

 

The performance curve in Figure 2 from Part Four (P&S, May 2009-reprinted below) is for 3,550 rpm. Calculate the approximate NPSHR, for 1,750 rpm, at the BEP.

NPSH2 = NPSH1 (N2/N1)1.5 = 14 (1750/3550) 1.5 = 14 (0.346) = 4.8 = ABT 5 FEET

(This 5 ft NPSH requirement would be at the 1,750 rpm BEP of about 225 gpm.)

 

Although the above 1.5 exponent is based on the author's experience with centrifugal pump NPSH tests at different speeds, Fang (7) reported different findings (varying exponents). Some continue to use the traditional two exponent (instead of 1.5).

This author concludes that the exponent would be two if NPSHR were based on incipient cavitation, but the 1.5 is more accurate for stepping NPSHR based on the more common 3 percent head‑drop NPSHR.

This characteristic (the 1.5 exponent) has caused significant field problems (1, 8). High energy pumps, tested at the higher speeds, indicate lower NPSH3 than that obtained by stepping the NPSHR3 from a lower test speed using the traditional two exponent. The 40 MW (54,000 hp) boiler feed pump described in reference (9) was NPSH tested at 1,490 rpm, and the results were extrapolated to the 4,620 rpm rated speed using an exponent of two.

Such a procedure eliminates the potential problem created by testing at the high rated speed. Incipient cavitation was observed to occur at an NPSHA about two times NPSHR3. In the plant, a 1,320 rpm booster pump provides NPSH almost three times the extrapolated NPSHR3.

 

References

1.   Vlaming, D. J., "Analysis of Cavitation Provides Advanced NPSH Estimates for Centrifugal Pumps," Oil & Gas Journal, November 19, 1984.

2.   Hydraulic Institute Standards, Hydraulic Institute, 6 Campus Drive, First Floor North, Parsippany NJ, 07054-4406.

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