Do Pumps Require Less NPSH on Hydrocarbons? Stepping NPSHR to Different Speeds


Written by:
Terry Henshaw, P.E.

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.

3.   Engineering Data Book, Natural Gas Processors Suppliers Assoc., Tulsa, OK, 1966.

4.   Stahl, H. A. & Stepanoff, A. J., "Thermodynamic Aspects of Cavitation in Centrifugal Pumps," ASME Paper No. 55‑A‑136, 1955.

5.   Salemann, Victor, "Cavitation and NPSH Requirements of Various Liquids," ASME Paper No. 58‑A‑82, 1958.

6.   Whistler, A. M., "A Yardstick for NPSH Requirements," Petroleum Refiner, Jan. 1960.

7.   Fang, Ken‑Shou, "How Accurate are Predictions of Required NPSH When Based on Speed­scale Effect?", Power, February 1979.

8.   Ross, Robert R., "Theoretical Prediction of NPSHR for Cavitation Free Operation of Centrifugal Pumps," United Centrifugal Pumps, about 1982.

9.   Elsässer, Thomas; Brecht, Bernhard; Baumgarten, Sven, "The Gentle Giant of Niederaussem," Pumps & Systems, December 2006 (and letters to the editor, February 2007).

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