At ambient temperatures, hydrocarbons are closer to their critical pressures than cool water. For the same drop in suction pressure below vapor pressure, less volume of hydrocarbon gas is liberated. Therefore, to get the same 3 percent head drop with the hydrocarbon, the suction pressure must be reduced farther below vapor pressure, so less NPSH would be available, i.e., NPSHr would be less.
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.
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.
(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 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.