To account for the effects of changing atomospheric pressure, simply convert all pressures to absolute pressures. NPSHr (head) converts to an absolute pressure.
Let's consider a suction pressure of 2 psiG. Assume water and a vapor pressure of 0 psia.
At sea level, atmospheric pressure (14.7 psia) converts to a head of 33.9 feet. A suction pressure of 2 psig would convert to 4.61 ft Absolute suction pressure = 14.7 + 2 = 16.7 psia. Total suction head available at sea level = 33.9 + 4.61 = 38.51 ft
At 10,000 feet atmospheric pressure would be about 1/2, so say 8 psia, or some 17 feet. A suction pressure of 2 psig would convert to absolute suction pressure of 2 + 8 = 10 psia and 23.07 feet.
If you need an NPSHr of 25 feet, that converts to 10.83 psia
At sealevel you'd have 16.7 psia, or 38.51 ft, > 25 ft, so you're OK.
At 10,000 ft you'd have an absolute suction pressure of 10 psia, or 23.07 ft < 25 ft, so you're NOT OK.
If vapor pressure was significant, then you'd have to subtract that vapor pressure from the absolute suction pressures above. Vapor pressure always decreases available NPSHa.
**********************
"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that
99.99% for pipeline companies)