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what is npsh and how to calculate it for a centrifugal pump
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Answer Posted / nasser khattab
Given:
• Atmospheric pressure = 14.7 psi
• Gage pressure =the tank is at sea level and open to atmospheric pressure.
• Liquid level above pump centerline = 5 feet
• Piping = a total of 10 feet of 2 inch pipe plus one 90° long radius screwed elbow.
• Pumping =100 gpm. 68°F. Fresh water with a specific gravity of one (1).
• Vapor pressure of 68°F. Water = 0.27 psi from the vapor chart.
• Specific gravity = 1
• NPSHR (net positive suction head required, from the pump curve at 100 gpm) = 9 feet
Now for the calculations:
NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings
• Static head = 5 feet
• Atmospheric pressure = pressure x 2.31/sg. = 14.7 x 2.31/1 = 34 feet absolute
• Gage pressure = 0
• Vapor pressure of 68°F. water converted to head = pressure x 2.31/sg = 0.27 x 2.31/1 = 0.62 feet
• Looking at the friction charts:
o 100 gpm flowing through 2 inch pipe shows a loss of 17.4 feet for each 100 feet of pipe or 17.4/10 = 1.74 feet of head loss in the piping
o The K factor for one 2 inch elbow is 0.4 x 1.42 = 0.6 feet
• Adding these numbers together, 1.74 + 0.6 = a total of 2.34 feet friction loss in the pipe and fitting.
NPSHA (net positive suction head available) = 34 + 5 + 0 - 0.62 - 2.34 = 36.04 feet
The pump required 9 feet of head at 100 gpm. And we have 36.04 feet so we have plenty to spare.
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