The net positive suction head available (NPSHA) is calculated:
NPSHA = AP + SP – HL - VP
AP = Job site atmospheric pressure, in feet of water SP = Static water pressure at the pump, in feet of water HL = Head loss between GARN® and pump inlet, in feet of water VP = Vapor pressure at desired HWS temperature, in feet of water
A simple equation for calculating the head loss between the GARN® and the inlet of the pump:
L = Length of pipe between the GARN® and the pump inlet EL = # of 45° and 90° elbows between the GARN® and the pump inlet BV = # of ball valves between the GARN® and the pump inlet GV = # of gate valves between the GARN® and the pump inlet T = # of tees between the GARN® and the pump inlet
HL, is the summation of pipe, fitting, and valve pressure losses between the GARN® unit and the inlet of the pump. All losses are to be calculated at maximum system design flow (GPM).
NPSHA must always be greater than the net positive suction head required (NPSHR) for the pump at design GPM, or cavitation and suction boiling will occur. The NPSHR is provided by the pump manufacturer (see the Pump Selection and Installation Guideline s section of this manual)
The following tables list atmospheric pressure (AP) at various elevations and vapor pressure (VP) at various HWS temperatures.
ATMOSPHERIC PRESSURE (AP)
Boiling Point of Water (°F)
Elevation (ft)
Atmospheric Pressure (ft)
Sea Level, 0
33.9 32.8 31.5 30.4 29.2 28.2 27.2 26.2
212 210 208 206 204 202 200 198
1000 2000 3000 4000 5000 6000 7000
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Hydronic System Design Manual ©DECTRA CORPORATION - March 2013
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