2.31 feet of water (SG = 1) produces a pressure of 1 psi. So, 25 feet of water head (assuming pump suction elevation = bottom of your tank) would be about 10.8 psi suction. Line losses would slightly reduce this pressure at the pump suction (assuming it's a well design suction line).
the pressure due to 25 ft water colum can be calculated as follows:
p=density x gravity x height
= 10.8 psig
ur observation of 18 psig is not understood. in case the tank is open at the top add the atmospheric pressure of 14 psi to it. so in case of open roof tank it will be 24-25 psig and in case of closed roof tank it will be 10-11 psig at the tank outlet at the bottom most point.
the pressure losses in piping will depend on the piping )O.D., Wall thickness and flow rate, the same can be calculated using MIT's formula based on reynolds no. Pressure losses in fittings in the piping can be calculated by converting the equivalent lengths. these pressure losses shall be deducted from the figures obtained above to get the suction pressure at the pump suction.
Suction pressure available at the pump flange = (pressure in the suction vessel) + (static head of liquid from the suction centerline to the top of the liquid) - (friction loss across the suction piping at your intended rate) - (friction losses across control valve, suction strainer, heat exchangers, etc.) Did I forget anything?
From there it's just a matter of units. Do the calc in all absolute or all gage units, you'll get the same answer; just be consistent. Thanks!
Pete
Atmospheric pressure is not absolute pressure, as another post may suggest.
if 10.8 is your guage pressure
and 14.7 is the atmospheric pressure
then 25.5 is your absolute pressure.
Use a barometer to measure atmosphere.
A vacuum measurement is implicitly a pressure below atmospheric. It is assumed that any measured quantity given as a vacuum is subrtacted from atmospheric and not added. This is not your case, as the tank is supplying pressure. We call it vacuum sometimes because it is the inlet to the pump. We call it NPSHA, (net positive suction head available)
newengr,
depends what do you need the pressure for... there is not just one pressure.
-absolute pressure available for suction conditions?(NPSHA - Net Positive Suction Head Available)
-Static Pressure (due to fluid head only) to determine if the pump seals will sustain the static pressure when not running
-Dynamic pressure (due to pipe entrance losses + friction losses + velocity head) to determine suitability of pipe size selection (combines with NPSHA), if these are too high you may want to increase the pipe size.
-Total pressure (dynamic + static) to determine system curve.
