Head
The energy imparted to a fluid by a pump is measured as the head (in metres)per unit weight of fluid.
dH = (Z2 - Z1) + ( P2 -P1 )/ (ρ g ) - (v22 -v12)/ (2 g )
| dH = Fluid Head developed across pump ..m Z1 = Fluid Supply Level..m Z2 = Fluid Discharge Level..m P1 =Pressure over Supply Fluid..N/m2 P2 =Pressure into which fluid is discharged ..N/m2 ρ = Fluid Density..kg/m3) g = Accelaration due to gravity 9.81 m / s2 v12 Fluid velocity at pump inlet m/s v22 Fluid velocity at pump discharge m/s |
Normally the velocity head and the height head is ignored for initial selection of thepump.
The head developed is therefore approximated to the static pressure difference expressed asa column of the liquid pumped in metres of fluid.
Note: For accurate calculations the difference in height of the pump suction and out discharge flangesand the difference in velocity head should be considered.
To convert the differential head developed to pressure difference in N/m2
p (N/m2 ) = dH * ρ g
NPSH
(Net Positive Suction Head)
As liquid flows from the pump inlet flange into the energy conversion region eg.impellor orpiston chamber, the head initially falls as the velocity of the fluid is increased. If the liquid is near it vapour pressure the drop in head may be sufficient to cause the liquidto boil this results in "cavitation". Cavitation is detectable as a rattling noise andresults in low efficiency, and high risk of damage to the pump.
The head drop for any pump design at a selected flow is provided by the manufacturer as the NPSH required NPSHR measured in metres of fluid pumped.
If the head of the fluid at the pump suction flange is not greater that the vapour pressure, at the pumping temperature, plus the NPSHR then cavitation will occur.
The head at the pump inlet flange - the relevant vapour is called the NPSH available NPSHA.
The NPSHA must be at least 1-2m of fluid greater than the NPSHR to avoid cavitation.
| Pi = Surface pressure of fluid (N/m2) Pb = Barometric pressure (N/m2) Pv = Vapour pressure of fluid (N/m2) ρ = Fluid density (kg/m3) Ve = Velocity of fluid at pump inlet (m/s) Ze = Fluid Surface level above pump inlet. (m) Fe = Friction loss is pipe to pump. (m Note: 1 bar = 105 N/m2 |
