Beranda » Oil Pump » THD & NPSH Pump Formula

THD & NPSH Pump Formula

This is a quick Overview of the Most common operating formulas in the pump industry:

Total Differential Head (THD)

System Head = total discharge head – total suction head

H = hd – hs

hd = hsd + hpd + hfd hs = hss + hps = hfs

hd = total discharge head hs = total suction head
hsd = discharge static head hss = suction static head
hpd = discharge surface pressure hps = suction surface pressure
hfd = discharge friction head hfs = suction friction head

Available NPSH

Available NPSH is “the difference between the total suction head and the vapor pressure of the liquid, in feet of liquid, at the suction flange.”

hsv = hsa – hypa

hsv = available net position suction head, in feet of liquid
hsa = total suction head, in feet of liquid, absolute
hypa = vapor pressure of liquid at suction nozzle, in feet of liquid, absolute

Energy Required

The energy required to carry 1 kg of liquid from the suction tank to the delivery tank is:

Hi = (Hm – Ha) + Hg + Hp (m)


  • Hm e Ha, expressed in C.L., are the pressures on the free surface of the liquid in the delivery tank and in the suction tank respectively. If both tanks are at atmospheric pressure, the term (Hm – Ha) is equal to zero;
  • Hg is the geodetic difference in level, in meters, of the system: the examples given in figure 3 show how it should be measured;
  • Hp is the total load losses, in meters, of the suction pipe (if fitted) and of the delivery pipe. Load losses are the energy losses that take place during movement of the liquid; they may be “continuous” or “accidental”. The former are those due to friction between the fluid and the pipe whereas accidental losses are due to a variation in speed (widening or narrowing of the section, mouths and outlets, valves, etc.) and to changes in direction (curves, elbows, etc.). To calculate load losses, graphs and/or tables are generally used that may be easily found in good books on hydraulics or on pumping systems.

While Hm, Ha, Hg generally remain constant as the flow rate varies, the load losses Hp increase as the flow rate increases following a fairly quadratic law: as a result the value of Hi also increases as the flow rate increases.


There is the power supplied by the pump to the liquid, expressed as:

Pu[W] = g[m/s2] * γ[kg/m3] * Q[m3/s] * H[m C.L.]


g[m/s2]: is the acceleration of gravity, generally equal to 9,81 m/s2. Then there is the power Pnom absorbed by the pump, that is, in the case of electropumps, the power transferred by the electric motor to the pump axle. Then there is the electric power Pabs absorbed by the electric drive motor from the power mains.

Pressure Conversion

Head (ft.) = psi x 2.31 / sp. gr.
PSI = head (ft.) x sp. gr. / 2.31

Lbs./sq. in. = In. of Mercury x .491
Lbs./sq. in. = Atmospheres x 14.7

mm hg. = Atmospheres x 760


Centistokes x 4.64 = SSU(approx.)
Centipoise Sp. Gr. = Centistokes


Gallons per minute (GPM) x 3.785 = Leters per minute

Temperature Conversion
°C = 5/9 (°F – 32)
°F = 9/5°C + 32

Specific Gravity

sp. gr. = weight of liquid
sp. gr. = weight of water

weight of water = 62.4 Lb./cu.
weight of water = 8.3 Lb./gal.


BHP = GPM x TDH x sp. gr.
BHP = 3960 x Eff.
BHP = 1715 x Eff.
KW = 0.746 x H.P.
Torque = (Ft. Lb.) = HP x 5260
Torque = (Ft. Lb.) = RPM
HP = E(volts) x I(Amps) x Eff. X P.F. x 1.732
HP = E(volts) x I(Amps)746
I(amps) = HP x 746 I(amps) = E(volts) x Eff. X P.F. x 1.732

O-Ring Temperature Limitations

Polytetrafluorethylene [TM]
Low: -100 °F (-73 °C) High: 300 °F (149 °C)
Viton [VA]
Low: -20 °F (-29 °C) High: 400 °F (204 °C)
Perfluoroelastomer (Kalrez) [KAL]
Low: -36 °F(-38 °C) High: 550 °F (287 °C)


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