Total Dynamic Head Calculator
Friction loss of fittings shown in equivalent feet of pipe
Pumps are kinetic machines in which fluid is accelerated to some velocity. As stated earlier, head represents the total energy supplied by the pump to the fluid. At any given point in a system where flow is present, some of the available energy is in the form of velocity. To obtain a true head reading at flow rates greater than zero-flow the head associated with the fluid velocity head must be taken into account. Velocity is determined by dividing the flow rate by the cross-sectional area of the pipe.
= Qm3/sec ÷ Am2
or Vft2/sec = Qft3/sec ÷ Aft2
Velocity head(hv) may then be calculated from the equation hv = V2/2g where ‘g’ = the acceleration due to gravity (9.9 meters per second per second, or 32.17 feet per second per second, at 45o latitude.) Most pipe friction tables include a value for velocity head for each listed flow rate and pipe size.
Friction head is the head associated with the friction of fluid through piping, valves and fittings. It may be obtained from pipe friction tables for any given flow rate or pipe size. Most friction tables include values for standard valves and fittings.
Total Discharge Head(hd)
Total discharge head is the gage pressure at the discharge of the pump (hgd) converted to head at the elevation of the impeller centerline, plus the velocity head at pressure gage location, plus any system friction losses (hf) between the pump and pressure gage location.
Total Suction Head(hs)
Total suction head is the gage pressure at the suction of the pump (hgs) converted to head at the elevation of the impeller centerline, plus the velocity head at pressure gage location, plus any system friction losses (hf) between the pump and pressure gage location.
Total Head(H) – Often called Total Dynamic Head(TDH)
Total head is the difference between total suction head and total discharge head.
H = (hgd ± zd + hv + hf) – (hgs ± zs + hv + hf)
- One problem that confronts users is that flowrate is needed to calculate the velocity head and friction. This is sometimes not available. Fortunately, these are often relatively insignificant in the total head calculation. Under certain conditions, as described below, they may be negat
- Velocity head may be ignored If the pipe diameters are the same for both the suction and discharge gage locations.
- At zero flow there is no velocity, therefore there is no velocity head and no friction head.
- Friction head is usually a relatively small number and may be ignored in pump installations where the pressure taps are within a few feet of the pump and the piping is relatively straight.
- For vertical submerged pumps without tailpipes, total head is equal to the head at the discharge pressure gage, plus the velocity head at the discharge pressure gage, plus the vertical distance to the free liquid surface of the sump or pit.
- For vertical pumps with a tailpipe and a suction lift solve for total suction head using the methodology described above. If readings are in mm or inches Mercury covert the values to feet and use a negative(-) value for head.
1 mm hg = 0.0133 m(H2O) 1 in. hg = 1.13 ft.(H2O)