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Measuring TDH

The total dynamic head of a pump is the energy at point 2 (pump discharge) minus the energy at point 1 (pump suction) plus friction losses incurred between the gage reading point and the pump flanges.  Referring to the drawing below, the line to the suction gauge is air filled (suction vacuum), therefore the gauge to datum correction shown (Hz1) is for the elevation difference between the point of the gauge connection and the pump datum rather than between the actual gauge and pump datum.  If the line to the suction gauge was water filled (positive suction pressure) then the correction would be from the gauge centerline to the pump datum as is shown for the discharge side gauge.

Pressure gauges read static pressure; therefore the pressure gauge reading must be corrected for velocity head. The values of velocity head are most easily determined by looking them up for a known pipe inside diameter and flow rate at the gauge tap location (see Hydraulic Data Tables).

For each flow rate the friction losses are calculated and expressed in feet. Friction losses are given for various flow rates in terms of feet of head loss per 100 feet of pipe length for the different pipe sizes.  Losses through various pipe fittings can be found in Hydraulic Data Tables for pipe fittings.

Example:

Total Dynamic Head = Energy at Discharge – Energy at Suction

Energy at Discharge = P2 + HZ2 + (V2D/2g) + HL2

All terms are converted to feet of head for calculation of total discharge head @ P2.

Energy at Suction = P1 - HZ1 + (V2S/2g) - HL1

All terms are converted to feet of head for calculation of total suction head @ P1.

1. Pressure measured in P.S.I is converted as follows:

      P.S.I  X 2.31 = Head in Feet

2. Suction Pressure, P1, read in inches of mercury (vacuum) is converted as follows:

      InHG X  1.133 = Head in Feet

  • P1 is entered as a negative for suction lift (vacuum).
  • P1 is entered as a positive if it is a flooded suction and read in PSI (PSI X 2.31).

3. Velocity head, V2 /2g, is expressed in feet and can be found in hydraulic data tables.

4. Elevation correction, HZ, is measured in feet

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