Total Developed Head Calculations for Horizontal Pumps

The total developed head (TDH) of a centrifugal pump is equal to the discharge head minus the suction head and is typically expressed in either “feet” or “psi”. Pump manufacturers prefer to express total developed head in feet because this measurement is independent of the liquid being pumped.

Suction Head

The suction head is equal to the static height that the liquid is above the pump centerline less all suction line losses (including entrance loss) plus any gage pressure existing at the suction supply source. When calculating TDH, the suction head may be determined at any convenient elevation as long as the discharge head is also computed based on that elevation.

tdh

where

hs = suction head (ft)

Zs = static height (ft)

fs = suction line losses (ft)

Ps,G = gage pressure at suction source (psi)

s.g. = specific gravity at operating temperature

EXAMPLE: OPEN TANK ABOVE PUMP CENTERLINE

A miscellaneous pump service is taking suction from a tank that is open to the atmosphere. The free surface of the fluid in the tank is 10 feet above the centerline of the pump. The friction loss in the suction piping is 2.92 feet.

tdh

EXAMPLE: OPEN TANK BELOW PUMP CENTERLINE

A miscellaneous pump service is taking suction from a tank that is open to the atmosphere. The free surface of the fluid in the tank is 5 feet below the centerline of the pump. The friction loss in the suction piping is 1.90 feet.

tdh

EXAMPLE: PRESSURE GAGE LOCATION

On an existing installation, the suction head would be the reading of a gage at the suction flange converted to feet of liquid and corrected to the pump centerline elevation (or any convenient elevation when determining TDH) plus the velocity head (in feet of liquid) at the point of gage attachment. If the suction pressure gage is sufficiently upstream of the pump, then the friction loss must be subtracted as well.

To determine the velocity head, the inside pipe diameter at the pressure gage attachment and the approximate pump flow must be known. The velocity head (V2/2g) is a small number compared to the TDH, but must be calculated to obtain a true representation.

tdh

Discharge Head

The discharge head is equal to the static height that the liquid is above the pump centerline plus all discharge line losses (including entrance loss) plus any gage pressure existing at the discharge destination. When calculating TDH, the suction head may be determined at any convenient elevation as long as the discharge head is also computed based on that elevation.

tdh

where

hd = discharge head (ft)

Zd = static height (ft)

fd = discharge line losses (ft)

Pd,G = gage pressure at discharge source (psi)

s.g. = specific gravity at operating temperature

Example: Open Tank

A miscellaneous pump service is taking discharging to a tank that is open to the atmosphere. The free surface of the fluid in the tank is 10 feet above the centerline of the pump. The friction loss in the discharge piping is 2.92 feet.

tdh

Example: Closed Tank

A condensate booster pump is discharging into a deaerating tank. The temperature of the fluid in the tank is 350ºF, and the tank is not super-pressurized. As such, during steady-state conditions, the pressure acting on the fluid is the fluid vapor pressure. The vapor pressure and specific gravity of the feedwater at 350ºF are 134.6 psia and 0.8904, respectively. The free surface of the fluid in the tank is 50 feet above the centerline of the pump. The friction loss in the discharge piping is 7.02 feet.

tdh

Example: Pressure Gage Location

On an existing installation, the discharge head would be the reading of a gage at the discharge flange converted to feet of liquid and corrected to the pump centerline elevation (or any convenient elevation when determining TDH) plus the velocity head (in feet of liquid) at the point of gage attachment. If the discharge pressure gage is sufficiently downstream of the pump, then the friction loss must be added as well.

To determine the velocity head, the inside pipe diameter at the pressure gage attachment and the approximate pump flow must be known.

tdh

Total Developed Head

Total developed head (TDH) is equal to the discharge head less the suction head and is typically expressed in feet (ft).

tdh

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