The step4 of the ESP design consists on determining the total dynamic head required to pump the desired capacity. It is common to simplify the procedure by combining or summarizing the additional energy that the pump must supply into a single term, Total Dynamic Head (TDH). TDH is a summation of the net vertical distance ﬂuid must be lifted from an operating ﬂuid level in the well, the frictional pressure drop in the tubing and the desired wellhead pressure.
TDH = HD + HF + HT
- TDH: total dynamic head in feet (meters) delivered by the pump when pumping the desired volume.
- HD: vertical distance in feet (meters) between the wellhead and the estimated producing fluid level at the expected capacity.
- HF: the head required to overcome friction loss in tubing measured in feet (meters).
- HT: the head required to overcome friction loss in the surface pipe, valves, and fittings, and to overcome elevation changes between wellhead and tank battery.
PS: HT is normally measured in gauge pressure at the wellhead. It can be converted to head, in feet (meters) as follows: HT = (psi / (0.433 psi/ft x sp. gr.)
The following plot depicts the curve of the lifting head required by the well as a function of the flow rate:
The operating point of the pump is the intersection of pump characteristic curve and the curve of the lifting head required by the installation (Head = 5500 ft, Q = 900 bpd):
Effect of the fluid density:
The Dynamic Head generated by a centrifugal pump is independent of the density pumped. Hence, the same pump at the same rotational speed will generate the same head, regardless of the density of the fluid pumped (the dynamic head expressed in length unit will be the same whether the fluid pumped is pure water (density=1), crude oil (density < 1) or brine (density > 1).
However, that does not mean that the pressures or horsepower required are the same! Pressure delivered by the pump (discharge pressure) depends on the density of the pumped fluid.