Pump Curve
Figure 1. Dynamic head in feet doesn't directly give pump power required.
Pump efficiency varies with flow rate. In addition, the efficiencies of the drive system (belts, gears, couplings, etc.) and driver come into play. For an electrically driven pump, the driver efficiency is the motor efficiency. This can vary as a function of total load. Motor efficiencies drop at low loads.
Dynamic head in feet doesn't change as density changes. Imposed pressure in psi and power demand will vary as liquid density varies. For many processes and conditions, the density is well understood and known in advance.
However, densities can vary dramatically in some common situations. For instance, at startup many units are cold, so densities are higher, increasing power demands. Let's look at a situation where startup density differences created motor problems.
Figure 2 shows the bottoms of a high temperature distillation tower. The startup procedure required circulating cold tower bottoms to the fired heater reboiler. Limits on both the pump (suction specific speed) and the fired heater (minimum safe firing with air control) restricted flow rate to 1,500 gpm for starting conditions. The design operation was 2,250 gpm. Obviously, the pump can handle the lower flow rate.