what is head? Assume that you have a pump designed to move oil clamped into a process line. You have a suction line and a discharge line, both running horizontally. Now imagine that you are able to “move” the discharge line so it pumps straight up into the air. The pump is then turned on. Once the pump is running, it will move the fluid to some height measured in feet. That height to which the pump can raise the oil to is its head. It’s that simple.
Is head affected by the suction conditions? The answer to that question is yes. If you lower the suction level, the head measured will be less. The opposite is true if you raise it. Why is that? Well, a pump is not very smart. It has no electronics other than a motor coupled to it. It is mindlessly providing energy. The motor converts electric energy into mechanical energy and that mechanical energy is used by the pump to impart that energy (pressure) into the fluid. If we raise or lower the suction level of the fluid, we are adjusting the potential energy of the fluid. The potential energy of ten foot column of oil is greater than the potential energy of a five foot column of oil. That is why we always calculate total head requirements based on an suction empty tank, as that is when we will need the pump to provide the most energy (the fluid isn’t giving us any “help” in the form of potential energy when the tank is empty).