Head Pressure is needed to pump the liquid through the system at a certain rate. This pressure has to be high enough to overcome the resistance of the system, which is also called “head”. The total head is the sum of static head and friction head:
1. Static head
Static head is the difference in height between the source and destination of the pumped
liquid (see Figure 2a). Static head is independent of flow (see Figure 2b). The static head at a certain pressure depends on the weight of the liquid and can be calculated with this equation:
Head (in feet) = (Pressure (psi) X 2.31)/Specific gravity
Static head consists of:
- Static suction head (hS): resulting from lifting the liquid relative to the pump center line. The hS is positive if the liquid level is above pump centerline, and negative if the liquid level is below pump centerline (also called “suction lift)
- Static discharge head (hd): the vertical distance between the pump centerline and the surface of the liquid in the destination tank.
2. Friction head (hf)
This is the loss needed to overcome that is caused by the resistance to flow in the pipe and fittings. It is dependent on size, condition and type of pipe, number and type of pipe fittings, flow rate, and nature of the liquid. The friction head is proportional to the square of the flow rate as shown in figure 3. A closed loop circulating system only exhibits friction head (i.e. not static head).
Electrical Energy Equipment: Pumps and Pumping Systems
In most cases the total head of a system is a combination of static head and friction head as shown in Figures 4a and 4b.
