Pump Plumbing Head Curves and Operating Points

Pump Plumbing Head Curves and Operating Points

Before reading this article, you may want to first familiarize yourself with Pump Basics.

Hydraulic Head

In fluid dynamics, head is a concept that relates the energy in an incompressible fluid to the height of an equivalent static column of that fluid. From Bernoulli's Principle, the total energy at a given point in a fluid is the energy associated with the movement of the fluid, plus energy from pressure in the fluid, plus energy from the height of the fluid relative to an arbitrary datum. Head is expressed in units of height such as meters or feet. - Wikipedia

Hydraulic Head is a term that originated in the water distribution industry and relates to the "head" or top of the water level. It is also another way of expressing the amount of pressure (PSI=pounds per square inch) lost or gained in a plumbing system. For example, a pool pump adds head or pressure to the plumbing system while the friction loss in piping and equipment removes head or decreases pressure in system. In addition, there are two basic types of head loss and gain in any plumbing system; Static Head and Dynamic Head.

Static head is the net elevation change of the water which can be either positive or negative and is directly related to the height of the water. So water moving from a higher elevation to a lower elevation has head gain while water moving from a lower elevation to a higher elevation has head loss. For example, a pool solar system installed on a roof and where the panels/pipes are primed and completely filled with water, there is static head loss when the water rises to the roof but then there is also static head gain when the water falls back to the ground. The static head loss on the way up is directly offset by the static head gain on the way down so there is no net static head change due to the fact that the solar is installed on the roof. However, there can be temporary static head loss or gain during the priming process where there is head loss due to the elevation lift but since the return pipe is not yet full of water, there is no static head gain to offset it.

The second component of head is dynamic head loss which is due to the friction loss of water inside of pipes, fittings and other equipment. As water travels through a pipe, the friction against the internal structures reduces pressure. A pool's plumbing system will experience dynamic head loss on both the suction side of the pump and return side of the pump since water is moving through plumbing on both sides. The faster water moves through a pipe, the more head loss there will be. A pool pump adds dynamic head gain to the plumbing system so as to create positive pressure and thus water flow through the pipes. The dynamic head loss in the pipes then reduces the pressure until the water returns to the pool where the pressure is once again at 0 PSIr (PSI relative to atmospheric pressure).

A common mistake made by some that are new to hydraulics is to assume that the head loss in plumbing is equal to the length of pipe since they both use the same units. While the dynamic head loss in plumbing is related to the length of pipe, it does not equal it. Addition factors are necessary to calculate the head loss in plumbing and can be quite challenging to accomplish.

In summary, pumps and elevation drops add head to a plumbing system while elevation rises, pipes, fittings, valves, filter, heaters, etc. subtract head from a system. For a swimming pool, the total head loss is always equal to the pump's head gain since the water is returned to the same atmospheric pressure as where it came from.

Pump Head Curves

A residential pool pump is actually made up of two separate "machines". The first machine is often called the "wet end" and this is the part that actually pumps the water and converts rotational energy into flow and pressure. The second machine is the electric motor which converts electrical energy into rotational energy and drives the wet end. Together, these two machines make up a residential pool pump.

Most manufactures of pumps include head curves in their pump manuals and/or sell sheets. The curves have two axes, Feet of Head vs. Gallons per Minute (GPM). Feet of head is used instead of PSI because a centrifugal pump will always deliver the same head for any liquid of any density while the PSI will be different for different liquids and/or densities so traditionally, manufactures have used head instead of PSI.

The head curve is useful in determining the flow rate of the pump if the head loss of the plumbing system is known. Below is an example of a head curve for the Pentair Intelliflo published by Pentair:

The maximum head of the pump is reached when the flow rate is near zero GPM and at full RPM. This is equivalent to an equal length of open ended pipe straight up in the air. The pump will fill the pipe to the top but no water will spill out.

This particular chart also shows that the energy use of a pump is fairly linear with the flow rate. This is true for all residential pool pumps. The higher the flow rate of the pump, the more energy that is used. However, flow rates tend to increase faster than the energy use which is why the energy factor of the pump as measured as gallons per Watt-hr, increases with increasing flow rate effectively reducing the energy consumption. However, there is a flow rate limit that a pump can produce which is called "run out" but most pool installations will never reach this limit because there is always at least some plumbing head loss.