When sizing a pump, there is a lot of verbiage thrown around that can be confusing and hard to understand. At Holland, we have been sizing and putting pumps into sanitary and high purity applications for over 60 years. In that time, we’ve found the best success comes when our customers are knowledgeable about their applications, can provide good information, and can “fact check” our sales engineers. One of the most important factors in sizing a pump is figuring out your total head requirements. Head is not quite the same thing as a pressure. The differences are nuanced and this post will aim to explain.
To begin, what is head? Assume that you have a pump designed to move water 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 water 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 water is greater than the potential energy of a five foot column of water. 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).
So how can a pump manufacturer tell you how much help their pump will produce if they don’t know the suction conditions? It’s pretty simple, actually. They subtract the head available at the suction for them head produced at the discharge. This is called total head. By doing this, it doesn’t matter what the suction tank level is, we know what the pump can do regardless of the suction conditions.
So what is the difference between pressure and head then? While some pump manufacturers that use pressure (differential pressure, that is) to characterize their pumps, this is not the most “accurate” way to do so. Head is useful because it evaluates a pump’s capacity to do a job. Most pump applications involve moving fluid to a higher level. If you have to pump a liquid up 30 feet and your pump doesn’t have at least 30 feet of head, then there is no chance it will work. Your pump will need at least 30 ft. plus the friction loss to get the required flow at the discharge point.
Another nice thing about head is it is independent of the type of fluid being pumped (assuming the viscosity is relatively low and similar to water). Whether you’re pumping water or a heavy caustic solution, the head achieved will be the same. The pressure at the discharge of the pump, however, will be higher for the heavier solution. The relationship between head and pressure can be characterized by the following formula:
Where p= pressure (psi)
And SG= Specific gravity of fluid being pump
As this formula shows, by using head, we are able to take into account the density or weight of a product and accurately figure out what are discharge head requirements are. We would not be able to do this by simply looking at a pressure gauge.
At Holland, when we size pumps, we try to identify a pump that has a total discharge head at the flow rate the customer requires. It is not difficult to establish this and we have proprietary software that helps us to calculate friction loss. Friction loss depends on the length and diameters of the pipe through which the fluid flows. A simple discussion will help us figure the main component of total head/pressure- static head- and calculate the other sources of energy loss.
Hopefully this post sheds some light on the nuanced difference between head and pressure. If you need help with your next pump application or have additional questions of hydrostatic fluid head, contact a Holland Sales Engineer today.