In today’s post, we’re going to look at one of the terms that gets thrown around a lot that can make newcomers eye’s pop out of their head- pump head. We use the term all the time- but what is it, why is it measured in feet, and how is it different than pressure?
Everyone knows what pressure is. You look at a gauge and it reads 35 PSI. We all think we know what that means. But when a Triplex Sales Engineer asks you what your head pressure is, you struggle to answer. To begin understanding this concept, stop for a second imagine you have a pump connected to a water discharge line. Disconnect it from the discharge line and pipe the outlet straight up into the air. The “head” is the height to which a pump can raise the water up.
OK so far? Sure. But how is head linked to pressure? We’ll get to that next. First, let’s talk about what more “head” means. Simply put, it means raising that fluid column higher, i.e. putting out more head pressure.
So that’s all that head is? How high we can pump a fluid? Not quite. Imagine for instance, your pump is located below a tank that has a fluid level 10 feet above the inlet to the pump. In this case, you’re getting a tremendous amount of help from gravity. The converse is also true- if the fluid is located below the inlet to the pump and we want to pump it ten feet high, not only do we need to overcome the 10 foot discharge header, but we also need to overcome the gravity holding the fluid down at the inlet side of the pump. Both of these help illustrate the concept of “total” head. The formula for which is:
Total Head = Static Head + Static Lift + Friction Loss
Where does friction come from, you may ask. Well, not only are we fighting gravity, we’re also fighting any resistance from the container the fluid is flowing through. In other words, friction.
So why doesn’t my pressure gauge read in feet? Well, it can. More commonly, we see this in mmHg, or millimeters of Mercury, generally on compound vacuum gauges. But really, the relationship between feet of head and pressure is:
Where H is equal to head in feet, p is equal to pressure in PSI, and SG is the specific gravity of the fluid. Specific gravity is a fluid’s weight relative to water. This makes sense- if a fluid was light a pump could lift it higher with less “work”. If a fluid was heavier, it would take more “work” to lift it just as high. So when we’re pumping water and we see the gauge reading 60 PSI, this really means that if we took that energy and directed it straight up into the air, it would go 138.6 feet high!
So the next time you’re talking to a Triplex Sales Engineer and they want to know you’re head pressure, don’t be overwhelmed. We just want to know the resistance we need to overcome relative to the weight of the fluid. This is so critical to sizing pumps that we’ll revisit pump head many times in upcoming posts about pump sizing and application.
Until then, if you have any questions about pump head or sizing your next sanitary pump, contact a Triplex Sales Engineer today!