# eli5: What is hydraulic head?

323 views

I’m adding mechanical pumps to a large stormwater treatment system. I’m a scientist but I’m completely new to engineering, and I cannot seem to get my mind around the concept of “hydraulic head.” This means that I am utterly incapable of understanding pump curves. Can someone please explain what hydraulic head is, and why it falls as flow from a pump increases?

In: Engineering

Hydraulic head is kind of like the potential energy of a fluid.

This is usually measured in vertical feet/other height unit, but could also be measured with pressure.

But it’s basically, if you had water somewhere high, like a pond, and then in flows 10 feet down a hill into a second pond, it’s lost 10 feet of hydraulic head, or potential energy.

This is important in things like water towers that feed towns, because it is the hydraulic head of all the water stored way up on the air, basically the pressure of that water so high up, that pushed the water through the water system pipes and to your house.

A pump typically adds head, because it is putting energy into the system.

A turbine typically loses head, because it is taking energy out of the system (like in hydroelectric power, turning the head into electrical energy)

So, if a pump is saying it can add 10 feet of hydraulic head, that means it can (ideally) pump water 10 feet up.

This is obviously in an ideal system, the friction of the pipes on the water, along with bends and turns in the pipe will slow the water down and pull energy out of it, I can’t tell you exactly how much you’ll lose with each turn, though I’m sure there’s tables for it somewhere, but typically the harsher the angle, The more head you’ll lose.

Head is a way of measuring water pressure. Basically it means if you attached the pump to a hose and attached the other end of the hose to a really high tower, how high could the pump pump the water?

> why it falls as flow from a pump increases?

It means that if you only want to pump a tiny trickle of water, the pump can push that trickle really hard. If you want a torrent, the pump can’t push as hard.

Think of it this way: Suppose you’re building a skyscraper and your pump’s on the ground level. The same pump could run a firehose on the second floor, or a drinking fountain on the twentieth floor.

Now it’s crystal clear. Thank you for the excellent explanations. Cheers!