Do You Know The Difference?

In the hydraulics community we can often hear people talking about pressure and flow as if they are interchangeable, or using them as synonyms.?

Unfortunately, things aren’t this simple and misunderstanding the differences between pressure and flow can stop us from progressing in our careers from maintenance (changing and maintaining parts) to being able to troubleshoot and design effective solutions to the problems we face.?

So today I wanted to give you a simple run through of the difference between flow and pressure so that we can start to understand why systems can still not work efficiently when we replace certain parts.?

What is flow??

The flow of a fluid describes the movement of that fluid as it, for example, passes through a pipe or runs along a surface. It has two components: flow rate and flow velocity.

Flow rate is the movement of a specific volume of fluid in a set amount of time. Flow rate is typically measured in litres/minute (lpm) using a flow metre.

An understanding of the different ways in which liquid flows – and what causes these differences – is absolutely essential to hydraulic system design.

Flow can be characterised in a number of ways, for example: steady or unsteady flow; laminar or turbulent flow.

Laminar flow: People usually think that fluid flows as one single mass, but that is not the case in reality. At low speeds, fluid flows in distinct, parallel layers – each of these moves at a slightly different rate. This phenomenon is known as laminar flow.

Turbulent flow: As the velocity of a fluid increases, tiny imperfections in the surface of the hose, tube or pipe, disturb the flow path. This creates a chaotic state disrupting the normally organised layers of laminar flow.

In a hydraulic system, turbulent flow can be found anywhere where bends and restrictions occur. Larger hoses and fittings can therefore help to reduce this effect. Important to note, the friction of this turbulent flow also leads to an increase in heat.

We measure and explain this flow phenomenon using a statistic called the Reynolds number. This equation was developed by physicist George Gabriel Stokes and named after the nineteenth century scientist Osborne Reynolds. A low Reynolds number indicates a smooth, laminar flow and a high Reynolds number usually indicates a flow that is more turbulent.

Now that we’ve covered the basics of flow and how it describes the rate at which fluid moves and the distinct way that it moves let’s talk a bit more about pressure.?

What is pressure??

In a hydraulic system, pressure is usually created by restricting the flow of the fluid, or by gravity.

As fluid flows through this oddly shaped section of pipe, Bernoulli’s Principle predicts that the pressure will fluctuate as the pipe’s diameter changes. Once we understand this, we can see how pressure is used in a hydraulic system to create the force that drives the system in the way it is designed to do.

So now you might be able to see that low flow doesn’t necessarily indicate that a pump or mechanical part is failing and similarly there is a lot of maths and physics involved in making sure that you have consistent pressure in a hydraulics system.?

If you would like to learn more about measuring pressure in a hydraulics system check out our training on that HERE.?

Or if you would like a quick handy tool that helps you keep on top of all your hydraulics check out our free ebook HERE.