Many of the regular and custom hydraulic power unit inquiries we take start off with a simple question – “I need a hydraulic power unit that will provide X gallons per minute at X psi”. In some cases the customer knows about where they will be with respect to engine size / horsepower, in others they have no idea. So how do you determine how much horsepower you need to deliver a specific flow and pressure? There is a pretty straightforward formula that you can use to answer this question.
Let’s walk through an example. A customer is looking for an HPU that will deliver 15 GPM @ 1800 PSI. The formula below is where we start:
The only thing that is helpful to know ahead of time is the efficiency. Typically at Hydra-Tech Pumps, we will use 85% efficiency for gear and vane pumps, and we use 90% efficiency for variable volume piston pumps. So we will calculate it based on using a gear pump – (15 GPM x 1800 PSI) / (1714 x 85% efficiency) = 27,000 / 1456.9 = 18.53 horsepower required. So now we know that we need a minimum of 18.53 horsepower to produce 15 GPM @ 1800 PSI.
The one thing to keep in mind is that this calculation doesn’t include any kind of parasitic load on the engine like running and alternator or cooling fan – you always want to round up on horsepower when choosing the engine / motor or you might not be able to get to your flow an pressure requirement. In the case of our example above, we would end up using a 23 horsepower (gas or diesel) engine or a 20 horsepower electric motor.
Another thing to keep in mind is this – you can move flow and pressure up and down without changing the horsepower requirement, but you can’t move both up. Below are 3 different requirements (with our example from above in the middle) to illustrate this point – all three can be achieved with the same 18.5 horsepower.
6 GPM @ 4500 PSI
15 GPM @ 1800 PSI
30 GPM @ 900 PSI
Now if you took the greatest flow and pressure requirement from the examples above, 30 GPM @ 4500 PSI, you would need a 100 horsepower unit. We are always here to do the math with you, or for you – let Hydra-Tech Pumps know how we can help you with your next hydraulic power unit requirement!