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Calculating Pump Flow Rates and Efficiency

Replacing/repairing suspect components is one of the most financially significant decisions you'll make as a hydraulic troubleshooter.

But how do you decide whether a pump is operating correctly?
This lesson will walk you through the process used to determine a pump's theoretical flow rate.
You'll also learn how to use the pump's actual flow rate to calculate its volumetric efficiency.
There will be math. Oh yes, there will be math.

Overview

There are three types of efficiencies used when talking about a hydraulic pump:

  • Mechanical
  • Volumetric
  • Overall

This lesson will focus on volumetric efficiency: what it is, and how it is determined.

The volumetric efficiency of a pump is one of the most important considerations in the decision to change out a pump or not.

The examples in this lesson will use a positive, fixed-displacement pump. (The pump moves a specific volume of oil with each complete revolution of the pump shaft.)

Variable-displacement pumps are a whole other ball of wax!

We will get in to that at another time.

Lastly, this lesson is not a "how-to" guide on testing a pump.

Testing a hydraulic pump, especially a fixed displacement pump, is potentially dangerous and should never be attempted without proper training and oversight with your specific pump.

Calculating the Theoretical Flow Rate

The first step to calculating a pump's theoretical flow rate is to determine how fast the pump is running.

This system uses an electric motor as a prime mover to drive the pump. So we will need to find out how fast the electric motor is running.

Most electric motors have a tag that will give us the speed at which its shaft turns.

We can see here that this electric motor is turning at 1140 RPM (min-1)

Now that we know how fast our pump is spinning, the next thing we need to figure out is the volume of oil delivered with each rotation.

There are multiple ways to determine this.
We are going to use the simplest method!

We need to collect a couple of pieces of information. We need to know the manufacturer of the pump and its model code.

This will help us find the factory data sheet.

This information is normally found directly on the identification tag of the pump itself.

Here we have a pump manufactured by Eaton/Vickers,
with the model code 25VQ21A 1B20.

All we need to do now is come up with the factory data sheet for our particular pump.

This may require some sleuthing as not everyone has their data sheets readily available.

Using the manufacturer name and model code you should be able to get the required information by calling your parts supplier.

Or, failing that, most manufacturers have their factory data sheets available on their website.


Source: EATON VickersĀ® Vane Pumps V Series - Low Noise Vane Pumps 560 November 2011

With the factory data sheet now in hand we are able to look up the oil displacement per revolution of the pump.

We can see here that our pump displaces 4.13 cubic inches (67 cubic centimeters) of oil for every full revolution of the shaft.

Now we come to the math!

With the data we have collected, the theoretical flow rate of our pump can be calculated using this formula.

Flow Rate = Displacement x Shaft Speed

By plugging our numbers into the formula we get:

Flow Rate = 4.13 cu.in./rev x 1140 RPMs

Flow Rate = 4708.2 cu.in. per minute

It is far more common to talk about flow in terms of US gallons per minute (USGPM) rather than cubic inches per minute.

To make this conversion all we have to do is take our initial value and divide it by 231.

4708.2 / 231 = 20.38

This gives us a final value for our theoretical flow rate of 20.38 USGPM

Why 231, you may be wondering?
This is because there are 231 cubic inches in one US gallon.

Alternatively, if we are doing our calculations in metric, it will look like this.

Flow Rate = 67 cc/rev x 1140 RPMs

Flow Rate = 76380 cc per minute

Dividing this by 1000 will give us the result in liters per minute.

We use 1000 because there are 1000 cubic centimeters in a liter.

76380 / 1000 = 76.38 LPM

We have now determined the theoretical flow rate our pump is expected to deliver.

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We hope you enjoyed Calculating Pump Flow Rates and Efficiency

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