Understanding Pump Flow Rate vs. Pressure and Why It Matters
by Adam Berquist, Senior Engineer, on Jun 8, 2021
If you’re a pump sprayer operator and you’re performing a soft wash application on a two-story residential home to remove lichen or mold, how do you get the spray pattern to reach the eaves of the home and provide even coverage without having to climb a ladder?
Should you increase pressure or increase flow to achieve a higher spray pattern?
There’s a common misconception that to achieve a broader vertical or horizontal spray pattern, you simply need a pump that puts out more pressure. On the contrary, increasing the flow rate is often the key.
The Difference Between Pump Flow vs. Pressure
It’s not uncommon to hear pump operators complain that their sprayer doesn’t have enough pressure when, in fact, the issue is the flow rate. In fact, some people use the two terms interchangeably, as though they’re the same thing. They’re not, and knowing the difference and the role each plays is the key to achieving proper pump performance.
A pump’s job is not to deliver pressure; rather, it is to deliver a rate of flow, pumping a certain amount of liquid over a given amount of time from a tank or reservoir to the outlet. Flow rates are often referred to in gallons per minute or GPM. There are some smaller pumps that rate flow at gallons per hour or even gallons per day, outputting extremely small amounts of fluid over a given time.
Pump pressure, however, is a measure of resistance to flow. Without flow, there is no pressure. In a positive displacement pump, such as a plunger pump, the rating in pounds per square inch, or PSI, outlines how much resistance the pump is designed to withstand.
A pump’s PSI rating is important because it indicates that the pump was manufactured out of materials and designed to handle a certain amount of pressure. But for pump operators, they should be equally concerned with a pump’s flow rate which determines how much you want to dispense, spray, or inject.
Will Increasing Pump Pressure Increase Flow?
In general, when pump pressure increases, flow will decrease. Take, for example, a misting pump that needs to produce an ultra-fine mist for cooling or dust suppression. Many misting pumps are rated at 1,000 PSI, yet their flow rate is quite low at .25 GPM.
Clearly, high pressure in this instance is very important to produce the right size droplets. However, it doesn’t indicate more output or vertical or horizontal throw. Instead, the higher pressure combined with the right nozzles results in a fine mist with very little flow that might only cover a small patio area.
On the other hand, a soft wash sprayer can be rated at a mere 100 PSI and have a GPM of 5.4, yet it can achieve a vertical throw of nearly 40 feet and a horizontal throw of more than 50 feet when paired with the proper nozzles.
More pressure changes the velocity of the fluid, but it also decreases the flow or output. The cause of the flow decrease is due to two factors: volumetric efficiency of the pump and reduced motor speed. Volumetric efficiency is a measure of the actual flow compared to expected theoretical (calculated) flow — volumetric efficiency decreases as pressure increases. Our positive displacement plunger pumps have about 90–100% volumetric efficiency compared to centrifugal pumps that range from 0–100%. This means that plunger pumps only lose about 10% of the flow when pumping against back-pressure, while centrifugal pumps will lose all the flow when pressure climbs too high.
Reduced motor speed occurs when motors are loaded heavier. So, when pressure in the pump causes more load on the motor, it slows down. When the motor slows down, the flow rate drops at the same percentage. A motor that operates at about 2000 RPM at low pressures will typically slow down to about 1750 RPM when the pump is pressurized to the maximum rating.
It stands to reason, then, that increasing pump pressure will not increase flow. In the soft wash example, more pressure won’t help the operator reach the eaves of a two-story home with the same amount of coverage. The operator needs a pump motor with an ideal combination of pressure and flow.
How to Achieve Proper Flow and Pressure
Engineering pumps for any application requires an understanding of fluid dynamics, and each industry has varying needs. Too often, a company will choose an off-the-shelf high-pressure pump to get a job done and wonder why it doesn’t perform as expected. It’s likely because the operators lack a full understanding of the relationship between flow and pressure.
Using pump performance charts for various pump models can help identify GPM flow rates, PSI, and how many amps the pump will draw.
The engineering experts at Pumptec have a thorough grasp on fluid dynamics and help OEMs and pump distributors pinpoint their exact needs. They make recommendations based on scientific principles and years of experience serving multiple industries, and can even customize pumps to your application’s precise needs.
In fact, we’ve developed a Guide to GPM and PSI that provides some of those industry recommendations. Take a look through it and then contact the pump experts at Pumptec. We’ll be happy to discuss your needs and determine the right pump for your application.