Understanding Pressure Washer Flow vs. Pressure

In searching for a new pressure washer, customers are required to determine the correct pressure and flow for the job they are working on.  Most folks have no idea what they need, the difference between the two, and why it’s important. Let’s take a deeper look into what you need and why you need it. 

Flow is measured by Gallon per Minute (GPM), which is a measure of the amount of water being pumped out.  Pressure is measured in Pounds per Square Inch (PSI), which is simply the force being produced by the water.  So, do you need pressure, flow, or maybe a little of both?  To understand the difference, let’s look at an example of extreme pressure and then extreme flow.

When I was a child, my dad worked at a factory that made youth football uniforms. These were the kind you’d open on Christmas morning, so you could dress up like the star player of your favorite team. They would stack fabric up in batches about a foot thick.  The next step was to make the patterns with a water jet cutter.  The water jet could cut thousands of pieces at a time.  The cutter used extreme pressure with a very low flow.  Water Jets can operate at 100,000 psi with as little as half a gallon per minute.  The cuts were very clean, extremely precise, and reduced waste.  Because there was so little water flow, the fabric was nearly completely dry after the jet made its cuts.  Water jets are capable of cutting granite up to 6 inches thick, bullet proof glass up to 4 inches thick, and steel up to 6 inches thick.  There are even water jets used in coal mining that cut up to 100ft of coal. These machines use very fine tipped nozzles, making the water stream typically only 1 millimeter in diameter. The more pressure that’s supplied, the more cutting force you’ll have.

Back in my Army days, I served on an Abrams tank.  During our field exercises, tons of sticky clay-like mud caked to every surface of the tanks.  Sometimes, it would dry into a concrete-like substance.  It was at our wash rack that I was introduced to a water cannon.  From there, the tank would drive down into a pit where there were stands of amiable guns of water.  These guns looked a lot like the nozzle one would see on top of firetrucks.  These water cannons wielded approximately 60 gallons per minute of a very wide stream of water, but at a much lower pressure of only about 100 psi.  The massive water flow worked well to move the mounds of mud off the tanks and into a collection pit.  Water cannons can also be used by police during riot control.  Water cannons can push up to 320 gallons per minute. Mining operations often use water cannons to move huge rocks and chunks of sediment.  So, for these purposes, Flow = ability to move material. 

Pressure washers can be purchased well between these extremes.  Gallon flow for a pressure washer will generally vary from 1 to 10 gallons per minute and 1000 – 5000 psi.  Choosing the correct flow and pressure depends on your application.  If you are moving a lot of material, say, off construction or agricultural equipment, there is a need for gallon flows over 5 gallons per minute.  If you need to scrape a material clean, such as gum off concrete or algae off brick, you’d want to look for higher pressures of 4,000 – 5,000.  In an application where you don’t need a lot of flow and are concerned with damaging a delicate surface like a car, a machine that operates around 2-3 gallons per minute at 1,000 – 1,500 psi would work perfectly.  This is identical to the way a typical coin operated car wash will work.

Water blasters / Water Jetters / Ultra High-pressure washers fill a different role.  These machines generally range in flow from 3 to 7 gpm and pressures range from 10,0000 to around 36,000 psi.  These are used to remove paint stripes from parking lots, strip barnacles off ship hulls, remove bark from lumber, or to even strip paint.  

Still confused?  Give me a call and I'll be more than happy to discuss your situation and make a recommendation.

-Michael Fields