Pressure Loss and Its Effect on Airless Spray Patterns
If you’re experiencing poor spray patterns and can’t figure out where the problem is, one often overlooked cause of the problem might be pressure loss through the hose. There are many factors, which contribute to this loss. Here are some of those key factors: and the correct method for checking pressure loss between your Graco airless sprayer and the spray gun.
The bigger the diameter of the hose, the smaller the pressure loss. For airless spraying, 1/4” ID hose is the most common. The next hose size is 3/8” diameter. 3/8” ID hose is substantially more expensive and heavier than 1/4” ID hose. But the pressure drop in a 3/8” hose is substantially less than in 1/4”. Consider the benefit versus cost when choosing hose diameter. Below is a chart showing the pressure drop difference between 1/4” ID and 3/8” ID hose at 2,000 psi.
Flow Rate/Tip Size
When there is no flow, there is no pressure drop. This is a static situation. With no flow, the pressure in the system is the same at every point. Different size tips result in different flow rates – the larger the tip, the greater the flow rate. With material flowing through the hose, the system is now in a dynamic situation. The greater the flow rate, the larger the pressure drop. The chart shows how pressure drop increases as the tip size is increased.
Tip Size Flow Rate* 1/4” Hose** 3/8” Hose*
.015 .20 gpm 125 psi 45 psi
.017 .27 gpm 140 psi 50 psi
.019 .34 gpm 160 psi 57 psi
.021 .41 gpm 182 psi 65 psi
.023 .48 gpm 204 psi 73 psi
.027 .67 gpm 248 psi 90 psi
*gpm at 2,000 psi ** Pressure drop was tested using 50 ft of hose with latex paint S.G. 1.36
Understanding the Chart
If you’re spraying with a .023 tip, the sprayer is maintaining 2,000 psi at the pump. With 100 feet of 1/4” hose, the pressure drop will be 2 X 204 or 408 psi. The pressure at the gun will be 1,592 psi. You probably will notice tails in the spray pattern at this pressure. If you use 3/8” hose, the pressure drop will only be 2 X 73 or 146 psi. The resulting pressure will be 1,854 psi, enough pressure to atomize most latex paints.
The thickness and viscosity of the material is also a major contributor to pressure drop in hoses. The pressure drop with stain is less than with block filler. Temperatures also play an important role in material thickness and viscosity. Just like motor oil, paint will become thicker and more viscous as the temperature is lowered. The pressure drop in a hose will be greater at 50oF than at 90oF.
With all other variables being equal, the pressure drop with a .023 tip from the chart below is 204 psi in 50 feet of 1/4” hose. It follows then that the pressure drop in 100 feet will double to 408 psi and will be 612 psi ( 204 psi X 3 ) with 150 feet of hose. So, if you have 2,000 psi at the pump, the pressure at the gun using 150 feet of 1/4” hose while using a .023 tip will be 1,388 psi.
Contrary to what most people think, elevation does not have a significant effect on pressure. For average latex paint, the pressure drop due to a 100 ft. rise in elevation is only 60 psi.
So, if you are having trouble atomizing the coating you are spraying, the problem may be pressure drop. Understanding the possible causes of the problem is important to being able to solve it. A fluid filled pressure gauge rated at 4,000 psi minimum is essential in tracking down pressure related problems.
Connect the gauge at the pump outlet and measure the pressure while spraying or in the dynamic situation. Then install the gauge at the inlet to the gun and again measure the pressure in the spraying or dynamic situation. The difference in pressure is the pressure drop. If the pressure drop is significant, look at what you can do from the list below to reduce this difference. If the difference is small, but the pressure at the gun is too low to atomize paint, you have a different problem which could be a worn tip, a tip that is too large, worn pump parts, or a clogged filter in the gun and/or pump.
By Tim Whelan
Originally Published in the November/December issue of ‘The Scouting Report’ an ALLPRO Publication.