Is It OK To Operate 2 Spray Foam Guns Off One Proportioner?

By Jake Jacobson and Bob Jutras
Intech Equipment & Supply

The spray foam insulation market experienced impressive growth during the housing boom of the mid 2000’s.   With that growth came more awareness of the benefits of spray foam.  Contractors diversified their businesses by adding spray foam to their offering.   Home Builders started offering spray foam as an option for insulation.  In fact, some home builders are using spray foam exclusively for insulation in the house.  Business has become increasingly competitive, as contractors are competing for the same business.  Doing business with a home builder leaves very little margin for error, as margins are thinner and the time window to complete projects is much tighter.  Getting the jobs done quickly without mistakes is critical to maintain profitability with any spray foam job.  In order to increase productivity, many spray foam contractors have implemented dual proportioners in their mobile rigs.   The system consists of a supply system that feeds two heated proportioners, two sets of fluid heaters, two sets of heated hose, and two spray guns.   There are systems out in the field that are trying to run two spray guns off of one proportioner.  Is this a smart practice?

Before I get into the technical aspects of this, I am going to try to paint a picture for you in an attempt to have you visualize how fluid flows.   I want you to think about a smooth meandering river perfect for a canoe.  We are going to assume that the width and depth of the river remains constant.   Therefore the total volume of water and the speed of the river is the same assuming the width and depth of the river remains constant.   Will the volume of the water in the river change?   No, it remains constant, unless of course it rains.   For the sake of this example, there will be no rain and no evaporation.  If the river gets narrower, what happens to the velocity or speed?   It goes up, the flow rate is higher.   The volume of the water is the same, but the speed of the water must increase in order to pass the same amount of water.   What happens if the river gets wider?   The speed of the water slows down, the flow rate is lower.   Again, the total volume of the water is the same.  The same concept applies to spray foam proportioning systems.   The volume of the A and B fluid sections are the same using identical pumps for A and B, and also using fluid hoses of the same length and inside diameter.  If there is a restriction on one side, the pressure will show higher on the side with the restriction.

Let’s go back to our meandering river.  Suddenly, we come to a fork in the river.   Will the volume of the water going to the right, #1 fork and the left, #2 fork be equal? Probably not.  Why?  Fluids will choose the path of least resistance, so the side of the river that has the least resistance will have more volumetric flow.

Now, let’s put a dam 300 feet down the right 1 fork of the river.   What happens to the flow of water on the left 2 fork of the river?   The volume of water goes up on the left 2 side, because the water has to go somewhere and when the right 1 fork is dammed or restricted, the water takes the path of least resistance and starts diverting down the left 2 fork.  In fact if the dam stops the flow of water completely, the quantity of water flowing on left 2 fork will theoretically double.

Now, switch gears back to our spray foam equipment.  Think about running two spray guns off one pump, and spray gun 1 stops spraying.   Does that impact the flow and pressure at spray gun 2?

You bet it does.  If both guns were working perfectly with no restrictions, you would notice greater pressure at gun 2.  The flow rate at spray gun 2 will be restricted by the tip size in the gun, but there will be increased pressure, which will increase the flow rate

If there is a restriction in spray gun 1 on the B side, will you notice a difference in pressure at the proportioner? Yes, you will.   The B side gauge will read higher than normal.

Is there any way to tell whether the restriction is in gun 1 or gun 2? Not with both guns spraying at the same time.  You would be able to see it if you sprayed with only one gun at a time. The gun with no restriction would display normal pressures when it is spraying and the gun with the restriction would display higher than normal pressure on the B gauge when it was spraying. The applicator spraying foam should see a difference in the foam pattern, consistency, and quality of the foam.

If spray gun 1 stops spraying, does that magnify any potential errors on spray gun 2?   Yes, it does.

So, is it possible to spray foam using 2 guns connected to one proportioner? Theoretically, it is possible, but everything has to be absolutely perfect.  Ask most any applicator and they will tell you the most common problems they deal with is a dirty gun. If one gun has a restriction, it is going to affect the ratio of both guns.  One will be lacking “A” and the other will be lacking “B”.   The magnitude of the restriction determines how much you will be off ratio.

We are going to get a little technical now, so hang in there; we will try to make it easy to understand.  The flow rate of water through a .029” orifice at 2000 psi is 0.9 GPM.  The flow rate of water through a .025” orifice at 2000 psi is 0.67 GPM.  Decreasing the orifice size by a mere 0.004”, about the thickness of a human hair will decrease the flow rate by a little over 25%.  This small restriction will definitely put you in a situation where you will be spraying off ratio material when both guns are spraying. If only one gun was spraying at a time, you would be on ratio with both guns.

And why would the guns stay on ratio when only one gun was spraying and be off ratio when both were spraying? Without the restriction, all of the fluid streams would be flowing through the same size .029” orifice. When both guns were spraying, the pressure at both ports on the   A side would be equal and the pressure at the B side ports would also be equal.   When you restrict just one of the orifices, let’s say on the A side, the pressure on the A side is going to be higher than normal because one fluid stream is being directed through a .025” orifice and one stream through a .029” orifice. The pressure will be pretty close to the same at both orifices, but the larger orifice is going to allow more material to flow through than the smaller orifice. What happens when just the gun with the restriction is spraying? The proportioner dispenses equal volumes of A and B components each time it completes one cycle. That material has to go somewhere, and barring any leaks on the high pressure side of the system, the material is going to be forced through the .025” orifice on the A side and through a .029” orifice on the B side. Since the A side port is restricted by .004”, it is going to take more pressure than normal for that same volume of material to be forced through it.  If you look at the graph below, the horizontal axis is pressure and the vertical axis is gallons per minute of water. The blue line represents the .025 orifice and the red line represents the .029 orifice. If you follow the horizontal line across from the .6 GPM mark, you will see that both the red and the blue line cross that horizontal line. Now follow the horizontal axis and draw an imaginary vertical line where it intersects with the .6 GPM line and the red line and you can see that it requires about 1300 PSI of force to push .6 GPM of water through a .029 orifice. If you do the same thing for the blue line, you will see that to push .6 GPM through a .025 orifice, it would require about 1750 PSI. The .025 orifice is going to require about 450 PSI more force than the .029 orifice to get equal flow.  In the graph below, the material used was water, and it is important to note that regardless of material type, you will see similar relationships between pressure and flow.  For this example, what you would notice is that the pressures at the proportioner would be normal when the gun with no restriction was spraying.  When the gun with the restriction was spraying, the pressure on the A side would be about 450 PSI higher than normal, but still on ratio. When both guns are spraying, the A pressure will be around 1525 PSI. This would result in a flow rate that exceeds the .6 GPM of the .029 orifice and a flow rate that is under the .6 GPM of the .025 orifice. One gun would be A rich and the other gun would be B rich! And as we say in the foam business, that is “Muy Malo” or very bad!

I guess what it all comes down to is how much risk are you willing to take?  There are many variables that come into play when spraying plural component materials such as polyurethane foam.  Dirty gun screens, partially obstructed impingement ports, differing hose temperatures, a restriction in one of the foam hoses just to name a few.  When a contractor brings their rig in for service, the most common issue is a dirty gun.  It is usually a restriction of some sort on the A side, and usually in the gun.  Repairs resulting from restrictions in a spray foam gun accounts for greater than 80% of the repairs we do.  The more variables you can control and maintain the better off and more productive you are going to be.  Ask yourself this question.  Is your material supplier(s) going to give you their blessing using this type of equipment to spray their product?

In summary, you loose control of your process when you spray with two guns running off of a single proportioner.   The downside risks far outweigh the short term monetary savings of buying only one proportioning pump.

We recently polled some highly respected industry experts on the subject of running 2 spray foam guns off of one proportioner.  Please see quotes listed below:

As a general rule we want to see 125 degrees and 1,000 psi at the gun, not just at the proportioner.  During cold weather there can be significant processing problems if the equipment setup has too much hose and too large a tip size.  In our classes we find that many applicators do not understand the effect of product temperature on the amount of pressure lost between the proportioner and the gun.     Thinking about splitting the energy into 2 lines then brings up questions about how to keep the pressure balanced between the 2 guns and still sufficiently high for proper mixing, and the applicator must also ensure that the material is in the heater long enough to get the delta T needed.”
Bob Creighton
Key Accounts & Warranty Program Manager
Bayer MaterialScience

“You are absolutely correct and this is not something we would recommend.  I know of instances with that configuration in the past that caused too many problems for us to consider.  As you referenced, and one of the primary reasons, is that the delta T rating on those machines is not great enough to handle 2 guns on a regular basis especially at the temperatures at which almost all contractors store their product.”
Dennis Meissner
Bayer MaterialScience

“It is not possible to run two impingement mix type applicators from one proportioning source at the same time.  If gun “A” were to restrict the ISO then gun “B” would be at near 2:1 ratio.  Also if gun “A” were on prior to gun “B” then the material would simply follow the path of least resistance and hardly flow to the second gun triggered.  You could conceivably run two guns as long as only one was triggered at any one given time, provided you figured out how to heat the two lines.”
Barry Taft
Technical Assistance
Graco, Inc.

“Running two guns simultaneously off of one proportioner is a bad idea.  Off ratio conditions generally start off as gun problems, usually on the iso side.  Typical two gun problems start like this:  Gun 1 develops a restriction on the iso side, the higher pressure transfers to Gun 2.  Gun 1 sprays resin rich, Gun 2 sprays iso rich and nobody knows there is a problem because the cross-talk with the guns masks any pressure imbalance.When checking the gauges on the system, they will look balanced.  The only time its “safe” to use 2 guns on a proportioner is to NEVER use them at the same time. “
Mark Weinberger

Engineering Manager
Graco, Inc.

“Besides the obvious needs of adequate heat and pressure, providing those needs are met and no other problems exist, it works just fine. Then comes a problem, for example, let’s say gun #1 has a restriction on the “B” side causing high pressure on the B side or a lack of B on gun #1.  The pressure differential seen by gun #2 is just the opposite as is the lacking component. At gun #1 you see A rich product and gun #2 you see B rich product, WHERE DO YOU START TO LOOK FOR THE PROBLEM. Starvation issues should be easier to ID. Worse case may be a circumstance where there is just enough of a pressure imbalance to cause the product to behave differently at each gun, resulting in a “let’s call the material manufacture” response or just simply a lot of head scratching and a “go on with show attitude. It takes someone who is tuned up to ID and solve the imbalances issue. Conclusion, it would not be something I would recommend”.
Jerry Whitaker  
Former Owner – UCSC

“We have not recommended this practice for many reasons:
1.  In case of an equipment problem causing off pressure or off ratio, 2 guns are crossed over not one.
2.  Heaters and proportioner are not designed to handle 2 guns, so pressures and heat will be hard to maintain. 
3.  Hose length would be reduced to half of the equipment’s normal capacity.
4.  When one gun is shut down, it will affect the pressure gauges.
5.  Makes it difficult to troubleshoot using the pressure gauges.”

Mason Knowles
Mason Knowles Consulting

About Intech Equipment & Supply
Intech Equipment & Supply, founded in 1995, is an independent, full service spray foam equipment integrator and parts supplier, offering equipment, parts, turn-key mobile spray systems and expert technical assistance.  Their main customers are commercial roofing contractors, insulation contractors, and the materials suppliers that serve those customers. . Typical markets served are spray foam roofing, spray foam insulation, single ply roofing, and commercial roof coatings applications.   Intech Equipment & Supply has been recognized by Graco as a Top 20 Graco HPCF North America Distributor for 3 years in a row 2010, 2011, and 2012.   For more information, visit