30 Gage Metal Duct Pipe can Collapse

[William C Rogers]

My previous post regarding 30ga metal duct pipe was if 30ga would work. Some yes, some no. Searching the Internet there were a couple of possible solutions. One was to leave a gate open all times, one to put plywood rings around the pipe, and one to make a vacuum relief. Since I hadn't dismantled the DC I decided to test all three before I put all this up and had a failure. I measured the thickness of my pipe and it was 0.011 (0.001 under nominal ). My DC is a 3hp Jet 1900. Specs say 1900 CFM, 5629 FPM, 12.5 static pressure @ 8". My test set up was three 5' sections with a wye on the end so I could incorporate my vacuum relief. There were blast gates at the end and before the wye. I used flex hose to connect to the DC. I made three 6" plywood rings and placed them around the pipe center of the joints. My vacuum relief was only about 20% effective with the springs I had on hand. My testing sequence was A) plywood rings, open gate open vacuum, B) plywood rings closed gate, open vacuum, C) plywood rings closed gate, closed vacuum. Then repeat without the rings. The results were with the plywood rings there was no failure. When I tested without rings and the vacuum open and gate closed the pipe collapsed. Not totally, but significantly. It didn't collapse completely as there was some vacuum relief and it did bounce back when the DC gate was open. The first photo is the test set up. The next two photos are with the DC running. The second tries to show the pipe distortion. The third picture shows 1/2 of my wood ring to give an idea of the distortion. I will use this pipe with the plywood rings. The rings work. The vacuum relief is not worth the effort.

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[Rod Sheridan]

Of course the rings work, that's why I suggested them.

I use them as hangers on the pipe for my Oneida cyclone.

Nice test photographs.........Rod.

[Paul Stoops]

I would be reluctant to use the option of leaving a gate open. Chances are you will lose a substantial amount of your suction at any given tool, significantly reducing your ability to collect the dust and debris. It may also be difficult to keep the debris in suspension within the ducts.

Paul, in Auburn WA

[William C Rogers]

Rod, you always give sound advice/good information. I had not found where someone had run any test and wanted to see if the vacuum relief would work. With what springs I had on hand I could only get about 20% opening with the gate closed and the pipe collapsed in this test. Without the vacuum relief I think the pipe would have completely collapsed. Since I will use the plywood rings it's not worth the effort to make the vacuum relief work.
Paul, my system is not as strong as some and I agree leaving a gate open is not an option. Also should I end up with all gates closed my pipe would collapse without the plywood rings.
It was fun running the test. I know first hand how to use 30ga duct with my system. I am going to play a little bit today before I dismantle everything.

[Ole Anderson]

Thanks. It is always nice to see someone run tests and post results.

[Peter Aeschliman]

I love this. It's very common for people to repeat what they've read elsewhere (i.e., never use 30 ga duct) without any personal experience. And then it because this "truth" that grows and is continually repeated.

But you've cut through the BS with a simple and logical test, and took the time to post it. Thanks a lot for doing that!

[William C Rogers]

Update: I ran the system again and measured the pipe. When collapsed it measured 3 1/8 X 7 3/8. In the free state after the test it was 5 1/2 X 6 1/2. When I set up this test I intentionally put all the seam in line. One of my theories was the seam placement would affect the outcome. Today I replaced the center pipe with a new one and installed ithe seam 90 degrees from the seams in the end pipes. The pipe did not collapse in this configuration. I even got brave enought to close the vacuum gate ( all gates closed) and it still did not collapse. There was about a 1/8 change in the pipe diameter. I'll install the seams 90 degrees but I will still use the plywood rings. This is likely why some have been able to use 30ga without problems. I'm done testing, time to install!

[Lee Schierer]

Before reading all the posts, I would have said that placing rings on the outside of a vacuum pipe would have no effect, but based on your tests the rings keep the pipe from going out of round and probably force it back to round. The duct work you buy that comes sort of curled and you have to slip one edge into the fold on the other edge, never ends up perfectly round, which would make it more likely to collapse than it would if the ducts are nearly perfectly round. Any kinks or dents would also weaken the duct.

[Ole Anderson]

I love this. It's very common for people to repeat what they've read elsewhere (i.e., never use 30 ga duct) without any personal experience. And then it because this "truth" that grows and is continually repeated.

But you've cut through the BS with a simple and logical test, and took the time to post it. Thanks a lot for doing that!

Am I understanding you to say that his test implies that 30 gage snap lock is ok to use for dust collection ductwork? I draw a very different conclusion.

[Chris Parks]

Instead of using springs make an adjustable counter balance. Have a shaft running across the duct with a swinging gate fixed to it and outside the duct have an arm fixed to the end of the same shaft. Put a sliding weight on that arm and move as necessary to counter the the suction as needed. I would think it could be tuned to close when a gate at a machine is opened and having it open at other times when the machines are not in use will clear the air of dust which is a plus.

[Frederick Skelly]

I love this. It's very common for people to repeat what they've read elsewhere (i.e., never use 30 ga duct) without any personal experience. And then it because this "truth" that grows and is continually repeated.

But you've cut through the BS with a simple and logical test, and took the time to post it. Thanks a lot for doing that!

+1. Thanks for posting your results!

[Brian W Smith]

Cool deal on the plywood stiffeners......we prefer sheet metal sleeves but hey,whatever works,right?

And put me down in the Ole Anderson corner....with a reservation,I don't even like 26 G,haha.If it can't be spot welded with one of our Mig setups(google that,they ARE available....Miller may have discontinued it,but Lincoln still catalogs a "tip" for ya'lls Mig guns that does "spot welding"...we have one and it does work as advertised)....it's too thin,haha.Most of our stuff is older 24G and thicker.Best,BW

[Peter Aeschliman]

Am I understanding you to say that his test implies that 30 gage snap lock is ok to use for dust collection ductwork? I draw a very different conclusion.

I sense that I'm being baited into an argument.

So instead, how about you get right to it and share your concerns?

[Michael W. Clark]

Instead of using springs make an adjustable counter balance. Have a shaft running across the duct with a swinging gate fixed to it and outside the duct have an arm fixed to the end of the same shaft. Put a sliding weight on that arm and move as necessary to counter the the suction as needed. I would think it could be tuned to close when a gate at a machine is opened and having it open at other times when the machines are not in use will clear the air of dust which is a plus.

This exactly. I think I mentioned this in your other thread. I have neaver sean a spring loaded vacuum relief in dust collection (yes for pneumatic conveying systems with PD blowers, but that is different altogether).

As far as 30 ga used for DC, if you can make it work in a home shop, why not? Most of the duct mentioned on here is not meant for DC anyway. S&D, PVC, 30ga or 26ga KD (knock-down), metal spiral...none of it was designed for wood shop DC, but we are adapting it for home shop hobby use.

Industrial/Commercial DC ductwork should be metal (conductive) for wood dust and meet the appropriate SMACNA standard. I think minimum SMACNA is 18 gauge for dust collection for full pen welds. HVAC systems do not handle the dust loading (neither do HVAC filters!) of a DC system nor do they approach the negative static pressures often present in dust collection systems.

Mike

[Bill George]

Just want to toss this out for consideration. If you wanted automatic vacuum relief a barometric damper could be used, you could add your own spring or weight as needed. You won't find them in 30 ga more like 24 ga as they are used on coal and wood burning stove chimneys. Vent pipe is usually 26 - 24 gauge more expensive and harder to find than the 30 ga warm air pipe.