30 Gage Metal Duct Pipe can Collapse
Let me say to start with I am not recommending the use of 30ga or the use of 26ga snap lock for ductwork. I am posting for information only. I did this testing for myself, but thought it would be good to share.
I have been woodworking a long time dragging the DC hose from machine to machine. When I built my new shop I decided to install metal duck work. Not researching much I just went to Menards and bought 6” snaplock and fittings. Well what they sell is 30ga. Now doing some research 26ga is the recommendation. I wasn’t sure 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 ). However the wyes I bought at Lowes were 28ga. My DC is a 3hp Jet 1900. Specs say 1900 CFM, 5629 FPM, 12.5 static pressure @ 8”. My inital 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 6” plywood rings and so I could place them around the pipe center of the joints. My vacuum relief was only about 20% effective with the springs I had on hand. So I put that aside. I also thought placement of the seams would impact the test. I expanded the test to five 5’ sections and one vertical off a wye at the end. I opened and closed the gate multiple times on each test and results did not vary.
The first test I assembled all straight pipe with the seam on top and when I closed the gate the straight pipe went oval. No permanent set occurred, came back close to original shape when gate was opened.
I then rotated the seams side to top and retested. The pipe didn’t collapse when the gate was closed, however you could see where it was on the verge of going oval.
Now for the fun, I slammed the gate shut in this configuration. The straight pipe “bounced” oval back to round. No permanent set in the straight pipe, but the vertical pipe did take a set which surprised me.
Finally I put the plywood rings on the first four sections (that was all I had at the time and forgot to take picture). Repeated slamming the gate shut and there was no collapse.
This photo is from initial test showing plywood rings (forgot to take photo last test)
My conclusions: I can get by with 30ga pipe using the plywood rings. Rotating the seams does help resisting pipe collapse. I will put one, maybe two plywood rings on each section of straight pipe and rotate the seams 90 degrees top to side. None of the elbows or wye showed any sign of collapsing. If I had a “do over” I would buy 26ga pipe, but use the 30ga fittings. I am using 6” pipe, so different pipe sizes and DC’s will have different results. I will save some money using 30ga, but will involve more installation time.
-- Bill R