How Do Threads Affect How Pipe Handles Pressure?

[WizardNoodle]

So I am going to be making a piston hybrid cannon over the summer, and have pretty much finished all of the designing of the cannon. I want the chamber to have an FNPT Sch 40 Cap so that I can easily access the innards of the gun in the case of a failure or maintenance.

The problem is that I'm not sure how having some threads machined onto the end of one end of the chamber for the cap will affect how well the pipe will deal to pressure. At first, I thought that running Barlow's Formula with the initial wall thickness subtracting the depth of the threads would give me what I need. But then I realized that the threads will be joined with the cap's threads as well, along with an Anaerobic resin compound. How much more is it safe to assume this factored in, will resist pressure?

Here are the specs of the chamber: OD: 3.5" Wall: 0.3" ID: 2.9" (I am wondering while writing this why the ID is not the OD - the Wall..? That's what onlinemetals says for the specs, though)

Thank you for your time.

[Gippeto]

Use the thinnest wall cross section that isn't fully supported by the cap. The threads create stress risers in the material and you need to further de-rate for those.

As a starting point and unless you receive a better answer, would use the wall thickness as determined above and a heavy safety factor of 5 or more.

3.5" od minus the id of 2.9" leaves .6"....the thickness of BOTH walls.

JM .02

[wdr0]

I would verify but the rule of thumb I always hear is 50% reduction from the working pressure of the same pipe unthreaded.
here is a table with thread pressure ratings, not sure of its accuracy though.

http://www.phoenixforge.com/PTR/Pressur ... 0Final.pdf

[WizardNoodle]

Use the thinnest wall cross section that isn't fully supported by the cap. The threads create stress risers in the material and you need to further de-rate for those.
JM .02

Not sure what you mean by what isn't fully supported by the cap. Are there leftover/exposed threads even if the cap is fully screwed on? Or maybe the hand tight threads? And how would I "de-rate" for the threads?

3.5" od minus the id of 2.9" leaves .6"....the thickness of BOTH walls.

So OnlineMetals, where I am ordering the pipe(I will have the mnpt threads machined) says that the wall thickness is .3... logically you'd subtract the ID from the OD so that is weird, but I think I'd rather safely underestimate than make a mistake that ends with a large explosion.

Also, the cap will be Sch 40 as that is the thickest I could find, while the chamber is Sch 80.

As a starting point and unless you receive a better answer, would use the wall thickness as determined above and a heavy safety factor of 5 or more.

Onlinemetals recommends a safety factor of 4 for a normal pipe. I'll go ahead and use 5 like you suggested, but how does the safety factor change relative to threading? And if the cannon will only be operating in milliseconds, do we even need a heavy safety factor for working pressure? If we assume the combustion lasts roughly 6 milliseconds, it would take 10,000 shots to have the pressure generated by the combustion last for a total of 60 seconds.

I would verify but the rule of thumb I always hear is 50% reduction from the working pressure of the same pipe unthreaded.
here is a table with thread pressure ratings, not sure of its accuracy though.

http://www.phoenixforge.com/PTR/Pressur ... 0Final.pdf

Pheonix Forge's table's working pressures seem pretty close to what Barlow's gave me.

[wdr0]

Just a note, the demensions you have indicate sch 80 pipe, you could get such pipe already threaded from most any commercial plumbing supplier or online from places like McMaster. For a cap I believe class 2000 would match the maximum rating of threaded sch 80 pipe

[Zeus]

Don't forget shock loading though, gradual continuous pressure isn't as harsh as fast peaking pressure, especially when you're working near the limits of your materials.

Stress risers, if you cut a square part out of paper then try to tear it, you'll see it'll tear right where the two pieces meet. Cut a round part out and it'll be somewhat harder to tear. Now picture that with steel and more than two dimensions. Bit simplistic, but you understand

[Gippeto]

Use the thinnest wall cross section that isn't fully supported by the cap. The threads create stress risers in the material and you need to further de-rate for those.
JM .02

Not sure what you mean by what isn't fully supported by the cap. Are there leftover/exposed threads even if the cap is fully screwed on? Or maybe the hand tight threads? And how would I "de-rate" for the threads?

There are always left over threads on a pipe. De-rate by increasing the safety factor.

3.5" od minus the id of 2.9" leaves .6"....the thickness of BOTH walls.

So OnlineMetals, where I am ordering the pipe(I will have the mnpt threads machined) says that the wall thickness is .3... logically you'd subtract the ID from the OD so that is weird, but I think I'd rather safely underestimate than make a mistake that ends with a large explosion.

Nothing weird....3.5" minus 2.9" is .6". Which makes the WALL thickness .3". Should go without saying that the wall thickness at the thread root will be less.

[Hurricane Air Arms]

I would make a conscious effort to replace any PVC with copper or steel. Then buy a tap/dye kit and thread it yourself because tbh, a PVC explosion, especially from a hybrid, can't be pretty.

Safety first.

If I'm not comfortable with the pressure rating of something then I really wouldn't risk it. But try testing it behind a closed door somehow?