External PVC Pressure Ratings

[Gepard]

Not sure if this belongs here but figured it was information. Please move if you want.

I do research and systems design for underwater vehicles and hobby-level ones use PVC pipe for their frame. The following is a table for the EXTERNAL pressure ratings for both Sch40 and Sch80 pipe. I can't say I've tested this but it's from an experienced engineer who spoke to the company himself so I personally believe it's very accurate.

Please note these are the External Pressure Ratings which means you can only use them in a Co-axial type cannon for the barrel. They should not be used for applications where there is compressed air on the inside. <Insert>

[Fnord]

Since I'm too lazy to do those crazy formulas... These numbers are the safe working pressure and not the crush pressure right?
What is the safety factor that these ratings give?

[Gepard]

I haven't done them but judging how the formulas are for Crush pressure and there's no mention of a safety factor in the formula. I think it's fairly safe to say that they are the pressures at which the pipe will fail and so you need to add in the safety factor yourself.

[jimmy101]

If I'm reading the table correctly the crush pressure of PVC pipe is typically less than the pressure rating? So the burst pressure is actually much more than the crush pressure?

Seems kind of surprising, aren't most containers (especially symmetrical ones) stronger in compression than they are in tension?

[Gepard]

From the email:

On the use of schedule 80 PVC for the pressure hull, I have attached an Excel file that shows crush depths for both Sch40 and Sch80 PVC. The failure point pressure figures are for externally pressurized pipe and were taken from a PVC pipe manufacturer's data*. I calculated the depth required to reach that pressure based on fresh water operation at something like 68 or 70 degrees F. The two formulas were also supplied by Harvel, and they suggested using both and designing to the lower valued result. It an interesting material because the primary failure mode for hull crushing is compressive buckling and PVC is for all practical purposes a brittle material under this type of loading. Even when the cylinder is loaded by internal pressurization and the predominate failure mode is tensile yield, PVC doesn't yield much before it breaks.

*I'm assuming he's talking about the dimensions since there is no external pressure numbers on the site

To be honest it makes sense....take a Coke bottle for example it can take a much higher internal pressure than a higher external pressure before buckling. And PVC is no where near as flexible as PET.

Feel free to dispute it but it certainly makes sense to me....

I saw noticed there wasn't much info (if any) to do with external ratings so thought I'd post it.

[frankrede]

From the email:

On the use of schedule 80 PVC for the pressure hull, I have attached an Excel file that shows crush depths for both Sch40 and Sch80 PVC. The failure point pressure figures are for externally pressurized pipe and were taken from a PVC pipe manufacturer's data*. I calculated the depth required to reach that pressure based on fresh water operation at something like 68 or 70 degrees F. The two formulas were also supplied by Harvel, and they suggested using both and designing to the lower valued result. It an interesting material because the primary failure mode for hull crushing is compressive buckling and PVC is for all practical purposes a brittle material under this type of loading. Even when the cylinder is loaded by internal pressurization and the predominate failure mode is tensile yield, PVC doesn't yield much before it breaks.

*I'm assuming he's talking about the dimensions since there is no external pressure numbers on the site

To be honest it makes sense....take a Coke bottle for example it can take a much higher internal pressure than a higher external pressure before buckling. And PVC is no where near as flexible as PET.

Feel free to dispute it but it certainly makes sense to me....

I saw noticed there wasn't much info (if any) to do with external ratings so thought I'd post it.

Thats irrelevant, a piece of pvc pipe is more rigid than a coke bottle.
A coke bottle will buckle under the force of a childs hanf too.
Try that with a more rigid material.

[Gepard]

It's not at all irrelevant....it shows that (at least one) symmetical container is weaker is compression than tension which was Jimmy's point.

The fact it buckles in a child's hands is exactly my point....

If you did that with a piece of pipe the exact same thing happens - you get compressive buckling which due to the brittle nature of PVC causes it to break.

That's why you have a lower external pressure rating than internal rating.

[jimmy101]

What a child's hands can do is irrelevant. There is a big difference in how a structure responds to a point load versus a load evenly distributed over its surface.

Besides, if you could get a child's hand inside the bottle and have it push outwards the bottle will buckle as well.

I wonder what the crush depth of a sealed, but unpressurized, scuba tank is?

[Gepard]

Yes I know that it isn't quite the same but the same effect happens when you exert a force all over the bottle. Haven't you ever seen when they put bottles in a hyperbaric chamber?

It won't buckle if you apply a force from the inside since the material wouldn't be in compression.

I don't know, look up the Modulus of Elasticity for the steel used and the dimensions and the formulas will tell you.

Surely the story of the imploded golf ball barrel shows that this is the case?

Don't argue with it just "cos it doesn't sound right" find the formula to prove it's wrong and I'll delete it, otherwise keep quiet since your just adding confusion.