Crush strength of a heated polymer pipe

[TylerMartin]

I'm looking to calculate the force required to crush various polymer-type pipes through the use of intumescent materials in fire collars. The force contact patch would be around 20mm in length on the outside of the pipe, with the force crushing the pipe from all directions circumferentially on the outside. I cant seem to figure out an accurate formula(s) to use to calculate the strength of the pipe and how the pipe strength would be affected as it softens due to the heat in a fire.

Any help or ideas would be greatly appreciated on this topic. Thanks!

 

[MikeHalloran]

What's inside the pipe probably affects the outcome.




Mike Halloran
Pembroke Pines, FL, USA

 

[TylerMartin]

There will be nothing in the pipe, just looking at a section which will penetrate a ceiling with about 20 cm out at each end, not sealed.

 

[MikeHalloran]

AFAIK, intumescent collars work by expanding a bit while they are burning to a char, and the char fills any pre-existing gaps. ... but while the stuff is burning, I'd expect it to be soft, and not capable of exerting much radial force.

I think you need to conduct a physical experiment to see if a burning collar will cause _any_ deformation of the specific pipes you intend to use, using an actual fire, and a pretty good mockup of the actual roof you intend to use.



Mike Halloran
Pembroke Pines, FL, USA

 

[Pud]

I'm with Mr Halloran - compressive force = zero to not much. Definitely requires a physical experiment. You are never going to be able to calculate it.


H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[TylerMartin]

Okay thanks for the tips! I'm looking at hard char intumescents which do produce some expansion pressure. Do you have any idea how much strength would remain in the pipe as it gets up to high temperatures or does it just melt away? I'm really looking for a way to calculate what an applied force would be required to crush the pipe and then compare to experiment.
Thanks again!

 

[MikeHalloran]

I worked on a yacht whose FRP mufflers were exposed to WOT exhaust gas (maybe 800F, not more) when the water injection system failed to work properly. They sagged from their normal oval shape to a flattened oval, under the influence of their own weight. They did not close up completely, and they did not melt away, rupture, or leak. The crew was alerted to a problem by the odor of hot plastic, and proceeded safely home at a leisurely pace.



Mike Halloran
Pembroke Pines, FL, USA

 

[MacGyverS2000]

Could the WOT exhaust gasses escaping have prevented the mufflers from closing completely under their own sagging weight?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Pud]

What's the pipe made from?

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[TylerMartin]

The pipes im looking at are various common water pipes. ie. PP, PVC, HDPE, PEX.

 

[berkshire]

TylerMartin
What is your ultimate goal in this ? Are you trying to make sure the firewall stays sealed so no flame can penetrate ? Even if the pipe melts completely.
I have a sneaking suspicion that in a fire, the firewall coating will not expand enough to seal a hole if the pipe melts completely, especially if the pipe is bigger than 25 to 35 mm .
B.E.

You are judged not by what you know, but by what you can do.

 

[TylerMartin]

I am looking at how various materials react to conditions in a fire and how different materials soften and then are ultimately crushed by an intumescent fire collar in a penetration. Industry standards (Australian) require no fire and smoke to be able to move through the penetration and only a limited amount of heat transfer. I am looking at what it takes to crush the pipe theoretically through calculations, and then try and transfer that knowledge into how much force an intumescent material can apply. Also looking at how different pipe dimensions affect the strength at this elevated temperature.

 

[berkshire]

Tyler Martin,
Here is a draft copy of our local (California USA ) specifications for your perusal. They cover a lot of the things you are working on.
B.E.

You are judged not by what you know, but by what you can do.

 

[TylerMartin]

Thanks for that B.E.

I am more looking for actual calculations/equations which might give a required expansion pressure to crush the pipe for different materials.

 

[berkshire]

TylerMartin
I am afraid Mike Halloran already answered that for you I think you need to conduct a physical experiment to see if a burning collar will cause _any_ deformation of the specific pipes you intend to use, using an actual fire, and a pretty good mockup of the actual roof you intend to use.
I know he said roof when you are actually dealing with firewalls, but the principle is the same, I think you are going to have to assemble empirical data by make and model number of product.
B.E.

You are judged not by what you know, but by what you can do.

 

[TylerMartin]

I understand that it would be easy to just conduct a physical experiment but I am trying to find some way to model this theoretically if it is at all possible. Thanks for the help though.

 

[MikeHalloran]

WRT to the yacht mufflers, the engines are turbo-Diesels, which are sensitive to backpressure in the sense that power output would fall off if the pressure went up beyond a few tens of inches of water. No reduction in power was mentioned, so I'd _guess_ that the backpressure was not terribly abnormal. It's a guess, because there is no backpressure sensor or related shutdown mechanism installed. There is an overheat sensor, a simple temperature switch, on the water injectors, and AFAIK, that didn't trip; the water flow was not interrupted, it just didn't get sprayed to the top of the pipe.

For reference, the mufflers look rather like oval automobile mufflers, except for being much larger, and made of FRP instead of steel. The pipes are 8" OD or so. I'm guessing the muffler bodies' elliptical shells have major dimensions of ~24" and minor dimensions of ~12", and are ~6 feet long. The walls are about 1/2" thick. The resin used is nominally a thermoset, but it behaves more like a thermoplastic when it gets hot. At some point it does burn and char, but the shells clearly didn't get quite that hot. By the time I got there to inspect them, they had cooled to ambient temperature and were quite rigid and strong, just misshapen.







Mike Halloran
Pembroke Pines, FL, USA