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cold weather?

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cold weather?

Unread postAuthor: geardog32 » Fri Sep 25, 2009 10:41 pm

this may be a dumb question but, at what temperature does pvc become too brittle and unsafe to use for spudding?
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Unread postAuthor: noname » Fri Sep 25, 2009 11:30 pm

An easy rule is: If you need a jacket, don't shoot with PVC. Of course, that's not very "scientific" and someone else will probably have a better answer...
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Unread postAuthor: jonnyboy » Sat Sep 26, 2009 12:32 pm

pvc's working pressure is usually rated at 73 degrees Fahrenheit. If you can figure out another working pressure at a colder temperature then you can work out the math to find the pressure rating at any given temperature.

The left column is temperature in Fahrenheit the right column is the de-rating factor.

73 1.00
80 0.88
90 0.75
100 0.62
110 0.51
120 0.40
130 0.31
140 0.22

The chart is right off of harvels web site found at the following link.

So a change in 7 degrees bring the rating down .12 so the chart can now be shown to look like this

31 .31
38 .40
45 .51
52 .62
59 .75
66 .88
73 1.00
80 0.88
90 0.75
100 0.62
110 0.51
120 0.40
130 0.31
140 0.22


EDIT-I don't think you can use a direct formula with this chart so you would have to use a recursive forumula which is a PITA because to get the right answer you have to get the answer for all of the previous answers so if you want the answer for term 10 you would have to get the answers for all the previous terms

I might make a wiki article on this and include a chart for temperatures 32 to 100 and pipe sizes .5 to 6

EDIT2- I forgot to say that if the working pressure is 100 psi then multiply this by the related de rating of the temperature

EX: 10” PVC SCH 40 @ 120°F = ?
140 psi x 0.40 = 56 psi max. @ 120°F
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Unread postAuthor: btrettel » Sat Sep 26, 2009 1:43 pm

You can't do what jonnyboy is suggesting (i.e. extrapolate backwards) without some knowledge of the material. The assumption that jonnyboy is making is that the material acts the same in cold temperatures as hot, which isn't true as I'll explain.

High temperatures reduce the stress the material can handle. Below 73 degrees F, PVC's ultimate and yield strengths should be relatively constant. The ultimate and yield strengths are not the issue here.

Low temperatures make PVC brittle. Here's a page with some information about PVC failure: http://www.madisongroup.com/services/fa ... lysis.html

Of interest to us is the impact strength vs. temperature graph:

Image

The plot is VERY steep in the temperatures we normally operate in. This means the a "small" change in temperature on the order of 5 to 10 degrees F results in a big change in impact strength.

What level is safe? I don't know. Just looking at the plot, I'd avoid going under about 60 degrees F. Don't take this as an endorsement of any temperature in particular. I'm generally wary of most plastic under gas pressure.

Edit: It's worth noting that ABS, as a much more ductile (i.e. the opposite of brittle) plastic, would in general not have these issues with temperature (unless there's something I'm not considering, which is a possibility :lol: ).
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Last edited by btrettel on Sat Sep 26, 2009 5:42 pm, edited 1 time in total.
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Unread postAuthor: spudamine » Sat Sep 26, 2009 3:39 pm

Thanks btrettel, that's a really interesting link. I knew brittleness was a problem at low temp but I never guessed the drop off would be that steep. I'll definately be putting my cannon to bed for the worst of the winter this year.
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Unread postAuthor: D_Hall » Sat Sep 26, 2009 6:46 pm

ANd just to muddy the waters even more....

In polymer science there exists something referred to as the "glass transition temperature." If a material is below Tg it is brittle. Above it and it's not.

For "normal" PVC Tg is about 80 C. Thus, PVC is brittle at room temperature, right? Duh! We knew that.

HOWEVER....

If you add certain plasticizers to PVC the glass transition temperture drops dramatically. Suddenly PVC is flexible at room temperature (garden hose, anybody?) and it's failure mechanics alter radically.

So.... What is the exact composition of the PVC you have in your hand?
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Unread postAuthor: psycix » Sun Sep 27, 2009 9:36 am

Heating pvc is a whole other thing then making it cold.

When you heat it, it becomes SOFTER and thus weaker (but less brittle!).
If you cool pvc, it becomes HARDER, and thus stronger... what?! wait, read on:

The strength increases, and it can hold MORE pressure, however, it becomes brittle, and it is far more likely to break on shock.
So: the pressure it can hold increases, but there is a new danger: it is too brittle and fragments on shock.
When firing a cannon, this often delivers more then enough shock to "shrapnelize" the frozen PVC cannon in one go.

To answer the topic: I think you should stay above 50*F or 10*C at all times.
You could, however, load the cannon inside, take it out, and fire it quickly before it cools. Never use ice projectiles though.
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 27, 2009 10:21 am

psycix wrote:When firing a cannon, this often delivers more then enough shock to "shrapnelize" the frozen PVC cannon in one go.


If you're going to make up words like that, you deserve to have razor sharp shards of PVC embedded in your anatomy! Henry would be turning in his grave! :roll: :wink:
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Unread postAuthor: spudtyrrant » Sun Sep 27, 2009 10:43 am

Man jack, knowing you i thought you would make a comment about what cold does to a certain part of a males anatomy :roll: :lol: :roll:
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 27, 2009 10:49 am

spudtyrrant wrote:Man jack, knowing you i thought you would make a comment about what cold does to a certain part of a males anatomy :roll: :lol: :roll:


Had I done that, I would have been accused of being predictable :P
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Unread postAuthor: geardog32 » Sun Sep 27, 2009 1:04 pm

so could one say that a sprinkler valve or qev would be safer in colder temperatures because they do not have a piston to slam around? would cold weather have any long term effects on the pvc?
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Unread postAuthor: psycix » Sun Sep 27, 2009 2:44 pm

a sprinkler valve or qev would be safer in colder temperatures because they do not have a piston to slam around?

Sprinklers have a diaphragm, QEV's have a piston or diaphragm. Both "slam around".
But there are more shock sources then that. Any part that's on the other side of the valve will get dumped full of pressure in a very small time, possibly breaking it. That's why some people shooting a frozen projectile blew their barrel, even when the environment is warm.

On top of that, a frozen chamber under pressure may decide to go boom when you accidentally drop it.

would cold weather have any long term effects on the pvc?

As long as you don't drop it (creating possible micro-cracks), you should be OK when the PVC is back up to temperature.
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