[camper92663 (OP)]

There are lots of stories about frozen pipes and flooded basements. After all it's that time of year now.
But I have a travel trailer and it has a gray-water waste tank (gray water is sink water waste).
There is a 30 gallon gray water waste tank hugging the bottom of the trailer and a two or three inch pipe extending out of its bottom and through a series of straight lines and elbows it makes its way to the side of the trailer where there is a valve used to dump the waste.
I have had the trailer out in the mountains and the temp suddenly fell to an over night low of 25 degrees.
The gray water pipes froze solid. But nothing broke! How come?

Camper

[ukmicky]

What are the pipes made from.

Also is the system pressurised and  can water be forced from the pipes back into the tank as the water freezes.

[camper92663 (OP)]

What are the pipes made from.

Also is the system pressurised and  can water be forced from the pipes back into the tank as the water freezes.

PVC Pipe and it is a gravity system. No pressure.

[eric l]

1) Your "grey water" obviously contains dissolved products.  Now all dissolved products cause in increase in the boiling point and a lowering (making more negative) of the freezing point.  See also this thread :  http://www.thenakedscientists.com/forum/index.php?topic=6197.0
2) The suspended matter in your gray water is probably much more plastic than ice would be, so that the expansion of the ice will be absorbed by these suspended matter first, well before the tubing.

[Soul Surfer]

Its probably the pipes.  PVC is much more stretchy than lead (not used now) or copper water does not expand all that much when it freezes the sort of pipes that bust when they froze were mostly lead ones.

[daveshorts]

Is the pipe completely full of water or could there be air in the system. Normally you don't have a problem with waste water pipes because they are rarely full of water so when they freeze there is an air space above for the waterto expand into.

[camper92663 (OP)]

All interesting replies so I will have to narrow the parameters some by asking you to verify something for my scientifically if you can:
1) The pipe is full of water
2) the pipe does not stretch enough to matter

So does ice freeze from the outside inward?
The only place where things freeze "instantly" is in cartoons so one expects the freezing of the water inside of the pipe to be progressive and that the water closest to the outside conditions would freeze before the water in the center.
If that is the case then wouldn't the ice that is forming on the walls of the pipe grow their 10% and squeeze out the water in the center?
The tank above the pipe is vented to atmosphere. If the tank does not freeze shut first then the water displaced by the pipe simply flows into the tank above it.

What do you all think of that answer?
Can anyone verify this with physical laws?
I would be very interested in links and stuff.

Thanks

Camp

[Soul Surfer]

If you put liquids into a freezer in containers they usualy peak in the middle as they freeze so a cerain amount of flow to relieve the pressure as the pipe freezes from the walls towards the centre can be expected.  The rest is till probably the stretchability of the pipe

[camper92663 (OP)]

I doubt that Young's modulus for PVC pipe is great enough to matter here but since all materials have *some* elasticity I'll concede some "stretch" if you will.
However I am still looking for a scientific explanation of the freezing outside in. I remember this problem from thermodynamics class in my previous life. I'm wanting to get a little studious here if possible.

Thanks

Camp

[Soul Surfer]

If freezing took place mostly by conduction of heat from the cold air through the solid pipe walls and into the water or ice layers it would go from the outside in.  You said the air temperature had dropped rapidly.

[daveshorts]

Thinking about it, what you need for a pipe to burst, is for two plugs of ice to form and then the water in the middle to freeze an burst the pipe. so if the ends of the pipe are warmer than the middle, there would always be somewhere for the expansion to go, hence it wouldn't break. I guess that a plastic pipe is less likely to form a plug than a copper one, because it's thermal conductivity is so much lower, that one spot is less likely to be a lot colder than another.

[JimBob]

Thinking about it, what you need for a pipe to burst, is for two plugs of ice to form ...

Not necessarily my learned colleague. All one needs is one ice plug and a cap of some sort on the other end of that section of pipe. A water tap with good gaskets is all that is needed. I know. I had the head pop off of a outside tap to put a garden hose onto for tending the lawn, creating mud, scaring off cats that are breeding or whatever else is needed.

[daveshorts]

Good point, although I guess this still means that if his pipe froze from the tap end first and back to the tank last, it probably wouldn't burst, as the water could always expand into the tank.
This would be what you would expect, as the tank is going to be relatively warm, as it is a big heat store and the tap being furthest from it will be relatively cold, so you would expect it to freeze that way.