Use of anhydrous ammonia to break pipe freeze? 1

[zdas04]

One of my clients has a buried 4 mile 8-inch natural gas line that normally runs at 1,000 psig that has been out of service for a couple of months. It was left pressurized with 7 lbm/MMCF water content. Apparently the water has condensed and frozen solid.

They've tried just about every possibility of pressure on either side of the plug with no success. They've pumped several hundred gallons of methanol with no success.

One of their contractors has suggested using anhydrous ammonia to melt the ice. This was a new concept to me and I've been searching both eng-tips.com and the Internet in general all afternoon without any hint of success.

Has anyone ever heard of using ammonia to break freezes? If so has it been effective? My client thinks that they fully understand the potential risks of using ammonia and have planned what seems to be appropriate safety precautions.

David

 

[Zapster]

This might be a stupid idea (I seem to have plenty), but have you considered pulling a vacuum on the line. If there are no issues with the mechanical strength of the line, it seems like the ice should go straight to vapor phase from the solid phase when you pull a vacuum. Not sure how long this would take or if it would even work. Take a look a phase diagram and do the math. It looks good on paper; however, it may not be practical???

 

[Zapster]

Now that I have though about this for a few minutes, the pressure must be dropped below the triple point .006 atm. I don’t think it would be practical to pull that type of vacuum on a 4 mile line. Oops.

 

[MikeHalloran]

I found a single reference to squirting household ammonia on frozen bicycle brake pads to thaw them ... written by people crazy enough to ride bicycles on ice.

... and a lot of ways to die or suffer by handling anydrous ammonia incautiously, or just being unlucky.

I think it could work ... but be careful out there.



Mike Halloran
Pembroke Pines, FL, USA

 

[pipehead]

The only way to break the HYDRATE is to lower the pressure on the line. get the pressure off and let the methanol inhibit the water so it will not freeze at ground temperature. Google Hydrates if you need more details.

The technique of putting ammonia is to dilute the water with super dry gas. You could use super dry air, natural gas, whatever and get the same result. I wouldn't use super dry air, I'd use liquid nitrogen, pumped up and vaporized and pumped into the line to dry the line instead of ammonia, if thats what you think you need.

 

[zdas04]

pipehead,
This is really benign gas in the line, over 95% methane with a touch of butane. I think fluids that froze left the dehy as water vapor, condensed, ran to a low point prior to freezing, and froze. I'm betting that it is just H₂O in the really solid state, I don't think that they're having hydrate problems.

Using really dry gas to increase evaporation rate sounds like a slow way to break an 8-inch diameter freeze, 30 ft(?) long (could be 50 ft, who knows, if the line was absolutely dry at the start then the trapped gas only held about a half gallon of water, have to assume that they started with some standing water, but no way to tell how much).

David

 

[ApC2Kp]

zdas04,

If the frozen plug section could be identified and located, would they consider using the electrical current from a portable generator (welding rig) to heat up the pipe? If the underground pipe has good coat and wrap plus some junction boxes for cathodic protection, then it might be a setup similar to thawing out ductile iron water pipes.

It would have less potential of leaking ammonia, if the frozen plug has possibly cracked the pipe.

 

[dcasto]

Here is the hydrate curve:
Hydrate Pressure Hydrate Temperature
psia F

100 -8.9
200 21.5
300 34.8
400 39.3
500 42.7
600 45.4
700 47.6
800 49.5
900 51.1
1000 52.5

Here it is if the remainder of the stream has CO2
100 -22.2
200 8
300 27.7
400 35.2
500 38.8
600 41.7
700 44.4
800 46.7
900 48.8
1000 50.5

My experience says topical methods won't fix hydrates or ice. Methanol poured over the plug at low pressure and gravity allowing the methanol to sit on the plug will work, IF the pressure is dropped enough. at 300 or 400 psig, you are at the water freeze point of 32 F.

 

[EOIT]

Don't know too much about using it to break freezes but from what I have researched for my company's uses of anhydrous ammonia is that when you add water to it the chemical reaction generates excessive heat.

The thought could be to get it to the frozen section, let it interact, and the more it interacts the more it will heat up and kind of carry on its own reaction.

Of course if the ice in the pipe has broken through the wall at any point then an ammonia leak could occur and I am not sure what the ramifications for that would be.

Sorry I couldn't be of more help.

Regards,

EOIT

 

[zdas04]

EOIT,
You really were of amazing help. I couldn't make the dry-gas evaporating the ice scenario work. I didn't realize that ammonia and water created an exothermic reaction.

Thank you.

ApC2kP,
The pipe is buried 4-6 ft for 4 miles, seems like that is a pretty big toaster. They're prepared to deal with the gas, I don't think they're prepared to add that much heat to the glue holding the coating on the pipe.

David

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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[zdas04]

Follow up.

While they were evaluating options and considering possibilities, the weather warmed up and the thaw broke on its own.

Maybe we'll need to re-visit this next year.

Thanks for the good comments above.

David

 

[vpl]

When I saw a new post added to this thread, I came primed to advise to wait on the weather....

Thanks for the update zdas04.



Patricia Lougheed

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[Ashereng]

He He He

If you just wait, sometimes the problem does go away!



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[btrueblood]

zdas,

As a further note, ammonia gas will dissolve in water, and the resulting mixture has a lower freezing point, thus it would work to unblock a freeze plug the same way methanol would. The benefit of ammonia is that the gas would be able to travel to the plug site more easily than methanol, (provided you could vent the line down first, and then pressurize), and maintaining the ammonia at pressure in the line would keep the gas moving into solution as more of the plug melts. You could get the concentration of the gas to increase more quickly at the plug by pumping the ammonia in, then pumping it back out, then in again (hmm, there's probably a better way to say that, but I'll quit now).

From this website:

http://www.tannerind.com/aqua-ammonia.html

comes the quote "The freezing point of a 26° Baumé solution is about -110°F. See the freezing point of other concentrations by downloading our aqua ammonia manual (pdf). "

Elsewhere they say 29.4% by weight ammonia in water is 26 deg. Baume'.

So, I think that it's a workable solution, since you get much lower freeze point depression than methanol/water mixtures (methanol+water only goes down to -73F according to Engineer's Toolbox - not sure if it gets better than that at higher concentrations than 50%?).