Carbon Steel, Icing in Ammonia Service

If you study coincident temperature-pressure conditions, you will likely find that this is not a problem, unless there are mechanical impacts to the pipe at the time. What grade of CS is the piping?

It is API 5L Grade B.

It was wrongfully selected and usually the line is not in service. In case of shutdown a little bit of ammonia flows throw this blow down line in controlled manner, but valve leakage has made it frosted

Then fix the valve too!

I don't off the top of my head know the cross-reference(s) for API 5L-B. (In ammonia refrigeration the specs ASME SA53-B and SA106-B are common). Unless a partial vacuum exists on your piping, the lowest possible temperature is -28F. But at that temperature, the pressure is zero. There is a whole coincident temperature-pressure relationship that applies to this. I'm not totally sure, but it sounds as though you are on the safe side of that relationship.

Continuous bleeding of Ammonia should be your first concern than bothering of frosting. Fix the valve leak or replace the valve during a shut-down.

1. Stop the liquid ammonia being blown down the piping.

2. If pipe is deemed not suitable for service - replace it.

If point 1 and 2 is followed not steam tracing is necessary.

Carbon Steel pipe or Black iron as it is sometimes called works very well under the conditions you stated. There is no need to heat trace or and thing else is you fix the issue of the leaking valve. You are producing a chiller with the leak. Study the effects of low pressure high pressure through and orifice. Hence the ice forming.

The frost is on the outside of the pipe,but causes no harm to the pipe,unless it is

creating interference with other objects nearby.

To prevent the ice from forming,you must insulate heavily against outside atmosphere.

Chances of the pipe getting brittle is rare as CS can withstand temp up to -20 C. Just check the hardness of the pipe where it had iced and non-iced portions. If nearly equal pipe is not brittle. Yes if pipe is healthy you can steam trace it or electric trace it if steam is not handy.

Raghava

To prevent icing you may move the valve 200 ft downstream.

Certainly that will prevent icing, but the line will get pressurised. It look like this line is used to dispose the Ammonia to a distance place in case of shut-downs. And it look like he doesn't want to elaborate this practice fearing the comments on safety issue

Where does this wasted ammonia go?

Do you have an absorber stack or column to reclaim the NH3?

The CS piping is not in danger of becoming embrittled,but if the ice is becoming a

problem by interfering with valves,etc,then INSULATE the exterior of the pipe using the

proper material and methods for refrigeration piping.( Hint:Not the same as for steam).

The ice is frozen moisture from the atmosphere so insulation is paramount.

Good luck.