surface corrosion on hot/cold ammonia pipe

[Service1927]

After only 2 years a leak occurred on a 3/4" seamless carbon pipe (schedule 40) carrying ammonia liquid and gas.

The pipe is insulated with pre-moulded pu and the cladding are with plastic tape. The pipe wills every 4 hours be cold (-25 dg C/-13 dg F) for approx 3 min, chilled (5 dg C/42 dg F) for 20 min and hot (50 dg C/120 dg F) for 2 min.

The pipe was not surface treaded before it was insulated and no damage of the insulation was found.

The pipe is on the sealing in a food processing plant and the roof space is normally good ventilated.

There is many other refrigeration pipes in same space with same conditions but only with constant temperature ranging from (-25 dg C/-13 dg F) to (5 dg C/42 dg F), those pipes have no corrosion.

I have a feeling that the temperature is the main reason and this is due to the insulation is not moisture tight enough since the leaks occurs where the pre-moulded insulation is joining and the rest of the pipe has a high but constant surface corrosion.

My intention is to change the pipe to a schedule 80 and then use a good insulation.

Any comments or experience will be very welcome.

 

[hacksaw]

You might try aluminizing (flame sprayed)

 

[kenvlach]

Are you 100% sure that the leak was from outside in?
You mentioned joint in insulation. Was this at an pipe elbow where erosion could occur? What is flow rate?

Presuming that the pipe rusted from condensation, I prefer coating rather than using Sch. 80.
It's probably cheaper to use PVC-wrapped or epoxy painted pipe rather than aluminized.
Re using Sch.80: your system might then require 1" to get sufficient flow, plus it's heavier to support.

If your plant is going to be around for a long time and your piping has long runs with few fittings, consider 316 Sch. 10S. Weighs less, better flow, won't corrode & has lower heat transfer coefficient (1/3 of steel).