Ammonia and SCC

[seanf]

Not much knowledge of ammonia systems, but do people increase the moisture level in the system to help prevent stress corrosion cracking? If refrigerant grade ammonia is meant to be 150ppm water and the level thats meant to help prevent SCC is 2000ppm

Thanks

[NH3LVR]

You might want to look over this old thread.
http://www.refrigeration-engineer.com/forums/showthread.php?5059-Stress-Corrosion-Cracking-in-NH3-Vessels&highlight=stress+corrison+cracking
Some one did offer to do a test for me, but they were out of town and would have had to travel at the plant owners expense.
I would talk to a water treatment person if if you have a good one. They might be able to point you in the right direction. But only if he is a good one, not just a salesman.
If the system ever operates in a vacuum, or is older it most likely has water in it allready.

[seanf]

Thanks NH3LVR,

Seems like purging and using the correct materials and manufacturing is the way to prevent it.
Was just curious to find out if was a regular thing to purposely increase the moisture levels as an additional measure.

Thanks

[NH3LVR]

I have never heard of it being done.
A nightmare to calculate and maintain as an oil pot or intercooler can remove water.
If the vessels were stress relieved after manufacture it should not be a problem.
I have never heard of any issues with SCC aside from HP receivers.

[seanf]

Thanks NH3LVR

[Glenn Moore]

Hi Sean
I have heard of companies in Europe who introduce moisture into NH3 systems to prevent SCC but have never experienced it in the UK or seen it happen. But I have experienced NH3 systems where there is excessive moisture in the refrigerant and this leads to a wire drawing effect to the inside of the pipe work, this causes metallic particles to return into the compressor causeing pistons and cylinders to score up badly with major component failure. I had such an experience on a Majjonier Carbo Cooler at a large soft drinks manufacturer in Tottenham. The internal maintenance guys used to change the compressor oil on a regular basis for some reason . They would shut the suction and discharge valves etc and then purge the ammonia in a dustbin full of water but they often left the machine while it was purging ,on several occasions the system sucked the water back into the compressor. This obviously after a while caused the refrigerant moisture ppm to escalate and this to erode the internal pipe surface and wreck the compressor. When the pipe work was opened up it looked like the internal surface of the pipe had someone scrapping their fingernails down the pipe. Me and a colleague spent 2 weeks overhauling the machine pressure testing the system vaccing and recharging to get it all back up and running . After that we took over the oil and filter changing . I always held the hose when purging as you can feel when the water is sucked back and simply removing the end of the hose from the water causes the gas to blow the water back out of the hose once the water has exited simply put the hose back into the water. We had more problems with water saturated ammonia than dry ammonia, and I believe a lot of systems have too much water due to poor care when purging.
I had several instances I was asked to investigate where the Danfoss control valves where sticking on the low side of the system , this was often caused by excessive moisture causeing the valves to corrode due to the high moisture ppm in the low side of the system , where we had to fit molecular sieve core driers to dry and clean the system. It took several core changes and time but it sorted the problem.

[seanf]

Thank you Glenn

[cricri]

Once in Grimsby I had to add water in a plant after commisioning. it was thirty years ago....
it was supposed to change the classification, using then non anhydrous ammonia.
it was a customer request. no matter with SSC.

[seanf]

Thanks Cricri

[RANGER1]

This article explains bit, but have never added water to a system, on purpose anyway!


https://www.irc.wisc.edu/?/paper&filter=all

UK water content below 200ppm
https://www.boconline.co.uk/en/products-and-supply/packaged-chemicals/gaseous-chemicals/ammonia/ammonia.html

Volume 5 No1 2005 stress corrosion cracking

[seanf]

Thanks RANGER1