Ammonia in Refrigeration Cycle 4

[poli60]

Dear Experts,
during a technical discussion about a refrigeration cycle with ammonia as refrigerant I've heard the following statement:
"We add 0,2%wt of water to circulating anydrous ammonia to avoid corrosion caused by hydrogen".
Is someone aware of / using this practice?
Could someone explain how water would avoid the corrosion?
General comments are appreciated.
Thanks and kind regards.

 

[cloa]

From what I've read water is no good for copper (stress corrosion cracking) in anhydrous ammonia but good for Titanium- what metals are in the system. In refrigeration, they want to keep the water content as low as possible less 33 ppm

 

[EdStainless]

A small amount of water acts a corrosion inhibitor for carbon steel in ammonia service.
I don't recall the exact amount.
The amount to water may be function of the metallurgy of the system, but some water is always added.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[poli60]

Dear EdStainless,
thanks for your help (and star to you).
Kind regards

 

[cloa]

https://www.iiar.org//aar/aar_green2.cfm?

Here's the second reference- they really don't like water added to ammonia as a refrigerant- maybe in other uses.

 

[poli60]

Dear cloa,
Thank you for your help and reference.
Anyway, I see some conflicting and cryptic info:
- just a clarification from my side: there's neither copper nor titanium in the system. Only carbon steel.
- which is the origin of 33 ppm you mentioned?
- in the reference you provide (thanks and star), I don't see where "they really don't like water added to ammonia as a refrigerant". My fault!
- which is the 1st reference? you only provided "the second reference".
I'd be glad to receive your further comments.
Generally speaking, it would be interesting to obtain hints from specialized operating/design people.
Kind regards

 

[poli60]

Another clarification from my side: it's a screw wet compression system, with lube oil.
My main worry is about the (possibly negative) interaction between water and lube oil.
Any comment/warning on that?
Thanks

 

[poli60]

Browsing for ammonia specifications I've found this reference

http://www.tannerind.com/anh-specifications.html

The web page contains the commercial specs for three anhydrous ammonia grades (ie metallurgical, refrigeration and commercial).
I think the 2 water concentrations mentioned in this thread are (partially) "justified":
- 33 ppm is the max content of water in metallurgical grade;
- 0.2 %wt (ie 2000 ppmw) is the min content of water in commercial grade
On the other side, refrigeration grade reports 50-150 ppm as typ-max content of water

 

[cloa]

http://www.osha.gov/SLTC/etools/ammonia_refrigeration/ammonia/index.html

The normal spec is much lower than the maximum permissible because ammmonia is hydroscopic- it will pick up water from the air.