A few comments.
Anhydrous ammonia (NH3) can be stored in two different ways:
a) pressurized, at ambient temperature. "Pressurized", in this case, means the equilibrium pressure of ammonia at ambient temperature, that you'll find on chemical engineering handbooks. "Ambient temperature", in this case, means a temperature that will never be reached in your location, like 100ºF (nearly 38ºC) in Alaska. The reason is that the inside pressure of the tank will never surpass the design one. In this case, the storage vessel will fall under ASME VIII Div 1.
b) at low pressure and cryogenic temperature. In this case, the storage vessel will fall under API 620 Appendix R, in which the design pressure is (or at least was years ago) 10 psig. "Low pressure" means 10 psig and "cryogenic temperature" is the equilibrium temperature of ammonia at 10 psig, that you'll find in chemical engineering manuals.
The material your tank is made of, A 212 Gr B, that has been long replaced by A 515 Gr 70, would suggest that you tank belongs to the a) class.
In my opinion, your cracks were not produced by stress corrosion cracking, because stress corrosion cracking is the one that develops in austenitic stainless steels due to the presence of chlorides, even in very low concentration, when the steel is submitted to stress.
This is not your case, because your steel is plain carbon.
Now, anhydrous ammonia isn't a strong corrosion agent when in contact with carbon steel. 1/8 of an inch of extra wall thickness as corrosion allowance would be sufficient. So, also in my opinion, your cracks have been produced by another reason that should be investigated.
First of all: are you sure that they're cracks and not another type of corrosion, pitting for example? The presence of moisture and/or oxygen in the ammonia will produde pitting, not cracks. If they're definitely cracks, has the vessel been subject to sudden temperature changes?
Regarding the repair you're concerned with, I don't see great difficulties in doing them, as your steel is plain carbon, equivalent to A 515 Gr 70. Grind the cracks until they've been completely ground off, which you'll see through a liquid penetrant test, and fill the groove with a plain carbon filler: E 7015 or 7018 if you're using SMAW or an equivalent wire if you use MIG. After the repair it should be carried out a hydro test. However, with your client's approval, it may be replaced by full Xray test.
The idea of PWHT shouldn't be discarded, as suggested by the other gentlemen that have answered your posting.
In my opinion, your main problem is not how to repair the corrosion that your tank has developed. The main problem are the safety precautions that should be taken for the people that will work inside your tank, especially for the welders. Ammonia is a poisonous gas, impossible to breath. You know already how ammonia smells like.
Gas masks are difficult to wear by welders, who must also wear their visual mask. You should get rid of all ammonia before your welders walk into the tank. One way to do this is to fill the tank completely with water and draining it. If the ammonia smell still persists, do it again.
Giovanni S. Crisi
Sao Paulo - Brazil
I merged the two seperate threads into this one...so if there is confusion in the way this thread flows, that is why.
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