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Is Metallurgical Grade Ammonia Acceptable for Refrigeration? | Blog No. 33

No one doubts that refrigeration grade ammonia is acceptable for ammonia refrigeration systems. But what about metallurgical grade ammonia? Is this also acceptable since it's a higher quality of ammonia with less impurities? The answer is not so simple...

According to how the current IIAR standards are written, the metallurgical grade of ammonia is unacceptable. The material specifications do not meet IIAR's material specifications. Consider the ammonia purity specification from ANSI/IIAR 2-2021 §5.2.1 which requires a minimum water content of 50 ppm:

Then, compare the water content specified in Tanner Industries' metallurgical grade of ammonia:

If the minimum water content required by IIAR is 50 ppm, but maximum water content in the metallurgical grade is only 33 ppm, this clearly is insufficient. And in theory if facilities were to use this dry form of ammonia, the pressure vessels would be put at a greater risk of stress corrosion cracking (SCC). In fact, IIAR 2 includes a whole appendix on this topic and one of the contributing factors is having ammonia which has a low water content.

One would think the case would be closed after all of this information. Come to find out, Tanner Industries and AirGas have been providing metallurgical grade ammonia to their ammonia refrigeration facilities for years. Have we in the ammonia refrigeration industry been hearing of an epidemic of stress corrosion cracking in pressure vessels due to poor ammonia quality? I certainly haven't and I haven't met anyone who has (correct me if I'm wrong!). What appeared to be the case is that the IIAR standard is too strict in their purity specifications. When I reached out to Tony Lundell, IIAR’s Senior Director of Standards and Safety, to request clarification, here was his response:

Here is the relevant section that Tony Lundell references:

Unless there is sufficient research proving the opposite, it seems that sufficient pressure vessel design (including post-weld heat treatment) is adequate protection against stress corrosion cracking and that the metallurgical grade of ammonia is acceptable. While this is likely true, it does not coincide with the way IIAR 2 is presently written. While we wait for IIAR to clarify this issue, I recommend clients document their variance on this issue within their process safety information and operating procedures. If you receive your ammonia from Tanner Industries or AirGas, you can reach out to our Project Coordinator for a free copy of a document which justifies the variance:

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