This topic has always been one that seemed to be controversial in the ammonia refrigeration sector, with decades of denial that Stress Corrosion Cracking could occur in an ammonia refrigeration process due to the water content in the ammonia. But last month, Technical BC (British Columbia) discussing their SCC concerns related to pressure vessels in a refrigeration process. I know, that ammonia in Canada is different than ammonia in the USA (sarcasm), but seriously, please consider checking out their concerns and their discussion of Post-Weld Heat Treatment (PWHT) as a means to manage this concern.
The ammonia refrigeration trade groups only “recommend” Post-Weld Heat Treating (PWHT) of the pressure vessels in the ammonia refrigeration process. But when we look at pressure vessels in ammonia service within other processes, we can find RAGAGEPs that specifically call for the vessel to be PWHT’ed…
How to manage stress corrosion cracking in new ammonia vessels (30 April 2021)
Here is the RAGAGEP for ammonia used in “fertilizer”
CGA 2.1-2014
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5.2.2.1 The entire container shall be postweld heat-treated after completion of all welds to the shells and heads. The method employed shall be as specified in the ASME Code, except that the provisions for extended time at a lower temperature for postweld heat treatment shall not be permitted. Welded attachments to pads may be made after postweld heat treatment [17]; and Exception- implements of husbandry will not require postweld heat treatment if they are fabricated with hot-formed heads or with cold-formed heads that have been stress relieved. 5.2.2.2 Carbon steels used to fabricate the pressure retaining parts of a container shall not exceed 70 000 psi (483 MPal minimum specified tensile strength in order to minimize the potential for the development of stress corrosion. cracks during service (does not apply to Sections 8, 9, and 10) [23). Experience indicates that containers fabricated from fine grained and low- to moderate-strength carbon steels (70 000 psi [483 MPaJ minimum specified tensile strength) and that have been postweld heat treated in their entirety after completion of all weldments are essentially free of the problem. Exception-Implements of husbandry may be fabricated from steel having a specified tensile strength of 75 000 psi (517 MPa). See also 5.2.2.1. NOTE-As an example. SA-516-70 may be used in conjunction with full heat treatment of the pressure vessel. |