Luis For storing anhydrous ammonia in a pressure vessel (either cylindrical storage, spherical storage type vessels) you have to follow the same guidelines as what is applicable under NACE MR 0175. Hardness limits in welds and HAZ should be restricted to a max. 220 BHN ( Rc 20) for prevention of SCC. In extreme cases, with contaminants of CO2, oil traces from pumps or compressors (carried over in the system) or other mercaptans may contribute to minor SCC at lower than HRc 20 ( < 220 BHN) depending on the quantity of oil, moisture and CO2 present in anhydrous ammonia. The same is applicable for liquid ammonia with contaminants of CO2, carry over oil traces or moisture in the system. Trust this helps you C.V.Srinivasan Nishi Engineers Pvt Ltd India April 19, 2007 E-mail: nishi@vsnl.com >Dear Sirs, > >If I have a pressure vessel with ammonium inside, which >it%E2%80™s the maximum hardness in welds that can I have to >avoid SCC (ex: material HII). > >Many Thanks > >Luis