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Discussion Forums - The Hendrix Group
HomeHomeDiscussionsDiscussionsGeneral Corrosi...General Corrosi...SCC attack on Low temp Ammonia LineSCC attack on Low temp Ammonia Line
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5/25/2010 10:00 AM
 
[updated:LAST EDITED ON May-25-10 AT 08:28 PM (CDT)]Hi, it is related to a similar question I asked previously, we have provided a drain line on an elbow which is part of the ammonia loading arm. The average hardness on its HAZ comes to 214 and 227 HB while the partent metal is softer. As the hardnesses measured are more than 200 mark, it is required to do the PWHT to bring down the hardness. As heat treatment may distort the eblow as there are close tolerances in the speacial flanges welded to this elbow. This raises few questions which I like to take your opinion on: 1. Is the hardness on welds kept below to avoid SCC and or to avoid the risk of brittle failure (the temp is -33Deg C when operating) 2. The ammonia will generally have 0.5 percen water, still there are chances of SCC if this is the main concern? Since the use of loading arm is non continuous, the welded area is exposed to oxygen after each use which adds to the risks of SCC. 3. The hardness is measured from outside, does this truly represent the hardness the inside surface which will be exposed to liquid? does this call for internal harness test to get accurage results? 4. Which section of B31.3 details the hardness requirement for Ammonia service. regards regards
 
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5/26/2010 10:00 AM
 
What's the material of piping? Regards, Ashfaq Anwer http://forums.thepetrostreet.com
 
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5/26/2010 10:00 AM
 
Bigpond Piping material if made of C.S to A 333 and fittings made of C.S to A 350 grade, chances of this hardness slightly close to doing PWHT limit to bring down hardness < 200 BHN may not affect performance of the same materials at - 33 deg cent in liquid ammonia service. Risks of SCC on C.S fittings and pipe at this low temp - 33 deg cent for ammonia service may not be there with 0.5% water. Hardness measurement is always done from outside for each weld with an assumption that the internal weld hardness will be more or less the same. Since the loading arm fitting is not operated continuously the only care you may have to take is not to subject the loading arm to sudden shocks or drop after pumping operations B 31.3 will not specify hardness limit for a specific service. This is drawn from experience by users / fabricators / manufacturers and operators of ammonia plants - mainly based on experience on SCC in ammonia service after long service hours usage. US committee on Agriculture - E.H.Phelps Committee report - on failures experienced in ammonia road tank recommended that 0.2% water in low temp ammonia will inhibit SCC trend in low temp ammonia service using A 333/ A 350 fittings. If you are afraid to carry out PWHT due to special flanges made to close tolerance limits are likely to get distortion from PWHT, the only option you have is to operate carefully without sudden shocks to C.S made of A 333 and fittings made to A 350 even with this slightly higher hardness around 230 BHN. If you wish to be doubly sure that you could tolerate some heating by a controlled PWHT at slightly lower temp around 500 deg cent but giving more time for the fitting to reduce hardness as well as maintain without distortion effect on the special flange with close tolerance limits, you could do the same to be free from slight worries of sudden shock effect on the materials used Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India May 27, 2010 E mail: nishi@vsnl.com >Hi, it is related to a similar question I asked previously, >we have provided a drain line on an elbow which is part of >the ammonia loading arm. The average hardness on its HAZ >comes to 214 and 227 HB while the partent metal is softer. >As the hardnesses measured are more than 200 mark, it is >required to do the PWHT to bring down the hardness. As heat >treatment may distort the eblow as there are close >tolerances in the speacial flanges welded to this elbow. >This raises few questions which I like to take your opinion >on: >1. Is the hardness on welds kept below to avoid SCC and or >to avoid the risk of brittle failure (the temp is -33Deg C >when operating) >2. The ammonia will generally have 0.5 percen water, still >there are chances of SCC if this is the main concern? Since >the use of loading arm is non continuous, the welded area is >exposed to oxygen after each use which adds to the risks of >SCC. >3. The hardness is measured from outside, does this truly >represent the hardness the inside surface which will be >exposed to liquid? does this call for internal harness test >to get accurage results? >4. Which section of B31.3 details the hardness requirement >for Ammonia service. >regards >regards
 
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