PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
(OP)
Is PWHT'ing of Carbon Steel Pipe For Anhydrous Ammonia Service just a customer requirement or a particular code requirement? If a code can you tell me which one and what paragraph? Our design temperature is -50°F. If the weld hardness is kept below 200 BHN is it really needed? Thanks for any input.





RE: PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
RE: PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
Always remember that any code is the minimum requirements.
RE: PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
RE: PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
RE: PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
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Cracking of Steel in Liquefied Ammonia
RE: PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
Most everything we deal with can be harmful and in fact cause permanent health issues including death, but few are treated as lethal. This is a real flaw that lethal service has been identified with its special precautions but a comprehensive list was not submitted.
I knew a man that was killed in an ammonia mishap and I have known people that have drowned. Water is not considered lethal service.
All that being said, I am in favor of PWHT'ing if using carbon steel. I am also in favor of chucking the carbon steel and using stainless. If you are using carbon steel to -50°, you are likely using A333 which is not easy for the end user to find and maintain the system after the initial installation. It can also be easily mistaken for a standard grade carbon steel several years from now when a maintenance repair becomes necessary.
RE: PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
RE: PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
The original question was regarding PWHT of pipe and not vessels. I am not aware of any code requirement for PWHT'ing of pipe. I think it is a very good idea for pipe if it is carbon steel since the mechanism that causes SCC in vessels would have a similar effect on piping.
I just think that for a variety of reasons, stainless makes more sense for pipe if the customer requires PWHT for carbon steel pipe. If the pipe is upgraded to stainless, there is no reason to PWHT or paint. Certainly for a vessel, carbon steel is the economic choice and must be PWHT.
Also, threaded pipe would not require PWHT but there is a lot of stress in piping when it is threaded and I just plain do not like (not very scientific here) threaded pipe for ammonia service.
RE: PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
I've put in a number of these systems following ASME B31.3 (normal) without incident. I've been following up with users and tracking every accident report I can find. I have not seen even a hint suggesting PWHT as an improvement(have seen leaks for every other reason). Will post any new info
RE: PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
The mechanism of SCC can only be stopped in carbon steel by the addition of 2-5% water by weight in ammonia.
RE: PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
Carbon steel, PWHT'd pipe will prove less expensive than stainless steel pipe depending on the size and number of welds in the system.
During the past 10 years, we have installed carbon steel without PWHT, carbon steel with PWHT and stainless steel in anhydrous amonia piping systems. All were designed for similar service conditions. The materials/PWHT requirements were based on the desires of individual Engineers or Owners. All systems have operated without incident to date. Most of our Customers have specified either carbon steel without PWHT or stainless steel. Most recently, one of the LARGEST petrochemical firms has specified carbon steel with PWHT for a similar piping system and we shall comply with those requirements.
If you are the Owner/Operator, the decision is yours to make based on your design requirements and any added jurisdictional requirements in conjunction with your Company's risk assessment of possible liability or negligence should an employee or other individual be injured or die as a result of an SCC caused leak exposure.
Often times, industry standards prevail in damage awards. And, unfortunately, industry standards are often set by the LARGEST and most influential companies - rightly or wrongly.
RE: PWHT'ing of Carbon Steel For Anhydrous Ammonia Service
I believe that PWHT is a very good improvement for the weld and HAZ in reducing risk of a SCC failure mode. since it will reduce the stress level variable of SCC..
Hardness requirement must be much lower than 200 BHN...it looks high for carbon steel in a service susceptible to SCC.
It is also important to consider besides water content the O2 level during service, specially when getting the material expose to air during shut downs.