PWHT'ing of Carbon Steel For Anhydrous Ammonia Service 3

Because it is lethal. See ASME SecVIII Div1 UW-2(a).

In addition to what Deanc replied, there can be a problem with severe cracking of steel in anhydrous NH3 service. Lower stresses, along with other measures, helps to prevent this. This is probably part of the customers requirements.
Always remember that any code is the minimum requirements.

Ammonia service piping systems are coming under greater scrutiny by a number of jurisdictions. If leaks were to occur due to Stress Corrosion Cracking, penalties will be severe. The Owner/Operator has decided that taking all precautions to assure that leaks will not occur during service is in its best interest and is willing to pay the added price for PWHT. I would advise that you not take issue in this regard.

Just because the weld hardness is measured below 200 BHN doesn't mean that the heat affected zone is necessarily that low or that the root pass is that low. I would STRONGLY advise a PWHT of the steel. Saving a few <insert monetary units here> on fabrication leaves you wide open for litigation (remember the Ford Pinto?) and a whole lot of heartache for the family of the poor schlepp who was standing next to the line when it failed.

This one is a bit old,but at the time I could not remeber where I had seen this.

Technical Articles
Cracking of Steel in Liquefied Ammonia

I disagree that Ammonia is lethal service. OSHA has detailed ammonia piping/storage more than most materials but has not declared it lethal 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.

Rregarding stress relief of NH3 vessels... I believe that you should check ANSI K61.

Steve Braune
Tank Industry Consultants

OSHA 29CFR1910.111 (Storage and Handling Anhydrous Ammonia) requires vessels larger than 36-inch diameter or 250 gallons to be stress relieved.

A quick clarification regarding my comment regarding stainless and carbon steel and PWHT.

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.

I have the same PWHT concern, except I'm only operating to -28F. I'm in a low pressure (less than 400 psig) sytem and feel perhaps PWHT is excessive for the ammonia piping - especially using commercial grade (0.5% water to inhibit SCC). There is no applicable code requirement and should be way under stress limits.

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

Keep in mind that handling anhydrous ammonia requires some minor amount of water to assure NO stress corrosion cracking in carbon steel. The presence of oxygen with anhydrous ammonia results in SCC regardless of PWHT. The presence of water inhibits the formation and growth of SCC.

The mechanism of SCC can only be stopped in carbon steel by the addition of 2-5% water by weight in ammonia.

BenThayer,
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.

Base on experience, it will be a very good practice to PWHT in a ammonia service.....after long service time SCC can developed ... it has been observed in welds w/o PWHT in a ammonia process tanks.

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.