Anhydrous Ammonia Vessel Fabrication

[sahsanb]

Dear Experts,

We have been facing recurrent cracks issue on our Ammonia Let Down Vessel. I have already discussed on the issues being faced on this vessel a few years back as well. Link to the discussion is given below:

https://www.eng-tips.com/viewthread.cfm?qid=408459

More cracks were observed during future inspections as well and we have finally concluded that owing to high consequences of any in-service leakage its better to replace the vessel with new one. Given below is some design information of existing vessel.

Service: Ammonia
Shell Material: A516 Gr. 60 N
Operating Temperature : 2 oC
Operating Pressure: 26 kg/cm2 g
Shell Thickness: 32 mm
PWHT: Yes (But no record available)
Vessel Orientation: Horizontal

While we suspect poor workmanship & improper heat treatment as the most likely cause of our troubles, I am writing this post to seek your expert advice on weather we can get the new vessel fabricated from a material other than A516 Gr. 60 N that will avoid this issue reoccurence.

Regards,
SAB

 

[Dhurjati Sen]

Dear sahsanb,

Properly fabricated (including documented PWHT) ASTM A 516 Gr 60 N vessel would work just fine.

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[MJCronin]

Good information here:

https://www.tannerind.com/sto-anh-ammonia.html

https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.111

https://www.tannerind.com/PDF/blue-anhy-amm.pdf

https://ammonia-safety.com/wp-conte...B_Davis_Ammonia_Standards_and_Regulations.pdf

MJCronin
Sr. Process Engineer

 

[blacksmith37]

It depends on what you mean by "anhydrous" ammonia . If it has "no" water ( less than about 500 ppm,) it can stress corrosion of moderately hard steel. I do not know the regulation ,but DOT sets a minimum for water in anhydrous ammonia. So if the water is very low and the weld or HAZ is roughly 200 BHN it could be stress corrosion cracking. Dr Phelps of USS did the definitive work in about 1969. Cracking of T-1 steel in tank trucks got USS involved.

 

[cap1a79]

I would focus on "no record" for PWHT.

Referring to API 571, clause 4.5.4 Ammonia Stress Corrosion Cracking, it says....

clause 4.5.4.3 of API RP 571
"Anhydrous ammonia with <0.2% water may cause cracking in carbon steel."

"PWHT eliminates susceptibility of most common steels (<70 ksi tensile strength)"


Check whether PWHT was carried out properly or not. If not available, check the hardness of weldment. It should be below 225 BHN based on API RP 571. If possible, keep it as below 200 BHN as recommended.




Lee SiHyoung,
WorleyParsons Oman Engineering,

 

[MJCronin]

Just a couple of minor notes:

Your stated operating pressure, (26kg/cm2 or 370 psig) is considerably higher than the design pressure of most commercial grade ASME VIII Anhydrous Ammonia storage tanks sold in the USA (250 psig)

This will have to be a special design ....

Also, This CGA document discusses the unique steel materials requirements for Anhydrous Ammonia

https://global.ihs.com/doc_detail.c...-2.1&item_s_key=00023390&item_key_date=851231

MJCronin
Sr. Process Engineer