Acceptable O2 level to avoid SCC in anhyd-NH3 service for PWHT vessel

[llls]

Hi, I work in a ammonia-urea plant and our NH3 stage is done in pressurised vessels. The vessels were design and fabricated to BS5500 in 1981 and have been in service since 1982. The vessels were manufactured using BS1501-224-32A-RT (with spherical dish heads) and all weld joints were post
weld heat treated (PWHT). With regards to water and oxygen levels during service we have been able to maintain:-
* 1000 ~1500ppm of water measured in liquid phase;
* 35 ~ 50ppm of Oxygen level measured in the gas phase (with the exception of high levels for one to two months after internal inspection of the vessels ex 950ppm at start up for several days and then down to 200ppm within a week or two).
The current inspection interval of the vessels is set at a 6-yearly interval and we use wet fluorescent MPI surface NDE method to test welds/heat affected areas for SCC cracks. Fortunately up to date we have not found any SCC cracking problems in the bullets (nor in other anhydrous intermediate process vessels).
We are reviewing the inspection frequency for the ammonia storage bullets and planning to extend the interval to 9-years. The basis for extended inspection interval is based on the fact that the opening of the vessel for internal inspection could create more risk (due to the high oxygen levels we get after these inspections can cause SCC cracking problems as we typically operate the vessel with 1000ppm of oxygen and after many purging cycles operations manage to get the levels down to 100ppm in about two months time) of initiating a SCC cracking problem.

I would appreciate feedback on the oxygen level/s that is acceptable in anhydrous ammonia service vessels that were PWHT. Based on our past vessel inspection experience I am currently using the figure of 50ppm (in the gas phase) as acceptable for out PWHT vessels, pending that the water level is kept above 1000ppm in the liquid level.
I would take this opportunity to thank you in advance for your time replying my inquiry.

Regards

Leonard Seneviratne
Senior Mechanical Engineer

 

[BenThayer]

you are on the right path with regards to determining your inspection interval based on your own experience.

I know that you have been in other threads regarding ammonia scc. i am assuming that one of the sources of your oxygen is due to the air/oxygen that you are adding to the urea process for the reactor passivation. you are basically stuck with some residual oxygen although you may review your purge sampling techniques. the oxygen level may be elevated in your purge gas from the condensers and be higher than you are actually seeing in the ammonia recycle receiver. you may want to look for scc in the condensers and assoicated piping.

one other thing that occurs when commissioning ammonia equipment is the potential for extreme cold temperatures because you are normally adding liquid ammonia to a vessel that only has nitrogen in it.

after all your pressure tests are done to confirm the vessel/piping integrity, you can then commission the equipment.

the correct way to commission a liquid ammonia vessel is to purge with nitrogen to get the oxygen out, followed by purging with ammonia vapor to get rid of the nitrogen and any residual oxygen.

now that you have ammonia vapor in it, pressurize it to about 50 psig prior to adding liquid. this will prevent the liquid from flashing to extreme cold temperatures. these extreme cold, sudden temperatures cause a significant contraction force on the inner surface. this is something you want to avoid with a vessel/service prone to cracking.