Cleaning ammonia vaporizer

[BRT549]

I am trying to improve the operation of an ammonia vaporizer in fertilizer production service. We switched ammonia providers last year, and am seeing a drop-off in vaporizer performance. The bottom of the bayonet-style vaporizer is full of water after a run, and there are significant rust deposits on the tubes. The rust and water seem to be coming in from the rail cars and ending up in our storage vessels, eventually ending up plated out on the vaporizer tubes. To do an annual cleaning, I'm looking for rust removal via chemical cleaning, since it's only a thin layer. The tubes and tubesheet are 316ELC and the shell is SA-516-70.

 

[Montemayor]

Brt549:

You say that “there are significant rust deposits on the tubes”. However, you also state “The tubes and tubesheet are 316ELC”. Therefore, the rust deposits you are finding on the tubes do not originate in the tubes; the rust must be coming from the SA-516 Gr 70 shell.

I would not be concerned with the condition of the tubes as I would be about the weakening of the carbon steel shell. Cleaning out the rust does not resolve the problem of losing wall thickness. You are obviously buying cheap or contaminated anhydrous ammonia (that isn’t anhydrous!). I would be cautious about cleaning out the vaporizer with chemicals. Firstly, the 316ss is sensitive to chloride ion attack and getting rid of rust and water is not a job for chemicals – it’s a job for draining and manual cleanout.

If the rusting starts to take a toll on the vaporizer shell, you may have to change out the shell for 316ss if you continue with the same quality(?) of wet ammonia. But you will still be required to drain out the vessel and inspect it. I would think about changing the quality of the purchased ammonia as an option. It may not be justified, but the exercise should be done. If you are witnessing rust formation, then a monitoring of shell integrity is warranted for the vaporizer – which I suspect is an ASME Section 8 pressure vessel.

 

[cloa]

Any chance its a terminology stuff up and he really means corrosion products not rust.

 

[BRT549]

Thanks for the replies. The 'rust' is coming in from the ammonia supply, either during NH3 production or from the CS rail cars. It sticks to the magnetic float follower in the level sensor. As you mentioned, Montemayor, we are getting crummy ammonia, with typical 0.5% water and ~1 ppm iron in it. The bid process is as much politics as economics, and we are stuck with it at least until June. We replaced the shell after 7 years due to pitting corrosion - the rest of the vaporizer is circa 1995 (ASME 8 div 1 1992 A94). That was with the 'good' ammonia supply.

I'm planning on pulling the shell off to do a hydroblast of the crud I figure is in the bottom of the tube bundle. We run the vaporizer at a very low level to do some superheating and prevent liquid NH3 from entering the process. This puts the condition of the bottom 1/4 of the tubes at a premium. I'm investigating adding a superheater downstream so I can run the vaporizer like it was intended, but capital $$ is pretty tight.

Anyway, I still want to clean up the tubes, even after a manual hydroblast. I was either going with a citric acid or ammoniated citric acid solution, but I thought I'd check here first. Maybe even a phosphoric acid wash, since we have plenty of that here.