SCC of ammonia storage tanks

[MadDog88]

I have read that between 100ppm and 1000ppm of water should be added to liquid ammonia being stored in carbon steel tanks (-33 deg C and ambient pressure). How does the water addition eliminate the risk of SCC? Also, can SCC occur if there is no oxygen present?

Thanks in advance for your comments.

 

[metengr]

MadDog88;
I am involved with the National Board Inspection Code main committee, and we deal with this issue. I have attached an interesting NBIC article that should be of interest to you;

http://www.nationalboard.net/classics/classic8.html

 

[MadDog88]

Thanks for that metengr - very informative. Cheers

 

[unclesyd]

A little more information on this thread.


thread330-79163

 

[MadDog88]

Thanks again, great stuff. We have a 5000t ammonia tank approx 4 years old. AE testing was done prior to initial filling and I am now looking at what needs to be done on this tank in the future as part of the inspection program.

Approx how large does a SCC crack have to be before it is picked up by a reputable AE operator? SCC cracks are intially very fine so is there potential to miss cracks in their initial stages? Although it would be costly and time consuming, should mag particle testing be carried out on welds also?

Cheers

 

[unclesyd]

It was our experience that AE was picking up cracking in the sphere below threshold of detection of MT (WF). The trigger level was set by the operator by the use of gain on the preamps and with gates with UT.
There is a specific program design for testing NH3 Spheres called Monpac. A lot of work on this program was done on our NH3 Spheres by Dr Fowler of Monsanto. These spheres stayed wired with tranducers until taken out of NH3 service.

The company that sells the AE programs and Equipment.

http://www.pacndt.com/index.aspx?go=technologies&focus=Technology Pkgs.htm

A company that does AE inspection.

http://www.matrixie.com/nh3.htm

The sensitivity 25 years ago was amazing I can only imagine what it is today.