Hydrogen Induced Cracking : Hardness criteria for NH3 storage tank 3

[purnawan]

For ammonia (NH3) storage tank (plate material of A537 Cl.1) of capacity 20.000 MT, is it a must to apply API 620 appendix G (G.3) Hydrogen Induced Cracking ??

 

[metengr]

Is it a must? I reviewed Appendix G and from the standpoint of weld heat affected zones and hardness, yes, this would apply to the storage of anhydrous ammonia. I would be much more concerned with the risk of stress corrosion cracking of the weld heat affected zones in the storage of anhydrous ammonia if no PWHT is performed during fabrication. Also, the water content of the anhydrous ammonia must be monitored to reduce the risk of stress corrosion cracking. So, I would say it is good design practice to follow Appendix G for reducing heat affected zone hardness.

 

[weldstan]

In the USA for carbon steel pressure vessels in anhydrous amonia service, PWHT is mandated.

 

[purnawan]

Dear metengr & weldstan, scope of Appendix G.3 is the corrosion because of presence of hydrogen sulfide in the service condition, isn't it?. In the existing ammonia storage tank (I have 3 exisiting tanks) we inspect every 5 years, no corrosion problem I found. Normally liquid and vapour ammonia are not corrosive media. Do we still have to do hardness test? please advise.

 

[metengr]

No, not completely. The general concept in G.3 is the fact that the weld heat affected zones should be at or below 22 HRC scale in hardness to reduce susceptibility to hydrogen-induced cracking. Hydrogen-induced cracking is a subset of stress corrosion cracking, and as I mentioned hardness must be below 22 HRC scale for anhydrous ammonia, so PWHT is necessary.

 

[SJones]

The degradation mechanism in anhydrous ammonia is 'ammonia stress corrosion cracking.' Measures for its control can be reviewed in API RP 571, 4.5.4 and include the use of PWHT and restricting weld hardness to a maximum of 225 HBW.

In terms of inspection, a useful paper would be:

M Abdullah, et al, Stress Corrosion Cracking, Crack Growth Prediction, And Risk Based Inspection Of Industrial Refrigerated Ammonia Tanks, Corrosion, Vol 67, (4), 2011, pp 046001-1 - 046001-12

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[purnawan]

thank you very much gentlemen for the responses and advise

 

[MJCronin]

Information from the National Board is always useful:

http://www.nationalboard.org/index.aspx?pageID=164&ID=182

Also, it is important to be aware of this US Safety standard:

ANSI K61.1 "Safety Requirements for the Storage and Handling of Anhydrous Ammonia"

Purnwa, if you are aware of a similar ISO or DIN standard it would be useful for you to tell us.....

MJCronin
Sr. Process Engineer