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New Anhydrous Ammonia Storage Vessel Fabrication & Material Susceptibility to Ammonia SCC

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sahsanb

sahsanb

Materials
May 31, 2013
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Dear Experts,

We are getting a new Ammonia Storage Vessel fabricated. Given below are major equipment design details

Material (nozzles/ shell & heads): SA-333 GR.6 / SA-516 GR.60
Design Pressure: 30 kg/cm2
Design Temperature: -20/70 OC
Material thickness: 32 mm

As per equipment manufacturing quality control plan, hardness testing will be performed before and after pwht to assess pwht effectiveness and identify if there is any anomaly. However I would appreciate your thoughts on what additional NDEs can be performed to assess pwht effectiveness and material susceptibility to Ammonia SCC specifically.

We also have a test coupon of same material heat that was used for vessel construction and was subjected to the same pwht as vessel. Would also appreciate your thoughts on destructive test(s) along with guidelines for acceptance criteria that can be performed to assess material susceptibility to Ammonia SCC.

Regards,
 
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Normally ammonia does not cause SCC in CS.
The whole vessel could be of SA 333.
Else, the vessel could be of SA 516 with nozzles of SA 106.
Is the new vessel being fabricated is identical with the old one metallurgy wise?

DHURJATI SEN
Kolkata, India


 
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I disagree ...

Sen states: Normally ammonia does not cause SCC in CS.

But long term US/EURO experience with AA pressure vessels indicates SCC cracking is a significant design concern !!

Page 52 of this venerable NIST guideline says:

Careful inspections of vessels used for storage of ammonia (in eithervapor or liquid form) in recent years have resulted in evidence of serious stress corrosion cracking problems. Statistics reported at a meeting on this problem [37] indicate cracking in approximately one-half of the vessels
examined.

The vessels for this service are usually constructed as spheres from one of the carbon steel grades, and they operate in the ambient temperature range. The water and oxygen content in the ammonia has a strong influence on the propensity of carbon steels to crack in this environment. Figure 15 shows the U.S. and European guidelines for operation and inspection frequency


MJCronin
Sr. Process Engineer
 
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If my memory is correct, OSHA requires stress relieving heat treatment (PWHT) of carbon steel vessels in anhydrous amonia service. That is all that the manufacturer did for such vessels. I believe that the EPC may have required full RT as well.
 
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welstan, et al....

I believe that OSHA 29CFR1910.111 (Storage and Handling Anhydrous Ammonia) requires vessels larger than 36-inch diameter or 250 gallons to have PWHT



A personnal note here: .... I believe that it is COMPLETELY MORONIC to put PWHT requirements in an OSHA standard .... Who the hell would think to look into OSHA equirements for pressure vessel design criteria..

IMHO, the USA would be WELL SERVED to develop complete and individual NEW AiCHe Pressure Vessel and flat-bottomed tank standards for commonly traded deadly liquids. These would include standards for Sulfuric acid, HCL acid, Caustic Soda, Anhydrous Ammonia, etc.. etc

We should follow the wise, chemical specific regulation examples of the international/EURO communtity..

Anybody have any thoughts about this obvious scattershot of hot defecation that are the US vessel regulation ?????????

MJCronin
Sr. Process Engineer
 
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Thank you very much gentlemen for your detailed feedbacks. As ammonia fertilizer manufacturing plant operators we have faced Ammonia SCC related issues sporadically. RCA for most of the issues observed at our site have pointed to non-compliances to required fabrication requirements at the time of original manufacturing.

In this particular case we will be proceeding with Impact Testing and Macro-examination of test coupon, additionally we will be doing hardness testing and insitu-metallography of both the vessel and test coupon.

Would appreciate your feedback if you think we should be doing any additional test.
 
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Sunshab...

1) You state that your new Anhydrous Ammonia Storage Vessel will be designed to a Design Pressure of 30 kg/cm2

This translates into a design pressure of ~427 psig .... This seems high, IMHO .Can You explain ?

Most of the AA ASME pressure vessels I have been involved with are a MAWP of 250 psig, ASME, Section VIII, Div 1


2) Commercially available everywhere, massive AA vessels are almost always horizontal axis CS with hemispherical heads. The shell MOC is typically SA 515 Gr 70/SA 455. The same PV design pressure (250 psig/265 psia) is sold for propane storage as well...Many old vessels 40-50 years old are used, reconditioned and sold over and over ...... With all of the huge inventory of used tanks, why are you buying a brand new vessel with your odd choice of Carbon Steel Materials ?




Oh, yeah .... You gave us a tiny bit of information aboit your new tank .... but, what is the diameter, shell length and Code of Construction ??

MJCronin
Sr. Process Engineer
 
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