Ageing Steel

[GL431]

I have following situation (as an example):
I have to evaluate the inlet an outlet boxes of an aircooler.
Operating Pressure: 5.000 psig
Operating Temp: 200°C
Medium: Natural Gas with up to 10% H2S (presumably by volume)
Operating time until now: 35 years.
Right now I cannot say which steel was used and which fabrication techniques were employed. (will know this soon)

The question is, can the steel become unsuitable for the service because of its age? Do changes occur in the steel microstructure and chemistry which should be considered? Does anybody know some good literature or case studies on this subject? I could not find anything in the API 571 which was directly relevant to the question.

 

[CoryPad]

Hydrogen sulfide damages metals, including steels.

You can consult ISO 15156-1, -2, & -3.

http://www.iso.org/iso/en/CombinedQueryResult.CombinedQueryResult?queryString=15156

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[metengr]

GL431;
By the nature of your question, it sounds to me that you need to perform a condition assessment of the air cooler, is this correct?

If so, in addition to the above for background information on damage, once you have identified the MOC, you can select various nondestructive testing methods (ultrasonic examination and wet fluorescent magnetic particle testing) to assess the condition of the vessel welds and base material.

Basically, if you keep the air cooler dry at all times, the exposure of carbon steel or low alloy steel to dry hydrogen sulfide will not be a problem because hydrogen induced cracking (HIC) occurs at higher temperatures. The web site below has an interesting report that might be of interest to you;

http://www.ornl.gov/~webworks/cppr/y2001/rpt/8435.pdf

 

[GL431]

CoryPad and metengr

Thanks for your help. We have a handle on the cracking mechanisms and NDT. My specific problem is the age of the metal (30 years plus). Even in the absence of cracking, blistering and other forms of damage, could the metal undergo changes over such a long period of time in service and in contact with process fluids, which should be considered in a condition assessment? Changes which cannot be seen by the NDT methods.

Another question: Does HIC occur only at the weldment or does it also occur at other parts of the pressure vessel plate which could be some distance of a weld? NACE RP0296 addresses only weldments and the examples given show HIC in HAZs. This question is for defining the scope of NDT for HIC detection during an upcoming shutdown.

 

[CoryPad]

HIC occurs preferentially at areas of high hardness and/or chemical segregation. This makes welds most susceptible, but not the only location for failures.

What is/are your materials? 200 [°]C is not that high for most steels, so aging might not be a concern.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[metengr]

could the metal undergo changes over such a long period of time in service and in contact with process fluids, which should be considered in a condition assessment?

Given your stated service conditions, no.

Does HIC occur only at the weldment or does it also occur at other parts of the pressure vessel plate which could be some distance of a weld?

HIC can occur in base metal, HAZ and in welds. Normally, weld regions are given highest priority because of metallurgical changes to the vessel steel as a result of the thermal affects from welding.

 

[GL431]

Thanks for your help. It is clearer to me now. Our plant inspector will inform me of the materials tomorrow. I was surprised that HIC is a concern in the parent metal, but now I realise that the plant was built before people started to specify HIC resistant materials at the requisitioning stage already.

 

[GL431]

The material is SA 516 Gr. 60 or Gr. 70