PWHT of Piping components in Sour service 2

I can answer the question below

1. Most of the literature discuss about Hardened micro structure control & Hardness, but not about the effect of residual stress. Is threshold stress for initiation of SSC is high?.But NACE Tech committee report 8x294 touches this subject.

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Reply; SSC or even SCC require three conditions acting simultaneously; environment, tensile stress and susceptible material. In the case where PWHT is performed you get the benefit of reduced residual tensile stress and lower hardness, which equates to eliminating two of the three conditions necessary for SSC or SCC.

I can answer 2, 3 & 4 based on my own experience at sour crude refining.

2. Upsets and unforeseen events such as equipment breakdowns happen all the time in refining and suddenly piping diets change unexpectedly. So now, with a pipeline higher levels than expected of H2S or anything else related to SSC will be present and can run for X amount of time until the problem is fully understood and remediated. When PWHT is recommended (not required) by industry experts due to service, we usually heed the advice and perform it.

3. Typically is required based on the process and to some degree past experiences and lessons learned.

4. Yes.

Metengr,


Agreed. Its understandable that "Stress" Plays a role in "Stress" corrosion Cracking & Sulfide "Stress" Cracking. But my question is, is residual stress generated during normal welding practice is a determining factor in SSC? if yes, then why PWHT is not mandated in NACE for carbon steel or in other standards Practices.

For example in a large scale refinery project, PWHT of piping weld consumes lot of time & money. Unless it is technically proven that non-PWHT will lead to SSC in wet H2S environment, its not justified to enforce PWHT.

Please share your opinion and experience of such failures...

DVWE,

Thanks for reply.

It has been my experience over the past 40 years on Refinery projects that a number of Owner's were specifying PWHT is Sulfide SCC service environments. Based on dissimilarities in carbon equivalents of pipe heats and global manufacturing sourcing, it became more difficult to assure meeting the hardness requirements in all cases. This is further compounded by the actual welding parameters encountered in both shop and field.

ispengg said:

is residual stress...a determining factor in SSC?

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Residual stress is a contributing factor, rather than the determining factor. The tenet of the NACE/ISO standards with respect to SSC of carbon and low alloy steel is that, with the correct heat treatment condition and hardness value, the likelihood of initiation of SSC is reduced. It is not guaranteeing immunity.

PWHT is not mandated partly because of the reasons that you have cited. It will be down to the end user to assess the risk associated with an SSC failure in order to reach a decision as to whether additional, more costly measures are of value. Part of that risk assessment may feature the issues that DVWE and weldstan have correctly highlighted.

Process licensors may have legal and reputational considerations in mind when specifying requirements, thus they could tend to err on the side of conservatism. Those that don't may just be thinking "how lucky do you feel."

Steve Jones
Corrosion Management Consultant


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

Sjones,

Thanks for the reply