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Retirement Thickness
5

Retirement Thickness

Retirement Thickness

(OP)
Can anyone help with determining the minimum wall thickness required for structural strength (aka retirement thickness) for ASME B31.3 pipework?

I have manged to find some figures but these look to vary depending material, diameter, service and which company is involved and do appear to be that scientific. I was wondering if there is a technical specification that I could refer to?

RE: Retirement Thickness

I would refer to API 579 for guidance regarding thickness - general wall thickness wastage and locally thinned areas.

RE: Retirement Thickness

Nakka1,
        As metengr says API579 is the way to go. You need to assess the total stress levels within the pipework, not just the pressure stresses. A company I used to work for would not allow the thickness to be less than 2mm. Anything less than that was repaired/replaced. This was considered the lower bound for structural strength.

RE: Retirement Thickness

(OP)
DSB123 & Metengr,

Many thanks for your responses.

I can see that API 579 would be useful in assessing particular pipes that have been in service and have corroded/erroded.

What I am putting together is a number of new pipe classes and would like to report a retirement thickness even though the pipework is brand new. This is to give guiance to the operator. Hopefully that has clarified things?

I did manage to find an API report (attached).

RE: Retirement Thickness

But the starting design retirement thickness must be taken as tmin, from the piping design code, based on the initial design analysis. Later on in service, that retirement thickness may be changed as result of running a fitness for service analysis such as API 579-1 Appendix A. For new piping it may not be best to second guess a future operational retirement thickness as it could get badly misinterpreted.   

Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
 

RE: Retirement Thickness

I agree with Steve.

The retirement thickness (tn - corrosion allowance) must be ternmined before the design starts. Once the corrosion allownac has be stated the designer will use tn-CA in the design process.

Kevin

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