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01:53 Jan-18-2009
In service pipe thickness limits

I been doing UT thickness gauging of process line in a petrochemical company, and have been asked a question by a colleague I cant answer. I understand that on piping the ASME code allows a 12.5% reduction of wall thickness and still remain acceptable. I understand this to be true for new fabrication.

But wahat about pipes that has been in service for several years?

At what point would a pipe be considered to be worn beyond its service life are require replacement? Is this a judgment call by the owner/consultant?

14:17 Jan-18-2009

bob sudharmin

Engineering, Reliability and Integrity Eng
Shell Malaysia Trading,
Joined Jan 2008
Re: In service pipe thickness limits In Reply to Spirochete61 at 01:53 Jan-18-2009 (Opening).

My recommendations: Look up API570

15:57 Jan-18-2009

Michel Couture

NDT Inspector,
Joined Sep 2006
Re: In service pipe thickness limits In Reply to bob sudharmin at 14:17 Jan-18-2009 .

I'm not too familiar with the petro industry, but if it is like anything else, the amount of wear should be measured from time of fabrication.

20:11 Jan-18-2009

John O'Brien

Consultant, -
Chevron ETC ,
Joined Jan 2000
Re: In service pipe thickness limits In Reply to Spirochete61 at 01:53 Jan-18-2009 (Opening).

You may need to do a little more research. Pipe specifications for seamless pipe are set at +/- 12.5% as new due to the inherent fluctuations in wall thickness from piercing mills. For pipe made from plate i.e that with a seam weld you may well find the allowable new variation is 0.1" max. If in a refinery or similar you should look at API 570 for guidance on assessing the acceptance of remaining wall thickness as measured in-service. Following that you can also consult API-ASME FFS1 (Used to be API RP 579) for methods on performing fitness for service on in-service piping.

02:54 Jan-20-2009

Roger Duwe

NDT Inspector, API-510, 570, 653
Joined Jan 2009
Re: In service pipe thickness limits In Reply to John O'Brien at 20:11 Jan-18-2009 .

The typical 'in-house' limit is 1/2 the original nominal thickness. The pressure limit calculated per API-570 will be minute -- usually 3 to 10% T [or thinner]. At these thicknesses, the pipe it too mechanically weak to support itself safely, let alone with a liquid in it. If you list a line for replacement at 1/2T, there is no rush to replace it. If you wait until 1/4T, and a 325# pipefitter walks on it between the hangers, you may have a catastrophic failure.

14:39 Jun-19-2009
Re: In service pipe thickness limits In Reply to Spirochete61 at 01:53 Jan-18-2009 (Opening).

Why not you compare actual thickness with the design one, can be calcluated from ASME B31.3 (Min. Required design thickness).. Usuall nominal thickness is higher than designed especially in low pressure and temperature service.(Inherent factor of safety)..

The best decision will be from API 579RP, first check what type of corrosion there, select proper chapter from API 579RP and carry out Level 1 or Level2 assessments. Hopefully, required decision can be drawn after Level2 calculations easily but if still problem a detailed stress analysis as per Level 3 assessment will be required. But usually Level3 assessment is not required!


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