TML & API 570 1

[Corgas]

According to API 570 TMLs can be eliminated for piping systems if "Noncorrosive systems, as demnostrated by history or similar service, and systems not subject to changes that could cause corrosion".

How many isnpections do I need (without finding thinning) to do in order to drop a particular TML?

Any suggestions or guidelines?

Thanks,

 

[SJones]

The key phrase in the quote from API 570 is "systems not subject to changes". It isn't only about the thickness readings it's also about process stability, operating history and, possibly, prior treatment of inspection data. Process monitoring data must be one of the key input parameters for the decision process. If the process corrosivity assessment hasn't changed over the inspection life then it could be possible to statistically infer that there is no future effect on remnant life. The first real question is: is the process data collection a) sufficient enough and b) reliable enough for judgement to be made about fluid corrosivity changes at the TML. The second question is: what confidence is there in the inspection data to say that corosion is at a sufficiently low level at that location not to monitor for it.

Personally, I would say that if the TML in question is not consuming a wholly disproportionate amount of resources, and is not in such a situation as to give worthless reliability of measurement, then it would probably be considered an ALARP (as low as reasonably practicable) measure to continue to take readings and possibly just reduce the frequency.

Guidance on determining the number of measurements may be found at:

http://www.hse.gov.uk/research/rrpdf/rr016.pdf

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04