api 579 rt factor

[Aberswn1982]

Hi all, tried this in the API forum but not had any replies so trying to post it here.......any views welcome

thanks:

Conflicting Rt values?? Any body have any views or guidance for me;

When completing a local metal loss assessment to API 579 part 5, I complete the steps for a level 1 assessment as per section 5.4.1 for a certain size defect of x longitudinal width and x amount of pit depth and it will satisfy the criteria and is deemed acceptable.

However I then try to 'grow' the defect to find the remaining life and estimate when the Rt/lamda values change to unacceptble region. In doing so i would continue to use the Rt = (tmm-FCA)/tc.

But in section 5.5.1.1 the Rt definition now changes to Rt=(tmm-(CR*time))/tmin. The tmin essentially replaces tc? In one example I was working on this has a significant affect on the Rt value. Potentially, a defect deemed unacceptable with Rt using tc, can then jump up to the acceptable region when using Rt with tmin???

Does anybody have any experience or found this previously, do you always assess a defect in the present condition with Rt and tc, and then move to Rt and tmin for remaining life??

Does anybody have an idea what is the engineering justification or difference between the two Rt definitions??

thanks