Hello there.

What exactly is the coincident ratio as per UCS-66.1. I know it is used to determine the exemption temp for impact in case UCS-66 fails. I also know that there are 4 parameters involved, namely, tr, tn, c and E. I am really confused about tr and tn. Appendix L has some examples. For example, lets say we have a shell to which UCS-66.1 is to be applied. The formula for tr and tn is the same (i.e. PR/SE-0.6P) but for tr the stress is taken at MDMT while for tn the stress is taken at design temperature. Why is it so?

Thanks a lot. Appreciate your advice in this regard.

Sharjeel

You are correct that "tr" is to be based on allowable stress at the temperature corresponding to the MDMT. "tn" is simply the nominal thickness of the component and does not refer to design temperature, etc.

The purpose of the coincident ratio is that if the component is not "fully stressed" then its impact test exemption temperature can be decreased to a colder temperature by the use of Figure UCS-66.1.

Tom.

Thanks a lot for all your five replies. I really appreciate it. In continuing with the same topic, some examples illustrating the rules of UCS-66 are provided in Appendix L.

In L-9.2, while calculating the value of

tr (required thickness in corroded condition for all applicable loadings)

and

tn (nominal uncorroded thicknes),

MAWP plus static head is used in the value of P. Now I still have two doubts.

1. Why do we use MAWP, why not the design pressure. (I believe
2. On top of it, the example adds static head of fluid to MAWP. Now static head was already used in the calcualtion of MAWP, becasue it was added to the design pressure.

My line of thought says that in this example, MAWP was not calculated, and hence the value used in this example is the design pressure used as MAWP.

Thanks a lot.

AL