Reduced Rating

Reduced rating is common where a conservative approach is applied in defining the piping materials while shaping the piping material classes.
Conservative... in the context of reduced thickness of pipes and fittings and having in some cases a reduced pressure rating of flanges. Subjectively, handling such cases is a little bit complex due to the nature of broad aspect of the parameters involved.

In a process plant, there will be few border cases of design conditions in the process streams, by which I mean the borders of the ANSI B16.5/B16.34 rating tables. On certain cases reducing to the next lower pressure rating will benefit a turnkey contractor, considering down the line cost benefit depending upon the quantum of piping involved in those streams. During such cases the piping material engineer who optimizes the PMC-piping material class concludes with process engineer to reduce the design condition.

In these border cases there are two aspects.
1) The simple safety factor reduction of the process condition judged by the process engineer.
2) The process stream may not be subjected to the defined design condition continuously, but on a cyclic or only for a short period of duration. ANSI B31.3 / 1 allows such temporary surging under certain criteria (Refer the code for details). Under such limitations, the piping material engineer works/designs the PMC with reduced pressure rating conditions also called as a STC - short term condition.

So in conclusion such conservative approach helps in avoiding Over Designing the piping system with the ANSI code as an excuse/backing. Addtionally, such concept has significance when the quantum of piping is extremely high and if the streams are in higher temp. ranges where material selection plays a very important role.

With regards,
Kannan

********

Can you please explain about " Flange Reduced Ratings".

Regards

Srinivasa Murthy