Newb qc question

[Welderturnedqc]

Hello all. I have been lurking around here for a while and finally joined. It seems as though there is an abundance of knowledge on this site. I was a welder for a long time and now I have taken on a role as an inspector. During my time as a welder I didn't pay too much attention to some of the finer details. Now I am required to pay attention to this stuff. Anyway, my question is how is it that a line can be designed using say a 900# flange with a branch having a 1500# rating and 3000# valves. I don't exactly understand the difference between how a valve might be rated vs. a flange or how a line would be flange rated with different classes involved. I would assume the line could only be rated to the lowest rated flange class. And what about o-let ratings? Any insight to this inquiry would be greatly appreciated. Thanks

 

[JJPellin]

Every component in a piping system is rated to operate up to a certain maximum pressure. The pressure rating will often depend on the temperature. The line would be rated to the lowest rating of the components. A 900# flange is rated to much more than 900 psi.

Johnny Pellin

 

[LittleInch]

It gets a bit confusing sometimes, but some of this is about small sized equipment and practicalities. Also pipe is designed to a design pressure, which sometimes is chosen to maximise the flange rating, but often is lower than the max flange rating pressure in ASME B 16.5

The design rating is taken from the lowest pressure component, which may be the pipe as this is calculated by design pressure, not the step like approach used by flanges (i.e. you can't ask for a #750 flange, but you can use sch 40 pipe instead of sch 80). Small branches are sometimes over spec'd because they are physically stronger, but also below 2" #900 and #1500 flanges are identical and hence most suppliers stamp them #1500 as it then covers both.

#3000 valves again are simply good strong small diameter valves which you know will do the job and the difference in cost is very small.

O-lets are bought to match the pipe and hence don't strictly have a pressure rating, but if the pipe is good enough then so is the o-let fitting.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[GarethChem]

Hi Welderturnedqc, and welcome

If I can add to what the other guys said - the flange rating also has to do with the operating temperature - the higher the operating temperature, the lower the pressure rating. You might find this link useful, it has the pressure ratings for a few different classes:

http://www.engineeringtoolbox.com/steel-flanges-pressure-rating-d_49.html