Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Newb qc question

Newb qc question

Newb qc question

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

RE: Newb qc question

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

RE: Newb qc question

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

RE: Newb qc question

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:


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close