mixed ratings: Flange and Body

[YBM]

The subject has come up about placing a flange on a valve or fitting wherein the flange has a higher class rating than the valve or fitting body.

My position is simply that this is not to be done. However, I thought that I had once seen this published in a standard, but I do not see it in B16.34 or B16.5.

Does anyone know if this subject has been addressed in a standard?

Thanks

 

[gerhardl]

OK, so no one has any experience about this (?).

Anyhow (also without any experience here), let me try to put down some points:

a) mechanically you could of course (within the possible measurement and mechanical/material limitations) do this.

b) you could also produce material certificates and test certificates for your mixed product (within limits).

c) you would (my opinion) not be able to refer to this as anything else than what you have: a product of components according to different specifications, with no common specification covering the total product. This will again lower the juridical strength of installer and end-user if any damage should occur, now or later, caused (or suspected caused) by the installation.

d) I can imagine situations where the 'product' could come up as a suggestion. Usually this type of suggestions is motivated by
economics: unwillingness to do what is technically correct.

e) If this is the reason: I agree fully with your reaction: do not accept.

f) On the other hand I would be sceptic, but willing to listen, if the whole installation is degraded to lowest pressure class (valve body and seat).

Good luck! Would be interesting to have the end of this story with more details!

 

[PeterIgg]

I would suggest that B16.34 para 2.1.1 (g) covers this. Basically it says that the body, bonnet and cover and associated bolting shall be capable of withstanding the pressure at 38°C. I would suggest that the flange rating necessarily defines the valve class.

Of course there are instances where a valve (ie a PRV) will have differing rated flanges, but I am pressuming that this is not where your question comes from?

 

[zdas04]

It is pretty common for the valve body to be rateable to pressures higher than the flange rating (e.g., I've seen valve bodies tested to 1,500 psig before ANSI 300 flanges are installed--it is a manufacturing-effeciency issue) and as long as you rate the whole valve at the flange rating (weakest component) you are fine, happens all the time.

Going the other way would be just plain wrong. Putting ANSI 300 flanges on a valve body rated for 280 psig would be misleading, incompetent, and probably actionable.

David

 

[bcd]

I have seen this happen before. In my case, the reason was that the customer was having a hard time gettign enough bolt loading out of a class 150 flange to compress a graphite spiral wound gasket. So tehy went to class 300 flanges even though the system was only rated for class 150 pressures. The valve tag was marked with the class 150 ratings.

 

[YBM]

Thanks to all for your comments. We will not sell a valve with flanges having a rating higher than the body rating. It is just asking for problems. It was good to see that the above comments generally agree with this practice.

As to B16.34 2.1.1(g)this does agree with this what I have said, except it offers an "out" in para.4.3.3.

I would prefer a stronger statement, but I realize that there are legitimate applications. (In my case we do not have this situation.)

Thanks again for an interesting thread.