To my knowledge, the answer is no.  There more important question is why would you knowingly install a critical component such as a flange in a service it is not rated for?  Do th right thing and use a 600# flange.

You could try rerating the flange per Appendix 2 but I doubt it will work. You could substitute the the flange material for a carbon steel lapped flange and get a higher rating. Maybe this is your problem - ss vessel with cs piping ?

Totally agree with jay165 given your design information. Change out the flange to the proper Class per ANSI B16.5.

paddyharry-

I generally agree with the above postings. But I've been in enough sticky situations where you are grasping at straws to make a design work on paper to commiserate with your situation.

As codeeng pointed out, you are not restricted to the B16.5 flange rating if it calc's out per Appendix 2. Before the rest of you scream that the piping also has to meet the rating, I'd ask you to read B31.3 (assuming that would be the applicable piping code) section 302.2.4. I've been involved with enough flange leaks to prefer increasing the flange rating, but sometimes that's just not feasible (notice I didn't use the word "possible"). To extend codeeng's other suggestion a bit, consider a 304 overlay on a CS flange.

Another possible option is to discuss the situation with your client and try to get the design pressure lowered to 495 psi (or drop the temp to around 370°F). "Wait, wait," you say, "the flange rating for F304L is only 420 psi at 500°!" Yup. So find some flanges which are dual rated F304/F304L. The L grade gives you the weldability you need while the straight grade gives you the extra 75 psi. Been there, done that. Unfortunately the extra boost of the straight grade doesn't get you to the necessary conditions, but it just might be close enough...

Again, if it is a whole new system, go with the CL-600 flanges. People like me will not have to deal with them if/when they leak. But if you can't, explore other options.

jt

A carbon steel lapped flange enables you to still retain the stainless nozzle with a stainless lap face. No need for any weld overlay. Perhaps you could confirm where the design pressure is coming from (I still suspect its driven from carbon steel piping).

You may interpolate and find you can use the 304 material exactly at 400F, ( i did not look the spec) so you must.
What I know is that when dual cert mats can not be used dually, you have to choose the rating at design in this case you design with 304 or 304L, can use both on one material.
ER

I found this calculator for max pressure of different flange classes and materials:
http://www.gowelding.com/pv/flange.htm
Enjoy

Keep the wheels on the ground
Bob
showshine@aol.com

paddyharry,

You are allowed, under the rules of ASME B31.1 and B31.3, to have short excursion in temperature and pressure over the original design.

I believe that these excursions are to 15% above the design pressure for a very short period of time.

You should investigate these provisions of the piping codes and see if they are of benefit to you...

Otherwise, I agree with the comments noted above......buy the Class 600 Flange

My opinion only

MJC