Briefly exceeding flange ratings? 3

[Canoman]

Are there any guidelines anywhere that would say if it's alright to exceed flange ratings in an upset case?

My client has installed a new pump, and we are designing a vessel that goes downstream of the pump. In order to set the design pressure of this vessel, I've investigated two cases. One is the normal suction pressure of the pump plus the deadhead differential pressure. The other case, which gives a higher outlet pressure is when the upstream vessel is relieving. The suction pressure during upstream relieving plus the normal differental pressure is the worse case.

The problem is, in this case, the flange ratings are exceeded. The piping is all existing, but the new vessels are not. So I'm afraid that I either need to tell them that they need to replace all of the piping with 300# flange ratings. If it is alright to exceed the flange rating for a brief period, then I think it will be alright since upstream relieving will not last very long.

If there are guidelines on this, where are they?

Thanks in advance.

John

 

[sshep]

You should check out ASME B31.3 paragraph 302.2.4(f)
This may be what you need.

 

[Guidoo]

For your convenience, the above mentioned ASME B31.3 paragraph 302.2.4(f) reads as follows:

(f) Occasional variations above the design conditions remain within one of the following limits for pressure design.
(1) Subject to the owner's approval, it is permissible to exceed the pressure rating or the allow-able stress for pressure design at the temperature of the increased condition by not more than:
(a) 33% for no more than 10 hr at any one time and no more than 100 hr/yr; or
(b) 20% for no more than 50 hr at any one time and no more than 500 hr/yr.
The effects of such variations shall be determined by the designer to be safe over the service life of the piping system by methods acceptable to the owner. (See Appendix V).
(2) When the variation is self-limiting (e.g., due to a pressure relieving event), and lasts no more than 50 hr at any one time and not more than 500 hr/year, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the in-creased condition by not more than 20%.

 

[jay165]

How 'bout lowering the relief valve setpoint on the upstream vessel so that your total pressure to the flanges is reduced to the rating level (285 psig?). Also, lower the pump discharge PSH or install a PSH between the upstram vessel and the pump , or will the pump shutdown if the upstream vessel is relieving (SAC out in API speak)?

There are other possibilities as well. You could trim the impeller on the pump to reduce the max pressure if you're close and the process can tolerate it.

 

[Canoman]

Thank you very much for the direction. That exerpt helped a lot. I looked at the code, and it's exactly what I was looking for.

Thanks again.