Discussion Two

From Mr. Don Gregurich:

Dear Mr. Leckner,

I read your about Relief Valve Set Pressures with interest because the issue came up several years ago and we had quite an internal debate (at a different company).

I don't remember the details, or resolution, but I was on the opposite side of the argument. Essentially, the situation is that there is a vessel with a design pressure of 150 psig (and let's assume same MAWP) and we want to use a relief valve set at 30 psig (There could be a number of process reasons for doing so.) Then, if this is the only relief on the vessel, does it need to be sized to accommodate the worst credible non-fire overpressure case flow at a relieving pressure of 33 psig or 165 psig? I felt that the answer was 165 psig, if, of course, all of the design was appropriate (valve body can handle the pressure/temperature at the relieving conditions, the inlet and outlet piping are sized correctly for those relieving conditions etc.) It certainly seemed to me that this was consistent with the spirit of the code, after all, the whole point is to limit the pressure in the vessel and this would meet that requirement.

Of course we must also be consistent with the letter of the code, if we are to avoid jail time, but I didn't see anything in the code that contradicted the above interpretation. Although I see your quote regarding rating of valves, don't see where it explicitly states that a valve can not be operated above its "official rated capacity" to perform the required service. I see that the code states that "the valve must have the capacity to relieve the load*" and that "the official rated capacity is that which is stamped*" but I don't see where it says that the valve must be able to relieve the load at the official rated capacity. So if I look at this in the strictest sense (by the letter of the code) I do not see that we can't size it based on the 165 psig. I don't think this is unreasonable: rating a pump at a given pressure/flow point serves to define the pump's capacity - but the pump can operate at different flows and pressures then at the rated point. Likewise for the relief valve. In fact, we could have selected this same valve, used a spring for 150 psig, and it would be functioning the same at the relieving conditions, as if it would with the 30 psig spring.

I confess that I haven't taken the time to reread through the code, but I was convinced back then that my interpretation met the spirit and letter of the code. I'd be interested to hear what you have to say about my position.

Thanks

Don Gregurich

Phlip Replies:

Don,

First, call me Phil and thanks for taking the time to read my column. Second, I do not agree with your position even though on the surface it appears to be sound, and this is why.

First (and this really doesn't answer the question but I just couldn't help but comment) I couldn't figure out why anyone would want to have such a low set pressure and still allow the relieving pressure to go so high. It is not logical and seems to defeat the purpose of having a low set pressure. You might as well have just set the valve for 150 psig and taken the conventional 10% overpressure and eliminate the controversy. But then I thought of a run-away reaction scenario where you may want the valve to pop open early and allow the vessel to slowly relieve its contents but still allow the pressure to build-up. However, this uses the relief valve as a
depressurization valve and I am not in favor of this. There are designs with proper use of control valves to do this.

In your E-Mail, you say "Then, if this is the only relief on the vessel, does it need to be sized to accommodate the worst credible non-fire overpressure case flow at a relieving pressure of 33 psig or 165 psig? I felt that the answer was 165 psig,...".

It is very clear that the valve does not "need" to be sized for 165 psig. It only "needs" to be sized for set pressure +10% overpressure (non-fire, single device case) in order to be consistent with the calculation of the certified (stamped) capacity. However, you are correct in that on the surface (at least as far as I can tell), the code (ASME Section VIII, Division 1, 1998 Edition) does not seem to prohibit one from sizing the valve based on such a high overpressure. ASME appears to only be concerned that the MAWP is not exceeded beyond its requirements. And as you say in your E-Mail, "I don't see where it says that the valve must be able to relieve the load at the official rated capacity."

However, a relief valve vendor will only guarantee the stamped capacity and ASME is very clear how this is to be determined. Therefore, in the event of an accident, how would you have guaranteed to an OSHA inspector that the catastrophic failure of the vessel or attached piping/equipment was not a result of an improperly sized relief valve since you have no guarantee of the flow rate through the valve? I guess you could try to prove that your calculations were reasonably accurate using real properties, accurate vapor
flow equations and the correct thermodynamics. Is all this really worth the potential liability just to buy a somewhat smaller relief valve? Not where this Process Engineer stands! So Don, that's Process Engineering-As I See It!

Mr. Don Gregurich Replies:

Thanks Phil,

I appreciate your insight; I understand what you're saying about the rated capacity of the valve as the only real figure that you can hang your hat on.I honestly can't remember the details of the situation, only that it never did get to the point where we needed to make a final decision on the set point issue. It's those applications that lie outside of the norm that really make us think about what we're doing, which hopefully leads to a better understanding.

I do have another one for you, this one also seem "sensible" to me, but I don't know what the code would think:

If an ASME stamped pressure vessel is being used for an "atmospheric" operation with an open vent to atmosphere that has been properly sized, is a relief valve (or disk) required? "Proper sizing" of the vent line means application of the same analysis and calculations that would be done for sizing of a relief device. The vent line would be sized to accommodate the worst credible upset condition at a relieving pressure that is under 15 psig. The location is in a jurisdiction that requires compliance with the ASME code.

Thanks again,

Don

Philip Replies:

Don,

I am not on any committee so I can't give you an official answer. However, ASME specifically states in Section VIII, Div 1, 1998 Edition, paragraph UG-125(a)"All pressure vessels within the Scope of this Division, irrespective of size or pressure, shall be provided with pressure relief devices in accordance with the requirements of UG-125 through UG-137." The key here is what they consider part of the "Scope". The Indroduction, Paragraph U-1, goes into the definition of "Scope" and it can get rather complex. I would have to read through this very slowly and carefully to see if this exact situation is addressed. I do have some other sources I can
review for interpretations and your question may not be able to be answered without a direct interpretation from the ASME committee. I would agree with you that it doesn't make sense to need a pressure relief device for this situation. I can tell you that in the past, when facing a situation where we do have an ASME coded vessel but cannot come up with any credible scanrio, nothing at all, we put a 3/4" x 1" relief valve on the vessel and call it out for thermal expansion - end of story.

I think I have an answer for the second question you asked me concerning an ASME stamped pressure vessel being used for an "atmospheric" operation with an open vent to atmosphere. Based on ASME Section VIII, Division 1, Introduction, Paragraph U-1 para c(2)(h), vessels having an internal or external operating pressure not exceeding 15 psi (103 kPa) would not fall under the scope even though it has a stamp. As long as any pressure vessel meets all the applicable requirments of the Division, it may carry the Code U Symbol.