Philip,

I am responding to your column in cheresources.com (http://www.cheresources.com/asiseeit1.shtml).   I disagree with your
interpretation of the ASME code.  The capacity of a relief device that is used to determine adequacy of design is based on the
allowable overpressure.  If the overpressure is higher the flow will be higher.   You can take credit for this.  UG-125 makes it clear
that the only requirement is that the pressure not exceed 110% of the MAWP (121% for a fire).  The stamped capacity is there only for reference.  It could not possibly be used to make a judgement on two-phase flow capacity.

Also, contrary to what you have in your article it is many times advantageous to have a set pressure much lower than the MAWP.  This is especially true if runaway reaction is a possibility.  A low set pressure allows the reactants to start venting much earlier, thereby removing reactant from the vessel before the temperature and reaction rate get too
high.  Check out the DIERS literature.

I think you should remove your column.

Mr. Niemeier,

Thank you for taking the time to read my article "Relief Valve Set Pressures" in the "Process Engineering-As I See It" section of "The Chemical Engineers' Resource Page and for your feedback.

Allow me to respond.

You write,
"I disagree with your interpretation of the ASME code. The capacity of a relief device that is used to determine adequacy of design is based on the allowable overpressure. If the overpressure is higher the flow will be higher.  You can take credit for this. UG-125 makes it clear that the only requirement is that the pressure not exceed 110% of the MAWP (121% for a fire). The stamped capacity is there only for reference."

You are falling into the same trap that most people fall into and this is precisely why I wrote the article in the first place. The certified capacity which determines adequacy of design of a relief valve (which is the 'device' I assume you are referring to since the capacity of a stand alone rupture disk is different) is based on the criteria described in ASME Section VIII, Divison 1, Paragraph UG-131, not Paragraph UG-125. Paragraph UG-131 states that, except for some very specific exceptions, the overpressure is to be 10% or 3 psi, whichever is greater and is to be referenced to set pressure. If set pressure happens to be equal to MAWP, fine, but as you point out
later in your letter, this is not always the case. There are no provisions that I can find that allow a vendor to certify a relief valve for
overpressures greater than these. In addition, the certified capacity is the only flow that is guaranteed by the relief valve vendors, nothing more or less. Using greater overpressures than the 10% or 3 psi described above in calculations for required relieving rate does not alter the guaranteed capacity provided by the vendor and which is required by ASME, Paragraph UG-129.

If you require further evidence of these points, feel free to contact Farris Engineering, a well-known relief valve vendor at www.cwfc.com. However, I can save you some time because I didn't write the article without doing some research first.

Now, you think capacity certification (stamped capacity) is just for reference?!? It is this certified capacity, not the calculated maximum relieving rate, which must be used when sizing inlet lines to the relief valve (using the 3% rule, ASME Section VIII, Division 1, Appendix M, Paragraph M-7) and in many cases, the outlet lines as well.

I invite you to re-read the article, specifically the paragraph titled "So, Why the Confusion? and also read ASME Section VIII, Division 1, Paragraph UG-131. The code is like a bowl of spaghetti. You have to weave through all the pertinent paragraphs to get to the end... and the full story.

For your information, the stamped capacity for relief valves in vapor/gas service is usually given in terms of SCFM of air at set pressure plus 10% overpressure and 60 degress F.  Valves in steam service are stamped in terms of pounds per hour of steam at set pressure plus 10% overpressure at the saturation temperature. ASME Section VIII, Division 1, Appendix 11 gives you a means for converting to your particular vapor/gas. Valves in liquid service are given in terms of gallons per minute of water at set pressure
plus 10% overpressure and 70 degrees F.

You continued in your letter with the following (still referring to the stamped capacity):

"It could not possibly be used to make a judgement on two-phase flow capacity."

I don't know why you mention two-phase flow since I didn't in my article. However, I agree, relief valve manufacturers at the present time do not have capacity certification capability for two-phase flow.

You then added:

"Also, contrary to what you have in your article it is many times advantageous to have a set pressure much lower than the MAWP."

I invite you to re-read my paragraph titled, "Relief Valve Set Pressure" specifically the sixth line where I write, "There are even times when the relief valve must be set even lower than design pressure."

Are we talking about the same article here? Did you actually read my article?

You finish your letter with:

"I think you should remove your column."

MAWP, design pressure, relief valve set pressure, what is allowed overpressure, calculated relieving rates, certified capacity, etc. appears to be a very confusing and misunderstood concept that most people are having when dealing with safety relief systems; you included. And this is precisely why my column must not only stay but also grow. My only regret at this time is that I obviously did not get the point across to you and for that I apologize. I hope this response clarifies those points you misunderstood. I will be more than happy to answer any other concerns you may have or even further discuss the points you already brought up.

I still disagree with you.  It is basic physics that you will get more flow if you have a higher driving pressure.  There is no reason not to take credit for overpressures above 10% as long as you don't go above the maximum allowable pressure for the valve.  See the following memo from Paul Papa, director of engineering for Farris:

As you mentioned, all of our certified capacities are based on testing that is done at 10% overpressure.   Nameplates are therefore marked with capacities at 10% overpressure in the appropriate certification fluid (steam, air, or water). From a selection standpoint, valves are typically sized based on 10 % overpressure.  As you correctly pointed out, Section VIII of the ASME Code indicates that ASME Code stamped pressure vessels require a relief device that must pass all of the required flow without allowing the pressure to go any higher than 10% above the maximum allowable working pressure (MAWP).

There are many cases where a vessel is being used at a pressure at much less than its MAWP.  In those cases, you can size and select the valve at an overpressure greater than 10%.  This will allow you to use a smaller valve as
the valves capacity will increase as the pressure increases.  Per ASME Code, the pressure must still be kept to the 10% accumulation pressure, that is no more than 10% above the MAWP.

For the most part, this is not a problem for the valve as long as the higher overpressure does not exceed the maximum pressure limit of the valve at the relieving temperature.

Jeff,

I'm not saying, nor have I ever said that you can't calculate the required relieving rate for the MAWP + overpressure and then choose a valve based on this. Remember fire cases? They go to 21% overpressure (set pressure basis) don't they and are often the sizing criteria for the valve. However, this still does not alter the guaranteed (stamped capacity) flow rate of the valve which is based strictly on set pressure + 10% (for the most part). And it is this flow that your maximum calculated relieving requirement cannot exceed and which is subsequently used in the hydraulic calculations, per ASME. I don't know how many times I have to say this. And this is what my article was strictly about; the criteria used to obtain the certified (stamped) capacity. Also, I also don't see in the Farris response you sent me where they will guarantee any number other than what they are allowed to stamp even if MAWP is 100 psig and you want to allow the pressure to go to 110 psig. If a valve is set for 50 psig, it will be certified based on 55 psig, clear and simple! Neither Farris nor any other vendor will guarantee that your valve will pass a different flow rate at any other condition. Nor will they tell you to use any other value in subsequent hydraulic or downstream sizing equations.

I would love to see you prove the calculations that give you that extra margin you keep talking about. You think it's simple? Try doing the calculations at a high pressure for something very non-ideal such as ethylene. All of the equations shown in ASME and API fall apart rather severely since they are all based on ideal gas. Try convincing an OSHA inspector that at conditions which exceeded the capacity certification test at the time of equipment failure that your valve was properly sized and should have been big enough. If you can do this via flow equations derived properly for non-ideal gases from the sound use of thermodynamics, great and there is no argument from me. Heck, I'll even write about it!! After all, ASME and API specifically state that the engineer is to use good engineering judgment and if in question, work with the relief valve vendor. As a matter of fact, you can even deviate from the code if you can prove that a given relief valve will not be adversely affected from whatever you are trying to do. As an example, I am currently on a project where the inlet line loss for one particular valve is in the 6% range. Notwithstanding the fact that the 3% rule is "non-mandatory" (but is still considered good engineering practice), we are working with Farris to determine if this would adversely
affect the valve. If it doesn't, we can live with the current piping configuration.

What I wrote was about stamped capacity and how this is what is recognized by Code, nothing more or less. Stamped capacity being the only guaranteed flow rate is fact, not interpretation. When to use stamped capacity has been set down by both Code and interpretation. You can disagree all you want with what I wrote. That is fine and I just love getting into these types of discussions. How else can we learn and grow? But you made some rather inaccurate statements in your first letter about what was stated, or not
stated in the article and that still leads me to feel you didn't totally understand it.

I've been giving this debate some further thought and I hope this will finally put it to rest. You and I seem to be disagreeing on the concept of sizing versus what you do with the valve once it has been sized.

My article on the Web site starts off with a statement made by the author of an article published in the May 2000 issue of CEP. This person made a statement that basically said if you want to decrease the size of a given relief valve, find a way to increase it's set pressure. However, the following Letter to the Editor in the October 2000 issue inspired my article. It stated, "This is not true; for a certain MAWP, the capacity of the relief device is not a function of its set point, but of MAWP alone." Now, had you really read my article, you would have seen these quotes! As I've said before and written in my article, per ASME, capacity for a given valve (which I am defining as certified capacity) is a function of set point only, not MAWP. Therefore, to increase the certified capacity of a given
size valve, one can increase the set pressure. Alternatively, to reduce the size and cost of the purchased valve, one can increase the set pressure of a smaller valve and maintain the same certified capacity.

The whole point should be "safety", not who can buy the smallest relief valve. If you want to stretch what ASME is trying to convey just to buy a smaller relief valve, go ahead; you don't work for me. If you did work for me, I wouldn't allow it. I'll take the conservative approach, it just isn't worth the potential liability. After all Jeff, you'll never know if the valve you bought is the right one unless the system over pressure is caused by the controlling scenario and the valve does or does not work. And guess what? This rarely happens since most controlling scenarios are loaded with conservative assumptions that are never achieved in real life.

One last point if I may. I still can't understand why you would want to set the relief valve at a lower pressure (50 psig) and still allow the system to achieve the much higher pressure (110 psig). This makes no engineering sense to me. You should just specify the set pressure at MAWP and go with the allowable overpressures. Then you won't have any debates. You mention run away reaction? I'm sorry but this is bad engineering practice and if you do this at your site, I would stop the practice. Run away reactions can occur
too fast for relief valves to react. I would use a stand alone rupture disk set at the lower burst pressure (your 50 psig example) for the run away reaction scenario and use the relief valve for other scenarios set at the 100 psig MAWP. This is a much safer design.

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

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!

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

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(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.