134 replies to this topic

[gvdlans]

Does anyone know how to size a Vacuum/Relief valve on a storage tank? I looked into API Standard 2000 (appendix C), but that only says that "The size of a vent valve is based on the venting device's tank connection". API RP 520/521 only deal about Pressure Relief Valves...

[Diederik Zwart]

Normally sizing of low-pressure relief and vacuum relief valves is done by the relief-valve vendor. The process engineer specifies the required relief load (based on API2000) and from this the vendor calculates and selects a relief valve. In my experience, when requested, vendors are willing to show you their calculation method (especially if you buy their valve). I don't think there is one generic sizing method, because each vendor has its own valves, with vendor-specific flow-factors (K-values).

If you do come across a generic sizing method, please post it, because I would be interested as well.

Regards,
Diederik Zwart

[gvdlans]

Thank you, Diederik (or may I say dank je wel?)

After I posted my question I did some more searches on the web, and came across a PDF file from Tyco valves containing flow curves for their vacuum/relief valves. See Tyco Brochure.

This illustrates your answer that sizing of the valves is normally done by the valve vendor...

[Art Montemayor]

Guys:

For my benefit, let's define what "sizing" means. I define the sizing of a vacuum/pressure relief on a storage tank as the exercise an owner's process engineer has to do to arrive at the capacity of the required tank relief devices sought. This requires, of course, that the process engineer carefully and accurately detail out all the possible, credible relief scenarios surrounding the tanks and their operation. The worst case is the relief design case.

The process engineer should also, in my opinion, decide if he will handle normal pressure relief and emergency pressure relief separately - not the instrument supplier.

The instrument supplier "sizes" (mechanically designs and rates) his instruments in accordance with the required relief capacity specified by the owner's process engineer. The owner and his process engineer are the responsible parties for the relief capacity installed on the storage tanks -- never, never the instrument supplier.

At the risk of being redundant, let me say it another way: In the USA (& in Europe) the government's regulatory agency holds the owner totally responsible for determining the correct relief capacity that will prevent an accident or incident. The owner is subject to being inspected and is held legally liable for producing up-dated and accurate calculations determining the true design case for each relief device. I believe that I am correct in all the prior statements I have made in this regard. I hope that we are in agreement on this issue, because it is a very, very important point.

I have designed and produced an Excel Workbook that leads you through the proper and documented calculations for pressure and vacuum relief of storage tanks. If anyone is interested, let me know by emailing me with your name, location and interest. I will gladly share this information with anyone that qualifies.

Art Montemayor
Spring, TX

[Diederik Zwart]

Art,

I totally agree: What I meant by the term 'sizing' in my previous post is (only) the mechanical sizing by the valve vendor based on the relief loads calculated by the process engineer. I can see that there could be some misunderstanding, so thanks for your comments.

I would like to receive your excel workbook, so I'll e-mail my address etc. to you.

Regards,
Diederik Zwart

[gvdlans]

Art and Diederik, thank you for your replies.

I also agree with post from Art Montemayor. With the word "sizing" in my question I meant the determination of the required orifice size inside the relief device, which is of course based on the required capacity determined in earlier steps.

[pleckner]

Allow me to add my two cents worth here.

We, the Process Engineer size them, period. And Art gave the correct definition of sizing. I am amazed that there are engineers out there that still think the vendor sizes rupture disks, relief valves and even relief piping and all they think they need to do is supply the flow rate and some fluid properties.

I also pretty much tell the vendor what I want even mechanically after discussing the applications with them. They are a great source for this but the owner is still responsible, not the vendor. I bring this up a number of times in my series on Rupture Disks, which, by the way you can find on this site (I got my plug in).

[gvdlans]

The discussion seems to go down to a matter of definitions. Question is whether sizing includes determination of relief requirements or not. When I look at API RP 520, I see that there are separate subsections called "Determination of Relief Requirements" and "Sizing For Gas or Vapor Relief, etc.". So there is some ground for seeing it as different steps. Anyway, as I already explained, this was what I meant in my question.

Who is responsible for the exercise is a different issue. I don't think there is any doubt that the process engineer is responsible.

[Art Montemayor]

Guido:

I consider this such an important subject in the area of process design that I tried to emphasize the following (which I may have failed to do):

1. As Phil Leckner has inferred, the process engineer does The Design. What I have always understood (and what I believe Phil is saying) is that regardless of what is "out-sourced", sub-contracted, or done by others in the total system design -- the Owner and his process engineer are the responsible parties of record. I think we all understand and accept this basic, legal principle; let's go into the subject a little further.

2. The basis of a Pressure/Vacuum relief design scenario is composed of several factors, but basically the underlying need is one of total safety. Those factors affecting or canceling out the system's safety are the responsibility of the Owner/process engineer - as stated above. Therefore, if a correctly defined and identified worse case scenario for a storage tank is submitted to a conservation vent vendor as a capacity specification, this does not mean that the selected (or vendor-proposed) vent valve is the decision of the vendor. It is the Owner's responsibility to ensure that he has selected a reliable vendor that will furnish the vent that will, indeed, fulfill the capacity (& pressure drop) requirements at the conditions identified as the hazard scenario. How the Owner goes about in making damn sure that he is dealing with a reliable and credible supplier should be of concern to him and especially his process engineer.

3. In this example, all tank vent suppliers will furnish you with their free software programs that calculate the "correct" vent model and size. What the process engineer should concern himself with (& what I think you are concerned with) is that the vendor calculations should be correct, that the correct model is selected and that the correct vent installation is done on the tank. What I mean by the "correct" installation is that you're not done with your job until every detail that could go wrong is nullified or erased. You essentially start the process by identifying the worse-case scenario capacity required. Then you entrust the supply to an outside contractor to design and fabricate the required vent. You check his calculations, method, logic, and proof of capacity and pressure drop. You accept delivery of his product at your plant and subsequently go forward and install the apparatus. You also, if you're smart, have to document the critical details of what was calculated, "sized", purchased, and installed. If you fail to ensure that the correct orifice, model, etc. is truly installed and left to operate and, by bad luck or error, the wrong item is installed -- you are guilty of failing to meet the law's requirement for safety. What legal action occurs between you and your supplier does not affect the fact that the law holds you/the Owner responsible for the installation. This is basic Caveat Emptor.

4. The above, I think, is the thrust of Phil's statement. It is not enough to "trust" a supplier (as I know you are concerned); you have to "trust" your own organization to basically identify the worse-case design scenario and logically and correctly proceed to resolve the problem. The Owner and management (as well as everyone else in the plant) depends on your correct assessment and logic decisions. Needless to say, this is why they pay you the "big bucks".

5. Additional areas for thought are the capacity constraints in the vent inlet nozzle and the discharge nozzle/piping/header(s). Any and everything that can affect the correct safety situation must be addressed. This is another, but related, area of concern that many engineers either discount as "minor" or as un-necessary. You will note in the workbook that I sent you that even the method of cleaning certain tanks - such as steaming out - is studied as part of the vacuum scenario. And the credible potential situation must be addressed and resolved - such as the correct vent nozzle size for vacuum relief during a steam-out + rain shower. Process blow-through and steam heating coil rupture are just more scenarios that might affect you. All of this is the Owner's responsibility through the process engineer.

I don't think responsibility is another issue in this case. It's at the bottom of every action and step that is taken towards the final, safe solution. That is why I can relate to your concern in making sure that you understand and dominate the calculations done by your sub-contractors. You will have to defend them, should you accept them tacitly or not. This is why I believe I fully understand Phil's statement of dominating the process design and mechanical solution. You must interject yourself in the total process - or at least be in total control of what is happening. Otherwise, somebody else's interpretation of the solution will be installed and operated without your knowledge.

An American president, Harry Truman I believe, once put a sign on his desk that stated "The buck stops here!" - meaning that he was not going to let responsibility pass him by. I think the same sign rightfully belongs on every process engineer's desk.

I have not tried to get into Phil's brain and read his thoughts - nor much less try to speak for him. But I think this subject is so important that I anticipate and welcome his valued comments in the event I have mis-interpreted anything or have spoken out of line.

Art Montemayor
Spring, TX

[gvdlans]

To Art and Phil,

First of all, thank you for your efforts!

You answers seem to suggest that there is any disagreement from my side. There is not: Process engineer is responsible for determination of relief requirement and size of the relief device, period. I never suggested anyting else, so we can stop that "discussion".

If anything, there is some confusion on the word "sizing", whether it includes determination of relief requirement or not. Again, both API RP 520/521 and API Standard 2000 leave room for interpretation that "sizing" is a different step from "determination of relief requirement". I do not see anyting wrong or unsafe in that. If anyone feels that "sizing" also means "relief requirement determination" that's fine with me.

[pleckner]

To "gvdlns"

I did not mean any direct criticsm to you when I stated,

"I am amazed that there are engineers out there that still think the vendor sizes rupture disks, relief valves and even relief piping and all they think they need to do is supply the flow rate and some fluid properties."

I meant this as a general observation in my experiences so I apologize if it came across anyother way.

And, to all who read this, I don't care how you want to interpret the word "sizing" or "Determination of Relief Requirements" or how many steps you want to break it down, the owner is responsible, not the vendor.

To "Art".

You did just fine!

[Sadananda Konchady]

Art,

I am a chemical engineer in the ethanol industry, where bulk ethanol fixed roof storage tanks need to be protected with inline flame arrestors and pressure cum vacuum relief valves.

I would appreciate receiving your excel workbook.
My E-MAIL is skonchady@chemengg.com

Thank you,

S. Konchady

[rxnarang]

Art,

The spread sheet! Me too! Thanks rxnarang@bechtel.com

Mr. Art,

I would like to get your excel workbook. If possible please email me at
amit.joshi@cdicorp.com

Amit Joshi.

[Art Montemayor]

Carlos:

It will be my pleasure to email a "serious drinking man" a copy of my workbook. Be looking in your inbox.

Saludos
Art Montemayor

[trilok_sontakke]

Dear Art

Me too waiting for your valued spreadsheet.

Thanks
Trilok