4 replies to this topic

[Travesh]

I am currently specifying relief valves for a certain plant. ASME Section 8, Div 1 paragraphs U-1 & Ug-125 & ASME Section 8, Div 2, para AG-121 call for all pressure equipment rated at more than 15 psig(1.05 barg) and larger than 6" (152 mm) in diameter, width, height or cross section diagonal (no limitation on length) to be protected by a relief device.

I have identified equipment with design pressure greater than 15psig & larger than 6". Do these still require a relief device if the design pressure of the equipment has been increased to withstand the relief scenario. eg. If the relief scenario for vessel has been identified as a blocked discharge. The pump shut-off pressure is 51 barg. The vessel is then designed for a design pressure of 51 barg. Is a relief device necessary in terms of ASME 8?

Essentially, does anyone know if ANY exemptions exist in ASME 8 (eg increasing design pressure) to prevent the installation of relief valves?

Thank you.

Regards,

[Art Montemayor]

Travesh:

I know of NO exceptions within ASME Section VIII design - except the 15 psig and the small size exception that you cite - that will allow you to omit a pressure relief device on a pressure vessel.

I caution you on continuing along this line of reasoning in trying to achieve a situation where you don't install a pressure relief device on a pressure vessel. This does not make for safe, logical engineering thinking. ASME (& many other sister engineering organizations) has gone to many lengths in putting together the reasoning and logic that make up the Section VIII Code, and it should be taken very seriously. Its basic intentions were (& hopefully remain) to apply a safe and conservative design and to mitigate the worst case scenario possible.

By mandating that a PSV MUST exist on a pressure vessel (no matter how inane this might appear to an engineer), ASME has almost guaranteed that should someone get a "bright idea" and use the protected vessel in another application (where a hazardous pressure can be achieved) it will remain PROTECTED and cause no potential harm to humans or the environment.

I can tell you from personal experience accumulated over the past 47 years that it is very, very possible to cause grave harm by not mandating the ASME code related to PSVs on pressure vessels - regardless of what the pressure level is or what engineers think about it at the time.

Many people have died stupidly and for no reason at all simply due to the fact that someone failed to consider the need to protect against a pressurized hazard.

I could cite you many personal bad incidents in my experience - but I'll stop here and allow other, knowledgeable and experienced engineers like Phil Pleckner to give their comments and advice on this matter.

[JoeWong]

Travesh,
I am absolutely agree with Mr. Montemayor on the approach...

Base on my understanding, exception of ASME was based on Risk Based Analysis...

I have identified equipment with design pressure greater than 15psig & larger than 6". Do these still require a relief device if the design pressure of the equipment has been increased to withstand the relief scenario. eg. If the relief scenario for vessel has been identified as a blocked discharge. The pump shut-off pressure is 51 barg. The vessel is then designed for a design pressure of 51 barg. Is a relief device necessary in terms of ASME 8?

Your vessel may be good for blocked discharge considering your pump is centrifugal type. Somehow i believe your vessel will potentially be isolated (during short term device checking) and exposed to external fire, you may need a PSV to protect the vessel...

An example...reciprocating pump discharge...a PSV is always installed at the pump discharge regardless of it design pressure...simple reason...

Power = function (Q x DP)

Blocked discharge ==> Q approaching zero
With fix power motor ==> DP increase to infinity ==> Discharge P to infinity (THEORITICALLY)

The pressure can develop to any pressure and weak point will just fail...Higher maximum allowable of weak point, higher the risk...

Last but not least, please use engineering justification with SAFETY mindset.

Hope this help.

JoeWong

[pleckner]

OK, what Art said is totally true except.......

There are two alternatives to installing a PSV on an ASME Code vessel.

1. Invoke Code Case 2211. This was a case brought up before the ASME committee just for this type of issue. The Code Case allows you to design for containment, which is what you want to do. However, (isn't there always a however?), you must do the same calculations you would do to determine the requirements for a relief device. You must go through scenarios and show by calculation that you can never, never, and never and ever exceed the vessel's MAWP without the relief device (or other alternative I'll talk about). Joe brought up the point of fire and this may or may not be a credible scenario in your case. If it is, then forget about Code Case 2211. The other however is that the vendor of the vessel is told up-front that you will be invoking Code Case 2211 so that he can apply the proper documentation. THEN, you must contact your local municipality and tell them you intend to invoke Code Case 2211 (hopefully, they'll know what you are talking about). They will request all your back up calculations. Even then, they DO NOT have to accept it and you'll need the PSV (or other alternative I'll get into). One more thing, you need to have the municipality approval BEFORE the vessel manufacturer builds the vessel. Have you ever tried to get your local municipality to approve anything in less than a decade?

2. You can use an approved safety instrumented system in place of a relief device. In some cases, especially if your facility is already set up for this, this can be a cheaper alternative than installing relief devices. However, the relief device is usually the less expensive and easier way to go.

So bottom line....there are valid, acceptable and legal alternatives but stick with the relief device. It will be a lot less trouble.

[Travesh]

Thank you to all of you for your responses (Phil, Joe, Art & Jack)!