Pressure relief valves and burst valves question?

[airhog]

Hello,

I am currently looking into a nitrogen pressure vessel design and have noticed that is uses a pressure relief valve (set a mawp).

It then has a burst valve in parallel (with the relief valve) set above mawp (over 20 percent). Which I believe to be a mistake.

My question is, is there any code that requires a relief valve and burst valve to be used in parallel?

Also, is this even necessary, or would a relief valve suffice?? Its actually a 1/2" ID relief valve and a 3/8 ID burst valve...

Any assistance or thoughts would be greatly appreciated.

Thank you,

 

[Jboggs]

My reply is based on experience, not any knowledge of codes. The rupture disc is your failsafe. ANY mechanical device can AND WILL EVENTUALLY fail. If your relief valve fails, and you have no rupture disc, you effectively have a bomb waiting to explode. Yes! You want a rupture disc.

 

[airhog]

I can see your point of view.

However, I still feel that having both pressure relief systems, when the system in 10+ years old and has never had a NEED to vent is overkill. (not to mention how the rupture disc is well above mawp)

I will re-state this "as only using a rupture disc as an emergency relief, instead of using a relief valve and rupture disc in parallel"

To be honest its the 3k in plumbing that is why I want to streamline this design.

 

[hugin8]

There are two other threads that discuss general pressure vessel codes. Just search for "pressure vessel", maybe something there will get you pointed towards the applicable standards.

You might provide a little more detail about the system. What's the size of the vessel? You said 10+ years old, is the system already manufactured? What is your mawp? What is your operating temperature? What is the pressure vessel being used for? What is connected to it? What would be the consequences of a fail-safe failure be?

 

[Cockroach]

The purpose of the rupture disc is to obviously override the relief valve in the case it fails. The design typically calls for 10% set point over that of the pressure relief valve. This is a valve code, not a pressure vessel requirement, so varies according to your geographic location.

What I quoted here is a Canadian Standard, CSA Z245.15 with Boiler Branch amendment for Alberta. You should look at the valve code and apply your experience to relief valves. What happens if the pilot fails?

Regards,
Cockroach

 

[vpl]

You have not provided sufficient information for anyone to help.
In what country/ area are you located? That will give the codes that apply.
What pressure does the tank operate under?
What does the nameplate say?

Just because you don't think you need both doesn't make it a good idea to remove one. In fact it probably makes it a very bad idea.


Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[airhog]

You have not provided sufficient information for anyone to help. In what country/ area are you located? That will givethe codes that apply. What pressure does the tank operate under? What does the nameplate say?

Just because you don't think you need both doesn't make it a good idea to remove one. In fact it probably makes it a very bad idea.

Hello,

I am located in the United states in the midwest. The tanks actual operating pressure is 45psi. The tanks nameplate states 175psi mawp. The pressure relief valve is 175psi while the rupture disc is 220psi.

From my research it is perfectly acceptable to use a rupture disk on its own... a pressure relief valve would be used if frequent higj pressure were commonly nseen

 

[GeraldJ]

The relief valve is there to protect the vessel and the people in the area as is the rupture disc.

Monkeys may monkey with the relief valve but the rupture disk is NOT ADJUSTABLE. As long as it is not painted or other wise tampered with it will still be there protecting the vessel and the people in the area. If this is on an air compressor which cycles on and off and the system is not horsepower limited, all it takes to have a serious over pressure event is for the motor starter contacts or unloader to stick. If and when that happens, the relief valve will pay for itself. If some yo-yo thinks the relief valve is malfunctioning and screws it down, the rupture disk will blow and the resulting shutdown of the equipment may bring more a knowledgeable head around to check things out.

At the best it is a protection device which can protect the system from a dangerous over pressure and which may or may not be required by code. At worst, it may eventually degrade and rupture prematurely and cause a shutdown.

So, IMO, far less trouble to leave the protection in place.

Now, just why did you think it would be a good idea to remove this protection device? You had better have a very good answer if something goes wrong after you do it!

 

[airhog]

To answer you question gerald j. It is part of my job description to reduce the costs of this particular system.

Upon investigation it looks like the rupture disk and pressure vessel are not designed to asme standards and i was hoping to find out from someone more experienced, why that is the case?

I understand the reasons for a redundant saftey system, but due to the nature of a rupture disk feel it would be an acceptable solution. It also appears to be a asme approved solution that will meet asme standards when lowered to within 10% of the vessels mawp.which the system currently does not meet.

Also the cryogenic relief valves used in the system are non-adjustable.

 

[berkshire]

air hog,
""Also the cryogenic relief valves used in the system are non-adjustable.""

Is this a Dewar ?
Does this vessel contain liquid nitrogen?

B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[vpl]

airhog,

You started by asking what codes applied and implied you wanted to remove the rupture disk. Your last post implies that the tank wasn't built to the ASME Code. Are you sure? If so, then you are sure that it's acceptable to operate that tank in your state? Some midwest jurisdictions take very dim views about use of non-ASME Code pressurized vessels and you as well as your company could be in very big trouble. However, that's a legal issue and is strictly your problem.

There's whole sections of the Code that talk about relief valves and rupture disks.

Of course I'm just guessing, but it's all together possible that your tank was built to Code and then someone decided to operate it a lower pressure without resizing the relief valve. If that's the case, then you probably aren't currently meeting the Code and should be -- which doesn't mean removing the relief valve, it means replacing it with a properly sized one. There's a number of threads on forum794 that talk about resizing tanks to lower pressures. You might find searching that forum helpful. Please don't double post your question over there though, unless you specificaly cross reference it to this thread.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[airhog]

Hello Berkshire,

The vessel does contain liquid nitrogen.


Vpl, my goal is to come up with an alternative solution, without explicitly stating what system i am working on.

I do have a pressure vessel design book, however it is a few hundred miles away from me at the moment.

 

[rogerbours]

Please check

http://www.fike.com/documents/presrel/adrd/technical_bulletins/TB8105 Best Practices for RD_PRV.pdf

for some good guidance on PRV & RD combinations