API 520 questions and rupture disc sizing 1

[limeister]

Hello

I have been charged with calculating rupture disc sizes.
I do not have access to the entire standard but I do have a formula.

A = [Q/(38*Kd*Kw*Kv)] * sqrt of [G/(P1-P2)]

A = Required discharge area
Kd = Effective coefficient of discharge
Kw = Correction for back pressure
P1 = Pressure 1 with 5% overpressure
P2 = Pressure 2 (vented to atmosphere so will be zero)
Kv = Viscosity correction factor
G = S.G. reference H20 = 1.0 @ 70 F
Q = Flowrate from production plant


There is a worked example for me but I can't follow where the S.G came from. Is it the specific gravity ratio of liquid (nitrogen) to H20? I think if I was sure about this I would then just plug the numbers in.


There is a lot of jargon that I am not familiar with so I was wondering if you could assist me in this calculation.


Thanks in advance

 

[TD2K]

Stop. A phrase like 'There is a lot of jargon that I am not familiar ...' really concerns me.

Get the whole standard first and read through it. Get some information from vendors on the sizing of their rupture disks and have someone in your organization who is properly qualified/experienced in this area to go over it with you AND review your work.

This is your final safety device to protect people and equipment. You should not be 'assigned' to size rupture disks and dropped into it as this sounds like you have been.

We'd be happy to help you out with questions but I'm really not comfortable with what I'm seeing here.

 

[limeister]

Dear TD2K

Last time I asked a question in this forum the help was overwhelming. I was really suprised. I didn't realize the seriousness of my question until I read the responses.

You're right and thanks for your concern. I am not qualified to do this calculation but it has been given to me. I'm not really sure why I was given this task. I think all they want is a rough estimate. I think they just want to know if I could come up with a reasonable number. They will review it with me after I make the first attempt.

There are people here who have 10 years plus experience but I doubt if they have done this sort of thing before hence they gave it to me. Or they are just too busy.

In regards to obtaining API 520, I'm not sure if my superiors are willing to get this standard for me. I will ask tomorrow.

I am also on a time frame of about 4 days so if you would reply soon it would be greatly appreciated.

I am wondering if anyone here is willing to mentor me? I know it's a long shot but I know I need it.

 

[pleckner]

I think we all understand your situation. Forget about that equation you have. Consider the rupture disk as just another piping component. As such, the rupture disk, like any valve or fitting, contributes to the frictional losses of the system as defined by a certain 'Kr' value or, as many engineers like to use (but I detest), an equivalent length.

You get this 'Kr' value from the rupture disk's manufacturer as it is a certified value. If in your case you don't have a specific disk to work with then you will have to defer to ASME Section VIII, Division 1, paragraph UG-131(k) which says to use a Kr of 2.4. Take the flow that you know from the relief scenario, increase it my 10% (I'll explain) and choose a ruptue disk size the same as you would choose a pipe size. Set up a hydrualics calculation starting from the final discharge point and work back towards the vessel in question. The pressure at the vessel must not be any greater than 10% above the MAWP or design pressure of the vessel (21% if this was for a fire relief scenario). If the pressure exceeds this, then you have to increase the rupture disk and line size.

Since this is a liquid system, then using DARCY is good enough.

Why not use the above formula? You present a standard relief valve sizing formula but is only applicable if you meet the 8 and 5 rule and discharges directly to atmoshere. The 8 and 5 rule states that rupture disk must be installed within 8 pipe diameters from the vessel nozzle entry and the length of discharge piping not be greater than 5 pipe diameters from the rupture disk. This is usually never the case with real installations. So, we use the resistance to flow method which is what I described above.

Why multiply the flow by 10%? ASME wants you to derate the rupture disk's maximum capacity by at least a factor of 0.9 to account for inaccuracies in this method (the friction factor and pipe roughness are never really known for sure).

For liquid properties, you need viscosity and density/specific gravity. The specific gravity is a ratio of the liquid density to that of water at 60 degrees F. Most people just divide the liqud density by 62.4 lb/cu.ft.

I hope this helps.

A last bit of my two cents. I can't believe any plant does not have a copy of the latest codes and standards for process safety. They must be around somewhere so I suggest you find them. Also, you must have a safety group, find them!

 

[MortenA]

I dont think pleckner' advise is very good.

If you dont have access to the standard - contact a vendor. Explain your situation to him. He will then tell what info he need and you can forward the info to him. He will then do the calculations.

I dont know any vendors from expeirence - but searching google the first link was:

http://www.elfab.com/

they seem to manufactor these things.

they even have a on-line enquiry form.

Best Regards

Morten

 

[pleckner]

To MortenA:

My adice comes right out of ASME Section VIII, Division 1, not to mention the technical references from the FIKE Corporation and not to mention quite a number of years doing Process Safety evaluation, documentation and calculations.

Also, there is not a vendor on this planet that will do the sizing for you; it is a liability issue and not their responsibility. You have to tell them what you want. Yes, they will tell you what information you need to supply to them but they WILL NOT do the calculations for you.

The information they need is bursting pressure, coincident bursting temperature, maximum operating pressure and coincident temperature, size, whether it is to be non-fragmenting or not, allowable manufacturing range, material of construction, whether you need vacuum support (this along with the maximum operating conditions help them to determine if they should recommend reverse buckling disks or not and what kind of holders they need to provide) and whether you need a tell-tale connection.

Also, anyone responsible for doing any relief calculations is taking a tremendous gamble if they do not have a copy of the latest codes and standards. This is law my friend and is not to be taken lightly nor can one afford not to do it correctly.

 

[MortenA]

Im sorry i should have read all your post.

Upon having done that it look like good advice.

Best Regards

Morten