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Hazards of Oversizing a Rupture Disk 4

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Pavan Kumar

Chemical
Aug 27, 2019
400
Dear Friends,

I would like to understand the hazards in over sizing a Rupture disk. I have to size a Rupture disk for a few scenarios. The most conservative case is that of an internal explosion which results in 12" size for the rupture disk. For failure of regulator case the ruptures disk required is 8". The reason for the 12" size for the internal explosion is more because of the higher relieving temperature ( 3500 Deg C) as opposed to 580 Deg C for Failure of regulator scenario.

If I select the 12" size then the Rupture disk is over sized for the Failure of regulator scenario which is more feasible than the Internal explosion scenario. If select 8" size then it is undersized for the Internal explosion scenario which though is quite unlikely.

I would like to understand the hazards if any of using the 12" size as it covers for both regulator failure scenario and the internal explosion scenario. The set pressure is 34 psig.

Thanks and Regards,
Pavan Kumar

 
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Pavan-

If you continue to learn and gain experience in this area you'll be capable of safely performing this engineering task in the future. But to be honest, you're not there yet. This is just feedback, and you shouldn't take as an insult. The task we're discussing is one for an engineer with expert knowledge in this area. It's very clear that this needs to be passes to such a person, either inside your company or to an outside specialist. There are lots of engineering companies that specialize in process safety designs (e.g. ioMosaic, Equity Engineering Group, Siemens, Providence Consulting, Smith & Burgess). Those are just a few (and I'm not personally connected with any of them).

 
I'm pretty sure compositepro is referring to the pressure rise from a deflagration in a closed vessel. Starts with a combustible mixture. Assume it combusts to completion. The pressure will rise due to temperature increase. The product side of the stoichemetric oxidation equation may create more moles of gas than the reactant side; thus an increase in moles of gas in the final state. don1980 pointed you to NFPA 68 which is one of the governing documents on the subject in U.S.

Good Luck,
Latexman
Pats' Pub's Proprietor
 
Hi Latexman,

We could size explosion vents per NFPA 68 for detonation cases. In my opnion the case we are considering is internal deflagration and not detonation.

Thanks and Regards,
Pavan Kumar


Latexman (Chemical)
22 May 20 17:18
I'm pretty sure compositepro is referring to the pressure rise from a deflagration in a closed vessel. Starts with a combustible mixture. Assume it combusts to completion. The pressure will rise due to temperature increase. The product side of the stoichemetric oxidation equation may create more moles of gas than the reactant side; thus an increase in moles of gas in the final state. don1980 pointed you to NFPA 68 which is one of the governing documents on the subject in U.S.
 
I agree with don1980, and the best way you can help your company now is bring in a Consultant with experience in Burner Managment Safety and Process Safety Designs.

Good Luck,
Latexman
Pats' Pub's Proprietor
 
I think for ignition the mol% does not need to be at LEL or above LEL. It will ignite but not explode I guess.

If a gas is present in concentrations below the LEL, it will NOT ignite. You may still have gas phase oxidation (depending on temperature), but no flame. A flame is just a controlled deflagration where the gas speed is a bit higher than the flame velocity, which creates a sustained flame. I'm just guessing here, but I think you have a nozzle mix burner type ( bottom option in the pic below) since you are operating "below" the well-mixed LEL all the time. This style allows for LEL concentration in the flame to be controlled by O2 diffusion into the gas stream, allowing operation at much, much higher air-gas ratios. That explains your air-gas ratio.

image_z22uko.png


As everyone else has said, I would bring in a burner consultant who, in addition to providing a burner management system, can help you identify failure scenarios that are specific to your particular burner. Can a failed-open regulator cause flameout, followed by gas accumulation and subsequent deflagration? Maybe. It depends on your burner design. A wide-open regulator may simply just increase your exit preheater temp. A specialist is needed in order to evaluate your system and give you realistic failure scenarios.

"Should I use the 2700 Deg C which results due to the heat combustion adding to the Process air or should I sue 350 Deg C, the normal preheating temperature as the deflagration raises the pressure too quickly for the temperature to rise."

As other as said, during any deflagration the relief temperature will be the operating temperature of your system. Whatever relief you put in would very likely be set at 350 C unless the operating conditions changed right before the deflagration event. It's possible that deflagration occurs on startup and ambient conditions, which would necessitate a relief temp of ~20 C.

Bring in a specialist.
 
It also seems that you may not be including the nitrogen content of the air in your temperature calculations.
 
Thanks everyone for your valuable suggestions. I think your suggestion to bring in a Process Safety Consultant is very much agreeable, especially since this pertains to Process Safety. I will keep you all posted about the developments in this regard.

Hi TiCl4,

The burner in our plant is indeed of the type 2 shown in your reply. I will work with the Burner supplier to understand the how the flame is being generated in the first place. Thanks for clarifying that the NG cannot be ignited when its concentration is below LEL.


Thanks and Regards,
Pavan Kumar
 
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