When could one use a Cold Vent Stack instead of a Flare Stack? 4

[lcms]

Dear Friends,

The petrochemical plant where I work is being asked to get out of our present flare system header that is shared with another company. Therefore we have to design a new waste gas disposal for our releases that are in fact only generated in emergencies.

We start the project assuming that a Flare Stack might be the only option, but recently we paid a more close attention to item 4 of API-521 and realize that cold vent stacks could also be an option and, in our case, even more appropriated.

Since we are a rubber manufacturer company, the studies for emergency releases show us that the release in emergencies are only 1,3 cis butadiene (43 t/h during 12 minutes).

Could one of you guys tell me your impressions about this subject and if you already have a issue like this and what was the final decision (cold satck or flare stack) and why.

Thanks in advance.

LCMS.

 

[Chance17]

I also have a background in Rubber manufacturing.
Where I once worked, the high latex (modest amounts of BD)streams went to an emergency cold stack.
The latex was considered to have high plugging potential.

The streams with high BD concentrations (such as the reactor) were vented to Flare.
The 30,000 gallon BD tanks also went to Flare.
The much larger BD spheres went to atmosphere.
All safety valves had inlet rupture disks to prevent leakage and popcorn polymer formaion in the Flare header.

It is my understanding that a pressure relief device can legally discharge to atmosphere i.e Flare header not required.
The main legal issue is BD concentrations at the fence line.
I have seen Flares that were burning BD at 98% destruction and the dispersion would violate acceptable fence line BD concentrations.

Given the human health issues with BD, I would never consider a cold atmospheric vent.

 

[lcms]

Dear Chance17,

Thank you very much for your helpful hint. It was higly appreciated.

I fully agree with your understanding that streams with butadiene must be sent to the flare stack to be destroyed to avoid Butadiene emissions to atmosphere, concerning enviroment and health issues. Our plant, though, is located in a country where this emissions are still not estabilished by law.

Instead, our flare will bring us some problems related to NOx emissions related to purge and pilot natural gas continous burning that are restricted here by law. That is why we are evaluating the cold stack, but I tend to use as design basis the explosive limits in our fence to calculate the height of the cold stack. As you mentioned health issues, could you please tell me what criteria should I adopt in my fence limits?

Thank you again for your valuable help.

Best Regards,

LCMS.

 

[MortenA]

I think that cold vents should be considered for all applications, where sufficient distance to the vent is not a problem. It should always be in combination with a flare - e.g. a ground flare for use when a planned blow down is required e.g. for maintenance.

Best regards

Morten

 

[Chance17]

Our government regulatory folks required less than 300 parts per billion butadiene ambient air concentration at the facility fenceline.

 

[lcms]

Dear Chance17,

Thank you very much again for this valuable information regarding butadiene concentration limit at factory fence.

Now I can do my calculation (stack heigh) using the following three criteria:

1) 300 ppb at site fence;

2) 2 ppm at grade floor of the stack (inside the plant);

3) 2% v/v (explosive limit at ground)

kind Regards,

LCMS

 

[mbt22]

LCMS,

Our plant, though, is located in a country where this emissions are still not estabilished by law.

But what is best practice and the right thing to do?

With an OSHA limit of 1 ppm (so monitoring and medical surveillance at >0.5 ppm?), destruction by flare would seem the only option.

Atmospheric venting of heavier than air hydrocarbons is also prohibited by a number of major petrochem companies.

Matt

 

[lcms]

Dear Matt,

Allow me to explain my point: - I fully agree with you.

Nevertheless, I have to convince my Board (Who will have to invest a big amount of money in a flare stack) that this is the only option as you say. They were a little bit septical when I have used exactly your arguments (release of heavy weight hydrocarbons are forbidden all around world, and the OSHA limite will never be attained unless I install a very high stack). Then they asked me to tell them how high the chimney would be, and here we are making calculations since then.

Besides, my figures so far have proved that the chimney would have to be unfeasible high (taller than 100 m!). So, I think we would have no option other than installing the flare.

Thank you for your help.

LCMS.

 

[mbt22]

LCMS,

Excellent, best of luck!

Matt

 

[JLSeagull]

Use a flare system. Review the the disaster video in the attached link describing an explosion in a BP Texas City refinery.

A blowdown drum and atmospheric vent stack instead of a flare seemed to be one of the key causes among many operating and maintenance flaws. Hot liquid filled the blowdown drum and vent releasing a geyser of hot hydrocarbon liquid and vapor. The vapor ignited and exploded destroying lots of equipment and killing and hurting many people.

http://www.csb.gov/investigations/detail.aspx?SID=20

 

[MortenA]

See - that the problem. You cant compare one vent system to another just like that.

The Texas city incident shows what can go wrong with a cold vent.

But other application e.g. vent of natrual gas has entirely different problems and concerns.

You should evaluate based on the application.

Best regards

Morten

 

[lcms]

Dear Friends,

Thanks for you all. Fortunatelly my bosses are not stupid (they are quite clever in fact) and accepted very well that we have no other way than building a flare. I am already quoting and it must be working by september next year.

All the posts; hints; opinions you sent me so far were very enlighntingn and helped a lot to consolidate my first opinion.

If you need any help too do not hesitate to ask. I will be glad to try to help you guys.

Regards,

LCMS