6 replies to this topic

[binzee]

Respected Seniors,

i am need of gudinace from you on the following questions:

1. Difference between Flare system and Blowdown system
2. Conditions where flare is employed and where we use blowdown
3. When we use the term blowdown stack....what does that mean?
4. Finally, advantages and disadvantages of both systems....

Thanks,

[Art Montemayor]

Binzee:

I believe that you are asserting that a "blow down" system is the same thing as a Flare system - for disposing of unwanted vapors or gases. Am I correct in this?

A blow down system is not the same thing as a flare system from an engineering point of view. From a disposal point of view they may be considered to achieve the same end. A blow down is simply what it states: one expels gases or vapors (usually not liquids) from a high pressure source down to atmospheric pressure for disposal (usually). You are throwing waste material (some times noxious) into the same atmosphere we all breath.

A flare system is more sophisticated and engineered. It is designed for the disposal of unwanted vapors or gases by way of combustion with air under a controlled and designed procedure. In a properly designed flare system only the final, residual products of combustion (usually CO2 & water) are disposed into the atmosphere.

As you can see, the basic differences are very different - especially when viewed by an "environmentalist". So, what is it that you mean by advantages and disadvantages? I think these are all pretty obvious and straight-forward.

[binzee]

Thanks for differentiating....

My question here is there are installations where blowdown system was used and was replaced by flare at later stage. Is this change manly due to envirnoment point of view or are there some other technical issues involved.
I hasd the opportunity to see BP Texas refinery incident investigation on NATIONAL GEOGRAPHIC...there was a statement given that BP was using blowdown system and it was recommnded to be replaced by Flare system.This proposal was later dropped due to highy budgetry cost....

Also one supplementry question,
Do we have a stack in the blowdown system, if yes what is the purpose..

Thanks

[JoeWong]

Mr. Montemayor,
I am sorry to cut in...

Thanks for differentiating....

My question here is there are installations where blowdown system was used and was replaced by flare at later stage. Is this change manly due to envirnoment point of view or are there some other technical issues involved.
I hasd the opportunity to see BP Texas refinery incident investigation on NATIONAL GEOGRAPHIC...there was a statement given that BP was using blowdown system and it was recommnded to be replaced by Flare system.This proposal was later dropped due to highy budgetry cost....

Also one supplementry question,
Do we have a stack in the blowdown system, if yes what is the purpose..

Thanks

binzee,
Blowdown system and flare system are totally different from operation, objective, function, limitation, etc although they are connected and have some degree of interaction between each and other. Please get a copy of API STD 521. There is explantion in it. For Blowdown, please look for "depressuring".

My question here is there are installations where blowdown system was used and was replaced by flare at later stage. Is this change manly due to envirnoment point of view or are there some other technical issues involved.

Do you mind to elaborate a bit more about the "blowdown system" ? It is a vent system where release of gas from plant collected in a header and discharge to atmosphere without firing ?

Venting hydrocarbon without flaring may have at least the following impact :
i) Environment - From green house effect measurement, One mole of C1 equivalent to about 20 mole of CO2. Flaring one mole of C1 generate one mole of CO2

ii) Safety - Direct discharge hydrocarbon to atmosphere generate gas cloud. It is a big hazard once it get ignited e.g.lightning.

Do we have a stack in the blowdown system, if yes what is the purpose..

I presumed your "Blowdowns system" is venting system...
To answer your qeustion, yes or no subject to dispersion of the vented components.

With direct venting, you have to ensure vented material is properly dispersed and the concentration of the component at the interested point (e.g. man reach area) and explosive limit (LEL) below the limit. One way to achieve this is to elevate the venting point by means of vent stack.

Hope this helps.

[binzee]

Thanks Mr. Wong for your valuable inputs....

[milind kamat]

Thanks Mr. Wong for your valuable inputs....

Hello,

Blowdown is another term for depressurisation of an equipment or a part of the plant system which can be isolated by(usually) remote operated valves. Basically the idea is to carry out an emergency depressurisation of the system, usually when the system is in the circle of fire. The depressurisation reduces pressure in the equipment thereby reducing the stress on the equipment walls, thus preventing rupture. What is done is that the rate of blowdown or the depressurisation is controlled by use of restriction orifice in the blowdown vent( required rate is as per API). The vented gases are then guided to a stack, invariably to the flare header. Therefore, blown down gases which are flammable are most often sent to the flare. The flare and the blowdown are the same. Its only that the blowdown is one of the releases into the flare system.

In the BP accident case, perhaps the blow down must have been sent to a tall stack which is being later planned to be sent to a flare. In the former the vented gases are dilute in the atmosphere whereas in case of flare they are positivelt destructed. Flare is thus a sure way of destroying the flammable vent/blowdown releases in te plant.

Hope I clarified the point.

Regards,
Milind

[Art Montemayor]

Binzee:

I think Joe Wong has outlined very clearly just what are the characteristics and basic operations of each system. I also believe I now know what (or who) is the culprit in causing a misconception on your part and raising this query. The problem, as I see it now, is reading (and believing) National Geographic magazine about a technical disaster dealing with technical terms and operations.

You cannot expect such a journal to accurately describe or write about technical matters such as the BP Texas City Explosion of March 2005. Please refer to my post (among several) on this subject: http://www.cheresour...tart=#entry8557

As it turns out, almost 3 years later, almost everything I foresaw and commented on in this disaster turned out to be true. Ever since the day it happened, various interests have been trying to “back-engineer” the results to fit their needs or their agenda. The basic facts are still there in the record book for all to read and study: In the past, BP saw it fit not to employ a flare system on their Isomerization Unit and, instead, depended on nothing more than a large (not large enough, as it turned out) vessel to receive any and all relieved liquids and vapors in the system. This vessel was nothing more than a temporary “wide spot” in the line – it served only to receive the liquids and vapors being relieved (the “blow down”) and hold them (with the exception of the vapors that were allowed to escape to the atmosphere) until they could “safely” be handled and disposed. It was no more complex or sophisticated than what I describe. As history bears out, the vessel (i.e., “blow down drum”) wasn’t big enough; it overflowed with liquid & heavy vapors and it was instrumental in causing the disaster and ultimate death of 15 innocent people.

The main point I want to bring to the attention and understanding of all engineers reading this is that there is no option in this example scenario. Here, I refer to the title of the thread: “Flare vs. Blowdown”. It is not a question of whether you should use a blow down drum or a flare system. The moment you are contemplating a flare system, you have already accepted the inclusion of a blow down drum! I will repeat this one more time in order to make sure I make myself understood: A flare system having to deal with possible liquid or 2-phase pressure relieving streams always has to include a blow down drum – or an adequate means of separating the liquids from the vapors that ultimately are vented to the flare stack where these same vapors are burned. The liquid can’t be burned in a conventional flare stack – and that is the problem when you attach a flare system to a series of relief headers in a processing plant. You must secure the manner in which any liquids are safely separated from the associated vapors meant for the flare stack. Getting rid of or safely dispensing of the liquids is yet another disposal problem that has to be also dealt with safely. Wherever you have the possibility of relieving both liquids and vapors you have to address the safe disposal of BOTH – not just the vapors that are visually seen burning through the flare stack.

This simple fluid handling process gets convoluted and totally misunderstood when newspapers and non-qualified magazines (such as National Geographic) try to relate what they believe happened or is happening. People who don’t have the credentials, experience, or knowledge of what technically is going on should leave it to qualified engineers to describe. However, the communications media doesn’t like to deal with engineers. They simply don’t understand what we are stating or describing since most of them slept through all their high school classes involving math, chemistry, and physics – or simply have never taken one such course. This continues to be their problem and one that will probably never be resolved.

The issues at Texas City were very, very simple and easy to understand – and also to remedy and prevent any disaster. However, as I have explained in the thread I started in our Safety and Chemical Processing Incidents Forum, BP decided or failed to take simple remedial actions to prevent such an occurrence – something that they had been doing for quite some time in my personal experience and which was confirmed by the government investigators. What I foresaw and commented on in our Forum on March of 2005 was later documented and confirmed in the official government report more than a year later. And that is the real tragedy – the fact that 15 deaths could have been avoided so easily!

I hope I have succeeded in explaining myself and also esplaining what I have intended to convey to all who read this.

P.S. I am a great admirer of National Geographic Magazine. I have suscribed to it ever since I graduated from university and was able to afford the cost.