5 replies to this topic

[obbchao]

Dear All Sir,

Hello all, I am a new process engineer in an engineering company in Thailand. I've 2 questions:

1. What is the difference between Flash Gas and Boil Off Gas?
2. In an Ethylene Storage tank (Please see attachment) there is a BOG line in top of tank. Why does BOG change to Flash gas in the downstream branch line?

I am a new member of this board. If anyone knows the answer to my question, please explain it clearly to me.

Thanks,

OBB

Attached Files

•  BOG_VS_Flash_Gas.xls   17.5KB   91 downloads

[Art Montemayor]

OBB:

You will find as you proceed with your career that you will be working more and more with and cooperating with Mechanical Engineers – especially if you are in operations or operations support.

In doing so, you will find that because of their training and specialty, Mechanical Engineers are not interested in Process Semantics – in other words, the specific meaning of process words and titles. In the case of a compressor recompressing process vapors, they are only basically interested in the flow rate, the temperature and the compression ratio. Where the gas originates or what kind of gas it is has no meaning or interest to them – except if they don’t know the ‘k’ value or the specific gravity. Therefore, someone with little interest or knowledge of process descriptions titled the compressor being used in your operation a “Flash Gas” compressor, when in reality you and I know that the gas is really a “Boil Off” gas. Normally, in crude oil and NGL processing you have a lot of “Flash” gas originating when high pressure liquid fluid is expanded adiabatically. This type of flash gas is very different from boil off gas. The latter is usually a steady-state, pure composition vapor. The former is usually a varying flow rate of different compositions. So, to a Chem Engineer it makes a big difference in the way we handle and process both gases. I identify with your concern to make sure that the identification of the gas is as accurate as you can define it. However, I’m afraid you will never get the opportunity to re-educate our esteemed mechanical “cousins” and we all have to live with the titles that were given to the equipment named by them.

[Qalander (Chem)]

Wonderful/Accurate Response Dear Art. Esq.

Since, I also had very little knowledge and nil exposure to figure-out such Gas production (Operation) 'Jargon'.

Now updated via your above post.

Best regards
qalander

[Pilesar]

OBB:
The difference is likely in the quantity and source of
the gas. BOG usually signifies that a cryogenic liquid has
boiled off due to ambient heat input into the storage tank.
The amount of boiloff gas is relatively constant and occurs
even when there is no flow entering the tank.

Flash gas is likely a larger quantity and not as constant.
Flash gas would occur if the tank undergoes thermal rollover
or when the transport vessel returns gas to the tank during
filling. The potential gas quantity can be much higher than
the steady state boiloff.

I suspect your BOG compressor runs constantly and is smaller
than your Flash Gas compressor which runs only when the
BOG compressor is overwhelmed.

[obbchao]

Thank you for every reply, I think this bord very nice for process engineer like me.
I earn more knowlage and I will learn more, if next time i have confuse I will consult you again.

[jprocess]

Dear All,

I have one good spreadsheet for BOG calculation. I will be glad to share it with anyone who is interested.
Just send me your request to jprocessman@yahoo.com

Best of Luck,
Mojtaba