9 replies to this topic

[jprocess]

Dear all,
Section 5 (Relief Systems) of GPSA states that:
"Cooling water failure is
often the governing case in sizing flare systems"
I want to request from experianced engineers that explain their ideas about this statement.
I want to know about the reason.Is this statement is experience based?
Thanks in advance.
Cheers.

[djack77494]

A very short answer is that failure of a plant's cooling water system results in large flows of vapor that would ordinarily be condensed (by the cooling water) but which do not condense due to the failure. There must be a path out of the system for the vapors (e.g. flare system) in the event of this situation.

[Nirav]

hello,

Yes, cooling water (CW) failure is sometimes the controlling case.
There are basic steps to size flare system. I consider you know them. The one of the steps is to find out "governing relief scenario" or controlling case.

When CW failure occur, there will not be any condensing in the system. Vapors will start to build up the pressure inside system. There are more than one exchangers which use CW as medium to condense or to cool the vapors/liquids.
Therefore, when CW failure occur, all those exchangers will stop condensing. I would give simple example.

There is a condenser (using CW) at distillation column overhead. So, if there is no CW, overhead vapors in column will start to build up which will increase pressure of overhead system. The required relief capacity could be the "normal" vapor flow from top of of tower.
What will happen if you have 3 more columns in your unit? All will undergo same phenomenon. And simultaneous relief from all columns' PSV will start. Therefore, total relief requirement could be HUGE. This could be higher than any other case of simultaneous relief (e.g. Fire case) and become controlling for flare header and system sizing.

Refer to API- RP 520/521 for more clarity and details on the above scenario.

Thank you,

[jprocess]

Dear Djack and Nirav,
Thanks a lot for your answers,
Can we replace air with water as cooling medium to prevent or decrease the relief load caused by cooling water failure?
Cheers.

[Nirav]

Jprocess,

You have innovative mind !! It would help you somewhere sometime...

But here, unfortunately, air can not serve the purpose by replacing cooling water for condensers or coolers which are designed to use cooling water as cooling medium.

The reasons are more than one. I am sure you would be able to list out them by yourself as an engineer.

Thanks,

Nirav,

Lets say insted of overhead condenser that uses cooling water we have overhead air cooler then i think loss of power to this air cooler can cause same relief scenario.

Correct me if I am wrong. Just trying to lear something here.

Regards,

Amit

[Zauberberg]

Absolutely correct.
In both of these cases, complete loss of condensing capacity is occuring.

[jprocess]

I should add some points regarding to my last post(which is refered to more than one year ago)
Water cooling vs. air cooling is an interesting comparison and due to limitations of air cooling, in most of the cases can not be used instead of water cooling.
But my main idea is about relief load calculation at the onset of cooling medium failure.
Sea water failure is one of the major scenarios that results in high releiving loads this is also true for air cooling failure but as I asked from some experienced engineers for the second one if considerations show that such a scenario is credible in some zones, emergency power generator should be supplied to prevent loss of cooling and high relieving loads. I believe this is a cost effective design.Any hint?

[peeyar]

I should add some points regarding to my last post(which is refered to more than one year ago)
Water cooling vs. air cooling is an interesting comparison and due to limitations of air cooling, in most of the cases can not be used instead of water cooling.
But my main idea is about relief load calculation at the onset of cooling medium failure.
Sea water failure is one of the major scenarios that results in high releiving loads this is also true for air cooling failure but as I asked from some experienced engineers for the second one if considerations show that such a scenario is credible in some zones, emergency power generator should be supplied to prevent loss of cooling and high relieving loads. I believe this is a cost effective design.Any hint?

I dont know what is the position of api521 but some engineering practice consider 20pct or so duty available from natural draft
peeyar

[djack77494]

I dont know what is the position of api521 but some engineering practice consider 20pct or so duty available from natural draft

I have also heard folks talking about taking advantage of natural draft cooling in an air cooler when power is lost. Cooling duty would certainly not drop to zero since air can continue to circulate through the cooler even when the fan is off. In fact, in cold weather climates that is one strategy used to avoid excessive cooling when the weather moves in that direction. Another strategy is to actually reverse the fan's direction so that it becomes even less effective at rejecting heat. There are other more effective strategies as well.
Doug