4 replies to this topic

[processjha]

What is droplet size effect on flare knockout drum design.

[processjha]

What is droplet size effect on flare knockout drum design

[Erwin APRIANDI]

What is droplet size effect on flare knockout drum design

Dear ProcessJha,

I think what you mean is What is the droplet size effect on flare tip?
answer is:
Larger size droplet can result in incomplete combustion with excessive smoking, possible “burning rain”, and even flame-out.

But if your question is
What is droplet size effect on flare knockout drum design?
Possible answer is:
The droplet size will effect on the required vessel cross-sectional area for gas flow, smaller droplet size will required more cross-sectional area for gas flow which means bigger flare knockout drum, while larger size droplet will be the opposite.

Edited by erwin.apriandi, 27 January 2011 - 12:42 AM.

[paulhorth]

Processjha,

API RP521 provides a design method for flare drums based on meeting the criterion of separating droplets larger than 600 microns. The basis for this size is that smaller droplets can be safely burned in a conventional flare tip and so will not produce burning rain.
The flare drum sizing method assumes uniform gas velocity across the cross-section of the drum. Achieving this depends on a good design of the inlet nozzle, and API RP 521 gives guidance on this. I have seen some CFD modelling of a flare drum which showed that the velocity is far from uniform. However, the API design method seems to conservative enough to work, but I don't know if there have ever been any measurements of droplet size, to test the effectiveness of a flare drum. Basically, separation of smaller drops requires lower gas velocity, which requires a larger diameter for the drum. The length between inlet and outlet nozzles also comes into it because the length provides the time for a drop to fall to the liquid surface.

It is important that the liquid level in the drum does not exceed the maximum used in the design. Droplets can be re-entrained from the liquid surface and carried over to the tip.

Paul

Dear friend,

For knockout drum sizing the minimum droplet sizes used ranges from 400 microns to 600 microns. Basically as explained in API 521 and other related sources, droplet size will help you to arrive at critical velocity. Perticularly, if you managed to keep velocity of droplet below this velocity, the weight of the droplet (gravity force) will be governing and finally the droplet will seperate. For smaller droplet sizes, the value of critical velocity will be less. This means a KOD sized for smaller droplet size will certainly have larger size than the KOD sized for the larger droplet size ( assuming same same liquid hold-up volume).
The seperation of liquid droplet is required for the proper functioning of Flares i.e. to avoid buring rain and incomplete combusion.

What is droplet size effect on flare knockout drum design.