Entrainment mesh eliminators - Double pad

[landrover77]

Gents

I'm designing a vertical vessel, 70" oil separator, with knitted wire mist eliminator. For separating oil mist from a gas stream.
We have plenty of experience in designing horizontal vessels with these mesh pads , but less so in vertical vessels
The Vessel diameter is sized based on the design flow required and the allowable Saunders-Brown velocity through the pad
However the turndown is 5% of the nominal. My understanding is that the mesh pads, will turndown successfully to 30%.
Therefore for turn down below 30% we normally fit a second mesh pad in series with the first, this pad fits into the same vessel diameter, but part of its area is blanked off to effectively provide a smaller pad area. Thus we have two mesh pads with areas / velocities providing turn down across the required range.

In a Horizontal vessel, we would typically put the smaller area pad before the larger one as we have found the smaller pad does provide some separation benefit, even at high flow outside of its theoretical design range. We presume it collects larger diameter droplets and may help coalesce the smaller droplets, which are still entrained in the gas flow prior to entry to the main pad,
However in a vertical vessels, if we have two pads above each other then we have concerns of how the liquid will drain successfully to the sump, at the base of the vessel. Especially if the smaller area pad is below the larger one, ie when at design flow, the velocity through the smaller pad will be high and i'm concerned the oil may become re-entrained / or at least held up and saturate the pad, resulting in yet higher velocities and potential re-entrainment.

I'd be interested in anybody's experience and thoughts Plus direction to manual/paper that discusses this.

thanks in advance.

 

[landrover77]

Gents

I wonder if i can bring this to your attention again
thanks in advance

 

[rmw]

Have you discussed this with any reputable pad vendors?

I can't give experience for pads at different diameters, having only done two stage mist elimination with vanes, but I think I can discuss the logic. A mesh pad acts to coalesce entrained moisture particles into larger particles where in a vertical flow stream they can attain enough mass to be able to fall through the upflowing stream to the bottom.

In the case you propose, the first and smaller stage would be operating at normal conditions at higher than design velocities which tells me that there would be a propensity for re-entrainment. That means that your larger pads above them, operating below their design velocity are you last line of defense.

If you think you must do this, then, putting the smaller area pads as the upper pads would have them operating at design velocity at turndown while the larger diameter pads below would be operating at less than design velocity, but that said, that doesn't mean that they would be doing nothing. They would be less effective, but would do some collection thereby lessening the load on the smaller pad deck.

Note that anything that falls from the upper deck, regardless of which one it is, has to fall onto and pass through the lower deck in order to get to the sump. Which would you rather have droplets collected by the upper deck falling through, a mesh pad operating above its design velocity or one operating below its design velocity.

Were it me, I'd put the smaller flow area pads as the upper deck and the larger as the lower. In fact, I'm not so sure that I would mess with variable flow areas but would just put two decks of pads in. Without doing any calculations or seeing any curves for the pads in question, I'd suspect that the pertial flow efficiencies of the two decks in series is good enough to do the combined job.

rmw

 

[landrover77]

Thanks rmw, i tend to agree with your points.