3 replies to this topic

[racerX]

Hi all,

I have looked through quite a bit but cannot still come to a clear answer, please help me out

For horizontal vessel sizing (without mist extraction element) for a particular droplet cut off size API 521 gives a trajectroy method,that same method is also inthe GPSA data book, 12th edition

My question is whether there is a K factor method for horizontal vessel sizing without mist extractors,
I have asked numerous people and they always say there are 2 methods, K factor and gravity settling (API one), but after having gone through GPSA I understand that the K factor is a capacity factor for mist extraction elements, and its values correspond to certain removal efficeincy as well:

ex, from GPSA fig 7-12: Droplet removal efficiency: 99–99.5% removal of 3–10 micron droplets.
Gas capacity, K,ft/sec, 0.22–0.39.

So, is it relevant to apply this method to size a vessel that has no extraction element and therefore there is no concept of removal efficiency but only a droplet cut off diameter whcih alone will determine vessel size through the settling equations ?

Regards
racerX

[ankur2061]

racer X,

Knockout drums without mist eliminators are a rarity and hence there is very little technical literature related to KO drums without mist eliminators. Shell DEP 31.22.05.11-Gen. -GAS/LIQUID SEPARATORS - TYPE SELECTION AND DESIGN RULES gives some details about a horizontal KO drum without a mist eliminator.

The design approach in the DEP is to keep a gas load factor λ_max of 0.07 m/s for determining the minimum diameter of the vessel.

Try to get a copy of the mentioned company standard and read through the entire section related to Horizontal Knock-Out Drum. You should get a fair idea on the design and sizing of a Horizontal KO drum without a mist eliminator.

Regards,
Ankur.

[racerX]

Hi Ankur

It is very common for horizontal knokout drums to be used as vent scrubbers to avoid the risk of plugging and cause overpressure

After having gone through some more in this, I realized that the API method is a trajectory method and is purely theoretical, whereas the K value method gives an allowable vapor velociy based on experience based K values,

So, though the vessel in my case is sized for 300 microns as per API, the actual vapor valocity in the drum is more than the allowable vapor velocity as determined by the K value method,

For vertical drums, K value is straightforward, for horizontal drums it is said to be a multiplier (>1) of vertical K value, so it comes down to what value of horizontal K u can choose ?

[Erwin APRIANDI]

Hi racerX,

I don't know whether you notice, but the formula for basic separator sizing based on droplet size can be found in GPSA DATABOOK eq 7.1.

The critical part of separator sizing is to ensure enough vessel cross-sectional area for gas flow (above LLHH) which you have to get the actual gas velocity less than the Vt (critical or terminal gas velocity necessary for particles of size Dp to drop or settle out of gas; get from eq 7.1). See attachment for more clear explanation

From this point, separator sizing is just a matter of geometry calculation rather than chemical calculation

Attached Files

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