3 replies to this topic

[alexzo1990]

Hi all,

 

I am trying to size a vertical knockout vessel/drum that receives a number of relief streams via a common emergency relief header. This is a knockout drum that is venting to atmosphere where there is no vapour cloud explosion risk. In addition, it will only receive single-phase vapour relief streams (mostly VOCs in fire relief situations).

 

I've been trying to follow the API 521 7th edition methodology mentioned in 5.7.8, 5.8.7 and Appendix C.3, but this is not a requirement. Since it is venting to atmosphere and there is no vapour cloud explosion risk it should be sized using design criteria for flare knockout drums according to the standard. So far so good.

 

I read Appendix C.3 (which is a detailed example on sizing a HORIZONTAL KO drum)  and it all makes perfect sense until the point where at the end it starts explaining  how a vertical KO should be sized.  The formulae used basically are

 

Acs = Rv / Uv, where Acs is the required cross-sectional area, Rv is the vapour rate and Uv is vapour velocity which in this case is stated to be equal to the dropout velocity 

 

and

 

D = (Acs * 4/pi)^1/2, where D is the drum diameter.

 

Is that it really? Having the above information one would just need to calculate the vessel's height?

 

If there was a requirement for a certain volume of storage for miscellaneous draining (as per equation C.14 for the horizontal drum case) how would that impact the calculation? If I understand the methodology correctly the Acs (as calculated above) is the minimum required cross-sectional area for effective vapour/liquid disengagement so any other liquid accumulation will just make the vessel bigger. For example if D was calculated as 1.5m from above and there was an additional requirement of 2,000L of liquid accumulation would the vessel just have to be made 1.3m taller (disregarding the volume of the bottom dish).

 

Kind regards,

Alex

 

[Pilesar]

The diameter equation based on terminal velocity you presented is what I use for gravity separation without a demister. Although a simple equation, the parameters can be tricky.

 

Knockout drums should be sized using both vapor and liquid calcs. Sometimes the liquid holdup requirement overrides the sizing based on vapor velocity. Usually, diameter is constrained by the calculated terminal velocity of particles in the vapor phase. The terminal velocity depends on the particle size, densities, viscosity and Reynolds number and is determined by Stokes Law, Newton's Law, the Intermediate Law or by some other rule of thumb. For compressor or expander suction drums, the terminal velocity calcs should be derated by 75%.

 

After diameter is determined, the height will normally be between 3 and 5 times the diameter. Longer than that and you should consider using a horizontal drum instead.

Below are my height design criteria which are not intended to be a standard for anyone else:

I round the calculated diameter up to the nearest 6 inch increment.

The height from inlet to top tangent should be half the diameter with 36 inch minimum (some guides say 48" minimum.) If you need space for a demister pad, add another 12 inches.

I use 12 inches as the height from the bottom tangent to the low liquid level.

The height from the low liquid level to the high liquid level will be based on standard displacer lengths -- 14 inches for a small displacer in a 'no-liquid' design.

Add more required height if needed to satisfy liquid holdup criteria.

The height from high liquid level to the inlet (for vessels less than 8 ft diameter) will be 24 inches rounded up so that the tangent-to-tangent length is in even six inch increments.

 

If sizing includes a demister, there are diameter adjustments for pressure. Once the required demister cross-sectional area is determined, an additional 4 inches is added to the vessel diameter to allow for demister support rings before rounding up. 

 

Then there are inlet nozzle design criteria and sizing criteria for the vapor nozzle and liquid nozzle.

 

There are many references for sizing guidelines! Some to consider in the order I found them useful -- the first two especially good:

GPSA chapter 7

Watkins, R.N.: "Sizing Separators and Accumulators", Hydrocarbon Processing; November, 1967, pp 253-256.

Shell DEP

API 12J

 

To reduce vessel size, mechanisms other than gravity can be used. Engineered demisting devices and specialized inlet devices can sometimes make a significant difference in required sizing and overall vessel cost.

 

[breizh]

Hi,

Consider this resource:

CheCalc ‐ Vapor Liquid Vertical Separator Sizing

 

more info using the Cheresources's engine.

https://www.red-bag....sel-sizing.html

 

Breizh 

[Dacs]

In a nutshell

 

Diameter = determines the separation efficiency (aka minimum particle diameter separation)

 

Height = determined by liquid holdup (based on required holdup from process, which differs between services) + vapor space disengagement (varies between companies) + misc requirement (inlet box, demisters, etc)

Edited by Dacs, 28 January 2025 - 12:37 AM.