Barometric Leg Design

[patibar]

In one of our unit, the existing barometric leg from a two stage ejector-condenser system is routed to OWS. Because of some TOC(Total Organic Comp)at utilities issue, proposed to divert the stream to other existing vessel. We have the problem of space constraint. The proposed vessel is far away and we can not provide vertical or 45° angle line to that vessel. We can provide a minimum slope for free draining. I would like to know the allowable slope limit for vacuum condenser tail pipe design. Appreciate if somebody helps me in this. All the available literature speaks about the vertical or 45° angled lines only. Definitely there should not be any horizantal lines to avoid gas pockets, which may create additional resistance, there by condenser flood may happens. But i would like to know the acceptable slope other than vertical or 45°?

Thanks.

 

[katmar]

The 45° angle limitation applies only to the two phase section. The lower part of the leg will be full of liquid and it could be run at a shallower angle than 45°. Work out where the theoretical liquid level is in the leg, then allow about an extra metre below that for disengagement and then run the line to its new destination. You should still avoid any high points in the new section or you could have problems at start up.

If you cannot do it this way and you have to run the two phase section at an angle less than 45° then oversize the piping by one or two sizes to be sure that there will always be a continuous vapour phase layer (i.e. the liquid layer never reaches the upper wall of the pipe).

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[patibar]

Thanks "Katmar" for your quick response and useful info.

The height beween the condenser and hotwell is 16.5 m. The vacuum required is 49mmHgA. The Hotwell is connected to a wastegas header having pressure of 10psig. A static liquid head of 0.6m in the hot well giving me that condensate will fill the barometric leg up to 15m. I would like to know 1.5m space is sufficient for gas disengagement or self venting? The condensate flow rate is only 250 Kg/hr. and leg pipe is 11/2" line. If you see the velocity, it is nothing. We proposed a slope of

We are proposed to divert the existing barometric leg to a vessel at two different points. One is at the top and the other is at middle. If due to any reasons, the liquid level goes down, column looses its vacuum. That is the reason we proposed a loop seal at the inlet of the vessel(hotwell). According to literature, we proposed 10 ft height of loopseal. Is it mandatory to have that much height? Lastly i would like to know, why seal loop is provided with bigger size that barometric leg pipe size?


Thanks.

 

[katmar]

From my experience and what I have read 1.5m is adequate for disengagement. However, your 10 psi plus the vacuum you want at the condenser uses almost the full 16.5m. Do you really have 1.5m available?

The length of the loop seal must be enough to hold the vacuum at start up before both legs can be refilled. Having a depth of 10ft means that you can draw 20ft of vacuum - assuming all the piping is of the same diameter. One way of overcoming this is to make the loop seal a larger diameter than the barometric leg so that the volume in the seal is able to fill a longer length up the barometric leg. Don't forget that the effective length of your barometric leg is from the condenser to the height where the downstream leg of the loop seal enters the vessel - this can reduce your barometric leg length significantly and it must be checked against the vacuum you want to maintain.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[patibar]

Dear "Katmar",

You are right, only 0.7m is available for gas self venting. As you said, I have considered the leg length starting from condenser to a height of vessel inlet nozzle. the existing loop seal is of 10 ft height and 3" line. We are thinking to leverage the same in the proposed 11/2" line. I would request you to give some reference or suggest from where to get the reference document.

 

[katmar]

I don't know of any single reference that covers all of this. Probably the best is "Designing piping for gravity flow" by PD Hills, Chemical Engineering, Sept 5, 1983, pgs 111-114.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"