Sizing feed surge drum

[slntsam]

hi,

i'm trying to size a vertical feed surge drum with no vapor flow.

I know the basic equations are as follows:

Vt= Kt*((Dl - Dv)/Dv)^0.5

A = Vapor mass rate / Vt / 3600 / Dv
D = (4 * A / pi)^0.5

my question is pretty basic. how do i size this drum if there is no vapor feed to the drum. and there is no vapor coming out of the drum either.

also, how do you determine LLL, HLL, HHLL?

thanks for your help.

 

[TD2K]

Simple answer is, you don't. The formula you quoted is for setting the diameter of a vessel where you are trying to separate liquid out of a gas. That's not what you are doing.

If this is a liquid surge drum then its purpose is likely to be to even out flow swings in your process (I'm assuming since you use the term surge drum you aren't trying to separate out water from HCs or some other function).

You need to determine how much volume or residence time you need to provide between low and high liquid levels and that drives how large of a drum you need. How much residence time depends on the expected feed flow swings, ability of your downstream equipment to handle changes and total flow rate. More surge capacity directionally is what Operations will always want but on high flows, you can wind up with very large, very expensive vessels that if they contain flammeable or hazardous materials creates another hazard for the site.

Depending on your numbers, you'll then select a vertical or horizontal vessel.

 

[epoisses]

Agree, just wanted to add the missing piece of data, which is the time you allow the operator to react when he gets the LLA or HLA. This depends on your plant's standards (which again are probably a function of the operation conditions and/or the material in the drum). A typical value is 15 minutes.
BTW what would happen if you left it out completely?

 

[asv80]

TD2K:
I am interested in knowing the procedure to design a similar application - a surge drum to even out flucutations. Are there guidelines to the residence time? If I assume a L/D of 4 and find the D to hold the volume more than the residence time, would it make sense?
Thanks.
ASV80

 

[MortenA]

Are you trying to design a pulsation dampner (e.i. a small vessel connected to a liquid system but with a volume of air under pressure so that liquid surges will be dampned?

Best regards

Morten

 

[asv80]

MortenA:
We have a heat exchanger using liquid ammonia and need a surge drum to ensure there is always supply to the exchanger. The flow rate to the exchanger of liquid ammonia is ~ 4000 lb/h at -10°F and there is no vaporization of ammonia any time. It is only going to be a liquid surge tank.
Thanks in advance.
ASV80

 

[25362]

Depending on the plant and process characteristics it is sometimes advisable to consider installing redundant tappings, or different kinds of level sensing instruments, to preclude the possibility of having undetected false readings.

 

[asv80]

Another use for the surge tank is to use it as a storge tank during cleaning of the exchanger.
ASV80