Designing unflooded (Self Venting) Piping

[patibar]

In Ethylene Glycol unit, recently we have decided to route the vaccum condensate of PGU Refining column two stage ejector system to an existing vessel, which is having a spare nozzle at the bottom of the vessel. The vaccum required in the column is 49 mmHg. The vertical distance between condenser and vessel is 17.3 meters. Vessel operating pressure is 10PSIG. Including vessel liquid hieght, the back pressure is coming as 1320 mmHg. At present, There are two barometric legs, one from each condenser going to OWS. We would like to reroute the streams into the vessel.

I have gone through the article by P.D. Hills (Designing Piping for Gravity Flow). In that it has written that "To avoid having the liquid carrying forward, adequate free area must be left in the pipe to allow gas to pass backward". I would like to know what is the minimum gap to be maintained for this.

The vaccum condensate flow rate is about 250 kg/h. From both the condensers 11/2" self venting lines (slope of 1:200) have been proposed and they will connect each other at grond level, after that with an expander 11/2" to 3", a loop seal of 3" has been proposed of 1m height and finally it gets connected to vessel bottom (11/2" nozzle with 3" X 11/2" reducer). This is the schematic. I would request you to validate my scheme, whether it works or not?

Thanks.

 

[patibar]

I have forgot to mention the following.

Froude number has calculated based on the article by P.D. Hills, and it is coming as 0.1 which is < 0.3.
Vaccum condensate is having water plus glycols and having density of 1100 Kg/m³.

Thanks

 

[LittleInch]

Please sketch this out so we can understand it. You clearly have a lot of information available, but without a drawing it is very difficult to visualise from a description

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[patibar]

Please find the attachment.

Thanks.

 

[katmar]

This design is too tight in my opinion and I would avoid it if possible. The difference in elevation from the bottom of the lower condenser to the liquid level in the condensate vessel is 16.1 m. The sum of the vacuum in the condenser and the pressure at the upper surface of the condensate vessel is equivalent to about 15.2 m of your process fluid. The level in the barometric leg would therefore be only 0.9 m below the condenser.

Theoretically this is OK under steady state conditions, but any fluctuation in the condensate vessel pressure or in the composition of the condensate could cause the liquid to back up into the condenser. Only you can judge whether this is a real risk. If you have to do it this way then perhaps make the first 2 m below the condenser 3" diameter to ensure that there is no froth in the pipe to decrease the density. And keep that 2 m vertical.

Some general comments on your design:
Never include horizontal piping (even at 1:200) in barometric legs. Air or vapor will get trapped there, especially at start-up. Use 45 degrees as your minimum slope. I learnt this the hard way. If possible the sloped 45 degree piping should be in the section that will eventually be flooded. If not, use a pipe 1 size larger for the sloped portion. The loop at the end of the pipe is a good idea to ensure that the pipe remains sealed. It might be worth making the final up-leg significantly larger diameter than the rest of the piping to be able to fill the vertical down-leg section when the vacuum is applied.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[patibar]

Dear Katmar,

Thanks for your quick reply and comments. We can not provide 45° slope lines since nearest vessel is located in other area and because of routing issue, we have decided to go with this scheme (Near Horizantal Line with slope of 1:200). Acording to the literature by P.D. Hills min of 1:45 slope to be maintained and "To avoid having the liquid carrying forward, adequate free area must be left in the pipe to allow gas to pass backward". I would likt to know about this are. how much it requires? In my case will it works?

As you said 0.9 meter gas is enough for vapor disengagement in steady state conditions?

Thanks

 

[katmar]

The recommendation in the Hills paper is for an overflow pipe coming out the side of a tank. I had assumed that your horizontal section was more than 0.9 m below the condenser, but you have not dimensioned it that way so perhaps I was wrong. 2" is way oversized based on the Froude Number criterion so if it never gets flooded you may get away with it, but I wouldn't do it that way. Also, a slope of 1:200 is more horizontal than 1:45. Make it as steep as you possibly can.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[LittleInch]

Lets see if I've got this right. Your condensate at a high level is sitting under low vacuum conditions (49mmHg). You want to run the condensate to a vessel some distance away - based on your 1:200 routing and 17m drop, this is about 3,000m away (!) which has 10psig pressure on top. I will take katmars calculation of 15m required to overcome the static head, never mind any friction losses. Therefore you have 0.9m vertical m of pipe in which you have liquid and gas, which equates on your slope of 180m. And all this in 2" pipe.

As katmar says, apart for the issue of being able to fill this line, either from your receiving vessel or the column and getting it liquid only for that length, this means that your free area is only required for the initial 180m. I would aim for a min 50% area at that flow or add another vapour return line on top and tee into the liquid line at regular intervals (say every 50m) with the final tee just below your inlet connection into the pipe. Actually if your 1 1/2" pipes from the vessels are liquid filled where is this vapour going to go? Seem slike ideal situation for a vapour lock to me.

overall though I agree with Katmar, this looks like an idea which works on paper only and in practice will never work properly, if at all. Perhaps drop the 17m or more at the start and run it into a can pump or something similar and pump it to your vessel. Establishing an accurate 1:200 drop in 2" pipe over 3,000m in an existing plant looks nigh on impossible to me and likely to take a lot of time and effort in survey, set-up, adjustment and construction.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[racookpe1978]

What will you do for startup or out-of-ordinary operations when both units are not operating at normal pressures and fill levels? What will protect your vacuum (in one condition when the vessel is below operating conditions) or provide that vacuum (in the other condition)?

 

[patibar]

I have calculated the back pressure considering all maximum values (Worst case scenario only). In actual conditions, the vessel will see the pressure of 0.1 to 0.2 Kg/cm²g (flare header pressure)rather than 0.7 Kg/cm²g (10PSIG) and I have considered the liquid level in between two level tapings, which may not be the case in actual. So in actual the back pressure is less in significant number, which will ensure maintaining the required vaccum in the column.

Thanks