14 replies to this topic

[Fish]

Dear all,

Please tell me the exact methodolgy to calculate barometric leg? My ssytem pressure is about 20inHg. Density of fluid is 1.26g/cc.

Also, please calrify to me whether the barometric leg must be STRAIGHT vertical downward pipe? or, it can be anyhow but downward?

Can please anybody help me in this regard. Its way too important and urgent for me.

Regards,
Vishal N.

Edited by F vishal, 24 August 2009 - 06:28 AM.

[katmar]

Assuming your system pressure of 20"Hg is the absolute pressure and your atmospheric pressure is around 30"Hg you need to provide a leg to counterbalance a gauge pressure of -10"Hg. You can work out the height required from the relation
Pressure = density x gravity x height (all in consistent units)

Add a bit of height as a safety factor because your pressure may fluctuate. This height is the vertical height of the leg, not its length. I strongly recommend that you make the leg straight and vertical and that you design the diameter to allow self-venting flow. You could get away with a 45 degree section where the pipe is flooded with liquid, but only do that as a last resort. Never have sloped piping in the upper part of the leg where vapor will be present. Don't have any horizontal piping, or anything less than 45 degrees, anywhere in the leg.

There are manufacturers who use a smaller diameter so that some vapor will be entrained with the liquid and thus help the vacuum pump in removing non-condensibles, but this is dangerous in my opinion because it will work for only a very narrow range of flows.

[hollerg]

The entry at
http://www.cheresour...barometric-leg/ has a good description.

The leg could be at an angle other than vertical, but there are some restrictions.
1) The pipe that is not running liquid full (that is to say the paritally filled pipe above the barametric leg) needs to be large enough for liquid drainage down and vapor travelling out.
2) The pipe typically is usually no more than 45 degrees off of vertical.
3) Use 45 degree elbows to make gradual transitions in direction
4) There can be no air leakage into a barametric leg or the seal will be broken

Good luck.

[Fish]

Dear Katmar,

Thanks for ur valuable suggestion. Actually, the system what i have is an filtration unit(pan filter for alumina hydrate filtration). there is no vapor anywhere. Vacuum pump is directly connected to reciever of the filter and the wash water is sucked from the filter to the reciever. A vertical line runs from reciever to vacuum reciever situated downstairs. Vertical straight height is about 6.5mt and the total length is ~19 mt (reciever to vacuum reciever). I am looking out for some modification which would reduce this vertical height o 3.5 mt.

Atmospheric pressure is 27.2 in Hg and operating is close to 20in Hg. I doubt whether the modification works. what u have to say abt this?

Regards.

[Fish]

Thank you Mr. hollerg.

[DRS]

If the atmospheric pressure is 27.2 in Hg and you are maintaining vacuum of 20 in Hg, then this vacuum can hold mercury column of height 7.2 inches or 182.88 mm. Since your liquid density is 1.26g/cc, the liquid leg height will be 182.88*13.6/1.26 = 1974 mm or 1.974 meters. This will be including the liquid column height in the vessel. Liquid column above this will fall down by gravity and even 3.5 meters would be redundant unless this is height from floor to floor.

Your description of the setup is a bit confusing. When you directly connect the vacuum pump to wash water receiver then wash water receiver itself is the vacuum receiver. The receiver downstairs is not a vacuum receiver since it sees only atmospheric pressure. May be I am missing something.

[Fish]

Mr. DRS,

Thank you very much for your reply. The doubt that you have seems interesting b' it is way too easy to answer. Actually, vacuum pump suction is connected to a reciever chamber which is connected to table filter. Wash water generated in filter is sucked in the chamber first. This chamber breaks contact between water and vacuumed air. As wash water during its filtration carries some amount of soda with it, we dont want it to flow to cooling tower(seal water to the vacuum pump comes from cooling tower) and contaminate the cooling tower.

Finally, collected wash water in the reciever chamber flows to vacuum reciever placed downstairs (it is 2 floors down). And the distance between vacuum chamber and reciever (or hot well called in some cases) is called Barometric leg as we know. Hope I've cleared your doubt. You have any suggestions on this?

Regards.

[katmar]

The vapor that I referred to would be the air in this case, which must be present in your vacuum receiver (or else you could not use a vacuum pump). The guidelines I gave apply to the barometric leg, which as you point out is the line from the receiver (chamber) down to the hot well. If your barometric leg is 19m long but covers a vertical height of only 6,5m then there could be problems. I do not understand how you could change the vertical height difference to 3,5m. Will you be raising the hot well, or lowering the vacuum receiver? A sketch would be very helpful to convey what you are dealing with.

[DRS]

Vishal,

That is a standard set up and that is why I am reiterating that the bottom receiver is not a vacuum receiver.

Katmar,

As long as we provide a vertical leg with height in mlc equal to that of absolute pressure, just above the liquid seal, there shouldn't be any problem. Say, there is a vertical pipe of 3 meters above liquid seal level (in the bottom receiver) and then you run it horizontally for 16 meters, now the 16 meters pipe form the part of vacuum receiver. Liquid falling into the vacuum receiver will drop down the pipe and into the bottom receiver. If there is enough liquid in the pipe so that liquid in the vertical pipe rises beyond 2 meters then liquid seal rises and overflows out of the bottom receiver.

I would like to know what are your concerns about the length of the piping (provided there is no leakage).

[katmar]

DRS,

My concern is two-phase flow in horizontal piping. If you built the barometric leg as you have described with the 16 m horizontal section immediately below the receiver you would have the liquid running horizontally in a partly filled pipe. If the pipe is hugely oversized this will work, but if the horizontal section is the same size as the rest of the leg it is possible (indeed likely) that you will get slugging which will cause pressure fluctuations. This isn't just theory - I have seen this in practice.

On the other hand, if you have the 3 meter vertical section immediately below the receiver and then run the 16 meter horizontal section at the bottom where it is guaranteed to run full of liquid then it will have a better chance of success. It wouldn't be ideal, but I have seen it done this way.

The only two-phase flow section should be vertical, immediately below the receiver, and should be sized for self venting flow.

[Fish]

Katmar,

Please find attached a sheet showing the system. One with the continous line is present operating system. And the one with a dotted line is something which im proposing. But im doubtful whether the break in vertical leg from 6mt to 3mt will work.

Data:

Vacuum operation: 20inHg max. Normal 14 in Hg.
Atm pr. : 27.2inHg.
Density of liquid: take 1.2 g/cc

Attached Files

•  Vacuum reciever.xls   18KB   249 downloads

[katmar]

The sketch helps greatly. A picture is worth a thousand words. Whoever did the original design seems to agree with my philosophy of vertical first and keep the horizontal piping flooded.

There is now confusion about the pressure in the vacuum receiver. You previously said the pressure was 20"Hg, but now you say it is a vacuum of 20"Hg. If it is indeed a vacuum of 20"Hg (i.e. absolute pressure in receiver is 7,2"Hg) then you need the 6 metre vertical distance.

The dotted line bypassing the seal tank and going to the injection tank must extend to the bottom of the tank because you still need to have a seal. See my reply to DRS above for the reason I do not like horizontal piping. But I accept that engineering is always a compromise. If you have to go directly to the injection tank it could be done by sloping the line as steeply as possible and by using a large pipe diameter. It's not ideal, but you could make it work. I would do this as a last resort.

[Fish]

Oh! it seems like im trying to get confused. For an absolute pressure of 7.2"Hg how would i need to have 6mt of pipe length to counterbalance it?

[katmar]

The pressure at the base of the barometric leg, at the level corresponding with the surface of the hot well, has to be atmospheric pressure which is 27,2 "Hg abs in this case. The pressure at the upper surface of the liquid in the barometric leg is the pressure in the receiver, which is 7,2 "Hg abs in this case. The difference between these two pressures (i.e. 20 "Hg) is the pressure (or head) that is supplied by the liquid leg.

The calculation of the liquid leg height (like all other calculations) is most easily done in SI Units, i.e.

Pressure = 20"Hg = 67700 Pascal
Density = 1,2 g/cc = 1200 kg/m3
Gravity = 9,81 m/s2

Height = Pressure / (Density x Gravity) = 67700 / (1200 x 9.81) = 5,75 metre

You need to add a safety margin to this, although you may have already done this by designing for -20 "Hg gauge against a normal operating pressure of -14 "Hg gauge. Theoretically you also need to add the friction head of the liquid flowing down the leg, but if you design the leg diameter for self venting flow the friction loss will be negligible.

[Fish]

Dear Mr. katmar

Many thanks to u for your profound and pertinent help. I hope my system of modification would work out. Although it requires a max height of 6 mt, I'll try and see whether vertical head of 3mt with rest horizontal length works.

U strongly believe that 3mt of straight height might retard leg required?

Regards.