9 replies to this topic

[ayan_dg]

Vacuum line having inlet pressure of 230 mbar (abs). How to size such a line . What is the velocity range one should keep. What is the pressure gradient (mbar/100 m) one should keep ?

[sheiko]

ayan_dg:


For vacuum vapor lines below 50 mm, allow max. 5% of absolute pressure for friction loss.

Source: Branan C., Rules of Thumbs for Chemical Engineers, 4th Ed., 2005

[Zauberberg]

Bechtel's design standard recommends velocities up to 60m/sec, for pressure range -0.94barg to atmospheric. Depending on application (e.g. vacuum tower overheads) you definitely want to minimize pressure drop and in such circumstances line sizing will be governed by allowable dP.

[Art Montemayor]

Ayan:

As I continue to stress on all fluid flow applications: Forget about sizing a pipe using the simple and quick velocity equation. As Sheiko and Zauberberg are expounding, rely totally on meeting the strict and sensitive pressure drop criteria in the process --- ESPECIALLY IN A VACUUM SYSTEM!

Your entire process will come apart at the seams if you fail to maintain a constant and reliable vacuum within your process. You are working within a very narrow range of pressure limits --- less than an atmosphere! You must ensure that the pressure drop suffered through the piping is the lowest you can afford - and be conservative. Use a minimum of off-sets (Elbows, tees, fittings) and only employ bends (NOT elbows) when you have to make a turn.

Read my workbook on creating and maintaining a vacuum.

[ankur2061]

Ayan,

An extremely important factor in sizing of vacuum lines is density correction of the evacuated gas/vapor, which if neglected can lead to serious under-sizing of the vacuum line. This has been practically faced by me in my earlier part of my career.

I would recommend a density correction for the gas/vapor at an equivalent length of a maximum of 10 m (30 ft) for vacuum lines operating at 100 torr or below. This follows the fact that small volumetric flow rate changes due to density change have a big impact on pressure drop when dealing in terms of vacuum.

As usual you have been provided excellent guidance by Art, Zauberberg and Sheiko for vacuum line sizing.

Regards,
Ankur.

[djack77494]

Sizing lines in vacuum service is the most critical of all line sizing and it is extremely important to accurately calculate pressure losses. What would be small errors at "normal" pressures are catastrophic for vacuum conditions. Often unusual measures, such as the use of bends rather than elbows, are taken to minimize pressure losses. When you do your calculations, be sure to know the exact routing of your vacuum line. Fortunately, at very low pressures, gases often behave close to ideally.

[Qalander (Chem)]

Splendid explanation!and Assertive as well.

[sachindhopade]

hope this helps u
http://www.processca..._Condition.aspx

[Art Montemayor]

sachindhopade:

Can you please explain how the Critical Pressure and Critical Temperature play a role in determining the pressure drop for a gaseous fluid flowing under a vacuum condition?

What help, if any, can this information give to the O.P.?

[deltaChe]

Ayan,

An extremely important factor in sizing of vacuum lines is density correction of the evacuated gas/vapor, which if neglected can lead to serious under-sizing of the vacuum line. This has been practically faced by me in my earlier part of my career.

I would recommend a density correction for the gas/vapor at an equivalent length of a maximum of 10 m (30 ft) for vacuum lines operating at 100 torr or below. This follows the fact that small volumetric flow rate changes due to density change have a big impact on pressure drop when dealing in terms of vacuum.

As usual you have been provided excellent guidance by Art, Zauberberg and Sheiko for vacuum line sizing.

Regards,
Ankur.

Dear Sir

Could you please explain more how to use this numerical methodl to correct the density in vacuum state ? Because I am very confused on this topic, how can I do this kind of calcuation on sizing 10m of compressible gas in vacuum state ?

Thank you very much.