Air trap in inverted-u pipe, how to avoid/get rid of

[marl0]

I hope this is the correct forum for this type of question.

I have been thinking about this problem for a while, but have not been able to come to a definite conclusion.

Say there is an inverted-U style pipe, as seen at many large chemical plants, and the pipe in question is empty, thus filled with air. The pipe needs to be brought online, so the pump is turned on, liquid (or a slurry) flows (is pushed) through the pipe, up vertically, and then across the horizontal on top of the inverted-U. How could you ensure that there would be no air pocket formed along the top of the horizontal stretch, before it falls back down the other side of the U? Would it just be a matter of forcing the liquid/mixture through the pipe fast enough or would there need to be some sort of vent on the top of the pipe? I would think that there would be the possibility that the mixture would settle in the bottom of the horizontal and let some amount of air stay in the top. Wouldn't this potentially cause problems, especially if flow measurements were being taken in this section of the pipe?

I don't have any experience in this field, but I got to thinking about it after taking a tour of a chemical plant where a friend of mine works. Any references or books that might talk about this, or just an answer/explanation would be much appreciated!

Thanks,
Cody

 

[ash9144]

Vent on top of U. Trying to blow through with high velocity cannot guarantee you'll remove the air. If you are measuring flow you would want to measure on the upward flow side not the falling flow side regardless.

 

[toothless]

There are devices designed for this very purpose, to vent air (or gases) from piping systems. Google pipeline auxiliaries and air vents to see what you get.

 

[marl0]

Ahh, thank you guys, this has been bugging me for a while! I appreciate the responses.

 

[bimr]

Piping is not normally designed in the manner that you describe.

Typically, the piping designer would put a note on the drawings like "no pockets" which would prevent the installation that you describe. Another note is "slope to drain".

If you do have the situation that you describe, you can use a vacuum pump to evacuate the air, a manual air release valve, or a ball float type valve.

http://www.valmatic.com/airrelease.jsp

 

[quark]

Such piping layouts are not uncommon, though. When the pipes enter from a main pipe rack to the building, inverted Us are possible (due to height differences). We use thermostatic steam traps as air vents and they have been good to me, so far. No problems with water but care should be taken with chemicals, in terms of proper drainage.

 

[zerosum]

The "no pockets" design problem is a continual one in many process plants. All the replies given above are possible solutions, depending...

We often must (or elect to) hydro a line which will ultimately be carrying a gas or vapor. The necessary vent valve at the high point gets removed after the hydro to avoid future leak points, but then how do we qualify the new closure (plug or cap, screwed or socket-welded) for no leakage?

High points in piping lines are often unavoidable nuisances.

 

[djack77494]

Simple question, but is it necessary that all the air be removed? Probably not. If you do have a downcomer with a large enough diameter, it will be "self-venting", and will always have some vapor inside.

 

[SmilesAtTheMoon]

Would it be possible to pig the air out using a sphere? I've seen many plants with U's for thermal stress relief and vehicular traffic.

 

[gerhardl]

When selecting a correct air release valve, (and/or combined with air inlet valve if vacuum collapse of pipeline or tanks is an issue) you have to specify according to the actual process conditions. Following questions must be answered:

1. Amount of air in/out of pipeline. (Equals to amount/time unit of liquid in or out of system, if not vented elsewhere or open end system)

2. Pressure and pressure peaks at start up and at normal operating conditions.

3. Type of liquid and characteristic (able to be vented without sticking residues? Toxic or inflammable fumes? Temperature? Corrosive? Compatabillity to sealing materials?

4. Allowable leakage to atmosphere when closed?

5. Temperature?

With answers to above, you will be able to find commercial existing solutions or combined solutions for most applications.

Most common sized air outlet/inlet valves are normally mounted with a revision on/off valve below, similar to arrangement for PRV's or PSV's.

Air inlet/outlet valves are well known for water mains, as highpoints commonly can not be avoided.