Air in pipelines and associated losses
Air in pipelines and associated losses
(OP)
Simple question with a probably not-so-simple answer: How do you generally account for the losses in pipelines caused by air pockets, that (as far as I've been told) can intruduce a restriction in the pipe (by reducing the effective cross sectional area) at the point where they're located?





RE: Air in pipelines and associated losses
http:/
RE: Air in pipelines and associated losses
If you are continuously introducing large quantities of air and not using air elimination valves, you need to start considering the possibilities of two-phase flow, which may have many forms of flow across a partially to full cross-section depending on the gas to liquid ratios and pipe slopes encountered in your pipeline at various points. If you get there, you are definitely out of the simple-solution range. Search "2-phase flow regimes" and you'll get some pictures of what you're up against.
RE: Air in pipelines and associated losses
RE: Air in pipelines and associated losses
This paper has been recently posted by eng-tips member kacarrol in another thread.
RE: Air in pipelines and associated losses
One of the pipelines I'm considering goes down a mine.
One problem related to air entrainment is that this pipeline, which may be considered essentially as a pipe running down a very long hill (drift/mains), is half DN125 pipe and half DN140. This causes below atmospheric pressures, which I'm worried will lead to air coming out of solution too frequently.
Why half/half? We found using DN140 would reduce the transient pressures enough (by lowering velocity) in the higher pressure areas to allow us to continue using HDPE, which isn't available in > PN25 rating (otherwise for the lower portions we'd use DN125 PN30 or whatever). Using PRVs isn't an option as we actually require high pressures in the lower sections (the pipe travels back upwards for another 150m vertical / 4km horiz after the bottom of the 'hill').
The issue is, as doing some math showed me, if you have a downhill pipeline that goes from small to big at a high enough flow you can get negative pressures generated, especially at the junction where small pipe meets big pipe. And I've read this may cause air to come out of solution (or get sucked into the pipe if fittings are loose).
We could simply use DN140 for the whole run, which I'm considering doing if lower pressures lead to air in the pipeline and cause problems. The only reason I didn't is because of the cost saving over kms of pipe.
RE: Air in pipelines and associated losses
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Air in pipelines and associated losses
Where small pipe meets big pipe, according to the Bernoulli equation, velocity slows and pressure increases. Being that the larger diameter is at a lower elevation, the static pressure component would be higher too.
What it sounds like to me, you did say "pipeline flowing downhill", and assuming that the flow is downhill in this pipe, is that you probably are not getting too much steady state flow, probably because of a fully open end or an outlet valve open to much, thus giving you no, or still a too low an outlet backpressure to allow steady state flows. When you are drawing out a higher flowrate than what is flowing in, vacuum conditions can be developed in the lower sections. If outlet back pressures were to be controlled properly to allow steady state flow across the full pipe cross-section, you would be able to maintain pressures above vacuum. I think you probably have steady state two phase flow in the lower sections, where air is backflowing into the pipeline from the outlet until the water flow from the source gets blocked enough to push the air out again. Just a guess and I may be wrong, but I think it allows the physics to agree with what you are apparently describing.
RE: Air in pipelines and associated losses
The entrance and exit effects should in part affect the flow rate, but the main contribution to head loss is always given by the friction loss which should be the same in this case. This way you should have a gentle slope in the first segment and the a steeper line in the second segment, instead a "more consuming" head in the first segment. I think this way the issue of having negative pressures generated could be moved downstream.
RE: Air in pipelines and associated losses
RE: Air in pipelines and associated losses
RE: Air in pipelines and associated losses
RE: Air in pipelines and associated losses
I should have mentioned the hydraulic grade for this pipeline under normal operating conditions does follow the grade of the earth fairly closely and negative pressures will not occur. However, under normal operating conditions the flow is throttled.
If someone were to come along and fully open the throttling valve at the bottom of the pipeline the negative pressures will occur (hydraulic grade becomes steeper than earth, etc). This is probably more likely than not, i.e. someone wants to throttle the flow further and asks 'Joe' to adjust the valve, but Joe doesn't know which way to turn it and takes a guess... and fully opens the valve....
If this means people have to go along and bleed the air out of the line at the high points - fine - but is it possible that this pipeline, already operating close to the limit of the pipes rating in terms of pressure, could overpressurise? Perhaps as a result of collapsing vapor cavities...
RE: Air in pipelines and associated losses
But as BigInch has said, until we can calculate the likelihood of air being drawn in and the minimum pressures attainable, we are proposing solutions to unknown problems.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Air in pipelines and associated losses
RE: Air in pipelines and associated losses
As for fast closing the manual valve you could fit a gearbox. Even then I have seen ingenious operators make up a yoke that fits into a pneumatic powered drill. They use this to close the valve quickly because otherwise it takes toooo long!
RE: Air in pipelines and associated losses
You've got a valid input by fellow katmar (make impossible for "Joe" to create flow conditions which can be detrimental or catastrophic).
You'll probably be aware of the stuff in the attached paper anyway......
ht