Entrapped air in horizontal pipe
Entrapped air in horizontal pipe
(OP)
Does anyone have experience with this? Here's my situation:
Vertical pump sends 15500 gpm of water into an empty 30" diameter 120' long horizontal pipe that turns down 10' vertically, turns and runs another 60' horizontally and then turns up another 20' vertically. The 60' long "trap" holds water every night when the pump is turned off, but the 120' long horizontal pipe drains completely.
Is the pump flow (~7 ft/sec) enought to push all of the air out of the horizontal pipe? Is there any information out there that might help? Should I see huge slugs of air coming out of the discharge? I have flow issues and I'm thinking that a huge air bubble in the horizontal line is restricting the flow. Thanks for any information.
Vertical pump sends 15500 gpm of water into an empty 30" diameter 120' long horizontal pipe that turns down 10' vertically, turns and runs another 60' horizontally and then turns up another 20' vertically. The 60' long "trap" holds water every night when the pump is turned off, but the 120' long horizontal pipe drains completely.
Is the pump flow (~7 ft/sec) enought to push all of the air out of the horizontal pipe? Is there any information out there that might help? Should I see huge slugs of air coming out of the discharge? I have flow issues and I'm thinking that a huge air bubble in the horizontal line is restricting the flow. Thanks for any information.





RE: Entrapped air in horizontal pipe
In real life, a pump discharge in hilly country is rarely related to final outlet pressure. The deviation from theory is air bubbles prevent accessing the rebound effects of the downhill sections and your pump discharge pressure is usually dominated by the height of the highest hill. This phenonenum is the most common version of "gas lock" or "gas interference".
In a line with minimal elevation differences and your huge flow rate, the liquid being pumped will drag this tiny amount of air (600 SCF is an infetisimaly small number) out of the line within the first few seconds of pumping. Gas interference just can't be causing you flow problems.
On the other hand, my handy-dandy reference book shows the onset of erosion to occur around 5 ft/sec, Your Reynolds number works out to around 1.5 million. I wouldn't be suprised if your flow problems were related to excessive friction.
David
RE: Entrapped air in horizontal pipe
RE: Entrapped air in horizontal pipe
I you are indeed filling an empty pipe every time you start the pump, then you will have to purge the air out of high point vent every time. There should definately be a high point vent before the pipe turns down the 10' run.
Tim
RE: Entrapped air in horizontal pipe
RE: Entrapped air in horizontal pipe
timbones- The pump is situated in a wet-sump without a check valve. When the pump is shut off for the night, the water in the horizontal pipe drains back to the sump leaving the 120' horizontal pipe empty. The entire 10' down vertical, 60' horizontal and half of the 20' up vertical remain full of water. Does that help?
Artisi- Shut off head is about 65 ft. According to the pump curve, the pump is capable of pushing 15500 gpm @ 21 ft of head.
Has anyone ever used software to model strange pipe flows? How about physical modeling?
RE: Entrapped air in horizontal pipe
What are the flow issues, you need to expand a little so we better understand the likely problem.
RE: Entrapped air in horizontal pipe
RE: Entrapped air in horizontal pipe
RE: Entrapped air in horizontal pipe
RE: Entrapped air in horizontal pipe
For vertical downflow of liquids the Froude number must be less than 0.31 to ensure that self-venting is possible. On the other hand, a Froude number of greater than 1.0 will ensure that bubbles are swept down and out of the system (i.e. siphon flow). A flow of 15,500 USgpm in a 30" pipe gives a Froude number of 0.78 so the bubbles do not easily rise upwards in the 10' downward-flow section, but neither are they easily swept downwards and out. The trapped air will greatly reduce the density in this section so you do not get the expected pressure recovery. Plus the water has to accelerate through this section because the bubbles take up a considerable fraction of the cross sectional area of the pipe and this will raise the pressure drop further.
If your pump has a 65' shut-off head, and you are operating at 21' head, you are probably in quite a steep section of the pump curve. At start up, while you definitely still have the air in the system and you are attempting to flush it out, you will not have the pressure recovery in the 10' section and your static head will be the full 20'. Add to this the friction and acceleration losses and you probably have a pump discharge pressure significantly over 21' and a flow of much less than the desired 15,500 USgpm. This start-up flow is very likely not enough to flush the air out and you never get to your 15,500 USgpm. What discharge pressure do you actually measure at the pump?
Under ideal conditions (no trapped air) your friction and exit losses are likely to be about 2 or 3 ft of head. Adding this to your net static head of 10' indicates that the pump should be OK.
How to fix it? An air vent near the end of the 120' horizontal section is probably the way to go. I would also like to have a foot valve or non-return valve in the sump to avoid the venting being required at every re-start, but there may be process reasons why you have not done that.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Entrapped air in horizontal pipe
Quick and dirty model: Is there any way you can measure the volume of water entering the system at pump start vs volume of water exiting to see if there is some resident volume of air remaining trapped in the system. Remember to reduce the 120' of 30" air volume to compensate for the increased pressure on that air volume when starting.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Entrapped air in horizontal pipe
Elevations:
sump water level = 81'
120' horizontal @ pump discharge = 93'
60' horizontal run = 83'
discharge = 100'
katmar- Our pumps do not currently have gauges for pump discharge, but we are working on getting them installed next week. I'll update after the installation.
RE: Entrapped air in horizontal pipe
RE: Entrapped air in horizontal pipe
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Entrapped air in horizontal pipe
RE: Entrapped air in horizontal pipe
Static heads add or subtract to get total static head
= 19 ft - 10 ft + 20 ft = 29 feet total static lift
Now you must subtract flow losses of 15500 gpm in a 30" diameter pipe that is a total of,
120' long + 10' vertically, + 60' +20 = 210 feet long
If that's more than 31' - 29 ft, you've run out of head. In any case this is a marginal installation... at best!
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Entrapped air in horizontal pipe
RE: Entrapped air in horizontal pipe
RE: Entrapped air in horizontal pipe
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Entrapped air in horizontal pipe
At what elevation is the pump? The 19' of static head could have a different effect if your pumps are mounted on the level of the 120' horizontal section rather than if they are mounted below the sump liquid level. The impact will be on the NPSHa.
You are correct that you need to consider all alternatives. But from my calcs I am convinced that trapped air will be a problem, and if other things are causing the flow to be lower than the 15500 gpm (you mentioned 12000 gpm) then it will make it even more difficult to flush out the air.
I strongly believe that your first action should be to get pressure tapping points on the pump discharge. This, together with the pump amps and the flowrate, will tell you if your pump is operating on its curve. If possible also get pressure points at the far end of the 120' section and somewhere near the middle of the 60' horizontal section. In addition to telling you the pressure, the tapping point on the 120' section will tell you if air is trapped there before you go out and buy an automatic vent.
It seems to me that you will have to do a "first principles" study before you can pinpoint where the problem is. It could be anything from a pump running in reverse to a welding bottle left in the pipeline. We have all experienced these things!
Good luck
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Entrapped air in horizontal pipe
katmar- What calcs are you refering to? I assume it has to do with the self-venting vs. siphoning, but I'd like some more information on the numbers you mentioned. I can calculate the Froude number, but am unfamiliar with the 0.31 and 1.0 numbers. Do you have links to information? Thanks.
RE: Entrapped air in horizontal pipe
I'm at home now and I only have a 5th edition of Perry (Chem Engrs Handbook) here. Under "Drain Pipe" Perry gives almost nothing other than the reference to Simpson. The newer editions may have more info.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Entrapped air in horizontal pipe
15500 USGPM thru 30" pipeline length 120' plus bends velocity etc = approx 2' loss.
Total head is therefore approx 13'.
Back to my earlier posts- I would be looking at entry problems as the pump is probably way out on the curve resulting in NPSH problems or vortexing or etc. Still interesting in what the "flow issues' are?
RE: Entrapped air in horizontal pipe
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Entrapped air in horizontal pipe
RE: Entrapped air in horizontal pipe
RE: Entrapped air in horizontal pipe
Inlet bell elevation = 72'
Sump water level = 82'
Pump discharge elevation = 93'
RE: Entrapped air in horizontal pipe
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Entrapped air in horizontal pipe
The fact that comments are not threaded just adds to the general confusion, IMO.
RE: Entrapped air in horizontal pipe
Can we clear up what the static head is - please advise the vertical distance from the sump level to the dischrage point - NOT the pump discharge level - the level at which the pipeline discharges.
The following is a free image posting site - http://www.freeimagehosting.net/
RE: Entrapped air in horizontal pipe
RE: Entrapped air in horizontal pipe
If you scroll back to orlandobill's posts of 22 Feb 07 10:11 and 22:07 he made the relative elevations pretty clear. Where I went wrong was to assume that he had a horizontal centrifugal pump mounted somewhere between the sump level and the 120' horizontal section, i.e. somewhere between the 81' and 93' levels. In fact "Vertical Pump" was almost his first words in his first post so it was my mistake.
It seems the pump suction is 10' below the sump surface, which agrees with the HPSHa of ~40' (i.e. 32' atm plus 10' depth less a foot or two of vapour pressure and friction). On this basis I can't see anything fundamentally wrong with the setup. I suspect the air would be a real problem at start-up, but being realistic I would expect that if you ran the installation a whole day the air should eventually be flushed out at 12000 USgpm. I don't think we can make any more deductions until we have some basic data like discharge pressures and motor amps. I asked Bill for a reference to the pump curve, and that would also be a help.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Entrapped air in horizontal pipe
Is it air in the pipeline, poor intake design, free vorticies or sub-surface vorticies etc etc.
Coupled with this a curve, discharge pressure, motor detail and current draw would also go a long way to clarifing a few points.
RE: Entrapped air in horizontal pipe
My "flow issue" is that while I expect to get 15500 gpm from the setup I am only getting 12000 gpm or so.
For clarification:
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RE: Entrapped air in horizontal pipe
The sketch makes it look like the discharge point is visible above the water level. If this is so, can someone get close to it and observe it during start-up? If they see massive bubbles of air being ejected for a few minutes, and then it dies down and becomes a nice steady flow of water then you can be fairly sure you are getting the air out. On the other hand, if they only see an ongoing stream of tiny air bubbles you will know that the air is not being removed rapidly at start-up. Have you tried this observation?
Once you have the pressure gauges installed next week that will give you more pointers. Was the pump shop-tested before installation? Is there more than one pump that is behaving this way? I assume you have a "-1" impeller size. If it is, you should easily get >13,000 gpm with the calculated hydraulics but running at 15,500 gpm will be very close to the edge of the curve.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Entrapped air in horizontal pipe
I will try observing the startup for bubbles. We have not been able to shop test this pump. Our rebuilder does not have the facility to do this. We thought about setting something up to try that, but haven't gotten there yet. We have a few pumps acting this way. Look at my forums and you will see my other question about our sump. I will post more info (pressures, amps, flows, etc.) Thanks for the help everybody.
RE: Entrapped air in horizontal pipe
Is this a new installation or an existing one?
If an old installation, did the pump operate ok before going to the rebuilder?
When you get some discharge pressure and power information we might start to see what is happening.
I keep asking questions but it's the only way to get a feel for what is going on.
RE: Entrapped air in horizontal pipe
RE: Entrapped air in horizontal pipe
If you want to get sensible answers I suggest you start again and put the whole story on the table. There is simply too much confusion, contradiction and missing information.
In the sump posting you said that you run 2 pumps at once, each at around 14400 gpm. Because your static head greatly exceeds the friction head, running 2 pumps at once will give you close to twice the flow rate in the pipeline. This would certainly flush out the air, or are you now running only one pump at a time?
In the sump posting you said this problem has arisen every year for 10 years. It seems that at the beginning of the year, after the pumps have been rebuilt, they run for a while and then gradually deteriorate. After rebuilding they are back to spec, but then the deterioration cycle starts again. This is a very different picture from what you have painted here.
In the sump posting you said that you would be inspecting the sump for build-up and the pipeline for scale and blockages. Was this done? What was found?
At what point did you decide that it was not a sump problem and that it was a trapped air problem? Why?
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Entrapped air in horizontal pipe
http://www.urban75.com/Mag/troll.html
Having spent more than my fair share of time on usenet years ago I like to think I know one when I see one, but based on katmar's observations now I'm wondering.
Some trolls are harmless in the sense that they get members talking about an issue amongst themselves. If 'orlandobill' is in fact a troll and has carried it this long, he must be a master baiter.
RE: Entrapped air in horizontal pipe
I originally was looking for information on entrapped air in a horizontal pipe. I intentionally neglected the additional information about my system since it had no bearing on the issue of trapping air in a single pipe.
In lieu of the accusation in Gator's latest post, this will be my post in this thread. I appreciate everyone's input on the entrapped air question, I think I have a direction to head.
RE: Entrapped air in horizontal pipe
Having now seen your pump curve it is clear that even with little or no static head recovery in the 10' vertical section you should still have enough head to pump about 13000 gpm. Even more so if you are running two pumps.
If you are able to observe the discharge to confirm that the air is being flushed out, and you can get a pressure gauge at the start of the 120' section you will be in a much better position to home in on the problem area.
Also, if the pumps do pump according to spec straight after a rebuild and then gradually deteriorate it is more likely to be a problem with the pump than the entrapment of air. I would imagine that an air problem would manifest itself from day 1.
I hope you will not make this your last post. I agree that you have a direction to head and I would be very interested to know the answer once you find it. I spend time at Eng-Tips to learn, not to teach. You should not take too much notice of Gator's question or my venting - technical geeks just want to get their teeth into nice juicy problems and we get frustrated when we are held back.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Entrapped air in horizontal pipe