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Entrapped air in horizontal pipe

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.

RE: Entrapped air in horizontal pipe

If you look at monometer theory, you'll see that uphills and downhills in a line completely filled with liquid are immaterial and the only thing that matters to hydrostatic head is the elevation difference between the inlet and the outlet.

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

Not claiming to have experience in this, but (if I understand your layout correctly) it would seem at least one air release valve say at the top of the horizontal 120' section of pipe exiting the pumps but maybe right where/before the flow dips down in the 90? and 10' riser, might allow the air to exit without need to scavenge it out with flow?

RE: Entrapped air in horizontal pipe

The whole situation sounds a little strange, but that may be because the information we have may not be complete. Where does the water drain to? Is this a wet-pit pump? Is there a check valve on the discharge side of the pump to prevent water backflow? Is there a discharge isolation valve?

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

What is the rated head capability of the pump?

RE: Entrapped air in horizontal pipe

(OP)
rconner- I agree that an air release valve at the point where the horizontal run transitions into the down vertical run would be a good idea, but I need to have extreme confidence that there is air being trapped and reducing the flow. The cost of installing one now would be very significant and management would want to be confident that the air release valve would help.

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 is the difference in elevation from the sump water level to the discharge point, is it 10ft?
What are the flow issues, you need to expand a little so we better understand the likely problem.

RE: Entrapped air in horizontal pipe

I get about 11.5 ft pressure drop including 10' elevation in the discharge (only discharge side). Velocity and Reynolds number match with that of David's. You may have excess flow, if your pump is designed for 21 feet head.

RE: Entrapped air in horizontal pipe

I agree with rconner. A vent with a discharge leg higher than the 20' discharge pipe section should vent the air.

RE: Entrapped air in horizontal pipe

I also came up with similar total head - hence the question of elevation before addressing what could be excessive flow and possible cavitation, inlet vortices or other inlet problems.

RE: Entrapped air in horizontal pipe

I believe that the trapped air is the problem. It looks to me like the cause is a combination of two conditions. The first is that your design velocity is in the transition zone between the self venting and siphoning conditions. The second is the transient start-up conditions which we often overlook.

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

With a typical electic motor, I would not think its start-up torque related, however it may be such that air is never totally evacuated and a resident air pocket is restricting flow.  Vent with a foot or discharge check should eliminate these problems.  

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.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Entrapped air in horizontal pipe

(OP)
The sump water level is 19' below the discharge level.

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

If there is Significant air at the top of the pipe, (enough to reduce the cross-section) it may be detectable by sounding the pipe with a hammer, or looking for a condensation line on a humid day.

RE: Entrapped air in horizontal pipe

The 12' height difference between the sump water level and the level of the 120' horizontal section is also part of the head that the pump has to provide. The design head of 21' leaves no margin of error with the theoretical head of 19' static plus 2' friction. If there are any valves, or more than the three elbows you have mentioned, you will be undersized on the pump. On this basis you cannot even consider installing a foot valve at the inlet.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Entrapped air in horizontal pipe

(OP)
katmar- Thanks for all your input, but I'm confused. The BEP of my pump curve is around 31' and 13500 gpm. Is this what is refered to as "designed for" as others are indicating? My understanding is that the pump will operate at any point along the curve. In our case (with only ~21' of head) the pump should discharge 15500 gpm @ >80% eff. We will be fine as long as we are pushing more than 13000 gpm, but we can't even get that. My thought was since we are not getting the expected 15500 gpm that something is definately off. Please confirm whether you still think the pump is undersized.

RE: Entrapped air in horizontal pipe

What Harvy is saying is,

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!

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Entrapped air in horizontal pipe

Check your inlet condition. If you see whirlpool, your submergence is not enough and you will be drawing in air causing cavitation. Also check the pump required NPSH. I estimate total 32.1 ft wg loss but I included (2) gate valves & a check valve. I did not have a strainer though. This also includes 19' differential head, borda entrance and exit out the 30" pipe. The friction loss/100' of pipe is estimated to be 0.452 ft wg/100' of pipe. This is bases on an open system & additional 15% allowance for aging. The valves came out to 161 ft equiv length. Friction & head loss to pump suction is 14.3 ft. so available NPSH is 33.96-.783-14.3 = 18.87 say 17' to be safe. Compare w/ NPSH required

RE: Entrapped air in horizontal pipe

(OP)
We've gotten a little off track. I must have confused everyone. My total static head is only 19' (100'-81'). Add another 2' for dynamic losses and I'm up to 21' or so. I'm wondering if trapped air in the long horizontal pipe might be causing my reduction in flow (from 15500 according to the curve to <12000). I'm investigating other possibilities (incorrect pump rebuild, obstructions, inproper sump conditions, etc.), but I wanted to hear if other felt as though entrapped air might be contributing to the issues.

RE: Entrapped air in horizontal pipe

If you don't mind some little fountains (its water heh?) Drill tiny little holes along the top of the 120'.  Kidding... Or maybe not.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Entrapped air in horizontal pipe

I did not mean anything too specific or technical by "designed for". It was the 15500 gpm at 21' head that I took as the design point.

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

(OP)
The vertical pump intake bell is at 72' and the centerline of the discharge pipe is 93'. The sump water level is 82'. I know my NPSHa (~40') is marginal to my NPSHr (~34'). That's material for another post.

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

Bill, yes I was referring to the self-venting and siphoning calc. I don't have any online links, but then I have not Googled it. The best reference that I have found is an old one from Larry Simpson, Chemical Engineering, June 17, 1968, pgs 192-214. In many ways this article is more useful than the Crane TP410 manual. A more recent article, with a slightly different slant and not quite as relevant to your situation is P.D. Hills, Chemical Engineering, Sept 5, 1983, pgs 111-114.

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

seems we are back to where we were sometime back - a static discharge head of around 12ft ( 93' pipe centreline + half a pipe dia. minus 82' (sump level))
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

Artisi, The discharge is at 100' with the sump level at 81' giving a static head of 19'. The pump has to supply all this differential head, even though 11' of the 19' is on the pump suction side. I agree that the pump is probably way off on the right hand side of the curve, and operating at a head of approximately 1/3 of the shut-off head. It could well be an NPSH problem. Bill, do you hear any cavitation?

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Entrapped air in horizontal pipe

Take back what I said,  seems I was confused on the static head - I assumed the 93' was the discharge point whereas it seems to be the pump discharge from the sump.

RE: Entrapped air in horizontal pipe

What is the elevation of the pump with regards the sump water level?

RE: Entrapped air in horizontal pipe

(OP)
It's a vertical pump.

Inlet bell elevation = 72'
Sump water level = 82'
Pump discharge elevation = 93'

RE: Entrapped air in horizontal pipe

Bill, do you have a single pump supplying the whole 15500 gpm? If there is more than one pump (duty or installed standby) what does the valving look like. Can you give us a pump make and model so that we can view the curve on the web somewhere?

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Entrapped air in horizontal pipe

You guys really need webspace to post sketches so you are not all running around in circles guessing about actual installed configurations/proposed solutions.

The fact that comments are not threaded just adds to the general confusion, IMO.

RE: Entrapped air in horizontal pipe

Gator is correct - a total confusion at the moment as to what the static head is.


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

orlandobill -- If you think that the air in the pipeline is a problem a quick fix to prove or disprove this point is an extension of BigInch earlier advice - (hole/s in the pipeline) - fit a 4 or 6 vavle to the pipeline which you can manually open to allow any trapped air to escape - see if this overcomes the problem - if it goes why not fit a standpipe to the pipeline with the highest point above your discharge point by a foot or so.  

RE: Entrapped air in horizontal pipe

Artisi, thanks for the reference to the image hosting site. That would be a great help. If orlandobill lives in Orlando the chances are that he is still asleep right now.

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

Like myself I didn't read some of the info all that well, however, I agree that the head static head is probably as advised - although I would like clarification to make sure for my own peace of mind. Also, orlandobill hasn't really addressed the "flow issues" so we don't really know what the issue is, - making it difficult to advise with any certainty.
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

(OP)
Thanks for the image hosting site. I hope these help. While I certainly appreciate all of the free advice, my focus at this point is the possibility of entrapped air as shown. I also should have pointed out that all of the pipe is either buried under 10'-20' of dirt or encased in concrete. Adding an air vent at the supposed collection point will be very difficult, but not impossible, which is why I am looking for information to predict whether the air could collect or not.

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:

[url=http://img2.freeimagehosting.net/image.php?c1baeb795b.jpg]


and:
[url=http://img2.freeimagehosting.net/image.php?f387f1c74f.jpg]

RE: Entrapped air in horizontal pipe

orlandobill, there are defintely other things that you can do before you go and dig down through 20' to install an air vent! While I am still sure the air will be an issue at startup, the more I think about it the more I believe that if you run this pump all day the air should all come out eventually.

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

(OP)
katmar- Do you ever sleep? I'm kidding!
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

The curve makes a big difference, at first look I would say you could have a hydraulic problem. Being so far out on the curve you could have a pre-rotation problem which will reduce flow or intake design problems.

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

What size is the pump inlet & discharge?

RE: Entrapped air in horizontal pipe

orlandobill, if these are the same pumps as you previously discussed in your sump problem ( thread378-173908 ) then we are all barking up the wrong tree here.

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

Have you guys been trolled before? A sophisticated troll can be hard to detect.

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

(OP)
Whoa! I have no idea what a troll is, but I think I can guess. If you notice, I never asked for advice on my flow issues. That conversation was started by others and I only responded to questions that were asked of me. In fact, at one point (22 Feb 07 13:34) I tried to get everyone back on track, but not everyone listened.

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

orlandobill, unfortunately it is not possible to decouple the trapped air question from the flow issues. That is why everyone (or at least Artisi and I) have been asking you what the flowrate and pressure drop are. If you have a low flowrate (Froude number < 0.31) the air will remain in the pipe.  If the Froude number is > 1.0 the air will be rapidly flushed out. In between these limits you will have "gradual" removal of the air - I don't know if anyone would try to estimate how long it will take to completely remove the air.

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

Good luck - but certainly come back if you still have a problem.

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