siphons....why variation in flow??

[miningman]

Can anyone suggest reasons why five identical 12 diameter siphons, all with the same head, all at the same location draining a lake, would have a variation in flow of 40% between the highest and lowest?? Obvious suggestions are partial blockages, etc but are there any really obscure engineering considerations I might have overlooked??

 

[BobPE]

what are the entrances like? are you sure they are identical? How would you prove this statement? I am saying that to be a devils advocate. What is on drawings and what is inthe field are two different things most of the time. How about apprutenances, are they all in working order? Sounds like a neat problem to study.

BobPE

 

[prex]

Air pockets of different sizes trapped at top of siphons? prex

http://www.xcalcs.com

Online tools for structural design

 

[miningman]

Air pockets?? Yes a reasonable suggestion but why would pockets of different sizes develop in five identical systems?? Which is another way of asking what should I physically be looking for in order to get the lower flowrate units running closer to the higher flowrates.

Drawings?? I wish I had some!! These 12 inch sclairpipes have been running sucessfully for 5-7 years. In previous years the flowrates were adequate to meet production demands but things are a bit different this year and we need to improve things.

The siphons are actually draining a tailings pond, the height of the dam being 16-20 feet above the water surface. I am new to the project and I recognise that 20 feet is getting uncomfortably close to the 32 feet theoretical maximum. My colleagues who have worked with this system assure me that it functions OK at 20 feet, at the end of each season. This year we have to draw down to a 23 foot elevation difference.

How do I know each entrance is identical?? I dont. All I know is that each pipe is marked with the same manufacturing identification markings.But we will have divers on site later this year. What do I ask them to look for if I send them down??

What is an apprutenance?? I'm not familiar with the term. I am familiar with the way the old timers used compressed air to pump water and the degree of submergence was important in getting maximum flow. Is the depth of each siphon inlet below water surface relevant?? I cant think why it would be but I've got enough grey hairs that Im not as sure about anything as I was 30 years ago.

Thanks in advance for ANY suggestions

 

[TD2K]

How long are the syphons, roughly speaking in terms of feet of pipe?

40% is a pretty good difference, that represents almost 2x as much driving force to overcome the additional line loss at the higher flow. Are the flows all different over this +/- 40% range or are 4 at one flow and 1 at the lower flow (I assume the former since you'd likely have mentioned such an obvious clue)?

What range of velocities do you estimate? That should give someone here an idea if air pockets are a potential factor or if they would be swept out once the syphon establishes itself. Personally, I doubt an air pocket would last.

The depth of the syphon below the water surface theoretically does not affect its capacity, that is set by the height difference between the lake and the receiving body. But, if one inlet (or several) was higher, it could be vortexing and drawing in air which would reduce its capacity, is there any sign of that?

I'd suspect some sort of restriction on the inlet side. Or if that doesn't work, something has been sucked and and jammed within the syphon.

 

[miningman]

Siphon length is approx 460 feet. Pipe is 12 inch scliar , SDR 17, nominal ID 11.16 inches ( from the catalog). Flowrates at the beginning of last season were, 2031, 1950, 2246,2209,and1550 gallons (UK) per minute. Flows at the end of the season were 85-91% of these values...which makes sense as the available head drops thru out the season. 2200 gpm corresponds to 519 ft /min or 8.65 ft per sec.

Available head at the beginning of the season 11.68 feet.

There is an additional pipe which consistently refuses to maintain its prime.....air leakage is the obvious expanation or there anyother possible reason??

 

[TD2K]

Losing prime sounds like an air leak to me. Can you get it flowing at all or does it stop soon after you get it flowing? 8 ft/sec would likely flush out any air pockets IMO or keep they from building up though I haven't worked with syphons.

As a point of interest, how do you prime these in the first place? Do you have a portable eductor of some type to remove the air from the high point?

I did a quick calc for a head loss of 11.68 feet for 11.16 ID piping. I got about 2100 UK gallons/min assuming an entrance/exit loss and about 25% additional straight feet of piping over your 460 feet to handle fittings such as elbows. This flowrate is pretty close to your numbers for 4 of the syphons.

I think you have something restricting the inlet to the one syphon flowing 1550 gpm or something in the syphon itself as the others are all within about 5% of each other and could be due to corrosion in the syphons. I can do a more detailed calc if you can give the the number and type of fittings in the system.

Last question. How are you measuring the flow? Is each syphon being measured in the same place for flow, eg. one isn't being measured right after an elbow where the others are all in nice straight runs of piping?

 

[miningman]

As I said Im new to the project and dont have first hand knowledge of the exact problem with the one siphon that refuses to continue operation. As for eductor, yes there is a valve towards the discharge end and a portable vacuum pump is used for priming purposes. Im not exactly sure how/where flow is being measured but my colleagues are competant and doubt that the measurement location varies that much..... its straight sclair, no bends elbows etc deliberately installed. And yes, I could well imagine that the actual length of pipes could vary by 10% so Ive no problem with actual flows varying by +/- 5/10% of theoretical.

On a related subject, with the 12 lines , the velocity head at discharge is around 1.1-1.2 inches. Since this is 10% of the total head available, if I modify the outlet area by construction of a small dam so that the siphons discharge submerged into a small pond with essentially still water.. say 50-80 feet a minute , do I regain enough velocity head to improve siphon performance. I know that theory says yes, but the life's bitter experiences have taught me that theory doesnt always apply. And yes I can see why a partial blockage of just one unit makes sense but we are also running 4 10 inch lines which have a 20% variation in min/max flowoutput

Thanks again for the suggestions