As I understand your system you have one pump that splits the flow into two 3" pipes at a tee or something. If that is correct I think you have a dynamical system where turbulent flow at the tee creates instability causing the flow to divide unevenly and variably. You can check this by temporarily hooking the output up to a single pipe. If this flow is regular, the problem is the dynamical instability. If it is still unstable, the problem is something else on the intake side. Another test would be to block one of the outlet pipes. If this causes the flow from the other pipe to become regular, you have dynamical instability at the tee.

If the problem seems to be instability at the tee you might see if a different fitting is available besides the tee. Maybe a Y? Another possibility might be to try smaller pipe. Three inch pipe sounds pretty large. Unless you are running a huge pump that really needs the big pipe, smaller pipe might solve the problem.

If my understanding of your system is incorrect, straighten me out and I'll take another crack at it.

larryl - your response makes sense. Here's my system. The pump with this situation is an Artesian delivering in the vicinity of 7500 gph, 2" pressure line outlet to tee to ball valves then increased to two 3" lines. The problem line goes to the head of one stream where a 1.5" pipe splits off and goes down hill to the flow out of the rock face. Ball valve near the end. The majority of the discharge goes into the head of the stream. At the end of each pipe is where I have noted the surging.

The second 3" line splits with a wye near the top and discharges into a filter pond and a second stream. I just noticed a lot of fine bubbles rising above the discharge into the pond, same time as the surging began.

I recently added another big pump as a separate system to increase flows and have made adjustments in flow with ball valves to achieve what I want.

The line with the surges has been dialed back some with ball valve and the ball valve at the rock face opened fully, which I never have done before. I didn't think about this and will experiment later.

What do you think--surging on one line, bubbling on another? I thought suction problem (2" pipes from skimmer and retro bottom drain to pump). Your explanation sounds reasonable altho the mechanics are beyond me.

I intend to run all winter but without the Artesian. I could change the plumbing (which looks odd with too many fittings anyway) then, tee to wye maybe.

But why did this start now after months of smooth flow?

A long response but it seems there a lot of potential problem areas.

Thanks - Mike

Do the pumps have a strainer basket pre-pump? Check the seal/o-ring on the lid.

Mike, your system is more complex than I thought. It sounds like the pump output is split and then each half is split again into four outlets. This multiple splitting does increase the possibilities for dynamical instability, but the bubbling on one line makes me think there is a leak somewhere that is pulling air into the system, which might be a contributor to the instability. I guess I would first try to track down the possible sources of the air introduction and worry about the instability as a secondary issue. One thing I would try is to use the existing ball valves to restrict the flow at the outlet points, which will increase the internal pressure somewhat. This might stop the air introduction, or maybe even reveal the problem point by causing it to leak water. This might not be how you want to run your system but it might be a helpful diagnostic.

If I had to guess I would venture that one of the ball valves is the likeliest source of the air leak.

By adjusting and trying to balance flow I seem to have minimized, but not eliminated the problem. larryl, I have two 3" pipes split from the pump and each splits again one time, not twice.

My filtration system is nothing more than net, brushes and pad in the skimmer and leaf baskets. And a filter pond. Only a retro bottom drain--a joke. I know your view curb1 on bottom drains and some days I wish I did have one. A friend checked the leaf baskets the other day and possibly didn't get the O-ring seated--something I have to check.

My life has revolved around natural elements of soil, water, rock and altho I started out to have a modest goldfish pond my wife said look what you could do here. I consider my system a landform--high volume of water circulating through two steep streams with several falls each. Fish and plants would never survive but that is fine with us. Loads of peripheral plants, more on the way.

Being at the base of a hill I struggle with erosion and siltation but am gradually winning the battle. A lot of stuff does go through my pumps yet I have clear water much of the time.

I never thought about the ball valves as a leaking spot since they are on the output side, not the suction. Gate valves are and I forget about them--they can leak?

I do hate to slow it down - maybe I should do the opposite first and turn off one line and then the other, blow them out, see what happens.

When I can I will check for leaks in the suction line (pressure line should make not difference?).

I will get back with whatever happens and if necessary get more information on dynamical instability. The question now is, can this situation harm my pump--any opinion.

Thanks for the valuable help - Mike

To follow the excellent advice above.

If one leg of the split is lower, you could easily suck air from the lower/upper OUTLET and have it come out the other.

If you google Venturi effect, you'll see it.

You only want to have a split at the same level, unless you tweak the valves.

Why it happened Now? It's a Mystery....

You are playing with Gravity as well as flow mechanics. Remember Pressure and Flow are DIFFERENT things. Even though in most cases they are the same, you don't have MOST cases. You don't want to do this, but if you used the same Outflow pipe size, as the inflow size, I'd bet you wouldn't have this issue (and that's what you are doing by playing with the valves).

Harm to the pump, unlikely unless you are sucking air on the intake side. If you have water swirling in the strainer, you should be OK. The moving parts are Water Lubed, no water means friction and wear, water means less friction and wear.

Greg

I understood that the bubbles show up on only one fork of the output. Re-reading your post I'm not sure if that is the case or not. If the bubbles show up on all forks the source is likely on the input side. If the bubbles are only on one fork, they almost certainly come in after the pump. The actual pressure on your system, with its massive 3" pipes, is probably very small. It wouldn't take much venturi effect to produce negative pressure at one of the ball valves and suck in some air, if there was a loose packing in the valve.

Is the 1.5" line to the rock face coming from the 3" pipe and does the 3" pipe divide into 2 1.5" lines or a 3" with a 1.5"? Because I think the different velocities of water in the 2 lines could also set up a venturi effect and suck air if there is a large discrepancy in sizes.

A little more info to consider. This pump is rated for 7500 gallons per hour at 13' head. With lift and friction I needed 3" pipes to get max gallons at head according to the pump flow chart.

One three inch pipe (the problem) goes uphill and discharges into the head of a stream. A 1.5" pipe splits off (a 3x3x1.5 tee I think) and does go downhill at least 25 feet. There is no valve of any type at that junction but there is a ball valve near the 1.5" discharge at the stone face.

The problem may have started when I fiddled with that 1.5" ball valve but I am uncertain.

The other 3" line goes to the head of a second stream and the filter pond. I reduced the 3" to 2", installed a wye and a 2" ball valve after the wye, leading to the stream head. A second 2" line split off before the ball valve and goes slightly uphill for maybe 15 feet and goes into the bottom of the filter pond. I found it necessary to install a check valve underwater to keep water from backdraining if the pump was off. This underwater pipe bubbled slowly but I think it was just water coming into the pond. Now it is bubbling fine bubbles fairly rapidly.

No where have I noticed a drop in flow, but with as much water as I am circulating it isn't easy to tell. Honestly, I don't know how long this has been going on as I didn't notice it until I opened the 1.5" valve and it was evident in the heavy flow arcing out.

Maybe this sheds a little more light on the problem? I thought this was a simple system but was never sure about the splitting, but it all seems to work and be adjustable.

Beginning to think there may be more than one cause.

There is a lot to think about in your responses. Any more ideas are welcome. Thanks - Mike

After some pondering, I think Greg/Watershed has it right. Splitting with the outlets at different heights allows gravity to strange things, especially with the large pipes that offer little or no backpressure. The suggestion to tweak valves based on relative height seems right on.

I agree that may be the cause. The split for the small pipe was the only place I didn't place a valve--put the valve downhill near the outlet. I guess I figured gravity would do the job for me. As I try to remember, the problem may have started when I changed that valve setting.

I'll check all the intake fittings. O-ring etc, then turn off the small pipe and watch, then experiment. Do you think installing a ball valve or diverter at the top would be helpful?

My number one concern is not hurting the expensive pumps. I guess this is why it is called a hobby.

Thanks - Mike