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By now everyone knows how much I LOVE plumbing. I am at my wits end with microbubbles.
I can vouch that my entire plumbing is thoroughly glued, o-rings well lubed with silicone grease, and all threaded connections well teflon taped. Many are also now silconed as part of my bubble crusade. It's air and water tight. I have the skimmer turned off and I have a filter sock under the return to catch bubbles, which I usually don't use, but am for the purposes of this experiment.
Meanwhile, my pump's intake is 10" under water and doesn't slurp. It's a 1" inlet and is not restricted anywhere; the bulkhead is on the bottom of the sump on the same level as the pump. There *is* a 90 degree elbow from the bulkhead to the pump intake -- problem?. Pump is brand new and squeaky clean. I didnt have this problem with my previous pump (same model, same plumbing setup).
I have the pump throttled back a significant amount on the outlet of the pump, which has significantly reduced the bubbles, but not eliminated them. (At some point, throttling it back stopped helping.) It does, however, prevent bursts of bubbles in addition to a steady stream. Fish are no longer acting aggravated with the bubbles at this level, but I'm not very happy about the amount I am throttling back the return pump and with cavitation still going on, of course I am reducing the lifespan of the pump.
Relevant summary found on the internet:
[i]Cavitation is a two-part process caused by the changes in pressure as the liquid moves through the impeller. As the liquid enters the suction nozzle of the pump and progresses through the impeller, there are a number of pressure changes that take place.
...As the liquid enters the pump through the suction nozzle, the pressure drops slightly. The liquid then moves into the eye of the rotating impeller where an even more significant drop in pressure occurs. The first part of the cavitation process occurs if the pressure falls below the liquid’s vapour pressure in the eye of the impeller. This causes vapour bubbles to be created in that area (in other words, it boils!). The second part of the process occurs as the centrifugal action of the impeller moves the bubbles onto the vanes where they are instantly re-pressurized and thus collapsed in a series of implosions.
...it is evident that cavitation can be avoided or stopped simply by increasing the pressure of the liquid before it enters the suction nozzle of the pump. This will ensure that the pressure in the eye area does not fall below the vapour pressure, and therefore no vapour bubbles will be created and no cavitation will exist.
...To avoid cavitation in the vast majority of instances, it is necessary to follow one simple rule:
The net positive suction head (NPSH) available must be greater than the NPSH required.[/i]
Okay, so how do I modify the NPSH, in plain English?
Don't count your gobies before they've metamorphosized.
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There *is* a 90 degree elbow from the bulkhead to the pump intake -- problem?. Pump is brand new and squeaky clean. I didnt have this problem with my previous pump (same model, same plumbing setup). if you hadn't said that you had the exact same plumbing previously and it worked... I would have said the 90 degree elbow was a significant cause here. Actually it still is. It would not be a surprise if the identical pumps as new still produced different flow rates. Really very common with mag drive pumps especially (they rate them at X galls per hour, but there is actually a variance of sometimes more than 10% difference between units that come off the assembly line... and most always higher than rated when theres an exception. That weay the mfg can reckon the difference by providing you with the same or more than the pump was rated at/promised). In short... your new same pump has more oomph than the last one (and hence... greater pressure) That 90 degree fitting on the draw side is a huge handicap. I have a pic of exactly that in one of my slide presentations called "Things You thought Your Knew" where it illustrates common plumbing mistakes. I do wish you could change this. If you cannot... can you at least use a sweeping elbow instead of a hard 90 degree? Before I/we give anymore armchair advice... can you show us a pic of that pump and tap? My apologies for slow replies this weekend too... will be on and off a bit. Hoping others chime in 
. Anthony Calfo
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Correction, this is the third unit of the same pump model; the only one with these difficulties. Who knows, maybe it's a rouge pump. 
I am not sure I could get rid of the 90 degree angle without elevating the pump, which is also a no-no. I have about 9-10" of clear space for a pump. This would also seem to preclude a sweep elbow, and I think it's too tight to run a length of flex PVC without placing a lot of stress on the bulkhead and pump inlet.
Photos of pump. 1 1/4" bulkhead (reversed) to 1" PVC, to elbow, to 1" union on the pump. Outlet is also 1" union to 1" flex PVC, up to ball valve continuing on with 1" flex PVC.
Last photo shows rear where 1" flex PVC connects to hard 1" plumbing and up to manifold. The other flex PVC line you see is the drain from the goby tank running to the opposite side of the sump.
Don't count your gobies before they've metamorphosized.
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I agree with Anthony that the 90 elbow may be the cause of the problem. You may also want to check out those disconnects to make certain that they are sealed tight & not sucking any air. I would try to change that 90 to a 45, or better yet a straight outflow to the pump if you can. Good luck.
Steve
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Well, after further review last night and this morning, I am much more skeptical of the idea that the pump is so much stronger than the others. I have the pump dialed back to about 800gph just to acheive bubble reduction. (This is a 1500gph pump.) It certainly isn't that much stronger than the others.
A 45 angle is a distant maybe -- as you can see from the photos and straight input is out of the question, even if I removed the union. And there is only about an inch of room between the pump rear and the front of the stand, which is why I think a 45 doesn't have much of a change of fitting.
Don't count your gobies before they've metamorphosized.
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| The 90 degree elbow is your problem. It is to close to the pump intake. The water velocity around the inside radius is a lot greater than the outside radius of the elbow. This cause the water velocity to be at different speeds or flow rates when it hits the impeller, thus causes it to cavita. If you need the elbow the rule of thumb is 10 times the inlet pipe diameter for the pump to be located from the elbow. If I have a pump with a 1 1/2" inlet I would place a piece of pipe 15" long between the elbow and the pump. This gives the water velocity time to equalize before it hits the impeller.
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TERRY (7/5/2006)
The 90 degree elbow is your problem. It is to close to the pump intake. The water velocity around the inside radius is a lot greater than the outside radius of the elbow. This cause the water velocity to be at different speeds or flow rateswhen it hits the impeller, thus causes it to cavita. Ifyou need the elbow the rule of thumb is 10 times the inlet pipe diameter for the pump to be located from the elbow. If I have a pump with a 1 1/2" inlet I would place a piece of pipe 15" long between the elbow and the pump. This gives the water velocity time to equalize before it hits the impeller.
Yikes. I see what you are saying about the velocity changing as it hits the elbow. I don't have 10" space, though; if I did, I wouldn't need the elbow.
As far as I can tell, my options are:
1) Change 90 elbow to 45 elbow, if it fits. But from what you are saying, this will not really resolve the issue, right? Because I would still need some pipe extension to allow the flow to equalize?
2) Raise the pump up several inches, permitting a curved inlet, but this would make the pump try to suction up. However, a curve would not totally solve the unbalanced velocity problem.
3) Move the bulkhead (ugh!) up several inches and leave the pump on the floor of the stand using a curved inlet, but then the intake would be near the surface and probably suck air there. And again, a curve would not totally solve the unbalanced velocity problem.
Thanks for your input, Terry. I completely understand your explanation of the problem, I'm just at a loss for how to fix it.
Don't count your gobies before they've metamorphosized.
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| I have never tried this . It is one of those things that sounds good on paper. You could try it an see. If you were to put another elbow on the tank side of the bulk head it may cause the velocities to equal out if you place the second elbow the opposite direction of the first. In other words form a "Z" shape with the second ell. Inside flowing to outside and outside flowing to inside. Just a thought. Maybe someone else could suggest something else or has figured out a way to solve this.
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| Ok, Reefmeister to the rescue Nicole, what days and times are you availble? Damn, I'd better start posting to this forum again....you've almost caught up to me!  ..
------------------------------------------------------ "Many people can do great things, but few can do great things for other people" KA
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No, there's not enough clearance. The elbow, if it fit, would be jammed up against the side of the sump.
Hmmmm..... I wonder if I could use the flex PVC (to at least get some curve in there and reduce the problem) and somehow reinforce the curve so that it doesn't place too much stress on the bulkhead? I do at least have some space to play with.
The only other thought is to reverse the bulkhead (again with the ugh!) and then maybe I'd had room to sit the pump straight on. The 2x4 supporting the corner might be in the way, though, and I'd make it really hard to remove the pump for servicing.
Don't count your gobies before they've metamorphosized.
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