How to locate suction leaks?

[coolfish5]

I have a suction leak on a system at work causing air to enter the plumbing system. Is there any trick to locate this leak without just replumbing or globing pvc cement on every connection?

This is on a fish only system, so aside from bubbles, it is causing supersaturation and killing my fish. Tank returns are through spray bars about 6" from water surface.

Drain 2" pvc, pressure 1 1/2" pvc. System is ~500 gallons. Some plumbing connections are located under the tank and there is no access without cutting appart they existing plumbing. I'd like to check if the unions and threaded fitting on the pump intake are causing the leak.

Thanks,
John

[BeanAnimal]

I suppose there are several ways... shutting the system down and looking for small leaks is one. Of course sometimes suction leaks do not drip when the suction is removed.

You could try food coloring in suspect joints. You could also try wrapping suspect joint in saranwrap or using vasaline to change the bubble density in the output.

Cappin the outputs (or closing some valves) and puttings a few PSI of air into the plumbing may also help you to find the leaks.


I would think the turbulance from the spraybars would help reduce the supersaturation.

[coolfish5]

The leaks do not drip when the pump is off, at least not that I can locate. So that kinda doesn't work unfortunately.
The spray bars help a little. It is 1 1/2" pvc that is slotted on a miter saw some 30 or so times, (every 1/4"). I measured saturation today, 117% directly from the filter before spray bar, 115% in tank with spray bar and no additional aeration, and 113% with heavy aeration. Still too much for my fish.
I'll have to check out some of the suggestions tomorrow or monday.
Thanks for your help.
John

[H20ENG]

Is the pump making grinding noises at all? Be sure its not simply cavitating before going through any more trouble. Slowly closing the discharge valve should tell you. If you close it slightly and the noise and bubbles goes away, then the pump is not getting enough water to the suction end. Sometimes the plumbing will get clogged with something and cause cavitation.

Other than that, the methods Bean provided work well, especially capping the plumbing and pressurizing the line.

[mc-cro]

my money is on pump cavitating. I have had more leaks and holes in my plumbing, and I have never had the problem you are describing. the only way I can think a leak would even cause such a problem is if it were right infront of the pump, like a venturi.

what size pump are you running, how long is the section of pipe feeding into the pump? I have heard that a bend right before the intake can cause cavitation on larger pumps

[coolfish5]

Well this is not on a typical aquarium.
It is 2 round fiberglass tanks, 2" bulkhead with a 2" standpipe/strainer (slotted pvc). 2" feed up to the pump, pump intake is 1 1/2" it has a threaded 2" slip reducer and a union directly infront of the pump. Pump is hayward 1 1/2hp (spa style setup). After pump is 1 1/2" flex pvc to a sand filter, after it splits to feed two the two tanks through slotted spray bars.
http://reefcentral.com/gallery/data/500/15260Fish_4_tank.jpg
I would think the sand filter would be sufficient head pressure to prevent cavitation, but I could be wrong. I have not noticed the bubbles in the pump strainer basket clearing after reducing flow to tanks. There is also two identical systems side by side and the problem is only in one.
I pressurized the system from our air system (48" h20 pressure) and still can't find any leaks using soapy water on all exposed fittings. If it isn't cavitation then its the threaded fitting on the bulkhead under the tank, which will require a tear down since its hard plumbed.
I'll look some more tomorrow.
Thanks for all of your advice.
John

[BeanAnimal]

Cavitation would not increase the O2 level either. It would appear that oxygen is getting in somehow. Could it be entrained air in the sand filters? IS there any way to replace a section of INTAKE plumbing with clear PVC or VINYL hose, to see if there are air bubbles on the intake side?

[mc-cro]

beananimal is right, cavitation doesnt inject more air, it just stips
it from the water column.

shot in the dark, what about the spray bar? could it be working like a down draft skimmer? sucking in air from somewhere?

[coolfish5]

I can see air in the intake of the pump. The pump has a filter basket with a clear lid. This air, a few bubbles usualy, is somewhat intermittent. It will show up for a while, but like today there was no bubbles in the pump intake. So I'm not really sure what would cause it to come and go.
I would suspect this air is getting trapped in the sand filter under pressure (10psi or so) and causing the supersaturation. I just need to locate the leak, possibly in a threaded fitting under the tank that I have no access to.
Thanks again for all the advice.
John

[AZDesertRat]

I don't think the 2" suction piping is large enough to feed the pump. I don't know what the configuration is inside the tanks (standpipe/strainer) but on the exterior I see lots of 90 degree elbows, valves and a tee which all contribute to head loss and ultimately starving the suction side of the pump. The ideal setup would be turning the tanks so both outlets are facing each other with the pump directly in the center, raising the tanks so the centerline of the pump is the centerline of the piping or better yet below and use 3" pipe so you have fewer fittings, less restriction due to the C factor of the pipe and a greater overall positive suction head.

[H20ENG]

"I would suspect this air is getting trapped in the sand filter under pressure (10psi or so) and causing the supersaturation."

We had a big problem with supersaturation from the pressurized lines feeding many satellite tanks. I had to build large degassers (basically a glorified trickle filter) to get the O2 back to normal before feeding the tanks.

You can test for cavitation real easy, just pinch down on the outflow like I mentioned. Is the pump quiet? OK, well its a Hayward, but is there any rough noise?

Also check your strainer lid. Goop it up with silicone grease to make sure its not drawing in air there. I have had that problem before.

I agree with AZRat about your plumbing.

I have used multimedia filters (same sand filter body) that did not place much backpressure on the pump like sand. Did you say it was 10psi?

[coolfish5]

Sorry its been a while on a response.

Yes the filters run at 10psi after backwashing, more on other systems and more when dirty.

They are 18" Hayward sand filters, running aquatic ecosystems mixed media, which is different sizes of sand. The pumps are hayward powerflow 1.5hp.

I'd agree that the pumps are oversized for the filters and a 1/2hp maybe less would be more appropriate.

Thanks for the tips. The air did not show the leak, but I suspected it was the threaded fitting under the tank, so I cut the plumbing and tightened that up and the leak is gone.

No more supersaturation problem.

Thanks again for all your help.
John

[H20ENG]

Great news. Glad you got it licked:)

[BeanAnimal]

It is amazing how a leak will pull air but not drip water! I am glad you got it fixed. That BTW is why I do not like threaded PVC.