Milky water

[buzz41]

Our small muni water system uses a ~10,000 gal hydro-pneumatic tank in combination with booster pumps to deliver water to our customers. A suitable elevated tank is not feasible. The system works OK, but air infuses into the flow in the tank causing a very objectionable temporary cloudiness at the tap.

The tank is about 40 feet long by 6 ft dia. It has a small port at one end. Water flows from the boosters through the tank to the system.

What are some of the possible solutions ? I have some ideas, but could use some help.
Thanks,
Ron

 

[LittleInch]

By "hydro-pneumatic tank" I assume you mean a tank part filled with water and part compressed air or gas??

Clearly the gas is dissolving into the water at pressure so you need to separate the two. If you allow the tank to fill completely and use bladder type accumulators attached to the tank or within the tank you should avoid this problem once you've cleared the current water out.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[buzz41]

Thank you for the response Littleinch. ("Littleinch", an epic pipeline I heard of as a kid. What a country we were! ) -

Our system flows about 100 gpm to 2000 gpm and the boosters are on/off 25 to 75 hp pumps. So, the railroad car sized air-over-water tank/accumulator seems necessary.

I should probably ask this vast pool of knowledge here if bladders are readily available and could a tank of this size be retrofit?

BTW, I even thought about the feasibility of floating a plastic membrane, maybe polypropylene or LDPE in the tank to separate the air and water.

Or maybe, re-plumbing it to a single connection tee'd into the system would reduce the dissolved air being pushed into the system? I am sure this issue is common and nearly everything has been tried. So, I am hoping that an affordable solution is possible. Thanks.
Ron

 

[operatorjoe]

I have operated several systems that have hydro-pneumatic tanks and never had one that would add air to discharge. The only way I could see this happening would be if there was too much air in the tank. The very large one I work with now has a glass tube on it to gauge water level. I would try to check water before it enters tank and be sure air is not coming from a problem around pump, like a obstructed intake, cracked housing. Check suction pressure or if it is a vertical turbine pump the water level in sump may be falling low enough to cause vortexes of air to be pulled into flow. If the cause is connected with flow to the pump you could be damaging the impeller.

 

[LittleInch]

It all depends how much water volume you have between low and high levels when you either turn off your booster pumps or turn them back on and at what pressure you keep the tank at - what is it by the way??

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[buzz41]

The pressure varies between about 70psi to 75psi as the pumps cycles on/off every few minutes. The sight gauge shows about half full. A pump run raises the level an inch or two from the turn on point.

Opjoe: If air was being pulled in by the pumps, the sight gauge would show falling water level as additional air entered,I would think. In this case, however, an air compressor is necessary to replenish the air which dissolves in the water. This is expected behavior for any air/water pressure tank without air/water isolation.

Ron

 

[buzz41]

Operatorjoe addl: Re: your suggestion that about the pumps as a source of air, the water is clear at a tee connection between the pump and the tank. I will double chk it though. ron

 

[operatorjoe]

I have been reviewing the plans for our system. The pressure tank is offset on a tee and not in or over any direct line of flow. Ours also has a air compressor to maintain the level.

 

[bimr]

Is this a new problem, a new system, or a problem that has occurred for a long time.

 

[buzz41]

bimr: "Is this a new problem, a new system, or a problem that has occurred for a long time."
Thanks, should have said. This prob has been there for several years.
Short history: The tank date is 1999. There hadn't been much talent around or motivation to get things right. Recent newspaper articles pointed out deficiencies including this problem. That produced a jolt of motivation and your truly is standing by with, I hope, the talent.

Ron

 

[LittleInch]

What I think you need to do has been laid out above. Fill your original tank all the way to the top and connect a separate pressure tank or tanks with a baldder arrangement, or even insert one or two air filled bladders into your tank, with a connection to the out side so that you can release the air if you need to access the tank or top them up occasionally


It would appear that you only need a relatively small volume to be maintianed at pressure before you pumps kick in.

Sizing bladders and pressure tanks is a balance between the variation in flow and how often your pumps start and stop. Make it too big and you've wasted a bit of money. Make it too small and you burn your motors out due to constant start / stop. Only you can really judge what the right volume is based on the flow rate and amount of times the motors can start and stop.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[buzz41]

Thanks Inch, I think I have the hydraulics nailed pretty well. With a base demand of 200gpm and a 75psi to 70psi bleed down time of 2 minutes, an air volume of 5600 gallons is required. That is about what we have now with a 10000 gallon tank.

A bladder of several hundred cubic feet for that tank would certainly do the job. Now where to start looking for that? Some tank people maybe. To get the badder in the tank, it would have to modified to increase access and re-certified anyway.

There was an interesting hint in a prev post by Operatorjoe. His tank is teed off to the side with no flow-through. It apparently functions without producing temporary turbidity. Promising. Need more info.

Ron

 

[hydrae]

A supplier of bladder type pressure vessels as a replacement
they are NSF-PW ASME

http://www.charlattetanks.com/

Hydrae

 

[bimr]

You need a bladder. With the high pressure that you have, it is causing the air to dissolve into the water. When the pressure is released, the air is coming out of solution.