Minimum pressure to run a Fleck 700 SXT on a WS and a GAC

[CraigC]

Good Evening all
Thank you for reading my question. I would like to run a low pressure system for processing chlorinated/hard water to a storage tank. The system is a 2.5 ft3 GAC with a Fleck 7000 SXT then running through a 70K 2.5 ft3 WS with a Fleck 7000 SXT. My question is how low a pressure would still yield adequate backwash of the GAC and also create adequate brine draw. Flow is not important this is a 600 gallon reservoir it will be filling which will be used infrequently. From the data on the Fleck it looks like 40 psi will create adequate brine draw in a 3/8 line but I am quite uncertain about the GAC. Any and all thoughts on the matter would be most appreciated.
Kindest regards
Craig

 

[Reach4]

You probably have a 13 inch tank, and that would have a 0.92 ft3 cross section. How warm will the backwash water be? There are some graphs in http://www.dot.ca.gov/hq/esc/Structure_Design/wwe/documents/high_activated_carbon_2348.pdf
So your backwash rate would be about 9 to 10 GPM. I am not a pro, but I expect 30 PSI would be able to do that, as long as you don't restrict the drain much. I would use 3/4 or more tubing for the drain if you want to be able to backwash at lower pressures.

Do you have that much flow available coming in?

The last page of the version F service manual has a graph of service flow rates vs drop for the controller. The tank would add a little more. I tend to think that the pressure drop for backwash would be similar or less than the downflow pressure drop.

You plan to de-chlorinate water on the way into a storage tank. I would be sure to sanitize everything after the GAC after you do plumbing work.

"My question is how low a pressure would still yield adequate backwash of the GAC and also create adequate brine draw. " You will schedule these to happen at different times, so that would not come up. Softener backwash is usually 4 GPM for a 13 inch tank.

 

[Sponsor]

 

[CraigC]

You probably have a 13 inch tank, and that would have a 0.92 ft3 cross section. How warm will the backwash water be? There are some graphs in http://www.dot.ca.gov/hq/esc/Structure_Design/wwe/documents/high_activated_carbon_2348.pdf
So your backwash rate would be about 9 to 10 GPM. I am not a pro, but I expect 30 PSI would be able to do that, as long as you don't restrict the drain much. I would use 3/4 or more tubing for the drain if you want to be able to backwash at lower pressures.
Hi Reach 4. Thank you for your kind and helpful reply. Water temperature should be about 4 degrees C 37-38 F for 8 months maybe rising to a max of 15 C or 60 F for the other 4 months. Thank you for the sizing on the drain, I am not restricted so I can use as large as the fitting on the Fleck will accept.
Do you have that much flow available coming in?

The flow actually comes from an above ground Chlorine retention tank so there is not "much" restriction to flow to the pump and I can size the pump accordingly for GPH.

The last page of the version F service manual has a graph of service flow rates vs drop for the controller. The tank would add a little more. I tend to think that the pressure drop for backwash would be similar or less than the downflow pressure drop.

You plan to de-chlorinate water on the way into a storage tank. I would be sure to sanitize everything after the GAC after you do plumbing work.

Wise advice to be sure

"My question is how low a pressure would still yield adequate backwash of the GAC and also create adequate brine draw. " You will schedule these to happen at different times, so that would not come up. Softener backwash is usually 4 GPM for a 13 inch tank.

Sorry for my poor explanation! As you mention they do not happen at the same time. What I was trying to explain was I was unsure which factor was the limiting one on the pressure I could use; either the backwashing of the GAC or ensuring adequate brine draw. Sorry for the confusion.

 

[Reach4]

Sorry for my poor explanation! As you mention they do not happen at the same time. What I was trying to explain was I was unsure which factor was the limiting one on the pressure I could use; either the backwashing of the GAC or ensuring adequate brine draw. Sorry for the confusion.

Definitely the backwashing of the GAC. The softener backwash is the big user for the softener at 4 GPM. During brine draw, the water use is less than 1 GPM for the 7000 softener.

 

[CraigC]

Definitely the backwashing of the GAC. The softener backwash is the big user for the softener at 4 GPM. During brine draw, the water use is less than 1 GPM for the 7000 softener.

Thank you again Reach 4
I was just reviewing rev F of the service manual you kindly mentioned. I am not the best with these graphs to be sure. Am I correct in interpreting the pressure drop across the valve (+ a little for the tank) would be about 3-4 PSI at 10- 12 GPM? If I may beg your indulgence a little further; would I be correct then in estimating I need to ensure a GPM of say 16 to the unit to ensure adequate backwashing of the GAC? All of these calculations assuming my media has similar characteristics to the Clack media, you kindly posted a link to, and that the drain size is optimized.
Thank you again for your kindness

 

[Reach4]

would I be correct then in estimating I need to ensure a GPM of say 16 to the unit to ensure adequate backwashing of the GAC?

I would say 10 GPM.

What is the source of that water? Note that the PSI that matters is the PSI into the 7000, and not the static pressure when no water is being used.

I am wondering why you have the 600 gallon tank. Is that open to the atmosphere?

 

[CraigC]

I would say 10 GPM.

What is the source of that water? Note that the PSI that matters is the PSI into the 7000, and not the static pressure when no water is being used.

I am wondering why you have the 600 gallon tank. Is that open to the atmosphere?

Hi Reach 4
The source of the water is originally well. It goes through a ph adjustment and air injection tank first. It is then chlorinated and "sits" in the retention tank. From there through the system I described into a 600 gal stainless tank. all tanks including the storage tank are closed. Why? simple answer is self sufficiency in a very remote and unpredictable location

I am a bit confused by your note "Note that the PSI that matters is the PSI into the 7000" What you say makes perfect sense however I am left wondering do I need to supply a certain PSI at a specific flow or is really just about the flow across the unit? Forgive my ignorance.
Cheers

 

[ditttohead]

Pressure is not critical at all on the 7000 GAC unit, it is a function of the DLFC button to regulate at highly varying pressures. The system will backwash fine at near zero PSI (IE: Open Vessel system designs), but the flow button may not work with the same accuracy below 20 PSI as it does above 20 PSI. I have not really tested them below 20 PSI but I don't think it will be that critical. You can simply measure the flow and determine if the flow control is working adequately.

 

[Reach4]

From there through the system I described into a 600 gal stainless tank. all tanks including the storage tank are closed. Why? simple answer is self sufficiency in a very remote and unpredictable location

If that tank was not pressurized, then there would be a worry that bacteria or other microorganisms could be introduced into the de-chlorinated water.

Ditttohead has the better info. I was only concerned that if the piping was long or small from the source, the pressure at no-flow could have been much higher than when loaded. With your own pump, that should not be a problem.

 

[CraigC]

Pressure is not critical at all on the 7000 GAC unit, it is a function of the DLFC button to regulate at highly varying pressures. The system will backwash fine at near zero PSI (IE: Open Vessel system designs), but the flow button may not work with the same accuracy below 20 PSI as it does above 20 PSI. I have not really tested them below 20 PSI but I don't think it will be that critical. You can simply measure the flow and determine if the flow control is working adequately.

Thank you for your kind reply Dittohead and Reach4. It all makes perfect sense now. Dittohead if I may ask one more question: is there any danger to a GAC system if the flow is set a bit on the high side? That is If I estimate the sizing of the DLFC button for the "worst" case (say when the water is closer to 65F rather than 38F) which would result in a higher flow than needed at times of cooler water. Naturally the system will use more water than necessary.
Cheers

 

[ditttohead]

Just watch that you don't hit the top screen with the media. You can tell this by hitting the side of the tank with the palm of your hand. The tank should feel hollow like a drum when it is backwashing. If the flow it too high, after a minute or two the system wont feel hollow. Not exactly scientific but fairly accurate.

 

[CraigC]

Just watch that you don't hit the top screen with the media. You can tell this by hitting the side of the tank with the palm of your hand. The tank should feel hollow like a drum when it is backwashing. If the flow it too high, after a minute or two the system wont feel hollow. Not exactly scientific but fairly accurate.

Brilliant pragmatic in the field reference, love it! Ahh such common sense.
Many Thanks Dittohead