Pressure tank sizing.

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Leeelson

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I've recently replaced a pump with a Grundfos Model 25S20-11 and a CSV CSV12560-3. I have an 8 gal pressure tank and have problems with pump cycling. There's a whole thread on this here:
https://terrylove.com/forums/index.php?threads/well-pump-help.63737/
I'd like to run the pressure switch at 45/65 but right now, even if I set the top pressure to 72, the lowest flow that has no pump cycling is 4.8 gpm. I've decided that I need a larger pressure tank and have heard that larger is better (except for the price penalty). I have limited space but can easily accommodate a Wellxtrol WX-203- 32 gal tank. One issue is whether this is large enough and I have some specific questions:
1)If I replaced the 8 gal tank with a 32, will I see about a factor of 4 decrease in the pump cycling for long running applications (irrigation) assuming all else is the same?
2) For a given pressure limit (e.g. 65PSI) I assume that the minimum flow for the pump not cycling will be the same. E.G. if 8 gpm is the lowest flow that stops the pump from cycling with the 8 gallon tank, this will be unchanged with a 32 gal tank. Is this correct?
 

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You have a 25 GPM, 2HP pump. At that depth and pressure it will produce about 20 GPM. So without a working CSV any flow you use less than 20 GPM will cause the pump to cycle.

A WX203 only holds 8 gallons of water. According to correct pressure tank sizing you need a tank that holds enough to make your pump run for at least 1 minute and 2 minutes is best. So you will need a WX302 at least and two of those would be highly recommended, especially since you have drip irrigation.

I know you are frustrated and I am afraid the problem is a pump installer who is "grumbling" instead of trying to make the CSV work properly.

I have the exact same 25S20-11 Grundfos pump at my house and a 3 GPM drip system. The pump is at 200' with a water level of about 140' and a tank with 3.5 gallons of draw. This system has been running on a CSV12560-1 valve for 25 years now.

You can get frustrated and just spend the money on a big tank or two, or you can work through this and figure out what the real problem is. I am more than willing to help anyway I can.
 

Leeelson

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A WX203 only holds 8 gallons of water. According to correct pressure tank sizing you need a tank that holds enough to make your pump run for at least 1 minute and 2 minutes is best. So you will need a WX302 at least and two of those would be highly recommended, especially since you have drip irrigation.
So my pump runs for 13 seconds now. If I replace the tank with one 4 times bigger, why won't it run 4 times longer?
I know you are frustrated and I am afraid the problem is a pump installer who is "grumbling" instead of trying to make the CSV work properly.
Huh? He didn't grumble. Apart from moving the CSV, which would negatively impact the way the pump hangs, how can he "make the CSV work properly"?


You can get frustrated and just spend the money on a big tank or two, or you can work through this and figure out what the real problem is. I am more than willing to help anyway I can.
How do we figure out what the real problem is?
 

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Pressure switch off set at 73 PSI. Tub faucet run at 4.8 gpm. Pressure rises slowly and stops at about 72. Pump runs, no cycling. For this cutoff pressure, it seems that the lowest flow that has no cycling is 4.8 gpm.

With the 60-3 valve set 20' below the surface the minimum flow should be less than 3 GPM at about 60-62 PSI. If you are using more than 3 GPM and the pressure gets any higher than about 62 psi, the CSV is not working correctly for some reason. It may have a rock or other debris in it. It may have been damaged in shipping. I don't know.

Other than opening up 15-20 GPM worth of hoses and trying to flush out the CSV I don't know what else to try. If there is something in the CSV this may flush it out. But if it goes back to 72 PSI while you are using more than 4 GPM, the only thing I can do is double test another CSV and get it to you, which I would be glad to do if you like. If it turns out to be my fault, I will make it right.
 

Texas Wellman

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If it was mine I would put a 220 gallon galvanized tank on it and call it a day. The tank doesn't necessarily have to go in the same spot as the old one. You can put it anywhere in line with the pumping line parallel with the old tank.
 

Leeelson

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I've got limited space and budget. I think the largest tank I can reasonably accommodate is an 80 gal tank. I have to believe the CSV is doing something for me in terms of limiting the flow so that I don't really need a huge tank as Valveman and Texas Wellman have suggested.

I'm trying to predict the performance of my system when I replace the 8 gal tank with an 80 gal tank. Since I will have 10 times the drawdown, why won't I have 10 times longer fill (cycle) time with no demand? This would at least get me a minimum of 2 minutes run time for the pump. It may not eliminate cycling for flows less than about 8 gpm, but at least the number of cycles should be reduced by a factor of 10. Where's the error in this logic?
 

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Since I will have 10 times the drawdown, why won't I have 10 times longer fill (cycle) time with no demand? This would at least get me a minimum of 2 minutes run time for the pump. It may not eliminate cycling for flows less than about 8 gpm, but at least the number of cycles should be reduced by a factor of 10. Where's the error in this logic?
I see no error in your logic if you are running a 20 PSI differential. If you are running a high differential as a workaround, then that would change things.
 

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I don't think the CSV is doing anything. At 72 PSI and a water level of 153' that pump will only produce 5 GPM, CSV or not. If the CSV was working you would have a constant 60 PSI and there would be no cycling for any flow above 3 GPM, so you would not need a larger tank.
 

Leeelson

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I don't think the CSV is doing anything. At 72 PSI and a water level of 153' that pump will only produce 5 GPM, CSV or not. If the CSV was working you would have a constant 60 PSI and there would be no cycling for any flow above 3 GPM, so you would not need a larger tank.
Something's not right. The water level is 122' and I can get 19 gpm (40 PSI) with the CSV. Without the CSV, I get 19 gpm at 60 PSI.
 

Leeelson

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Other than opening up 15-20 GPM worth of hoses and trying to flush out the CSV I don't know what else to try. If there is something in the CSV this may flush it out.
Ran a yard hydrant (19 GPM, steady 40 PSI) for about 4 minutes. No change in fill rate. Still about 13 seconds (45 to 68, no demand)
 

Reach4

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How about this idea. You put ..... Oops... bad idea. Never mind.
 
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Leeelson

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How about this idea. You put an adjustable CSV inside by the pressure tank. You let your yard plumbing have higher pressure when the irrigation is not going, and you tune your irrigation to get the right pressure while irrigating. If that does not work out, you could add a PRV before the yard load.
View attachment 32838
Not sure why the irrigation pressure would be higher. If the pressure switch is set to e.g. 40/60, won't the irrigation see that pressure (same as house)? The irrigation is not really adjustable. It's drip and runs one zone at a time for long periods. I'm guessing the flow rate is somewhere in the 6-8 gpm range.

I have 2 (related) problems: short pressure tank fill time and cycling for irrigation (CSV not doing its job for > 3 gpm). Even if I could lengthen the pressure tank fill time, I wouldn't be able to change the cycling due to irrigation, correct?
 

Reach4

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Removed my proposal, which just would not work.
 
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Leeelson

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I am envisioning a system where the CSV is inside just before the pressure tank+switch. That makes it handy to adjust. Because the irrigation would tee off before the CSV, it would see higher pressure. As I understand it, the possible adjustment is from using particular drip heads.
With this configuration, I would still get frequent cycling when running the drip. The drip heads are adjustable, but they are configured to give the proper water, which varies among applications. I have some drip lines with 50 heads, and some with 10. Those with 10 will likely never draw enough flow to keep the pump from cycling.
 

Reach4

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With this configuration, I would still get frequent cycling when running the drip.
Yes. I obviously did not think this through correctly. I will modify my erroneous posts above.

Could you bury a valve box where you could place an adjustable CSV below the frost line before the tee?
 
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Leeelson

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Yes. I obviously did not think this through correctly. I will modify my erroneous posts above.

Could you bury a valve box where you could place an adjustable CSV below the frost line before the tee?
Thanks for your creative ideas. I think I will just bite the bullet and buy an 86 gal pressure tank. Should take about 2 weeks to get here and be installed. Hopefully at that point I will finally have a good working system. I'll report back at any rate.
 
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