Tank Air Pressure

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Wondering

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I was just reading about having multiple tanks(current and an old thread) and they said as long as 1 tank has the correct air charge you are ok. I have more than one tank and I always thought you needed to keep them at the same pressure for them to all empty completely and together. I know it is hard to keep them exact but I figure within 1 lb to 2 of each other. Am I right or wrong or does it only matter for one tank? I have Amtrol tanks if that matters.
 

Reach4

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They would not have to be at the same pressure, but no reason for them to be fairly close. The exception is that if the altitude at the bottom of the tanks varies much, you would adjust by 0.433 per foot of elevation difference. The one by the pressure switch is the important one to prevent fast cycling.

The precharge pressure is set at about 2 PSI lower than the pump cut-on pressure if you have a submersible pump. That precharge pressure is measured while the water pressure is zero. Go about 4 PSI lower than the pump cut-on pressure if you have a jet pump.
 

Wondering

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I have a 3/4 hp F&W submersible. Both tanks are together, well one is right at the pressure switch, the other one 2-3 ft away.
 

Reach4

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Your system should work great. Do you have a problem? If the pump goes on for at least a minute each time, there is not a problem. If it goes on longer, that is better.
 

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No problems. After reading that about different air charges I was just wondering. Mine runs 2 minutes give or take and was 15 years old this year, tanks and pump. Keeps up 2 houses(sometimes 3 when their pump messes up) and 20 cows right now. Currently its 96 degrees so they drink a good bit during hot weather. Hope mine keeps holding up. I also run 40/60 psi.
 

Jeremy Harris

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Out of interest, is there any reason why the pump cut in pressure shouldn't be a higher than 2 psi above the pre-charge pressure?

It seems that this is the "golden rule", but if you have a limited pump delivery capacity (the flow rate, not pressure, of mine's limited because it sucks sand from the well at high rates), is there a problem if the tank pre-charge is set for, say, 35 psi and the pressure switch set to cut in at 55psi and off at 70psi?

I have had mine set like this, so that the pump can "catch up" with demand. The pump can just about deliver enough for a shower, so having the cut-in pressure set a bit higher gives a bit of leeway from the tank if someone turns a tap on briefly when the shower is running. I've been wondering if I'm storing up a potential problem by having settings like this.

The pump never short cycles, as the flow rate limiter means it always takes a fair time to refill the tank, even with just a 15 psi differential on the switch.
 

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You reduce the draw down amount of the tank and overstretch the bladder a little, but that is OK. Actually having a lower air pressure charge than the cut in is the only way to be sure to have a little water stored during a power outage.
 

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It seems that this is the "golden rule", but if you have a limited pump delivery capacity (the flow rate, not pressure, of mine's limited because it sucks sand from the well at high rates), is there a problem if the tank pre-charge is set for, say, 35 psi and the pressure switch set to cut in at 55psi and off at 70psi?
The precharge is normally selected to minimize the amount of extension that the diaphragm has to go through, yet to supply water between the switch turns on and the pump delivers water.

There is some variations in the pressure switch actions and in the overshoot and undershoot that occur after the contacts open and close. I am actually tracking that on my system currently.

If you set your precharge to say 47 PSI and had your pressure switch set for 55/70, that would seem to be in about the same range of diaphragm stretch as 47 precharge and 50/70 PSI switch setting. 35 PSI precharge with 50/70 is going to be harder on the bladder.

If you wanted to go 55/70 or even 60/70 with your extra-big tank, you would want an electronic switch rather than the classic mechanical switch. Note that drawdown is less with higher pressures.
 

Jeremy Harris

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Thanks very much, that is really helpful. I have an electronic switch, one of the optical ones that uses beam-break sensors to detect a pressure gauge needle, so can set the difference between cut in and cut off to as close at about 15psi difference. I can easily increase the pre-charge a bit to reduce the stress on the bladder, that makes a lot of sense, thanks again.

I can say that I'm impressed with these optical pressure switches. I was taking a risk in buying one, as they are Chinese, and I have a certain wariness when it comes to Chinese stuff, but having had a UK made spring-type pressure switch fail from frost damage within a year, I saw these Chinese ones for a lot lower price, so bought one to try. It was cheap, but surprisingly well made. I took it apart to see how it worked, and check out the quality, and was impressed, so I bought a couple more as spares. They were cheap enough to just keep in the cupboard if I need them, and have the big advantage of being really easy to set (you can independently set the turn of, and turn off pressure with a screwdriver in a couple of seconds).
 

Reach4

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A pressure relief valve is often suggested if the pump can develop high pressures at the surface, in case the pressure switch fails. I don't have one. Maybe your SQ pump has protection built in. I don't know.
 

Jeremy Harris

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It seems to, as I (accidentally!) shut the pump line off and it just holds steady at 9 bar (around 130 PSI), but that's a good thought I could easily fit a PRV, as I have a spare new one from the a heating tank that I could fit, that works over the right pressure range. Could be a good extra safety feature in case the Chinese switch fails (frankly I don't have a lot of faith in Chinese stuff, but even iPhones and stuff are made out there now, so I guess it can't be all bad.........)

PS: Just realised I'm writing this on a four year old Chinese made laptop!
 

Wondering

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Haven't had any problems with it(knock on wood). It is now 15 years old and still going. If the cow waterers are filling up or the house next door is using water I never know it as far as my pressure is concerned. I hope it last several more years--or longer!!

So, F&W have a new line of pumps now? Are they using different motors or the same as mine?
 

Wondering

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So what is the best motor now? And Pump? Or is it about like everything else these days, close your eyes and pick one and hope for the best!
 

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It seems to, as I (accidentally!) shut the pump line off and it just holds steady at 9 bar (around 130 PSI),

That means your pump has a max pressure of 130 PSI. That is called "deadheading" the pump, and it will only last about 5 minutes if you let that happen again. A pressure relief set at 100 PSI or less would keep the pump from deadheading.
 

Jeremy Harris

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Thanks once more - the pipe from the pump now has a pressure release valve fitted, as of this morning, set to blow at 90 psi and drain into the sump drain in the concrete chamber at my well head. Hopefully that should be belt-and-braces protection in case a pressure switch gets stuck on and keep everything at a safe pressure. I put the PRV before the flow restrictor, so it should also provide protection in the unlikely event that that should block up.
 
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