# Diaphragm tank drawdown Q:

### Users who are viewing this thread

#### Buddy

##### New Member
Can anyone tell me what the drawdown should be for 2 -119 Gallon diaphragm pressure tanks?
We have a 60/80 pressure switch. We turned off pump and drained it out and got about 42 Gallons.
Does that seem about right?

#### Reach4

##### Well-Known Member
usually you expect about 25% of the nominal capacity. That is reduced some at higher pressures.

25% would be 59.5 gallons, so you are low.

With a submersible pump, the air precharge should be 58 psi.

When the water pressure is zero, the pressure tanks should be empty of water. So they are relatively light. With water pressure zero, see if one seems heavier. Maybe try knocking on the sides of the empty tanks.

#### Valveman

##### Cary Austin
Staff member
With a 60/80 pressure setting a 119 gallon tank will hold 25.13 gallons or 50.26 with two tanks. You should get a little more than that when draining the tank completely out as the air charge pressure is lower than the pressure switch on setting. The amount of air charge in the tank has an effect on how much water a tank actually holds.

Pressure tanks are not designed to "store" water. Even though those tanks can have 50 gallons in them when full at 80 PSI, when the power goes off you have no way of knowing how much water is still in the tanks. If the pressure is at 61 PSI when the power goes off, those two huge tanks wouldn't have enough water to flush a toilet.

With a Cycle Stop Valve controlling the pump, even at 60/80 pressure a house only needs a 10 gallon size tank. One of those 119 gallon size tanks is more than needed to supply a city of a quarter million people when working with a CSV.

#### Buddy

##### New Member
usually you expect about 25% of the nominal capacity. That is reduced some at higher pressures.

25% would be 59.5 gallons, so you are low.

With a submersible pump, the air precharge should be 58 psi.

When the water pressure is zero, the pressure tanks should be empty of water. So they are relatively light. With water pressure zero, see if one seems heavier. Maybe try knocking on the sides of the empty tanks.
Yes we charged both ups bit in the spring to 58 each. Last week they were both at 64psi for some reason?
We had the same 42 Gal come out in the spring as well.
One tank did seem a bit more moveable but might be the fittings. They both had the same condensation and coldness level not too high up from the base. Is it possible that air could be sitting on the water and below the diaphragm?

#### Buddy

##### New Member
With a 60/80 pressure setting a 119 gallon tank will hold 25.13 gallons or 50.26 with two tanks. You should get a little more than that when draining the tank completely out as the air charge pressure is lower than the pressure switch on setting. The amount of air charge in the tank has an effect on how much water a tank actually holds.

Pressure tanks are not designed to "store" water. Even though those tanks can have 50 gallons in them when full at 80 PSI, when the power goes off you have no way of knowing how much water is still in the tanks. If the pressure is at 61 PSI when the power goes off, those two huge tanks wouldn't have enough water to flush a toilet.

With a Cycle Stop Valve controlling the pump, even at 60/80 pressure a house only needs a 10 gallon size tank. One of those 119 gallon size tanks is more than needed to supply a city of a quarter million people when working with a CSV.
We have 4 properties on the 2 tank system. It works not too bad but I'm farther up a hill.
I asked Reach4 this question as well about if there could be air sitting above the water below the diaphragm?

We have had an issue with one well user using their yard hydrant to fill up multiple bottles and containers up to 80 gal sometimes. It drains the tanks then just runs the pump for 8 minutes maybe more as tanks don't fill up.
I got air in my uphill line and wondering if the pump could be pulling air if the water table is getting low?

#### Reach4

##### Well-Known Member
Submersible pumps can pull air if the well is running dry. Another possibility is that the big water use is lower down, and air is entering from above via a vacuum breaker.

Once the water pressure falls to the air precharge level, nearly all of the water and air below the diaphragm comes out.

#### Buddy

##### New Member
Submersible pumps can pull air if the well is running dry. Another possibility is that the big water use is lower down, and air is entering from above via a vacuum breaker.

Once the water pressure falls to the air precharge level, nearly all of the water and air below the diaphragm comes out.
Yes I've thought that air is being pulled down my line because when I turn a faucet on air is right there coming out. Sometimes its mid stream after a while on too.

Another Q I have is I've seen how the lower elevation yard hydrant is being used to fill containers. It has 25 GPM from a bucket test we did. They are opening and closing it for average 8 minutes sometimes. We have 42 gal total drawdown right now so our 2 tanks are being emptied, pump come on, bypasses the tanks and goes straight to hydrant and never shuts off until the last container is filled and finally 2 minutes later the pump shuts off once at 80 psi. if its running 4-5 times longer than usual is it damaging the pump? considering the water table is probably lower right now? It's a 65' well. In the spring it was taking 1.25 minutes to fill and now its 2 minutes.

#### Reach4

##### Well-Known Member
Not damaging the pump.

How much static (no flow) pressure difference is there from the lowest tap to the highest?

If during the yard hydrant using water, the pressure drops to less than that difference, then air could be being pulled in from higher. One thing that would seem to stop that would be to put a check valve in series with the pipe(s) going to the higher places.

Another thought is to somehow limit the flow from that yard hydrant. https://www.agriculturesolutions.com/water-flow-regulator-kit-choose-1-2-3-4-or-5gpm
You could also use GHT adapters with a Dole valve. Those come in a lot off different flows.

#### Bannerman

##### Well-Known Member
We have 42 gal total drawdown right now so our 2 tanks are being emptied, pump come on, bypasses the tanks and goes straight to hydrant and never shuts off until the last container is filled and finally 2 minutes later the pump shuts off once at 80 psi. if its running 4-5 times longer than usual is it damaging the pump?
Perfect!

Pumps are designed to operate continuously, and will achieve longest life if running 24/7/365 without cycling On/Off. It is constant cycling that will reduce the lifespan of not only the pump but also the pressure switch, check valve and pressure tank diaphragm. Ideally, you will always want to use 100% of the pump's capacity, just as you stated has been flowing to the yard hydrants since that will prevent the pump from cycling.

When water use is less than the pump's flow capacity, then the excess pump capacity will fill the pressure tank. Once the system pressure rises to the pressure switch cut off setting, then the PS will shut down the pump, but if water consumption continues, then the PS will reactivate the pump again and the entire process will be continually repeated for as long as water use continues.

To minimize cycling, the pressure tank usually needs to be sized so the pump once activated, will run for at least 60 seconds minimum, but preferably 2-minutes after no further water is being utilized.

I stated the word 'usually' as Valveman's Cycle Stop Valve device will change how your existing pump will deliver water, and will prevent cycling when at least 1 GPM is being utilized, even when using a very small 4.5 gallon pressure tank.

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#### Buddy

##### New Member
Not damaging the pump.

How much static (no flow) pressure difference is there from the lowest tap to the highest?

If during the yard hydrant using water, the pressure drops to less than that difference, then air could be being pulled in from higher. One thing that would seem to stop that would be to put a check valve in series with the pipe(s) going to the higher places.

Another thought is to somehow limit the flow from that yard hydrant. https://www.agriculturesolutions.com/water-flow-regulator-kit-choose-1-2-3-4-or-5gpm
You could also use GHT adapters with a Dole valve. Those come in a lot off different flows.
When I see they are opening and closing the hydrant below 400' away and 50-60' below, my kitchen faucet goes from 2 Gpm to .5 Gpm and some times Zero.

I know it can be stepped down and suggested try a 50' hose even to reduce the flow but they haven't done anything.

#### Buddy

##### New Member
Perfect!

Pumps are designed to operate continuously, and will achieve longest life if running 24/7/365 without cycling On/Off. It is constant cycling that will reduce the lifespan of not only the pump but also the pressure switch, check valve and pressure tank diaphragm. Ideally, you will always want to use 100% of the pump's capacity, just as you stated has been flowing to the yard hydrants since that will prevent the pump from cycling.

When water use is less than the pump's flow capacity, then the excess pump capacity will fill the pressure tank. Once the system pressure rises to the pressure switch cut off setting, then the PS will shut down the pump, but if water consumption continues, then the PS will reactivate the pump again and the entire process will be continually repeated for as long as water use continues.

To minimize cycling, the pressure tank usually needs to be sized so the pump once activated, will run for at least 60 seconds minimum, but preferably 2-minutes after no further water is being utilized.

I stated the word 'usually' as Valveman's Cycle Stop Valve device will change how your existing pump will deliver water, and will prevent cycling when at least 1 GPM is being utilized, even when using a very small 4.5 gallon pressure tank.
I realize these pumps can run continually as you and Reach4 are stating but my concern is damage to the pump as the water table is much lower now so running it for 8 minutes compared to 1.5 minutes might be dropping the well level and its sucking air. We don't have an underground lake.

#### Reach4

##### Well-Known Member
There are devices that can monitor the current to the pump, and shut the pump down for a programmable amount of time if you use up the water.

Is it your well? If not, maybe you could appeal to the landlord, or the other tenant.

#### Valveman

##### Cary Austin
Staff member
One of these will keep you from having to worry if you are pumping the well dry.

#### Reach4

##### Well-Known Member
When I see they are opening and closing the hydrant below 400' away and 50-60' below, my kitchen faucet goes from 2 Gpm to .5 Gpm and some times Zero.
When the pressure looks like zero, it might become less than zero. So the air could suck in via your open faucet. A check valve on the incoming water could prevent that, but the other high-house should also have a check valve, or the check valve should be in line with both high-houses. But even if it was just for your house, it should cause less air to be in your line. Your high neighbor could still admit air if that neighbor opens a faucet while the pressure is below zero.

#### Buddy

##### New Member
NEW PROBLEM with Diaphragm tank. This thread explains the 2 tank system we have but now there is a new issue.

We did a shut down and pressure test of the 2 x 119 gal tanks and we got 90 gallons that came out when pressure went to 0 psi.
I figure something is wrong as previous drawdown amounts of a regular cycle with our 60/80 pressure switch should be around 55 gallons I've read. 6 months ago we got 42 Gallons. I figure that there is always a bit more water in the tanks when you completely drain once the system is off but 90 gal seems off.
One tank had 57 psi when empty and the other had 24 psi so figure it's toast maybe. I charged it up to 40 psi when I figured I'd stop filling as there was creaking sounds. maybe the diaphragm was sticking. Should I have just kept filling it to the 58 psi it should be?
Anyway we powered up the system and the tanks filled up, then we did a regular drawdown test stopping at the cut in pressure and we got 30 gallons.
Another question is can one tank be leaking air out the valve and that's why it had more water volume? We didn't put water on the valve but will next time to test.

#### Reach4

##### Well-Known Member
Another question is can one tank be leaking air out the valve and that's why it had more water volume? We didn't put water on the valve but will next time to test.
Yes. With a Schrader valve, the valve cap is supposed to be the main seal, rather than the valve core.

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#### Valveman

##### Cary Austin
Staff member
If you put 58 PSI air in a tank, it should be completely empty when the pressure drops to 57 PSI. If it did not go "thump" and the bladder hit the bottom of the tank at 57 PSI, something is wrong with the bladder. Draining all the way down to zero pressure slowly is a sign the bladder is bad. There could be water on top of the diaphragm, and the tank will not deliver as much draw down. Or, the diaphragm could be torn so badly it is doing nothing, letting the tank hold way more than the 25 gallons it should, but not delivering much draw down above the 58 PSI mark as it should.

With a Cycle Stop Valve the pump doesn't cycle on and off like a clock, which keeps the tank diaphragms from going up and down over and over until they tear. Without a CSV it is not a matter of if but when the tank will fail, as the diaphragm goes up and down with every cycle of the pump.

#### Buddy

##### New Member
If you put 58 PSI air in a tank, it should be completely empty when the pressure drops to 57 PSI. If it did not go "thump" and the bladder hit the bottom of the tank at 57 PSI, something is wrong with the bladder. Draining all the way down to zero pressure slowly is a sign the bladder is bad. There could be water on top of the diaphragm, and the tank will not deliver as much draw down. Or, the diaphragm could be torn so badly it is doing nothing, letting the tank hold way more than the 25 gallons it should, but not delivering much draw down above the 58 PSI mark as it should.

With a Cycle Stop Valve the pump doesn't cycle on and off like a clock, which keeps the tank diaphragms from going up and down over and over until they tear. Without a CSV it is not a matter of if but when the tank will fail, as the diaphragm goes up and down with every cycle of the pump.
It does go "thump". There's no splashing sound above the diaphragm when moving the tank empty or full. During our pressure test when empty it only had 24 psi and we got 90 gallons out when draining both tanks out to zero.
If the tank lost air then more water was able to enter I figure. Then powered up we only got 30 gal drawdown between 80-60 psi. Could the diaphragm just be stuck sometimes?

Reach4 : wouldn't the actual Schrader valve be the problem and not the cap?

#### Valveman

##### Cary Austin
Staff member
Rarely is the Schrader the problem. When a tank loses air charge it is nearly always from a torn bladder/diaphragm. With only 24 PSI air in the tank, yes the tank has much more draw down as it won't go "thump" and be empty until it gets down to 24 PSI. With a 60/80 pressure switch it should have 55-58 PSI air in the tank and go "thump" and be empty at that pressure. But it will give more draw down between 80 and 60 if it has 55 PSI air rather than 24 PSI air.

With 24 PSI air charge the bladder is being way over-stretched at 80 PSI, which is why it holds more water when draining to zero pressure.

#### LLigetfa

##### DIYer, not in the trades
With 24 PSI air charge the bladder is being way over-stretched at 80 PSI, which is why it holds more water when draining to zero pressure.
YMMV since some diaphragm tanks have internal dome limiters.

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