Two pressure tanks at different pressures?

[snotman1]

Hi there!
I have a large (too large!) capacity well pump suppling irrigation, which cycles on and off too quickly. Pressure activated, currently 40-60, with a 6 gallon pressure tank at 37psi. I am planning on adding a 24 gallon tank, and was thinking of charging it with air to 50psi - what do you think?? The smaller tank will provide that little bit of flow between the cut-in sensor activating and the pump actually pumping water, and the larger tank will allow the pump cycles to lengthen (longer to fill with pump running, longer supplying water when pump is not running). Is this crazy???
Thanks in advance>

 

[Valveman]

Yeah, kind of crazy. Lol! Different air charges in the tanks won't work. If you want to stop the cycling use a Cycle Stop Valve. The only other way to stop cycling is to use all the water the pump can produce, so there is no extra water to fill the tank. It is almost impossible to match every irrigation zone to the size of the pump. The Cycle Stop Valve is an easy answer to your problem.

 

[Sponsor]

 

[snotman1]

Thanks Valveman. I believe you, but why won't different air charges in the different tanks work?

 

[Bannerman]

why won't different air charges in the different tanks work?

Your 6-gallon tank will store only ~1.5 gallons water between 40-60 psi. Because you are setting the air pre-charge to 37 psi, a small quantity of additional water will remain in the tank under normal operation, but whenever the water pressure is less than 37 psi, the tank will contain 0 water.

Likewise , the 24-gallon tank you are considering, when pressurized with 50 psi pre-charge, will contain 0 water when the water pressure is less than 50 psi. Setting 50 psi pre-charge on a system running 40/60 psi, will result in the 24 gallon tank storing only ~3 gallons water as opposed to ~6-gallons (+ additional small amount) when setup with the identical air pre-charge as currently used for the 6-gallon tank.

Even when calibrated with an identical 37 psi pre-charge in both tanks, both tanks combined storage capacity will be only 7.5 gallons. Once the pressure switch shuts off the pump @ 60 psi, only 7.5 gallons further consumed will cause the system pressure to drop to 40 psi whereby the pressure switch will reactivate the pump. Cycling will continue, although less frequently.

To eliminate cycling while water is being consumed, install a Cycle Stop Valve as Valveman recommended.

 

[LLigetfa]

As Bannerman said, setting such a high precharge just squanders drawdown.

 

[snotman1]

Thanks all, makes sense to me now.

 

[Valveman]

Good stuff! But just an adder. Setting the air charge in a pressure tank higher than the pump on setting will cause a water hammer as the bladder/diaphragm will bounce off the bottom of the tank like a basketball on a hard wood floor.

 

[CountryBoy19]

Good stuff! But just an adder. Setting the air charge in a pressure tank higher than the pump on setting will cause a water hammer as the bladder/diaphragm will bounce off the bottom of the tank like a basketball on a hard wood floor.

This^^^

My setup involves 2 pressure tanks, 1 at my cistern that is 1200 ft from the house, at approximately 25 feet lower than the house, and 1 at the house to help buffer demands (it takes a couple seconds to get the water in a 1200 ft water line moving so we would always get significant pressure drops before I added the 2nd pressure tank at the house). Unfortunately, when I added the 2nd tank I didn't precisely calculate the elevation (pressure) difference between the 2 tanks and got them slightly off. My low pressure cutoff switch kept tripping and it took me a while to figure out that it was water hammer at the lower tank tripping the switch. I bled 2 psi out of the lower tank and haven't had any issues since then.

 

[Valveman]

Yeah, with too much air in a tank, when the bladder hits the bottom the pressure also bottoms out and will trip a low pressure cut off switch. But you should not have needed the second tank. You must have too many check valves or something for there to be a delay in pressure? A check valve at lower pressure tank or anywhere except for the one right on the pump can cause this problem.

 

[CountryBoy19]

you should not have needed the second tank. You must have too many check valves or something for there to be a delay in pressure? A check valve at lower pressure tank or anywhere except for the one right on the pump can cause this problem.

No extra check valves, the lower pressure tank is 1350 feet away from the house (and upper tank), the cistern & pump is 1200 feet away. Any time a valve is opened demanding more flow rate from a line that long there will be a pressure drop until the water gets moving faster. There is also a pressure spike when that valve is closed. Adding the additional tank at the house buffers the supply and therefore smooths the pressure drops and spikes.

I could understand the argument that I simply could have moved the pressure tank to the house, but the only tank that I could fit through the crawlspace access door was a 20 gal and o didn't want a single 20 gallon pressure tank. And a cycle stop valve with a small tank wouldn't work because I have points of use at several points along the 1350 feet of waterline. So I put a pressure tank at both ends and did my best to get the pressures balanced between them.

 

[Valveman]

The friction loss in the long pipe between the two tanks makes the tanks fill at different rates/time. The one closest to the pump will fill first, then the first tank will drain to fill the second tank. There should be no hesitation is pressure when you open a tap. Water is not compressible and 50 PSI at one end means 50 PSI at the other. As soon as a tap is opened the whole line of water starts moving from the pressure in the pressure tank. You may just need to turn up the "pump on" pressure to make up for the elevation difference. The CSV also helps with this as the pump is running continuously when water is being used instead of the pump cycling on and off while water is being used.

There will be a pressure spike at the bottom of the hill when a tap is closed quickly. A riser pipe with a cap or a small bladder tank before the valve being close can help if the tap must be closed quickly.

I have some systems where the pump is thousands of feet away from the house(s) and there is no pressure tank at the house(s).

Again, a CSV can help with pressure spikes and dips as it keeps the pressure constant while using water. The CSV just needs to be installed at the pump, no matter how far away from the houses.

 

[Reach4]

If things work with the two tanks OK, great. If not, the oscillation-like effects could be eliminated by putting a check valve in the line between the two tanks, and after the pressure switch. This could be at the house or at the well house.

 

[Valveman]

A check valve doesn't help because the oscillation only goes forward, not backward. The first tank will fill until the pressure switch shuts off the pump. Then the first tank will slowly drain into the second tank until the pressure equalizes. Sometimes this can even cycle the pump on again, before both tanks are full. At the least it will drain the first tank that had 60 PSI when the pump shut off down to maybe 50 PSI before the pressure equalizes. The more restriction between the two tank, friction loss, check valve, etc., the worse the problem.

If the two tanks in different locations are helping with the delay and surge in water at the end of the long lines, all you need to do is make sure both tanks fill at the same time/rate. If you have a large enough pipe that there is basically no friction loss between the two tanks, they should fill at the same rate.

A Cycle Stop Valve on the pump will also insure both tanks fill at the same rate. While you are using water the CSV will hold a strong constant pressure to the house. When you stop using water the CSV will fill the tanks or finish filling the tanks at the rate of 1 GPM. At 1 GPM there is no friction loss in the long line between the tanks, and both tanks will fill to 60 PSI before the pump shuts off.

 

[LLigetfa]

And a cycle stop valve with a small tank wouldn't work because I have points of use at several points along the 1350 feet of waterline

The CSV and tank can remain at the well and still have a second tank at the house. When the water use ceases, the remaining 10 PSI fill rate would be at 1 GPM and at 1 GPM, friction loss becomes a non-issue WRT the tanks equalizing.

 

[Valveman]

The CSV and tank can remain at the well and still have a second tank at the house. When the water use ceases, the remaining 10 PSI fill rate would be at 1 GPM and at 1 GPM, friction loss becomes a non-issue WRT the tanks equalizing.

Yep. And with the CSV at the pump, as long as there are no check valves except the one on the pump, the CSV will work with any tap anywhere on the line.

 

[CountryBoy19]

The CSV and tank can remain at the well and still have a second tank at the house. When the water use ceases, the remaining 10 PSI fill rate would be at 1 GPM and at 1 GPM, friction loss becomes a non-issue WRT the tanks equalizing.

CSVs are rated for use right at the top of the pump or direct burial in earth? I didn't realize that. There is nowhere to put a CSV at the pump unless it's buried underground.

 

[CountryBoy19]

The friction loss in the long pipe between the two tanks makes the tanks fill at different rates/time. The one closest to the pump will fill first, then the first tank will drain to fill the second tank. There should be no hesitation is pressure when you open a tap. Water is not compressible and 50 PSI at one end means 50 PSI at the other. As soon as a tap is opened the whole line of water starts moving from the pressure in the pressure tank. You may just need to turn up the "pump on" pressure to make up for the elevation difference. The CSV also helps with this as the pump is running continuously when water is being used instead of the pump cycling on and off while water is being used.

There will be a pressure spike at the bottom of the hill when a tap is closed quickly. A riser pipe with a cap or a small bladder tank before the valve being close can help if the tap must be closed quickly.

I have some systems where the pump is thousands of feet away from the house(s) and there is no pressure tank at the house(s).

Again, a CSV can help with pressure spikes and dips as it keeps the pressure constant while using water. The CSV just needs to be installed at the pump, no matter how far away from the houses.

The rule about the pressure being equal at both ends is only true when the water is stationary.

As soon as the water starts flowing the pressure drops, it drops due to the inertia of the water and friction loss in the pipe. The drop caused by inertia is only temporary, the drop due to friction loss lasts as long as the water is moving and when the water is shut off the opposite happens due to inertia, a pressure spike.

The same thing happens if one tap is flowing and another is opened, a temporary drop in pressure due to inertia. This is where it was noticed the most, when one person was in the shower and a toilet was flushed etc, the pressure would drop noticeably for 3-4 seconds then recover a bit. The longer the line the more the pressure will drop due to inertia.

Likewise, in your example to me in another thread about multiple check valves and how the water column above the pump can slam into the column at a check valve, that is only a problem if water has inertia (it does).

When a well pump kicks on it's not pumping it's full rate instantly, it has a small delay as the pump comes up to speed and gets that column of water moving.

Last comparison, water is not unlike electricity, volts is pressure, current (amps) is volume (gpm), resistance is friction loss, inductance is inertia loss. On really long runs of wire, a motor starting up (tap being opened) can cause large voltage drops due to the inductance of the wire. Thus, conductors are upsized to offset the voltage drop. My 30 gal air compressor will start fine plugged into a 15 ft, 12 ga extension cord, but it won't even turn over a full revolution on a 100 ft, 12 ga cord. Because when I open the tap, there is a voltage (pressure) drop.

That drop due to inertia cannot be avoided without adding a tank near the tap (to decrease the amount of water that has to get moving quickly) or boosting pressure to mask the issue.


And yes, you're right that the pressure tank closer to the pump fills faster (sort of). But it's not bad. The tank nearest the pump is downhill and set higher than the tank at the house. When the pump kicks off it will settle back down about 2-3 psi as some of the water is still moving up the hill (through the 1300 ft of pipe) to equalize the pressure. Having 2 tanks decreases the pressure variations at both ends when the pump kicks on and off. The fact of the matter is, the further the distance from the pressure source, the more inertia is a factor in pressure drop because it's dependent upon the mass of water sitting between the pressure source and the taps that are being used.

 

[Bannerman]

CB19, the issues you describe are common symptoms of an undersized (diameter) supply line. Although you didn't specify the diameter or type of the current supply line between the cistern and house, I anticipate there would have been no need to compensate with a second pressure tank in the home if the supply line had been sized appropriately in consideration of the 1350' length of run and the home's flow requirements.

I suspect the pressure tank at the cistern is larger than the PT in the home. If the PT within the home was larger than the other, then as Valveman mentioned in #13, it is likely the pump will become reactivated each time the pressure falls below the pressure switch cut-in setting when it fills the larger tank after the pump was shutdown initially.

Friction will be greatest for the layers of water nearest the inner surface of a pipe whereas the water nearest the center will have the least friction (water on water) and so will flow more rapidly. Because friction loss is cumulative along the length of a run, increasing the pipe diameter appropriately can compensate for friction loss which is calculated in relation to the flow rate needed.

Another potential source of flow restriction is a restrictive or partially closed isolation valve. Full port ball valves are the preferred choice, but gate valves will also provide full flow, but the gate may become disconnected from the stem, which can result in a flow restriction even as the valve seems to be fully open. Since the flow path through a globe valve results in multiple changes in flow direction, globe valves will be most restrictive and should be avoided if possible.

Your motor analogy is not a true comparison as starting a motor from dead stop will be most similar to a short circuit since that is when the motor will briefly consume a much greater amount of power compared to power draw while up to speed and operating within design load. Opening a faucet to allow 2 GPM flow, will not cause a brief 12 GPM flow through the faucet before reducing down to 2 GPM.

the further the distance from the pressure source, the more inertia is a factor in pressure drop because it's dependent upon the mass of water sitting between the pressure source and the taps that are being used.

As Valveman stated, water is not compressible and once pressurized, will flow immediately in any direction once there is any opening in which to escape.

Consider the many miles that water will often travel between the water source and each home in many municipal water distribution systems. Opening a faucet that is miles away from the source, does not result in any delay before water will flow from the faucet.

 

[Valveman]

There should be no delay for 3-4 seconds when you open a tap as Bannerman says. There is not much friction loss when just opening a 2-3 GPM faucet and that is not what causes the delay. The pressure tank should start the water moving down the line when you open a tap at the other end. The pressure tank only has to supply as much as you are using. So, the tank should push 2-3 GPM down the line as soon as a tap is opened. However, if the inlet/outlet of the tank is restricted or too small, it cannot supply the water as fast as needed.

The pressure is like voltage and the pressure tank is like a capacitor. There should be no complete loss of pressure at the far tap as long as the pressure tank is charged enough and can push water into the line without any restriction.

Then with water already flowing from the pressure tank down the line to the open tap, a drop in pressure at the tank causes the pump to start. If there is a noticeable difference in pressure when the pump starts, the pressure tank is not able to feed the line as fast as needed.

There are some pumps that take 3-5 seconds to come up to speed once they are started, but the size of the tank and the air charge needs to be set to allow the tank to supply the water needed until the pump comes up to speed.

Like Bannerman said, there are no pressure tanks in a municipal system and people miles away from the source do not have a hesitation in water when they open a tap.

Post a picture of the pressure tank(s) at the bottom of the hill as there must be something causing a restriction.

 

[LLigetfa]

CSVs are rated for use right at the top of the pump or direct burial in earth? I didn't realize that. There is nowhere to put a CSV at the pump unless it's buried underground.

There is one model of CSV that can be installed in the well casing.