Bladder Tank Sizing for Variable Speed Pump

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Alan Waterman

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That is an interesting graph. It shows the tank to be filled in a few seconds, and to drain in a minute or so. I would like to see what the pressure gauge is doing in relation to this graph? I think you are running about 2 GPM and getting about 2 gallons out of the tank before the pump starts. When the pump starts it puts 2 gallons back in the tank in 10-20 seconds. This would mean the gauge on the tank only moves up and down by 1-2 PSI. That is what I would expect with an 80 gallon tank and the little mechanical switch with the rubber bell shaped cover.

The pump peaks quickly and then drops a bit, which is the 50% speed thing it does for a few seconds before shutting off completely. With a small tank the switch is made before the pump shuts off completely and it bounces like that 45 times a minute.

Also curios about the 14.7 rate during the time the pump is off? Is that the parasitic loss from the Monodrive?

Yes, the 14.7 watts is just the standby power of the controller.

At 50 psi, I get about 18 GPM from my 25 GPM pump when run continuously. At 40 PSI, it pumps out about 22 GPM. 0 psi, it's about 30 GPM at the well head with the water at 120 feet deep and the pump at 200 feet. There's a little bit of TDH from the friction of the pvc so it's slightly less than if the pump were higher right at the water level. The more TDH it has to go through, the less water it will pump at the same horsepower. Deeper water, more pipe, more bends, etc add to TDH. Working pressure and water depth are the two biggest contributors to TDH. I've tested many many scenarios at different flow rates and different amounts of TDH at various points in the system.

For the graph above with the master bedroom shower, it's 2.3 gallons in a 5 gallon bucket for each cycle. So the 8 seconds to fill up and the 56 to empty result in 2.3 gallons put back into the pressure tank in 8 seconds.
 

Alan Waterman

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That is another interesting graph. You can see the pump running at 1/2 speed for a few seconds after each peak. I would love to see that when using a small tank, as is normal. Using that big tank has taken a lot of abuse off the pump, which is why it has lasted so long. But one of the benefits of constant pressure is not needing a large pressure tank.

I think that is a stated benefit that is not true. With a 2 gallon pressure tank, the cycling would be maddening. Yes it would work but you'd wear the pump down AND the pressure tank very quickly.

Perhaps with a true PID control system that can run the pump at a true variable speed, it would be fine. But since I already had this pressure tank, I didn't see a need to replace it. When it finally it finally goes I guess I'll probably replace it with an 80 gallon or larger.

The way I see it, in order for the system to last, it needs a regular large pressure tank as if you still had a traditional system. The benefit I get is that the pipes in my house (copper under slab) don't get stressed with millions of 40 to 60 PSI changes over decades. Nearly everyone in my neighborhood with a house as old as mine has been re-piped with Pex. When I turn of the hot water to my water heater so that I don't have a cooling or expanding body of water messing up my pressure test, then turn off the main valve to the house which is after the pressure tank, and then measure the pressure, it still holds without dropping a single PSI for hours.

I do this test about twice a year. Every now and then I get a small drop and then I kind of freak out and run around the house only to realize that there's a faucet or something that has a small drip leak. Once I fix that the pressure holds again. I realize some day I will need a re-pipe, but for now, along with constant pressure and using a phosphate feeder to coat the inside of my copper, I can hopefully get by another decade. Fortunately I'm lucky to have water that is perfectly ph neutral.
 

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8 seconds of run time is not good for the motor, even with soft start. One minute of run time is minimum, two minutes is better, and running continuously while using water is best.

You really can't change the TDH. The water in the well will always be at a certain depth, and you will always need 50 PSI in the house. The TDH stays about the same. You can greatly change the water flow by opening or closing faucets. Working pressure and depth are the two biggest contributors to TDH as you say, but flow rate is the biggest consideration to horsepower or Kw.
 

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That is another interesting graph. You can see the pump running at 1/2 speed for a few seconds after each peak. I would love to see that when using a small tank, as is normal. Using that big tank has taken a lot of abuse off the pump, which is why it has lasted so long. But one of the benefits of constant pressure is not needing a large pressure tank.

Also, with that tank holding 20 gallons of water and only expressing 2-3 gallons at a time before refilling, the tank is not getting flushed very well. You may find some real crud coming in from the tank when the power goes off and all 20 gallons gets dumped to the house for the first time in years.

There's a turbulator on the intake of the wx-302 so the water is well agitated even with only 2 something gallons going in and out. I have a very large clear whole house filter housing that has a phosphate pellet cartridge that is also clear so if anything makes it that it would get trapped in the pellets. I'm blessed to have soft water (1 grain and no iron) with no sediment or anything.
 

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I think that is a stated benefit that is not true. With a 2 gallon pressure tank, the cycling would be maddening. Yes it would work but you'd wear the pump down AND the pressure tank very quickly.

Perhaps with a true PID control system that can run the pump at a true variable speed, it would be fine.

Exactly, that is not a true variable speed pump. It is basically on and off between 2 PSI, which is maddening with a small tank. There are lots of so called VFD's or constant pressure pumps that do not actually vary the speed. There are problems with determining the minimum speed to make a real variable speed pump shut down when needed, so they make these type contraptions instead because they are easy to set up. VFD's are not really made to last anyway. They are made to work just well enough that you think it is OK to need and purchase a new one every five years or so.
 

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8 seconds of run time is not good for the motor, even with soft start. One minute of run time is minimum, two minutes is better, and running continuously while using water is best.

It's not even 1 minute. For household use like taking a shower, it's 8 seconds every minute. It can't have been too bad for it. My pump has lasted longer than anyone else's in my neighborhood and I'm the only one with a VFD.
 

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There's a turbulator on the intake of the wx-302 so the water is well agitated even with only 2 something gallons going in and out.

The "turbulator" is proof the tank doesn't get circulation with a VFD. They started adding the turbulators for just that reason, because it does help in a situation like yours.

Everything that is added as a Band-Aid for something a VFD does or doesn't do is just another opportunity for a failure.

The simplest way to solve a problem is always the best way. You don't need complicated and expensive electronic controls, a large tank, turbulator, torque arrestors, wire stand offs, indoor clean and cool air for the controls, and the list goes on. The Cycle Stop Valve simplifies everything, which makes pump systems cost less, last longer, and work better. But glad you have beaten the odds on your system so far.
 

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The sprinkler argument doesn't apply. No matter what kind of system you have installed, your sprinkler zones should be tuned for constant maximum pump operation. All 16 of my zones flow enough water to keep the pump on 100% of the time at 43 to 48 psi.

Having your sprinkler system matched exactly to the output of the pump is the only reason that system has lasted so long. Most people don't have a sprinkler system, much less a perfectly matched one. The Monodrive is cycling the pump on/off several times for just a shower, it would have cycled millions of times with a hose sprinkler or smaller irrigation zones, and the pump would not have lasted.

VFD's are good because they deliver constant pressure like a Cycle Stop Valve. But no matter how long it last with a VFD or conventional pressure tank, a pump will always last longer when controlled with a CSV.
 

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Having your sprinkler system matched exactly to the output of the pump is the only reason that system has lasted so long. Most people don't have a sprinkler system, much less a perfectly matched one. The Monodrive is cycling the pump on/off several times for just a shower, it would have cycled millions of times with a hose sprinkler or smaller irrigation zones, and the pump would not have lasted.

VFD's are good because they deliver constant pressure like a Cycle Stop Valve. But no matter how long it last with a VFD or conventional pressure tank, a pump will always last longer when controlled with a CSV.

Having a sprinkler system not tuned for the pump flow is bad for the system no matter what. The VFD gave me constant pressure and eliminated the mild water hammer I was getting when the pump shut off. And although there wasn't water hammer when the pump turned on, the pressure spiked pretty from coming on instantly. The soft start and stop is marvelous and I'm convinced that the wear and tear from hard starts is far worse than more frequent cycling.

Running a pump at 100 psi when you're only using 50 psi in the house does use more kwh. There's no way around it. It increases TDH a ton.

For example, I have a 4" 25 GPM V series 2 hp motor and PE at 200 feet deep with PV that has in I.D of 1". The water depth is 125 feet as of this morning. For 18 gpm, that gives 276 feet of TDH + another 30 for the bends and pipe resistance in the horizontal pipe before I get to my first outlet and about the same to the first sprinkler valve for a total of about 315 TDH. It's a bit of a back and fourth because the flow rate ALSO effects TDH so you have match it.

https://www.tuhorse.us/total-dynamic-head-tdh-calculator/

My pumps GPM falls to ZERO at 375 TDH. As the water level drops, the TDH will increase. If I increase pressure, it also increases. In my case, if I increase pressure to 95 PSI, my flow rate will drop to 1 gpm at the full 2700 watts that my pump runs on.

At 97.5 PSI, my pump flow rate is 0 GPM. It's spinning but cannot overcome 375 feet of TDH. It's using the full 2700 watts and flowing zero at this point....and then the pump burns up because the motor which is below the pump end is not having water forced around it be the PE.

THDVSeries.JPG
 

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FYI, if one decides to run at a high PSI from the pump, like when installing a CSV, picking a pump end for the given hp could make a lot of difference. For example, a 2 hp 15 gpm V series is way more efficient at deeper levels or higher pressures than the 25 gpm pump. The 25 gpm pump may pump more water than the 15 gpm pump at shallower water levels or lower psi, but it will lose very quickly as the water gets deeper or pump pressure is increased.

The same 2 hp V series pump at 15 gpm hits static 0 gpm at a much higher TDH of 596 feet. Where my 25 gpm pump stops at 375 feet of TDH, the 15 gpm version of the same pump is able to hit 14 gpm still. Now if I switched to that pump end with my same 2 hp motor, I'd lose flow right now at my current depth and pressure. In a few years, I'll need to switch out the PE with a 20 gpm to account for our dropping water table.
 

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Having a sprinkler system not tuned for the pump flow is bad for the system no matter what.

My pumps GPM falls to ZERO at 375 TDH. As the water level drops, the TDH will increase. If I increase pressure, it also increases. In my case, if I increase pressure to 95 PSI, my flow rate will drop to 1 gpm at the full 2700 watts that my pump runs on.

At 97.5 PSI, my pump flow rate is 0 GPM. It's spinning but cannot overcome 375 feet of TDH. It's using the full 2700 watts and flowing zero at this point....and then the pump burns up because the motor which is below the pump end is not having water forced around it be the PE.

Well if you have a Franklin brand pump the amps won't drop much, but they should drop at least 10%-20% when the flow is restricted. That is just how pumps work. If you picked a better pump like a Goulds it will drop 30% and a Grundfos will drop 55% in amperage when restricted. So, your 2700 watt motor would only be drawing about 1300 watts when the CSV restricted it to low flow. That is what my 2HP, 25 GPM Grundfos pump does. Here is a video of the amp meter.

The energy needed to run the Monodrive or any VFD is added to the draw of the pump/motor. So, a 2HP motor controlled by a VFD is using 2.1HP to pump 2HP worth of water. Add to that the parasitic losses of the VFD as it is using 14+ watts 24 hours a day, even when the pump is off. There are several studies that prove a VFD uses considerably more energy than a conventional pressure tank system where the pump is either running at its best efficiency point or it is off.

With a good pump the CSV can reduce the amp draw of a pump by 50%, which makes it comparable to the energy use of a VFD. However, it would take 77 years for a conventional pressure tank system to save enough energy to make up for the 800 bucks saved by using a CSV instead of purchasing a VFD or a big pressure tank.

As for the soft start, simply running 390' of #10 wire to a 2HP pump will make it soft start. The longest length of the smallest wire possible for the size of pump will produce 36% less torque and eliminates the need for any complicated, expensive, electronic controller.

Matching the sprinkler system to the pump is rarely practical. You may need a 25 GPM set in the back yard. But the side yard only needs 10 GPM and the driveway just need 5 GPM for the bubbler's. Even when I set up irrigation systems to match the pump, someone always decided to run a garden hose or hose sprinkler to some new addition later on. Very few people know how to keep their pump from cycling. I find it is best to use a CSV and be ready for anything the homeowner wants to do with the water. I always plan for the worst case, as that is usually the way it ends up working.

The CSV can never close to less than 1 GPM and will never burn up your pump. When the pump can produce 97 PSI back pressure and the CSV is set to deliver 50 PSI, the 47 PSI of difference across the CSV is what causes the 1 GPM minimum flow that keeps the pump cool forever.

I am not bashing VFD's as I think the idea of constant pressure is a good thing. But when there are two or more ways to get constant pressure, the simplest way is always the best way, and that is the CSV.
 

Alan Waterman

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It is true that the 14.7 watts adds up. In a year at 18 cents / kwh, the standby cost of the controller is $22 / year. I'm willing to live with that :)

My pump draws 2700 watts at 40 psi and at 55 psi. It makes no difference. The higher the psi, the less water it flows but still takes the same power. It was the same the with the previous pressure switch (I won't call it a controller).

Even if my pump dropped to 70% of the 2700 watts at 97 psi pumping 1 gpm, I'd still be using 70% of the power continuously to pump 1 gpm.

The Franklin pump still needs the full power to try and overcome the 375 TDH in my scenario so I wouldn't even get the 30% pullback so Franklin pumps are probably a really bad choice to use in combination with a CSV.
 
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Valveman

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When taking a shower, the pump will run for 8 seconds and then stop for 56 seconds so basically cycling on and off about 55 times a an hour.

That is lots of cycling. That would be 1,320 cycles per day if used with a sprinkler or two like the way many people irrigate.

It was the same the with the previous pressure switch (I won't call it a controller).

If you already replaced the pressure switch then the pump has probably been cycling more than you think. Bad pressure switch is the first sign of a cycling problem.

The Monodrive is not variable speed. It is basically just a soft starter with a 2 PSI pressure switch. It is always running at full speed, drawing 2700 watts, running 8 seconds and off for as long as the tank can supply with a 3 PSI drop. But again, I am glad you got 15 years out of it. They need a few of them to last that long. Most people have to replace that $2400 controller 3-5 times in 15 years. Most people give a 5 year average for a VFD like this one.
 
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