Bladder Tank Sizing for Variable Speed Pump

I have recently installed a new variable speed pump and I saw that the old very large bladder tank have been replaced with a small tank. My plumber said that with VSD pump I only need a small bladder pump. When I ask him how do you size the bladder tank for the VSD pump and he just said that was all I needed.

Therefore, can anyone tell me

1. Why a VSD pump only need a small bladder tank?

2. how to size a bladder tank for the VSD pump?

3. Can we run the VSD pump without the bladder tank? I have tried with the bladder tank being isolated from the system and the VSD pump run as normal.

Thanks.

I understand that the small bladder tank only softens the many stops and starts of the pump. If you brush your teeth, it turns on and off.

As for the pressure setting, Franklin Monodrive recommends 70% of set pressure.

So a system set for 50 psi, would have a pressure tank setting of 35 psi.

See attached page from manual.

Although that page says you can "use an existing tank with much larger capacity", it will be of absolutely no help. A pressure tank can only deliver water as the pressure decreases. When pressure remains constant, no water ever enters the tank or is drawn from the tank. It doesn’t matter if the tank is the size of a baseball or the size of your car.

You are correct that this type system functions the same when the tank is isolated. For these VFD or VSD type systems, a tank is simply a shock absorber. It gives the system a second to see that you have turned on a faucet before the pump gets started. It also keeps the pressure from spiking extremely high when you turn off the faucet, before the pump gets shut down. The pump is still going to start every time you rinse a toothbrush, the ice maker is filled, or water is used anywhere.

With the SubDrive/MonoDrive type systems, the pump is also going to start 45 times per minute when you are using water for longer periods of time, as when taking a shower or running a sprinkler. This type system is going to start and stop the pump 2700 times for every hour it is running. This adds up to about 2 million cycles per month, and is one of the major reasons for premature system failure. It is still going to do this, no matter the size of pressure tank.

A CSV and a regular pressure switch will solve this problem. It allows the use of the entire amount of water stored in any size pressure tank, and also delivers constant pressure after the pump is finally started. But of course then you would not need the VFD/VSD type system at all.

Manufacturers promote products that help keep the cash flowing. Making your pump system last a long time would be counter productive to their efforts. It is more important to find out what the manufacturer is not telling you, than to simply accept what their glossy ads are telling you. If you were convinced that the VFD system would save you energy, then you have just fallen for another "green lie" that only perpetuates "planned obsolescence". VFD systems do not save energy.

CSV, try it you'll like it!

Valveman said it best.

Thank valveman for the clarification.

Another question: do I still set the pressure tank 2 PSI below the VSD pump pressure?

No! The chart above shows you the precharge amount. It is usually about 30% below the constant pressure. Constant set at 50 PSI, air pre-charge in the tank should be 35 PSI.

The VSD pump that I installed is Grunfos Sqe. Is the 30% pre-charge pressure below the pump pressure applicable to my system too?

No difference!

See this link for a searh on CU301 controlleer for the SQE.

https://terrylove.com/forums/search.php?searchid=2524111

I installed a Monodrive 15 years. It is not a true variable speed controller. It can't hold the pump at exactly the needed speed to keep a constant flow required to keep the exact same pressure. It's basically on/off with a very soft start and very soft stop that allows the pressure to drop several PSI before the pump turns on. It still uses a pressure switch that is electronic rather than mechanical. It's not a variable transducer. Some of the Franklin controllers do use a 4-20ma transducer with PID conrol, but the monodrive system is not one of them. Yet it does not have any of the aforementioned shortcomings.

The SQE system is a true PID constant pressure system and will adjust the speed of the pump for continuous constant flow operation while taking a shower to keep the same pressure.

A CSV valve will allow you use the entire range of the pressure tank but only at the stop pressure so if you're cycling between 40 and 60 psi, your constant pressure is 40 psi. Still much better than no constant pressure at all.

With a Monodrive, the pump does NOT start and stop 45 times per minute as you take a shower. I have an 86 gallon WX-302 preset to 38.5 PSI with a control pressure set to 55 psi. 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. If I set the "steady flow" dip switch, which is only intended for installations where there's a spigot between the well and the pressure tank and the pressure tank is a long ways from the well, then the cycle doubles and the pump runs for half that long with 28 seconds between.

My pressure tank is now 24 years old most likely because it has not had to deal with constant emptying/refilling. It's exercised for a small fraction of it's range. My pump and motor have done just fine.

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.

The CSV is a marvelous invention and if I didn't have access to a VFD I'd have a CSV.

Alan Waterman said:

The CSV is a marvelous invention and if I didn't have access to a VFD I'd have a CSV.

Click to expand...

Thanks for that, but you are thinking about it the wrong way. The VFD is a marvelous invention and if I didn't have access to a CSV I would be putting up with all the negative side effects of a VFD to get constant pressure like you are. Although, as you say the Monodrive doesn't really deliver constant pressure.

Actually with that large tank and a 40/60 switch a CSV would need to be set to hold 58 PSI constant, not 40 PSI. This way the CSV only fills the pressure tank at 1 GPM from 58 to 60 PSI, which gives 2 minutes of run time to fill and 80 gallon size tank.

With a small tank the Monodrive does bounce the pump on and off about 45 times a minute. Having to use an 80 gallon size tank to get the cycles down to 55 times an hour is still not good. However, I am amazed the Monodrive has lasted 15 years, most would have been replaced 3-5 times by now.

At least with a Monodrive you still have a regular single phase motor and can easily switch to a regular control box and a Cycle Stop Valve. The Cycle Stop Valve would work with any size tank from the 80 gallon down to a 4.5 gallon size, and there would be no cycling at all when water is being used at any flow rate above 1 GPM.

15 years is already way past the average life of a Monodrive system. With a CSV you could still be bragging about your pump system working in 30-40 years.

I also believe you will find out the little pressure switch on a Monodrive is mechanical not electronic and is subject to wearing out from all that switching.

Oh and BTW you should not have to run your sprinkler zones at 100% pump flow. With a CSV you can run a zone as small as 1 GPM and the pump will not cycle at all.

Yea, I still have the old control box to swap just in case. The pressure tank is the same one that I had before the VFD install. But it works basically like what you're describing with the CSV. It allows the pressure to go down a few PSI and then the pump turns on full speed (with a soft start) to fill it back up to the preset.

But why isn't it good to have a large pressure tank with a VFD? It slows the cycling and from your description works just like a CSV if you have the CSV set to cycle between 58 and 60 psi?

If its true that monodrives typically fail after 3 or 4 years, maybe my large pressure tank is what has extended it's life with far less cycling than a small 2 gallon tank?

I went back and checked my records. I bought it in 2008. It's a MonodriveXT NEMA4 but has been indoors its entire life even though it's rated for exterior use in pretty severe conditions.

I paid $945 for it. The exact same unit is still sold today as a current model and the prices are in the $2500 range.

The mechanical switch for the Monodrive only has about 1 PSI between on and off. NO matter what size tank you should see the pressure going up and down by 1 PSI several times a minute. But the big tank slows it down a lot. You should not be seeing the pressure drop a few PSI before the pump comes on. With only 1 PSI between on and off the 80 gallon size tank should be giving you only 1 gallon before the pump starts. But that is still much better than only 1 PSI using a little 4.5 gallon size tank. The Monodrive doesn't completely shut the pump off while using water, only dropping to 50% speed and then back to full speed over and over to try and maintain the 1 PSI differential.

With a CSV and big tank the pump would not be cycling between 58 and 60. The CSV would keep the pump running and the pressure at a constant 58 as long as you are using water. The pressure would only build to 60 and the pump be shut off when you finally stop using water. Then you would need to use all 20 gallons from the 80 gallon tank as the pressure dropped from 60 to 40 before the pump would be started again.

If you use a regular 40/60 switch with the Monodrive you would be getting 20 gallons out of the 80 gallon tank before the pump starts. But that would mean the Monodrive is basically doing nothing.

Being installed indoors helps a lot. Other than that you are just lucky you haven't had to replace that Monodrive controller 3-4 times like most other people do. Check valves failing is another problem as the Monodrive causes them to open and close 45 times a minute with the small tank.

Advances in technology are not driven by what is best for the consumer as much as they are driven by what makes the manufacturer the most money. VFD's are heavily advertised by the manufacturers because that is how they make the most money. That alone tells you VFD's are not the best thing for the consumer.

valveman said:

With a CSV and big tank the pump would not be cycling between 58 and 60. The CSV would keep the pump running and the pressure at a constant 58 as long as you are using water. The pressure would only build to 60 and the pump be shut off when you finally stop using water. Then you would need to use all 20 gallons from the 80 gallon tank as the pressure dropped from 60 to 40 before the pump would be started again.

Click to expand...

Curious, if the pump runs continuously while you use it and it say pumps 15 GPM at 58 psi and you're using 2 gpm, where is the other 13 GPM going? Back down the well casing?

This is power usage while using the shower in the master bedroom. The time between cycles is about 2 minutes when running the shower in my daughters bedroom which uses a lot less water. For the master bedroom, it's about a minute between. It takes about 3 to 4 seconds from stop to full on and visa versa.

Alan Waterman said:

Curious, if the pump runs continuously while you use it and it say pumps 15 GPM at 58 psi and you're using 2 gpm, where is the other 13 GPM going? Back down the well casing?

Click to expand...

You would have 58 PSI going to the house because that is the set pressure of the CSV. Before the CSV there maybe 100 PSI or whatever it takes to make that pump only produce 2 GPM. The CSV maintains 2 GPM (because that is how much water you are using) by making the pump think the well is so deep it can only pump 2 GPM. There is no extra water left to dump anywhere. The pump uses less energy when pumping less water, so you wouldn't want to dump any water and use more energy.

There's about a 3 psi range when continuously running.

valveman said:

You would have 58 PSI going to the house because that is the set pressure of the CSV. Before the CSV there maybe 100 PSI or whatever it takes to make that pump only produce 2 GPM. The CSV maintains 2 GPM (because that is how much water you are using) by making the pump think the well is so deep it can only pump 2 GPM. There is no extra water left to dump anywhere. The pump uses less energy when pumping less water, so you wouldn't want to dump any water and use more energy.

Click to expand...

It takes a lot more power to pump 2 gpm at 100 psi than at 50 psi. The CSV is increasing TDH a ton in this scenario and the pump is put under a lot more stress....unless I misread your statement about the pressure before the CSV being held at 100 psi.

Alan Waterman said:

This is power usage while using the shower in the master bedroom. The time between cycles is about 2 minutes when running the shower in my daughters bedroom which uses a lot less water. For the master bedroom, it's about a minute between. It takes about 3 to 4 seconds from stop to full on and visa versa.

Click to expand...

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?

Alan Waterman said:

There's about a 3 psi range when continuously running.

Click to expand...

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.

Edit; On seconds thought after looking at that graph I would just make sure there is about 48 PSI air in the tank. May give even longer off times and won't hold much crud.