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# Thread: 3M recommends pressure tank size for their CBF100 carbon filter

1. The basic calculation is simple on paper, but a little more difficult in the field. That is why I recommend the Chemilizer pump system, they are not affected by varying flows, pressures, etc.

Clorox bleach (not generic) contains approximately 60000 ppm of chlorine. You need to reduce that to below 10 ppm. We use water to reduce that 60000 in the chemical storage tank. We then inject that dilution into the water stream in a controlled amount depending on the water condition. I am travelling for a few days so I do not have my charts in front of me, in general, if you keep the total chlorine level below 15 ppm, the carbon tank should have no problem. Of coure, you have to consider a few variables such as contact time, what type of storage, water quality, etc. to know the proper amount of chlorine to inject. Chlorine in a storage tank will also lose effectiveness over time, so mixing a huge batch that will last for several months would not be a good idea.

I apologize if my calculations are off a bit, but these should be close. With a Chemilizer HN55 1-128 ratio pump, a dilution of 35 gallons water to 1 gallon of bleach should give you between 10-15 ppm chlorine after the pump/injection system. Other pumps, stenner, LMI, etc are usually adjustable in some way. The math is fairly simple if you can determine what the ratio of pumped/injected chemical to water used is.

65000 / 35 / 128 = ppm chlorine

Since I dont do these calculations daily, hopefully somebody on this forum who is familiar with proper chemical injection systems, (not the chlorine pellet systems) will add some better information for you.

2. Well, tell me if my math is wrong, but from your figures (using 60000ppm for 6% Clorox):

.06/35gal = .0017143...
1:128 = .00001339...
or 13.4 ppm

So I figure the concentration/solution that my injector is doing right now is approx. 11ppm. Previously it was maybe twice that, before I reduced it, using a trial and error approach.

Thanks ditttohead, for helping me to understand how to figure this!

So 10 to 15ppm is the common concentration in most systems?

How would I know if I could do less than that?

Here's another whopper:

When the system was first installed, no one thought to install the carbon into the tank. The installer blamed the supplier, and the supplier blamed the installer.

It ran this way, at perhaps 24ppm, for two weeks (maybe at even 3 times that concentration for a few days).

Any ideas what kind of damage was likely to have been caused to the rest of my system with the high chlorine concentration, esp. to the resin in the softener tank?

If the resin fails, will it be obvious to me, perhaps in the shower?

Thanks again!

3. Gary, I should give you the whole picture about my system. (Sorry, it's kind of long...)

Our well was probably drilled in the early 80s. The driller's long gone. The old pump was a Red Jacket that sat on top of the 20gal pressure tank. The pump was rated for @ 5GPM.

We've updated with a new submersible and softening system.

- the well is 47 feet deep
- "10GPM" is marked under the well cap with a 6" casing
- the water level in the casing is 5 to 8 feet from the surface of the ground
- the pressure tank is @ 4 feet below ground level

The plumber ordered an 8GPM 1/2hp submersible with a 20gal pressure tank. 8GPM was out of stock, so the plumber got the 12GPM 1/2hp - I questioned it at the time, though my knowledge is nil about these things.

The new submersible hangs @ 10 feet from the bottom of the well - 25 to 30 feet below the surface of the water.

We now have a brand-new 12 GPM pump (the cost of which I've deducted from my final payment to the plumber), and a new 20gal bladder pressure tank.

If we were starting where the plumber should have, trying to figure out what pump to put in: the well recovery rate was rated at 10GPM by the driller, doesn't it make sense to put in a pump that pumps no more than 10GPM (or even 8GPM because of the well's age), and then size the pressure tank to match the pump's capacity?

A couple of reasons I'd prefer starting by replacing the pump, rather than the tank:
- The existing 12GPM pump will pump the well dry if it runs any amount of time, judging by the driller's capacity rating.
- Isn't it easier to pull the pump and reinstall a new one, than it is to disconnect/reconnect all the plumbing that's necessary to remove and replace the tank?
- doesn't the draw-down measurement depend on the pump being sized right?

One other question. You're now mentioning a 60 second OFF time with the pump cycle. This conversation started with the question being about a minimum 60 second ON cycle. What amount of time should the OFF time be?

4. Originally Posted by hcw3
Well, tell me if my math is wrong, but from your figures (using 60000ppm for 6% Clorox):

.06/35gal = .0017143...
1:128 = .00001339...
or 13.4 ppm

So I figure the concentration/solution that my injector is doing right now is approx. 11ppm. Previously it was maybe twice that, before I reduced it, using a trial and error approach.

Thanks ditttohead, for helping me to understand how to figure this!

So 10 to 15ppm is the common concentration in most systems?

How would I know if I could do less than that?

Here's another whopper:

When the system was first installed, no one thought to install the carbon into the tank. The installer blamed the supplier, and the supplier blamed the installer.

It ran this way, at perhaps 24ppm, for two weeks (maybe at even 3 times that concentration for a few days).

Any ideas what kind of damage was likely to have been caused to the rest of my system with the high chlorine concentration, esp. to the resin in the softener tank?

If the resin fails, will it be obvious to me, perhaps in the shower?

Thanks again!
Thanks for doing the calculation with a calculator, I was doing it in my head in the middle of the night.

The softener resin can handle a certain amount of resin, over time, the exposure is cumulative, so for arguments sake, lets say that standard resin can handle .5 PPM for 1 million gallons, Not knowing what type of resin you have, I am not sure of its total tolerance. A simple calculation and you can see that you may have used up a few months of the softener resins life in a week or two. Fortunately, resin is fairly inexpensive and easy to replace Assuming you get no more Chlorine into it, then it is only subjected to normal wear and tear which include sudden pressure changes, backwashing (physical damage), etc. All of these cause additional wear to the resin, but for the most part, they are almost nothing compared to oxidative damage. I would not worry about it considering how minimal the exposure was.

5. ok, that's nice to hear.

I really didn't want to make time to learn about changing the resin right now.

Thanks.

6. Originally Posted by hcw3

- the well is 47 feet deep
- "10GPM" is marked under the well cap with a 6" casing
- the water level in the casing is 5 to 8 feet from the surface of the ground
- the pressure tank is @ 4 feet below ground level
The 10 gpm is the recovery rate gpm. You can take that 10 gpm out of the well and the 5-8' static water level in the well stays right there. You never get to use the rest of the water stored in the well which is all that is above the inlet of the pump. That is between the motor on the bottom and the wet end on top of the motor.

Originally Posted by hcw3
The plumber ordered an 8GPM 1/2hp submersible with a 20gal pressure tank. 8GPM was out of stock, so the plumber got the 12GPM 1/2hp - I questioned it at the time, though my knowledge is nil about these things.
An excellent choice for a pump for your well.

The 20 gal tank is not but... it is the norm for guys that don't know what they are doing or... like to have service calls for burnt out pumps and broken bladders over the years.

Originally Posted by hcw3
The new submersible hangs @ 10 feet from the bottom of the well - 25 to 30 feet below the surface of the water.
There is 1.47 gals/ft of water in a 6" casing/well.

You are not wanting to use any of that storage, which you should, otherwise the water gets stagnant. You could draw the well down to say 5' or less above the pump inlet every day and cause few to no problems. That's how all wells are operated, the water level is dynamic and goes up and down; not usually to only 5' above the pump though.

You just need a larger pressure tank that allows enough gallons of water to be used (draw down gals) from the pump shutting off until it turns on again; minimum of 60 secs off. That would be roughly 12-15 gals draw down but any pump supply or plumbing supply house that sells pumps and tanks can tell you what tank for a 12 gpm 1/2 hp submersible pump. Well unless the recovery rate is higher than the max draw down gals used. Fifty feet and more is quite normal.

Originally Posted by hcw3
We now have a brand-new 12 GPM pump (the cost of which I've deducted from my final payment to the plumber), and a new 20gal bladder pressure tank.
Pay the man for the pump 'cuz he was/is right. See about the tank below.

Originally Posted by hcw3
If we were starting where the plumber should have, trying to figure out what pump to put in: the well recovery rate was rated at 10GPM by the driller, doesn't it make sense to put in a pump that pumps no more than 10GPM (or even 8GPM because of the well's age), and then size the pressure tank to match the pump's capacity?
Nope, see above and stay with the pump you have and pay for it but...

I suggest you go after him for the right size tank at his cost for the time it will take him to swap out the smaller tank for the correct size. And pay him the difference for the larger tank's cost.

He shouldn't need any different fittings than he has used but... if your tank is buried in a pit without space for a larger tank, yer stuck with the smaller one than required/strongly suggested by the pump manufacturer.

Originally Posted by hcw3
A couple of reasons I'd prefer starting by replacing the pump, rather than the tank:
- The existing 12GPM pump will pump the well dry if it runs any amount of time, judging by the driller's capacity rating.
- Isn't it easier to pull the pump and reinstall a new one, than it is to disconnect/reconnect all the plumbing that's necessary to remove and replace the tank?
- doesn't the draw-down measurement depend on the pump being sized right?

One other question. You're now mentioning a 60 second OFF time with the pump cycle. This conversation started with the question being about a minimum 60 second ON cycle. What amount of time should the OFF time be?
The pump would have to pump the 10 gpm recovery rate plus a few more gallons for a fairly long time before the well will go dry with 10 gals running back in the well each minute you are using water.

The pump does not cool off when it is running, only when it is off. Unless it is off'n on every 15-20 secs and it warms up the water around it.

Originally Posted by hcw3
You're welcome.

About your chlorine dose, most chlorine systems are setup to provide a residual of .2 or .5-1.5 ppm of free chlorine after the retention tank. Free chlorine is what's left of the total chlorine after the chlorine demand has been met. If you don't maintain that residual and the demand for chlorine rises, you don't oxidize all the iron etc. or kill all the bacteria in the water.

IMO 20 something ppm of chlorine for a couple weeks would seriously damage most if not all resins. I would insist on having the dealer/installer/plumber replace it at their cost since he didn't order the mineral for the filter he ordered.

7. Originally Posted by Gary Slusser
The pump does not cool off when it is running, only when it is off.
You are wrong on that point. The pump does not have to be off to cool. It cools while it is running and should run for at least a minute to cool from the heat of starting. A larger tank is not to extend the off time, it's to extend the on time.

8. I agree the tank is not big enough. It's a common mistake made by drillers and plumbers alike. I suspect because they are afraid to try and sell a larger tank because of the increased cost. Bigger tanks let the pump run longer during the cycle and give you more draw down between pump cycles.

9. Gary, thanks for the clear and very detailed explanation.

I understand. And I'm learning humility... 8-) I'm glad you got through to me before I went and picked up a new pump.

So in this case, the drawdown would be continually increasing, because the pump gpm is greater than the replenishment gpm. To get a drawdown measurement, what's the right time while pumping to take the measurement?

10. Originally Posted by LLigetfa
You are wrong on that point. The pump does not have to be off to cool. It cools while it is running and should run for at least a minute to cool from the heat of starting. A larger tank is not to extend the off time, it's to extend the on time.
Really?

The only cooling when the pump runs is when water flows past the motor/pump.

In many cases, like in rock bore wells, there is no cooling when the pump is running because there is no flow past the motor.

Any pump manufacturer will tell you the pump has to be OFF for 60 secs minimum for up to 1.5 hp submersibles and OFF for a min of 120 secs for larger hp pumps.

Anything using electrical power heats up when turned on, that includes submersible pump motors. Anything electrical cools off when turned OFF. Check it out and prove me wrong if you can. Let me know what you find out.

Question, why does short cycling burn up pump motors? Short cycling is having the pump go ON too soon or frequently. That's because the most heat is generated when the pump turns ON, no heat is generated when the pump is OFF and a hot motor cools when it is turned OFF, or do you disagree with that?

If so then explain why the draw down gals of the tank is used to size the tank based on the 60 sec output of the pump.

When the pump is running, draw down gals are refilling the tank with excess flow from the pump. Excess flow based on the demand for water in the building. You don't get the draw down gals into play until the pump is turned OFF. That's the only time the draw down gals are used.

We could get rid of the pressure tank if we would control the pump with a flow switch and turn on the pump as we open a fixture and off when we stop using water, right? Well that doesn't work well and burns up pump[ motors and that's why pressure tanks where invented, and especially bladder tanks; so the pump could be turned OFF to cool the pump motor.

A CSV works with very small tanks because the stored water prevents the pump from coming on for each itty bitty water use AND the pump runs to refill the draw down gals after the water use stops. That's so the pump doesn't come ON before the motor can cool.

11. Originally Posted by hcw3
Gary, thanks for the clear and very detailed explanation.

I understand. And I'm learning humility... 8-) I'm glad you got through to me before I went and picked up a new pump.

So in this case, the drawdown would be continually increasing, because the pump gpm is greater than the replenishment gpm. To get a drawdown measurement, what's the right time while pumping to take the measurement?
NO.

The draw down gallons are the gals you get out of the tank from the time the pump shuts OFF until the pump starts again.

If the total draw down galls are fewer than the 60 sec run time of the pump (the out put of the pump in 1 minute), that causes starting the pump again in less that 60 secs and that is called short cycling.

Short cycling burns up pump motors because the motor has not had sufficient time OFF to cool before starting again. So the larger the tank, usually the larger number of draw down gals it depends on the cut out (turn off the pump pressure switch setting), the longer the pump is OFF to cool before being restarted.

The higher the pump operating pressure switch settings, the faster the draw down gals are used and the sooner the pump comes on again. So, you get more draw down gals with lower switch settings and less with higher switch settings. Higher pressure moves more water faster and the pump comes on more frequently, generating more heat, hence it takes longer to cool it OFF, so you need a larger tank (more draw down gals) to keep the pump OFF longer or... to lower the switch settings but that usually will not have the pump OFF for the minimum 60 secs all pump manufacturers call for.

12. Gary, you got yourself all turned around. Pumps are sized to the fill time of a tank, not the drawdown time. The drawdown time is a wild variable, highly dependent on use. The fill time is fairly constant when there is little or no use.

As for a CSV, how would it, when coupled with a small tank and it's limited drawdown, provide a minute of "off" time for the pump? A CSV extends the fill time, not the off time.

13. Well you might size a pump that way but the right way is to size a pump based on the peak demand gpm of the building and the total dynamic head (TDH) of the system.

You select a pump that can deliver at least the peak demand gpm required by the peak demand flow rate of the building and then the hp required to do deliver that gpm.

Then you size the tank to be able to allow the pump to be off for a minimum of 60 secs for up to 1.5 hp and 120 secs for larger hp pumps up to I think 5 hp.

Head (TDH) is the operating pressure the pump will be run at plus the depth of the water level the pump will have to pump from; meaning the maximum depth the water may fall to in the future and the height of the highest fixture in the building, and the pressure loss of the check valve, drop pipe and all tubing in the building along with all the fittings in the well and plumbing including any valves used in the plumbing. And a good pump guy will usually add some gpm for future additional water use if there is a possibility of that. Now the building owner may nix buying the correct pump but that's on him or her.

As to the CSV, it delivers the gpm being used and anything over that refills the tank bit normally it fills the tank after the water use stops. You need to read up on its operation but, the 4.4 gal tank's (or the regular "20 gallon" most people have as the OP here) draw down provides for small flows before the pump comes on, otherwise, if the tank were refilled while water was being used (like all other pressure tanks) the pump would turn on every time a faucet was turned on. If the draw down gals are the same as the gpm delivered to the pressure tank by the output of the pump or higher, the pump stays off for the required 60 secs because 60 secs is one minute and gpm means gallons per minute.

A shower head is usually limited to 2.5 gpm (in the US), there's more than a minute off. Same for flushing a toilet or getting a glass of water or brushing teeth, rinsing a cup/glass or plate etc..

So back to pressure tanks.... that means the tank draw down gallons has to be at least the same as the delivered gpm of the pump at the tank or the pump will be coming on prior to 60 secs (1 minute). BTW most if not all well pumps are rated continuous duty, meaning you can turn them on and never shut them off BUT, turn them on and then off and then on again without letting them off for at least the minimum 60 or 120 secs and you will burn up the motor eventually. Short cycling has nothing to do with how long the pump runs. It could be on 24/7 until it died and that would be ONE cycle and there's nothing in that about short cycling. Short cycling means turning it ON too soon after it shuts off.

14. Just my two cents. Pumps have to be off for a minimum of 1 minute to cool down properly after running at max amps. Starting a pump before it cools down is like starting an overheated car engine. The split second the pumps runs before the thrust bearing gets lubrication will cause a hot thrust bearing to shave off a layer. The motor won’t last long this way.

A pump also has to run for at least a minute to dissipate the heat caused by the inrush of amps from an across the line start. So the pump needs to be off for a minimum of 1 minute and on for a minimum of 1 minute. These minimums are designed to just make the pump last through the warranty period. Longer run and longer off times will increase the life of the pump/motor.

In a rock well where the pump is being fed from the top, the motor actually needs to run longer than a minute after an across the line start. This is because it can only dissipate the heat from the top end of the motor, close to the pump, where there IS some flow.

However, all of this changes when using a CSV. The CSV makes the pump start against a closed or almost closed valve. This greatly reduces the inrush current on startup. As a matter of fact it makes the motor start under a no load condition, as if it wasn’t even attached to a pump at all. This is a recent discovery. We are in the process of graphing the inrush amps of pumps started under three conditions. The first is the normal full amp across the line start. The second is starting the pump against a closed valve as with a CSV. The third way is when the pump is slowly “ramped up” as with a Variable Speed Pump or VFD.

What we find is the old across the line start causes the motor to pull 6 to 9 times the running amps for a split second. When used with a CSV, the startup amps are basically the same as starting the motor without the pump attached. So the startup amps are barely higher than the running amps. What is fascinating is that the startup amps when ramping up slowly with a VFD, are higher than start up with a CSV, but still less than full across the line starts.

What this means is you don’t need the minimum of one minute of run time, when running the pump on a CSV. Since there is no inrush current, the motor doesn’t have to run for a minute to dissipate extra heat. Although with a CSV, the pump always runs a minute to put 1 gallon in a pressure tank anyway.

With a CSV, the pump never shuts off when there is more than 1 GPM being used. So basically the pump never shuts off while you are using water, as it does when not using a CSV. You don’t need much of a tank to keep the pump off for a minute, when there is no water being used.

Also when the CSV is filling the tank at 1 GPM, the motor amps are reduced, basically de-rating the motor. If you are not using the full amount of water the pump can produce, the CSV is de-rating the motor even while you are using water. No matter if you are using small amounts of water or if the tank is filling, the CSV is de-rating the motor, making it use less amps and run cooler. With a CSV, the pump no longer needs to run for a minute or be off for a minute. This is because there are no inrush amps on startup, and the motor is de-rated (running cooler) when supplying you water or when filling the tank.

Now having said all that, which proves you only need a 4.4 gallon tank that holds 1 gallon of water when using a CSV, chlorine injection adds another wrinkle. When the CSV varies the flow to match the amount of water you are using, the amount of chlorine injected also needs to vary with the flow rate. That Chemilizer HN55 1-128 ratio pump is a good way to go. Thanks to dittohead for that. And there are other injectors that will vary according to flow and can be used with a CSV.

My problem with chlorine injection is contact time. The chlorine needs to be in contact with the water for 15 to 20 minutes to do what is needs to do. A bladder tank only has one inlet/outlet connection, and doesn’t hold much water. So the water with injected chlorine can go right past the tank and straight to the house without the proper time to work.

With a galve tank the water goes in one side of the tank and out the other. At least the chlorine can’t come straight to the house. But I still think the tank needs to be large enough and hold enough water to give the amount of chlorine contact time needed. That means if your house can use 10 GPM, then the tank needs to hold 150 gallons to give 15 minutes of contact time.

I know nothing about softeners and very little about water treatment. But I believe a larger tank is needed when injecting chlorine. I have also bushed up to a length of 12” pipe underground, so it holds enough water to give the proper chlorine contact time when the tank was too small. Sorry for the long post.

15. Thanks valveman, for the clear and concise explanation. WRT to contact time, chlorine-to-water ratios, and variable flow, I'm thinking that if one had a 150 gallon contact tank, that the ratio would average out as long as there wasn't long term low flow usage.

WellMate makes composite retention tanks, so galvanized is not the only option.
http://www.wellmate.com/en-US/produc...onnect-series/

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