Help With Pressure Tank Selection Needed....Please

[w30bob]

Hi Guys,

Looks like a great forum you have here.....maybe you can help me out. I want to replace my well pressure tank but need help sizing a new one. My current tank is original to the house, which was built in 1977. The label on the tank says;

Medalist Manufacturing Co
Posi-Pressure II, Model PP2-42
36 gal capacity
Draw Down = 11.6 (20/40), 9.5 (30/60)
16” Dia, 50” H
150 psi working pressure

I can't find any info on the web for this tank or the company that made it, but from the looks of it I don't believe it has an internal bladder or diaphragm............I think the air pressure acts directly on the liquid inside the tank, but I don't know what you call that type of pressure tank. There's a Schrader valve on the top of the tank, but that's it. The tank itself is a single piece of steel, not two half tanks like most bladder tanks. Regardless, the tank loses pressure within a few days of me setting it........so I'm going to replace it. I checked the Schrader valve and it doesn't appear to be leaking, but even if it has a very slow leak I don't know what the internal condition of the pressure tank is and want to replace it (along with the hot water heater, which is also original to the house). I'd rather replace these things now when it's convenient for me and not when one of them fails and makes a heck of a mess (usually late on a Sunday night when we have friends over).

When the house was built water demand was from 2 full bathrooms, kitchen, washing machine and 2 outside water spigots..........so I assume the tank was sized for that demand. Sometime before I bought the house an addition was put on, which added a third full bath and another outside water spigot. We're a family of 3 people with frequent guests visiting on weekends, so I want to make sure I size the pressure tank correctly. My well pump is located in the well, and I don't have the specs for it. So based on the tank label info and my current water demand..........can you tell me how to determine the correct size tank to use? If not what other info do I need?

I'm leaning toward a Flexcon Flex-Lite composite tank from what I've been reading. Does anybody think that's a bad idea? Oh, I'm a bit limited in the space available to put the tank, so I really don't want to go wider than 16" diameter............but I can cross that bridge after I figure out what capacity tank I need.

Thanks in advance.

regards,
bob

 

[Valveman]

Tanks are normally sized to the pump, not the demand. A 36 gallon tank holds about 8 gallons of water at 40/60. And I really think it is a bladder tank of some kind as you have a Schrader on top of the tank and didn't say anything about another fitting on the tank for an AVC. It has lasted a lot longer than most, so going back with at least the same size tank would be recommended. However, that was a long time ago and there are better ways of controlling pump now than there was years ago. If space is a concern you could go back with a PK1A kit that only uses a 4.5 gallon size tank. The kit includes a CSV1A Cycle Stop Valve, which works with a much smaller tank. The whole kit will fit in a space 14" X 14" X 24". Not only does the CSV kit save a lot of space, but the CSV will deliver constant pressure to the house, which makes for much stronger pressure in the shower and other things. Tanks are only designed to reduce the number of pump cycles, not to store water. So when you have a CSV to eliminate the pump cycling the size of pressure tank is almost a moot point. The CSV makes water go right past the tank to the faucets, so it doesn't matter if it is a 1 gallon or a million gallon size tank.

 

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[Reach4]

Without a CSV, sizing is such that the pump runs a minimum of a minute each time it goes on. So a 10 GMP pump delivering 10 GPM would get a pump with a 10 gallon or more drawdown.

A pump may deliver more or less than its nominal flow, depending on things including the water depth compared to what that pump's design depth to water. Also, the pressure switch setting affects the flow rate. So while you see the tables saying the drawdown decreases with pressure, the effect of the increased backpressure is to decrease flow. So the fill time may be more or less with a given pump and given tank at higher pressure.

There is a procedure for measuring the drawdown of an installed tank and the GPM of an installed pump into the pressure tank. It involves running water until the pump just kicks on. Start your timer and turn off the water faucet immediately. Let the pump fill the tank fill without any water use. Time the run time. When the pump stops, turn off the power. Then measure the water available from a faucet. You know how long it took to fill the tank, and you know how much water it took. You can compute the pump flow. You can compute the drawdown needed to get at least a minute. If your tank is a "conventional" tank with no diaphragm or bladder, expect a blast of air at the end of drawing water. It could also give a blast of air if it had a failed bladder.

A conventional tank relied on a method of adding air into the incoming water, and releasing that w

Flexcon Flex-Lite has a diaphragm and is a well-regarded pressure tank from what I have seen. With a submersible pump, you will set the precharge to 2 PSI below the cut-on pressure.

http://www.flexconind.com/products/well-tanks/flexlite-well-tanks/
Dimension B is diameter, A is height, and C is the nominal pipe thread for the connection.

16 inch diameter is a tough one. I remember somebody saying that for the same drawdown, a wider diameter tank would tend to last longer. A CSV fits a small space.

 

[w30bob]

Hi Guys,

Thanks for the quick replies. After I posted my question I started reading other posts and learned of CSVs. Really didn't know that option existed. I was up to the wee hours of the morning reading Valveman's website. Very interesting stuff.........I need to read more. This will sound dumb, so no laughing, but forgetting about the "soft start" feature of CSV controllers for a second...........if my pressure tank isn't doing what it's supposed to do, and only acting as a reservoir of sorts.......wouldn't I have a CSV system now if I simply set my pressure switch to turn the pump on at 45 lbs and off at 50 lbs?

In regards to the regular pressure tank, it makes sense that wider is better than taller........as the diaphragm or bladder travels a shorter vertical distance for the same volume change if wider because the width is constrained by the tank......so less stress on the rubber. Looks like I got more homework to do.......I'll get right on it.

thanks,
bob

 

[Reach4]

wouldn't I have a CSV system now if I simply set my pressure switch to turn the pump on at 45 lbs and off at 50 lbs?

No. That would cycle the pump quickly. The CSV adds backpressure to limit the flow when the pressure is above its setpoint. Thus the run time is extended as the pressure tank is nearly full.

Also, a mechanical pressure switch cannot give such low differentials.

 

[Valveman]

No. That would cycle the pump quickly. The CSV adds backpressure to limit the flow when the pressure is above its setpoint. Thus the run time is extended as the pressure tank is nearly full.

Also, a mechanical pressure switch cannot give such low differentials.

Thanks Reach, good explanation above. Let me try this about your last quote. The CSV limits the pump output to match usage, and therefore the pump run time is for as long as you are using water. Only AFTER you stop using water will the CSV allow the tank to fill and the pressure switch to shut off the pump. AND then the CSV refills the tank at 1 GPM above the CSV set point instead of max pump putput, so you can get a minute of run time for every gallon the tank holds above the set pressure of the CSV. Whewwww! Mouth full! The CSV is such a simple little valve to have such a complicated explanation, sorry. But I think it is well worth people's time to figure out how it works, as it solves a lot of the everyday problems with pump systems.

One more thing. Your tank has 8 gallons of draw and you get 1 minute of run time with a 8 GPM pump BECAUSE it is working between 40 and 60 PSI. If you reset the pressure switch to 55/60 that same tank will only hold 2 gallons of draw and the pump will only run 15 seconds to fill the tank. You can do the 55/60 with a CSV, although it isn't needed, but not without a CSV unless you quadruple the size of the tank.

And they do make mechanical pressure switches that will go that low of a bandwidth. I use a lot of Allen Bradley 836C7J switches that will work as low as 2 PSI between on and off. But I only use those on systems with huge tanks like city water towers.

 

[w30bob]

Hi Guys,

Thanks again.....I think I get it now. I was afraid to look at the prices for the PSide Kick kits thinking they'd be incredibly expensive for all the features provided.......but was pleasantly surprised.........they're cheaper than what a large pressure tank would cost. Looks like an easy DIY installation, so I think I'm becoming a convert! I'll jump in head-first tonight when I get home from work and research my brains out........I'm an engineer, so I can't help myself. Hey........I saw that eye roll! I guess you really do learn something new every day. Talk to y'all later.

regards,
bob