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Information seems vague or non-existent from pump manufacturers. I need to know what size pressure tank is required for my setup, and all I'm getting is "doesn't need much" or "x gallons" and nothing about draw down.

My Setup
I have a 38ft deep well and the submersible will sit at about 35ft deep. I will be pumping 30-40gpm at max demand (when irrigation is on etc.). We are going with a 230v, 3 phase, 2 Horse Power Pump. PVC drop pipe (2") and 200ft of 1 & 1/2", 200lb poly pipe to the house where the controller and pressure tank(s) will live.

A local supplier says a single 7 gallon tank is fine. Another says get 2x7 and the a 3rd supplier says 2x5. No one seems to know why they hold those opinions.

I'm lost!
Shouldn't I have enough draw down for my pump to spin up (assuming it takes it a second or two?) and keep a constant 35gpm and 40-50psi? How do I know how much time it will take the pump to catch up to demand? Now, I don't even care about not enough flow to instantly run the entire irrigation system. It's more, do I have enough to:

a. Not damage my pump or pipes or appliances (this is #1 consideration!)
b. Not be annoying if I turn the shower on (not having enough pressure for a minute)

NOTE: I do NOT need advise static water level, water rights/local laws etc. I can legally pump 35gpm and my well could handle 100gpm or more all year round.

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  • Larger is never a problem. How small you can get away with is a matter of the variable speed controller, and you seem to have not mentioned that at all, other than in your title. If you are unsure or don't mind the space taken up/expense, just get a bigger tank. It's literally never a problem to have more tank than the minimum you "need" - for a pump without variable speed/constant pressure control, you'd want a drawdown of 35-40 gallons, which might take a couple of 80 gallon bladder or diaphragm tanks. VS/CP control does drop the minimum size a lot. – Ecnerwal Apr 8 at 20:31
  • It's based on the controller for the variable speed pump? Maybe i'm putting too much emphasis on the pump itself and it's really the VFD that does the work of spinning the pump up/down responsibly. Space actually is a bit of a consideration here. It's all planned for wall mount in a utility closet. I was just going to get two, 7 gallon ones. But, seems like they aren't even made right now / no one can get them because - "Covid". So, I was going to just do 2*5 gallons. Just not sure if it's enough. I guess I probably have space for a 3rd if it's not. – maplemale Apr 8 at 21:23
  • The normal arrangement for a variable speed pump is a controller with a pressure sensor. Since the pressure sensor controls the start, stop, and speed, yes it normally depends on (and is documented in the manual for) the controller. I've not opted to buy one for my setup, but as far as I recall for my 7 gpm pump they claimed a 2 gallon tank was sufficient. Mail order I have not seen much of a tank shortage. – Ecnerwal Apr 8 at 22:04
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Maintaining 40-50 psi is the key. Yes you can do it with a 7 gallon bladder tank and it would be better to go to a 35 gallon bladder tank but depending on your flow you could set your pump to run at 80% all day and maintain 50 psi so you really don’t need much of a pressure tank in that case.

If your pump is producing 40 gallons at 70% then you want a big tank to reduce cycling of the pump.

Water cooled 3 phase pumps can start more often than single phase pumps because 3 phase start better than single phase with a capacitor.

You don’t want to cycle your pump because this is where the cost $$$ to run comes in. Remember a motor draws 3-5x normal full load amperage at startup and even higher amounts called locked rotor amperage until they start turning. Starting often can double your cost to run so that 35 gallon tank provides just under a minute were 7 gallon only a few seconds.

Will it hurt your system to go big on the tank if it would run at 70% 50 psi nope won’t hurt anything but it provides a buffer if a head gets plugged so you are not pumping enough for constant run.

So go big with the pressure tank it will help in most cases and won’t hurt. Note I did not talk about dual set points 50 or 60 psi pump drops to 70 then at 40 it jumps to 120% never stopping lowest power consumption and the pressure tank being big helps maintain control. Of pressure because there is a slight lag from pressure reading, pump commanded up to pressure increased.

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    I agree with most of what you said, Ed Beal. But for an individual residence, I'd just "GO BIG" on the pressure tank to minimize pump cycling....again depending upon type of pump, but I agree, cycling a 2HP pump often isn't a good idea. BTW, who the heck has 3 phase power at a residence? Not very common. Is this some sort of mansion? A water right for 35 GPM?!?! with a 100 GPM well? I think we are missing some information. – George Anderson Apr 8 at 21:46
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    Irrigation tends to imply farm and farms often have 3-phase if it's available. – Ecnerwal Apr 8 at 21:54
  • There's also the possibility of a 1 phase to 3 phase VFD, as researching my answer reminded me. – Ecnerwal Apr 8 at 22:09
  • @George I thought I said to go big 3-4 times but it would not be right to mention a small one could be used if everything was at a sweet spot on the pumping curve. 2hp submersible can be cycled 4-6x what a standard TEFC motor can but it’s not heat but power that kills you. I had a 30hp pump on original water rights for my 1930 farm but the power was getting almost as high for that pump as my home and the horse barn (they wanted to get rid of the remaining 230v 3 phase and since I only have a portion of the original property it was flood irrigation). I had 100 gpm water rights at a rural farm. – Ed Beal Apr 8 at 22:12
  • @EdBeal This all makes sense and helps me understand. Though, I still don't get why the manufacturers aren't referencing draw down as it seems to me, that's what really matters. – maplemale Apr 8 at 22:52
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As you have still not clarified what VFD you have or plan to buy, I'm looking at the documentation for the Goulds Aquavar Solo2, which will power a 2 hp 3phase pump. Link is via a vendor site I have purchased (other things) from but have no other association with.

In Table 1 on Page 4 it specifies:

For pumps up to 36 gpm a 7.3 gallon pressure tank is reccomended. Up to 70 GPM (if your 40 applies, rather than 35) 13.9 gallon is recommended.

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  • I believe those are the minimum sizes as a larger tank won’t be a problem. – Ed Beal Apr 8 at 22:19
  • ...as I already commented, but then a reply comment said space was an issue. And I realized that my reply to that comment was headed answerwards, so I wrote one. – Ecnerwal Apr 8 at 22:20
  • Thank you very much! That's actually super helpful. I have a friend who's done this before helping (but is just saying, use the biggest I can get) and I believe that IS actually the exact VFD he has ordered for me. I guess what is confusing me is the draw down. I would think the draw down is what truly matters here but i'm not understanding something because it doesn't seem to. The documentation you listed even states "Use Total Tank Volume, not drawdown volume, to select the proper tank size. The total tank volume should be approximately 20% of the pump’s maximum flow." – maplemale Apr 8 at 22:48
  • Draw-down is for on-off control, not variable-speed control. For a new install, the savings on huge tanks you don't have to buy can actually pay for the variable speed controller. – Ecnerwal Apr 8 at 23:03
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WOW, that's one heck of a well! You got lucky! There are some types of pumps that are variable speed and ramp up and ramp down due to demand They are good for community water systems with many users. For an individual user, I'd go with a large pressure tank (like 60-80 gallons) to avoid short cycling of the pump (depending upon type of pump). I think you need to find better pump guys.

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  • Home-scale variable-speed constant-pressure systems have been in the market for more than a decade, and gaining ground. Not just for "systems with many users." – Ecnerwal Apr 9 at 2:45
  • Shoot! To everyone: I missed the part in the title about a variable speed submersible pump. DOH! so yeah, with one of those you don't need much of a tank at all, as long as you have a very productive well that can meet on-demand needs, which it seems the OP has. and to Ed Beal.....I didn't mean to say you didn't say "go big"....just echoing your sentiments. Take care all. – George Anderson Apr 9 at 8:55

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