Pump and pressure tank sizing for water to 4 homes

[msoultan]

Hi,
In our community we have two large water storage tanks (80k gallons each) that gravity feed all of the homes. For the most part everyone has good water pressure, but there are four homes that aren't much lower than the storage tanks and end up in the 20-40 PSI range with low flow rates (due to small pipes). We would like to increase the water pressure and flow rate to all four homes, so I was hoping for some advice. Here's what I think I figured out...

For the four houses, we're going to make the assumption that we want to be able to handle a peak demand of 50 GPM - this is probably on the high side, but we'd rather over-design the system than under-design it. Next, we'd like to make sure each of the homes has at least 50+ PSI. Obviously pipe size to the homes will affect this and we can up-size the piping if the increase in pressure doesn't fix the issues.

Here is what I think we need, but I'm wondering if I'm getting this correct - I'm thinking that we would want to have a pump that can supply that peak demand of 50 GPM. We would get a 30/50 PSI pressure switch which, in conjunction with a Flexcon 119 gallon pressure tanks, for example, would give us a drawdown capacity of 40.6 gallons per tank:

https://www.flexconind.com/wp-content/uploads/2021/09/Challenger-Spec-Sheet-LR.pdf

I was thinking that realistically we'd only need to handle a 40 GPM peak, so we could get a 50 GPM pump and throttle it down to 40 GPM - and if I'm understanding minimum runtimes and drawdown capacity, we would want at least an 81.2 gallon drawdown which would be two tanks. At a minimum we would have an additional 30 PSI per home.

I was thinking that we would design the system such that in the event we needed to handle more peak flow, we would already have the tank manifold set up to accept a third tank, and we could also bump up the pump GPM.

We have considered an in-tank pump, but we'd much rather have an external pump unless it's ridiculously more cost-effective to go with an in-tank pump.

It would seem like when searching for these pumps, they don't list the PSI output of the pump, but how many feet they can pump, so I'm assuming if I want to use 30/50 PSI setpoints, then I'd probably want to have a pump that can do 55 PSI, or a pump that can pump 50 GPM at 127.05 ft (2.31 ft/PSI * 55 PSI).

ADDITIONAL DETAILS:
- Max water level in the main tank is 19.1ft
- The proposed location for the pump is next to the tank because the water company owns the land and there is electricity nearby.
- If the pump is placed at the base of the tank, it would already have 19.1 ft of head, or 8.29 PSI - tank level usually hovers around 18.7ish ft, so le's say 8.1 PSI pump inlet pressure.
- There is a 2" pipe that runs to the four water meters, and the pressure at the meters is around 20 PSI, so that's about a 27.5ft elevation drop from the base of the tank.
- The water meters are located a couple hundred feet from the tanks and there is no power there.


So here are my questions:
- Am I calculating this stuff correctly?
- How do you go about calculating these peak usage values - in some regards I'm wondering if we'll ever even hit that peak water usage, but I also don't want to end up on the other end where it's under-designed. I was hoping for some real-world advice on sizing these kinds of systems.
- Should the pump be sized to match the peak demand, or should there just be enough pressure tanks to handle the theoretical peak demand for some given amount of time? So in the case of two 119 gallon tanks, we'd be able to provide 40 GPM for 2 minutes (assuming the pump wasn't helping), correct? Granted, once the tanks drained down and the pressure switch kicked on, the pump would be handling the entire load, and if the demand is higher than the pump's output, there will be a drop in pressure.
- If I am getting this correct and I need a 50 GPM pump that can push up to at least 127 ft (or 55 PSI), are there pumps that will provide this flow at the pressure we're looking for? If so, does anyone have any pump recommendations?
- What type of pump would we look for? Centrifugal seems to be a popular one, but I'm not sure if that's appropriate. I've also seen jet pumps and multi-stage pumps.

Thanks!!
Mike

 

[Valveman]

Like you said, pressure tanks cannot supply peak demands. With an in-line pump you can add the incoming pressure to what the pump can build. I would say you will have a minimum of 20 PSI inlet to the pump. That means the pump only needs to build 35 PSI to make the 55 PSI you want. 50 GPM is also way more than needed. 5 GPM per house is a rule of thumb. You also do not need any tanks for storage as you are drawing from huge storage tanks already. Just need a generator to keep the pump running during power outages.

I would use a Goulds J15S as it will do up to 25 GPM. All you need to control the pump is a PK1A kit with a 10 gallon pressure tank and a 50/70 pressure switch setting. If later you decide you need 50 GPM as more houses are added, you can just double up on this system and have some redundancy as well.

 

[Sponsor]

 

[msoultan]

Valveman,
Thanks so much for the info! First, I added a few additional details to the original post because your answer brought up some items that might be an issue:

ADDITIONAL DETAILS:
- Max water level in the main tank is 19.1ft
- The proposed location for the pump is next to the tank because the water company owns the land and there is electricity nearby.
- If the pump is placed at the base of the tank, it would already have 19.1 ft of head, or 8.29 PSI - tank level usually hovers around 18.7ish ft, so le's say 8.1 PSI pump inlet pressure.
- There is a 2" pipe that runs to the four water meters, and the pressure at the meters is around 20 PSI, so that's about a 27.5ft elevation drop from the base of the tank.
- The water meters are located a couple hundred feet from the tanks and there is no power there - well, there's power, but it's the resident's power, not the water company's.

Like you said, pressure tanks cannot supply peak demands.

I don't understand why you say this - if the assumption was that peak demand was 40 GPM on the low side, wouldn't two 119 gallon tanks be able to handle that load for two minutes?

With an in-line pump you can add the incoming pressure to what the pump can build. I would say you will have a minimum of 20 PSI inlet to the pump.

For the reasons stated in the additional details above, as much as I would like to put a pump where we're getting 20 PSI, doing so would be challenging - we'd most likely need to put the pump next one of the large storage tanks, so it would at least have around 8 PSI to the inlet of the pump. Otherwise we'd have to be paying one of the homeowners for their electricity, which I guess isn't the end of the world, but would like to avoid that.

That means the pump only needs to build 35 PSI to make the 55 PSI you want.

That being said, this is a really good point, and justification for putting the pump at a lower elevation - if I'm able to place the pump lower, I don't have to build as much PSI, meaning I don't need as big of a pump. I'm going to take a look at the site, talk to some people there, and see what we can do.

50 GPM is also way more than needed. 5 GPM per house is a rule of thumb. You also do not need any tanks for storage as you are drawing from huge storage tanks already.

Not only is this probably the most important data point for this whole project, but it was also the question I felt I was probably answering incorrectly and I'm really curious how you came to this conclusion? Is this just from experience working with these types of projects and you've found that household rarely exceed 5 GPM peak? Or that rarely would you see more than 20 GPM combined. Frankly, I'd be surprised to see all the houses use 50 GPM (or even 40), but I wanted to make sure I didn't underestimate and then we're having to retrofit the system due to poor planning.

Just need a generator to keep the pump running during power outages.

I was planning of T-ing into the original connection and the putting a swing check valve upstream of the pump - that way in the event that the power goes out, the houses still have water flow similar to what they have now.

I would use a Goulds J15S as it will do up to 25 GPM. All you need to control the pump is a PK1A kit with a 10 gallon pressure tank and a 50/70 pressure switch setting. If later you decide you need 50 GPM as more houses are added, you can just double up on this system and have some redundancy as well.

So, I thought that for pumps greater than 20 GPM you wanted to have at least 2 minutes worth of runtime handled by the pressure tanks - I'm assuming that's so that the pump isn't cycling on and off like crazy and has an opportunity to cool down? How does this solution mitigate that issue, or is it really a non-issue?

Thanks!

 

[Valveman]

- Max water level in the main tank is 19.1ft
- The proposed location for the pump is next to the tank because the water company owns the land and there is electricity nearby.
- If the pump is placed at the base of the tank, it would already have 19.1 ft of head, or 8.29 PSI - tank level usually hovers around 18.7ish ft, so le's say 8.1 PSI pump inlet pressure.
- There is a 2" pipe that runs to the four water meters, and the pressure at the meters is around 20 PSI, so that's about a 27.5ft elevation drop from the base of the tank.

If installing the pump at the tank with max 8 PSI coming in, I would size the pump for zero pressure to the inlet. That will require a muti-stage centrifugal or a submersible in the storage tank. You can get 25 GPM at 60 PSI with a muti-stage 25GB10, 1HP.

Or a 25GS10, 1HP submersible.

 

[Valveman]

- The water meters are located a couple hundred feet from the tanks and there is no power there - well, there's power, but it's the resident's power, not the water company's.

You could also install a little 1/2HP, J5S, with a PK1A at each house, and each house would have its own booster.

 

[Valveman]

I don't understand why you say this - if the assumption was that peak demand was 40 GPM on the low side, wouldn't two 119 gallon tanks be able to handle that load for two minutes?

Sure, if the tanks were not empty before the large demand occurred. But pressure tanks are completely empty before the pump comes on. If the pump cannot supply peak demands, an empty tank is not going to add anything. The large tank actually becomes an additional demand, as the pump is trying to refill the tank at the same time as supplying the demand.

 

[Valveman]

Not only is this probably the most important data point for this whole project, but it was also the question I felt I was probably answering incorrectly and I'm really curious how you came to this conclusion? Is this just from experience working with these types of projects and you've found that household rarely exceed 5 GPM peak? Or that rarely would you see more than 20 GPM combined. Frankly, I'd be surprised to see all the houses use 50 GPM (or even 40), but I wanted to make sure I didn't underestimate and then we're having to retrofit the system due to poor planning.

Cities use 1.6 to 3.5 GPM per connection, depending on the water storage and supply available. 5 GPM has been my rule of thumb for 50 years. Rarely if ever is every tap in the house open at the same time. The biggest problem with booster systems is that they are usually oversized, causing cycling, excess energy use, and more expensive maintenance. Don't know how many 50 GPM booster systems I have installed that never use more than 10-15 GPM. The two pump system is less expensive, more efficient, and gives redundancy. It will also give you 50 GPM for peak if that is ever needed. The pumps I figured will do 25 GPM at 60 PSI, which requires the CSV to be set at 60 PSI while using a 50/70 pressure switch setting.

Oh, and BTW, glad to help here, but you can call the company if you have more questions. 806 885 4445

 

[Bannerman]

Valveman, as msoultan has posted only 1X in this thread, where are his/her additional comment quotes obtained from?

 

[Valveman]

I approved the thread. New thing forum is doing. Thought everyone could see it. Don't see how to approve it again? Need a mod that knows more about it than me.

 

[msoultan]

Sure, if the tanks were not empty before the large demand occurred. But pressure tanks are completely empty before the pump comes on. If the pump cannot supply peak demands, an empty tank is not going to add anything. The large tank actually becomes an additional demand, as the pump is trying to refill the tank at the same time as supplying the demand.

I thought that was the whole purpose of setting the tanks 2 PSI below the cut-in point - that way water is still being supplied while the pump is starting up:

Why is the recommended pressure for a bladder tank 2 psi less that well pump activation pressure

If I want the well pump to start less frequently, why should NOT the pressure of the bladder tank be GREATER than the well pressure start setting. I have a 30-50 well pump pressure switch which ki...

diy.stackexchange.com

Also, if the pump is sized to handle peak demand, then I'd assume it would supply that peak demand to the homes while the tanks were empty - and since the tanks are pressurized they wouldn't fill up until the demand dropped such that the tanks could be filled -am I missing something?

 

[msoultan]

You could also install a little 1/2HP, J5S, with a PK1A at each house, and each house would have its own booster.

Yeah, that could be an option. I think I need to look at the costs and see which method would be more expensive and/or provide better service - handling it centrally or at each home - but I think it's worth looking into.

 

[msoultan]

Cities use 1.6 to 3.5 GPM per connection, depending on the water storage and supply available. 5 GPM has been my rule of thumb for 50 years. Rarely if ever is every tap in the house open at the same time. The biggest problem with booster systems is that they are usually oversized, causing cycling, excess energy use, and more expensive maintenance. Don't know how many 50 GPM booster systems I have installed that never use more than 10-15 GPM. The two pump system is less expensive, more efficient, and gives redundancy. It will also give you 50 GPM for peak if that is ever needed. The pumps I figured will do 25 GPM at 60 PSI, which requires the CSV to be set at 60 PSI while using a 50/70 pressure switch setting.

Oh, and BTW, glad to help here, but you can call the company if you have more questions. 806 885 4445

So I'm a bit confused that you say only 5 GPM per house as that seems a bit low - just one garden hose could flow that much, and add that to someone using the garden hose and a couple other fixtures in the house and you're well beyond 5 GPM per house. I saw in another one of your posts you recommended 5 GPM per bathroom:

How to select the well pump size

Hi All Does anyone have good rules of thumb (or real guidelines) for selecting the proper size well pump, pressure tank and how deep to put the pump? Also - comments on VFD versus a simple pressure switch with tank would be appreciated and any other suggestions or recommendations. Our new 2...

terrylove.com

Did you mean 5 GPM per bathroom instead of per house?

 

[Bannerman]

confused that you say only 5 GPM per house as that seems a bit low - just one garden hose could flow that much

Yes, but are all 4 homes likely to be using more than 5 GPM at exactly the same time? Many people expect they require way more flow capacity than is actually utilized.

Valveman said: "Cities use 1.6 to 3.5 GPM per connection" This is a reasonable method to estimate supply when there is a larger number of connections, otherwise, pumps and water mains would need to be massive if sized to provide the maximum demand to each connection at the same time.

5 GPM is often utilized for a single home supplied from a well. Because your installation will be supplying 4 homes, calculating an average 5 GPM each should be reasonable for household use. Each home is currently able to access ?-GPM at 20 psi, so the pump system will only need to boost the pressure, thereby increasing the potential flow rate available to each.

Valveman also stated: a "two pump system is less expensive, more efficient, and gives redundancy. It will also give you 50 GPM for peak if that is ever needed." The higher peak flow maybe an issue if irrigation at multiple properties at the same time is likely.

Suggest installing a CSV system using the small pressure tank mentioned, along with one 25 GPM pump with the pressure switch set to 50/70 psi. The CSV will then deliver 60 psi constant once the pump becomes activated. If it is found that 25 GPM is insufficient, then a 2nd identical pump in parallel may be added, utilizing a 45/65 psi pressure setting.

 

[Valveman]

Garden hoses or any outside water use should be added to the 5 GPM per house. Read further down on the thread from a decade ago and I said 10 GPM is way more than enough. I was trying to prove the point that when using a Cycle Stop Valve you can use as large a pump as you think you need, and the CSV will make it work like a small pump when only a small pump is needed. And you can. But experience tells me on a system with 4 houses, a secondary 25 GPM pump will rarely if ever even come on. We normally even add an exercise clock to force the second pump to come on for a couple minutes each week. Otherwise the pump is rusted stuck and/or the water in it green when the time finally comes that a spare pump is needed.

5 GPM for each house turns into 7 GPM when needed, as the pump can supply that much more during peak events, although the pressure drops a little. That will run 2 showers at the same time and still have some left if a washing machine is filling or a toilet is flushed. But with a 20-25 GPM pump you have a lot extra as long as the neighbors don't all time their showers to exactly the same time as you. Lol!

 

[Bannerman]

since the tanks are pressurized they wouldn't fill up until the demand dropped such that the tanks could be filled -am I missing something?

Simplified explanation using your proposed 30/50 pressure setting, 28 psi PT pre-charge and 50 GPM pump capacity.

Water use will cause the system pressure to gradually fall to 30 psi whereby the pump will become activated. With the 28 psi PT pre-charge, there will be a small quantity of water remaining in the PT when the pump is activated @30 psi.

When water consumption is less than 50 GPM, the pump will supply greater capacity than is being consumed, thereby causing the excess capacity to fill the PT until 50 psi is achieved, causing the pressure switch to shut off the pump. Continued water use will be initially supplied from the PT until the system pressure again falls to 30 psi whereby the cycle will be repeated and will continue for as long as water continues to be utilized. The only way the pressure tank will not fill fully during water use, will be to ensure the pump's maximum capacity (50 GPM) is always consumed, thereby preventing the system pressure from rising to 50 psi.


A CSV restricts the flow from the pump when consumption is less than the pump's capacity. The CSV maintains a specific constant pressure downstream, so the pump will always deliver the exact amount of water that is being consumed. With a 50/70 pressure switch setting, the CSV will provide constant 60 psi to each home, and the PT will only be filled ~50% (to 60 psi) until there is no further water needed. Once water consumption is less than 1 GPM, the CSV will continue to allow 1 GPM to pass, thereby causing the system pressure to rise to continue to fill the PT until the system pressure is 70 psi whereby the pressure switch will shut off the pump.

The recommended 10-gallon pressure tank, will store only ~2.5 gallons, so once any water begins to be consumed, the system pressure will quickly fall from 70 to 50, thereby causing the pump to be rapidly activated so it can then supply 60 psi constant for the entire remaining time water continues to be consumed.