Drinking Water Well Pump Control Strategy

[djcbgn]

Hi,

I am working on a very large development project (2000 homes at buildout), for which the city is requiring the developer to install wells to provide water supply. The developer does not want an elevated tank on the site and there is a connection to the county system for emergency water demands. I am asking for input on controlling the well pumps by system pressure or another option other than tank level. They are performing test wells now so I dont know the production capacity of each well yet but I expect each well to produce around 150-200 gpm and there will be several.

I dont have experience with hydropnuematic tanks so I was looking for some guidance here for sizing.

Thanks for your help.

 

[bimr]

The proposed water supply scenario is too large for a hydropneumatic tank system.

Could you explain how the county system will supply emergency water demands?

 

[jhnblgr]

A system of this size should be required to have storage tank just to meet fire flow and pressure requirements. Hydro pneumatic tanks are a poor idea in general. Ideally the development should provide a backup water supply to the municipality and become part of the district. Utilizing the existing system for storage. But likely this isn’t an option. The system needs to be designed to the municipal system pressure or have a pressure regulator installed and flow meter. You will need a whole treatment system for the water based on local regulations. The only other option is a booster pump system to pressurize the system but that is a really bad idea. The developer is always going to take the cheapest route and likely the shortest lifespan and with years of fixing systems like this because they end up in disrepair and burned out leaving the home owners with the bill and hating engineers.

 

[LittleInch]

Well first you need to work out your max and min demand and what you think the cycles are. but your demand is likely to be 10 or even 20:1 turndown.

For 2000 homes the flow is unlikely to ever be zero so you will need at least one or two wells running at all times. then for a network you will need to work out where the inputs are from the wells and where the lowest pressure will be in the network.

Forget any sort of micro hydropnematic tank. That's for single dwellings each with their own water pump. You're in a completely different league with 2000 houses.

Then you can figure out a cascading start for the wells so as pressure falls more wells start up. You probably need to program in a sequence of which wells operate all the time so its not the same one each time.

but this will cost money and effort to do so is normally why a simple elevated tank with level control provides some spare capacity of those peak 15minute periods in the day when everyone turns the shower on and flushes toilets etc or decides to water the grass all at the same time. It also give a very consistent pressure.

Basically users just want potable water at a decent pressure available 24hrs day/ 365. And water that doesn't make you ill. Fail any of those and the wrath of the homeowner descends on you with a vengeance....

 

[EdStainless]

Why are towers and standpipes so ubiquitous?
Because they are the best solution for this application.

 

[cvengr]

Hi,

I am working on a very large development project (2000 homes at buildout), for which the city is requiring the developer to install wells to provide water supply. The developer does not want an elevated tank on the site and there is a connection to the county system for emergency water demands. I am asking for input on controlling the well pumps by system pressure or another option other than tank level. They are performing test wells now so I dont know the production capacity of each well yet but I expect each well to produce around 150-200 gpm and there will be several.

I dont have experience with hydropnuematic tanks so I was looking for some guidance here for sizing.

Thanks for your help.

Off the cuff, depends upon the State and County possibly municipality or City, but as a rule of thumb, The Drinking Water Permit Requirements for California, see Title 22 Drinking Water Permit. Plan for Maximum Daily Demand, and the System Capacity as a whole must support the Maximum Daily Demand over a long period say 10 yrs. For Fire Flows, must meet 4x Peak Hourly Flow, say 2x 1800gpm Fire Hydrants ( 2 x 1800 gpm x 60min/hr x 4 hr = X gal/4hr PHF) Plus peak hourly flow normal conditions within the Distribution System (usually morning or dinnertime),
For a small system supporting about 20,000 people, it's Max Daily Demand was 6.4MGD, while Avg Daily Demand ranged from 1MGD to 3.5MGD, annual peak about 4-5MGD in late summer.

I suspect you are looking at a 1-4 MG Above ground Ground water storage tank, either a pressure tank or change in elevation for gravity feed, to keep say 30 psi in the system. (No more than 84-100 psi, throughout the Distribution System.) Probably fed by 2-12 wells with 1-3 running fairly routinely, say specific gravity well pumps producing about 1000 gpm each (no less than about 600gpm ea). Depends on groundwater chemistry, and production rates, how deep they are, how many aquifers they must pass through for drinking water quality production.

Some states require the developer to dedicate the funding up front for the utility buildout. Some do not, say Colorado, notorious for bonds placed on parcel owners costing more than any home built on the property to complete min utility costs.

Regarding well pump controls. Might look at a Wet SCADA system to assist your efforts.
Pump Controllers will likely used VFDs, but calling for pump startup will always be tied to the water level in the reservoir which needs to be filled. Typically each Closed Reservoir (Water Tank) will have a LLow, Low, High, HHigh level, which triggers how many wells need to operated to refill the Reservoir. If it is a gravity fed system, the pressure will be related to the water level in the tank. Some 1MG tanks in MUDS around Texas use Pressure Tanks sized to about 20,000-50,000gal (not uncommon in the 80s). Either the reservoir is pumped with water at an elevation from which it gravity feeds the Distribution System, or you use Pressure Tanks to pressurize the Distribution System. If the terrain doesn't support adequate change in elevation, then either pressure tanks or an elevated Water tank (Water Tower) would be required. Same amount of power required to lift the water to the higher elevation or maintain a pressure tank. Might be able to have pumps on line, but would require a high level of redundancy and continuous ops or high fluctuation in operations. I haven't designed or seen such a system, but controls and pump wise, the technology is there.

No matter what you do, the Authority Having Jurisdiction will likely require a Civil PE or EnvrEngr PE to submit an Engineering Report, from which the State would issue the Drinking Water Permit. That Engr Rpt would have to comply with all federal, state, county, and municipal laws, stds, and codes for approval to operate to serve as a public water supply.

For 2000 homes, a population of just under 5000, you'd still have to have evacuation routes, egress, ad ingress, Circulatory Routes, Potable Water and Fire Mains, .... 2024 costs, 0.5MG tank at about $3.5mil; Specific Gravity pumps and casing for existing well, about $350k ea. Old 2000 rule of thumb for DistSys piping was Line Size (Dia inches) x $10/LF for 36in deep to TOP OG buried PWTR lines.

Emergency connections to other systems, can also flow both ways. Verify with the County. Good luck, how this helps.

 

[djcbgn]

Sorry for not responding sooner to the comments I have been out of town and just got back. I wanted to provide some clarification on this project and I am waiting on some more information to come in. As I mentioned, the developer improvements (wells, possible elevated water tank, water and sewer lines, sewer pump stations) will be turned over to the city. There is an existing County 12 inch water line within the adjacent road and the developer project will connect to this main. The City wants the connection to the county water main for redundant supply to the development .Before my initial meeting with the City I did not know if this project would be connected to the County water main or stand alone. So I found out the project is connected and the City engineer said that the county could provide fire protection for the first phase of the project (about 725 lots) then the on-site tank would come online, which would provide fire flows. The County connection would still be available for redundancy. So my thought would be the tank will provide fire demand storage, operational storage and dead storage. I just received pressure log and fire flow test data taken on the county water line adjacent to the project. The static pressures are about 110 psi. Since the elevated tank would float on the County system the tank height would be about 250’ high. At this point the developer wanted to eliminate the elevated water storage tank due to the height. This left me with trying to figure out another way to control the well pumps. This is my current predicament. I thought maybe using hydropneumatic tank, VFDs or even a timer to control the well pumps but I am not confident of any of those solutions.

I am thinking about a PRV to lower the incoming county pressure therefore decreasing the height of the elevated water tank. The tank would not be built until later in the initial phase or the second phase so water demand would be higher to turnover the tank volume. I would run the well(s) in the first phase on a timer. The county connection will provide supplemental water (peak hour/fire protection) until the tank is built. Any thoughts?
Thanks.

 

[LittleInch]

So the height of the tank is assuming that you are matching the static height of the county water main?

That is cart before the horse stuff.

What pressure do you need at the base of the tower to feed the demand and your fire system?

All you need to do is throttle the incoming supply based on level control and thene the height of your system is up to you. You could be at ground level and just use booster pumps or booster pumps for high flow rates only.

Many ways to skin this cat.

As said before you could just plug all your wells into the development ring main and then develop a system to start / stop pumps as required based on either total flow or more likely pressure at the highest / lowest pressure point in the network.

An elevated tank though is just a lot easier.

 

[bimr]

The first thing you have to consider is the elevation across the development. You want to have enough pressure to serve all of the development,

Why is the county pressure so high? You should have a target of 60-80 psi as the final system pressure.

You need to prepare some type of agreement between the developer and the county on the water supply.

 

[fel3]

To piggyback on bimr's last comment....The reason for a maximum target pressure of 80 psi is because that is what building plumbing is designed for. If system pressures exceed 80 psi, then every affected lot will require an individual pressure regulator to protect building plumbing, or one or more master pressure regulators would be required to isolate the affected low-elevation areas. These measures are common in water systems with high vertical relief. If your site is relatively flat, this should not be an issue.

If the connection to the County's water system is intended to be one-way only (from County to development and not the other way around), then you could install a master pressure regulator at the connection to reduce the pressure to say 75 psi. This means that a tank on the order of 175 feet tall might be appropriate. However, you won't know the appropriate tank elevation until you estimate domestic water demands, learn from the AHJ what your fire flow requirement is, and model the distribution system with your wells, an elevated tank (model at both high water and low water levels), and with and without the supply from the County.

 

[djcbgn]

Thanks for eveyone's input, this was very helpful.