pressure tank question

[sam74]

Hello, this is not usually my problem to deal with but it is at the moment.

I'm dealing with a with a water well and a predesigned elevated storage tank by requirements of the project. I also have to meet minimum storage requirements and static and residual pressures. For this reason I will also require an at grade pressure storage tank to meet minimum storage requirements and a booster pump system to meet minimum pressures.

My plan was to have the well pump up to the elevated storage tank which would then gravity feed to fill the at grade tank and the water system until all aspects were filled and the float switch in the elevated tank shut off the well pump.

Since I have little practice at this the pressure tank is baffling me. At first I assumed there would be something similar to an air release valve on a water main installed on the tank so that it could fill completely with water and dispell the air pocket. Would this cause the booster pump to collapse the at grade tank when it kicked on?

So do you just live with pressurized air in the at grade tank due to the head of the elevated tank?

How do you tell how much of your at grade tank is full at static to be sure that you meet storage requirements?

Sorry if there is a simple answer I'm overlooking but I've been out of school a bit and this isn't what I'm normally used to doing.

Any advice is welcome.

Thanks

 

[SteveWag]

Both the elevated and at grade tanks will be vented and supplied with an overflow. Will the at grade fire tank/pump supply water to the drinking water system? You could end up with some very old, stale water if so. You could connect the overflow from the high tank to the low tank and put high level floats in each and stop the pump when both are full. Same way, low level floats in both tanks could be used to start the pump. The logic would be: either one of the low water levels can start the pump and both tanks need to be full to stop the pump. You may also use the well pump to fill the low tank and then pump up from the low to the high. This could keep the water fresher.
Steve

 

[sam74]

So the first sentence of your reply regarding vents and overflows you would consider a requirement?

I was orignally thinking a vented with overflow at grade tank but what keeps the overflow from flowing constantly since it is connected directly with the elevated tank?

By hooking the overflows together I'm assuming then that you don't discharge this somewhere and therefore are overflowing back into the system which would address the problem stated above.

The entire system will be for potable water needs at a border crossing in a country other than the US. It is not meant to be elaborate but functional and meet minimal desing/build requirements. Unfortunately with all the bits and pieces required to meet the requirements it seems elaborate. There was not a great deal of forethought by the people writing the requirements.

Thanks for the input

 

[bimr]

If you use a pressure tank at grade level, you can keep the tank full at all times. Use an air release valve to keep air out of the system.

See page 50 of this Apco catalog

http://www.apcovalves.com/classic/pdfs/AIR_VALVES/610.pdf

These air release valves function just like the valves used on water mains.

The water tower should have a bottom outlet, so you should not get a significant amount of air except when filling.

The type of tank that you should use will be determined by the volume of water storage that is required. If you need hundreds of thousands of gallons of water storage, it is more economical to use a gravity tank for water storage.

If you use a gravity tank, it would be most economical (as far as pumping costs) to pump from the well to the gravity tank. Then use a booster pump to pump into the water tower and distribution system.

If you have a pressure tank, you can pump from the well to the water tower. If your tower was built at the proper elevation, you would not have to repump.

 

[bimr]

Correction, it is a Model 50 on page 10.

 

[SteveWag]

Both tanks should have a vent (for filling and for empting) and an overflow. I can not imagine either tank withstanding pressure or vacuum. The vents are usually sized for the maximum fill or empty rate. Most gravity tanks (your elevated one) are vented big enough so the “broken” waterline event does not collapse the tank. I meant for the elevated tank to overflow into the lower tank, and the lower tank to overflow to waste (grade, storm sewer?). The overflow from the lower tank is just a safety measure, in the event the controls fail to shut off the well pump. You have two design options, fill, from your well, the elevated tank first OR fill the lower tank first. If the elevated tank is to be filled first, then overflow into the lower tank. If the lower tank is filled first, then pump from the lower to the higher. In the first case, you pump all the water to the higher tank, so bigger well pump. In the second case, you have a smaller well pump but another set of pumps to deal with.
Steve

 

[sam74]

Thanks for the reply.

There is not a lot of logic to the design. Just following requirments. Trying to introduce spec writers to logic currently.

We are specifically required to use a 20 m tall predesigned tank that holds 7000 gallons. The 20 m tall tank doesn't even meet static pressure requirements of 40 psi.

The storage requirements are based upon 3 days storage for a certain liters/capita/day which exceeds the 7000 gallons almost twice.

So we added an at grade storage and then a booster pump to meet pressure requirements. Simply put we have a tee with the elevated storage on one side and the at grade storage on the other. The booster pump is on the tee'd end feeding the system.

Any other input appreciated

 

[sam74]

SteveWag, I see what you're saying about withstanding the pressure or vacuum. The elevated tank is what it is so I'll assume it was designed correctly. For the at grade tank what do you think about using the valve on page 11 of bimr's reference, the model 2001. It would act as a vent when filling or emptying the at grade tank.

I still can't grasp the overflow on the lower tank but this isn't normally my thing.

My well construction spec requires drilling 150 m so I didn't figure pumping an additional 20 m to the elevated tank would be that big of a deal. That way everything else can be filled by gravity. But the elevated tank would be the last to be filled and would always be forcing water from the at grade tanks overflow unless there is some sort of valve that is normally used that I'm not aware of.

Thanks for the continued input

 

[bimr]

sam74

I understand the elevated tank is existing. What is the elevation of the tank and the range of the operating water level in the elevated tank?

I don't understand why you are planning to install a pressure tank. A pressure tank would have to be operated as a gravity tank because as water is consumed, the tank would drain. The gravity tank is also less expensive.

Regarding the installation of a 7,000 gallon pressure tank. What is the design pressure of the tank? Note that the design pressure should be at least 75 psi if you plan to pump through it.

What is the design flow rate to distribution?

I would think the best pumping scenario is to pump from the well to a gravity tank. Then pump from the gravity tank to the elevated tank. This assume the elevated tank was constructed at the correct elevation to provide a minimum water pressure in the distribution system.

 

[sam74]

bimr,

The tank actually is not existing but it is an existing detail we are required to use by contract. See detail link

http://www.aed.usace.army.mil/EngineeringDocuments/20 Meter Water Tower.pdf

Tank size is not specified other than metal 7000 gal tank submit for approval somewhere in the document notes. The 20 m high tower (really 19 m according to detail dimensions) would need to have a tank elevation up to 28.16 m to have a static pressure of 40 psi if my calculations are correct (and they may not be).

I was not planning on pumping through the at grade tank but using it and the elevated tank as connected storage for the booster pump (and it's hydropneumatic tank) that pressurizes the system to maintain a static of 40 psi and a residual of 30 psi. It seemed to be the cheapest remedy if it works. But like I said this is not usually my thing I was just willing to run with it.

I really wouldn't mind having an at grade gravity tank I just don't understand the overflow on the at grade tank with the two tanks being connected and the elevated tank exerting a constant head on the overflow to push water out.

I feel like from your and SteveWag's comments I am probably missing something.

 

[bimr]

In the water business, it is typical to have a minimum 20 psi distribution pressure. The 20 m high tank pedestal will provide a water pressure of approximately 30 psi at the base of the tank. The drawing shows a 20 m high pedestal, not 19 m, plus you should have some water in the bottom of the tank.

I would surmise that 30 psi at the base of the tank is probably the design pressure. This appears to be a water supply for a small base or something, so one would not expect anything beyond the minimum requirements.

Here is a typical process scenario:

Pump from the well into a gravity tank on the ground. Then use booster pump(s) to pump the water from the gravity tank into the elevated tank. The water distribution will be supplied by the elevated tank with approximately 30 psi water pressure.

Use a level transmitter on the gravity tank and a level controller to control the running of the well pump. The gravity tank will need an overflow nozzle, not an air release.

Use a level transmitter on the elevated tank and a level controller to control the running of the booster pump(s). The elevated tank will also need an overflow nozzle. The booster pumps need to be sized for the peak demand which will be a larger flow rate than the well pump.

I don't see any reason for connecting the overflows. If you connected the overflow from an elevated tank to the lower tank, you would have to double the overflow pipe size coming out of the lower tank.

You don't need any air release valves.

 

[bimr]

If you attempt to pump out of the ground level tank and the elevated tank at the same time, it will be complicated pumping scheme.

You would have to use different pumps with different pump curves for each tank. Different pumps are required because you have different pump suction pressures. Then you would have to try to match the pumping rates from 2 different pumps with the pumps fighting against each other to put out the water.

It may be possible to do this if you use VFD's on all of the pumps. Howver, the VFD's are somewhat less reliable and more expensive.

You would also have a more complicated control scheme to put water into both of the tanks at the same time.

The bottom line with attempting to pump out of both tanks at the same time, is that the control scheme will be complicated, expensive, and less reliable.

 

[sam74]

bimr, thanks for the input. I think I follow you but don't see how it meets my demand and pressure requirements. I'll try and explain more.

My original water model in epanet had an elevated tank as the source for simplicity (as most all my epanet models do). The proposed system was modeled using fixture unit demands for individual buildings to arrive at a peak domestic flow with no fire flow requirements. This was a value of 306 gpm.

In a static condition pressure requirements are 40 psi everywhere in the system by contract. What I have referred to as residual pressure and you have referred to as distribution pressure I believe are the same but minimum requirements by contract are for 30 psi everywhere (generally 20 psi on a local scale but this isn't local). The elevated tank and distribution pipe sizes were then adjusted accordingly to meet the pressure requirements at all junctions.

On sites previous to this sites design there were no elevated tank requirements so we took the peak domestic flow contacted the booster pump manufacturer and selected a pump system with a pump curve that could meet the peak domestic flow. The booster pump system is typically a 3 pump system. Storage requirements were specified as at grade tanks (typically (3) 50000 liter tanks).

This site was different in that a previously designed elevated tank was required by contract. Since we have no control over tank height and it is not tall enough to meet minimum pressure requirements how would pumping from gravity storage to the elevated tank meet the pressure requirements?

My path to what I was proposing on this site was then to add ground storage to the required elevated storage to meet contract requirements for storage and a booster pump to the system to meet minimum pressure requirements. It should be a smaller booster pump since we do have 20 m of pressure head already but it would seem that a booster pump directly to the system would be required.

What if the well pump fed the elevated storage, which fed the at grade storage (still assuming pressurized), and the at grade storage fed the booster pump to the system?

 

[bimr]

Thanks for the further input. There is a way that you can make that system work. Design the system so that the pressure tank will function as a standpipe.

Install piping from the bottom of the elevated tank to the top of the pressure tank. The piping would have to be installed without pockets so that air would not be trapped between them. Without pockets means that the pipe would be installed without dips (that will trap air) as the piping moves upward to the elevated tank.

The pressure tank would have to be rated for the water pressure at grade. It looks like a pressure of at least 40 psi would be adequate. The pressure tanks should have relief valves if it is possible that the water pressure may be increased above the pressure tank design pressure.

Install air release valves in any location if there is a possiblity to trap air. For example if you have a side outlet on the pressure tank, you need an air release in the tank top.

As you pump out from the system to distribution, the elevated tank will drain first; if you continue to pump, you will then drain the pressure tank.

You will only need one level control system on the combined tanks.

The three booster pumps will take suction from the pressure tank(s) with the pump inlet pressure varying between roughly 10-30 psi.

 

[sam74]

Okay great, I think we finally got on the same page. My lack of description and limiting of important information sometimes hampers these things. I really didn't want to start off with an extremely long explanitive post.

So what I was originally planning (with the addition of the apco valve model 2001 to my pressure tank) should work I think. The suction side of the booster pump would run to a tee with elevated tank and ground well on one side of the tee and the pressure tank on the other.

I was planning on using the same pipe to fill and discharge from the pressure tank. To try and use previous details this is typically a horizontal cylindar that will be updated with the apco valve.

So if the well feeds the elevated tank which will gravity fill the system and pressure tank and then finally the elevated tank gets filled to the shutoff elevation do you see any problems as long as the pressure tank is rated for 40 psi?

Thanks

 

[77JQX]

If I understand correctly, you have a contract that requires an overly complicated and expensive system. If you're going to have to pump to a closed loop system to meet minimum pressures, there's no need for an elevated tank (expect perhaps in case of power failure, but a genset would be cheaper). Or alternately, a larger, taller, elevated tank would also do the trick without the at grade tank and booster. But to build an elevated tank, at grade pressure tank, and booster pump station, just because a contract says you must build the wrong elevated tank is a waste of money - now and in the future. Is there any way you can talk sense to the client?

 

[sam74]

I agree completely. We are attempting to talk common sense with them. However we are going to stand down for a while on this job so we will propose something that will work based upon contractural requirements until the client changes their mind or the job gets built.

A couple of mistakes I've typed or omitted earlier.

- I used a reservoir not a tank as my source of flow in epanet.
- When speaking to the booster pump manufacturer I provided
them with my peak demand and the height of my reservoir to get
the booster pump sized.

What is making more sense now would be the these items in series - water well, elevated tank, pressurized tank, booster pump. That way water should be moving in all storage items to hopefully reduced stagnant water.

 

[bimr]

The pressure tank should be designed with a pressure rating above that pressure that it will expereince during operation. So if your gravity tank is 30 m above grade with an overflow, a pressure rating of 40 psi will probably be good enough. If you maintain an open connection with the gravity tank, you should not have to be concerned with a vacuum when you pump out of the tank.

 

[BigInch]

... unless somebody closes the valves between them. Put in the vacuum releif valve, even if you don't "need" it.

From "BigInch's Extremely simple theory of everything."

 

[bimr]

It would be preferable to specify the pressure tank as being designed for 40 psi and full vacuum than have to maintain a vacuum valve.