Resources on Small Distribution Systems with VFD pumps and no distribution tank.... 2

[ATLPipeEngineer]

Can anyone point me to any resources on small distribution systems that are fed with VFD pumps, but do not have a distribution tank? Thanks.

 

[swazimatt]

what is the water source? pumping directly from the mains? (normally not allowed by councils)

 

[coloeng]

What kind of resources are you looking for?

 

[ATLPipeEngineer]

It would be pumping straight from the clear well of a new small treatment plant. This development will be spread out and have their own treatment plant.

It will be for a development that wants no tanks.

I am just looking for some documents on lessons learned, typical design procedure, etc.

 

[1503-44]

There are various references on system design, but none that get that far into the vfd or constant speed pump questions. But there are other references for that.

Well vfd downhole is not great straight away. That is, if the vfd is in the well. I'm not sure if it is, but without tanks, I'd guess it will be that way.

VFD sets you up needing the biggest possible pipe diameters and pumps when everybody wants water and then drops to very slow velocities most other times.

But the killer is vfd is not great for the well pumps, because head is more or less constant. At low flow rates, you can't make head, so you always pump lots of water, but it's got nowhere to go.
If you operate in the high rpm range just to get up the well, then there isn't much more rpm left to pump through the pipe, so you wind up running at max rpm anyway. Constant speed will be more efficient, or maybe about the same. Pretty sure vfd will not be advantageous.

Is it some other configuration?

If its from surface pools to surface pool, you still get stuck buying big pipe and pumps, unless you can use the storage capacity of the pools somehow. If you can do that, then why do you need vfd?

System design should concentrate on finding a break point between using storage capacity and constant speed vs no storage capacity and variable speed pumps. Then deciding which is more economical after any power savings differences, vfd control and wiring costs, tank and pipe costs and maintenance considerations.

I had this vsd reference handy.[this website is not uploading right now. ill try again later.]

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Valvecrazy]

As was said, with a well pump or supply pump that must deliver a constant pressure there is a very limited range where the pump speed can vary at all. Being as power used by a centrifugal pump decreases when restricted as well as when the speed is reduced, a constant pressure valve may be a better alternative. Either will work without a pressure tank as long as the system is large enough to always have some flow. But smaller systems that will be at zero flow for some of the time also need a pressure tank. The pressure tank will supply water for small uses without the pump coming on. Without a pressure tank even a tiny leak or small use of water will cause the pump to run and then quickly turn off. Quick cycling is very bad for the pump and system. However, with a VFD or a constant pressure valve the pressure tank can be many times smaller than without variable flow control of the pump, like when just using a pressure switch.

 

[ATLPipeEngineer]

Who are the big suppliers of small distribution size pneumatic tanks?

 

[Valvecrazy]

That would be a hydro-pneumatic tank. What is the max gpm and pressure needed for the system, or what size pumps do you have?

 

[ATLPipeEngineer]

That would be a hydro-pneumatic tank. What is the max gpm and pressure needed for the system, or what size pumps do you have?

The maximum VFD pump rate will be 1,395 gpm. The average will be a lot lower.

The pressure needed is about 75 psi.

 

[Valvecrazy]

I would not use one pump for a system that needs up to 1395 GPM yet also needs to produce as little as 5 GPM and maybe even zero flow. I would probably set it up with a 200 GPM jockey pump and a couple of 600 GPM pumps. Each pump controlled by its own constant pressure valve or VFD with staggered pressure settings. The jockey pump could be set at 85 PSI, one of the 600 GPM pumps at 80 SI, the third pump to come on would maintain the minimum 75 PSI needed. In this way the smallest pump comes on first, and is the only pump to run until flow over 200 GPM is needed. Second and third pumps come on if needed and supply varied amount as needed. Basically the 200 GPM pump would be the only one working with the hydro-pneumatic tank using a pressure switch setting of like 85/95. When using a constant pressure valve the minimum flow from the pump will be like 5 GPM. Many times these large systems have enough leaks they never get below about a 5 GPM flow rate and the jockey pump will run 24/7. If that is the case a very small or even no hydro tank or diaphragm tank is needed as the pump will never build up to 95 an shut off anyway. But if the system is tight and has few or no leaks, I would use at least an 86 gallon size diaphragm tank to handle any flow rates less than 5 GPM.

 

[bimr]

Your system is too large to utilize a hydropneumatic pressure tank. In the State where I am located the State limits hydropneumatic tanks to water systems with less than 150 service connections.

Generally, the State will require a storage tank for the water system with a volume equal to the average daily flow. You might look up the Ten States standards as well as the State where the project is located.

You are probably looking at a system that has 2 VFD pumps with another smaller pump for the night use where there is little flow.

VFD

VFD Pumps

The State will probably require at two water wells, disinfection, and standby power as well.

AWWA, 1999. Design and Construction of Small Water Systems, 2nd Edition, American Water
Works Association, Denver, CO.
USEPA, 1991. Manual of Small Public Water Supply Systems, EPA 570/9-91-003.
WSDOH, 2007. Covenants for public water supply protection, DOH 331-048, Washington State
Department of Health, Olympia, WA

If you have a specific question, post it.

 

[Pinwards]

Look into Fire Flow Requirements too. If your water system is feeding hydrants or sprinkler systems there may be a minimum storage requirement volume that the wet well can't meet.

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