VFD vs Hydropneumatic Tank 2

[MesaMan]

Does anyone know if any state accepts the use of VFD's instead of Pressure Tanks in Water Systems?

Are there any advantages other than VFD soft start?

What are the disadvantages of utilizing VFD's in Water Systems?

 

[bimr]

Pressure tank are only used on smaller water systems.

VFD's have been used to control the pressure in water distribution systems. I worked on a project last year with a ground storage tank and pumping station with VFD's. Capacity was 500,000 gpd.

The biggest drawback to VFD's is probably the cost. The cost is higher than a mechanical pressure control setup.

The advantage to VFD systems is that such systems are simple to automate.

Soft start was not really an issue. But you need to study the potential for water hammer in the water distribution system.

 

[thewellguy]

You have to consider your application. The benifits of vfd's and cyclespots or other means of regulating flow only work well if the system use varies over time and volume. If the pump only comes on once a day and runs for an hour then none of the system will help much. If the flow and usage varies throughout the day then a vfd is a good option. I'm not a fan of cyclestop. Valve crazy I know you disagree so Mensaman please form your own opinions.

If you have more info on your application please provide it so that we can better help you.

 

[Valvecrazy]

“Does anyone know if any state accepts the use of VFD's instead of Pressure Tanks in Water Systems?”

A 1000 gallon pressure tank only holds 150 gallons of usable water at best. Other means of pump control needs to be utilized. In the past, the state of Texas has considered VFD’s an unreliable control for drinking water applications. They would only allow the use of a VFD if there is a backup system in place, including bypass relays as well as adequate pressure tank or elevated storage. There may be other states that allow the use of VFD controls but, I would bet there are other stipulations as well.

“Are there any advantages other than VFD soft start?”

The advantage is that the water supplied by the pump exactly matches the amount of water being used so, a pressure tank or elevated storage should not be needed. However, without a pressure tank or elevated storage, the soft start of a VFD can be detrimental. When the demand increases faster than the pump can get soft started, a negative pressure wave is created in the pipeline which is followed by a pressure spike when the pump does catch up. When demand decreases faster than the pump can be slowed down, the opposite occurs.

“What are the disadvantages of utilizing VFD's in Water Systems?”

Other than what has already been mentioned, all I can think of is harmonics, stray voltage, voltage spikes, electrical fluting of ball bearings, resonant frequency vibrations, nuisance faults, environment control, technical complications, and cost. There are alternatives to VFD controls that do not have these side effects. Thewellguy says he is not a fan of them but, I know thousands of people who are.

 

[Valvecrazy]

The following thread better describes what I was trying to say.

http://www.eng-tips.com/viewthread.cfm?qid=203504&page=1

 

[MesaMan]

thanks everyone. you've provided me with a great amount of information. I can safely say that utilizing VFD's w/o pressure tank is not to the benefit of the distribution system and customers.

 

[Valvecrazy]

I hope you are able to decide on a viable alternative.

http://www.cyclestopvalves.com