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Storage Needed Downstream of VFD

Rule of Thumb Question - In general terms, what is a rule of thumb in regards to bladder tanks or some hydropnuematic tank downstream of a VFD in terms of volume needed.
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  • Hi nkg2,

    Can you provide a bit more background on the system you're working on? Are you considering the hydropneumatic tank for transient protection? Are you conducting a steady state or EPS in WaterCAD or WaterGEMS, or a transient analysis in HAMMER?

    You might find some general information in the book Advanced Water  Distribution Modeling and Management

    In HAMMER, there are many factors to consider when sizing a hydropneumatic tank for surge protection, some of which are listed in this article:

    Modeling Reference - Hydropneumatic Tanks


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

  • Hi Jesse - Sorry I did not know you had replied or I would have replied sooner.

    VFD/VSD is supposed to help reduce the number of bladder tanks since water and pump continuously move. I am trying to figure out what is the rule of thumb in regards to what that reduction is? Is it 1/3 your peak demand flow?

    So for example, lets say that I have a standard pump and controls that have a set point of 40/60 psi and I want a minimum rest time of 30 minutes for the pump after shutdown. I then specify X volume of nominal volume from Y number of bladder tanks. In the case of VFD/VSD, the electricity to the windings are constantly there and can handle the varying frequencies applied therefore a reduction in volume is okay. The volume in the bladder tank is there to help keep a smooth HGL downstream of the pump so that it does not wind up and down so fast that it create a harmonic like condition. I just didn't know if there was an AWWA standard or generally accepted practice on that.
    Oops forgot to say I am doing EPS and Steady State modeling of the system as a whole, don't own Hammer.
Reply
  • Hi Jesse - Sorry I did not know you had replied or I would have replied sooner.

    VFD/VSD is supposed to help reduce the number of bladder tanks since water and pump continuously move. I am trying to figure out what is the rule of thumb in regards to what that reduction is? Is it 1/3 your peak demand flow?

    So for example, lets say that I have a standard pump and controls that have a set point of 40/60 psi and I want a minimum rest time of 30 minutes for the pump after shutdown. I then specify X volume of nominal volume from Y number of bladder tanks. In the case of VFD/VSD, the electricity to the windings are constantly there and can handle the varying frequencies applied therefore a reduction in volume is okay. The volume in the bladder tank is there to help keep a smooth HGL downstream of the pump so that it does not wind up and down so fast that it create a harmonic like condition. I just didn't know if there was an AWWA standard or generally accepted practice on that.
    Oops forgot to say I am doing EPS and Steady State modeling of the system as a whole, don't own Hammer.
Children
  • First, I'll assume that this is a closed/dead end pressure zone with no floating (elevated) storage. If you have storage, you probably don't need either a hydropneumatic tank (except for transient control) or a variable speed pump.

    For a closed system, most engineers use either a variable speed pump or a hydropneumatic tank. Either will enable the pumps to run fairly efficiently. Usually the hydro tanks are best for very small systems or systems with widely varying use (e.g. camp grounds, ski resorts, golf courses...). Variable speed pumps become economical for larger systems. Variable speed pumps must have a standby generator while hydro tanks need one if you are planning for long duration power outages.

    It is difficult to turn over the water in the tank if you have a pump controlled to maintain a constant head.

    The one instance where you may want both a hydro tank and a variable speed pump is to protect against a power outage where it takes a while for the generator to start. The tank can supply water while the generator is starting which could be as long as a minute. If you don't have a tank, it is likely that you are putting some people out of water while the generator sense the power outage, starts up, ramps to full speed and starts the pump.

    So the answer to how big should the tank be is "How long of an outage do you want to plan for vs. how much do you want to spend vs. how frequently do you want to cycle the water in the tank."

    Answer Verified By: nkg2 

  • I created a wiki for this question/answer because it applies some great step by step thinking in order to layout the solution to the problem. It can be found here for any future engineers who may have a similar question:

    communities.bentley.com/.../24986.how-much-do-vsp-svfd-s-help-to-reduce-the-number-or-size-of-hydropneumatic-tanks-in-a-system

    Mark

    Mark

    Answer Verified By: Sushma Choure 

  • In most of my dealings, I deal with small distribution and treatment systems ( less than 300 homes/units per site) where if you are lucky you have a large pressure vessel, hydropnuematic tank to supply the pressure and volume needed to supply the sites. In a couple of instances, I deal with ground storage to booster to end user. Most of my experience is from going in reverse meaning that I see the end product but try to reverse it into a model and try to incorporate what I learn from field into the model. In none of the cases, except for one personal experience (no model yet), do I have a VFD/VSD. That being said, the cost of inverters are coming down and most motors these days are inverter ready so instead of say 6 bladder tanks down stream, I can have two for example, and what I would pay for the other 4 would be more than sufficient to pay for an inverter. Not only that but it frees up real estate.

    I am commenting however because A) I am glad this is turning into a wiki because there was additional thoughts to consider and B) because I think I can add more thought to the process. I think you could set your VFD to act more like a soft start/stop thus being able to turn over the water in the downstream hydropnuematic tank(s). I know that is not what the intent was but at the same time you don't want stale water and you do want your windings to cool some. I would be concerned that if you didn't have some sort of cushion or tolerance setting on the inverter that any slight variation in hydraulic grade would cause the inverter to rapidly react thus putting wear and tear not only on the motor but also sending oscillating energy into the piping. In the particular case I am looking at is an option to do just that which is to reduce the number of bladder tanks and be a little more forgiving. As I look at my particular case with 6 bladder tanks (which takes up a ton of room), my boost pump run time is 1.5 minutes and my downtime is 11 minutes during my "worst period of the day based on idealized diurnal curve". Six bladder tanks only give me 180 gallons. It will give me about 16 minutes of storage provided that my instantaneous flow at that point does not exceed my average daily flow.
  • Using a VFD as a soft start when you have tanks to dampen out surges is pretty expensive. The air pocket in the tanks should absorb the surge.

    Six small tanks are probably more expensive , take up more floor space and require more plumbing than two larger tanks. But you need to run the numbers on that.

    If you decide to run a VFD instead of the tanks, you can program them so that they don't respond quickly to changes in pressure.

    If you send me your email address, I can send you a paper I did on this subject. (tom.walski@bentley.com)

    Answer Verified By: Sushma Choure