CSVs good or bad?

Discussion in 'Pumps and Tanks Well Forum & Blog' started by ncgeo, Mar 13, 2009.

  1. ncgeo

    ncgeo In the Trades

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    North Carolina
  2. ncgeo

    ncgeo In the Trades

    Messages:
    47
    Location:
    North Carolina
  3. valveman

    valveman Moderator Staff Member

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    Lubbock, Texas
    Thanks for bringing this to my attention. I have replied to their post. Maybe we can get them educated on how pumps really work, so they won't make such fools of themselves in the future.
  4. speedbump

    speedbump Previous member

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    I agree, the Pirate seems to be pretty one way on the CSV. I wonder what makes him feel that way? I didn't really hear him bragging on VFD's, so maybe he just likes to sell a lot of tanks.

    bob...
  5. Gary Slusser

    Gary Slusser That's all folks!

  6. valveman

    valveman Moderator Staff Member

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    They finally got them posted Gary. "Guess Who" is there agreeing with the person who doesn't understand pump systems? I would be embarrassed and humiliated if he agreed with anything I said.
  7. Gary Slusser

    Gary Slusser That's all folks!

    I'll bet he's from AZ or NM. I'll go read it.
  8. Mr.Nice_Guy

    Mr.Nice_Guy New Member

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    http://www.irrigation.org/ibt/0208/p29.htm

    The CSV article has much more clout because there's actually math used.

    "If a variable-speed drive had been used the submersible motor would overheat or "cycle" during low-flow situations."
    -Perhaps in 1992. Nowadays, decent vfd systems automatically detect flow and shut off accordingly. You can cycle a pump as many times as you want via a VFD, there is no inrush current and therefore no heat build-up.
    ---------------------------------

    Ronald, to answer your question -
    It completely depends on the application. Generally this forum is for Well pump applications - where the static head of the TDH is of a much higher percentage. And also the HP required for a single house is 1-3. You're not going to get a quick ROI (return on investment) when dealing with 1HP motors. If this is the case, the energy savings of a VFD are comparable or a little less than that of a CSV.

    However, as the water level in the well gets smaller and smaller, the VFD begins to save more money - because the speed of the motor can be slowed more and more. Of course this depends on WHO sizes the pump system.

    In most HVAC systems and pressure boosting Residential/Commercial/Industrial applications - CSV's just cannot compete; but at this point, you're comparing apples to oranges.
  9. valveman

    valveman Moderator Staff Member

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    Let me take this one thing at a time. The article in question was talking about a 75 HP submersible in 10" casing. A CSV would derate the motor to about 40 HP at low flow. Derating the motor would allow it to safely pump hot water, so it takes very little cool well water to maintain proper cooling. This is why the flow can safely be as low as 5 GPM with the CSV.

    A VFD does not derate a motor, it creates a smaller motor (40 HP) from the larger one (75 HP). Cooling flow still needs to be adequate for a fully loaded 40 HP motor, which is .5 feet per second. With an 8" motor in 10" casing that would be a minimum flow of 55 GPM with the VFD. When the flow needed is less than 55 GPM, the VFD would sense low flow and shut off the pump. Flow is still being used so the pressure would drop and the pump would be started again. This cycling would continue as long as the flow needed was less than 55 GPM.

    Seven years ago when that article was written, there was no such thing as "sleep mode" for a VFD. Because of that article and the many other times I said it, "sleep mode" was added to most VFD controls. When a VFD shuts the pump off because of flow being less than 55 GPM, "sleep mode" allows the pressure to drop, usually about 10 PSI, before the motor restarts. This allows a pressure bandwidth, so a pressure tank can supply some water during times when the pump is off. Before "sleep mode" the pump would have restarted almost immediately with a VFD control, which would have caused excessive cycling and heat. "Sleep mode" gives a little more time between cycles but, you would need to use a very large pressure tank to keep the pump from cycling excessively.

    A submersible must reach 30 hertz or ½ full speed in 1 second to prevent thrust bearing damage. You can't get from 0 to 30 hertz in 1 second without inrush current. This current would be less than if the pump went to full speed. However, inrush current will still be 200% to 300% of running amps with a VFD, compared to 600% with across the line controls. So I disagree that a VFD will let you cycle a pump as many times as you want. It is usually the fact that the motor has not had time to cool down before it is restarted that is the biggest problem.

    There is not quite as much heat build up from starting with a VFD as there is when starting across the line. However, once the pump is up and running, the VFD causes much more heat and is much harder on the motor than starting across the line. The harmonic currents created by the VFD cause additional motor heat. Even with required filters on the VFD, voltage spikes to the motor can be 400% of line voltage. With 480V, the VFD sends spikes to the motor of about 2,000 volts. This adds to the motor heat, and also causes partial discharges in the motor windings, that eats away and breaks down the insulation. These are the main reasons why a VFD controlled motor needs more cooling flow than a motor running on standard across the line controls.

    Now we can also talk about resonance vibration, harmonics that are fed back into the electric grid, bearing currents, and many other things that are problems when using a VFD. I am presently working with an engineer who swears that VFD's are causing the steel casing in his city wells to deteriorate. An example is two wells drilled at the same time, using the same casing, which are now 9 years old. One well used a VFD controlled pump, the other used standard across the line controls. The well with the VFD has had to have the casing replaced as it looked like Swiss Cheese. The casing in the other well still looks like new. VFD's are not the cure all that everyone wants them to be, and in most cases, VFD's cause more problems than they are worth.
  10. valveman

    valveman Moderator Staff Member

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    Generally the energy use at low flow rates is a little less when using a VFD. However, because of the parasitic losses of the VFD itself, and the decrease they cause in motor efficiency, VFD's cause more energy use at high flow. If you are using low flow most of the time, a VFD uses less energy. If you are using high flow most of the time a CSV saves more energy. If your flow rates are sometimes low and sometimes high, then there is very little difference in power consumption between VFD and CSV.

    A VFD cannot save energy unless you can reduce the head or pressure required.

    With a house well, this would mean deciding that you really do not need 50 PSI today, 20 PSI should be plenty. Of course you always need 50 PSI, so a VFD cannot save energy.

    If the water level in your well gets higher, then you can reduce the speed with a VFD. However, when you use water, the water level in the well will drop, and you need to increase the speed. If the water level stays high, you just didn't need as large a pump to start with.

    Even in HVAC systems, the only time a VFD can save energy, is when the load is mostly friction head. In these cases, a VFD can slow the pump more as the flow rate decreases. This usually happens because someone installed pipe that was too small to start with. There are some good uses for VFD's. However, in most fresh water applications, the static head prevents the VFD from being anything other than an expensive headache.

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