Help Needed from Any Scala2 or VFD Experts

Discussion in 'Pumps and Tanks Well Forum & Blog. Water is life.' started by HOOS1990, Jul 14, 2017.

  1. HOOS1990

    HOOS1990 New Member

    Joined:
    Jul 14, 2017
    Location:
    Virginia
    I have a SCALA2 pump, newly installed, that at certain times will cycle on and off constantly while a tap is open. Essentially what happens is that while a tap is open the pump gets the pressure up to to the set point (70 psi), cuts off, pressure drops to around 60, then the pump kicks back on and it does that all over again. This video (not made by me) is exactly what I am seeing:


    At other times, the thing runs like a champ. I open a tap and it runs until the tap is closed. I open another tap and it seamlessly keeps the pressure constant. Another tap, again, seamless. I've had 5 taps open at once all at 70psi, so when it works its pretty cool.

    I'll add that I have read the forums and understand there is some difference of opinion on VFDs and grundfos pumps. But, I've got the SCALA2 installed and would like to try to make it work. I have 10 more days to return it for a full refund, so if I cannot get it to work properly i will uninstall and perhaps go with a CSV type solution.

    Questions:

    1. Does anyone know what causes this issue?
    2. Could it be the internal pressure tank? It is precharged at the factory to 18.5 psi. Anyone know what it should be set to for incoming pressure of 40 and outgoing of 70?
    3. Could this be caused by fluctuating incoming pressure. I do notice that sometimes my incoming pressure is 40, others its 45, sometimes even 50. I am wondering if adding a pressure reducing valve to the supply side to keep the pressure at 40 (maybe even set it to 35) might resolve the issue?

    Thanks!
     
  2. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    http://www.burdickandburdick.com/grundfos lit/SCALA/98880508 I & O.pdf page 13 of 18 says
    Adjust the tank precharge pressure to 70 % of the required outlet pressure.​

    70% of 70PSI is 49 PSI.

    I don't know if that helps your symptoms, but it has to help the life of the diaphragm.
     
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  4. HOOS1990

    HOOS1990 New Member

    Joined:
    Jul 14, 2017
    Location:
    Virginia
    Thanks Reach4. I was instructions that did say to set the tank to 70% of pressure but did not see the part where it says outlet! This clears up that question.
     
    Last edited: Jul 14, 2017
  5. valveman

    valveman Cary Austin Staff Member

    Joined:
    Mar 15, 2006
    Occupation:
    Pump Controls Technician
    Location:
    Lubbock, Texas
    There is a lot of sales information on that pump, but very little on technical instructions. I see where others who had that same problem added some air to the tank, and it seemed to help some. It doesn't sound like the variable speed part is working at all. With other Grundfos products like the SQE, when the pressure transducer fails the pump reverts to cycling on and off as if it had a regular pressure switch. That maybe your problem here. But I don't see that symptom in any of the instructions.

    There sure are lots of problems discussed in the reviews of this product for it not to even be a year old. But hey it only cost 600-700 bucks, so just replace it every few months when it starts acting up. That is the exact goal of the manufacturer. They don't make any money on pumps that last 30 years. They made it real easy to just remove the old one and install another one.


    As with any variable speed pump, there is so much electronics in the controls, you could have many different problems. Grundfos still makes some good pumps like the JP series jet pump. The JP series doesn't have any electronics except for the old reliable 40/60 pressure switch. Adding a simple mechanical (no electronics) Cycle Stop Valve can make the JP series deliver the same constant pressure as a variable speed type pump. Keeping it simple and mechanical (no electronics) is the best way to get 30 years out of a pump like that.

    Grundfos is one of many pump companies who tries to build a Variable Speed Drive directly into their pump, so you won't have any choice over how it is controlled. This way you can just budget maybe 50 bucks a month for the rest of your life to cover the replacement pumps.
     
  6. HOOS1990

    HOOS1990 New Member

    Joined:
    Jul 14, 2017
    Location:
    Virginia
    thanks for the replies so far. I increased the tank pressure to 49 psi. That seemed to have solved the problem. But this morning, I noticed in running the sink that the pressure was fluctuating. I went to look at the pump and it was cycling on and off. Took the pump out of the loop and checked the incoming pressure. 60 PSI. Never have seen it that high, typically its around 40. So, my thought is that when the incoming pressure and outgoing set points are close (I have the Scala set to maintain 70), the pump has a hard time figuring out what to do. Sure enough, I raised the setpoint to 80, and the pump ran fine, no cycling.

    I'l keep an eye on it when the city pressure drops again and see if the pump runs fine. In the meanwhile, looks like I'll be ordering a PRV to deal with the city water incoming pressure fluctuations.
     
  7. valveman

    valveman Cary Austin Staff Member

    Joined:
    Mar 15, 2006
    Occupation:
    Pump Controls Technician
    Location:
    Lubbock, Texas
    Glad you got it working. I started working with VFD's over 30 years ago. I haven't had a chance to play with one of those models yet, but I don't know why high incoming pressure would cause a problem. It is a shame you have to use a simple mechanical pressure reducing valve to make all that high tech stuff work. I think you will discover in a short time that the simpler and more mechanical you make your pump system, the less likely you will be out of water, and the less it will cost you.

    A CSV works similar to that simple mechanical pressure reducing valve. The difference is you put the CSV on the discharge of the pump instead of the inlet, and it makes a constant pressure system without any of the high tech electronics that makes pump systems expensive and less dependable.
     
  8. phil Tuttobene

    phil Tuttobene New Member

    Joined:
    May 12, 2017
    Occupation:
    Inventor, Entrepreneur, VFD Consultant
    Location:
    Orlando, FL
    I would NOT run your pressure to 80 PSI. Most fixtures are only rated to 80. So surges may reduce the life of fixtures, especially toilet valves.
    If your pressure is 60 coming in, I question the need of a booster. Typical house pressure should be 60-70PSI but no higher.

    If your water supply can not keep up with demand the pump will cavitate. This is where a well designed VFD controlled pump comes in.
    A well designed variable speed pump will automatically detect cavitation and modulate its speed to the point that cavitation just stops.
    Once the demand falls below the GPM your supply can furnish the pressure automatically rises back to the setpoint.

    Be careful with cavitation, it will destroy the pump. I have written many articles on designing anti-cavitation algorithms and its a very big part of a well designed variable speed pump.

    Hope it helps.
    Phil
     
  9. valveman

    valveman Cary Austin Staff Member

    Joined:
    Mar 15, 2006
    Occupation:
    Pump Controls Technician
    Location:
    Lubbock, Texas
    Hi Phil
    Boy did you hit the nail on the head with most of that stuff. I can think of a few more, but you listed most of the problems associated with Variable Frequency Drives, or VFD controlled pumps. Whether there are no well built VFD's for domestic use because no one will pay the price required, or just because the manufacturers are taking out quality to build in planned obsolescence is a matter of opinion. Kind of like which came first the chicken or the egg. The fact is there are no well built VFD's made for the residential market.

    The few well built VFD's made for the commercial market are priced accordingly. Which would be fine if they actually did save energy. Most of the VFD's I have seen that lasted any length of time used an air conditioner to keep them from overheating. And in many southern installations the air conditioner for the VFD uses more energy than the pump and VFD combined. Not a good way to save energy.

    Then if you look at systems like the ones in the pump and well and fresh water supply business, there is always a static pressure to maintain. When maintaining a static or constant pressure, a VFD will always increase the energy used as the pump is slowed down. The drop in horsepower and flow rate is not linear. Reducing the RPM of a pump will cause the horse power to drop by 50% alright, but the resulting flow rate will decrease by 90%. This means a VFD can increase the cost per gallon of water pumped by 300% to 500%.

    When the water in the well is always 100' deep and the house always needs 50 PSI (115' of head), the pump must always produce 215' of head, regardless of the flow rate being used. Look at your pump curve. According to the Affinity Law when maintaining a constant head or pressure the pump can only be slowed by 10%-20% at most. With the drop in horsepower being many times less than the drop in flow rate, the cost per gallon pumped goes up, not down. Only with positive displacement pumps and pumps for friction dominated systems like large chillers could there possibly be any energy savings from a VFD. And nearly all pumps in the well pump and fresh water supply business use centrifugal impellers and are not positive displacement.

    Then like LLigetfa says your analogy of one foot on the brake and one on the gas does not work with centrifugal type pumps. When the output of a pump with a regular centrifugal impeller is restricted, the horse power is reduced almost exactly as much as if you reduced the RPM. If you are going to say a VFD reduces energy consumption, then you have to say restricting with a valve also saves energy, because the cost per gallon will be almost exactly the same between a VFD and a valve.

    It is a fact that restricting the flow rate of a pump with a valve will reduce the horsepower almost exactly the same as slowing the RPM with a VFD. This is a completely counter intuitive property of the centrifugal impeller. I would venture to say that only 2%-5% of the engineers in this world understand this counter intuitive property. Engineers don't do well with counter intuitive things. Everyone, even most engineers have it in their heads that restricting a pump with a valve makes the pump work harder, which is the opposite of the truth. Some of them get very angry with me for saying "restricting a pump with a valve makes the pump work easier", but that is a fact.

    Lastly, the engineers who work for the Federal Government are obviously not in that top 2% to 5% of engineers who really understand how pumps work. Mandating the use of a VFD on pumps of any size is ludicrous, as it will actually increase the energy consumption in any system maintaining a static head or constant pressure. When a review of the actual work needed to be accomplished is done, (ie; pump x amount of gallons at y pressure), a VFD will always increase the amount of energy used. The fact that a VFD reduces the energy needed to spin the pump and motor confuses people into thinking a VFD saves energy pumping water, and that is far from the truth.
     
  10. Sergio Zito

    Sergio Zito New Member

    Joined:
    Oct 13, 2018
    Location:
    Argentina
    From the description of your problem the key here to understand, is that when inlet pressure rises the pump fails to get a stable speed.
    Looking how a VFD controlled pump work you must know a low speed limit is established, typically 25hz.
    If running at its lowest speed the sensor detects the setpoint is being surpassed, it symply shuts off the motor.
    As there is flow, pressure will drop and pump will restart, over and over.
    When you rise setpoint (outlet pressure) the pump will need more rpm to get the setpoint, running ok. Same when inlet pressure is lower.
    The trouble occurs when minimum speed is too much for requiered setpoint /flow/pressure working point: pump will cycle.

    Another key is that if you can sustain 5 taps at 70 psi with this pump, is because inlet pressure and flow are very good, so the pump just “boosts”.
    Seems to be oversized for your needs.

    Installing a PRV on the inlet will make the system loose efficiency. More energy will be requiered for pumping all the time.
     
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