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Thread: VFD Savings

  1. #1
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    VFD Savings

    Customer with a 100 HP motor pumping from a large tank to two pressure regulating valves. Input pressure to the valves is 96 PSI. Output is 53 PSI on the load side of one of the valves. Assuming for the moment that they are both set the same.

    The only restriction on the inlet side is the short pipe connecting the centrifugal pump to the tank.

    The VFD savings calculator I found indicated payback was in less than 2 months by just changing the speed(s) for x amount of time for each. That would be a quick sell.

    Seems to me that I would also need to know how much water is actually moving at that 53 PSI in the 6"? pipes leaving the PR valves. What other values do I need to make a better SEWAssumptiveG in cost savings for a customer?
    Tom
    TBLO

  2. #2
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    Quote Originally Posted by ptonsparky View Post
    Customer with a 100 HP motor pumping from a large tank to two pressure regulating valves. Input pressure to the valves is 96 PSI. Output is 53 PSI on the load side of one of the valves. Assuming for the moment that they are both set the same.

    The only restriction on the inlet side is the short pipe connecting the centrifugal pump to the tank.

    The VFD savings calculator I found indicated payback was in less than 2 months by just changing the speed(s) for x amount of time for each. That would be a quick sell.

    Seems to me that I would also need to know how much water is actually moving at that 53 PSI in the 6"? pipes leaving the PR valves. What other values do I need to make a better SEWAssumptiveG in cost savings for a customer?
    Yes, you need head and flow.
    That said, two months would be unusual in my experience.
    Si hoc legere scis nimium eruditionis habes.

  3. #3
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    Does the system run continuously?

    Do you actually know the power being drawn by the motor?

    You need to somehow figure the power being drawn to estimate the savings. Since hydraulic power is flow times pressure, you are correct that one approach would be to know the flow in the pipe.

    But the electrical input to the motor is another approach. The motor is rated 100hp but is probably not being used at full capacity.

    As a rough estimate the change in power consumption will just be the pressure ratio across the valves.

    So if the system currently uses 65 hp, then with the vfd it would use 53/96 * 65 hp.

    This estimate ignores things like pump efficiency and pipe losses, and just focuses on the losses in the valves.

    -Jon

  4. #4
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    Quote Originally Posted by winnie View Post
    Does the system run continuously?

    Do you actually know the power being drawn by the motor?

    You need to somehow figure the power being drawn to estimate the savings. Since hydraulic power is flow times pressure, you are correct that one approach would be to know the flow in the pipe.

    But the electrical input to the motor is another approach. The motor is rated 100hp but is probably not being used at full capacity.

    As a rough estimate the change in power consumption will just be the pressure ratio across the valves.

    So if the system currently uses 65 hp, then with the vfd it would use 53/96 * 65 hp.

    This estimate ignores things like pump efficiency and pipe losses, and just focuses on the losses in the valves.

    -Jon
    I should be able to handle that with my old FLuke 43B.

    Renting a clamp on flow meter was my other option or measuring the tank and determining how many gallons the pump removes in x amount of time.
    Tom
    TBLO

  5. #5
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    Quote Originally Posted by winnie View Post
    Does the system run continuously?

    Do you actually know the power being drawn by the motor?

    You need to somehow figure the power being drawn to estimate the savings. Since hydraulic power is flow times pressure, you are correct that one approach would be to know the flow in the pipe.

    But the electrical input to the motor is another approach. The motor is rated 100hp but is probably not being used at full capacity.

    As a rough estimate the change in power consumption will just be the pressure ratio across the valves.

    So if the system currently uses 65 hp, then with the vfd it would use 53/96 * 65 hp.

    This estimate ignores things like pump efficiency and pipe losses, and just focuses on the losses in the valves.

    -Jon
    Would that ever be high enough to give a reasonable ROI on the installation of a VFD?
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

  6. #6
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    Does the thing ever have circumstances where the load is closer to 100 HP? If not best investment would have been a smaller motor up front. If it is determined you do see max load of ~65 hp then that probably means you at least need a 75 HP motor.

    Does cost of 100 HP drive justify that route vs changing to a 75 HP motor?

    This is presuming a centrifugal pump also. I have a client with a positive displacement high pressure pump application (50 hp motor on this one) where the load does vary to some degree depending on what product they are putting through it, but their main concern was maintenance costs on some pump components, slowing the pump down yet being able to main needed pressure and flow made VFD attractive on this application. I think they run it between 40 and 50 Hz most of the time, but has lessened the replacement parts needed since we put the drive on it. When running at 60 Hz the pump parts are moving at faster speed and wearing out faster, to regulate pressure and flow they simply adjust bypass valves in the system, which they still do running on the vfd but it is a balancing act of getting the right pressure and flow but at lowest input speed as possible without overloading the motor.

    This is pump that injects liquid product into a dryer, getting the right spray pattern into the dryer effects what kind of powder consistency comes out of the dryer, so pressure and flow rate through the nozzle is critical to final product, but not every product has same solids content plus temp, pressure and humidity of ambient has some impact and needs a certain amount of fine tuning every time they run it.
    I live for today, I'm just a day behind.

  7. #7
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    Quote Originally Posted by don_resqcapt19 View Post

    Quote Originally Posted by winnie View Post
    As a rough estimate the change in power consumption will just be the pressure ratio across the valves.
    Would that ever be high enough to give a reasonable ROI on the installation of a VFD?
    Let's imagine best case for the OP; system runs at a full 100 hp 24x7.

    100 hp is about 75 kW. 1800 kWh per day.

    Assume a 100% efficient pump; 75 kW of hydraulic power (pressure * volume) exits the pump.

    Pressure is being dropped from 96 to 53 PSI After the pressure reducing valves, there is only 41 kW of hydraulic power

    If we instead run the system at lower speed to get only 41 kW of hydraulic power out of the pump then we only use 984kWh per day, saving 816 kWh per day.

    This is a best case back of the envelope, and it would give a pretty rapid payback.

    As kwired noted, if the load isn't changing, it might make lots more sense to reduce how hard the pumps are being pushed, so that the pump output pressure is lower and there is less loss at the pressure reduction valves.

    -Jon

  8. #8
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    Does it run continuously? I’d have to say, yes. Especially now. 3000 Head Dairy.
    Tom
    TBLO

  9. #9
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    if running at constant flow and all that is required is a press drop resizing the pump is the best option

    you can get a rough approximation without knowing flow since it will cancel when looking at a ratio of the power vfd/valve

    since the dp is so high 43 psi ~ 100' a vfd may pay off

    assuming piping losses ~ 50% of the valve or 50' head
    we can calc an ~ flow assuming motor at 90%/hp, pump eff 60%
    hp = q x 150 x 8.34 /(60 x 550 x 0.6) = 90
    q = 1400 gpm

    hp loss due to valve ~ 100 x 1400 x 8.34/33000 = 35 hp or $25k/yr ($0.1/kwh, continuous)

    the best (only) way is to get the pump curve
    plot the system curve with valve
    apply affinity laws to get pump head to 53 psi/100' hd
    compare the hp for both

  10. #10
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    Quote Originally Posted by winnie View Post
    Let's imagine best case for the OP; system runs at a full 100 hp 24x7.

    100 hp is about 75 kW. 1800 kWh per day.

    Assume a 100% efficient pump; 75 kW of hydraulic power (pressure * volume) exits the pump.

    Pressure is being dropped from 96 to 53 PSI After the pressure reducing valves, there is only 41 kW of hydraulic power

    If we instead run the system at lower speed to get only 41 kW of hydraulic power out of the pump then we only use 984kWh per day, saving 816 kWh per day.

    This is a best case back of the envelope, and it would give a pretty rapid payback.

    As kwired noted, if the load isn't changing, it might make lots more sense to reduce how hard the pumps are being pushed, so that the pump output pressure is lower and there is less loss at the pressure reduction valves.

    -Jon
    Ok, I assumed that the reduction of pressure on the discharge valve also resulted in a reduction in flow.
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

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