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  1. #1
    Join Date
    Apr 2006
    Location
    Massachusetts
    Posts
    113

    setting up a vfd on a pump

    I have a system that has 2 15 hp pumps with by pass control. DP is set at 25 system runs at 30 to 32 with bypass 100% open. These pumps run 24/7 365 feeding about 30 reheats and a 10" pipe in heat exchanger.(sorry don't know Cap. of exchanger) 1 pump on, 1 on stand by. Supply and return Piping 12 inch, by-pass is 8 inch about 30 feet from pumps. Total pipe run one way is about 200 feet. Pumps on second floor pipes drop down to first floor. EMS system is controlling by-pass valve and right now on and off of pump motors. Using ALC controls.

    Thinking of putting vfd on pumps but not sure how to control them. Would I take the bypass out of the control loop or need to add a control point to the system?

    I'm more of a tech then control guy so would be installing the VFDs and wiring. Would have my control company come in for programing and start up.

    Any help or suggestion would be most helpful.

    Thank you
    Dean

  2. #2
    Join Date
    Jul 2005
    Location
    Pittsburgh, PA
    Posts
    258
    Typically I would see a pressure sensor out in the field at 2/3 controlling at a setpoint. Start one pump and run it for 200 hours, then swap to the other. Monitor your hours, status, and alarm point, from the VSD, send out a S/S and an AO for speed control. The number of AOs and S/S will be determined by the VSD setup. You're using ALC so you and program just about anything.

    Be cautious during a lead swap, sometimes you have to ramp the lead pump down, swap leads, and ramp pump 2 up. Depending on your piping, you may ramp up #2 and go into a cavitation situation. You shouldn't need the bypass since you will be controlling the loop on pressure, assuming that you have a 3 pipe system and not a 2 pipe. You may want to enlist the help of a balancer to be sure that you are supplying the correct amount of water. You can also look at your worst case and be sure that you have enough water at that unit.
    HotRod


    Controls..some days your a hero, some days your a zero. Direct acting since 1992.

  3. #3
    Join Date
    Apr 2006
    Location
    Massachusetts
    Posts
    113
    Didn't think about that until you said it. It is a 2 pipe system. The pressure sensor that's on the system now is just after the by-pass. It's not the way I would of install system, but this is what I got stuck with. Tring to get some kind of control of system and try to save some energy..

    I also forgot earlier that the by-pass valve has been replaced 4 times in the last 8 years. Complete blow out of valve and all packing. This is what I have been told, and what I can dig up from past work orders.

  4. #4
    Join Date
    Sep 2008
    Location
    Cedar Rapids, IA
    Posts
    476
    Quote Originally Posted by coolright View Post
    Didn't think about that until you said it. It is a 2 pipe system. The pressure sensor that's on the system now is just after the by-pass. It's not the way I would of install system, but this is what I got stuck with. Tring to get some kind of control of system and try to save some energy..

    I also forgot earlier that the by-pass valve has been replaced 4 times in the last 8 years. Complete blow out of valve and all packing. This is what I have been told, and what I can dig up from past work orders.
    You can feed the pressure sensor output straight to a VFD. Most of them have an ability to control to an analog input setpoint. Just set it for a pressure right below what the bypass would open at. Some VFD's can handle having 2 pumps lead / lagged off of their output. You could have one VFD do both pumps this way and leave the existing controls alone. If you go with 2 VFDs just parallel the DC output from the pressure sensor to both of the VFD's

  5. #5
    Join Date
    Jun 2001
    Location
    San Diego, CA
    Posts
    1,149
    just commissioned a system like this. Here's what we did.
    Open everything up, all the zone valves, close the bypass, and run the pump 100%. Pressure at your DP sensor in this condition becomes your bypass setpoint. Now close everything and open the bypass. Set the pump minimum speed for rated gpm's through the bypass (should be the minimum gpms for the heat/cold source.)

    The VFD will ramp down until it gets to the last one or two zone valves then the bypass will start opening up. Should save some wear on the valve over time.

    This assumes a properly designed system in good balanced condition.

  6. #6
    Join Date
    Apr 2006
    Location
    Massachusetts
    Posts
    113
    Thanks to all. I like that idea from you gbfromsd. That is kind of the way I was thinking but not sure if it would work.

    On the VFDs I would be putting 2 on. 1 for each pump. Controls right now are an analog output to motor starter to start stop. On a time base. So right now pumps run if they need to or not. They ran all summer with no need.

    Also will be needing to work on set points for boilers right now set for on at 170 off at 180. No OAT reset for them.. 7 boiler total, talk about a waste of energy. Shut 4 down for summer. Wanted to shut more down but managment feel we needed rest of them on..

    Dean

  7. #7
    Join Date
    Jul 2005
    Location
    Pittsburgh, PA
    Posts
    258
    Keep in mind that you can do this with 1 VSD, Danfoss makes a VSD with Lead/Lag and bypass. One VSD with 2 output contactors controlled by one AO. This will save you money but you don't have a backup drive. I guess with a bypass it really does'nt matter. One start signal, 1 control signal, 1 lead/lag signal, 1 input signal.
    HotRod


    Controls..some days your a hero, some days your a zero. Direct acting since 1992.

  8. #8
    Join Date
    Mar 2008
    Location
    Northeast
    Posts
    128
    Quote Originally Posted by gbfromsd View Post
    just commissioned a system like this. Here's what we did.
    Open everything up, all the zone valves, close the bypass, and run the pump 100%. Pressure at your DP sensor in this condition becomes your bypass setpoint. Now close everything and open the bypass. Set the pump minimum speed for rated gpm's through the bypass (should be the minimum gpms for the heat/cold source.)

    The VFD will ramp down until it gets to the last one or two zone valves then the bypass will start opening up. Should save some wear on the valve over time.

    This assumes a properly designed system in good balanced condition.
    unless the by-pass is EOL I would delete the by-pass no need for it with the VFD, If you had a by-pass near EOL you could go to a min flow to keep the water in the loop at a desired SPT.

    -V-

  9. #9
    Join Date
    Apr 2006
    Location
    Massachusetts
    Posts
    113
    I didn't know about the danfoss VSD. I'll have to check it out. Sounds like a good way to go.

    Other question is.. What is EOL? I know many be stupid question but have never hear that before.

    Dean

  10. #10
    Join Date
    Jul 2005
    Location
    Pittsburgh, PA
    Posts
    258
    End-of-line....

    There are no stupid questions, just sometimes stupid answers...LOL
    HotRod


    Controls..some days your a hero, some days your a zero. Direct acting since 1992.

  11. #11
    Join Date
    Apr 2006
    Location
    Massachusetts
    Posts
    113
    No the by-pass is at the being of the line about 30 feet from pumps. Plate heat exchanger is at of line about 200 feet from pumps.

  12. #12
    Join Date
    Jan 2007
    Posts
    735
    Start with what gbfromsd and hotrod53 said. Open everything up and close the bypass valve - you can actually eliminate the use of the bypass valve like hotrod53 says.

    If you know what your total GPM is supposed to be, and you have a way to measure it (ultrasonic meter, or flow meter) you slowly ramp the VFD up unitl you get the desired GPM. Take a reading at the DP sensor and that will be your setpoint.

    Your other option is to do a little bit of trial and error. If you're trying save energy, you could try setting the VFD based on trial and error. 30 reheat coils? Four boilers minimum? Why your not resetting that hot water loop is beyond me.

    How large is this building?

  13. #13
    Join Date
    Nov 2006
    Location
    SE Wisconsin
    Posts
    299
    Just a thought but I didn't see it mentioned earlier.

    Make sure your motors are inverter rated. I have a customer that (somehow) ended up with VFD's on motors not rated for inverter duty.

    He's gone through bearings like Grant went through Richmond until they got that figured out.

    Not necessairly what you were looking for but my .02

    Regards!
    Last edited by chesehd; 09-18-2008 at 10:02 PM. Reason: grammar

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