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  1. #1
    Join Date
    Apr 2006
    Location
    Columbus, Ohio
    Posts
    809

    Controlling a Variable Speed Pump off of Pressure Differential

    My company is wanting me to set up a system to control off of pressure differential. I was wondering how can I determine a good setpoint for this? When controlling off pressure differential does the pump need to slow down when the pressure differential is high?

    Thanks
    J. King

  2. #2
    Join Date
    Mar 2008
    Location
    Council Bluffs, Iowa
    Posts
    112
    Quote Originally Posted by apprentice3 View Post
    My company is wanting me to set up a system to control off of pressure differential. I was wondering how can I determine a good setpoint for this? When controlling off pressure differential does the pump need to slow down when the pressure differential is high?

    Thanks
    The Balancer will give you the setpoint, and yes it needs to slow down.

  3. #3
    Join Date
    Oct 2009
    Location
    Phnom Penh
    Posts
    31
    Yes you are right. When you PD get higher VSD should slow down. and PD get lower VSD should speed up.

    You need to write the LOOP for this operation. For Loop statement you need to carefully set the parameter.

    If you set wrong then VSD Variable is too fast and will make your entire system get problem. so you need to set Loop that VSD can change the speed slowly. and you can accept.

    yes correct the Balancing Team will tell you the setpoint of the PD. Cause when they do the balancing they will run the VSD manually. after they finish they will go to measure the differential pressure where you install PD sensor. and this set point you need to put in your loop statement in (BIAS). So mean the set point and your BIAS is almost the same.

  4. #4
    Join Date
    Mar 2005
    Location
    Pacific Time Zone
    Posts
    4,211
    Just to throw out there are you sure this is an application that a VSD with diff press control is worthwhile?

    I've had more than one job where such was done/planned, but it'd never work as the piping arrangement/valve setup wasn't correct - 3 way valves on all the valves, deadheading the pump, etc... Many it ended up just being a nice soft start/stop setup with a bit of 'motor protection'.
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  5. #5
    Join Date
    Jun 2010
    Location
    Southwest NM
    Posts
    22
    How big is the pump?

  6. #6
    Join Date
    May 2002
    Location
    Madison, Wisconsin
    Posts
    430
    You don't tell us enough to even guess a set point. Something in the piping needs the flow through it controlled. What is it?

    Typically, this control loop would include a VFD or a bypass valve. Bypass is the old-fashioned way, since it tends not to reduce the pumping energy when flow through the load is not required. VFD's are the new-fangled way, since they actually slow the pump down to reduce the the flow, so also reduce the required pumping energy. Typically, you might be controlling the pressure across, say, a heating/cooling coil with a variable valve which is performing proportional cooling. Maybe even a gob of such coils, each with its own valve. Give us a clue ...
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  7. #7
    Join Date
    Dec 2007
    Location
    Georgia
    Posts
    20
    Measure your pressure at the end of line and start out low. As previously stated we need to know an application. In my experience Trane wshps for example can operate 6-10 psid ...just depends. TAB would be beneficial here.

  8. #8
    Join Date
    Aug 2006
    Location
    New England
    Posts
    520
    Assuming there are 2-way valves on the job and the pumps were designed for a 100% call on all valves, the proper procedure would be to open all the valves, run the pump at 60 Hz, measure the differential pressure and that's your setpoint.

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