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  1. #1
    Join Date
    Feb 2010
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    MALAYSIA
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    Need Advice to place Pressure Sensor

    Hi expert forumer,

    I'm beginner in control automation. I'm need some advice from all of you regarding to this

    1) which is best location to place water pressure sensor to control Variable Speed Drive Pump? - refer picture below

    2) Which parameter (delta pressure or delta Temperature) to control bypass valve?

    Please refer to this picture for more info.

    Thank you


  2. #2
    Join Date
    Apr 2007
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    Amarillo by mornin'
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    I would put it almost to the furthest end of the piping run. Also, control the bypass to pressure not Delta T. If your running from a chiller, it will take care of the temperature for you.
    "It's not that I'm smart, it's that I stay with the problem longer”
    Albert Einstein

  3. #3
    Join Date
    Mar 2005
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    This tends to be argued but I'd basically agree w/Xarralu. 2/3 or further from the pump so you get more of a system pressure and can sense the further zones better. Sensing your 'furthest' zone helps ensure you have proper pressure out to those zones. Granted one many times can tune due to poor locations, but it's better just to have it right from the start, especially since sometimes it takes a whole lot of time to tune...
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
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  4. #4
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    Feb 2010
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    Quote Originally Posted by xarralu View Post
    I would put it almost to the furthest end of the piping run. Also, control the bypass to pressure not Delta T. If your running from a chiller, it will take care of the temperature for you.
    Thank you for suggestion, xarralu and crab master.
    my mistake for Delta T.
    So, Delta T for control bypass and Delta P for control Variable Speed Drive Pump. May I know why need to place the sensor furthest end ? Is it will effect the delta P ?

  5. #5
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    Mar 2005
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    Quote Originally Posted by azka View Post
    So, Delta T for control bypass and Delta P for control Variable Speed Drive Pump. May I know why need to place the sensor furthest end ? Is it will effect the delta P ?
    Quote Originally Posted by crab master View Post
    2/3 or further from the pump so you get more of a system pressure and can sense the further zones better. Sensing your 'furthest' zone helps ensure you have proper pressure out to those zones.
    Proper pressure out to those far zones ensures your valves to have the proper pressure so they will have their rated flow.
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
    BIG Government = More Dependents
    "Any 'standard' would be great if it didn't get bastardised by corporate self interest." MatrixTransform
    http://threedevilskennel.com/ - not my website.
    Versatile Hunting Dog Federation - www.vhdf.org/


  6. #6
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    Feb 2010
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    MALAYSIA
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    Quote Originally Posted by crab master View Post
    Proper pressure out to those far zones ensures your valves to have the proper pressure so they will have their rated flow.
    Thank you for clear explanation, I think that take a lot of time to tuning the PID for VSD and valve

  7. #7
    Join Date
    Mar 2007
    Posts
    272
    The differential pressure should be at the most remote AHU. Find out the minimum required Diff pressure for the coil of this AHU and that should be your setpoint. This setpoint should be maintained by modulating the pump speed. If you maintain this pressure then all AHU's will have sufficient water flow

    The Bypass control is a different story. Your system consists in essence of a primary and a secondary loop but without the use of primary and secondary pumps. I am seeing this setup more and more and this will probably be the future for chilled water control.
    The reason for the Bypass is to ensure minimum flow through the chillers. To make this work you will need differential pressure sensors for each chiller or flow meters for each chiller.

    When the load in the building decreases, the AHU chilled water valves will close and pump speed will decrease. Until you get to the point that there is not enough water flow through the chiller. At this point you will start to modulate the bypass valve open. Because the bypass begins to open, the diff pressure at your most remote AHU will drop causing the pump speed to increase. And system will find a new balance point at which Pump Speed maintains the most remote AHU Diff Pressure at setpoint and the Bypass Valve is opened to the point that the chillers have there minimum required flow.

    By the way in your picture, I believe your Chilled water labeling "To AHU" and "From AHU" and your 2 pressure sensors are reversed
    Last edited by exwtk; 01-22-2012 at 12:16 AM.

  8. #8
    Join Date
    Mar 2007
    Posts
    272
    Actually ... your labeling might be correct but then the discharge and suction of the pump is backwards. Typically with centrifugal pump the water line in line with the pump shaft is the suction and the one coming out of the pump housing is the discharge.

  9. #9
    Join Date
    Jul 2009
    Location
    Kansas
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    370
    Most Sensors should be put 10 Pipe Diameters from the closest fitting, as a general rule of thumb too. Flow, Pressure... anything that can be affected by turbulence.

  10. #10
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    Thank you Exwtk, I understand it the function to control it.

  11. #11
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  12. #12
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  13. #13
    Join Date
    Jun 2004
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    Eugene, Oregon
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    Quote Originally Posted by exwtk View Post
    The differential pressure should be at the most remote AHU. Find out the minimum required Diff pressure for the coil of this AHU and that should be your setpoint. This setpoint should be maintained by modulating the pump speed. If you maintain this pressure then all AHU's will have sufficient water flow

    The Bypass control is a different story. Your system consists in essence of a primary and a secondary loop but without the use of primary and secondary pumps. I am seeing this setup more and more and this will probably be the future for chilled water control.
    The reason for the Bypass is to ensure minimum flow through the chillers. To make this work you will need differential pressure sensors for each chiller or flow meters for each chiller.

    When the load in the building decreases, the AHU chilled water valves will close and pump speed will decrease. Until you get to the point that there is not enough water flow through the chiller. At this point you will start to modulate the bypass valve open. Because the bypass begins to open, the diff pressure at your most remote AHU will drop causing the pump speed to increase. And system will find a new balance point at which Pump Speed maintains the most remote AHU Diff Pressure at setpoint and the Bypass Valve is opened to the point that the chillers have there minimum required flow.

    By the way in your picture, I believe your Chilled water labeling "To AHU" and "From AHU" and your 2 pressure sensors are reversed
    Good answer.
    "The bitterness of poor quality remains long after the sweetness of low price is forgotten". --Benjamin Franklin
    "Don't argue with an idiot, they will drag you down to their level and beat you with experience". --Mark Twain
    http://www.campbellmechanical.com

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