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  1. #1
    Join Date
    Jan 2005
    Location
    Central NC
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    165
    I have a HVAC system that uses hot water and chill water primary and secondary loops. Each circulator pump has a differential pressure switch across it the purpose of which as I understand it is to shut it down in the event of a ruptured pipe. Because the differential pressure switches were not properly matched to the system, some of the pumps sometimes shut down under normal operating conditions because of switch bounce. I have heard that monitoring the current draw of the pump motors is a more reliable way to detect pipe breakage. Is this true? and does anyone have an opinion on which are the better of these monitoring systems currently on the market?

  2. #2
    Join Date
    Mar 2003
    Location
    Cincinnati
    Posts
    7,977
    If you are talking about the coupling breaking on the pump then the differential switch you have should work fine but if not "Hawkeye" makes a nice adjustable current sensing switch that you wire into the starter and you can set it to shut the chiller down if the pump amps go below a set-point.

  3. #3
    Join Date
    May 2004
    Location
    Los Angeles, CA
    Posts
    1,560

    Dear Pwn01,

    An alternate way to monitor the chilled water, and/or hot water circulating pump(s) for either return, or supply pipe and/or fitting breakage would be to have a simple pressure switch connected to the return side of the pump. At a predetermined value the pump would turn off due to insufficient suction water pressure, which of course would occur should a pipe break, should the makeup water refill system fail, or should a suction line strainer plug up, all problems demanding the cycling off of these pump(s).

    Respectfully Submitted,
    John J. Dalton

  4. #4
    Join Date
    Jul 2001
    Location
    Buffalo N.Y.
    Posts
    1,571
    We have pressure switches, but there main reason is to detect a pump shutdown, then switch over to the lag pump.
    I guess they could act as a break detection also, depending on the size of the pipe that broke and the distance from the pump to break point.

    If your having a switch bounce problem causing nusiance shutdowns your having issues with either flow, ie: valves opening/closing causing hi back pressures then dropping out or system cushion issues, or a faulty switch/piping to switch.

  5. #5
    Join Date
    Jan 2005
    Location
    Central NC
    Posts
    165
    It seems that the technical name for the system that uses the differential pressure switch is "loss-of-flow detection." From what I read, it should detect line breakage (low DP) and line blockage (high DP) and shut the pumps down under either condition to avoid pump damage.

    The DP switches currently installed only trigger on low DP and are adjusted to the bottom of (maybe out of) their range (I think that this is what is causing the false alarms). I looks like they actually need an operating window with trip-points for high or low DP.

    These switches are manufactured by JC/Penn. I found a company by the name of Load Controls Incorporated online yesterday that sells the current sensing loss-of-flow systems. I would certainly be easier to replace the present system with DP switches with high and low setpoints rather than converting to the current sensing version. Does Honeywell sell DP switches for this type of application? It seems that the websites of these companies are not user friendly to the average Joe. One must know very specific information to get any info off of their sites. Other than JC/Penn and Honeywell, what companies sell this kind of thing?

  6. #6
    Join Date
    May 2004
    Location
    Los Angeles, CA
    Posts
    1,560
    Dear Pwn01,

    Please post both the upper, and the lower limits of this pressure differential switch and the electrical requirements, and maybe we can find a switch for you.

    Respectfully Submitted,
    John J. Dalton

  7. #7
    Join Date
    May 2004
    Location
    Los Angeles, CA
    Posts
    1,560
    Dear Pwn01,

    Couldn’t you simply use two separate single input pressure switches with a single pole, double throw electrical configuration each? These are relatively inexpensive, easy to obtain, and come in a variety of operating pressures and differential ranges.

    Respectfully Submitted,
    John J. Dalton

  8. #8
    Join Date
    Jan 2005
    Location
    Central NC
    Posts
    165
    The lower limit would need to be 5 psi while the upper limit could be 50 or 60 psi. The present set-up uses 24 VAC as the signal voltage. Unfortunately, there is only one pair of wires pulled to the location (for each of four switches) and it would be a job to pull others. I suppose, though, that it would not be difficult to design a simple circuit that would take the output from two different DP switches and translate that into a single signal to send back to the control software. I'll have to think about that, John, that has potential especially in light of the fact that I have a surplus of those switches on hand.

  9. #9
    Join Date
    May 2004
    Location
    Los Angeles, CA
    Posts
    1,560
    Dear Pwn01,

    Based on the fact that you have these pressure switches on hand, I think that would be the best path to seek, and the accuracy, and dependable repeatability of these simple pressure switches would make installation, setting, and operation of these everyday components a snap for you.

    This would also be within the scope of the KISS (Keep It Simple Stupid) method as well.

    Respectfully Submitted,
    John J. Dalton

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