Plant Pump Protection
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  1. #1
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    Plant Pump Protection

    I have a larger chilled water plant with 2 chillers and many motorized valves to operate in any configuration ( either one stand alone or series with either one being lead ). This is a new start up and it was all running when I first got involved two weeks ago ( noticed some water on the floor then ). The programmer is telling me that he has the valves proving open before pumps can be enabled but I arrived on site the other day and found the plant down and pumps running with valves closed.

    I'm thinking that a hard wired interlock is the only way to be positive that the pumps will not operate if valve positions are not correct. Thoughts on this ? System is Tracer SC .

  2. #2
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    I would consider temperature monitoring of the pump casings if shut off head is a concern. Flow switches are another protection option but require taps and need a time delay. Does this plant have equipment operators or engineers around when changing lineups? Are they around to respond to alarms?
    Last edited by Ithos; 07-13-2013 at 09:13 AM.

  3. #3
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    Usually, I hard wire a permissive to the pump control circuit using pilot relays from the valve end switches. This ensures that even if somebody overrides the pumps from the control system, nothing will run.

  4. #4
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    The valve actuators each have 2 end switches that are currently not being used for anything is what I was thinking in my original post. I know that there are programmable relays available with multiple inputs available that I could use for a " this or that " scenario to either enable or disable the vfd's.

    There are currently no operating engineers on site and I'm afraid that eventually there may be persons with their own interests accessing the system. Also , the casings do currently have temperature indicators on them ( they don't seem to have any wiring connections ) , and I don't really know much about them.

  5. #5
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    Depending on the pump, it might be damaged by overpressure or cavitation long before it overheats. In that case, a pressure switch or two would be a good idea.

  6. #6
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    I guess that I was typing when the reaper posted , thank you reaper.

  7. #7
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    Quote Originally Posted by NiHaoMike View Post
    Depending on the pump, it might be damaged by overpressure or cavitation long before it overheats. In that case, a pressure switch or two would be a good idea.
    I appreciate the thought and the reply.

  8. #8
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    In that case Reaper's suggestion is an absolute minimum imo. If there are other configurations which may lead to shut off head then further measures should be considered. Installing a diff temp switch between the pump casing and the suction piping maybe one solution but there may be better ones depending on pump design. Human oversight is a significant layer of protection in plant operations. Without it you will need a much more sophisticated design from a control standpoint.

  9. #9
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    Thanks Ithos.

    As a controls contractor , are safeties like this typically specified by the design engineer or is this something that you decide upon review ?

  10. #10
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    Or anyone , is this something that " good " controls contractors pick up on at bid time ? It does not really matter in my case at the moment either way , just curious. I know everyone is hanging on every penny right now , so the question can read " back in 1999 would the controls contractor place this protection wiring in his submittal on his own or was this something always stated from the design engineer " ? Nothing is stated on my drawings regarding this f.y.i.

  11. #11
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    They should be specified in the design which presumably was done by an engineering firm. But since you have already experienced a significant problem the controls contractor may need to perform modifications. My advice was probably the least expensive option. If you have the budget then there certainly better choices. But with pressure and flow interlocks they will need to be bypassed upon start(delay on make) which may mean they have to be incorporated into the digital control system. Some examples http://us.magnetrol.com/PDFs/1/41-114.pdf . Mechanical flow switches like those used on many chillers are not that reliable in my experience especially paddle switches. If these are new pumps then I would consult the authorized dealer or manufacturer who could probably provide the best guidance. Also any place that repairs pump seals could give some expert pointers.

  12. #12
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    Quote Originally Posted by Ithos View Post
    They should be specified in the design which presumably was done by an engineering firm. But since you have already experienced a significant problem the controls contractor may need to perform modifications. My advice was probably the least expensive option. If you have the budget then there certainly better choices. But with pressure and flow interlocks they will need to be bypassed upon start(delay on make) which may mean they have to be incorporated into the digital control system. Some examples http://us.magnetrol.com/PDFs/1/41-114.pdf . Mechanical flow switches like those used on many chillers are not that reliable in my experience especially paddle switches. If these are new pumps then I would consult the authorized dealer or manufacturer who could probably provide the best guidance. Also any place that repairs pump seals could give some expert pointers.
    I have some good and bad feelings about valve interlocks. The first thing to determine is if there is a problem with the pump running dead headed. MOST pump designs should not have an issue, other than heat buildup. If heat build-up is the concern, then a temp switch would make more sense. My concern is that end switches can and do fail, and it would suck to bring down the entire chiller plant because a valve limit switch has failed. On the other hand, if volute temp switch fails, you just lose that pump.

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