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Burner Safety

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gwpurdon
gwpurdon Member Posts: 6
in Gas Heating

I have an industrial gas furnace with a Honeywell burner controller for the gas controls and a PLC for the furnace controls. I have discovered a major issue with the gas safety wiring. They did not wire the gas safety switches in series and directly to the Honeywell Controller. They took each switch back to a standard relay. From the relay it goes back to a PLC non safety input and out of a relay output card in the PLC to the Honeywell controller. The PLC also controls the gas modulating valve. I know this is a big safety issue that must be corrected.

I can only assume they did this so the PLC could provide diagnostics when a safety trips. That feature is desirable and would like to be able to keep that and meet safety regulations. I have a question about an idea that would provide both options but want to see if it meets regulations.

What if I wired each gas safety switch to it’s own single channel safety relay module like a Dold LG5924-48-61-24. All of the Dold modules safety rated contacts would be in series and connected directly to the Honeywell safety chain input. The non safety contacts would go back to the PLC for the diagnostics. This removes PLC from the safety circuit. I know this will work but will it meet safety standards?

Comments

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 12,168
    edited March 28

    Can you sketch the wiring diagram as you believe it is connected from one limit/safety switch to the next and back to the common area where they all come together? or maybe take a photo of the wiring diagram on the boiler or burner?

    Your PLC may have separate terminals for each type of limit or control switch in order to be useful in diagnosing faults. What is the model number of the PLC?

    This statement scares me a little "This removes PLC from the safety circuit. I know this will work but will it meet safety standards?" You would need to know the logic the original equipment manufacturer had in mind when the design was approved and certified for sale to a consumer/contractor for installation. Again a wiring diagram from the manufacturer would be helpful

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • gwpurdon
    gwpurdon Member Posts: 6

    I will upload a sample of the original print as the manufacture designed it.

  • mattmia2
    mattmia2 Member Posts: 16,762

    is the plc something that was furnished with the boiler or was it field installed as part of the building controls?

  • gwpurdon
    gwpurdon Member Posts: 6
    Honeywell.jpeg
    PLC Relau Output to Honeywell.jpeg
    Relay to PLC Input.jpeg
    Switch to relay.jpeg

    PLC is an Allen Bradley CompactLogix

  • gwpurdon
    gwpurdon Member Posts: 6

    The PLC came wired that way by the oven manufacture.

  • ratio
    ratio Member Posts: 4,236

    If you make changes to the manufacturer's wiring, I would imagine it would be your responsibility to prove to the fire inspectors that you didn't cause any problems. If it's wired up the way they say, it would be theirs…

  • gwpurdon
    gwpurdon Member Posts: 6

    I agree if I make changes, I take on responsibility. If I know it's wrong and dangerous and do nothing?

  • Larry Weingarten
    Larry Weingarten Member Posts: 4,096

    Hi, I'd be tempted to contact the manufacturer and talk with their engineer. Either they will satisfy you that their design is safe, or they will thank you, and then authorize you to make changes… hopefully in writing.

    Yours, Larry

  • gwpurdon
    gwpurdon Member Posts: 6

    I don't want to get into a tit for tat with them over their design. I am not a competitor because I don't build ovens, we are a controls group. I discovered the issue while working on the oven for my customer. I recommended to them that they should look into it and they asked me to fix the issue.

    I will recommend to them that the safety chain be wired straight back to the Honeywell controller. I will remove any safety circuits from the PLC. They will lose the diagnostics be it will be safe.

    Allen Bradley has just come out with burner controller for their PLC. It uses the GuardLogix PLC with safety rated I/O and a certain version of the software. It is very expensive but will also offer this to them as an option.

    The question I posed was more for my curiosity of the safety aspects of my idea.

  • mattmia2
    mattmia2 Member Posts: 16,762

    probably the way to cover your liability is to have a pe sign off on it. it is probably some interaction of the mechanical/fuel gas code and some listing standards so sorting it out is a little bit above the technician level.

    using double pole relays to give feedback to the plc and control the gas control directly seems functionally correct but it would really take an engineer to sort out all the regulation that apply.

    it is supervised, it isn't something just siting in a basement so there is more opportunity for someone to notice a malfunction before it becomes catastrophic.

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