Burner Safety

gwpurdon · March 27

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?

EdTheHeaterMan · 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

gwpurdon · March 28

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

mattmia2 · March 28

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

gwpurdon · March 28

Honeywell.jpeg

PLC Relau Output to Honeywell.jpeg

Relay to PLC Input.jpeg

Switch to relay.jpeg

PLC is an Allen Bradley CompactLogix

gwpurdon · March 28

The PLC came wired that way by the oven manufacture.

ratio · March 29

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 · March 29

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

Larry Weingarten · March 29

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 · March 29

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 · March 29

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.

EdTheHeaterMan · March 30

@gwpurdon said: "I don't want to get into a tit for tat with them over their design."

Then ask for their help in understanding their design. Let them know that you are a service company that is trying to make sure the system is safe to operate, and that you would like to understand the logic behind the design and where the safety feature you believe is missing is actually located.

As several others have mentioned, you do not want to take on the responsibility of redesigning their system.

Here is a tip you can use: there is a site called ChatGPT that will often give you incorrect technical information, but it is great at making your correspondence sound more friendly and less confrontational. I have used it on some occasions to reword my thoughts.

Here is an example of what I wanted to say to someone and what the ChatGPT reworked version looked like:

What I want to send:

What the f (Whiskey Tango Foxtrot) are you thinking, you idiot, you can not put the check in the same envelope with the request. can't you follow instructions? it clearly states that you submit the request then wait for the response before you send the check. how stupid can you be? Wait for the response and then send the stupid check in the envelope provided for checks. Will you ever learn you moron? I am mailing your check back and maybe I might think about forwarding your request to the proper channel

What ChatGPT sent:

Hello,
I want to clarify an issue with the materials you sent.
The instructions require that the request be submitted first, and that you wait for a response before sending a check. The check should only be mailed after approval, using the envelope specifically provided for payments. Including the check with the initial request does not follow the required process.
Because of this, I will be returning your check to you. Once you receive a response to your request, you may then send the check in the appropriate envelope as directed.
Please be sure to follow the instructions carefully going forward, as this helps avoid delays and additional handling.
Thank you for your attention to this matter.
Regards,

So take your query to the oven manufacturer and type it into ChatGPT. Start by asking the ChatGPT AI bot to make this non-confrontational and friendly. You will be pleased or even surprised what it comes up with.

No tit for tat when you ask nicely.

You may not get the reply you want and then you will want to ask again for them to be more clear. Run your request thru ChatGPT before sending.

You want this all in writing so you have the information for future reference. What someone says on the phone is rarely able to be used in any future conversation because they can always say "I never said that". So when you get an answer verbally always follow up with a text or email that sums up what the peraon said to you. Something like this:

“Thanks for the information you gave me on the phone. To summarize our conversation, I understood you to say that the Finigan pin must be placed into the Cronston valve before applying the throttle torque adapter to the machine.

If my understanding is correct, a brief confirmation from you would be appreciated. However, if I misunderstood anything, please let me know the proper procedure so I can be sure I understand your answer completely.”

Burner Safety

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