Hard wire circuit using relays to stop boiler fuel feeders

[SA07]

Hi
We have 2 boilers using coal or bagasse as fuel. There is a hardwire circuit using relays to stop boiler coal or bagasse feeders and close the gate valves. There is emergency push button boiler in Control Room and a Digital Output from Boiler PLC which open this circuit.

In the past, we have got several boiler trips following disturbance/faults on the utility network. We suspect this hardwire circuit is cause of the trips. There is 24 Vdc power supply.

Is it possible to suppress this hard wire and do all the programming in the boiler PLC for the fuel feeders?

We have other critical boiler safety like the following which are programmed in the boiler PLC.

deaerator low water level
drum low or high level
main steam pressure high
furnace pressure high or low
SA, FD, ID fan stop

 

[LittleInch]

Use a small UPS in the power supply into the 24V transformer?

Hard wire is there to increase the reliability of the emergency stop. Going via the DCS reduces this reliability.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SA07]

All control circuits are already supplied by UPS.
All PLC are redundant in pair Siemens S7 400

 

[dpc]

Boiler codes such as NFPA 85 have specific requirements for master fuel trip circuits. This is for understandable safety reasons. There are various schemes used for boiler trips, but it probably is possible to use an energize-to-trip scheme IF you have a reliable battery system. This should eliminate nuisance trips on momentary utility power issues. PLCs can probably be used if configured as fail-safe in the event of PLC failure. But you need to know which codes apply in your location. This isn't something you can just experiment with.

 

[LittleInch]

So if your UPS is working properly why does utility disturbance create issues with your 24V system?

Check the UPS first, then monitor the 24V supply to see if there is an issue or not.

Removing a hard wire system needs a high level of reliability in the system replacing it. I have no idea if that plc provides it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[FacEngrPE]

UPS are not all equal. If a line disturbance passed through your UPS it is probably the wrong one for your site. Depending on the disturbance you may need to add surge suppression.

If your 24 volt supply is DC, some come with ride through batteries (example

https://dcpoweronboard.com/din-rail-dc-ups/.

I believe this is a better approach than using a line voltage ups and a standard 24V power supply.
For critical circuits you can also use additional tricks - like dual 24 volt redundant power supplies - to reduce the risk of a power supply problem causing a trip.

 

[IRstuff]

Typically, in military systems, a circuit implementing a safety feature is not allowed to be implemented in software, for reliability.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[FacEngrPE]

Implementing NFPA 85 in software requires safety rated PLC's, accessories, and design professionals that are experienced with these documents, and Safety Integrity Level design.
[ul]
[li]FM 7605, Approval Standard for PLC-Based Burner-Management Systems.[/li]
[li]API-556, Instrumentation, Control and Protective Systems for Fired Heaters and Steam Generators.[/li]
[li]ISA-TR84.00.05, Guidance on the Identification of Safety Instrumented Functions (SIF) in Burner-Management Systems.[/li]
[/ul]
If you are not in a jurisdiction where NFPA 85 is mandated, find out what is, if not by your jurisdiction, perhaps by your underwriter.

https://www.nationalboard.org/Index.aspx?pageID=164&ID=439[/URL]]When a jurisdictional code inspector walks away saying everything passed, managers and others in a position of authority have a certain peace of mind – even about 40-year-old equipment that requires many manual steps to operate safely and puts their site at serious risk of improper manual start-up or shutdown daily. These managers need to learn that being technically “in compliance” – but nowhere near the current code’s level of safety – is not a moral victory.

Consider also that unless you are in a state that mandates ASME CSD-1 or NFPA 85 compliance, inspections rarely address gas trains and/or fuel system issues. Interlock and gas train testing is usually assumed to be a responsibility of the owner in these states. You can imagine that the level of compliance with these little known testing requirements is much less in states where the issue of combustion control testing is not even on the table.

I have been involved in a boiler refit, that involved the use of PLC;s in a Hamworthy-Peabody flame safety system that included PLC's. There are a bunch of things flame safety systems require in both hardware and software to meet requirements for safety, that I understand but would never attempt to design or modify.

 

[SA07]

Plz see below comments from the commissioning engineer of our boiler:

"From memory this hardwire stop is a requirement of the NFPA code that we used on the design of the control system
If you check the NFPA requirement there may be a clause that states that the designer/owner may ignore this requirement if they have experience that it is not required."

The best thing is that we keep the emergency stop in our control room to stop the boiler feeders through hard wire as a back up in case the PLC freezes.

Thanks all for your response.