Choosing relay and PLC output card

[Moebius]

I would agree with val_99, and suggest that you have the burner control relay control both of the valves, and simply let the PLC apply a command to the burner control relay...

The burner control relay is much more than a relay. It will include a flame monitor input and possibly an airflow proving input as well as time limits for flame proving and will be certified (if installed according to requirements) to handle the job.

I'm with OkiePC and val_99 - the point is that if you change anything about a control system, especially one that has been designed some years ago, you then become responsible for the design of that system: unless of course you are just replacing a fuse or swapping one relay for a like device.
Once you start adding a PLC YOU are LEGALLY responsible for the system .

If you want to cover your back, then any design you do MUST be checked and verified by a competent engineer with experience in the field that applies, in this case burner applications

Suitable solution would either be a safety PLC (S7-300F) with the right safety functions (eg burner monitoring) or use a standard PLC with a burner control relay (Honeywell, Landis & Staefa, etc)

jdbrandt's comment about hard wired timers is good, and we've used these for functions such as watch-dog, but not for failsafe controls like burners

Good hunting

 

[val_99]

Ok, sorry for delay. Here is my position on your situation:
1. I understand you are dealing with existing PLC control and only thing you are modifying - installing 2 valves instead of one. Some existing PLC code is also modified to acomodate 2 valves.
2. Let leave the question of legality of usage PLC controlling Flame safety aside, but you have to at least investigate it and inform the customer if potential misuse of PLC is indeed occures. Normally PLC based flame safety control is not right thing to do. Only some expert in burner control companies are certified to do it. Even safety PLC not rated for it.
3. I have asked about line size becouse it gave me an idea on the burner BTU rating. Assuming your burner is natural gas burner and gas pressure is in range of inches, total BTU may be below 400000 and you may use only one valve with no feedback, accordingly with FM (Factory Mutual) requirement. Two valves better then one, but still, not feedback is required. So you may use regular solenoid valves.
5. I don't know what country you are working in, you may have local regulations different then US and maybe all we talk, just doesn't matter for your situation.
Good luck

 

[Adenitz]

Sorry for late reply.
I have discussed these issues with the investor and decided that company that installed the burner will deliver the valves. It will be 24 VDC. We will make all necessary program correction, after which the other company will provide necessary certificates.
Thanks for pointing this out.