Over-temperature protection using PLC to control temperature in a vessal

[rmore]

I am using a PLC to control the temperature in a vessal. I am looking for opinions on using the same PLC for controlling the temperature, as well as, monitoring and protecting against over-temperature.

I have seen this done 2 ways.
1. Use the PLC to control the heater contactor, and use a seperate over-temp module to control a seperate contactor to protect against over-temperature fault conditions.
2. Use a PLC with 2 thermocouple inputs, one for the process temperature, and one for the over-temperature. The same PLC will then control 2 outputs, one for the heater contactor, and one for the over-temperature contactor.

Option 2 is obviously less expensive, as it does not require a seperate module. However, is option 2 legitimate? Is it safe, or can it be made safe? Is it code compliant?

Thanks, in advance, for your feedback.

 

[jraef]

Often the 2nd over temperature limit relay is a safety device, PURPOSELY separate from the controller as a last resort should the controller fail.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[IRstuff]

Typically, in military systems, safety devices are not supposed to be controlled by software, since software can be subject to design errors that might cause a missed detection of a failure. The simpler the logic and hardware, the more reliable the failsafe tends to be.

TTFN
faq731-376

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[dV8r]

It is generally best to check with your insurance carrier to verify what they need to cover the system in case of failure.
If you are self insured get a write-off from your boss whoever that is and file it.
If you are not insured, put in series two over-temp devices.

 

[itsmoked]

I think I saw "Safety Rated" PLCs that could do your 2) above but normally you don't do that for the reason stated by jraef.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[danw2]

NFPA 86 is the code for ovens, heaters, furnaces over 400K BTU. I don't know who regulates lower output devices, but the high limit safety concept is universal, even for the kitchen coffee makers.

- separate sensor
- separate logic solver/controller
- latching output that interlocks the energy/fuel supply, latches when tripped, requiring manual reset

NFPA calls it "excess temperature limit controller". The market calls them high limit controllers and they're submitted for FM approval (CYA).

What are you going to say when the prosecuting attorney has you read this, out loud in court,

"8.16.10 The operating temperature controller and its temperature-sensing element shall not be used as the excess temperature limit controller".

 

[jraef]

And there ya go... specifics.
That was a better answer.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[daveross100]

Use the TMP300 of TI as a sensor to control the Temperature

Any question : email - daveross100 at gmail dat com