There’s no such thing like a fail-proof thermocouple sensors. Over time, thermocouples fail. To compensate for that, a temperature controller will usually go into upscale burnout mode, and drive the furnace burner to low fire or reject the SCRs. Then again, you need to handle the downtime, rework, or maybe the potential of losing product.
Not long ago, a plant operator called to ascertain if there we had a means to work around this burnout mode, so he wasn’t putting things off and materials.
His heat treat load had almost finished its final soak once the control thermocouple broke open. The controller, as expected, drove the furnace burner to low fire. The operator then popped the controller into manual mode, so he could nurse the burden through the remainder of their soak cycle. He used the temperature reading over a recorder, fed from your second, unbroken thermocouple inside the protection tube as temperature indication for your load.
In the event the situation had happened during the night time, it might not have already been handled with similar attention your day-shift operator had provided.
So, he asked if there was any way to get the controller automatically “fail over” to a second thermocouple.
Multiloop controllers with rack-mount I/O, like Process Automation Controllers (PACs) or higher end PLCs, have multiple I/O points and programming that could implement a control strategy for fail-to a 2nd thermocouple as soon as the primary control thermocouple breaks.
WIKA T32 and Honeywell STT350 temperature transmitters with dual inputs for failoverFor some decades, temperature transmitters used at the same time industries have gotten connections and settings for a couple of sensors, and so the output would fail over when the primary sensor (or its wiring) failed due to a burn-out (open circuit) condition.
WIKA’s T32 transmitter and Honeywell’s STT350 smart temperature transmitter have the dual input fail-over feature (sometimes called redundant sensor operation).
However some industries or shops don’t use either multiloop controllers or temperature transmitters. They connect a thermocouple along with its extension wire right to just one loop temperature controller as shown below.
The Honeywell UDC 3200 or 3500 ¼ DIN controllers do it by alarming on thermocouple break conditions (upscale or downscale) and switching in the primary thermocouple on input #1 to 97dexhpky secondary thermocouple on input #2. It takes approximately two seconds to change over, so it’s fast and disrupts the burner minimal.
Implementing thermocouple fail-over for UDCs necessitates the Input #2 and digital input options. Here’s the application note detailing thermocouple fail-over setup for Honeywell UDCs.
The plant agreed with our recommendation to alarm with a failover condition by connecting the controller alarm via an interposing relay to some supervisory system that both annunicates the alarm and sends a text message or email on alarm.
A production schedule do not need to be disrupted from the failure of a control thermocouple. An inexpensive, simple thermocouple fail-over strategy could save the day once the primary thermocouple fails by permitting the furnace load through its cycle.