There’s no such thing as being a fail-proof temperature sensor thermocouple. As time passes, thermocouples fail. To make up for your, a temperature controller will often enter into upscale burnout mode, and drive the furnace burner to low fire or reject the SCRs. Then again, you suffer from the downtime, rework, or even the potential of losing product.
Not long ago, a plant operator called to see if there we had a means to work around this burnout mode, so he wasn’t wasting time 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 stress through the remainder from the soak cycle. He used the temperature reading over a recorder, fed from the second, unbroken thermocouple from the protection tube as temperature indication for your load.
In the event the situation had happened in the middle of the evening, it may possibly not are already handled with similar attention the morning-shift operator had provided.
So, he asked if there seemed to be any manner to offer the controller automatically “fail over” to your second thermocouple.
Multiloop controllers with rack-mount I/O, like Process Automation Controllers (PACs) or high end PLCs, have multiple I/O points and programming that could implement a control strategy for fail-over to another thermocouple when the primary control thermocouple breaks.
WIKA T32 and Honeywell STT350 temperature transmitters with dual inputs for failoverFor some decades, temperature transmitters used in the process industries have experienced connections and settings for a couple of sensors, hence 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, many industries or shops don’t use either multiloop controllers or temperature transmitters. They connect a thermocouple and its extension wire straight to a single 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 switch over, so it’s fast and disrupts the burner very little.
Implementing thermocouple fail-over for UDCs demands the Input #2 and digital input options. Here’s the application form note detailing thermocouple fail-over setup for Honeywell UDCs.
The plant agreed with this recommendation to alarm on the failover condition by connecting the controller alarm using an interposing relay to a supervisory system that both annunicates the alarm and sends a message or email on alarm.
A production schedule do not need to be disrupted through the failure of the control thermocouple. An affordable, simple thermocouple fail-over strategy could save the day when the primary thermocouple fails through getting the furnace load through its cycle.