Thermocouple Fail-Overfrom drift or burn-out

Honeywell UDC ¼ DIN ControllersHoneywell HC-900 Process Automation Controller

1

Thermocouple Fail-over

Situation• Fail-Over strategy

• Primary Control thermocouple burns out• Secondary control thermocouple is mounted nearby, but a

control setup is needed to switch over to the secondary thermocouple when the primary thermocouple fails

• Drift• The higher the working temperature, the more the

chemical degradation of the thermocouple hot junction which creates thermocouple drift.

• Thermocouple failure will include T/C drift detection, even though the thermocouple has not yet broken open.

2

UDC ¼ DIN controller: Thermocouple Fail-over

What’s needed• UDC ¼ DIN PID temperature controller

• two low level, thermocouple inputs, or temperature transmitter for 2nd input• 2nd input on UDC 3200 or UDC 3500 is thermocouple input• UDC 2500 would need a temperature transmitter for its 2nd input,

a high level input• digital input option• alarm relay output

• 2 control thermocouples mounted close to one another• Primary thermocouple for control• Secondary thermocouple for backup

• Configuration to get the controller to ‘fail-over’ to secondary thermocouple when • primary T/C drifts to the condition detected as thermocouple FAIL, or• primary T/C breaks open and upscale burnout is invoked

3

UDC ¼ DIN controller: Thermocouple Fail-over

Approach• An alarm is generated when either

• Thermocouple Health monitoring detects an immanent thermocouple failure (TC FAIL)

• Thermocouple burns out• temperature (PV) > X°F

• (2400° is upscale burn-out value for Type K)• The alarm relay output is wired into a digital input• When the digital input is activated by the alarm relay contact,

it changes the PID’s analog input• from PV 1, the primary thermocouple on Input #1• to PV 2, the secondary thermocouple on Input #2

4

UDC 3500 Thermocouple Fail-over

1st Thermocouple on Input #1

2nd Thermocouple on Input #2

Dual input: 2 thermocouples, one on input #1, the 2nd on Input #2

The drawings look identical. They’re not, the terminal numbers differ. Be sure to get the correct T/C on the correct set of terminals

The cold junction compensator is an external device, highlighted in red (on the right) that needs to be connected,

5

UDC 3500 Thermocouple fail-over Alarm #1 configured to alarm on Thermocouple FAIL or PV > 2400°F (TC burn-out)

Alarm #1 is relay #5 (Table 2-6), for current output.

Alarm 1 NC contacts (relay #5) wire to Digital Input #1:

17 to 2316 to 19 6

UDC 3200 Thermocouple Fail-over

1st Thermocouple on Input #1

2nd Thermocouple on Input #2

Dual input: 2 thermocouples, one on input #1, the 2nd on Input #2

The drawings look identical. They’re not, the terminal numbers differ. Be sure to get the correct T/C on the correct set of terminals

The cold junction compensation is internal on a UDC 3200

7

UDC 3500 Thermocouple fail-over Alarm Configuration

TC FAIL

PVTC IN 1 High Value *

HIGH

0.1

NoLATCH

- -- -- -

- -

DISABLE DISABLE ENABLE

* example: Type K IN 1 high value is 2400°F

8

UDC 3200 Thermocouple fail-over Alarm #1 configured to alarm on Thermocouple FAIL or PV > 2400°F (TC burn-out)

Alarm #1 is output #4 (Table 2-6), for a current output model.

9

UDC 3200 Thermocouple fail-over Alarm #1 NC contact is wired to Digital Input 1

Digital Input #1 is configured for PV2; meaning, change to PV2 (back-up thermocouple on Input #2, when the DI is active [closed switch on the input])

Alarm 1 NC contacts (relay #5) wire to Digital Input #1:

8 to 117 to 10

10

UDC 3200 Thermocouple fail-over Alarm Configuration

TC FAIL

HIGH

PVTC IN 1 High Value *

0.1NoLATCHDISABLEDISABLE

- -- -- -

- -

* example: Type K IN 1 high value is 2400°F11

HC-900 Process Automation Controller: Thermocouple fail-over Function block configuration for dual thermocouple inputs, output of

analog switch block determined by state of thermocouple Failure alarm