Too Low-Temperature Readings after a Failed Thermocouple Change

Instrumentation engineering root cause analysis of re-heating furnace too low-temperature readings after a failed thermocouple change.

Article Type:Root Cause Analysis (RCA)
Category:Instrumentation
Equipment Type:Instruments
Author:S. Raghava Chari

Note: This root cause analysis (RCA) is from real-time scenarios that happened in industries during the tenure of two or three decades ago. These articles will help you to improve your troubleshooting skills and knowledge.

Too Low-Temperature Readings

The re-heating furnace supervisor complained about too low-temperature readings after a failed thermocouple (TC) change.

The instrument crew repeatedly found correct recorder calibration. Hence, the furnace supervisor asked the author to resolve to enable using the furnace to increase rolling.

Author Solution

The author’s Optical Pyrometer check confirmed furnace temperature around 150O C more than the temperature recorder reading.

He complimented the furnace supervisor for his major disasters avert judgment – slabs if any charged into the furnace would have melted and made it unusable.

Cleaning the mess and repairing the furnace would have taken several weeks and reduced rolling mill output 10 to 15%.

He sketched figs A and B and explained to the crew how the error happened. He instructed them to take care of the following when making TCs and installing them in the field:

  1. Connect the thermocouple wires to head (Hd) terminals making sure of polarities
  2. Connect extension leads at thermocouple (TC) head, intermediate JBs if any and at instrument terminals making sure of polarities. In short, correct polarities are essential all the way.
Wrongly terminated Thermocouple (TC) wires
Figure A
Correctly terminated Thermocouple (TC) wires
Figure B

With the Thermocouple (TC) and Extension

Wires correct polarities all the way the recorder read correct and rolling resumed.

Author: S. Raghava Chari

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