Thermocouple Extension Lead Problem

Instrumentation engineering root cause analysis (RCA) of thermocouple extension cable lead problems and solutions.

Article Type:Root Cause Analysis (RCA)
Equipment Type:Sensors
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.

Thermocouple Extension Lead Problem

Study the previously discussed Problem: The thermocouple (TC) flange fire had burnt the 50-m long 30-pair chromel-alumel extension cable connecting the Mineral Insulated Thermocouples (MITC) JB to the control room instruments.

Airfreight from the US takes a minimum of one month. Vendors quoted min 6 months delivery and no plants could loan or sell.

Author solution

The author developed the below-given steps to solve the difficult unresolvable despairing problem (DDUDP).

  1. Run a rush purchased 30‑core 1.5 mm2 copper cable and single pair TC extension lead from JB1 to panel terminals. Luckily, sufficient length single pair extension lead was left after solving problem 19.. 
  2. Convertor TCs jointed extension leads (Read Previous Problem) already terminate on JB terminals; Cut measured length plus 3 meter extra EL from the EL drum used in previous problem. Connect one wires to a JB1 terminal and the other end wires to the panel instrument as in fig 1. In effect this TC is a thermocouple with measuring junction (jn) at JB1 terminals.
  3. Connect the 30-core copper cable wires to panel instrument according to fig 1
  4. The temperature system is ready
Thermocouple Extension Lead Problem
Figure 1

How does the above-described modification work?

Fig shown circuit explanation

 1Convertor TC emf outputTM-TJB1
 2Shorted Single pair Ext Lead emf outputTJB1 – TI
I-Panel Inst
 3Fig: connections add 1+2
 4Hence EMF at instrument terminals(TM-TJB1)+(TJB1-TI) = (TM-TI)
 5The instrument’s cold junction compensation adds emf = TI. Hence the instrument reads the measured temperature TM Correctly

Benefits Realized

Benefits Realized are A total plant shutdown (SD) causing DDUDP resolved inexpensively.

Instrument crew and supervisors learned that one can solve most problems by applying job knowledge and innovations; hence, all should relentlessly increase job knowledge by studying vendor literature, tech journal articles and ads, and remembering these for timely use.

Author: S. Raghava Chari

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