Instrumentation engineering root cause analysis (RCA) of fire burnt ammonia converter thermocouple (TC) problem threaten entire plant shutdown.
|Article Type:||Root Cause Analysis (RCA)|
|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.
Thermocouples (TC) Problem
Thermocouple (TC) flange leaks caught fire burnt the catalyst beds 24-nos.
Mineral Insulated Thermocouples (MITC) exiting the ammonia converter shell bolted TC flange welded compression fittings (fig 1 shows on exiting MITC) and terminating at the 3 m away located Junction Box (JB).
The NH3 convertor has 3 catalyst beds. ¼” dia 4-corners positioned duplex Chromel-Alumel measure the bed’s bed top (gas inlet) temperature and bottom (gas outlet) temperatures.
Thus, each bed has 8 duplex MITCs and the 3 beds 24 MITC total. Each thermocouple (TC) exists via a compression fitting welded to the 10” 2500 # RTJ flange bolted to the convertor wall.
Putting 24-Nos. spare MITCs involves the problems of:
- Catalyst evacuation which involves months of pre-shutdown preparation and 5-days for catalyst evacuation i.e. a a month to 1½ month unscheduled total plant outage
- The 10-15 more years residual life catalyst becomes unusable and needs expensive disposal
Hence, the author volunteered to try joining the 50-60 mm long MITC stubs to the JB with extension leads (EL) though it is constraints ridden as under:
- Field connecting MITC ends to extension leads is time intensive and mostly unsatisfactory. Hence, factory provided MITC leads terminate at the JBs as in this case
- Stripping the fire hardened incolloy MITC sheath requires considerable practice which the crew lack
- Necessary stripping tool is not available
Yet the author took up this task as the other alternative is unthinkable. A mostly used up erection surplus single pair EL drum and author purchased TC wire joining eyelets and crimping tools were the favorable factors.
MITC to EL joining the training
The author formed three teams each of an electrician and instrument mechanic and trained the team as under:
- They recovered 24 Nos. 1 m MITC good portions from those between the stubs and the JB
- They welded 150 mm long junior hacksaw blade to a 100 long ½’ pipe
- Deep scored the MITC sheath all around 25 mm away from an end
- Holding the long end with a plier and bending the 25 mm portion with another plier broke the sheath neatly
- Carefully tap the small end with a screwdriver blade shed its Mgo insulaing powder leaving bare TC wires without shorting with each other
Trial with tube-cutter to mark the MITC sheat invariably shorted the TC wires and hence abandoned.
The team practiced sheath stripping till each team member became proficient.
Other Thermocouple Associated Tasks
Other Thermocouples (TC) associated tasks done were:
During the training, a millwright tightened the TC flange studs soap bubble test leaks free and an inst technician TC compression fittings.
The convertor under near 100 bars pressure helped leak testing. The cramped 10-m elevation location and the 14 mm spanner frequent slips and its retrieval from 0 elevations tired the technician and delayed the job initially.
The author got the spanner twine tied to a nearby structural member and speeded up the job.
Once the TC flange and TC fittings were leak-free, a ‘graduate’ team set to join the stub wires to JB1 terminated extension lead wires thus:
- Peel the TC1 Stub – a map of the TC flange identifies the TCs) – sheath as learnt during ‘graduation’.
- Epoxy seal the sheath opening (fig 1) to prevent the MgO insulation absorbing moisture and rendering the TC useless,
- Identify +ve chromel wire using an AVO meter and slip a 2 mm diax2mm long yellow sleeve over it. As the convertor is still hot an AVO meter identifies thermocouple wire polarity
- Slip a 10 mm dia x 30 long yellow sleeve over the EL jacket and a 4 mm diax20 long yellow sleeve over the yellow EL 1 core. The crew terminating the ELs at JB1 tagging each EL converter end with TC Nos. help connecting to correct TCs.
- Slipped lose 50 mm long yellow colored sleeve over the EL. Inserted the stub TC wire and EL1 barred yellow lead into a brass eyelet and crimped the joint tight. Slipped the 4 mm dia sleeve bit over the joint
- Likewise, eyelet jointed the stub’s negative Alumel wire with that of EL1 red wire and slipped the large dia sleeve to cover part EL and part stub covering the joint entirely. Twine tie at EL and MITC stub.
- Likewise joined the leads of the duplex TC1 leads with EL1A
- TC 1 joint is complete
The tedious tasks are done in the cramped location tired the persons quickly. So he will move out after a joint for a break and another one would step in.
Thus the crew restored all stubs to EL joints in 5 shifts. Contrary to dire expectations, the crew made 22 out of the 24 thermocouples (TCs) work – remarkable indeed!
Other crews completed the rest of the connections, the converter was ready for use in just 4 days, and the threatened indefinite total plant outage disappeared.
The US Managing Director hailed this also “reconstructing an egg run over by a truck!”.
The convertor did fine 7-years more until the catalyst activity became too low and the plant unloaded it and loaded pre-reduced catalyst. The author put the spare thermocouples taking this opportunity in one day before loading the catalyst.
Author: S. Raghava Chari
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