Instrumentation engineering root cause analysis of level instruments of ammonia flash vessel stopped working.
Article Type: | Root Cause Analysis (RCA) |
Category: | Instrumentation |
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.
A Differential Level Transmitter (DLT), a Hi-Level (LG), a Lo-Level (LG), a Hi LS, a Hi-HI LS, a Lo LS, and a Lo-Lo LS – Seven level instruments – bolt to the 3” Sch 120 pipe connecting the vessel top nozzle (below figure; TN) to the bottom nozzle (BN).
The figure shows just two instruments to avoid clutter. Such instruments mounting avoids multiple taps welding on the 40‑bars operating pressure ammonia, N2, and H2 rich synthesis gas wetted vessel subjected to stringent weld codes.
All the level instruments behaved erratic three months after commissioning. Simultaneous erratic so many instruments behavior points blocked signals. Draining the line and steam cleaning if necessary seemed the answer.
All ridiculed the Author’s diagnosis, “how can liquid ammonia as clear as distilled water choke especially 3” pipe column feeding the instruments?”
The author patiently explained, “Friends, I also don’t know the oil source; but oil draining only we can do. Let us do it and hope for the best. The 0.8 Sp Gr oil settles at the tank bottom below the 0.64 Sp Gr liquid NH3.
I guess this oil emulsified and choked the 3” pipe in 3 months. Hence, let us try oil draining. Though half-convinced, the crew were reluctant to drain fearing the extreme pungency of ammonia escaping to the atmosphere. Even PPM levels of ammonia are highly irritating.
The author guided the operation engineer to get the oil drained safely as under:
Crew drained nearly ¾ drum of oil. Then they put the instruments back on service. Finally working all instruments cheered all.
The plant continued till the 18-months later turnaround by draining the oil two monthly.
The author got done below listed activities solved the problem forever:
Thanks to the supplementary nozzle, oil did not reach the 3” pipe, and the instruments performed flawlessly between the two yearly TAs.
Operators conveniently drained the oil from the depressured tank easily and were extremely safe.
Intermediate & final Solution Benefits are:
Author: S. Raghava Chari
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