Repeat Rupture Discs (RD) bursts shut down a Urea Plant each time.
| Article Type: | Root Cause Analysis (RCA) |
| Category: | Mechanical |
| Equipment Type: | Pipelines and Miscellaneous Problems |
| 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.
Problem
Urea Plant’s first Flash Separator (FFS) Rupture Discs Burst repeatedly a few years after commissioning.
Root Cause Analysis
Since the rupture disc (RDs) functioned well during the last 5-years fit-up problems are unlikely.
Perhaps, de-bottleneck steps achieved higher than rated capacities streams’ throughputs have increased first flash separator pressure higher than before.
Verifying was not easy because of the older generation pneumatic instrumentation and not saved charts.
The author found from the Process Licensor data sheet that RDs are of
1) Burst Pressure (BP) 21 bars and
2) 30% margin between burst and operation pressures and 5%burst pressure tolerance.
That means the RD could burst at 0.95*21 = 19.95 bars. In addition, the operation pressure shall be < 70% burst pressure i.e. <19.95*0.7=13.965 bars is necessary for reasonable RD lives!
The old batch RDs lasted perhaps because of +ve tolerance and lower vessel pressures at the former lower production rates.
Author RCA solution
Since the operation pressure now is 19.95 bars and temperature 165oC, the author insisted on getting 5% tolerance RDs of stamped BP = 19.65 / (0.9*0.95) =21.05 bars at 165o C. and holders to suit the Indian Co the present RDs supplier.
This meets the code requirements as vessel design pressure is 21.5 bars and thickness 16 mm against the calculated 11 mm +3 mm corrosion allowance.
The new RDs eliminated nuisance RD bursts, associated plant shutdowns, and lost production.
Author: S. Raghava Chari
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