Sledge open/close too hard to operate and too soon leaking plug valves pose safety hazards, disable maintenance pump changeovers and lead to consequent total plant shutdowns often.
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.
Operators sledging 220-bar liquid NH3 pipeline valves handle to open/close scared the author – instrument engineer then:
Sledging non-proficient operators mistakenly hitting and breaking the close by very small size gland sealant nipple and breaking it is easy.
Resulting uncontrolled enormous quantities 220‑bars extremely pungent liquid ammonia spills could force total plant, may even the village evacuation.
His reporting it to the then Maintenance Manager got him the task of finding a solution!
Author RCA queries (Q) and answers (A) are:
Q: Why sledge open/close?
R: The ¼ – turn metal to metal contacting plug valve requires very tight sliding fits and hence to high operating torques to open and close even at zero differential pressures across the plugs
Q: How do new valves operate easily?
R: The crew and supervisor had no answer. The valve instruction manual said, the factory applies a propriety ‘lo-mu’ coating on the finished plug and valve body mating bore. The author guessed 1 maybe the no lubricity liquid ammonia lubricated hard sliding removes the ‘lo-mu’ coating in few uses.
2 Small % highly corrosive ammonium carbomate in the recycled liq NH3 removes the ‘lo-mu’ coating and corrodes the plug and valve body bore 3-The shop repair by machining the body and plug tapers followed by hours of lapping restores the leak tightness for a short time; but requires so high operating torques that sledging is the only opening / closing method
Obviously, this valve selection is wrong; hence the author looked for other valve types.
A journal ad on ball valves appealed to him: the ad described, “very low-torque, one hand operable leak-tight even under across valve 300-bars ΔP ball valves.
It added their field-replaceable Nylon seats seals are the secret of the low torque operation and leak tightness over long periods.
Two valves tried confirmed the ad claim and were leak-tight for months and months. The delighted Maint Manager ordered enough nos. ball valves to replace all and two spares.
Benefits of Author RCA based solutions are:
The operators and valve repair crew shown gratitude overwhelmed the author.
Author: S. Raghava Chari
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