Instrumentation engineering root cause analysis (RCA) of butterfly control valve failure caused serious vicinity heavy fire in a plant.
|Article Type:||Root Cause Analysis (RCA)|
|Equipment Type:||Control Valves|
|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.
Butterfly Valve Failure Problem
A sudden huge fire near the ammonia converter occurred. Fire and volunteer crew put it out after lengthy considerable efforts.
Operations crew reported butterfly valve TCV 701 non-drive end (NDE) shaft missing; hence enormous 400o C, 250 bar syn gas (75% H2+25% N2) spill auto ignited into the huge fireball.
Dissembled Valve Observations
TCV 701– 10” 2500 # wafer body Butter Fly valve – shop-dissemble inspection findings are:
- The 10 mm dia shaft Non-drive end (NDE) missing; found later 50 mm away on the floor
- Obviously, the 250 bars, 400o C syn gas enormous spill gush out burst into very hard to put out fire
- The disc hub bore to the SS 316 shaft clearance was around 3-mm i.e. too loose fit up
- A 12 O’ Clock position taper pin and a 25 mm away 3 O’ Clock position taper pin (fig 6) lock the disc to the shaft. The one end lock allows stress free differential thermal expansion of the disc on the shaft; hence essential
- Flow turbulence flutters the one end secured too loose on the shaft disc, enlarges the taper pin holes, gradually the shaft cross section at pin hole decreases and eventually the too weak shaft breaks
- Line pressure ejected shaft NDE ejects out like a bullet. No injuries or fatalities were sheer luck!
Author solution follows
- As the immediately ordered 10% stronger K‑Monel shaft for abundant safety would take 6 months to reach, the shop make a SS shaft and super finish the gland areas
- Weld fill the disc hub original pinholes, Increase the hub bore dia and tightly fit a shop made 3 mm WT SS sleeve of shaft slide bore i.e. near zero play.
- Holding the disc concentric to body bore, assemble and place the shaft in position; Drill a 2.5 mm dia hole on disc hub passing through the shaft away 10 mm from disc end; drill another 90o displaced 35 mm away from disc end. Taper ream the holes
- Make two 30 mm long pins to pass through the reamed holes and project the hub’s both sides. Drove a pin tightly in each hole and slightly bend the small dia end. The valve is ready for use.
- Bolt the shop made shaft-guard (fig) – author added part – to prevent ejection – if ever the improved shaft breaks – and consequent fire – a redundant safety step.
RCA based Repair Benefits
Thus, repaired valve never failed. The manufacturer thanked the informative feedback appreciated the improvements and includes these features with thanks in their products.
Author: S. Raghava Chari
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