RCA Related to Trip Throttle Valve (TTV)

Root Cause Analysis (RCA) related to the Steam Turbine Stubborn Dissemble defying 12” 2500 # Trip Throttle Valve.

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

The plant manager believed removing the startup strainer from the Trip Throttle Valve (TTV) would enhance the current 110% NH3 production another 4 to 5%, He authorized one shift shut down for the task. 

Stubborn Trip Throttle Valve (TTV)

During, pry out, the 2 Nos. jack-bolts’ ¾” NF threads slip aborted the attempt. The crew had to tighten back the bolts and hand them over to operation for plant startup in 2 hours.

The author thought of threading 2-flange holes and jacking up the top flange. During the mock shop trial, the strongest technician using the longest lever could not turn the 3” NC first tap even by ¼ turn.

The stud holes dia was suitable for 3” NC taps only. Obviously threading the 150 mm thick top flange holes under inconvenient conditions of working from scaffold is just unthinkable.

Author innovation

The author using the steam pressure pried out the stubborn flange safely as under!

• Close the TTV upstream main steam block valve
• Loosen and back up the top flange stud nuts by 3-mm.
• Open the 1” bypass valve across main steam block valve; gradually pressurize the Trip Throttle Valve (TTV)

About 6-bar pressure pried out the top flange and butted it on stud nuts (SN), which prevented flange blow off.

Two more trials of depressurizing, nuts back up, and pressuring lifted the flange completely. There was no strainer! Either forgotten past removal or steam carrying away the disintegrated strainer bits were likely!

The author got the flange tongue dia reduced by 20 microns for easy future pry out, drilled the original jack bolt holes for 1” NF threads, tapped 1” NF threads, and screwed in 1” high tensile bolt pry out bolts, and boxed up the valve.

Author: S. Raghava Chari

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