Recently, Thielsch Engineering was contracted to provide engineering supervision and perform repairs by welding on the main steam control valve chest at a 500 MW facility located in Texas, thus returning it to a level of integrity suitable for continued service under the design operating conditions.  The mid 70’s vintage unit had experienced significant thermal fatigue as a result of cycling operations.

The cracking conditions had been identified three years ago and had been monitored for propagation. The repairs to the chest involved over forty-five separate locations of cracking conditions, each over an inch deep, with the deepest being 2-1/2 inches. The repair process utilized on the valve chest consisted of several phases. These included nondestructive examinations, preheating, and welding, and post-weld heat treatment.  Each of these processes was carefully monitored and implemented by Thielsch Engineering personnel.

A series of nondestructive examinations were performed throughout the repair process.  The nondestructive examinations performed included visual, liquid penetrant, and dry magnetic particle inspections.  The final examination included visual and dry magnetic particle inspections.

Although the extent of cracking excavation was significant and thorough removal was required, the more challenging part of this particular repair was the accessibility of many of the cracking locations. Some of these areas required incredibly creative techniques for access involving the use of some “not so specialized” equipment including a standard broomstick handle and a mirror at times. Typical views of the repairs in progress are provided in Fig.’s 1-6 below. The ability of Thielsch’ talented welders was proven by this challenging repair process; using over 300 pounds of welding rod, working two shifts around-the-clock for over four weeks, our group of talented and committed engineers and welders saved this client hundreds of thousands of dollars by refurbishing, rather than replacing, this critical component.

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To learn more about our experience with baseload and cycling units, please contact Peter Kennefick at