Healing the Natural Way

Background

The 320 MW HTC unit was shut down for inspection of the HP Emergency Stop valve (ESV) and Governing valves (GV’s) due to restricted valve movement experienced in operation. Upon opening the ESV it was found that the chest seat had come adrift and damaged the spindle seat as well as the steam chest location areas. The seat diffuser section had broken up and pieces went through into the GV’s.

 

The ESV chest seat moved in the cavity between the valve spindle seat and chest body, severely damaging the location surfaces. Additionally, a crack was found at the rear of the original rear interference fit landing.

 

Scanning of the actual components was done during the evaluation of the damage and repair options. The steam strainer was also damaged in the process and required replacement.

Original Design

The 320 MW HTC unit was shut down for inspection of the HP Emergency Stop valve (ESV) and Governing valves (GV’s) due to restricted valve movement experienced in operation. Upon opening the ESV it was found that the chest seat had come adrift and damaged the spindle seat as well as the steam chest location areas. The seat diffuser section had broken up and pieces went through into the GV’s.

 

The ESV chest seat moved in the cavity between the valve spindle seat and chest body, severely damaging the location surfaces. Additionally, a crack was found at the rear of the original rear interference fit landing.

 

Scanning of the actual components was done during the evaluation of the damage and repair options. The steam strainer was also damaged in the process and required replacement.

Recovery

The valve seat design was customised to cater for the damaged chest as well as incorporating design improvements as per the following criteria:

 

  • The valve function had to be restored to the original condition
  • Avoid weld repair
  • The repair time had to be kept to the minimum
  • Uncontrollable variables had to be avoided
  • Work had to be executing with activities in parallel as far as possible
  • The weaknesses of the original design that lead to the failure must be addressed
  • The solution had to conform to best practise engineering and design standards and had to be peer reviewed
Proposed Solution

The valve seat design was customised to cater for the damaged chest as well as incorporating design improvements as per the following criteria:

 

  • The valve function had to be restored to the original condition
  • Avoid weld repair
  • The repair time had to be kept to the minimum
  • Uncontrollable variables had to be avoided
  • Work had to be executing with activities in parallel as far as possible
  • The weaknesses of the original design that lead to the failure must be addressed
  • The solution had to conform to best practise engineering and design standards and had to be peer reviewed

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