D11 Issues – Cracks in the HP/IP Shell

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At STUG 2015, 50% of users of the GE D11 Steam Turbine reported cracking issues in the HP/IP shell, especially the N2 packing gland and shell fit. These cracks create the potential for forced shutdowns and relatively long outage cycle times.

Operators can mitigate this potential through a more rigorous NDE of the gland and fit any time the component is exposed, and monitoring HP/IP efficiency and/or pressure for a significant but gradual drop. This reduction may signal imminent failure and advise a shutdown of operations for a major inspection. GE indicates that the “cycle time for repairing cracked packing glands is approximately 25 days, which can become the critical path during a major outage”. This cycle time can significantly extend the OEM’s recommended 60 day plan for a major outage. GE also recommends having a spare N2 packing gland on hand if this additional cycle time will significantly disrupt operations.

TGM® recommends addressing this potential problem with a thorough and ongoing program of performance monitoring in conjunction with robust advanced planning for outages. Our Running Condition Assessment (RCA) program can help identify and evaluate this and other potential operational issues and form the cornerstone of the shutdown planning. A robust, flexible and contingent advanced plan can reduce major outage cycles from 60 days to 45 or even 30 days, depending on the amount of repair involved.

These results can only be achieved with a high level of coordination (well in advance of the outage) between the operator, the outage team, and an array of subcontractors that may (or may not) be called upon if various planned contingencies are realized. A case in point is the N2 packing gland and shell fit. Significant pre-outage planning is required to prioritize the disassembly and NDE of these components in case long-lead time issues are identified. Dust blast and NDE of the upper half of the N-2 packing case should begin as soon as it is exposed and removed; the lower half should be prioritized after the rotor is removed. A repair plan should be in place in case cracks are found, and the repair vendors should be identified, costed, and notified prior to the outage. TGM® has developed a 14 day cycle plan for most cracking issues, using pre-approved TGM®vendors. Our one-year warranty also applies to these repairs.

Most users who have experienced cracking problems have reported varied results. Repairing shell cracks involves many technical considerations and must be performed by experienced and knowledgeable personnel. Please refer to our previous 3-part series on weld repair (Part 1, Part 2, Part 3). We agree with others that once the N2 packing gland starts cracking, it will eventually have to be replaced.

TGM® additionally recommends providing on-site machining equipment in case adjustments need to be made to the packing gland or the shell fit, and for other potential modifications and known potential TIL requirements. Provisions should also be made to perform high-strength on-site weld repairs to correct dimensions or to repair shell cracks in these or other locations.

Summary
The high-efficiency GE D11 Steam Turbine is a valued workhorse in many combined cycle plants. However, the efficiency comes at the cost of many known potential problems resulting from the higher temperature and pressures, close tolerances, and overall complexity of the unit. The potential for forced shutdowns and relatively long outage cycle times can be significantly reduced. This reduction requires:

  1. A thorough and ongoing program of performance monitoring
  2. A high level of coordination (well in advance of the outage) between the operator, the outage team, and potential subcontractors to develop a robust and flexible outage plan
  3. A knowledgeable, proactive and creative program management / technical direction team to execute the many moving parts of the plan.

Please Contact Us for more information on this and other D11 issues.