Upon initial examination, significant issues were revealed with the HP inner casing, nozzle chambers, and selected components, all critical for proactive maintenance and long-term operation.

This 3600-rpm machine contains a double-flow HP (high pressure) turbine element, a double-flow IP (intermediate pressure) turbine element, and two double-flow LP (low pressure) turbine elements. The HP inner casing consists of 8 inlets, 4 per half.

MD&A’s Turbine-Generator Repair Facility team focused on the repair and restoration of the damaged steam turbine nozzle chambers, upper and lower half inner casings, and the center flow guide.  Various moderate erosion, fretting, and impact moderate damage was found on all nozzle chambers, nozzle plates, and inner casings.  All nozzle chamber flexplates were fractured and broken.  The B2 and C3 nozzle chambers inlet pipes were fractured from the inlet snouts and these two nozzle chambers were floating loose within the inner casing.

Nozzle Chambers

B2 nozzle chamber work called for the replacement of the fractured inlet pipe end, repair of horizontal and vertical face fretting damage, repair of admission-side impact damage, repair of broken and fractured axial locating fits, and repair of the nozzle plate eroded partitions, including all post-repair NDE inspections.  Wall thickness UT (Ultrasonic) inspection evaluated the remaining wall thickness due to the admissions-side impact damage using ASME® Codes.  The axial locating fits and other areas received weld restoration and machining, as needed.  MD&A Experts then supplied and join-welded the new nozzle-chamber inlet pipe end and prepared the chamber for reinstallation and welding into the casing.

The C3 nozzle chamber first required the removal of a broken and lodged bell seal and retaining nuts from the inlet pipe, lodged behind the nozzle plate.  The fractured inlet pipe end was then removed, followed by repair of horizontal and vertical face fretting damage, repair of broken and fractured axial locating fits, repair of the nozzle plate eroded partitions, and repair of the nozzle plate seal grooves, including all post-repair NDE inspections.  MD&A Experts then supplied and join-welded the new nozzle-chamber inlet pipe end and prepared the chamber for reinstallation and welding into the casing.

Inner Casings

For the lower half inner casing, MD&A Experts replaced various horizontal joint studs that failed UT inspection, and repaired broken key block screw holes.  Various minor repairs were performed on damaged center pins, damaged axial tongue fits, damaged key pockets and locating pin slots, and damaged bell seal bores.

The remaining installed nozzle chambers horizontal and vertical face fretting damage and nozzle partitions were repaired, along with admission-side impact damage, all followed by wall thickness UT inspections and evaluation.  The repaired and restored B2 nozzle chamber was then reinstalled into the casing and a new top hat and all-new flex plates welded into location.  Final heat treatment, machining, and NDE inspections were performed without issue.

Upper half work included various minor repairs on damaged center pins, damaged axial tongue fits, damaged key pockets and locating pin slots, and damaged bell seal bores.  The remaining installed nozzle chambers horizontal and vertical face fretting damage and nozzle partitions were repaired, along with admission-side impact damage, also followed by wall thickness UT inspections and evaluation. The repaired and restored C3 nozzle chamber was then reinstalled into the casing and a new top hat and all-new flex plates welded into location.  Final heat treatments, machining, and NDE inspections were performed without issue.

Once the two nozzle chambers’ reinstallations were completed, new nozzle plate seals were installed and machined, and new bell seals were supplied, final machined, and fit checked to each inlet snout bore.

Center Flow Guide

Center flow guide work included horizontal joint stud UT inspection, welding and machining of the axial and vertical key fits, centering-slot damage repair, removal and replacement of lower-half vertical keys, and final assembly fit checks to the inner casings.

Final Assembly Verification and Shipment

MD&A then performed and verified final assembly checks of the center flow guides into the casing halves and performed a final upper half casing “tops-on” check to ensure closure of the horizontal joint and all nozzle chamber positions and clearances were acceptable.

All components were completed and returned to the site six days ahead of the contract schedule.

Because of our vast knowledge and experience with different types of OEMs, MD&A experts can perform any welding repairs. Our commitment to excellence has earned customer trust for all turbine-generator needs. We focus on delivering consistent quality and value with fast response, superior communications, and innovative solutions. We are your full-service, OEM-alternative!

Have MD&A look at your casings today, call our Turbine-Generator Repair Facility at (314) 880-3000, or use our Contact form.

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THE CHALLENGE:   The subject power plant is an important electrical generating facility in the country it is located within.  Returning the unit to service in the shortest possible time was critical for the turbine owner to provide its customers with a reliable supply of electricity.  A typical replacement period for a steam turbine of this design is approximately two (2) years.  Having the steam turbine offline for that long would have a severe impact and as such, MD&A was called upon to develop an alternative plan to bring the replacement steam turbine online sooner.  In addition to the time challenge, there were additional logistical constraints associated with transporting, installing, and repairing large power generation equipment within the subject country.

MD&A’s SCOPE:  MD&A was awarded turnkey scope of supply and installation of the steam turbine and auxiliaries. MD&A’s scope included replacing the steam turbine and installing a new control system, as well as a comprehensive rebuild of all steam turbine valves and actuators and installing a new hydraulic power unit.  MD&A also provided turnkey installation, start-up support, and all project management.

THE SOLUTION:  MD&A in conjunction with its parent company, Mitsubishi Power, developed a customized plan to complete manufacturing and installation within one (1) year of the project start date.  The key elements of this successful plan included reducing manufacturing lead time, coordinating equipment shipping, utilizing MD&A’s extensive in-house capabilities, and overall extensive project management and planning.

Manufacturing lead time was significantly reduced through Mitsubishi Power working closely with their suppliers for rotor forgings and shell castings.  Mitsubishi Power also expedited manufacturing activities within their factory as well.

Shipping logistics were streamlined with over 300 tons of equipment flown from the factory via charter aircraft in five (5) carefully coordinated flights.

By utilizing MD&A’s extensive in-house capabilities, the MD&A Project Team was able to maintain control of the schedule more effectively and shorten the timeline. MD&A’s three (3) major divisions; MD&A Services, MD&A Repairs, and MD&A Parts supported this project.

For example, critical internal parts for the four (4) steam turbine main and reheat valves were machined at MD&A’s Valve Machining Shop.  These parts were then fitted and assembled in MD&A’s Valve Repair Shop (Figure 1). The actuators for these valves were repaired, rebuilt, and tested at MD&A Bearings, Seals & Hydraulics Repair facility (Figure 2). The new control system was developed and installed by MD&A’s Control Systems. MD&A also drew upon their internal expertise and deployed MD&A turbine specialty engineers and technicians to the site in support of such activities as technical direction, supervision, turbine alignment, valve casing repair, valve re-assembly, turbine seal fitment, controls, turbine commissioning, and startup (Figures 3 and 4).

Lastly, MD&A’s developed a staggered “Just-In-Time” plan that integrated delivery of the components with the sequence of the replacement turbine buildout. This approach coordinated and optimized the manufacturing, delivery, and site installation activities to further shorten the critical path schedule.  For example, the Intermediate/Low-Pressure Rotor Assembly arrived at the airport in the early morning (Figure 5) and later that same day, it was installed in the lower half of the turbine shell at the plant (Figure 6).

The RESULT:  MD&A completed installation 3 weeks earlier than the committed date. The total time from project start to turbine operation was less than 12 months. In addition to the key elements discussed above, critical to this success was MD&A’s responsive approach combined with extraordinary teamwork.  The Owner and MD&A approached this project as a team.  The Owner’s Plant personnel provided support in a variety of ways from the start of the project through turbine commissioning and startup.

A truly extraordinary accomplishment was achieved in the power industry with this Rapid Steam Turbine Replacement Project, resulting in a critical electrical generating unit returned to service for the Owner and their customers in less than half of the normal time.

In our 35 years, MD&A’s commitment to excellence has earned customer trust for all turbine-generator needs. We focus on delivering consistent quality and value with fast response, superior communications, and innovative solutions.

MD&A is easy to work with. We provide immediate access to expert help when you need it, where you need it. Around the corner or the globe, we ensure prompt, thorough communication and follow-through. For every repair job, large or small, the speed and effectiveness of our response team are matched only by the depth and breadth of our engineering expertise.

Find out today why so many power generators use MD&A to maximize operational effectiveness. Call us today +1 (518) 399-3616 or Contact us.

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The rotor was sent to MD&A’s Turbine-Generator Repair Facility in St. Louis, MO for standard blast cleaning, NDE and visual inspection.

Inspection

Stage 3 L-0 stage buckets and covers revealed extensive cracking. Further examination revealed multiple blade root, airfoil and cover cracks, and showed that the unit was experiencing high-vibration trip due to cracking and loss of blade cover sections.

Finite Element Analysis (FEA) steady-state and vibrational analysis followed, based upon the owner’s operating speed requirements.

Blades were found to be in resonance close to the customer’s preferred running speed. The root cause was identified as bucket natural frequency at that speed.

The primary cause of the extensive Stage 3 bucket cracking was a 5 nodal diameter natural frequency resonance with a 5 per revolution harmonic impulse in the steam path.

Modifications of the bucket geometry were deemed necessary to raise the assembled natural frequencies.

Options

Maintaining the original design while thickening the blade trailing edges was considered, but produced an undesirable speed-avoidance range for the customer. Long-arc covers, often effective in suppressing response to resonant stimulus, were also considered but the resulting differences in cover lengths would negate the harmonic benefits.

Resolution

The solution was bucket redesign, and included:
• Completely detuned vanes with a change of cover material from 403SS to Ti6A14V alloy.
• Thickened trailing edges of the vanes to provide additional vane robustness to withstand the moisture droplet recirculation erosion that was occurring on these edges.

The speed avoidance range of the option chosen is recommended to be lower than 5200 and higher than 5900 rpm, allowing operation within that range and limited operation at less rpm. The redesigned buckets with titanium covers also result in reduced centrifugal stresses because of the lighter mass of both the titanium covers and the modified bucket vanes.

Resonance avoidance is the primary benefit of the new design and is predicted to significantly reduce the vibratory stresses that caused the cracking. The turbine bucket redesign & modification has additional beneficial features:
• Lower centrifugal stresses (because of the lighter mass of both the covers and vanes).
• Thickened trailing edges for additional life.
• Increased radius of the tenon hole (location of cover crack initiation) to decrease stress concentration.
• Rounding of the sharp corners of the bucket root (to lower stress concentration effect).
• Chamfering of the wheel dovetail sharp edges for damage protection.
• Robust assembly of covers (10 and 11 buckets each) compared to the original design (16 and 17 buckets each).
• Reduced steady-state stresses of lighter-mass modified blades at the first hook locations.

MD&A designed, produced, installed, and final machined the new blades and covers, and performed the rotor high-speed balance.

The customer was instructed to avoid running speeds close to 5000 and 5900 rpm, an acceptable operating parameter.

Inspection also showed an axial crack on the outer hook admission side of the 3rd stage wheel. The crack was excavated and blended, modeled and analyzed by FEA, and the repair accepted by the customer.

MD&A’s experts brought this critical unit back online in full compliance with the owner’s operating requirements, and for the long term. The coordination of field response specialists and the vast knowledge of the repair facility team demonstrated our ongoing commitment to innovative gas and steam turbine-generator diagnostics and repair.

Call MD&A today for any turbine repair at +1 (314) 880-3000 or use our Contact form.

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