MD&A experts arrived in Abu Dhabi and immediately established all support systems and protocols necessary for an efficient state-of-the-art response.

Initial assessment and engineering analysis showed heavy scale buildup but no significant rub on the covers. Runouts of the steam turbine rotor showed a total indicator runout reading (TIR) of 0.0324 in., confirming that the rotor was bowed and must be straightened.

All options, including rotor machining, thermal straightening, and mechanical straightening, were considered and discussed with the owner/operator in detail. MD&A’s technical expertise, experience, and recommendations were critical factors in the final decision.

The selected solution was the thermal process known as thermal straightening, viewed globally as one of the most successful and least damaging long-term steam turbine rotor straightening techniques. This thermal straightening technology, properly applied, would produce the desired immediate and long-term results while preserving the integrity of the rotor’s original structure and geometry.

The decision was made to keep the rotor horizontal throughout the entire process. Thermocouples, heating pads and insulation were prepared. Scaffolding was erected for electric wire control. A custom fixture was designed to check rotor location during turning.

The rotor was first inspected (including non-destructive examination), cleaned and prepared for initial stress relief to remove any stresses imposed by the bending. Initial post-stress-relief runouts showed the TIR now reduced to 0.0247 in., improved but not sufficient for full-service, long-term operation.

Controlled and precise thermal applications at well-defined points would follow.

A series of thermal straightening, runouts and stress relief followed. Progress was monitored as hot spots were applied and stress relieved.

Three iterations of thermal straightening would be applied along with runouts, blast cleaning and NDE/hardness testing, and axial measurements.

After the third iteration of thermal straightening and stress relief, MD&A experts determined the maximum TIR to be 0.009 in. Then, in consultation with the owner/operator and with careful review of data from a companion unit, all parties agreed to terminate the hot spotting. MD&A confirmed that limiting the number of hot spotting iterations and thus reducing the metallurgical impact is the ideal goal in any steam turbine rotor thermal straightening project. With this agreement, the rotor was again cleaned and inspected, and prepared for proper low-speed balance.

With the TIR measurement greatly reduced and low speed balance achieved, the unit was successfully placed back in full operation.

This project exemplifies many of the experience-based benefits brought to a project by MD&A:

• Responsive communications
• Solid in-depth analysis
• Strict attention to detail
• Ability to develop and perform “out of the box” repairs to support continued, reliable operation
• Project management skills and coordination of multi-disciplined operations
• Focus on the end goal
• Consideration for the owner/operator’s long-term use of the equipment
• MD&A’s commitment to steam equipment as part of our gas turbine/steam turbine/generator portfolio of expertise and experience
• Global commitment to the power industry

MD&A invites you to discuss any steam turbine, gas turbine or generator rotor issues with us today. Call our Turbine-Generator Repair Facility in the U.S. at (314) 880-3000 or use our Contact Form.

The post Steam Turbine Rotor Straightening in Abu Dhabi appeared first on MDA Turbines.

Units modified for higher efficiencies and compatibility with 7FA Gas Turbine exhaust temperatures are subject to the even higher rotor to casing differential thermal expansions in the more tightly-packed seal areas of this single-casing design.

The key to the continued efficiency and safe operation of these machines is highly specialized NDE inspections, remaining life analysis, and forward-looking repair option solutions.

HPIP Outer Casing/Shell Cracking

The HPIP Outer Casing has been shown to be susceptible to Low Cycle Fatigue (LCF) as well as Creep Rupture cracking, from different operating conditions.  Both types of cracking are found adjacent to the N-2 male rabbet fit, which supports and positions the internal N-2 packing casing.

During a cold startup, the Hot IP inlet steam enters the IP steampath thru the two lower IP steam inlet bores. This steam rapidly heats the IP side of the HPIP Outer Casing N-2 male rabbet fit. The HP side of the N-2 male rabbet fit does not yet have any HP inlet steam so differential thermal expansion of the HPIP outer casing diameters occurs across this fit.

The differential thermal expansion puts the HPIP Outer casing into high compression adjacent to the fit. This compression creates LCF on the HP side of the N-2 fit. The LCF is cumulative with each cold startup. The accumulated LCF can then create radial and circumferential cracking on the HP side of the N-2 rabbet fit. The cracking is most severe in the lower half as this is closest to the IP steam inlets.

On the upstream (HP) side, Creep and/or LCF cracking has sometimes been known to sever the 4-inch-thick alloy steel HPIP Outer Casing wall in less than 5 years from the time of crack initiation.

N-2 Packing Head Cracking

The N-2 Inner Packing Casing separates the HP inlet steam from the IP inlet steam near the midspan of the HPIP rotor and Outer Casing.  It is located by the N-2 rabbet fit noted above.

During steady-state full load operation at peak temperatures and pressures, there is a steady-state steam pressure difference between the HP and IP sides of the N-2 packing casing. The net difference wants to force the N-2 packing casing towards the IP section. The female fit on the outer perimeter of the N-2 packing casing is resisting this unbalanced steam force.

This creates a steady-state tension force at the HP side of the female rabbet fit.  This steady-state tension force occurs at the full operating temperature. At this high temperature, the N-2 Packing casing material is vulnerable to creep rupture damage. The creep damage is again cumulative and radial circumferential cracking has been sometimes known to occur at the female fit at about 100,000 operating hours. This damage has led to the loss of axial positioning of the N-2 inner casing and internal damage to the rotor and diaphragms.

Solution

The key is to identify the cracking early, then pursue the optimum repair solution for safe and reliable long-term operation.

Most owners/operators do not know that the hidden internal damage exists until it surfaces as an emergent issue during a normal inspection outage.

Proper repair by Mechanical Dynamics & Analysis (MD&A) for outer shell IP inlet is on-site stress-relief weld repair and in-shop repair and upgrade of N-2 cracking casing. If needed, our experts also perform thermodynamic analysis, evaluating cycle efficiency to help isolate problems.

MD&A has more than 30 years of experience successfully applying its technology, knowledge, and professionals on more than 500 casings worldwide on a variety of OEMs (Original Equipment Manufacturers.)

Horizontal Joint Leakage

The D11 lower HP/IP casing is long and slender, supported on the ends with the highest temperatures in the mid-span section.  Over time the casing sags, with the lower half experiencing higher temperature transients during startups and high enough steady state midsection steam temperatures to permanently yield portions of the casing.  The result can be steam leaks from inside to atmosphere and eventual steam cutting off the top to bottom flange, entering the insulation. The impacts are efficiency (heat loss), operability, and safety.

The solution requires localized repairs and re-machining to correct both the steam cutting and the casing sag.

MD&A offers laser scanning equipment to determine the amount of distortion as compared to the original joint plane. Following weld buildup of the horizontal joint, a laser-guided mill blends the repaired section to the original plane. MD&A’s skilled technicians have been performing this on-site work successfully for more than two decades on a variety of OEMs and steam turbine models.

A related issue is mid-span outer shell creep and associated axial clearance shift from the N-2 downstream as rotor length remains constant. Offset packing rings and modified diaphragm seal faces are two of several solutions offered by MD&A.

Bowed HIP and Clearance Control

The D11 rotor is long and slender with a significant distance between bearings. If the rotor rubs, it can easily become bowed, and there is no access point for a mid-span balance. Vibrations can become excessive during operation and roll-up/down through critical speeds. The heat from metal-to-metal rubbing and water quenching during cooldown are the two main damage mechanisms. All lead to performance loss and high vibration.

During a site inspection, MD&A technicians look at the specified clearances, the levels of distortion in the casing, rotor, and diaphragms, and all other components. For the casing and diaphragms, MD&A alignment engineers can perform a realignment for improved efficiency.

Traditional bowed rotor repair is a shop process performed by MD&A’s Turbine-Generator Repair Facility in St. Louis, MO USA. If needed, straightening can be performed on-site with vertical or horizontal thermal straightening and stress relief. When required, high-speed balance can be performed in our state-of-the-art balance facility.

Diaphragm Dishing

Diaphragm dishing/deflection is also a concern with these units due to high operating temperatures, insufficient main weld depths, reduced axial spacing, and problematic weld processes utilized in the original construction.

MD&A offers specialized diaphragm dishing repair services ranging from diaphragm inserts for upstream displacement, offset packing segments, restoration of diaphragm fabrication welds including increasing main weld depths, diaphragm stiffness improvements when possible, and all required machining.  MD&A also offers new custom–engineered diaphragms with improved materials, deeper weld penetrations, and increased stiffness.

D11 Fleet

MD&A is known for its catalog of field services, fast on-site and in-shop response, parts supply, and, above all, its approach to long-term better-than-original solutions.

Although most D11 problems are in the HP/IP area, MD&A’s overall philosophy is to eliminate and resolve all reliability problems. The intent is to improve, not just recreate.

The best solution moves beyond simply restoring a unit to its original specifications. MD&A’s working philosophy is to consider all repair/replace options, look beyond the original, and move forward in consultation with the owner/operator with the optimum solution for real-world service going forward.

Call MD&A’s Turbine-Generator Repair Facility today at (314) 880-3000 or use our Contact form.

Also read how, through its services experience and detailed engineering study of the D11 fleet and configuration, MD&A/Mitsubishi Power has developed an HP/IP “Drop-In” retrofit solution based upon proven steam turbine technology.

The post D11 Problems & Repair Solutions appeared first on MDA Turbines.

The Turbine end shaft end was bent and found to have a 0.007” total indicator runout (TIR) at the coupling outer diameter. The generator end shaft end was bent and found to have a 0.053” TIR at the coupling outer diameter. MD&A was able to straighten both shaft ends to within 0.0015” TIR using a combination of thermal and machining straightening processes. Then, MD&A completed the final high-speed balance and final machining of the shroud covers.

Shaft End Thermal Straightening
The rotor thermal straightening process included an initial rotor stress relief and multiple thermal straightening iterations. MD&A expert technicians performed detailed visual, dimensional, and non-destructive examination (NDE) of the rotor forging before and after each thermal process to confirm that no issues were present. Rotor NDE inspections consisted of both hardness testing and wet fluorescent magnetic particle testing.

Based on the amount of shaft end distortion and the experience and expertise of MD&A technicians, it was recommended carrying out an initial stress relief of the rotor and additional thermal straightening iterations of the shaft ends to further reduce the shaft end runouts.

The initial stress relief of the rotor forging was performed on both shaft ends to relieve any internal stresses and determine the final relieved state of the shaft end runouts. This involved removing all installed blades and seal strips, stress relieving the rotor, blast cleaning, NDE, hardness checks, and runout checks along the length of the rotor.

Additional thermal straightening iterations were also performed on the generator end of the shaft to further reduce the amount of runout. This involved thermally moving the shaft end, stress relieving the rotor, blast cleaning, NDE, hardness checks, and runout checks along the length of the rotor.

Due to the high temperatures of both couplings during the stress relief processes, a vertical stress relief was not possible. As a result, the stress reliefs were performed in the horizontal orientation while rotating the rotor periodically 180° during the heating process so as not to set a thermal bow into the rotor.

DFLP Rotor Machining
Based on results of the thermal straightening iterations, machining straightening of both shafts was performed. The work involved final straightening machining of both shaft ends and final dimensional, NDE, and runout checks along the length of the rotor.

The thermal straightening operations reduced the Turb End shaft end TIR from 0.007” at incoming to 0.003” after completion. The Turb End shaft end straightening machining included final machining of the steam gland diameter, all steam gland seal grooves, all transition radii, and the coupling to remove any remaining runout.

The thermal straightening operations reduced the Gen End shaft end TIR from 0.053” at incoming to 0.015” after completion. The Gen End shaft end straightening machining included final machining of the journal, oil deflector, steam gland diameter, all steam gland seal grooves, all transition radii, and the coupling to remove any remaining runout. Also, the coupling holes were line bored to set the bolt circle pattern back on center.

High-Speed Balance
In parallel to the blading and seal strip installation, MD&A produced a new stub shaft in support of performing the high-speed balance.

The balance work involved the installation and aligning of the new stub shaft, installation of the rotor in the high-speed balance facility, balancing the rotor at operating speed, and performing an overspeed test of the rotor.

The rotor work was completed on schedule and returned to the owner.

For every repair job, large or small, the speed and effectiveness of our response team is matched only by the depth and breadth of our engineering expertise.

Have us look at your bowed rotor today. Call our Turbine-Generator Repair Facility at (314) 880-3000 or use our Contact form.

The post DFLP Rotor Straightening and Machining appeared first on MDA Turbines.

Incoming inspections and evaluations consisted of blast cleaning, NDE, visual inspections, material verification, as well as a detailed runout check of the rotor.

The rotor straightening involved an initial stress relief operation, as well as multiple iterations of thermal straightening, to correct the incoming rotor runout condition.

In support of the rotor straightening, machining modifications were made to the shaft N-3 packing gland area as well as the inboard thrust collar.  The N-3 packing gland area on the rotor was machined to a smaller diameter and new packing rings were supplied and installed.  The backside of the inboard thrust collar was modified to add a much-needed balance weight plane on the turbine end of the rotor.  The thermal straightening process did not require machining of the rotor journals, re-sizing of the journal bearings, machining of the couplings, or line boring of the coupling holes on-site during reinstallation.  After completion of all repairs, the High-Pressure rotor was balanced at operating speed and overspeed tested at 110%.

SUMMARY

The High-Pressure rotor was received with a bow of 0.040” TIR (total indicator reading) in the N-3 packing area of the rotor.  This amount of runout would not allow the rotor to spin up through critical speeds, let alone get to rated speed without severe vibration. MD&A successfully straightened the HP rotor to 0.003” TIR using a thermal straightening process.  The HP rotor was balanced and overspeed tested without incident and returned to the site for reinstallation and startup.

Have us look at your bowed rotor today. Call our Turbine-Generator Repair Facility at (314) 880-3000 or use our Contact form.

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