Industrial steam turbine/generators are used globally in chemical and petrochemical plants, refineries, pulp & paper mills, and mining operations. They are also used in biomass and waste-to-energy facilities, combined heat and power plants, geothermal and concentrated solar power installations, and desalination operations, including many others.

Ongoing operation of these units ensures stable, reliable use of proven technologies to convert the expansive power of steam into useful mechanical work. The processes are both fundamental and critical as the world exploits all available resources to meet its present and future needs for heat and power.

The professionals at MD&A are uniquely qualified to serve these needs by applying their deep and time-tested experience with steam turbine/generators, regardless of OEM.

For this particular industrial unit, onsite work was performed by MD&A’s Outage Services division, Generator division and our Turbine & Generator Controls division. All valves were sent to MD&A’s Turbine-Generator Repair Facility in St. Louis, MO for inspection and repair. MD&A’s Bearing, Seals and Hydraulics Division in Euclid, OH, inspected bearing assemblies, thrust pads and oil deflectors.

ST-660 Industrial Steam Turbine Major Inspection

After mobilizing, MD&A’s experts removed the upper-half shell and turbine rotor, then performed blast cleaning for complete non-destructive examination. The rotor, packing carriers and blade carriers were aligned to OEM specifications. The nozzles were blast cleaned and NDE’d in place.

Inspection revealed evidence of steam cutting that was photographed and recorded, as well as some deformed bolts. Various other specifics were recorded in the Recommendations Report.

Upper half and lower half shells were also cleaned and inspected.

A key benefit of MD&A’s inspections is written and photographic documentation of lessons learned that can assist the owner/operator in future outages. One lesson learned was on the rotor lift. For this inspection, the turnbuckles were removed and chain falls were used in their place. For future outages, two 15-ton and two 10-ton chain falls should be on hand due to minimal lift clearances.

This recording would benefit both owner/operator and contractors involved in future work.

In-depth inspections then proceeded. This included axial and radial blade clearances, rotor radial runout checks, nut rotation and tightening, thrust bearing clearances, and complete visual and NDE inspections (general, HP impulse wheel, and all 36 rotor stages).

Generator

For the Siemens® conventionally air-cooled generator (71.18 MVA), the coolers were cleaned, visually inspected and pressure tested. The field was removed and generator inspection and testing were performed.

MD&A completed major inspections of both the generator field and stator including visual, borescope and electrical testing. Minor contamination was found on the field turn windings beneath the retaining ring. Signs of field turn migration were also identified and addressed.

One field ground detection bolt was found already removed prior to the MD&A Generator Team’s arrival. It was reinstalled according to OEM specifications. To complete electrical testing, the field winding was isolated from the brushless excitation components. Testing then proceeded on the generator field (IR with PI, DLRO and AC Impedance), rotating armature and rectifier wheel, stationary field, and stator.

At completion, field windings were reconnected to the shaft-mounted brushless excitation equipment and a final IR test was performed.

Conductive paint was applied to the affected partial discharge areas to reduce sparking effect and increase stator bar life.

With testing complete, MD&A issued a recommendation for power factor testing at yearly outage intervals to monitor health of the insulation system.

Brushless Exciter

The brushless exciter was cleaned, inspected, and realigned. The ABB Unitrol 6000 Exciter Regulator Cabinet was also cleaned and inspected.

Steam Turbine Valves

MD&A’s facility in St. Louis disassembled, cleaned and inspected the control valve, main stop valve and other valves. The emergency stop valve was also disassembled, cleaned and inspected during the outage.

For all, a final blue check was performed, 100% blue contact was achieved. New rope packing, supplied by MD&A, was installed during valve reassembly.

Other Key Elements

All turbine bearings were replaced due to wear. Generator bearings were NDE’d and reused. Replacement thrust bearings were installed at reassembly due to wear, New RTDs were installed at MD&A’s Bearing, Seals and Hydraulics facility.

Auxiliary system work included the lube oil tank which was drained and cleaned under MD&A supervision. MD&A performed additional cleaning of the tank and baffle plates. Two lube oil coolers were unstacked, washed and disassembled.

Alignment

Topless laser alignment was completed before startup. This method utilizes laser accuracy to eliminate the time-consuming and expensive process of reassembling a steam turbine for “Tops-On” alignment measurements, saving hundreds of man-hours labor per turbine section. This included horizontal joint measurements, bolt roundness checks, stationary component position verification, and side slip readings among others using laser equipment. Our alignment engineers have the expert knowledge needed to interpret the data.

Why MD&A?

The experts at MD&A are not only dedicated to the fundamentals of turbine and generator longevity. They apply their vast experience in all OEMs to understand and import precise lessons learned throughout the industry, then direct their knowledge and skills to the ongoing development of using steam effectively and efficiently for the long-term benefit of owner/operators throughout the world.

MD&A’s technical directors and service technicians bring decades of experience to each industrial and utility project regardless of OEM, a process now proven in more than 20 countries worldwide. Committed to both industrial and utility customers.

We invite you to discuss any industrial steam turbine major inspection or other steam turbine, generator or gas turbine issues with us today. Call us at (518) 399-3616 or use our Contact Form.

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High Pressure Turbine

HP/IP – MD&A’s Outage Services Division disassembled and inspected the HP/IP rotor onsite, as well as reassembly. The HP/IP rotor, HP inner casing, nozzle, N1 and N3 packing cases, and all diaphragms were sent to the MD&A’s Turbine-Generator Repair Facility for inspection, testing, NDE, blast cleaning, and repair. MD&A’s technicians replaced the spill strips and shaft packing. They also drilled out and tapped all diaphragm elevation key screws and clearance key screws. The clearances were good radially and axially.

Bucket Replacements – The team also replaced both stage 6 and stage 7 buckets. Rotor NDE revealed concerning crack-like indications on two of the blades on stage 6, the last of the HP section. Excavation was attempted, but unsuccessful. A decision was made to replace the row of blades with new buckets, which were expeditiously manufactured during the outage. The replacements were produced by reverse engineering the existing ones, to help ensure proper functioning and fit. Once received, MD&A’s experts then machined and installed the new buckets.

Stage 7 buckets were replaced as part of the original scope of work for the planned outage. In addition to the buckets, new covers, axial lock keys, and grub screws were installed as well. The wheel dovetail showed minor mechanical damage in two locations, which were polished prior to installing the new buckets.

Diaphragms / Nozzle – Severe erosion was found on stage 7 at opening inspection. FOD was also noted.

At our Turbine-Generator Repair Facility, dimensional inspection revealed no major findings. NDE revealed some typical issues, such as impact damage, rubs and other relatively minor damage.

Other repairs included blending mechanical damage, removing impact damage, and resolving some indications. The team also machined the T1 and T2 journal surfaces.

MD&A’s Topless Laser Alignment (TLA) specialists aligned all diaphragms. Once complete, diaphragm clearance keys were measured and adjusted. The nozzle keys required significant scraping for contact. Large indentations were also noted and corrected. The nozzle clearance keys were adjusted to proper clearance, and the hold down bolt clearance was also adjusted. Minor repairs were performed on nozzle box components.

Alignment

Stationary component alignment for the HP/IP turbine section was conducted using our Topless Alignment® service. This method utilizes laser accuracy to eliminate the time-consuming and expensive process of reassembling a steam turbine for “Tops-On” alignment measurements, saving hundreds of man-hours labor per turbine section. Our alignment engineers have the expert knowledge needed to interpret the data. Coupling alignment was performed as well.

Outer Shells – Several issues were found on HP/IP outer shell as follows:

Severe dishing and indentation was noted on the steam inlet flanges, which were machined. Cracking in the reheat bowl was identified in the HP/LH casing crush pin appendage. These were drilled and mapped.

The blowdown pipe was found seized in the LH/HP casing. The seal rings were cracked locking rings and distorted seal rings and were replaced. The blowdown pipe was also machined to correct out of roundness.

The TC pipes were found to be severely bent during disassembly and were replaced with new pipes fabricated by MD&A’s Machining Services division.

Inner Shells – The HP inner shells were inspected at MD&A’s Turbine-Generator Repair facility. They performed visual and dimensional inspections, as well as NDE. The team prepared a report detailing observations and repair recommendations, and then conducted certain repairs.

On visual inspection, the HP inner shell, both lower half and upper half, revealed typical conditions including galling, scoring, dinging, fretting, thread damage, and the presence of raised material. The lower half blowdown seal rings were noted to be bound up and loaded to one end. No significant issues were otherwise noted.

Dimensional inspections were conducted on the snout pipe, blow down pipe, diaphragm grooves, and various critical fit widths. Dimensions were found to be mostly within range, except for a snout pipe diameter and float. Recommendations were made to remedy these issues.

MD&A performed repairs on the HP inner shells, including destructive removal of broken bolts & screws, stoning and blending of various indications and surfaces, and replacement of the blowdown seal rings.

Oil Deflectors – MD&A Bearings, Seals, and Hydraulics Division inspected and refurbished T1 and T2 oil deflectors. The team noticed that both were rubbed hard in the vertical plane. After evaluating the clearances with the customer, and previous experience with increased clearances in areas with 750 degree temperature potentials, T1 and T2 clearances were increased.

Boiler Feed Pump Turbine

MD&A’s experts at our Turbine-Generator Repair Facility removed and inspected boiler feed pump turbine (BFPT) A and B valves. For BFPT A, the LP SV stem was found and replaced. The team refurbished the LP SV disc and replaced the seat. The HP SV and CV were both replaced, and the HP SV strainer basket was repaired.

For BFPT B, MD&A’s experts refurbished the LP SV disc. Bushings on the LP SV rack were noted as oversized and had severe wear. The bosses were refurbished so OEM bushings could be installed. The HP SV was replaced, and the Machine Services team repaired the HP SV strainer basket. They also machined the bonnet to resolve excessive clearance.

Conclusion

Impressively, all of this was completed three days ahead of schedule! This is despite the challenge of resolving the unforeseen bucket replacement, which included manufacturing.

Several of MDA’s divisions participated including Outage Services, Generator, Steampath, Machining Services, Bearings Seals & Hydraulics division, Topless Laser Alignment, High Speed balance and Turbine & Generator Controls Division. This engagement underscores MD&A’s extremely diverse range of turbine inspection and repair capabilities, in-house expertise, and the ability to get the job done on time.

These are exactly the reasons why operators worldwide rely on MD&A for exceptional steam turbine inspection and repair services as a full-service, OEM-alternative. Call us at (518) 399-3616 today or use our contact form.

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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.

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Unlike steam turbine alignment, gas turbine alignment is performed with the upper halves installed, also called “Tops-On” alignment, and moves must be made in a specific sequence, adding to the overall challenge.

MD&A Alignment Engineers have experience with a variety of OEM’s allowing them to service a large portion of the installed fleet.

The most common issue with three bearing gas turbine models, such as the GE® 7B, 7E, and 7EA, is with casing slippage in the turbine/exhaust frame casing joints. The casings serve an important function as they support the bearings, rotor, and hot gas path hardware. It’s necessary that casings stay in position to keep the unit’s performance and efficiency at an optimal level.

On affected turbines with this issue, the casing slips downwards in relation to the exhaust frame. This causes misalignment of the #2 and #3 bearings, as well as, causing a vertical step change in the stationary component positions near the joint.

Two bearing gas turbine models, such as GE’s® 7F turbines, can also become misaligned over time. This is due to conditions including gravity sag, casing slippage, and thermal creep. These conditions cause shifting for both the casings and the T2 bearing, requiring the unit and its components to be precisely realigned.

Occasionally, turbine compressor discharge casings need to be replaced on older machines. Originally, these casings were fabricated with welded joints, which have been shown to be susceptible to casing creep. As the metallurgy changes over time, the casings distort, leading to significant performance issues. This is resolved by completely replacing the compressor discharge casings.

The process for our specialized Gas Turbine Alignment starts with an internal inspection of the unit. Partial disassembly is required to identify the issues. The most common signs of misalignment are blade rubs. These are caused when the rotating blades come in contact with the casings after they have slipped or become distorted. This condition is visually evident on both the casings and the blades themselves. This type of damage can only be seen and assessed if the unit is taken apart. Rotor position to casing measurements is also taken and recorded. Once the cause of damage is understood, the repair phase begins.

The typical Gas Turbine Alignment consists of realigning the bearings and changing the dowel pins in the turbine/exhaust casing vertical joint. On the 7F series units, only the T2 bearing is alignable, since the T1 bearing is secured within a cylindrical fit. Alignment is a fairly complex undertaking due to very small clearances and tight tolerances. The process requires specific expertise, equipment, and experience to properly diagnose and resolve the casing alignment issues.

In decades past, on earlier models, misalignment was less of an issue. Older units were more forgiving and kept running even when misaligned. Today, it’s much more of a concern. Alignment affects clearances and clearances affect output and air quality, which is now heavily regulated.  With today’s modern units and high-efficiency levels, even small rubs of rotating blades with the casings open clearances and degrades the efficiency and output of the unit.

Our Alignment Engineers have the knowledge needed in order to interpret the data. Each engineer’s broad base of experiences contributes to the overall value MD&A provides to our customers. You can expect dependable and accurate results every time you hire MD&A to align your turbine.

MD&A’s specialized services range from Compressor & Turbine Rotor Repairs to Plant Performance Testing and Analysis. We provide immediate access to expert help when you need it, where you need it. 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.

Call our Gas Turbine Services at (518) 399-3616 or use our Contact form.

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