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.

In MD&A’s experience, significant mechanical failures, particularly in the High Pressure (HP) and Intermediate Pressure (IP) sections, such as rotor cracking, shell, and N2 packing head cracking, continue to worsen in both severity of the defects as well as the frequency of occurrence.  In addition, other reliability issues such as poor vibration characteristics bowed rotors, and diaphragm dishing further contribute to increasing losses in a unit’s thermal performance.

The underlying reasons for the early maturation of the D11 fleet are varied.  The application of low-alloy, single-shell configurations operating at 1050 F is a major factor in the reliability issues of the D11.  However; the evolution of operational demands for combined cycle power plants also plays a major role in minimizing life expectancy.  Early industry expectations were that combined cycle plants would be primarily base-loaded.  However; as the industry matured, combined-cycle plants were eventually required to be capable of sustained, frequent load cycling, and start/stop operational profiles.  By itself, frequent cycling can be a sufficient cause for early-onset mechanical failures. The coupling of operational cycling with single-shell, low-alloy components of the D11 has a multiplying effect.  Based on reported feedback, the severity and range of mechanical issues for the D11 fleet continue to progress with time.

In circumstances where it is determined that the best approach is to repair and maintain existing turbine hardware, Mechanical Dynamics & Analysis (MD&A) offers a full complement of turn-key field services, repairs, and parts.  MD&A has a 16-year history of supporting the D11 fleet including such services as weld repair of cracked packing heads, straightening of bowed rotors, diaphragm repairs, installation of advanced sealing packages, and steam valve internal component manufacturing and replacements.  MD&A is also recognized as an industry vanguard in the area of fossil steam turbine shell weld repairs for over 20+ years.

Additionally, for units that experience progressive reliability issues, spiraling maintenance costs lost thermal performance, and electrical generation, a 15-20 year life-cycle Cost of Electricity (COE) evaluation suggests a retrofit which, in comparison to continuous maintenance expense, significantly outperforms the later on a net present value basis (NPV).  The magnitude of NPV spread is primarily dictated by unit/customer-specific data.  As an example, shell cracking at the main fit ultimately necessitates shell replacement for long-term resolution as a weld repair is considered only a temporary solution to return the unit to service.  Shell cracking is also common at other locations such as the HP and IP inlet areas.  The Discovery of shell cracking during maintenance inspections or as a root cause for forced outages results in high repair costs and disruption to the owner’s operations.  Additionally, the discovery of rotor cracking, depending upon the severity and location, may necessitate rotor replacement prior to the unit returning to service.  As rotors are a long lead component, this scenario suggests it prudent to plan for turbine replacement prior to reaching end-of-life on critical components such as rotors.

MD&A, in conjunction with its parent company Mitsubishi Power, has developed a “Drop-In” retrofit solution for the D11 HP/IP which addresses fleet reliability concerns, providing excellent availability, and enhanced thermal performance.  The MD&A/Mitsubishi Power design was created by leveraging proven steam turbine technologies from Mitsubishi Power.  Given Mitsubishi Power’s extensive combined-cycle fleet experience, our design team was able to develop a retrofit solution that maintains plant interfaces.  Each retrofit offered by MD&A/Mitsubishi Power is custom designed and optimized for its unique application and includes replacement of the existing flexible, low alloy shell with a robust, double-shell construction utilizing high chrome material within high-temperature locations.  The new shell facilitates the implementation of other improvements such as a larger diameter rotor with excellent vibration characteristics and incorporation of a high-efficiency steam path.  Similar to the double-shell, these components also utilize a robust mechanical design including high chrome material for superior creep characteristics.

Inherent with the retrofit, an opportunity exists for re-optimization of the steam bottoming cycle including an increase in duct firing.  In situations where plant operating missions are modified, combustion turbines are upgraded, or duct firing profiles are altered, resetting the unit design point can provide the end-user with significant value.  Design re-optimization can yield major cost-saving benefits beyond the resolution of reliability and thermal performance deficiencies.

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 with a projected payback within 2-3 years.  For more information, contact MD&A at (518) 399-3616 or use our contact form.

Also, read how MD&A has expertise in innovative D11 repair solutions while delivering consistent quality and value along with superior communications.

The post D11 Retrofit Solutions in light of Early Life Expectancy  appeared first on MDA Turbines.

Many units in the combined cycle setup, which can be comprised of two MS7001FA gas turbines and a D11 steam turbine, are also equipped with 7FH2 generators. Many of these machines have unbolted babbitted hydrogen seals. If there’s any contamination of the oil or improper assembly, those conditions may cause the unbolted seals to tilt, which could lead to a hydrogen leakage and oil ingress into the generator.

The OEM recommendation is to upgrade the hydrogen seals to a bolted style which involves replacing the housings. MD&A offers a more cost-effective option to modify the existing housing to accept bolted seals.

The customer requested an upgrade of their existing unbolted hydrogen seals to a bolted design for two of the two generators.

MD&A supplied the two casings with a bolted style along with the upgraded hydrogen seals and all the hardware required. We also supplied a spare set of D11 valve parts and hardware.

For upcoming maintenance, MD&A also supplied one set of 7FA refurbished third-stage hot section parts (including buckets, nozzles, and shrouds).

Our Bearings, Seals & Hydraulics division stocks a large number of hydrogen seals at our Euclid, OH facility, and we can service any size machine. If we don’t have the seals in stock, our skilled technicians can fabricate them using materials we have in the shop.

When a customer orders a set of hydrogen seals from us by part number, we don’t just manufacture the seals to the standard original size. Instead, we always have a dialogue with the customer to learn what their journal size is and what their housing cavity thickness might be. Then we custom-size the seals to fit properly. Some of our craftsmen have been doing hydrogen seals for 20-plus years, so they are well-seasoned and trained in doing the work.

Since 1984, MD&A Bearings, Seals & Hydraulics Division has designed, manufactured, retrofitted, and repaired hydrogen seal rings to correct operational issues such as excessive oil flow, excessive hydrogen consumption, and electrolysis. We have also developed a level of service designed to substantially reduce the manpower traditionally associated with the field ­fitting of critical seal assemblies.

Call our MD&A Bearings, Seals & Hydraulics division today at (800) 446-4776 or use our Contact form.

The post Hydrogen Seal Upgrades appeared first on MDA Turbines.

The post Power Engineering Magazine’s D11 Retrofit Solutions advertorial appeared first on MDA Turbines.