Methods to Extend Turbine Life and Improve Performance
By Muhammad Saqib Riaz, Ph.D.
A steam turbine is designed and optimized for a specific set of steam conditions. After long term operations either the steam path components deteriorate or due to changes in steam conditions, a re-optimization of the steam path components is required to regain higher turbine performance. Steam Turbine retrofits are performed to achieve various goals such as improved turbine efficiencies, heat rate or power output. Retrofits are also performed to address turbine reliability and or maintenance issues. Availability of newer technologies and materials for steam path components can be utilized to not only increase steam turbine performance, but also to extend maintenance intervals, and extend life. Retrofits can also accommodate required changes to the thermal cycle based on emerging needs of the power plant. The effectiveness of the retrofit is achieved by utilizing the best available technologies configured to the plant’s specific needs.
Common Steam Turbine Fleet Issues
Steam turbines face challenges that are either associated with turbine deterioration or due to original design limitations. Often times certain modes of operation can lead to reduction in turbine component life. Below sections discuss some of the common issues observed in the steam turbine after long term operation.
Typical High Pressure Steam Turbine Issues:
Based on the steam conditions, each section of a steam turbine faces different challenges. High pressure (HP) section of the steam turbine is exposed to highest pressure and temperature condition. Any deterioration in HP section results in highest loss in the steam turbine performance compared to other sections. Due to higher pressure, tighter clearances are required to reduce steam leakage. A common issue observed in older units is inner casing and blade ring distortion that leads to rubbing of components that open up clearances and increase leakages. At the HP steam inlet, failure of the Bell Seal Ring and welded joint failure at the nozzle chamber have been observed on multiple units. Many units have steam turn-around in the steam paths that result in loss of steam pressure without doing any useful work. Bolt cracking due to material or high thermal conditions is also observed in multiple units. Older rotating blades and vanes mostly had cylindrical airfoils providing much lower level of performance.
Typical Low and Intermediate Pressure Steam Turbine Issues
By the time steam reaches intermediate pressure section (IP), the pressure is dropped but due to reheating of steam, the temperature can be as high as 1000 – 1150°F. In the low pressure (LP) section both temperature and pressures are lower compared to other sections of the steam turbine.
The challenges observed in these sections are somewhat different compared to HP sections. Casing distortion issues still exist in the IP section due to high temperature observed by the steam path components. Older rotors have center bore and shrunk-on discs in the LP section that reduce rotor reliability and also contributes towards shaft vibrations. Similar to HP section, the rotating blades and vanes mostly had cylindrical airfoils providing much lower level of performance. Typically older LP section blades are grouped blades with tennons that generally have reliability and maintenance issues.
Advances in Steam Turbine Technologies:
To improve competitiveness, every original equipment manufacturer (OEM) develops newer and improved technologies to address issues identified in above sections. These newer technologies focus on improving performance along with reliability and maintainability of the product.
One of the most common improvements provided in a retrofit design is the upgrade of existing blades with three dimensions blade vanes that are bowed and twisted to reduce secondary losses in the steam path. Shrouds are made integral to the blades to improve reliability of the blades. Roots with generous radiuses are designed to reduce local stress. Advanced computational fluid dynamics (CFD) tools are used to study the flow characteristics to ensure losses are minimized. Multiple rows of blades are modeled in these analyses to understand interaction between different stages of rotating and stationary blades. Unsteady CFD analyses are performed to study flow behavior at various operational conditions and blade profiles are optimized to reduce losses.
Validation is a key part of the design cycle. Mitsubishi Hitachi uses a test rig, where all newly designed blade profiles are tested. Extensive instrumentation is applied in the steam path to collect pressure and temperature data that is used to calculate blade efficiency, pressure distribution etc. These results are then compared with design calculated parameters for validation purposes.
In the design of the steam turbine, the goal is to extract maximum amount of energy from the steam before the steam enters into the condenser. As the steam reaches towards the end of the LP section, the pressure of the steam is much lower resulting in large volumetric flow. At this stage longer blades are required to handle the large volumetric flow to reduce exhaust losses. Longer blades are designed to achieve higher turbine performance and cost reduction. With longer last stage blades, a 4 flow LP section performance can be achieved with a 2 flow LP section. Mitsubishi Hitachi being a world leader in steam turbines has one of the largest offerings of last stage blade designs for various application ranges.
Common Features Applied on New Generation of Last Stage Blades – 1
Last stage blades are one of the most important and complex part of the steam turbine that produces more than 10% of the total turbine output, and demand high reliability design under complex loading. New generations of last stage blades are designed as one piece with shroud as integral part of the blade. These integral shrouded blades (ISB) not only makes assembly easier but also provide higher levels of damping compared to conventional grouped blades. At the time of assembly, ISBs have some gap at the shroud and snubber connections but with increasing speed, the gap closes and entire row of blades acts as one continuous coupled structure. With continuous coupling it is much easier to control synchronous and non-synchronous vibrational characteristics of the row of the blades. To improve fatigue life of the blades, the tennons are removed to reduce stress concentrations and larger roots with generous fillet radiuses are applied to reduce local stresses to prevent stress corrosion cracking (SCC). Figure 1 shows some of the common features applied on latest last stage blades.
During the development of last stage blades, significant resources are used to ensure the robustness of the design. For design validation, full scale blades are manufactured and assembled on a test rotor that is especially manufactured for the new blades. This assembled rotor is then rotated with the help of an electric motor in a vacuum chamber to study vibrational characteristics of the blades. To excite various vibration modes, air excitation is used that is provided via nozzles located at different radial positions of the blades. A Campbell diagram is generated from the strain gage data that are installed on the blades to understand the vibratory response.
To study the blades behavior in steam environment, Mitsubishi Hitachi uses their Steam Turbine Load Test facility located in Takasago, Japan. The last three or four stages of the LP section are manufactured in full scale and assembled on a test rotor. The entire steam path is instrumented heavily with pressure, temperature and strain gages. Pressure transducers are traversed along the length of the blade to capture pressure profile. The data obtained from this test is used to validate CFD and other design calculations. Mitsubishi Hitachi’s steam load testing facility is the largest testing facility in the world in terms of steam flow rate. The testing is performed at a wide range of condenser vacuums and steam flows to study blade behavior at extreme operating conditions. For low load operation, a drive turbine is used to operate the turbine.
The next level of design validation is performed at an in-house power plant located at Mitsubishi Hitachi’s Takasago factory. This combined cycle power block consists of a gas turbine, steam turbine, HRSG and an air cooled condenser. New technologies such as last stage blades, seals, and coatings developed are applied in the steam turbine for long term reliability and performance testing. The steam turbine is heavily instrumented to collect turbine data throughout the unit. This facility helps design engineers understand the long term deterioration modes of the steam turbine components. Mitsubishi is the only company in the world to have such a type of test facility. Figure 2 provides an aerial view of the power plant and images of the steam turbine.
Mitsubishi Hitachi In-house Combined Cycle Power Plant – 2
In some cases, to push design validation to next level, validation testing is also performed at the customer turbine site. This type of testing can range from torsional testing of the entire rotor train to last stage blade testing to study blade behavior at certain operating conditions. Relevant components are instrumented to collect desired data that is later compared with design calculations for validation purposes.
Advanced Sealing Technologies:
Turbine performance is greatly impacted by steam leakages. Any steam that is not passing through the steam path is a loss. In high pressure turbine sections, steam leakage is more crucial as any small clearance opening will result in much larger amount of leakage than in the lower pressure sections of the turbine.
The following section describes seals developed by Mitsubishi Hitachi for application in various sections of the steam turbine which are shown in Figure 3.
Active Clearance Control (ACC) Seals:
ACC seals have been in use for more than a decade with remarkable results. Conventional spring back seals are assembled with a fixed, generally larger clearance. In case of ACC seals, at the time of startups and shutdown, the clearances are larger to ensure no rubbing during transient events, but during steady state operation the clearances are reduced by the steam pressure applied at the seal rings. This ensures small clearances during steady state operation to reduce leakages and maximize performance.
The non-contact nature of leaf seals provides long term performance sustainability. A stack of thin leaf plates are housed in a casing which is accompanied by additional labyrinth seal teeth. At the time when rotor is stationary, the leafs touch the rotor but as the rotor starts to rotate, due to hydrodynamic force, the leaves lift upward leaving a very small clearance between rotor and leaf seal making it a none contact seal. The wider geometry of the leaf seals provide higher axial stiffness and can be applied at locations of higher differential pressures. Long term operation has shown negligible wear on the leaf seal and on the rotor.
Abradable seals have a layer of softer abradable material applied on the packing rings with sealing tooth on the rotor. On conventional seals, in the event of rubbing, the seal tooth wears down due to rubbing with hard rotor material. This results in larger clearances for long period of time until the seals are replaced. For abradable seals, in case of rubs, the seal tooth does not wear rather it wears the abradable material. The sharp seal teeth keeps steam leakage low and help sustain performance for longer periods of time.
Guardian Packing & Vortex Shedder® Seals:
Guardian seals have guardian posts made out of low friction wear resistance material. In case of a rub event, the guardian posts come in contact with the rotor, while remaining seal teeth that have slightly larger clearance are protected.
Vortex shedder seals have dimple like features on the seal fins. These dimples generate flow vortices that provide resistance to the path of leaking steam thereby reducing leakage.
Types of Steam Turbine Retrofits:
Retrofits on a steam turbine are performed to achieve different goals. Based on the requirements from the customer, the scope of the retrofit can vary significantly. The below list provides a few high level retrofit classifications:
- Small modification to the unit to achieve specific turbine operation conditions. This may involve changing one or more rows of blades.
- Entire steam path upgrade along with new inter-stage packing seals (with or without rotor replacement)
- Steam path upgrade with new bladed rotor along with new seals and new inner casing (if applicable)
- Entire turbine section replacement.
The benefits of the above retrofit scopes can be greatly increased through the application of new technology components.
Different Sealing Technologies Applied to Reduce Steam Leakages- 3
Retrofit Case Studies:
The following retrofit examples show different levels of modifications performed in each case.
Retrofit Case 1: HP /IP Steam Path Retrofit
In this case an old HP/IP steam path was replaced with an upgraded steam path. A new mono-block rotor was applied along with latest technology integral shrouded blades to improve performance and reliability of HP and IP sections. At the main steam inlet, the Bell Seal Ring configuration was converted to stack ring inlet design to improve reliability and reduce leakages. To reduce inner casing distortion, a thermal shield was applied at the inlet of the IP section. Seals were improved throughout the steam path with application of ACC and leaf seals where applicable. The steam path was optimized to swallow larger amount of the steam as more steam was available for power generation. The outer casing of the turbine was reused and the upgrade was designed to fit within the existing outer casing. The project surpassed the guaranteed efficiencies with around 4.4% improvement in HP efficiency and around 7% increase in IP efficiency beyond degradation recovery. Figure 4 provide a schematic of the steam path after upgrade.
Retrofit Case 2: LP Steam Path Retrofit
This case study is related to the retrofit applied to the LP section of the steam turbine. A boreless new rotor with high efficiency ISB blades was provided as part of the retrofit. LP end blades were replaced from grouped blades to longer integral shrouded blades that provided larger annulus area to reduce exhaust losses. The exhaust flow guide for the last stage blade was redesigned to guide steam smoothly out of the LP section. Similarly the inlet throat area was redesigned to ensure a smooth transition of steam to the first stage in LP Section. The inner casing was optimized to accommodate longer last stage blades. As a result of this upgrade, the LP section was able to achieve more than 10% higher efficiency including degradation recovery.
Case study 3: Industrial Steam Turbine Retrofit
Industrial steam turbines are used not only for power production but they are also commonly used to supply process steam. Due to multiple functions of the industrial units, the changes required in the turbine over the life span of the unit are more common. In many cases the need for the process steam changes over time or the economics of the plant operation does not allow certain modes of operation.
All these scenarios can be addressed during retrofit opportunities.
HP/IP Steam Path Retrofit – 4
In this case study, new steam conditions drove to the optimization of the steam path.
The need for extraction steam was also changed and the goal of the retrofit was to close one existing extraction and add one extraction that has new pressure and flow requirements. During the retrofit, the main objective of the customer was to minimize the number of modifications while maximizing the unit power output. As a result of this effort, multiple options with varying impact and complexities were provided to the customer to select the option that best matched their needs.
HP/IP and LP Section Replacement Retrofit Project – 5
Case study 4: HP/IP or LP Section Replacement
In this type of retrofit, the entire section of the steam turbine is replaced. This provides maximum flexibility to the design engineers to achieve retrofit goals due to reduced constraints associated with retained components. In such types of retrofits the main constraint is to fit the new outer casing within existing bearing span. The steam path can be optimized to achieve higher level of performance by adjusting blade path base diameters, applying latest technology ISB blades, best possible sealing combinations etc. Casing distortion and cracking issues are addressed by redesigning the casing and with the application of thermal shields. Welded rotor (HP Section) and mono-block boreless rotor with 12% chrome material (LP Section) are applied. New inner and outer casings allowed flexibility to put larger last stage blades. Inlet and exhaust steam flow path were designed using state-of-the-art CFD programs by making 3D models of relevant sections. This helped in reduction of losses in the steam path resulting in higher turbine section performance approaching that of new units. Figure 5 provides an image of an HP/IP and LP section retrofit project showing improvements applied in the steam turbine.
Steam turbine retrofits are performed to achieve higher performance and improved heat rate coupled with improved reliability and maintainability.
Changes in thermal cycle can be accommodated along with addressing the customer’s operational issues. Newer technologies and material can be applied to improve both turbine performance and component life. The scope of the retrofit project can vary significantly based on the needs and requirements of the customer from modifying several stages and extractions, to complete section replacements.
Muhammad Saqib Riaz is manager of Steam Turbine Engineering at Mitsubishi Hitachi Power Systems Americas in Orlando, Florida.