The latest testing of the compressor for the Westinghouse 501ATS gas turbine was recently completed. When it enters service early next century, the 501ATS system will be capable of a combined cycle output of 420 MW, and efficiencies of 60 per cent.
Four manufacturers are currently developing gas turbines as part of the advanced turbine systems (ATS) programme, with financial support from the US Department of Energy. Allison and Solar are working on machines in the 5 to 15 MW range, while GE and Westinghouse are developing 300 MW class units that will give combined cycle outputs of around 400 to 420 MW.
The overall purpose of the ATS programme is to develop low cost, high efficiency gas turbine systems, with lower environmental emissions than existing units. The specific objectives are:
efficiencies greater than 60 per cent (LHV) on natural gas for large scale utility turbine systems, or a 15 per cent improvement in efficiency for smaller industrial systems
NOx emissions lower than 9 ppm, and CO and unburned hydrocarbon emissions lower than 20 ppm, without post-combustion cleanup
fuel flexibility – initially designed for natural gas but with the potential to use coal-derived and biomass fuels
busbar energy costs ten per cent lower than the best available (in 1992) turbine systems meeting the same environmental requirements
reliability, availability and maintainability equivalent to, or better than, current state-of-the art systems.
Both the GE and Westinghouse systems under development use closed-loop cooling to improve system efficiency and reduce environmental emissions. Closed-loop steam cooling utilizes the superior heat transfer characteristics of steam, compared to air, and also enables better integration between the gas turbine and steam turbine cycles.
GE’s ATS, the 107H, is a 400 MW combined cycle system with an overall efficiency in excess of 60 per cent LHV. The high system efficiency is achieved by increasing turbine inlet temperature to 2600°F (1427°C) and incorporating many design features from GE’s aircraft gas turbines. An 18-stage compressor, scaled up from GE’s CF6-80C2 aircraft engine, is capable of delivering 1230 lb/s (558 kg/s) of air at a pressure ratio of 23:1.
Westinghouse is developing a combined cycle ATS system capable of producing 420 MW, with an overall system efficiency of 60 per cent LHV. Turbine inlet temperature is 2750°F (1510°C) and the 20-stage compressor is capable of delivering 1200 lb/s (544 kg/s) of air at a pressure ratio of 27:1. Compressor tests were completed in October 1997, and testing of the combustor and turbine is now nearly complete.
The 501ATS gas turbine is the next frame in the Westinghouse series of utility turbines. The evolution of Westinghouse’s product range started with the 45 MW 501A in 1968, which was developed in stages, up to the 100 MW 501D5 in 1981. The next turbine in the series was the 160 MW 501F, introduced in 1991.
This was followed by the 230 MW 501G, which formed the basis for development of the 501ATS.
When the 501ATS is used in combined cycle configuration, the powertrain will comprise the gas turbine, generator and the steam turbine, connected in an in-line arrangement with a self-shifting and synchronizing clutch located between the generator and the steam turbine. The gas turbine exhaust will pass through a three-pressure heat recovery steam generator (HRSG), which will incorporate additional features to recover rotor cooling air heat and to preheat the fuel.
The high pressure steam turbine exhaust steam will be used to cool the transitions and the first two stages of stationary vanes. The reheated steam will then be returned to the steam cycle for induction into the intermediate pressure steam turbine. The two-case axial exhaust reheat steam turbine will use 3-D bowed impulse and reaction blades on the high pressure and intermediate pressure turbines, respectively. The low pressure turbine will incorporate 1.07 m long titanium blades in its last stage.
Compressor
Testing of the Westinghouse ATS compressor, was completed on 22 October 1997. It shares many common parts with the 16-stage compressor from the 501G machine and its mass flow is identical to that of the 501G. However, the higher inlet temperature of the 501ATS and closed-loop cooling meant that an increase in pressure ratio from 19:1 to 27:1 was required. This was achieved by adding three stages to the rear of the 501G compressor.
The latest 3-D viscous codes and custom-designed airfoils were used in the design of the compressor. Variable stators have been added to stages 1 and 2 to improve starting capability and part-load performance.
The full scale 501ATS compressor verification tests were carried out at a specially designed facility at the US Navy base in Philadelphia. To reduce the power needed to drive the compressor to the level that is available at the site, the facility was designed for sub-atmospheric inlet pressure. The inlet system comprised a filter house, a straight pipe with a flow straightener and a flow meter, inlet throttle valve, diffuser with flow straightening devices, 90° bend with turning vanes and a silencer.
Because of the sub-atmospheric operation, two stages of compressor bleed air were ducted into the inlet diffuser, after passing through coolers. The exhaust system included a large diameter back pressure valve to provide control over the test pressure ratio. A small diameter quick-acting valve, located in a bypass line around the large back pressure valve, was used for recovery from compressor surge.
The compressor was instrumented with static pressure taps, fixed temperature and pressure rakes, thermocouples, tip clearance probes, blade vibration monitoring probes, rotor vibration probes, acoustic probes and strain gauges. Provisions were made for radial traverses in eight axial locations in the compressor and four radial locations in the inlet duct.
More than 500 individual measurements were recorded. A dedicated data acquisition system was used to collect and reduce the test data, and important performance and health monitoring parameters were displayed on computer screens in real time.
After the compressor test facility was commissioned, an extensive test programme was performed. This included design point performance verification, blade vibration and diaphragm strain gauge measurements, inlet guide vane and variable stator optimization, compressor map definition, and optimization of starting characteristics.
Compressor testing was successfully completed ahead of schedule, and all mechanical and aerodynamic performance expectations were confirmed. Since the front stages of the 501ATS compressor are the same as in the 501G, this test was also confirmation of the mechanical integrity and performance of the 501G compressor.
Lowering NOx
The piloted ring combustor was selected for the 501ATS as the most successful of several dry low NOx combustors developed by Westinghouse over the last ten years. This combustor comprises a pilot and two axially staged premixed zones.
Premixed fuel and air is introduced into the primary zone, where the combustion is stabilized by a swirl-induced recirculation zone and a centrally located pilot. The secondary zone, downstream of the primary zone, is supplied with premixed fuel and air through an annular duct surrounding the primary zone. The piloted ring combustor has achieved single-digit NOx emissions and excellent stability on low pressure tests.
It is now undergoing high pressure testing to optimize emissions and expand the operating range by improving fuel and air mixing in the primary premixing passage.
Increasing efficiency
Like the design of the compressor, the design of the four-stage turbine is based on a 3-D design philosophy and viscous codes. The airfoil loadings were optimized to enhance aerodynamic performance while minimizing airfoil solidity. The reduced solidity resulted in lower cooling requirements and increased efficiency. To further enhance plant performance, the following features were included:
turbine airfoil closed-loop cooling
active blade tip clearance control on the first two stages
improved rotor sealing
optimum circumferential alignment of the airfoils.
The 501ATS utilizes advanced thin wall turbine airfoil designs with thermal barrier coatings and aero engine cooling technology. Closed-loop steam cooling is used on the first and second stage vanes, while closed-loop air cooling is used on the first two stages of blades.
Air was chosen for blade cooling to eliminate the risk of steam corrosion, deposition and complexity that closed-loop cooling with steam poses. An added advantage with air cooling is that the air can be cooled after it is removed from the combustor shell so that only relatively small amounts of cooling air are required. In service, the cooling air will be filtered to remove dirt particles before being ducted to the rotor blades.
The difference in plant thermal efficiency between blade closed-loop cooling by air rather than steam is estimated to be about 0.2 per cent.
Westinghouse has been using thermal barrier coatings on turbine airfoils since 1986 and they are now a standard feature of 501D5, 501F and 251B11/12 machines. Recent field trials on coated airfoils have demonstrated excellent results after 24 000 operating hours. To achieve further improvements, the 501ATS turbine incorporates advanced coating systems that use new ceramic materials and improved bond coats.
In addition, the latest aero engine blade and vane nickel-based alloys are used in the 501ATS turbine design. To provide increased creep strength and fatigue resistance compared to conventional materials, single crystal nickel alloy CMSX-4 is used for the first stage vanes and blades.
Casting development for the first stage blades and vanes is underway at the Allison Single Crystal Operations Division of Rolls- Royce. To verify the critical cooling designs, a comprehensive three-part programme is being undertaken. Various parameters were measured on plastic models, at Carnegie Mellon University. A liquid crystal thermochromic paint technique was used to measure the internal heat transfer coefficients, while outside heat transfer coefficients are being measured using model turbine tests. The first stage vane cooling design will be verified at ATS operating conditions in a hot cascade test rig.
The one third-scale model of the first two 501ATS turbine stages is being tested in a rig at Ohio State University. This is a shock tube test facility, where the shock tube is pressurized, the test chamber is evacuated, exit traverse is spun up to speed, and the turbine rotor is accelerated up to speed by an air drive. A diaphragm is then exploded and the shock tube discharges, resulting in a shock at the test section inlet. The turbine is accelerated to the desired test speed, exit area chokes, and the turbine reaches stable design speed, pressure ratio and mass flow. The test data are then recorded in a fraction of a second.
The main objectives of the turbine test programme are to verify the aerodynamic performance with reduced airfoil solidity, to quantify the performance benefits achievable through optimum circumferential alignment of the turbine airfoils, and to measure outside heat transfer coefficients on the airfoils. The test programme started at the beginning of this year.
TablesTable 1.
ATS/501G compressor specification