This article presents recommendations resulting from a study of the safety of enclosures at the Teeside combined-cycle gas turbine (CCGT) power plant that may help other plant operators assess and reduce the risk of explosions. Many of the gas turbines in CCGT plants like Teeside are housed in acoustic chambers to reduce noise and permit turbine cooling by ventilation. The Eutech study at Teeside showed that the substantial number of pipes and flanges in a gas turbine enclosure, combined with high pressures, presents the hazard of explosion in case of gas leaks. The hazard analysis of Teeside was carried out to develop several fault trees to quantify the current level of risk to an operator from an explosion inside the enclosure. Eutech made several recommendations to Enron to improve safety at Teeside. They were: increasing the number of gas detectors, locating them more closely to points where leaked gas would collect, and improving the detectors' reliability by examining their potential failure modes.
Despite the best efforts of design engineers and plant managers, natural gas turbines are not beyond mishap. The volatility of the fuels they use can cause explosions, while electrical faults can cause gas turbines to overspeed, damaging them.
Strategies to resolve both of these problems were presented at the 43rd Annual American Society of Mechanical Engineers International Gas Turbine Institute Turbo Expo '98 held in Stockholm, Sweden, from June 2 to 5. The first involves an explosion risk study conducted by Eutech Engineering Solutions Ltd., a wholly owned subsidiary of Imperial Chemicals Inc. (IC I), based in Winnington, England, and the second is the Safeset Turbo Coupling designed by Voith Safeset of Hudiksvall, Sweden, to disconnect overspeeding turbines.
The background of the Eutech study dates to 1996, when British health and safety inspectors raised concerns about the safety of gas turbine enclosures. Enron Power Operations Ltd. of Middlesbrough, England, commissioned Eutech to conduct a study of its Teeside combined- cycle gas turbine (CCGT) power plant in June. Eutech was charged with studying the explosion potential inside Teeside's gas turbine enclosures and recommending solutions to achieve an acceptable level of risk.
The Teeside plant produces 1,875 megawatts from eight Westinghouse 701 DA gas turbines and associated heat recovery generation and steam turbine generators, making it the world's largest gas-fired combined- cycle heat and power plant.
At 12:25 a.m. on July 17, 1996, a fire and explosion occurred in the unit 106 enclosure at the Teeside power station. The incident was caused by the ignition of a naphtha leak from a joint during fuel changeover. The blast injured an operator, who had entered the gas turbine enclosure to verify the operation of the equipment, and also seriously damaged the enclosure, dislodging equipment, walkways, pipework, roof panels, and a door. The fire activated the automatic Halon fire protection and emergency fuel shutdown systems, extinguishing it. This incident made Eutech's assignment all the more critical.
CCGT power plants like Teeside are popular because utilities and industrial users take advantage of their higher efficiency-up to 60 percent, compared to the limits of 39 percent offered by simple- cycle power plants. The energy efficiency of CCGT plants is derived from the recovery of heat energy from turbine exhaust by heat recovery steam generators that produce additional power, in the form of steam, electricity, or both, as opposed to simple-cycle operation, which involves the burning of fuel to turn a single turbine.
Many of the gas turbines in CCGT plants like Teeside are housed in acoustic chambers to reduce noise and permit turbine cooling by ventilation. These enclosures also contain fire suppressants, such as Halon. These turbines typically rely on complex fuel supply pipes with multiple high-pressure joints to deliver the primary fuel, natural gas, with a liquid fuel backup. For example, a 40-megawatt gas turbine's pipework can include up to 30 flanges or flexible joints , while a 250- megawatt turbine's pipework may have more than 200 flanges and 90 flexible joints.
"The large number of flanges and joints, combined with high pressures, presents an explosion hazard within the enclosure in the event of fuel leaks, in combined- cycle gas turbine plants ranging from three to more than 1,000 megawatts," noted Peter Hunt, a chemical engineer at Eutech who led the Teeside Power Station risk assessment study. The managers of most CCGT plants with enclosed gas turbines install gas detectors made by companies such as Detector Electronics of Slough, England, in their exhaust ventilation streams and use ventilation to spot and dilute any fuel leaks before they become explosive.
"However, most ventilation systems are designed for cooling the enclosure rather than diluting a flammable mixture," noted Hunt. The apparent solution of increasing ventilation flow rate would also make the gas detection system less accurate. " Indeed, depending upon flow rates and gas detector alarm settings, there may be a range of leak sizes that are actually too large to be diluted below the lower explosion limit in the vicinity of the leak, but are too small to register an alarm on the gas detector when mixed with the total airflow through the enclosure. This phenomenon can lead to a range of undetected leak events that can cause an explosion," said Hunt.
Eutech took a risk- based approach to its study composed of three main steps: hazard identification, risk evaluation,' and risk reduction. These steps led to the identification of software improvements, including safety management systems, incident response procedures, training, and alarm testing. There were also hardware improvements, including ventilation, gas detection, interlocks, and protection systems.
The Eutech team began by preparing a three-dimensional flow map of the air movement within the turbine enclosure, based on a series of measurements in order to assess the ventilation around potential leak points. The Teeside gas turbine enclosures were approximately 635 cubic meters with 14 air inlet points. Ventilation was provided by three roof fans with a specified airflow rate of 4.8 cubic meters per second.
"We used a vane anemometer to take airflow readings on all 14 air inlet points, defining the number of readings by the air inlet area. We also established a mean velocity, and thus total flow, for each inlet," explained Hunt. The total measured airflow into the enclosure at 200e was 13 cubic meters per second, including a leakage allowance. This corresponded to a total flow of 15 cubic meters per second at 70°C in the fan exhaust.
The internal static pressure was measured at 50 Pascal, with the calculated loss through the fan casing of 44 Pa, to cross check the total. airflow. Thus, the total static pressure develop ed by the fans was 94 Pa. This was checked against the fan performance curve, which correlated to approximately five cubic meters per second, according to Hunt.
The Eutech team prepared a 3-D velocity grid by measuring air movement velocities at each point and using tracer smoke to provide directional data. "This information was used later to evaluate the dilution ventilation rates at potential fuel leak points, a more practical approach compared with using computational fluid dynamics software, which would have been much more complicated than this application required ," explained Hunt.
The Eutech analysts used the fuel ring main as a reference point in their calculations, and established a 3-D grid at 0.5-meter intervals. This enabled the analysts to create a complete velocity and, thus, volumetric flow picture within the turbine enclosure consisting of approximately 500 data points.
"We also generated smoke with a similar density to air and fed it into the enclosures to produce visible airflow patterns that we recorded on video tape. This demonstrated that the solid flooring of the enclosure reduced the effectiveness of air movement, so we recommended it be replaced with open grid flooring, which was done by Enron," said Hunt.
The smoke tests and 3-D grid proved there were no dead spots, although there was considerable flow variation inside the enclosure. Both the smoke and velocity measurements were taken on a turbine during operation and when off-line, to represent startup as well as operational conditions.
Calculating Acceptable Risk
Eutech applied the process hazard review (PHR), originally developed by its parent company, ICI, to serve the chemical industry in the 1990s as a way of rapidly assessing risks in existing plants by focusing on events like fire, explosion, or loss of containment. This' approach also draws strongly on the practical operating experience gained by the power station personnel. "One of the interesting things I found in the Enron project was that our background in the design, installation, and operation of complex and high-hazard chemical and petrochemical plants was very applicable to the study of Teeside Power Station," remarked Hunt. The PHR was chaired by a Eutech lead safety consultant, and carried out by engineering and operations personnel from the Teeside Power Station. The review concentrated on the risk to operators within the enclosure by studying fuel leaks and their ignition.
WITH MANY COUNTRIES tightening their emissions standards, gas turbine pollution control technologies were also highlighted at the Turbo Expo in Stockholm. For example, Catalytica Combustion Systems Inc. of Mountain View, Calif., presented its XONON Combustion System at the conference. This system is designed to hold nitrogen oxide (NOx) emissions to less than 3 parts per million, comfortably meeting the less than 5 ppm of NOx currently specified for new gas turbines in the United States under regional provisions.
The XONON (literally "No NOx" spelled backward) Combustion System prevents NOx formation within a turbine combustor through catalytic conversion, a less expensive and cumbersome alternative to installing selective catalytic reduction equipment to clean up pollutants from the gas turbine exhaust. XONON also eliminates the need for steam injection, which can affect combustor efficiency, and dry low NOx systems, which can cause flame instability and its associated vibration problems.
"Instead, the XONON module is attached directly within the gas turbine combustor," explained Joseph Cussen, director of marketing at Catalytica. The XONON system consists of four components. The first is a lean, premix preburner used to heat the air to the required catalyst inlet temperature. Second is a fuel injection and fuel/air mixing section that provides a well-mixed, uniform fuelair mixture to the XONON catalyst module. There, a portion of the fuel is combusted without a flame. Lastly, in the homogeneous combustion region immediately down stream of the catalyst module, the remainder of the fuel is combusted. The entire process is accomplished at temperatures below the point that NOx is formed. Without a flame, the carbon monoxide (CO) and unburned hydrocarbons (UHC) are also reduced to low levels.
Flexibility is a hallmark of the XONON technology. "Our engineers incorporate a chemical thermostat within the catalyst module that limits the catalyst temperature even at very high fuel/air ratios, so that the catalyst temperature is significantly below the combustor outlet temperature," said Cussen. The thermostat extends the performance life of the catalyst module and enables the technology to be used on a variety of gas turbines that have inlet temperatures ranging from 1,000•C to 1,6OO•C.
In addition, the catalyst module is composed of several sections, each designed to perform a specific function and achieve an exact operating temperature. This enables engineers to optimize the system to serve a specific gas turbine.
Catalytica conducted 1,000 hours of testing on a 1.5- megawatt Kawasaki MIA-13A engine in a test cell in Tulsa, Okla. At base-load conditions, the XONON combustor produced less than 3 parts per million NOx, and 10 parts per million CO and UHC. The XONON-equipped Kawasaki turbine was scheduled be installed in a utility in Northern California this summer for field use. Cussen said that XONON "will become commercially available in late 1998, and will be installed in 1999 in several systems, in different gas turbine models."
Natural gas is the primary fuel at the Teeside plant, with naptha and propane used as the secondary fuels. Fuel is supplied at 25 bar gauge and reduced to 15 bar gauge before being sent to the nozzles. The gas enters the turbine enclosure through a 4-inch pipe connected to a circular manifold that feeds the gas turbine via 18 by 1.5-inch pipes. The PHR quantified small leaks as holes smaller than 1.5 millimeters, medium leaks as holes 1.5 to 6 mm in size, and large leaks as holes more than 6 mm.
"The process analysts modeled the flammable mixture potential by combining the air volumetric rates with the calculated fuel leak rates, and compared this to the flammability criteria of the lower explosive limit (LEL)," explained Hunt. Among their findings was that in the case of small leaks, ventilation reduced the fuel concentration below the LEL, but only 30 percent of medium leaks would fall below the LEL. All of the large leak scenarios exceeded the LEL. "In fact, the ventilation would have to be increased by 50 times to dilute all medium and large leaks, so other methods of safety were considered," noted Hunt. Ignition sources identified by PHR included faults in electrical equipment, gas turbine hot spots, fuel discharges that can cause static electricity, and the mechanical failure of rotating equipment.
The hazard analysis of Teeside was carried out to develop a nll1nber of "fault trees" to quantify the current level of risk to an operator from an explosion inside the enclosure. The fault trees included data collected as detailed from the causes of leaks, likelihood of a flammable mixture formation, ventilation reliability, detection system ignition probability, and the probability of an operator being present. This approach enabled the analysts to compare the quantified level of risk against acceptable target criteria and highlighted those areas that contributed most to the risk.
"This is a vital step, because you need to know the current level of risk to improve safety," stressed Hunt. This application of a quantified approach is commonly used in the chemical and petrochemical industry, he added.
Improving Maintenance Procedures
Eutech made a number of recommendations to Enron to improve safety at Teeside. They were: increasing the number of gas detectors, locating them more closely to points where leaked gas would collect, and improving the detectors' reliability by examining their potential failure modes-for example, poisoning, isolation in error, and power loss.
Couplings protect turbines by limiting surges in torque.
"We also recommended improving ventilation by ensuring that the fans are providing a positive airflow to push leak gas toward the detectors, and detecting ventilation failure by installing a current meter on the fans to check that they are on," said Hunt. He said all of these hardware changes were implemented by Enron on a single turbine for study and refining before they are retrofit to all eight turbine enclosures at Teeside.
Software changes recommended by Eutech and being put into practice by Enron included improving maintenance procedures. That included checking pipe joints before startup of a CCGT unit, checking for gas inside the enclosure, and sending that information to the control room before permitting a maintenance crew to enter an enclosure. Enron also developed emergency drills for its personnel in case of future incidents.
"Although the Teeside study was directed specifically at that power plant, the methodology we used can deter! Tune the level of risk in other gas turbine power plants," said Hunt, who noted that a similar explosion risk study has been commissioned by its 266-megawatt Corby Power Station in Northamptonshire, England.
Reining in Over Speed
While less dramatic than explosions, electrical faults can also damage gas turbines, particularly aeroderivative gas turbines, whose lightweight design makes them more • sensitive to the overloads that can appear when there is a malfunction, typically a malsynchronization or short circuit. These events ca use torque peaks at the generator output shaft 10 times full load torque, lasting for several seconds in the case of short circuit, and are transmitted backward into the turbine system.
Steps must be taken to protect the turbine by limiting the surge in torque, typically by means of a coupling such as the Safeset couplings developed by Voith Turbo in Sweden, to disconnect turbines from their output shaft during torque peaks. These couplings are used in many types of gas turbines, including the Allison 501-KB7 and General Electric LM 6000.
The Safeset couplings transmit the torque through a frictional joint whose torque capacity is controlled by hydraulic pressure. If the coupling is exposed to a higher torque than it can transmit over the frictional joint, it will slip there. This slippage causes a shear ring to cut open a shear tube, releasing the hydraulic fluid and reducing pressure (and thus torque) within milliseconds.
However, when the gas turbine is mechanically disconnected from the workload, it will increase speed momentarily, which can also damage the turbine. "The magnitude of the acceleration is controlled by the amount of fuel available and how it flames out, as well as where in the drivetrain the mechanical disconnection takes place," explained Bo Appell, a metallurgist and managing director of Voith Safeset AB in Hudiksvall, Sweden. Appell said that the best point to limit overspeeding would be between the gas turbine, which has relatively low inertia, and the generator, which has relatively high inertia.
"Thus, Voith engineers used their Safeset technology to develop the Safeset Turbo Coupling to be mounted as a spacer, between two membrane coupling flanges between those points without disturbing the gearbox or gas turbine itself," said Appell. In the new coupling design, the torque-limiting feature of the Safeset is combined with a hydrodynamic brake.
The brake is based on Foettinger's design principle, with its main components consisting of two bladed wheels: a pump wheel and a turbine wheel. The torque transmitted from the driving side is converted into kinetic energy of the operating fluid by the pump wheel. In the turbine wheel this kinetic energy is converted back into mechanical energy.
When the torque limit has been exceeded and the Safeset has been released, the gas turbine is mechanically disconnected from the large inertias of the gearbox and the generator. At that point, the gas turbine accelerates and creates a speed difference over the hydrodynamic brake. The braking action from. the hydrodynamic brake is proportional to the speed difference that is developing, so that the greater the overspeeding, the more effective the braking action. This provides a desirable soft buildup of the strong braking action required to limit the gas turbine overspeed. Simulations for the GE LM 6000. show that the maximum speed can be kept below 4300 rpm in overspeeding from the base speed of 3600 rpm. The new coupling will be introduced commercially in the near future, according to Appell.