Overview Modern combustion (or gas) turbine packages typically include an axial compressor, a combustion section and a turbine. Both compressor and turbine sections consist of one or more sets of fixed and rotating blades, called stages. The rapid expansion of exhaust gases is converted to useful work in the combustion turbine stages, and continuous output ratings of 16,000 kilowatts (22,000 horsepower) per unit are not uncommon. Two main types of combustion turbines are used today- the aircraft type, designed for aircraft propulsion, and the industrial type, which is designed for stationary service. Industrial turbines are typically mounted on a frame skid, and are used to drive rotating equipment such as compressors and electrical generators. Both turbine types utilize internal flame sensors to monitor the presence of flame within the combustion section, and initiate automatic shutdown of the combustion fuel system in the event of a flameout condition. This will prevent accidental fire or explosions upon combustion re-ignition. In addition to the internal combustion control systems, leading manufacturers and users of industrial gas turbines also install high performance, optical flame and combustible gas detection systems for turbine skid, enclosure, and facility protection. This document will provide guidance in the proper application of optical flame and gas detectors for total gas turbine area hazard monitoring. Exhaust Stack Pointwatch IR Combustible Gas Detector (3) Inlet Air filter UV/IR Flame Detector (2) Silencer Heat Recovery Steam Generator Transformers Controls Generator Gas Turbine Open frame gas turbine showing optical flame & gas detector locations Combustion Turbine Fire Protection Page 1 of 6
The first step in turbine facility fire protection is to establish multiple lines of defense, which include: 1. Engineered fire prevention design. 2. Reliable and fast detection of a flammable gas/vapor leak and fire. 3. Proper fire suppression using fixed automatic, and portable extinguishing systems. 4. Emergency shutdown capability. Engineered fire prevention design is the most fundamental level of fire protection. This line of fire defense must be implemented early in the engineering design stage of the facility to be effective. Significant research, planning, and hazard analysis is required. The second line of defense is most effectively provided by optical flammable gas and flame detectors. Proper detector selection, installation, and routine maintenance is required of all hazard-sensing detectors to ensure a high level of safety is secured and maintained. The third line of defense requires an NFPA72-compliant, fire suppression and/or deluge releasing system. This system must be designed and installed per NFPA recommendations, and also should be augmented with portable fire extinguishers located strategically throughout the facility. In addition to fire suppression, emergency shutdown of the turbine and/or other facility processes is often critical to prevent the spread of fire. An approved, fault-tolerant system with sophisticated I/O logic capabilities is recommended to perform this safety interlock function. Proper detector selection for turbine applications always starts with identification and analysis of all flammable materials present. After this is completed, the application engineer must then identify the most likely conditions and scenario leading to fuel leakage, accumulation, and possible ignition. After these steps have been completed and analyzed, it is then possible to specify detectors that will reliably detect the hazard. Combustion Turbine Fire Hazards Turbine combustion fuels, lubricating and hydraulic control oil systems, generator cooling systems, and pipeline transmission products are generally the most common fire hazard risks within most gas turbine applications. Although combustion turbines have been designed to burn many fuels, most are designed to Page 2 of 6
burn natural gas, light oils, or a combination of the two. Modern gas turbines utilize an annular group of combustors surrounding the machine between the compressor and the turbine. The fuel is mixed with air and burned in the combustors. A fuel delivery system, similar to those used on boilers but with additional safety interlocks, is used. Any continued fuel flow after a temporary combustor flameout can result in an explosion. Fuel leakage, either internal and external, is extremely hazardous in combustion turbine areas. Lubricating oil fires present an unusual fire protection problem. A turbine oil system fire can be controlled by shutting down the oil system, however this action will almost assuredly result in the destruction of the turbine. Therefore, the lubricating system must always be kept in service until the turbine coasts down. Spill drainage systems should always be trapped to prevent the spread of an oil fire from on area to another. Even small oil leaks can be extremely hazardous. High temperature valve bonnets and turbine exhaust shrouds can provide ready ignition of "misting" hydraulic oil leaks. Generators create, as a by-product of generating electricity, large amounts of heat. They must be continuously cooled while in operation. Many generators are cooled using hydrogen. Hydraulic oil pressure maintains pressurized oil seals that contain the hydrogen, and an oil system shutdown or a loss of oil pressure means that a hydrogen leak can occur. A fixed hydrogen gas and flame detection system utilizing catalytic combustible gas sensors and ultraviolet flame detectors is recommended in these applications. Natural gas, oil, and other pipeline products are often gathered, compressed, and lifted with power provided by gas turbines. The sheer volume of flammable material present within a typical pipeline means that a potential for disaster exists even in the event of a "small" pipeline compressor station leak and fire. Another well-known operating hazard of gas turbines is the powerful vacuum effect present at combustion air intake. High concentration flammable vapor clouds or leaks near the combustion air intake can have disastrous results due to pre-ignition within the compressor section. Many turbine users now monitor combustion and cooling air intake areas using infrared combustible gas detection systems. In summary, any flammable liquid/vapor leak or spill in a combustion turbine area can have disastrous consequences. The degree of hazard is dependent upon the type and size of the leak, conditions within the area, the temperatures present, and isolation from pumps, control circuits, and other flammable materials. Early detection and action is critical to ensure effective fire prevention. Most turbine fire suppression systems use carbon dioxide, FM-200, or other agent for extinguishment. Proper suppression agent selection is typically dictated by the specific fire Page 3 of 6
hazards present within the application, and numerous fire protection engineering standards and recommendations apply to the proper design and installation of these systems. Combustion turbines are typically more valuable than the surrounding structure. In many applications, numerous turbines will be installed within a single structure. In the case of gas turbine pipeline compressors, these structures are often situated in remote locations. This remoteness usually increases the fire hazard risk of the facility because the public fire department, which is usually heavily relied upon, is often under-sized and located far away. For all of these stated reasons, automatic fire detection and suppression systems are highly recommended for turbine facilities. Flammable gas and optical flame detectors are integral components of these systems. Benefits of Combustible Gas Detectors in Turbine Areas Combustible gas detectors will provide early warning of flammable gas concentrations before a catastrophic event occurs. Acoustic enclosures, fuel storage areas, cooling air circulation systems, pressure relief valves, and other enclosed spaces are all suitable areas for protection by gas detection instruments. High operating temperatures in these turbine areas often preclude in-situ sensor installation, requiring the use of extractive vapor sampling techniques to ensure satisfactory sensor performance. Floor drainage is another overlooked portion of many facility protection systems. Most liquid spills end up in the floor drains, and potentially explosive levels of combustible vapors are possible. Another hazard risk in turbine areas is toxic vapor accumulation. Enclosed spaces near combustion turbine exhaust systems should always be monitored for oxygen depletion, carbon monoxide, and other toxic gas build-up. Det-Tronics offers a variety of reliable gas sensing technologies. Catalytic and Infrared combustible gas sensors are available for detection of virtually any combustible gas, and electrochemical gas sensors are available for monitoring of toxic gases and oxygen. Page 4 of 6
Catalytic Combustible Gas Detector Pointwatch Infrared Gas Detector Benefits of Optical Flame Detectors Optical flame detectors will provide the fastest detection of a turbine fire in the early ignition stage. In addition, the field of view, sensitivity, and signal processing algorithms provided by Det-Tronics optical flame detectors makes them the best choice for monitoring areas where interposing equipment and nuisance radiation sources may be present in potential high risk ignition areas. These and other challenging environmental conditions exist within combustion turbine applications, including the previously mentioned high heat conditions which can preclude thermal and smoke detectors. With ambient operating temperatures in the 100-150 degrees F. (38-65 C) range, virtually all Det-Tronics flame detectors are capable of providing satisfactory area flame protection. However, in desert or tropical settings, or under equipment failure conditions (ventilation fan or damper failure), turbine area temperatures may reach 230 degrees F. (110 degrees C.) or more. Det-Tronics ultraviolet detector model C7050B with DE1888K sensor is suitable for continuous high temperature exposure of up to 257 degrees F. (125 degrees C.). In addition, other Det-Tronics flame detectors such as the U7602E unitized UV detector, U7698E unitized IR detector, and the U7652B unitized UV/IR detector have been tested and are confirmed to withstand repeated exposures to operating temperatures of 257 degrees F. (125 degrees C.) for periods up to 3 hours. The variety of flammable hydrocarbon materials present within gas turbine areas means a wide range of fire scenarios, fire propagation rates, and radiant spectral energy signatures are likely. A flame detector that is capable of detecting these fires is recommended, and the Det-Tronics model X3300 multi-ir Protectir offers Factory Mutual (FM) certified performance test capabilities for a variety of flammable hydrocarbon fuels. Certified performance with no false alarms has enabled the X3300 Protectir to set a new world standard in optical flame detection within gas turbine applications. Page 5 of 6
Model X3300 Protectir Model U7652 UV/IR Flame Detector Integrated Turbine Control and Safety Systems Historically, gas turbine hazard monitoring systems have been built using a variety of sensors, transmitters, controllers, and operator display systems. The Eagle Quantum now offers the turbine operator an addressable, total gas turbine hazard monitoring package. This system provides an NFPA72 compliant, shutdown and releasing system that combines the hazard monitoring and emergency action elements required for gas turbines into a single operator interface. Please contact Det-Tronics for additional information on the Eagle Quantum System for Gas Turbines. Additional Reference Materials Applicable Standards & Practices for Gas Turbines: USA: NFPA 37 API 616 ANSI B133 UK: BGC IM24 IP 15 Page 6 of 6