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Fire Protection for Turbomachinery
EVALUATING HYBRID SUPPRESSION SYSTEMS FOR AERODERIVATIVE GAS TURBINES
Testing was carried out to evaluate the effectiveness of a hybrid fire suppression system on aeroderivative style gas turbines. A utility company and its fire protection engineer provided the use of a Pratt & Whitney FT4 aeroderivative turbine generator to test the hybrid inert gas and water mist system on an operating unit under load, enabling the research team to assess its efficacy in realworld scenarios.
The utility established the test criteria: Testing would be deemed successful if the system could cool the turbine skin to less than 380°F (the auto ignition temperature of lube oil and turbine fuel plus a safety factor) within 10 minutes. The 10-minute timeframe matched the performance of the existing CO2 extinguishing system.
Testing involved: Operating the turbine off the power grid but at full speed; on the grid and allowing the unit to cool naturally; and on the grid allowing the system to discharge. Systematic changes in water flow and installation parameters were used to find optimal results.
Testing was conducted at a site in Holtsville, New York, housing 10 twin pack FT4s. Each pack consisted of a generator and two FT4 turbines at each end. The thrust of each engine was directed through a free turbine on the same shaft as the generator. The turbine generators were used during peak demands. Each twin pack was capable of producing electricity on the grid in just over two minutes, and could provide about 50 MW within eight minutes of startup. Upon shutdown, the turbine was no longer producing thrust. However, the independent turbine and generator shaft continued to rotate for 20 minutes until it stopped.
The FT4 enclosures created a challenge in that the rear exhaust portion was not sealed and open to the atmosphere. The thrust exiting the free turbine entrained air within the enclosure. The entrained air entered the enclosure through secondary dampers and provided cooling for the turbine enclosure when operating. Upon detection of a fire, the turbine generator was shut down, which involved the unit being immediately removed from the grid, the fuel shut down, and secondary air dampers closed, limiting the airflow through the enclosure.
Fires in turbine enclosures are generally caused by a leak or failure of a fuel line or lube oil line. If a fitting loosens or the hose breaks, the media could pool or spray onto a hot surface, igniting the fluid. Although the fuel line can be shut down upon detection of a fire, the lube oil must continue to circulate until the free turbine and generator stop rotating.
As a result, fire suppression systems should be able to extinguish the fire and quickly and uniformly cool the turbine skin to prevent re-ignition of the lube oil or turbine fuel. Non-uniform cooling can cause warping of the turbine skin, which could impact the blades.
Standard requirements for CO2 fire suppression systems are to provide a design concentration of 34% within one minute and maintain 30% for a period of 20 minutes, or 10 minutes plus the time needed for a safe shutdown of the lube oil. This is because such systems do not provide cooling, so suppression agent volumes must be sustained at levels unable to support combustion. During CO2 system testing at the Holtsville site, it took 10 minutes to reduce the temperature of the turbine skin below the auto ignition temperature.
Hybrid fire suppression
Hybrid fire suppression systems are a newer option for the protection of turbine enclosures. They offer advantages over CO2 systems, including the elimination of life safety and enclosure integrity concerns.
A hybrid system employs water and an inert gas to suppress a fire. The gas essentially atomizes the water, creating a swirling fog in the enclosure. The components attack the fire simultaneously, the water cooling the space, and the gas reducing the oxygen content and generating steam.
Hybrid systems produce water droplets that are smaller than 10 microns in size — up to 100 times smaller than water particles delivered through traditional water mist systems. The small droplet size combined with the minimal amount of water released per emitter prevents significant wetting of the space.
A total of eight tests were used to evaluate the cooling performance of the hybrid fire suppression system. Per FM Approval Standard 5580, two emitters were installed in the turbine enclosure based on the enclosure volume. The test apparatus involved a 60-gallon water tank to be pressurized using the outlet pressure of the hybrid fire suppression system automatic regulating valve (ARV).
A 20-port manifold was connected to 20 49-liter nitrogen cylinders. One end of the manifold had the fire suppression system ARV while the other end was connected to a tube trailer using a high-pressure (3,000 psi) hose. The tube trailer consisted of 62,000 cubic feet of nitrogen at approximately 2,700 psi and was used to refill the cylinders between testing.
A National Instruments data acquisition system recorded temperatures at the compressor section, combustion section front, combustion section rear, hot turbine section, exhaust diffuser, enclosure, as well as oxygen level under the unit near the fuel hose connections. In addition, hand-held devices recorded outside air temperature, relative humidity and air flow through the secondary dampers.
After completing these tests, the hybrid fire suppression system met the criteria established by the utility: It cooled the turbine skin to below 380°F within 10 minutes. One test demonstrated that it was capable of doing so within 33 seconds. Successful cooling results were obtained with the emitters installed overhead and aimed toward the hot section of the turbine.
However, the turbine body becomes an obstruction and reduces the velocity of the discharge below the turbine body where the fuel lines can reside. The design of the enclosure with an open exhaust plenum adds difficulty in that oxygen reduction is minimal.
To resolve the obstruction and provide a more uniform discharge around the turbine, it is recommended that additional nozzles be installed below the turbine. The additional emitters add cooling and fire suppression to protect the underside of the turbine, thus mirroring the overhead protection.
This indicates that hybrid fire suppression systems are able to protect aeroderivative turbines under normal operating conditions. They are a viable, alternative to CO2 systems.
