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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 real-world 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 consists of a generator and two FT4 turbines at each end. The thrust of each engine is directed through a free turbine that is on the same shaft as the generator.
During peak demands
The turbine generators are used during peak demands. Each twin pack is capable of producing electricity on the grid in just over two minutes, and can provide about 50 MW within eight minutes of startup. Upon shutdown, the turbine is no longer producing thrust. However, the independent turbine and generator shaft continues to rotate for 20 minutes until it coasts to a stop.
The FT4 enclosures consist of five walls with the rear being open to the atmosphere. The thrust of the turbine causes a phenomenon within the enclosure known as surface attachment, which multiplies the amount of air moving through the enclosure.
The multiplied air enters the enclosure through secondary dampers and provides cooling for the turbine enclosure when operating. Upon detection of a fire, the turbine generator is shut down, which involves the unit being immediately removed from the grid, the fuel shut down, and secondary air dampers closed, limiting the airflow through the enclosure.
Quick and uniform cooling
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 shut down of the lube oil.
During CO2 system testing at the Holtsville site, additional CO2 was required due to the design of the enclosures. Testing showed that the turbine skin temperature had been reduced below the auto ignition temperature within 10 minutes.
Read more in the July-August 2015 issue of Turbomachinery International magazine
(Bob Ballard is the Fire Suppression Technology Manager at Victaulic, a manufacturer of mechanical pipe-joining and fire protection systems. For more information, visit www.victaulic.com)