Steam turbine generators are widely used throughout the world as a common source of energy production for public utilities and industry. Fires in turbines are not frequent, but when they occur they can be dangerous for responders to handle, difficult for power plant operators to manage, and costly for the utility and the public they serve.
Water mist systems have the firefighting attributes necessary to make managing a turbine fire less complex. This article introduces the concept of turbine fire protection using high pressure water mist and how these systems can integrate with a power plant’s operation. Case studies featuring a recent large fire, research into turbine fires, and an installed water mist solution are presented.
WHAT DO TURBINES DO AND HOW CAN THEY CATCH FIRE?
A steam turbine utilizes superheated steam to spin a shaft. The energy imparted to the shaft can be used for various work outputs like running large compressors and pumps. The most common use is electric power generation—creating electricity by attaching a generator to the end of the turbine shaft.
Steam turbines range in size from 5 kilowatts to over 1500 megawatts. These are massive, heavy machines spinning at 3,600 RPM for 60 Hz power production and 3,000 RPM for 50 Hz. There are a number of subsystems supporting turbine operation, but the lubrication (lube) oil system is the primary fire concern.
Lube oils are Class IIIB combustible liquids with flflash points ranging from 375°F to 500°F. The flash point of lube oil is high enough that in many applications it would not be easy to ignite. But because of how it is used for steam turbines, the fires that do occur are very challenging and difficult to put out. As a result, responses to
turbine fires are usually slow and measured in order to keep responding personnel safe.
Lube oil is used to lubricate bearings along the turbine shaft. Pumped through steel pipes from a tank and pump skid in a recirculating system, lube oil systems contain hundreds of gallons of oil. If, for any reason, lube oil flow is interrupted, the spinning shaft will make metal-to-metal contact with the inner bearing surfaces. Bearing failure can result in catastrophic damage to the turbine, so the flow of lube oil must be available to the turbine shaft any time it is spinning, even if it is on fire.
HIGH PRESSURE WATER MIST: AN ALTERNATIVE TO
Water mist systems are used in power generation applications for combustion turbines and diesel generators. These pieces of equipment experience similar fires to steam turbines. Water mist designs for combustion turbines and generators are typically total flooding systems suited to the size of those pieces of equipment. Steam turbines present a different challenge; they are installed in large rooms with high ceilings, commonly exceeding the listings of most total flood water mist systems. For steam turbines, local application water mist designs are the only choice.
Three firefighting mechanisms of water mist have been documented: ultra-rapid evaporative cooling, local oxygen displacement, and radiant heat blocking of the flames to unburnt fuels. Manufacturers of water mist systems have performed many fire tests and are acutely aware of the abilities of specific systems to extinguish difficult fires. Snuffing pool, spray, and vertical surface fires of flammable and combustible liquids are what water mist systems are known for.