Coal, Gas, O&M

Steam Turbine Fire Protection will Reduce Repair Costs

Issue 11 and Volume 106.

By: Don Drewry and Dominique Dieken P.E., HSB Professional Loss Control

The mention of automatic sprinkler protection for steam turbine generators may evoke disagreement from operations personnel and turbine manufacturers. Typical objections include not using water to extinguish oil and electrical fires and concerns of thermal shock caused by cold water impingement on turbine parts.

While these concerns appear valid, it is important to gain an understanding of fires involving turbine generators, the damages they have caused and the proper methods of providing fire protection. Several studies have been conducted on the subject of turbine generator fires and fire protection systems, including the 1985 EPRI report, “Turbine Generator Fire Protection by Sprinkler System,” by Black and Veatch.

This study is based on a detailed survey of 151 utilities, Westinghouse, General Electric, NFPA and several insurance companies. The survey covered the period from 1930 through 1983. There have also been a number of large turbine generator fires since then whose outcomes support the EPRI study. However, no recent studies have been published. The EPRI study documented a total of 175 generator related fires and 33 hydrogen explosions.

Fire Damage History

Of the 119 fires involving lube oil, 39 fires occurred at the turbine bearings, 16 fires involved lube oil piping, 14 fires occurred below the turbine deck, and seven fires involved the lube oil reservoir. The exciter has also been identified as both a lube oil and an electrical hazard (seven electrical fires and two oil fires). Thus, it can be said that the greatest fire hazard for turbine generators is from lube oil.

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Most steam turbines use mineral oil with a flash point ranging between 375 F and 500 F. When sprayed onto a hot surface the oil will self-ignite at about 675 F. Pressurized supply oil lines, if damaged, can compound the problem as atomized oil would be sprayed onto hot surfaces. Either self-ignition or a nearby ignition source can result in a three-dimensional fire at the bearing or lube oil piping with the burning oil flowing downward and collecting at ground level in the form of a pool fire.

The flame temperature of a lube oil fire, up to 2,100 F, can cause heat to be transferred to various turbine components through conduction, convection and radiation. If there is flame impingement, the surface temperature of any exposed area can be expected to reach 2,100 F within five minutes. As the temperature rises the turbine generator components expand at different rates. If this expansion is prevented because of geometric limitations, thermal stresses occur, which can exceed the yield strength of the materials and cause the components to fail.

When a fire occurs the heat rising from the fire can also collect at the ceiling. If this happens the temperature can rise above 900 F and failure of the roof can occur. Likewise, a pool fire at ground level will quickly involve control and power cables beneath the turbine deck. Any extended exposure to heat will also damage the turbine’s concrete pedestal.

Likelihood of Fire Damage

Although the likelihood of a fire involving a turbine generator is low, several such fires have caused property damages upwards of $30 million with forced plant outages exceeding six months. In two cases permanent decommissioning of plants was necessary. The EPRI study concludes that based on a 30-year plant life there has been one fire in roughly 200 unit-years. This means that one out of seven turbine generators in operation will experience a fire.

While not every fire will result in an inferno, typical damages for a unit without a fire suppression system can be between 50 percent and 60 percent of the unit’s cost plus the cost of peripheral damage to auxiliary equipment within a 30 feet radius. If automatic fire suppression systems are provided the property damage is expected to be limited to repair of the component which caused the fire, water clean-up and repair or replacement of components damaged by the sprinkler water. Assuming four weeks of downtime, a conservative loss estimate for peripheral damage would not exceed 10 percent of the turbine’s installed cost.

Thermal Shock and Water Damage

Thermal stresses from rapid cooling of turbine components by the application of water generally exceeds the yield strength of the material which in turn causes a decrease of the material’s modulus of elasticity. While the thermal shock from water is greater than the thermal stress from the fire, thermal shock only affects the material surface up to the first inch of the material’s thickness. However, thermal stress affects the entire material.

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While the damage from thermal shock cannot be completely discounted, the damage from thermal stress (fire exposure) is a much greater concern. Proper spray nozzle placement and additional insulation or water shields placed over critical components will minimize the possibility of thermal shock.

In the EPRI report, water was used 24 times to extinguish fires and in nine of these cases hoses were used. The other 15 fires used waterspray or a sprinkler system. In all instances no turbine damage was reported and in only two cases did the sprinklers cause an electrical short in motors located below the turbine. Based on this logic, and the case studies supporting it, one can conclude that waterspray is a safe fire protection method.

For those who are still concerned about “accidental” water discharge the probability of an inadvertent actuation of a properly designed, installed and tested automatic waterspray system is extremely remote: less than one in 100 million years.

Fire Suppression

Some people will argue that the best method of fire suppression is operator response with fire extinguishers or a water hose to manually spray water over the bearing seal areas. However, unless a fire is stopped in the very incipient stage, portable extinguishers will be ineffective. Generally the majority of electric generating plants are not equipped to fight interior oil fires beyond the incipient stage. By the time the public fire department arrives and is ready for fire fighting, extensive damage has already been done.

Fires above the turbine deck occur most frequently at the bearings and typically result from a failure of the oil seal. When this happens the concern is a spray fire from the bearing seal rather than the bearing itself. In 1979, a study on extinguishing turbine bearing fires was conducted at the Fire Technology Laboratory in Finland. The results of the study showed that even high density water, sprayed at rates up to 0.90 gpm/sq ft, does not extinguish an oil spray fire. However, the study did find that the cooling effect of the water does control the fire enough to prevent damage to the components.

Further tests showed that using a medium density water spray rate of 0.47 gpm/sq ft, and completely enveloping the flame, will extinguish the spray fire. Although local application carbon dioxide systems have been used to some extent for protection against bearing fires, they are not a preferred method of protection. Local application carbon dioxide systems can be used effectively on two-dimensional oil fires but are not used on spray fires. These systems also have the disadvantage of limited supply. Total-flooding carbon dioxide systems do provide adequate protection for spray fires but are not practical unless the area around the fire is confined.

Fires below the turbine deck can result from an oil release above the turbine deck or failure of oil piping. In most cases, this type of fire will be a two-dimensional pool fire at the ground level. These types of fires can be effectively controlled and suppressed by ordinary sprinklers. Some plants use concentric or guarded lube oil piping where the supply line runs within the return line. Although this contains any oil leak, the concentric piping is not a substitute for sprinkler protection because the piping usually terminates under the machine housing where the piping splits.

Fire Suppression Design

One problem in the electric power industry is the lack of adequate and comprehensive design standards and guides. As a result, the specifications of a turbine generator fire suppression system are usually left to the designer or consulting engineer’s interpretation of NFPA 850 or general knowledge of sprinkler systems. NFPA 850, “Electric Generating Plants and High Voltage Direct Current Converter Stations,” specifies design densities for the bearing areas under the turbine deck and over the lube oil lines under the turbine lagging. The guide states that both bearing and under deck systems should be automatically actuated rather than manually operated.

Although NFPA 850 refers to the EPRI study, it lacks specific guidance on nozzle placement and how the bearing fire suppression system should be actuated. The EPRI report recommends protection by an automatic waterspray system with closed heads for all bearings except the exciter. It also provides recommended temperature ratings for both the heat detectors and the nozzles.

The current industry practice for the protection of turbine generator bearings set forth by a combination of the EPRI study, NFPA and insurance carriers, is by a pre-action waterspray system (Figure 1). This system comprises one to two closed 90-degree directional spray nozzles over each bearing and directed at the shaft seal (Figure 2). The nozzles should be rated at approximately 150 F above the highest ambient temperature of 286 F to 350 F. These nozzles should also be located approximately two feet from the shaft at the 10 and 2 o’clock positions, thus providing the proper spray pattern, cooling and flushing of any oil spray/leak below the turbine deck.

Additionally, one heat detector rated at approximately 100 F above the highest ambient temperature should be installed two feet directly above the shaft. In the case of a fire, the heat released by a fire triggers the heat detectors, which in turn opens the deluge valve. The water discharge should be 0.25 to 0.30 gpm/sq ft of protected area.

The area below the turbine deck and the lube oil lines under the turbine lagging should be protected by an automatic wet pipe or foam-water sprinkler system. It should also be designed to cover all areas where oil could flow, spray or accumulate. Spill containment curbs, or berms, should be used to define the area subject to the above criteria. Utilizing berms prevents the pool fire from spreading outside the sprinkler-protected area. Sprinklers also need to be provided under obstructions wider than 4 feet, such as large piping and valves, and under the condenser as this is an area where burning oil can accumulate.

If a mezzanine is present, sprinklers must be provided for each level below the turbine deck as per NFPA 13, “Installation of sprinkler systems,” which does not consider grated steel a substitute for sprinkler protection. Sprinkler coverage areas must not exceed 100 sq ft. The below deck sprinkler design needs to be based on an application density of 0.30 gpm/sq ft over the entire area protected by the sprinklers or 5,000 sq ft, whichever is less. The total water demand should combine both bearing and underdeck systems, including hose streams.

Although the extension of the bearing pre-action waterspray system to the exciter enclosure is an acceptable means of fire protection, the installation of an automatic total-flooding carbon dioxide system (CO2) is generally preferred over waterspray. The CO2 system needs to provide a 50 percent concentration per NFPA 12, “Carbon dioxide extinguishing systems.” The design should comprise an initial and extended discharge to maintain the concentration for the coast-down period of the turbine generator.

The risk to the turbine generator from fire damage significantly outweighs any potential damage from properly designed fire protection systems. Experience has shown that catastrophic damages can occur where adequate fire protection systems are absent or the wrong type of system is installed.

References:

D. Drewry, A Quantitative Analysis of the Thermal Effect of Water Spray Protection When Applied on Hot Turbine Components, M.S. Thesis, Worcester Polytechnic Institute, 11986

Turbine Generator Fire Protection by Sprinkler System, Electric Power Research Institute, 1985

NFPA 850, Recommended Practice for Fire Protection for Electric Generating Plants and High Voltage Direct Current Converter Stations, National Fire Protection Association, 2000

D. Dieken, “Designing for Plant Fire Protection,” Power Engineering, November 1998

L. R. Hathaway, Electric Generating Plants, Fire Protection Handbook, National Fire protection Association, 1997

Authors—

Don Drewry is Vice President of HSB Professional Loss Control in New York. Drewry has 27 years’ experience with various heavy industries and power generation projects. He holds a M.S. degree from Worcester Polytechnic Institute.

Dominique Dieken, P.E., CFPS, a Senior Engineer with HSB Professional Loss Control, has over 11 years’ experience in industrial fire protection, specializing in electric power generation. He holds a B.S. from Cal Poly San Luis Obispo and is a registered fire protection engineer. Dieken is also a volunteer fire fighter in Greenwich, Conn.