Fire protection systems for air-cooled hydroelectric generators have several special requirements due to these generators’ unique geometries. This survey of options will help plant owners and operators make the best equipment selections for their plants and thereby avoid unexpected surprises.
The global hydroelectric industry is very busy relicensing and modernizing existing facilities (Figure 1). In the U.S., hydroelectric generating station modernizations are multi-year projects that either upgrade or replace all the powerhouse systems, including the fire protection systems. If you are involved in a hydroelectric upgrade project, there are fire protection guidelines associated with air-cooled generators that must be carefully considered during the upgrade design and equipment selection process.
|1. Water, water everywhere. The Chief Joseph Dam, with its 27 turbines producing 2,640 MW, is the second-largest hydropower plant in the U.S. and the largest operated by the U.S. Corps of Engineers. Installation of the first 16 units on the Columbia River in Washington was completed from 1955 to 1958. The final 11 turbines were installed between 1973 and 1978 and later upgraded in the mid-1990s. Even though hydroelectric plants are surrounded by water, proper fire protection system design is very important. Source: Dominique Dieken|
Fires in hydroelectric generators are typically the result of a generator fault; however, not all faults are followed by fire. Generator fires are a low-likelihood event, but their consequence, in the absence of fixed fire protection, can be the complete destruction of the generator or warping of the stator and rotor frames from heat exposure. Replacing a 100-MW generator can cost over $20 million and require a downtime of a year or longer.
The fire risk associated with non-hydroelectric air-cooled generators in the electric utility industry is generally low, and fixed fire protection systems are usually unnecessary. However, the geometry of the typical hydroelectric generator provides larger surface areas that increase fire spread. Combustible materials in the generator—such as end turn insulation, winding insulation, end shields, cable, and contamination (such as dust and/or oil particles)—can also quickly increase the spread of a fire.
Does Your Plant Require a Fire Protection System?
One of the more difficult engineering decisions during a modernization project is deciding whether or not to provide a fixed fire protection system. As with many engineering questions, the answer is: “It depends.”
Equipment selection depends on a number of factors, such as the size of the machine, the type of winding insulation, and the risk tolerance of the stakeholders (including owners, operators, insurers, and financial institutions). It also entails considering the consequences of extended unit downtime, such as replacement power purchases, if the unit is damaged by fire.
As with many design questions, the answer you get often depends upon who answers the question. Past editions of National Fire Protection Association (NFPA) 851 suggested that units with a nameplate rating of less than 25 MVA do not need protection. The industry in years past thought the risk of fire at these smaller units seldom warranted the expense of installing a protection system. However, the 2010 edition of NFPA 851 now defers this decision to a formal Fire Protection Design Basis, which documents the decision-making process.
From a fire hazard standpoint, there are essentially two types of air-cooled generator winding insulation: thermoplastic and thermoset. Modern winding technology using epoxy resin (a thermoset polymer) is recognized as having a lower fire risk than the older thermoplastic (a mix of asphalt, cloth ribbon, and polyester) insulation. One common misconception is that Class F copper wire winding insulation does not need fire protection. This class rating pertains to the maximum temperature of the wiring during operation and is not necessarily indicative of fire risk. However, according to NFPA 851, protection should be provided for “generator windings consisting of materials that will not extinguish when de-energized.” That means, in addition to the winding insulation, other combustibles within the generator—such as various fiberglass parts, lubricating oil, and dust—must be carefully considered in any fire protection design.
Unfortunately, NFPA 851 is open to interpretation, and to my knowledge there are no representative tests that would quantify a specific threshold in the full geometry of an installed stator. For example, the 2005 edition of NFPA 851 stated that “[t]hermoset [insulation] does not require fire suppression systems.” However the technical committee that reviewed the standard reconsidered this position after receiving reports of several self-sustaining fires involving thermoset insulation and has deleted that sentence from the 2010 edition.
One major property insurer, FM Global (formerly Factory Mutual), takes a somewhat different approach to evaluating potential fire losses. The current FM Global standard, Data Sheet 5-3/13-2, Hydroelectric Power Plants, requires fire protection for generators rated 50 MVA and over, regardless of winding composition.
Good design practice is to determine if the risk of an extended outage for repairing or replacing a generator is unacceptable. If so, protection should also be provided for smaller machines. Accordingly, the U.S. Bureau of Reclamation guidelines state that fire protection should be provided for units over 10 MW.
In the end, a determination that fire protection is needed will probably be driven more by the potential for huge monetary losses during a lengthy forced outage than by physical damage to the generator alone.
The rest of this article looks at the types of fire suppression systems available for air-cooled hydroelectric generators. The table summarizes the advantages and disadvantages of each option discussed.
|Hydro air-cooled generator fire protection system options. Source: Dominique Dieken|
CO2 Suffocates Fires
Although carbon dioxide (CO2) is a very effective agent, the fire extinguishing success of CO2 systems requires enclosure integrity and the proper application rate of the gas. CO2 systems extinguish fires by starving them of oxygen, so a release of the gas into an enclosed space poses a risk to humans. CO2 released into an inadequately designed system or a leaky enclosure may not extinguish the fire.
CO2 is still the prevalent fire protection agent used for air-cooled hydroelectric generators due to its nonconductive properties, relatively low agent cost, and lack of residue. The downside of releasing CO2 in an enclosed space, such as a generator enclosure, is the danger it poses to personnel (Figure 2).
|2. Orderly CO2 storage. A well-arranged fire suppression system manifold for a generator enclosure, including high-pressure CO2 cylinders designed to provide an initial and extended discharge. Source: Dominique Dieken|
Personnel safety is the most important aspect of using CO2. Any CO2 release into a space that results in a CO2 concentration exceeding 7% is dangerous to humans. For comparison, the minimum concentration of CO2 required to extinguish a flame is usually between 34% and 50%. This high concentration of CO2 has many owners and operators concerned, and for good reason. There have been 119 documented fatalities involving total-flooding CO2 systems, many of which involved marine applications. According to some estimates, the actual number of fatalities exceeds 300. An additional concern: CO2 gas could migrate to lower areas of the powerhouse and form dangerous “pockets” in unexpected spaces that can remain a personnel danger even after the fire is extinguished.
These and other personnel protection concerns are addressed by the current edition of NFPA 12, Carbon Dioxide Fire Extinguishing Systems, which contains a number of new retroactive personnel safety requirements, including ANSI-compliant three-panel warning signs, manual lockout valves, and audible and visible alarms. If the system is not provided with a discharge delay, a formal procedure must exist that ensures that the CO2 system is disabled anytime someone enters a protected space. This procedure should also ensure that the system is reactivated once personnel have exited the enclosure. Other design and operating considerations include those that follow.
Evacuate Affected Spaces. Given that the quantity of CO2 discharged is usually for a relatively small enclosure, the likelihood of CO2 gas migrating to lower levels at harmful concentrations in the large volume of a powerhouse is remote. Nevertheless, in the event of a CO2 system discharge, it is prudent to evacuate levels in the powerhouse beneath the generator. Personnel entering these lower levels after the discharge should be provided with oxygen monitors and, preferably, self-contained breathing apparatus. The powerhouse should be thoroughly ventilated and atmospheric checks made before other personnel are allowed to reenter.
Use an Odorized CO 2 . Although not required by NFPA 12, CO2 gas with an added wintergreen odorizer can enhance personnel safety. Much as the natural gas industry adds mercaptan to odorize natural gas to produce that rotten eggs smell as a safety measure, the smell of wintergreen quickly warns staff that carbon dioxide gas is present. Odorized CO2 can be difficult to obtain from local suppliers, as it is not a common industrial gas. A better alternative may be to install a commercially available odorizer assembly, which injects a scent into the CO2 stream during a discharge.
Perform Baseline Tests. A discharge/concentration test is an ideal time to also measure oxygen levels throughout lower areas of a powerhouse to provide a baseline for which areas of the powerhouse could become dangerous in the event of a CO2 system discharge.
From a design perspective, the initial concentration of CO2 when released must reach 30% after 2 minutes and 50% after 7 minutes to effectively extinguish an open flame, in accordance with both NFPA 12 and FM Global Data Sheet 5-3/13-2. Next, the concentration must remain at a minimum of 30% for 20 minutes or the wind-down time of the generator, whichever is greater, to allow for cooling and to prevent re-ignition. These specifications could pose special problems inside a generator enclosure if generator cooling air is drawn through the enclosure using fans. If this is the case, the design must consider additional CO2 quantities to make up for losses caused by cooling air until the generator is brought to rest.
New CO2 systems require a full discharge test as part of the acceptance testing requirements. This test measures the concentration within the protected space with an analyzer to ensure that the design concentrations of CO2 are achieved. If this test was not initially performed, or if the generator was recently modernized, an ideal time to perform such a test on high-pressure systems (the gas cylinders) is after 12 years from the last hydrostatic cylinder test date, the date when the cylinders must be emptied and tested anyway. In the event of a discharge, the doors to the generator enclosure should remain tightly closed for at least 20 minutes after the discharge. Only then should be generator enclosure be carefully inspected to ensure that the required residual concentration of CO2 is present.
Many older CO2 systems use components, such as squibs on the discharge heads, that may no longer be commercially available. Squibs can be replaced with modern electromechanical actuators. In addition, these older systems typically lack the personnel safeguards required by NFPA 12. As long as they are properly retrofitted and/or maintained, older CO2 systems can provide a level of safety and reliability equivalent to modern systems.
CO2 continues to be an appropriate fire extinguishing agent for generators, and with the proper controls and safeguards, its hazards can be minimized. A safety poster seen during a recent plant visit encapsulates the purpose of these safety systems: We do not work in a dangerous environment; we work in an environment where hazards are recognized and controlled to safe levels.
Water Spray Douses Fires
Water spray systems are the second-most-common fire protection system, assuming that an adequate water supply is available, Because, like CO2 systems, water spray systems are engineered using common components and water is abundant at hydroelectric plants, spray systems are also the least expensive for air-cooled generators.
Water is a highly effective fire extinguishing agent, but the generator should be de-energized prior to the application of water—a disadvantage compared with CO2 systems. Water spray systems extinguish fire by direct cooling, by placing calibrated quantities of water directly on the fire. In most cases, water spray is applied through directional spray nozzles located on a ring just above the stator. The two water spray options have different system designs:
Deluge systems. A flooding device called a deluge valve uses an external fire detection signal to charge the piping with water. A deluge system uses open nozzles; once the deluge valve is actuated, all nozzles spray water (Figure 3). The deluge system gives a better level of protection because the entire stator is sprayed in the event of fire. The disadvantage is an increased chance of inadvertent discharge. For unoccupied plants, cycling deluge valves are available, which automatically shut off the flow of water when the fire is extinguished, thus minimizing water damage. The system design and installation are governed by NFPA 15, Water Spray Fixed Systems for Fire Protection.
|3. Deluge system. A stainless steel deluge water spray system with spray nozzles protects an air-cooled generator. Courtesy: Public Utility District No. 1 of Chelan County|
Preaction systems. A preaction system uses closed nozzles. The piping integrity is supervised using air pressure, resulting in a supervisory alarm signal if the piping system is breeched by leak or failure. Preaction systems require two separate events to cause a discharge and are thus considered to give the highest protection against inadvertent discharge. The preaction valve admits water to the piping system, but there is no water discharge until the fire has generated sufficient heat to actuate the thermal element in one or more spray nozzles. The disadvantages are the inherent delay caused by the two separate sensors and the likely lack of uniform water distribution onto the stator, which could result in increased fire damage (Figure 4).
|4. Preaction system. This is a close-up of a water spray line and a spray nozzle inside an air-cooled generator. Courtesy: Public Utility District No. 1 of Chelan County|
Although there are no known cases where water spray from a fire protection system has caused significant damage to a generator, that doesn’t mean damage isn’t possible. Therefore, water discharge in the absence of a fire should be avoided. In the event of a discharge, the accepted practice is to dry the generator by running it while it is electrically isolated for at least 24 hours before re-energizing it.
Water Mist Smothers Fires
Water mist is a relatively new player in the industrial fire protection market. This technology discharges about one-tenth or less of the quantity of water of a water spray system and is thus a good choice when ample fire protection water is not available or when larger quantities of water discharged are undesired. Water mist offers two distinct advantages: a plentiful agent supply, and the enclosure “tightness” is not critical.
The operating concept is that a small quantity of water discharged at high pressure through very small orifices produces a fine mist, which is more efficient in fire suppression than larger water drops. The primary extinguishing mechanism consists of heat extraction and displacement of air by the water vapor. The system design and installation are governed by NFPA 750, Water Mist Fire Protection Systems.
Water mist systems are among the most expensive fire protection options because of proprietary parts, their extensive use of stainless steel materials, and the requirement of a full-scale fire test for each manufacturer’s system. Unlike the other systems, which are engineered on a case-by-case basis, each manufacturer’s water mist system is evaluated and tested to prove that it will be able to extinguish fire within the expected parameters. Therefore, the system listing/approval stipulates limits as to the type of hazards and enclosure size that a particular system can protect.
There are currently few manufacturers whose systems hold listings/approvals from two agencies (Underwriters Laboratories and FM Global) for the protection of machinery enclosures. These include Marioff, Securiplex, and Tyco Fire & Building Products. Particular attention should be given to the listing/approval limitations of any water mist system to ensure that it is appropriate for the protected hazard and that it is also acceptable to the stakeholders.
There have been numerous water mist installations on air-cooled hydroelectric turbines, both in the U.S. and Europe, in recent years.
Clean Agent Gases Are Now Available
There are a variety of halon-alternative clean agent gaseous systems, including halocarbons such as DuPont’s FM-200 and inert gas agents such as Ansul’s Inergen. The design and installation of these systems is governed by NFPA 2001, Clean Agent Fire Extinguishing Systems.
Clean agents generally have the advantages of being nontoxic at their design concentrations and leaving no residue. Clean agents are not specifically listed for generator protection, and testing of halocarbon agents on Class C (electrical) fires has shown that flame can rekindle if the equipment remains energized. Therefore, it is imperative to de-energize the generator prior to actuating the fire suppression system. If this cannot be done, then the concentration of fire suppressing agent should be increased accordingly.
The cost of clean agent systems is generally higher than that of water spray or CO2 systems. In addition, as with CO2, the integrity of the generator enclosure is critical, as the agent concentration must build to prescribed levels to extinguish flames.
Due to the prohibitive cost of discharging the agent for the purpose of measuring its concentration during an acceptance test, NFPA 2001 allows an enclosure integrity test, also known as a door fan test. By measuring various pressure differentials while pressurizing and depressurizing the room using a calibrated fan, a computer program predicts the retention time of the minimum required concentration. Although NFPA 2001 specifies the minimum protection duration as 10 minutes, it is prudent to increase this to 20 minutes, following the methodology for CO2 systems. Because it is highly unlikely that a single-shot system will maintain the concentration for that length of time, an extended discharge time should also be considered.
As with CO2, special care must be exercised when evaluating the generator cooling air arrangement. Because of the many drawbacks, there are few, if any, clean agent systems installed on hydroelectric generators today.
Consider a Hybrid Suppression System
Hybrid systems use a dual fluid, water, and inert gas (typically nitrogen) to generate a micro fog. They share many properties of water mist and clean agent gas systems, but they operate at lower pressures than either system and produce even smaller water droplets than water mist systems (Figure 5). The extinguishment mechanism is through flame cooling and reduced oxygen concentration by displacement of air from water vapor and inert gas. The design and installation of these systems is governed by NFPA 750, Water Mist Fire Protection Systems and NFPA 2001, Clean Agent Fire Extinguishing Systems, with certain exceptions.
|5. A hybrid system. This spray mist system uses high-pressure water and special spray nozzles to produce a water mist that can blanket and smoother a fire. On the left is the stainless steel piping system that forms the mist. On the right are nitrogen-filled cylinders that provide the pressure to atomize the water droplets. Although it is an effective fire suppressant system, the hybrid microfog design is the most expensive of the fire protection system options. Source: Dominique Dieken|
Similar to water mist, listed/approved systems are currently only available from two manufacturers: Ansul (AquaSonic) and Victaulic (Vortex 1000). As with earlier systems, a hybrid system must be designed, installed, and tested in accordance with the manufacturer’s instructions and its listing/approval.
Detect, Then Release
The objective of the fire detection system is twofold: to notify plant operators of a fire and to actuate the fire suppression system. Quicker notification means faster release of the fire suppression system, less staff exposure to danger, and less damage to equipment. However, quicker response time must be balanced against the likelihood of false detection leading to actuation of the fire suppression system. Fire detectors should actuate the system in the presence of a real fire but not in the presence of minor upset conditions, such as particles released by friction brake systems or common dust.
A typical method of actuating CO2 fire suppression systems is to arrange them to discharge when a generator fault condition is experienced. This is not the best method, as it is possible that a winding insulation fire is not immediately detected by a protective relay or that the relay does not function properly. Conversely, because only a small percentage of fault conditions result in fires, discharging a fire suppression system every time a fault occurs is unwarranted. Because an automatic fire suppression system is really a backup to the protective relays, it is important that the actuation of the fire protection system also initiates an automatic trip of the unit to ensure that the generator is de-energized and comes to a stop.
The best fire detection scheme consists of a dedicated stand-alone fire detection system with the fire detectors installed in a circle directly above the stator at the top of the generator shroud. Detectors should be installed in strict accordance with their UL-listing limitations and with the most recent edition of NFPA 72, National Fire Alarm Code.
Smoke detectors respond significantly faster than heat detectors in the event of fire. Photoelectric smoke detectors are most appropriate for electrical hazards, as they pick up larger particles typical of those released by electrical fires. Fixed-temperature (spot-type and linear) and rate-of-rise heat detectors are immune to smoke and other air-suspended particles, but they have a longer response time than smoke detectors. In general, rate-of-rise heat detectors are the best all-around choice. Another option is to install both heat and smoke detectors. In that case, it is advisable to program the detection system to only release the suppression system if either a smoke and a heat detector, or two heat detectors, are actuated (Figures 6 and 7).
|6. No smoking allowed. This is a typical smoke detector located within a generator enclosure. Courtesy: Public Utility District No. 1 of Chelan County|
|7. Mercury rising. Another useful temperature-measuring device is the rate-of-rise heat detector, shown here within a generator enclosure. Courtesy: Public Utility District No. 1 of Chelan County|
Automatic or Manual Release?
Best engineering practice is a fixed fire protection system designed to actuate automatically and with provisions for at least one manual actuation station per system. This approach is endorsed by most authorities, including NFPA, FM Global, and the U.S. Bureau of Reclamation.
There is a school of thought that manual-only actuation of fire suppression systems precludes unwanted discharges; however, this reasoning is misguided for several reasons. Foremost, the success of fixed fire suppression systems depends on prompt and reliable actuation. Only under the best circumstances (prompt detection, well-trained staff, good accessibility, and proper human response) does a manual actuation scheme yield satisfactory results.
Loss history also shows that human actions in emergency conditions tend to be less than perfect. In addition, many powerhouses are either not constantly staffed with journeyman operators or are staffed with the minimum number of operators, often just one or two per shift. Essentially, having a fixed fire suppression system with an unreliable or slow method of actuation is akin to not having a system at all. Not surprisingly, an Electric Power Research Institute study (“Turbine Generator Fire Protection by Sprinkler System,” July 1985) found that the reliability of manually actuated fire suppression systems in the power generation industry is less than half that of automatically actuated systems.
What about the consequence of an inadvertent system discharge? In our experience, an unintentional CO2 system discharge seldom occurs; perhaps one event in 30 system-years can be expected. Assuming that the proper personnel safeguards are provided, the consequence of an unwanted discharge is relatively low and consists of refilling and possibly hydrostatically testing cylinders. The likelihood of an inadvertent deluge water spray system discharge is on the order of one event in 55 system-years, according to a study performed by the Idaho National Engineering Laboratory (“Firewater System Inadvertent Actuation Frequency,” August 1991). Other studies suggest that the likelihood of inadvertent preaction water spray system discharge is less than one event in 1,200 system-years, or about 22 times less likely than for deluge systems. Regardless, the risk of fire damage caused by failure of actuation far outweighs the risk of equipment damage caused by an inadvertent fire suppression system discharge.
If, however, a manual-only actuation for your fire protection system is still preferred, the following conditions should be present:
- The powerhouse must be constantly staffed by an adequate number of qualified operations personnel.
- A reliable method of early fire detection should be provided within the generator.
- The manual actuation station(s) must be easily accessible in case of a fire. Remote actuation stations in the control room are desirable.
- A written operations procedure should be in place that addresses the manual actuation of the fire suppression system.
- Refresher drills covering the procedure for fire system activation should be routinely conducted.
Note: The views contained in this article are solely those of the writer and do not necessarily represent the positions and views of C.V. Starr & Co., Inc. and/or Starr Tech. This article is for informational and educational purposes only. Any recommendations or conclusions in this article should not be interpreted as any guarantee that the reader will achieve the same results.
— Dominique Dieken, PE, CFPS ([email protected]) is a senior fire protection engineer for Starr Technical Risks Agency Inc., a member of the C.V. Starr & Co. Inc. group of companies.