Carbon dioxide is the most prevalent type of fire suppression system protecting air-cooled hydroelectric generators. But personnel safety concerns are leading many plant owners to replace such systems. Hybrid fire suppression systems, a newer option, have been installed by several hydro plant owners, including Salt River Project.
By Robert Ballard and Steven Owens
Salt River Project wanted to replace the carbon dioxide (CO2) systems at its Theodore Roosevelt, Horse Mesa and Mormon Flat dams and hydro projects. After reviewing the available options (discussed on pages 44 to 47), SRP chose the Victaulic Vortex™ hybrid extinguishing system for the facilities’ Westinghouse air-cooled generators. The systems have been in service since 2013 and no discharges have yet been required.
At hydro facilities, CO2 fire suppression systems have been installed in generator enclosures and oil storage rooms, large motor air housings and transformer vaults. At the minimum concentration required to extinguish a flame, CO2 is lethal. In addition, many hydroelectric facilities built in the 1950s and 1960s are undergoing relicensing by the Federal Energy Regulatory Commission and owners are considering the upgrade or addition of fire protection systems to previously unprotected generators.
Available fire suppression systems for generator enclosures are: CO2, water-based, clean agent and hybrid. Each are discussed below.
CO2 gas displaces oxygen and is popular for its nonconductive properties and low cost. Systems achieve a concentration of 30% within two minutes and 50% within seven minutes.
Advantages: The agent won’t damage the generator, there’s no residue after discharge, and it can be refilled or replaced by a local supplier. Disadvantages: CO2 interferes with central nervous system function.1 CO2 is heavier than air and can settle in lower levels of the generator or powerhouse, resulting in dangerous concentrations.1 Safety requirements include pre-discharge alarms, activation delays for personnel evacuation, and strict procedures for re-entry into affected spaces. Enclosure integrity is vital to maintain the concentration of suppressing agent required for extinguishment.
These include water spray (large droplets released by sprinklers) and mist (via nozzles with small openings). Water extracts heat from the fire, and the steam displaces oxygen and aids in radiative and convective heat blocking. Water spray is the second most common type of fire protection system in generator enclosures, with water mist gaining interest due to the smaller amount of water released.
|This illustration shows the swirling fog created by a hybrid fire extinguishing system.
photo courtesy Victaulic
The difference is in the size of the droplets and the amount of water released. In a spray system, droplet size is greater than 1,000 microns, and each sprinkler can release more than 25 gallons per minute (gpm). Water mist systems atomize the water into droplets ranging in size from 50 to 100 microns (high pressure) or 400 to 1,000 microns (intermediate pressure) and in flow from 6 to 10 gpm. The systems may be designed as a local application, where the fire suppression system is designed to protect the hazard located in a permanent enclosure or, where total flooding is not feasible, a “local application” system discharges the water droplets directly onto or about a defined burning surface or volume.
Advantages: The agent is free and easily available. There’s no downtime for agent refill. Enclosure integrity is not required. Water damage is unlikely with a water mist system. Water spray systems use common, widely available components that make them typically the least expensive system in terms of installation. Disadvantages: Water spray systems can cause damage in the event of discharge. Local water supplies are generally electrically conductive, so the generator should be de-energized before system discharge. NFPA 850 prescribes a “drying” time of at least 24 hours to avoid stator ground faults, in which the generator is mechanically run while electrically isolated.2
These employ a proprietary chemical or inert gas, and their use in hydroelectric generators remains limited due to drawbacks. Inert gas systems extinguish fires through oxygen reduction. Chemical agents (such as halon, widely used in maritime and aviation applications) rely on flame temperature reduction or disruption of the combustion process.
Advantages: Clean agent systems are nonconductive, leave no residue and are generally human-safe. Production of new halon ceased in the mid-1990s due to its damaging environmental effects, but environmentally friendly alternatives are available. Disadvantages: Without enclosure integrity, additional agent concentration may be required. They have limited thermal cooling capabilities and do not reduce radiative or convective heat transfer. The fuel is not cooled, and re-ignition from hot objects is possible. Systems can be expensive in terms of installation, maintenance and agent cost, and downtime to return cylinders to manufacturers for refill with proprietary agents can result.
These systems, introduced in 2009, employ water and an inert gas to suppress a fire. The gas atomizes the water, creating a swirling fog in the protected space. The components attack the fire simultaneously, the water cooling the space and the gas reducing the oxygen content and generating steam.
Advantages: The fine mist is electrically nonconductive. The droplet size (less than 10 microns) and minimal amount of water released per emitter avoids generator damage. Systems designed for occupied spaces maintain oxygen at levels within safe breathing tolerances. Evacuation and discharge delays may not be required before system activation. Enclosure integrity is less of a concern, with the ability to extinguish fires in open, naturally ventilated areas. The system uses common, low-cost agents available from local suppliers. The inert gas cylinders do not require semiannual weight tests – gauges indicate adequate agent levels – also limiting downtime. Disadvantages: The primary concern is installation and maintenance costs due to the limited number of manufacturers and proprietary parts. In addition, despite successful installations in power generation applications, questions remain as to the efficacy of the inert gas/water mist suppressing system, as well as its effect on energized generators.
SRP selects hybrid system
SRP has served central Arizona since 1903, providing electricity to nearly 1 million retail customers in a 2,900-square-mile service area. SRP deemed the risk of CO2 systems to personnel too great and sought a low-maintenance system for its Theodore Roosevelt, Horse Mesa and Mormon Flat dams and hydro projects on the Salt River near Phoenix.
Theodore Roosevelt was completed in 1911 and features one 36-MW generating unit. Horse Mesa Dam was completed in 1927 and contains four units: three 32-MW conventional units and one 97-MW pumped storage unit. Completed in 1925, Mormon Flat Dam features one 10-MW conventional unit and one 50-MW pumped storage unit.
The unit at Roosevelt and main units at Horse Mesa and Mormon Flat were equipped with high-pressure CO2 systems. Having experienced more than one accidental discharge, SRP concluded CO2 posed as much risk as a fire. The maintenance required was also a burden.
Alliance Fire Protection, one of SRP’s fire protection system service vendors, recommended the Victaulic Vortex hybrid system. This system deploys a high-velocity, low-pressure mixture of water and nitrogen. Nitrogen enters the emitter and is compressed to its operating pressure, then discharged from the outlet. The gas expands and becomes supersonic, generating a Mach disc perpendicular to the outlet. Water is injected in the area of the Mach disc, beginning the atomization process. Several other factors – such as the nitrogen gas velocity, water velocity, and foil design – play a role in the atomization and droplet size.3 The result is a dense homogeneous suspension of mist that quickly fills the hazard space in a swirling pattern.
For smaller fires, nitrogen is the primary extinguishing agent, reducing oxygen to a level where combustion cannot be sustained. In larger fires, the water mist is more effective, cooling the fire by absorbing the heat and reducing the available oxygen.
|The Victaulic Vortex system water and nitrogen piping runs within the tight confines of the generator enclosure. photo courtesy Joseph Nobile, AFPC|
The water droplet surface area is 90 times greater than that of standard sprinkler systems, providing maximum heat absorption efficiency and total extinguishing without the chance for re-ignition. Nozzles release 0.13 to 1.06 gpm, depending on the size of the emitter, thus preventing damage to electrical equipment.4
Alliance and Victaulic representatives demonstrated the Vortex system to SRP representatives, who saw the minimal amount of water used and specified the hybrid system for the main generator at the three hydro plants to protect the upper enclosure surrounding the exciter, collector, rotor, stator and winding. The main fire hazard is the lube oil system and, to a lesser degree, the winding insulation.
Alliance designed the system for each facility based on enclosure volume. In the event of a fire, the system is designed to discharge for 5 minutes, standard design for machine-space type environments.
From November 2012 to February 2013, the CO2 systems were removed and the Vortex 1500 hybrid extinguishing system was installed and commissioned within a 30-day window during each facility’s outage period. Despite the tight schedule and logistical challenges such as remote locations and confined work spaces, the installations were completed on time. System removal and installation took about three weeks, which left a week for testing and commissioning.
The Vortex hybrid system has exceeded SRP’s expectations, and personnel from engineering, construction and fire risk are pleased with the result. The system has not yet been called upon to extinguish a fire.
In addition, the Vortex hybrid system is installed on three generators owned by Sacramento Municipal Utility District in California and two units at the 289-MW E.B. Campbell facility owned by SaskPower.
Editor’s Note: This article was excerpted from a paper presented at HydroVision International 2015 and authored by Bob Ballard, Victualic; Joseph Nobile, Alliance Fire Protection (now deceased) and William J. Reilly, Victaulic.
Bob Ballard is fire suppression technology manager and Steven Owens is Vortex application engineer with Victaulic.
1FIST 5-12, CO2 System Operation and Maintenance, Bureau of Reclamation, Denver, Colo., 2005.
2NFPA 850, Recommended Practice for Fire Protection for Electric Generating Plants and High Voltage Direct Current Converter Stations, National Fire Protection Association, Quincy, Mass., 2015.
3Reilly, William, Dual Agent Extinguishing System: Victaulic Vortex, 2008, http://static.victaulic.com/assets/uploads/literature/White%20Paper/WP-04.pdf.
4Puzio, Matthew, “The Victaulic Vortex Fire Suppression System: Fire Suppression for Electrical and Electronic Applications,” 2011, http://static.victaulic.com/assets/uploads/literature/WP-08.pdf.