The U.S. power generation sector has been transitioning away from coal for some time. According to U.S. Energy Information Administration (EIA) statistics, natural gas generation surpassed coal as a source for generating electricity for the first time on a monthly basis in April 2015. This placed coal and gas on an even footing for the year, each with approximately one-third of all electricity generation. In 2016, natural gas outstripped coal on an annual basis, becoming the primary source for electricity generation in the U.S.
Today, natural gas is used to generate approximately 34 percent of the power in the U.S., while coal is used for about 30 percent. Statistics indicate that gas will be the primary fuel for power generation for some time to come and that it will continue to grow in market share.
As owners renovate old plants or bring new ones online, one of their main areas of focus is operational safety, and one of the most important components is fire safety. Every plant owner understands the inherent dangers to personnel and equipment and recognizes the importance of outfitting a gas plant with appropriate fire safety systems. This is a challenge, and using traditional methods is limiting.
Fire systems and safety
Fire suppression for power plant turbine enclosures traditionally has been a game of tradeoffs. Installing a CO2 system, which has been a common choice for power-generation facilities, introduces a mortal safety hazard for plant personnel. Anyone in the area when a CO2 system is activated becomes a fatality. Alternative systems use halon, a volatile organic compound similar to chlorofluorocarbons. While halons generally are not fatal, they can cause breathing difficulty and skin and eye irritation. Excessive exposure can cause unconsciousness. In addition to the negative effects they have on people, halons are harmful to the environment. Because of their long lifetime in the atmosphere, some enter the stratosphere, where they can damage the ozone layer.
Another danger in systems that use chemicals is that under extreme heat, some gases turn into hydrofluoric acid. When this chemical change occurs, it creates a serious threat to worker health. Hydrofluoric acid is a highly corrosive systemic poison that burns the skin and deeply penetrates the tissue. If even a small amount of hydrofluoric acid is ingested, it can be fatal.
Water systems also are used for fire suppression. While they introduce no chemical contaminants, they have other shortcomings. There must be a local water source or large on-site area dedicated to water storage. And if a water system is deployed, the deluge often results in damage to electronics and other equipment. Costs mount any time one of these systems is activated because of the time-consuming cleanup and the need for on-site containment and off-site disposal.
Both CO2 and halon systems are rendered ineffective in areas where there are enclosure integrity issues, such as in older facilities. Construction is nearly always required before either system can be expected to work with any degree of effectiveness. Preparing a plant to use one of these fire safety systems is often quite expensive.
Historically, plant owners have had few choices and have been forced to weigh the costs of available systems, knowing that every choice introduces serious limitations.
Innovative thinking has led to a new concept in fire safety that removes dangerous chemicals from the equation and introduces improved performance along with easier installation and minimal cleanup. The hybrid system, which is based on cyclonic distribution of nitrogen and potable or deionized water, represents a step change in fire safety for power-generation facilities.
The basic system consists of stored nitrogen, a captive water supply and integral overhead emitters installed in the hazard area. The system is scalable. The amount of nitrogen required is determined by the elevation and enclosure volume of the area being serviced. A twin piping network and emitter discharge the suspension of water and nitrogen into the hazard area. Sidewall emitters, placed at the top of the system, require no additional piping. As installed, the system is ready for activation. It is fully compatible with automatic hazard detection systems and is adaptable for remote manual activation.
The foundation of this new design is a vortex that uses supersonic distribution to create a homogeneous suspension of nitrogen and water molecules that are only 10 microns in size, which maximizes the system’s ability to rapidly extinguish large- and small-scale fires using only 1 gallon of water per emitter per minute at full capacity. The tiny droplets form a noncandescent cloud that absorbs heat and reduces oxygen to extinguish a fire. The design allows the system to be effective in openly ventilated spaces, which means there is no need for an air-tight room.
This hybrid system is particularly applicable for suppressing fires in areas where an electrically non-conductive medium is necessary, where cleanup of residual chemicals would present difficulties, and in areas that have to be free of toxins to accommodate workers. It is appropriate for machine spaces, turbine enclosures and data centers.
Because of its components, the system presents a completely green design that is safe for the environment and for personnel, so it can be discharged immediately upon sensing smoke or fire, unlike CO2 systems for which activation must be delayed so personnel can be evacuated to protect them from toxic agents. This design advantage allows a fire to be addressed more swiftly than other fire safety systems and ensures that anyone exposed to the cloud will be unharmed. Because of the nature of distribution, there is nearly zero surface wetting, which means little cleanup is required.
After a discharge, the nitrogen cylinders can be recharged quickly without disruption to day-to-day operations. No downtime is required for maintenance, which is simple enough that it introduces no safety risks for the facility or personnel.
As an alternative product designed for environmental safety, the system has received approval from the United States Environmental Protection Agency’s (EPA) Significant New Alternatives Policy (SNAP) program, formed under Section 612 of the Clean Air Act to identify and evaluate substitutes for ozone-depleting substances (ODSs). To determine if a new product qualifies, the agency performs a cross-media analysis of risks to human health and the environment from the use of various substitutes in different industrial and consumer uses that historically have used ODSs. The EPA reviews candidates for ozone depletion potential, global warming potential, toxicity, flammability, occupational and consumer health/safety, local air quality and effects on the ecosystem. The vortex system is listed by SNAP as an acceptable replacement for Halon 1301 in total flooding applications.
In addition to having SNAP approval, this design has undergone Class A and Class B testing by a third party and has FM 5580 approval. It has been approved to the National Fire Protection Agency (NFPA) 750, and its introduction has led to the formation of a new committee that will develop a documentation for qualifying hybrid fire extinguishing systems, NFPA 770.
The science was proven to be sound, but to test the system’s efficacy, it needed to be installed in a working facility. That opportunity came when owners of a power generation plant in Florida agreed to install the system and test its functionality.
First movers test innovative system
Facing enclosure integrity issues, the Putnam Power Plant needed to replace its halon system. The plant, owned by Florida Power & Light (FP&L), was built in the 1950s and converted to a gas/oil combined cycle plant in the 1970s. Each of its two 550-megawatt units has two combustion turbines and one steam turbine.
Putnam originally was outfitted with a halon fire suppression system, which requires room integrity to effectively suppress fire. Given the age of the building, achieving enclosure integrity posed a significant challenge.
FP&L has one of the lowest emissions profiles and one of the leading energy-efficiency programs among utilities nationwide and prides itself on being a clean energy leader. In support of its clean energy goals, the company had elected to replace its old halon system with a more sustainable option.
One of the goals in making this change was to avoid repeated interruptions to operations. Every time the plant experienced a major outage, the roof had to be detached so the turbine could be removed. When the roof was replaced, it was necessary to perform an enclosure integrity test to be certain the halon system would be usable in the event of a fire. All the steel and the roof of the building had to be tight to prevent leakage, which would compromise the effectiveness of the system.
With older turbines in the facility that have doors and dampers that do not close fully, the plant could not create a sealed enclosure without a considerable retrofit expense. Should an incident have arisen while the doors and dampers were open, the halon system would not have been able to work effectively.
Recognizing the need to address this safety issue, FP&L began evaluating alternative systems. Although CO2 systems are prevalent and have been used effectively over the years in unoccupied areas, the NFPA and EPA have taken an aggressive stand on their use. According to the EPA, CO2 is lethal at the minimum design concentration when it is used as a total flooding fire suppressant. A CO2 system requires pre-discharge alarms to allow for the evacuation of personnel, which delays activating the system, during which time, a fire can spread quickly. Like halon systems, CO2 systems require enclosure integrity to function properly.
A nontoxic system
With room integrity among the top considerations, the contractor and plant engineers decided to look beyond established alternatives, evaluating options that included the hybrid clean agent/water mist system. They were interested in learning more about how vortex distribution uses nitrogen to atomize water, creating a suspension that enters the protected space at 40 miles per hour. They examined data that showed how the unique swirling pattern allows the suspension to quickly fill the hazard space and attack the fire, overcoming aerodynamic forces that typically decelerate and diffuse water droplets, absorbing heat and starving the fire of oxygen.
Although FP&L was convinced that the technology had merit, there was concern about the effect the water would have on the turbines. Fire safety systems that use water had been avoided because water can damage or shock turbine casing, resulting in expensive repairs. The size of the water droplets in the cloud were so small, however, that the mist did not damage equipment. Residual moisture is barely detectable following discharge.
The new system was installed in the first unit at the Putnam Plant in 2012, with the second unit completed the following year. The contractor worked within shutdowns to replace the old system to reduce the impact on plant operations.
The hybrid system at Putnam is configured with zone control panels to isolate activation to just the unit affected by a fire. The two units share a nitrogen supply, which is stored in cylinders to simplify long-term maintenance. Rather than relying on yearly weight tests to determine nitrogen levels – a process needed for systems using CO2 cylinders – gauges on the hybrid system indicate adequate nitrogen levels. Maintenance best practices dictate that the cylinders be inspected for possible replacement about every 12 years. Hoses should be inspected every five years.
Success with the Putnam project led FP&L to install a hybrid system in its Lauderdale Power Plant as well. Installation and commissioning at Unit 4 of the Lauderdale Plant was completed in April 2013, with installation at Unit 5 finalized in 2014.
FP&L’s willingness to test this system not only allowed the company to improve fire safety at both plants where the system is installed, it allowed the product to be tested and proven in a real-world environment. The success of the hybrid fire safety system at the FP&L facilities proves the viability of the design and opens the door to broader applications.
Mark Martella is a vortex sales specialist at Victaulic, a manufacturer of mechanical pipe joining and fire protection systems. In this role, Martella provides his more than 10 years of industry experience to drive business growth. He is a member of the Fire Suppression Systems Association (FSSA) and holds a bachelor’s degree in Business Administration and Marketing from Southern Illinois University, Carbondale.