Putting out halon20 July 2004
At Kozloduy 5 and 6, it was necessary to replace halonfire extinguishers for object protection of electrical cabinets. By W Neugebauer and O Gechev
At Kozloduy 5 and 6 the existing halon extinguishing systems are designed as object protection systems for electrical cabinet rows. Each of these rows is protected by a small dedicated halon gas cylinder, directly installed on the sheet metal housing in front of each cabinet row.
Kozloduy nuclear plant units 5 and 6
In compliance with international restrictions on the production of certain halon fire extinguishing agents under the Montreal Protocol, signed 16 September 1987, replacement of this agent at Kozloduy 5 and 6 was required for environmental protection reasons.
ELECRTICAL CABINET PROTECTION
The technical investigation started with the aim to merely replace the existing halon with another gaseous extinguishing agent and reuse – as far as meaningful – mechanical and electrical components of the existing extinguishing system.
As a first step in the design process, the new extinguishing agent had to be selected and specified based on the NFPA 2001 and ISO 14520 standards. In principle, two groups of agents are possible, hydrogenated halocarbons with extinguishing effect by physical cooling or chemical reaction, or inert gases which decrease the amount of oxygen in the vicinity of the fire.
After a detailed analysis of the as-built situation of the mechanical portions of the existing halon systems, the idea of merely changing the gas bottles on existing systems had to be revised. Replacement of the complete gas systems was necessary.
The process of definition of a suitable extinguishing agent – with due consideration for specific environmental, and health and safety properties as well as technical (extinguishing) characteristics with regard to the special conditions at Kozloduy – resulted in the selection of an inert gas (nitrogen, argon or a combination of gases) which is absolutely clean and harmless to the environment and to people.
The new gas systems were designed as distribution systems for individual cabinet rows with the following additional requirements:
• To enable the design extinguishing gas concentration to be reached inside the cabinets, the fans at the top of the cabinets, which force ventilation, need to be switched off on actuation of extinguishing systems.
• Due to the ventilation openings on the electrical cabinets not being closed off, the gas amount had to be increased substantially (up to 300-400%) compared to the value needed for reaching the design gas concentration in the case of a protection system for a tight room.
• The holding time after reaching the design gas concentration should be at least two minutes, resulting in the necessity for gas discharge over the total holding time (see Figure 1).
Principle of gas discharge in cabinets with openings
VERIFICATION OF GAS AMOUNT
A test programme was developed for the verification of design values. The test configuration used two electrical cabinets comparable in volume, electrical internals and ventilation openings to existing cabinets at Kozloduy. The tests considered the use of different sizes of gas cylinders, nozzles and restrictors and featured the nozzles located at the top or bottom of cabinets (see Figure 2).
During gas discharge, the oxygen concentration versus time was monitored by three gas sensors installed at different levels inside the cabinets.
The necessary design concentration was reached in the tests. In addition, a large and very fast variation in gas concentration over discharge time was noted. This effect could be the result of fresh air being drawn from the outside into the cabinet through the gaps between the doors and the frames of the cabinet. This hypothesis was confirmed by pressure measurements for inside the cabinet which alternated between overpressure and underpressure.
On the basis of the tests it could be stated that a holding time for the gas concentration of at least two minutes would require additional sealing of cabinets (especially the doors) and a further increase in the design gas amount would also be necessary. Additional sealing of cabinets was rejected on the grounds that this measure could demand repeat approval of the electrical cabinets as it could possibly change the cooling by air flow during normal operation.
An increase in the holding time for the gas from two minutes to ten minutes was also discussed, to prevent reignition by the electrical power supply to the cabinets which cannot be switched off automatically on actuation of extinguishing.
The requirements for cabinet sealing and the longer holding time for the gas of at least ten minutes resulted in the necessity to change the design from object protection (cabinet rows) to complete room protection (total flooding systems).
TOTAL FLOODING SYSTEMS
The available extinguishing agents were then re-evaluated on the basis of further design criteria (for example, space for necessary gas containers, possibility of pressure build-up, personnel protection). As a result, the new systems are designed with FM200 as the extinguishing agent.
Restrictions have been incorporated in European guidelines such as 88/540/EWG, (EG) No 3093/94 and (EG) No 2037/2000, limiting the production and use of substances with ozone depletion potentional (such as halons and other halogenated hydrocarbons). This has resulted in the development of halon alternatives and new standards for clean agent fire extinguishing systems. NFPA 2001 is the technical standard that addresses the design, installation, testing, inspection, operation and maintenance of clean agent fire suppression systems.
NFPA 2001 was used as a basis for International Standard ISO 14520 Parts 1-15. The first edition of this standard, dated August 2000, includes nine halocarbon agents and four inert gases. Further restrictions by the European Union are currently being discussed. As a consequence of the re-evaluation of potential extinguishing agents with regard to environmental concerns as described above, fundamental changes to ISO 14520 will take place in the near future.
Health and saftey aspects
Tests have shown the acute toxicity of FM200 to be equivalent to that of halon 1301. FM200 has been evaluated for cardiac sensitisation through test protocols approved by the US Environmental Protection Agency (EPA). The EPA’s Significant New Alternatives Program (SNAP) classifies FM200 as acceptable for use as a total flooding agent in occupied spaces within specific limitations. The gas design concentration of FM200 for extinguishing plastic fuels such as electrical components is between 8-8.4% and therefore lower than the NOAEL (no observed adverse effect level) for FM200 of 9%. This would enable persons to be present in the area without needing respiratory gear and without any impediments to vision, thereby allowing safe evacuation of the fire area.
The extinguishing principle of halocarbons is based on mainly physical or chemical means. Inert gases reduce the oxygen concentration to below the level at which combustion of inflammable material can take place.
This results in necessary design gas concentrations for inert gases of approximately 40% and for FM200 of approximately 8% for plastic fuel fires.
FM200 is stored in liquid condition, superpressurised by nitrogen to 42 bar. Inert gases are pressurised gases, stored in gas cylinders at 200 bar (up to 300 bar). The specific weight of FM200 (as superheated vapour) is about five times as high as that of inert gas.
The mass of FM200 and inert gas needed for extinguishing does not differ greatly. This is despite the large difference in the volumes needed of the two agents. The space needed for inert gas storage cylinders is more than five times that for FM200 (or even more than seven times greater if large FM200 containers are used).
This allows much more flexible location of FM200 gas cylinders close to the extinguishing area within an existing plant, as compared to the large amount of space needed for inert gas cylinders.
A further advantage of the low gas design concentration for FM200 compared to inert gases is that this usually means that measures for venting in the walls or ceilings of the protected rooms are not necessary.
The time for discharge of 95% of the extinguishing agent is 10 seconds for halocarbons and 60 seconds for inert gases. The very short flooding time for the halocarbons leads to very short times for extinguishing, thereby reducing possible fire damage.
The following provides an example of a rough estimate of the necessary amount of FM200 gas for the design of a total flooding system for an electrical room of 500m3:
• For extinguishing this room volume, approximately 335kg of FM200 are needed (about 0.67kg/m3).
• Depending on the size of gas cylinders used (140 or 80 litres) and possible variations in cylinder content, a total of between three and six gas cylinders may be necessary.
Boundary conditions for total flooding systems
To ensure proper functioning of the total flooding systems, adequate room tightness is necessary:
• Existing cable, pipe penetration seals and doors need to be checked for tightness. Openings in walls and ceilings need to be closed off.
• Ventilation systems need to be switched off upon actuation of extinguishing.
• Ventilation ducts need to be closed by existing or newly installed fire dampers with automatic actuation.
After adequate room tightness has been established, ‘door fan tests’ need to be performed for all extinguishing areas to determine the minimum holding time for the resulting room tightness. Performance of such room integrity tests also avoids the discharge of extinguishing agent into the environment, as these make discharge tests unnecessary.
Single-area or multi-area systems
Depending on the local situation, distribution systems for several rooms as well as for single-area systems will be selected to give optimised technical and economical solutions.
Multi-area extinguishing systems with distribution valves for two or more extinguishing areas can considerably reduce the total amount of extinguishing gas necessary. This reduced amount of gas requires fewer gas cylinders and likewise fewer locations and less space for arrangement of the gas cylinder racks. The expenditure for maintenance and periodical tests is also reduced.
Due to its physical properties, FM200 has extremely poor flow characteristics (two-phase flow) relative to inert gases. This leads to very stringent restrictions on possible pipe runs.
Overall assessment of the advantages and disadvantages of multi-area systems compared to single-area systems is not limited to the aim of reducing the amount of gas. Further influencing factors for each local situation should also be taken into account, such as:
• Length and dimensions of piping, based on distance between gas cylinder rack location and extinguishing area.
• Reduced gas content of gas cylinders (filling ratio) for multi-area systems, especially those with long pipe runs, demands more gas cylinders in the cylinder rack and therefore more space.
• All gas cylinders of a cylinder rack should be the same size and with the same filling ratio.
• Cylinder racks share additional components such as distribution valves with electrical or pneumatic release.
• Lower reliability of more complicated multi-area systems.
• Gas cylinder racks could be located at a distance from extinguishing areas, possibly even on a different building level.
With regard to system requirements, the control devices for actuation of extinguishing should independently and separately perform all functions including detection and annunciation of incipient fire.
The new systems are actuated automatically by new gas extinguishing control stations with actuation logic that requires response of 2-out-of-2 fire detectors.
To ensure proper functioning of the extinguishing systems, additional room tightening measures must be implemented. This entails deriving input signals to automatic controls to cause ventilation damper actuation from output signals from extinguishing control panels. In parallel to this, the existing fire alarm systems must be replaced by a new processor-controlled fire alarm system featuring interactive modules and employing the loop technique.
As an optimised solution, the new gas extinguishing control stations are connected with the fire alarm system along a bus system using components of the same system family from the same manufacturer. This enables both standalone operation of gas extinguishing control stations as well as common information on the control panels close to the main control rooms and the terminal in the fire brigade building.
For each unit at Kozloduy, the new fire alarm system consists of three fire detection stations, corresponding to the three safety systems. In each unit, these stations are connected together and to an additional control panel located close to the control room along a bus system to form a loop.
The gas extinguishing control stations will be connected to dedicated loops for each separate unit of the plant. As the bus network itself is limited to 16 stations, the connected 10 gas extinguishing control stations, together with the station for the control panel and gateway modules mean that a certain scope is available for future extension. The resulting two loops for extinguishing control stations will be connected via gateways with the two loops for the fire alarm system by a Cerloop data link with a common terminal for information in the fire brigade building.
The gas extinguishing systems for the 17 rooms per unit at Kozloduy are designed as single-area systems, with some two-area systems with distribution valves and common gas containers.
The two-area systems use a common control station with two extinguishing modules. Some extinguishing areas located close together belong to the same electrical system (or to a non-redundant electrical system) and also use a common control station with two extinguishing modules.
This results in a total of ten gas extinguishing control stations (per unit for unit 5 and 6) which perform the following functions:
• Fire detection and alarm annunciation for the rooms to be extinguished.
• Automatic extinguishing system actuation.
• Switch to manual mode or switch to ‘mechanical blocking of extinguishing’ for maintenance purposes.
• Output of signals for optical and acoustic warning devices.
• Time delay for system activation.
• Actuation signals, for example, for closing of fire dampers.
• Interlocking of system actuation with contacts for closing doors and dampers.
• Control of cylinder pressure by pressure switches.
• Monitoring of system availability, system actuation and system fault status.
Based on a paper presented at the ‘Fire and Safety 2004’ meeting organised by Nuclear Engineering International, and held in Munich, Germany on 11-12 March 2004. W Neugebauer, Framatome ANP, Tour AREVA, 92084 – Paris la Défense, France. O Gechev, NPP Kozloduy, 3320 Kozloduy, Bulgaria