- 131 - FROM THE CLASSICAL FIRE FIGHTING WATER SUPPLY TO STRUCTURE AND SMOKE GAS COOLING TAKING GLEINALMTUNNEL AS AN EXAMPLE; PART II Kern H. AQUASYS Technik GmbH, Linz Austria ABSTRACT Traditionally fire fighting in tunnels is done by the fire brigade with aid of the existing fire fighting equipment such as fire hydrants or hose reels in emergency cabinets. This means seem to be sufficient for smaller car fires but recent catastrophic fires have shown that the fire brigade cannot access the scene when a fully loaded truck is involved, as the temperatures in the tunnel exceed live threatening limits in just a few minutes after the incident. Based on these facts, stationary fire fighting systems which are instantly available after the detection of the fire and which are able to cope with large scale fires have been developed. These fire fighting systems can either be sprinkler or water mist systems installed in the traffic area, or smoke gas cooling systems installed in the tunnel exhaust ducts for limiting the temperature of the smoke gases. The aim of both systems is to protect the tunnel by limiting the temperature of the structure to an acceptable level during a large scale fire in the tunnel. Keywords: water supply, fire fighting, water mist, tunnel ventilation, smoke gas cooling 1. INTRODUCTION Several catastrophic fires during the past years did change the design of the tunnels to make them more durable against these incidents by enhancing the ventilation system, installing stationary fire fighting means or change the concrete composition of the tunnel structure. In the project described in this document, the ventilation system was upgraded to cope with the new requirements, which included extracting hot smoke gases from the tunnel traffic area. Rather than replacing the existing ventilation fans by high temperature resistant units, it was decided to implement means to cool the hot smoke gases to a level where the existing fans can be further used to extract the then cooled smoke gases over the required period of time. This smoke gas cooling system works on the principle that fine water mist droplets are injected in the smoke gas stream upstream the ventilators where the heat energy from the smoke gases is withdrawn by evaporation of the water droplets. 2. SMOKE GAS COOLING IN THE GLEINALM TUNNEL 2.1. The Project The Gleinalmtunnel is situated on the highway A9 between Graz and Linz and belongs to the highway network of the ASFINAG. The tunnel has one bore with bidirectional traffic and has a length of 8.320m. The tunnel is equipped with a transverse ventilation system. The Gleinalmtunnel was opened in 1978 and since then several upgrade programs where undertaken to adopt the safety of the tunnel to the evolving standards. One of the major improvements was the installation of 84 exhaust air flaps in the intermediate ceiling to the exhaust duct, and the installation of fresh air flaps in the intermediate ceiling to the fresh air duct in 2002.
- 132 - With this modified ventilation system only one exhaust air flap is opened in case of a fire in the tunnel. This may result that in case of a fire the temperatures near the ventilators can raise very quickly to rather high values. When the ventilators were designed to the former specifications, they had to withstand a temperature of 250 C over 60 minutes. The current guidelines RVS call for a temperature resistance of 400 C over 2 hours for all constructions, which are in contact with the exhaust air or smoke gas respectively. Therefore, to further improve the safety of the tunnel and to prevent the ventilators from high temperatures, there is a need to cool the extracted smoke gas by means of a smoke gas cooling system. Figure 1: View of the North portal of the Gleinalmtunnel The employer of the project is ASFINAG. The basic planning of the ventilation system was performed by FVT, Graz University of Technology, and the planning of the water supply was done by Kaiser & Mach ZT GmbH. AQUASYS was responsible for the smoke gas cooling system on a turn- key basis. 2.2. Ventilation System of the Gleinalmtunnel The Gleinalmtunnel is a one bore tunnel with a length of 8.320 meters and operated with bidirectional traffic. The tunnel is equipped with a transverse ventilation system, consisting of six ventilation sections. Each section is equipped with a fresh air and an exhaust air ventilator, both designed as axial flow fans. Two ventilation sections are operated through the portal buildings whereas the remaining four sections are operated through the north cavern or the south cavern respectively. Each cavern is connected to a vertical air shaft where fresh air is drawn in and exhaust air is expelled. Figure 2: Ventilation Sections of the Gleinalmtunnel
- 133 - The Gleinalmtunnel is equipped with an intermediate slab and a separated fresh air and exhaust air duct above the traffic area. In the exhaust duct air flaps are installed approximately every 100 meters, which are capable to extract the smoke gases in case of a fire. Through smaller flaps in the fresh air duct, fresh air is injected during normal traffic operation. Figure 3: Cross Section of the Gleinalmtunnel 2.3. General Design of the Smoke Gas Cooling System in the Gleinalmtunnel As mentioned earlier the Smoke Gas Cooling System shall be capable of reducing the temperature of the hot smoke gases during a fire in the tunnel to a level that the axial fans of the ventilation system are capable to operate for a duration of 2 hours in this environment as specified in the current RVS guidelines. This temperature reduction is achieved by injection of fine droplets water mist into the smoke gas stream in the exhaust air duct. The water mist droplets evaporate and withdraw the temperature energy from the smoke gas in the equivalent of the energy which is needed to evaporate the water droplets. Based on this effect the temperature of the smoke gas is reduced to a level where the ventilation fans can operate for 2 hours. Figure 4: Principle of Smoke Gas Cooling System in the Gleinalmtunnel
- 134 - For designing the smoke gas cooling system the following parameters have been specified: Maximum Temperature of the extracted smoke gases upstream the gas cooling system is 400 C Maximum Temperature of the smoke gases downstream the gas cooling system and upstream the ventilation fans is 150 C Maximum smoke gas flow rate in each ventilation section is approximately 150 m³/s The velocity of the smoke gases in the exhaust air duct is approximately 18 m/s Maximum distance between ventilation fan and closest exhaust flap is 20 m Maximum available water flow rate 20 l/s Minimum operation time 120 minutes Based on above parameters the smoke gas cooling system has been designed as follows: Thermal energy for reduction of temperature of smoke gas from 400 C to 150 C kj Qp = mp * cpm *( T2 T1 ) = 40. 000 s Evaporation energy of water Q v = 2. 250 kj kg Amount of water for smoke gas cooling (including efficiency) l Q w = 20 s Maximum available time for evaporation of water droplets in exhaust duct A 20m t = = = 1, 1s v m 18 s With this figure the Water Mist Droplet Distribution was selected to D v 0,9 100μm Water pressure to produce water mist by means of water mist nozzles p 25bar Verdampfungszeit unterschiedlicher Tropfendurchmesser 12,00 11,27 10,00 8,00 t(s) 6,00 4,00 2,00 9,13 7,22 5,52 4,06 2,82 1,80 1,01 0,45 0,11 0,00 500 450 400 350 300 250 200 150 100 50 Tropfenø (ym) Figure 5: Evaporation time of water droplets versus size of water droplets
- 135-2.4. System Design of the Smoke Gas Cooling System in the Gleinalmtunnel The Smoke Gas Cooling System for the Gleinalmtunnel consists of four independent stations to supply the six ventilation sections. One portal station with underground water reservoir and high pressure pump for ventilation section A north portal One portal station with underground water reservoir and high pressure pump for ventilation section F south portal One station in cavern north with elevated tank ΔH = 365 m for ventilation sections B + C One station in cavern south with elevated tank ΔH = 285 m for ventilation sections D + E Figure 6: Schematics of Smoke Gas Cooling System in the Gleinalmtunnel In the portal stations the water is stored in underground reservoirs with a volume of 150 m³. Booster pumps supply the water to the main high pressure pumps which produce a water flow at a rate of 20 l/s at 30 bar. From these main pumps the water is transported through high pressure pipes to the four nozzle rings in the exhaust ducts. Depending on the required water flow rate, the water is expelled through a number of water mist nozzles into the smoke gas stream in the exhaust duct where the fine water droplets evaporate and subsequently cool the smoke gas. In the cavern stations the water is stored in elevated reservoirs at 365 m respectively 285 m above the tunnel. The water is transported in a vertical pipe through the fresh air inlet shaft to the cavern stations, where water control valves are situated to direct the flow rate to the nozzle rings. Likewise to the portal stations, the water is then expelled through a number of water mist nozzles into the smoke gas stream in the exhaust duct where the fine water droplets evaporate and subsequently cool the smoke gas. As the water reservoirs for the cavern stations are situated 365 m respectively 285 m above the tunnel, the geodetical pressure of 36 bar respectively 28 bar is used to drive the water through the nozzles without the need of an additional pressure pump.
- 136 - Vertical Line Control Valves HP Piping Cavern Stations Vertical Shaft Cavern To Water Mist Rings Figure 7: Cavern Stations of Smoke Gas Cooling System in the Gleinalmtunnel 2.5. Control System for the Smoke Gas Cooling System in the Gleinalmtunnel During normal operation of the Gleinalmtunnel the Smoke Gas Cooling System is on stand by and monitors all relevant states of the system and constantly reports it to the main tunnel control system. For each ventilation section respectively for each ventilation fan three temperature measuring grids are installed. The system is activated when the smoke gas temperature in the exhaust duct exceeds 175 C. Then the system controls the water flow rate such that the temperature at the ventilator inlet is kept between 150 C and 175 C regardless of the smoke gas temperature which could reach a temperature up to 400 C. In normal operation the system works fully automatically. However for emergency or maintenance operations the system can be controlled through touch panels directly at the control stations. Exhaust Duct 4x Nozzle Ring Exhaust Flap Figure 8: Nozzle Rings of the of Smoke Gas Cooling System in the Gleinalmtunnel during operation
- 137-3. REFERENCES Figure 1, Photograph ASFINAG Figure 2, Tender Document RGK GlTu ASFINAG Figure 3, Drawing 6 398_TuRQS ASFINAG Figure 4, Drawing Kaiser & Mach ZT GmbH Figure 5, Table AQUASYS Figure 6, Drawing 1 398_Schema_06 ASFINAG Figure 7, Drawing AQUASYS Figure 8, Photograph AQUASYS