THE IMPACT OF CONTROLLABILITY ON THE DIMENSIONING OF SMOKE EXTRACTION SYSTEMS FOR BIDIRECTIONAL TRAFFIC ROAD TUNNELS

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Laureen Kennedy
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1 THE IMPACT OF CONTROLLABILITY ON THE DIMENSIONING OF SMOKE EXTRACTION SYSTEMS FOR BIDIRECTIONAL TRAFFIC ROAD TUNNELS Abstract Electrowatt Infra, Switzerland To fulfil the requirements of the Swiss design guidelines, the ventilation system in road tunnels with bidirectional traffic with smoke extraction at the fire location must achieve a minimum air flow of 1.5 m s -1 towards the fire from both sides. To control this airflow jet fans are installed and controlled based on measurements of the air velocity in the tunnel. Inaccuracies in the airflow chain of measurement and the lack of flexibility in the controlling of the jet fans leads to the need for higher exhaust flows to ensure that the required airflows in the tunnel can be achieved. A method to estimate the minimum exhaust flow needed to satisfy the design requirements and the related problems of controlling the air flow in the tunnel are described and discussed in the first part of this paper. The impact on the exhaust flow and hence on the cost of the principal design variables such as the type of detector, data acquisition and analysis system and the control strategy are also assessed. In the second part of this paper an attempt is made to quantify the accuracy of several flow measurement systems (detectors, measurement chain, etc.) by statistically analysing a series of tests carried out in the Gotthard road tunnel between 2001 and Keywords: ventilation design, bidirectional traffic, controllability, exhaust flow 1 Background In tunnels with bidirectional traffic and smoke extraction, the strategy in the event of a fire is to exhaust the smoke from the traffic space near to the fire location through remotely actuated mechanical dampers (Figure 1). Figure 1: Ventilation of a bidirectional traffic tunnel with smoke extraction at the fire location 3 rd International Conference Tunnel Safety and Ventilation 2006, Graz

2 36 The Swiss design guidelines for tunnel ventilation (ASTRA 2004) require that jet fans should be used to ensure that a minimum air flow of 1.5 m s -1 towards the fire from both portals is achieved. However the air flows from the tunnel s portals towards the fire depend on the ratio of the pressure drops in the two sections of the tunnel and these are not known when the fire is detected. Furthermore there are significant time-dependent pressure fluctuations during a fire emergency that make it necessary to actively control the jet fans based on measurements of the air velocity in the tunnel. There are two main problems in controlling the longitudinal air velocity: 1. The jet fans usually have just two states on or off so their thrust can only be controlled in finite steps. 2. Errors in the measurement of the air velocity lead to inaccuracies in controlling the longitudinal air velocity. These two factors both have a negative impact on the ability to be control the longitudinal air velocity. With the minimum required exhaust flow equal to 3 m s -1 x A T, the required air velocity on each side of the fire cannot be assured leading to a reduction in the security for the people in the tunnel. Air Velocity [m/s] Impact of additional Jet Fan Impact of measurement Error Unfavourable Situation Favourable Situation 2 Required Exhaust Flow Exhaustpoint Exhaust Flow 3 x Tunnelsection Tunnel length [m] -1-2 Figure 2: Unsymmetrical flow yields unfavourable situations In the first section of this paper the issues associated with controlling the longitudinal air velocity during an emergency in tunnels with bidirectional traffic are discussed and analysed and solutions are proposed. Equally important when designing the capacities of the tunnel s ventilation system are the inaccuracies of the airflow measurement and the consequences of these are addressed in the second section where the results of a series of tests are statistically interpreted.

3 37 2 Impact of the jet fans 2.1 Methodology For a better understanding of the impact of the jet fans on the air velocities in the tunnel a study was carried out to determine the magnitude of the velocity changes when a jet fan is turned on and how large the exhaust flow rate has to be to satisfy the design requirements irrespective of the jet fans. The study was carried out in three steps: Step 1: The required thrust of the jet fans was calculated based on the most unfavourable combination of the tunnel and vehicle friction; the meteorological pressures and the buoyancy forces caused by the fire. Then, taking into account the redundancy conditions required by the guidelines and the numbers of jet fans that could physically be installed in the tunnel, the thrust per jet fan was defined. Step 2: The variable parameters were defined the exhaust flow rate, the exhaust location and the starting velocity in both tunnel branches. With these data and the boundary conditions from step 1, a pressure source or sink term was calculated to achieve a stable flow situation. The pressure term was given by the static pressure balance between the portals (see Eq 2 with Δp SV = 0). Step 3: Equations Eq. 1 and Eq. 2 were then solved with the unknown velocities v l and v r and Δp SV = F SV / A T as a equation system. The velocity independent pressure terms are irrelevant for these calculations so are not included in the pressure balance (Eq. 2). Portal Left Portal Right Figure 3: Topology and equations of the flow calculation 2.2 Boundary conditions for the study For the study a single tube tunnel with bidirectional traffic (one lane per direction) was used with the following principal dimensions: length 2 km, cross-sectional area 50 m², longitudinal gradient 2%. The dimensioning of the jet fans in this particular tunnel is not determined by the flows required during normal tunnel operation but by the most unfavourable fire situation. In this case the pressure required to be produced by the jet fans is 74 Pa of which 21 Pa is as a result of the friction forces caused by the walls and the stationary vehicles and longitudinal flows of 1.5 m s -1 ; 28 Pa is due to the thermal buoyancy forces created by a 30 MW fire; and 25 Pa is due to the meteorological conditions - a 10K temperature difference between inside and out and a 10 km/h adverse wind.

4 38 Taking into account the redundancy requirements, the effective thrust per jet fan depends on the numbers of jet fans that can be installed: Table 1: Effective thrust required for different numbers of jet fans Number of jet fans installed Thrust required per jet fan, N To calculate the impact on the flows when an additional jet fan is switched on the least favourable situation is when there are only a few vehicles in the tunnel because the friction forces due to the vehicles will be low. In all cases a traffic density of just 50 veh km -1 was used. In the calculations the blowing direction of the additional jet fan is in the positive direction, i.e. from left to right. With respect to the control of the longitudinal flows, the most unfavourable location for the exhaust is at the beginning of false ceiling near the left portal (see Figure 4) assuming that the jet fans blow from left to right. All the calculations reported here are with the exhaust point located 300 m from the left portal. Figure 4: Unfavourable exhaust location regarding the controllability of the longitudinal air flow 2.3 Discussion of the results regarding jet fans The flow situations before and after switching on a additional jet fan with two exhaust flows equivalent to 3.0 and 4.5 m s -1 for the five different jet fan thrusts are shown in Figure 5. In each case the starting condition was with equal air speeds towards the fire, i.e. v l = -v r = Q EX / (2A T ). The air flow in the tunnel reacts strongly to the thrust of the jet fan. With a low exhaust flow and large thrust from the jet fan, the flow in the right tunnel branch reverses. In such a situation the smoke would flow past the open dampers creating a particularly bad situation for the people in the tunnel as the smoke would travel all the way to the right hand portal.

5 39 With a higher exhaust flow rate the velocity in both branches is increased before the jet fan is switched on and the flow in the right hand branch does not reverse when it is turned on an improved situation regarding the ventilation aim. The results for four exhaust flow rates and five different jet fan thrusts are summarised in Table 2. v left -SV off v left -SV on v right -SV off v right -SV on Air velocity [m/s] Air velocity [m/s] Exhaust flow 3.0 m/s x A T 12/370 10/460 8/620 6/925 4/1850 Number jet fans / Effective thrust per jet fan [N] Exhaust flow 4.5 m/s x A T 12/370 10/460 8/620 6/925 4/1850 Number jet fans / Effective thrust per jet fan [N] Figure 5: Flow situations for the bidirectional traffic case, required longitudinal velocity = 1.5 m s -1 toward exhaust point Thrust [N] Table 2: Degree of performance Exhaust flow [m 3 /s] Legend Air flow toward the fire from both sides not satisfied Air flow toward the fire from both sides satisfied, v min > 1.5 m s -1 not satisfied Air flow toward the fire from both sides and v min > 1.5 m s -1 satisfied

6 40 With an exhaust flow of 3 m s -1 it is not possible to reach the design requirements in any situation. The higher the exhaust flow used the higher can be the maximum thrust of each jet fan, i.e. the larger the jet fans can be. An attempt has been made to calculate an approximate value for the minimum exhaust flow that is needed to achieve the design requirements. It is mainly dependent on the thrust of a single jet fan and the two loss coefficients z L and z R given by Eq. 3 and Eq. 4. z R = z L L = λ L N Cp EP EP FZ FZ Eq. 3 dhyd AT L L ( L L ) N Cp T EP T EP FZ FZ λ + Eq. 4 dhyd AT In many tunnels the distance between the left portal and the exhaust point (L EP ) is 300 m in the worst case so z L has a typical value of 2.7 to 3 and the lower value is the one used to obtain the results shown in Figure 6. required Exhaust Flow [m/s] z R = 5 z R = 10 z R = 15 z R = Effectively Thrust per Jet Fan [N] Figure 6: Minimum exhaust flow as a function of the thrust per jet fan and zr with zl = 2.7 For any particular tunnel the value of z R is constant and cannot be changed by the designer of the ventilation system. If the value of z R is low (i.e. the tunnel is relatively short), the exhaust flows need to be significantly higher than in longer tunnels with higher values of z R. The only approach available to reduce the required exhaust flow is to reduce the effective thrust from each jet fan. However, in many tunnels, particularly those with steep gradients, a large total thrust has to be installed to overcome the possible thermal buoyancy caused by the fire. Furthermore extra thrust is often included in the design as reserve. However, as the exhaust duct within the tunnel physically limits the numbers of jet fans that can be installed in the tunnel, this means that the thrust from each jet fan has to be high and a correspondingly high exhaust flow would be needed to able to control the flows in the tunnel as required.

7 41 To avoid unreasonably high exhaust flows the aim should be to provide the smallest possible steps in thrust and this can be achieved by: spreading the required total thrust over as many jet fans as possible, using jet fans with multiple speed motors, or using jet fans with frequency converter driven drives Although this analysis has been carried out for single tube tunnels with bidirectional traffic operation it is equally applicable to twin tube tunnels with smoke extraction systems, particularly those where congestion is likely and the required emergency ventilation is comparable to that in bidirectional tunnels. It is clear from the foregoing that the subject of controlling the longitudinal air velocities in the tunnel during an emergency should be considered during the early stages of the dimensioning of the tunnel ventilation system; it cannot be left until just before the system is commissioned. 3 Impact of measurement uncertainties 3.1 Boundary conditions For the dimensioning of the exhaust flow and the control strategies it is necessary to know the uncertainties in the air velocity measurements in the tunnel. Between July and March 2002 a series of tests were carried out in the Gotthard road tunnel to investigate the measurement behaviour of six common air velocity measurement devices (see Table 3) that were installed over a length of 30 m of the tunnel and monitored during normal operation of the tunnel (Zumsteg 2002). Table 3: Measurement devices that were investigated in the Gotthard road tunnel Name M1 M2 M3 M4 M5 M6 Measurement principle Impeller - 2 Point Pitot tube - 2 Point Thermal - 2 Point Ultrasonic - 2 Point Ultrasonic - Section Ultrasonic - Section To determine the measurement uncertainty the test results have been analysed statistically. The measurement devices each provided an averaged value of the air velocity every 10 s. To reduce the amount of data a further average value every minute was calculated and saved. For the statistical interpretation a total of measurements were used taken from 24 days distributed throughout the year with weekdays and weekends proportionally included. For every measurement time the expected value was calculated on the basis of the six devices (M1 M6) according to Eq. 5. Δ v = v Median v ) Eq. 5 ( Mi, t) ( Mi, t) ( M 1... M 6, t)

8 Discussion The results in Figure 7 show the arithmetic mean, the standard deviation and the 95% boundary value. The deviation for the thermal device (M3) is remarkably poor. For all others (M1, M2, M4, M5, and M6) the deviation is of the same magnitude and can be specified as (neglecting M3): Average value: m s % boundary value: m s arithmetic mean standard deviation 95% Boundary Velocity in [m/s] M1 Impeller M2 Pitot Tube M3 Thermal M4 Ultrasonic M5 Ultrasonic M6 Ultrasonic Figure 7 Key data for the analysed measurement devices The effect of the measurement uncertainty on the achievement of the required air velocity in the tunnel can be compensating with a higher target value and hence a higher exhaust flow. As a result the target value for the velocities has to be increased because of the uncertainty in the velocity measurement and the exhaust flow by the equivalent of double the uncertainty in the velocity measurements. The magnitude of this measurement uncertainty is dependent on the measurement interpretation and the acceptable confidence interval but it can be reduced if numerous and adequate devices are installed. It should be noted that particular attention should be paid to the outliers since they can adversely affect the control of the jet fans and it is strongly recommended to use an approach to filter out the outliers. An example therefore is to calculate the median value from the measured data (a minimum of three values is needed). Rather than the average value as the median value is almost unaffected by outliers (Figure 8). It is also worth using the measured volume flow through the exhaust fan in the data interpretation because of its high accuracy. Considering the above mentioned recommendations, a measurement uncertainty of m s -1 seems comparative, thus the exhausted flow has to be increased by an amount equivalent to m s -1.

9 Mean value Median value Velocity [m/s] Measurement- triple Figure 8: Comparison of arithmetic and median mean considering outliers 4 Conclusions 1. The thrust from the jet fans is normally only controllable in finite steps. 2. Increasing the exhaust flow is an effective method of improving the ability to control the longitudinal air velocity in the tunnel. 3. The minimum exhaust flow needed is determined by the thrust step per jet fan, the air velocity measurement uncertainty and data related to the pressure losses in the tunnel. 4. The uncertainty in the air velocity measurements in the tunnel means that the exhaust flow has to be increased by the equivalent of m s -1, favourable conditions presumed. 5. To achieve the required longitudinal air velocity in the tunnel regarding measurement uncertainly and thrust jump of jet fan, an exhaust flow of 4 m s -1 or more would be necessary in most cases. 6. The required exhaust flow can be minimised by reducing the steps in thrust due to the jet fans. 7. The steps in thrust from the jet fans can be reduced by either using a larger number of smaller jet fans to provide the total thrust required or by using jet fans fitted with multior variable speed motors using frequency converters.

10 44 5 Nomenclature A T Tunnel cross-section area [m 2 ] λ Wall friction factor [-] Cp FZ Vehicles friction factor [m 2 ] Δp D Dynamic pressure loss [Pa] D hyd Tunnel hydraulic diameter [m] Δp F Friction pressure loss [Pa] F SV Jet fan thrust [N] Δp J Junction pressure loss [Pa] L EP Length portal to exhaust point [m] Δp JF Jet fan pressure gain [Pa] L T Tunnel length [m] Δp S Starting pressure term [Pa] N FZ Vehicle density [veh km -1 ] Q EX Exhaust flow [m 3 s -1 ] Suffix v Air velocity [m s -1 ] l, r left / right 6 References Bundesamt für Strassen ASTRA (2004): Richtlinie Lüftung der Strassentunnel: Systemwahl, Dimensionierung und Ausstattung, Schweiz Zumsteg F., Steinemann U. (2002): Vergleichende Luftgeschwindigkeitsmessungen im Fahrraum des Gotthard- Strassentunnels, Tiefbauämter der Kantone Tessin, Uri, Graubünden und Zürich, Schweiz

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