Predicting permissible carbon monoxide concentration limits for tunnel ventilation under normal operation

Transcription

1 Predicting permissible carbon monoxide concentration limits for tunnel ventilation under normal operation Wei Ye 1,*, Xu Zhang 1, Xiang Zhou 1, Yuan Yuan 2 1 HVAC & Gas Institute, Tongji University, Shanghai, China 2 College of Environmental Science and Engineering, Tongji University, Shanghai, China * Corresponding realshow@live.cn SUMMARY Carbon monoxide is taken as a reference pollutant for assessment of the toxicity of the exhaust gases from vehicles due to its chemical and biologic property that inhibits the blood s capacity to carry oxygen directly by attaching itself to hemoglobin and forming a toxic compound known as carboxyhemoglobyne(cohb). The current code Specifications for Design of Ventilation and Lighting in Highway Tunnel of China lists admissible CO concentration limits for longitudinal, transverse and semi-transverse ventilation tunnels and vary from international standards both on principles and values. For each type of tunnel, a fitting equation for predicting permissible CO concentration limits based on simplified CO concentration profile in tunnels and difference form of CFK equation (Coburn et al., 1965) is presented. And the comparison results support the Chinese code to a very limited extent, especially the value of thresholds in this code may be inappropriate. KEYWORDS Tunnel ventilation, carbon monoxide concentration, code, CFK equation, modelling 1 INTRODUCTION The target pollutions for tunnel ventilation lists in Chinese current code Specifications for Design of Ventilation and Lighting in Highway Tunnel (JGJ ) are carbon monoxide, oxynitride, Pb, sulfur dioxide, particle matters and etc.. Even CO was no longer the dominating factor for designing of tunnel ventilation system in some countries since the CO emissions per vehicle have reduced significantly and in addition, as heavy duty vehicles are mostly powered by diesel engines, the visibility issue has become in many cases the driving force in ventilation design for normal operation (PIARC, 2004), CO is still taken as a reference emission for the assessment of the toxicity of the exhaust gases. Carbon monoxide inhibits the blood s capacity to carry oxygen directly by attaching itself to hemoglobin and forming a toxic compound known as carboxyhemoglobyne (COHb) (Modic, 2003). Although forming COHb is a reversible process, the damage it causes may not. [COHb] (COHb concentration in blood) levels can be roughly assumed at 0.8% initially in human bodies (Muller and Barton, 1987) and when [COHb] level reaches at 2.5% to 4%, decrease in the short-term maximal exercise duration in young healthy men can occur, and between 2% and 20%, COHb can cause effects on visual perception levels, audition, motor, and sensory-motor functions, and behavior (WHO, 1994). Despite the human activity, altitude and individual metabolism, the damage caused by COHb depends on exposure, and both the concentration and time should be taken in account. The Chinese code set a series of merely length (or time) related limits, which assumed [COHb] would only affect human health under the condition of only it s greater than 10%, for CO concentration in variable ventilation tunnels.

2 Modeling for predicting [COHb] based on CO exposure is another way to reveal the puzzle and has been researched since 1960s. The most well-known and widely-used mathematical model was first established and called CFK equation (Coburn, Forster and Kane, 1965) and proved for its accuracy by WHO (Nevers, 2000). CFK equation has several modified forms (Muller and Barton, 1987; Han and Zhang, 2009 and etc.) but none of them can predict CO concentration limits based on exposure concentration and time for tunnels. Ye and Zhang provided CO concentration limits predicting models based on integral forms (2010) and difference forms (2011b) of CFK equation. For each type of tunnel, a fitting equation is presented for predicting permissible CO concentration limits based on CO concentration profile and difference form of CFK equation which imports exposure concentration and time. 2 CURRENT CO CONCENTRATION LIMITS Current CO concentration limits from Chinese Code and PIARC regulation The CO concentration limits list in JGJ of China can be summarized in Table 1. Assuming that all the vehicles are at normal speed of 50km/h. And Table 2 shows the CO concentration values given by PIARC (Permanent International Association of Road Congresses) for road tunnel operation only (PIARC, 2004). Table 1. Permissible CO concentration limits from Chinese Code: JGJ Tunnel length [0, 1000m] (1000, 3000) 3000m Longitudinal 300ppm 344mg/m 3 ) 250ppm (286mg/m 3 ) Transverse interpolation 250ppm (286mg/m 3 ) Semi-transverse 200ppm (229mg/m 3 ) Table 2. CO concentration values given by PIARC 2004 Traffic situation CO concentration in design year of 2010, ppm (mg/m 3 ) Fluid peak traffic: km/h 70 (80) It is obvious that the permissible CO concentration limits given by Chinese Code are length (or time) related limits and the limits for longitudinal ventilation tunnels are 50ppm higher than those for transverse and semi-transverse ventilation tunnels. On the other hand, PIARC provided a unique limit, which is much lower than the value from JGJ , for all types of ventilation tunnels under normal operation. Current CO concentration limits from international hygienic standards Table 3 summarized five international hygiene standards which described maximum CO exposure concentration and time and equivalent [COHb] values which are calculated by original CFK equation under that particular exposure concentration and duration. Table 3. CO concentration limits from international hygiene standards From Limit, ppm(mg/m 3 ) Exposure type and time Equivalent [COHb] ACGIH [1] 25 (29) Time weighted average: 8h 3.35% OSHA [2] 50 (57) Time weighted average: 8h 6.25% NIOSH [3] 35 (40) Time weighted average: 8h 4.51% OEHHA [4] 20 (23) Acute:1h 1.21% GBZ 2.1 [5] 26 (30) Short term exposure limit: 15min 0.95% Note: [1] : ACGIH USA. Threshold Limit Values for Chemical Substances and Biological Exposure Indices. 2005; [2] : OSHA USA. 29 CFR (Occupational Safety and Health Standards - Air contaminants); [3] : NIOSH USA. Recommended Exposure Limit for CO. 1996; [4] : OEHHA USA. All OEHHA Acute, 8-hour and Chronic Reference Exposure Levels (chrels), Dec. 18, 2008;

3 [5] : GBZ Occupational exposure limits for hazardous agents in the workplace Part1: Chemical hazardous agents (in Chinese). The equivalent [COHb] values from hygienic standards suggested that for short time exposure (for most cases of tunnel travelling), [COHb] should be around 1.0%. Even for long time exposure, [COHb] that exceeds 6.3% may not be appropriate. 3 MODELING All the details of derivation process of CO concentration limits models based on CO concentration profiles and difference forms of CFK equation were presented in Ye and Zhang (2011a, 2011b). 4 Predicting Permissible CO Concentration Limits Assuming that: a)the tunnel s length is L (km); b)the tunnel is one-way traffic and the traffic speed is constant at v (km/h) and the pollutant generation rate is constant as well; c)the diffusion of CO is neglect; d)the effect for air volume through cross section by vehicles intake and exhaustion is neglect, so the air velocity in tunnel is constant; e)do not consider the temperature rise and gradient variability in tunnel; f)the directions of air flow and traffic in the tunnel are the same. Longitudinal ventilation tunnel with jet fan(s) and no vertical shaft(s) In longitudinal ventilation tunnels, usually CO volume can be accumulating along the tunnel. And for hygienic concerns, the CO concentration at the exit of the tunnel should be controlled. The effect for permissible CO concentration limits at the exit of the tunnel due to tunnel length or vehicle speed are shown in Figure 1 and Figure 2. Figure 1. The effect for permissible CO concentration limits due to tunnel length Figure 2. The effect for permissible CO concentration limits due to vehicle speed Assuming that [COHb] should be controlled within 1.0% due to the results of Table 3, and the PIARC limit would be secure from COHb damages for human health if the tunnel is less than 10.9 km in length while the Chinese code limit would be only suitable for tunnel that isn t longer than 3.0 km. Usually short length road tunnel are more common in urban areas, basically the PIARC limit is safer than the Chinese code limit. Take δ CO (L, v, [COHb]) as the permissible CO concentration limit for longitudinal ventilation tunnel. Since the permissible CO concentration limit is approximately in a direct ratio with vehicle speed and the increment of [COHb], and in an inverse ratio with tunnel length, then δ CO (L, v, [COHb]) can be described as a fitting equation (Eq(1)). When the tunnel s length is less than 20km, [COHb] is no greater than 5% and vehicle speed is between 50km/h to

4 100km/h, the correlation coefficient can be within [ , ]. CO CO(1, v,1.0%) [COHb] 0.8% ( Lv,,[COHb]) L 0.2% CO(1,50,1.0%) v [COHb] 0.8% 737 v [COHb] 0.8% (ppm) (1) Longitudinal ventilation tunnel with jet fan(s) and vertical shaft(s) Ye and Zhang (2011a) found out that the permissible CO concentration limits for longitudinal ventilation tunnels with vertical shafts can be set at the same level of common longitudinal ventilation tunnels due to several factors. So for tunnel operation, the permissible CO concentration limits can be calculated by Eq(1) as well. Transverse ventilation tunnel The CO concentration along the transverse ventilation tunnel can be roughly taken as constant. So the permissible CO concentration limits can be calculated by original CFK equation. Figure 3 shows the effect for permissible CO concentration limits due to tunnel length. Figure 3. The effect for permissible CO concentration limits due to tunnel length The PIARC limit can secure a transverse ventilation tunnel under 5.6 km in length while the Chinese code can only do the job for a tunnel within 1.9 km if the hygienic requirement is set high. A fitting equation can also be presented as Eq(2) for transverse ventilation tunnels. The correlation coefficient can be within [ , ]. As the CO concentration along the tunnel can be considered as constant, the permissible CO concentration limits should be controlled at every position of the tunnel theoretically. CO 371 v [COHb] 0.8% ( Lv,,[COHb]) (ppm) (2) Air supply type semi-transverse ventilation tunnel The CO concentration profile in a typical air supply type semi-transverse ventilation tunnel is shown in Figure 4. The profile can be changed dramatically as the neural point (NP) is uncertain due to pressure change at the entrance and exit of the tunnel. The CO concentration

5 when NP is in the tunnel is actually the average concentration when NP is outside the tunnel. And Figure 5 indicated that there is a lowest permissible CO concentration that can be set wherever the NP is and the lowest concentration is the one when the NP is in the tunnel. Figure 4. CO concentration profile in a 5km long tunnel Figure 5. The effect for permissible CO concentration limits due to neural Point position Since the air supply type semi-transverse ventilation tunnel can be equaled to transverse ventilation tunnel when the NP is in the tunnel. Then the permissible CO concentration limits can be set at the same level of transverse ventilation tunnels as Eq(2). Notice that, the definition and application of X n can be found in Ye and Zhang (2011b). Air exhaustion type semi-transverse ventilation tunnel As shown in Figure 6, for a typical air exhaustion type semi-transverse ventilation tunnel, the peak CO concentration cannot be stable due to different NP s positions. But the positions of peak CO concentration don't affect the permissible CO concentration limits to a great extent. And the maximum CO concentration wouldn t be infinite due to dilution by air flow in real tunnel. The fitting process imported a peak revise method to avoid such inappropriate matter. No describe here. Figure 6. CO concentration profile in a 5km long tunnel The fitting equation can be presented as Eq(3) for air exhaustion type semi-transverse ventilation tunnels. The correlation coefficient can be within [ , ]. 5 CONCLUSIONS CO 1161 v [COHb] 0.8% ( Lv,,[COHb]) (ppm) (3)

6 A fitting equation for predicting permissible CO concentration limits based on simplified CO concentration profile in tunnels and difference forms of CFK equation is presented for each five types of tunnel. And the comparison between calculation and current limits provided by Chinese code JTJ and PIARC 2004 shown that: (1) The permissible CO concentration limits for longitudinal ventilation tunnels with or without vertical shafts can be set at the same level, and for transverse and air supply type semi-transverse ventilation tunnels, the limits can also be set to the same. But the limits for air exhaustion type semi-transverse ventilation tunnels should be as much as approximately 3 times of the limits set for transverse ventilation tunnels. These results don t support the classification from the Chinese code. Furthermore, the length related limits calculation method didn t make sense from hygienic point of view. (2) Although the limits provided by PIARC are not specific on different types of ventilation tunnels, they are much more secure than Chinese code for short length tunnels. For long length tunnels, CO concentration limits should be set to a much lower value, but other technologies can be introduced to balance the economic cost. Further research should be done to set a new series of CO concentration limits for ventilation tunnels in China. ACKNOWLEDGEMENT This research has been supported by the Science and Technology Commission of Shanghai Municipality (Grant No. 08DZ ). REFERENCES Coburn R.F. et al. Consideration of the physiological variable that determine the blood carboxyhemoglobin concentration in man. Clinical Invest, 1965, 44(11): Department of the Environment. Expert panel on air quality standard. London: HMSO; Han X., Zhang X. Study on Ventilation Flow Rate of CO Index in Longitudinal Ventilated Tunnels Based on the Altered CFK Equation. Chinese Journal of Underground Space and Engineering, 2009, 5(4): , in Chinese. JTJ Specifications for Design of Ventilation and Lighting in Highway Tunnel, in Chinese. Jurij Modic. Carbon monoxide and COHb concentration in blood in various circumstances. Energy and Buildings, 2003, 35: K. E. Muller, C. N. Barton. A nonlinear version of the Coburn, Forster and Kane model of blood carboxyhemoglobin. Atmospheric Environment, 1987, 21(9): Nevers Nd. Air pollution control engineering Beijing: Tsinghua University Press, PIARC. Road Tunnel: Vehicle Emissions and Air Demand For Ventilation Ye W., Zhang X. A permissible carbon monoxide concentration limit model for longitudinal tunnel with shafts. Proceedings of the 7 th International Symposium on Heating, Ventilating and Air Conditioning, p a, Shanghai, China. Ye W., Zhang X. Discussion on CO concentration threshold in tunnels based on an altered CFK equation. Railway Standard Design, 2010, 2(S2): 51-56, in Chinese. Ye W., Zhang X. Integral and difference forms of revised CFK equation for prediction carbon monoxide thresholds in tunnels. Proceedings of the 7 th International Symposium on Heating, Ventilating and Air Conditioning, p , 2011b, Shanghai, China.