HOW TO COMPUTE RELEVANT METEOROLOGICAL PORTAL DIFFERENCES

Transcription

1 HOW TO COMPUTE RELEVANT METEOROLOGICAL PORTAL DIFFERENCES B. Höpperger, J. Croll TFD Consulting Engineer e.u., Austria ABSTRACT A ventilation system in road tunnels must be able to handle the relevant meteorological portal pressure difference mostly based on a 95 % or 98 % quantile of hourly values from a representative year. Meteorological forces often require more than 50 % of the ventilation power in the traffic area. Unfortunately there are different theories to compute the relevant pressure difference. Also guidelines don t describe a definitive way. A variation of up to 50 % results between the theories. In some projects it was necessary defining the way to compute the relevant portal pressure difference considering the point of view of the independent engineer as well as the point of view of the government and their experts. At the end there are different ways used in different projects depending on their actors. The presentation will show the possible variation between the theories using on-site measurement data as well as computed data by the meteorological institute ZAMG for several long road tunnels in Austria. Higher meteorological forces could require additional jet fans or in the worst case additional jet fan niches and therefore result in higher investment costs with a potential economic risk of several hundred thousand euros. A possible future way of required measurement sensors and meteorological data and a suggestion how to compute the relevant quantile will be outlined achieving a standard procedure to obtain relevant portal pressure differences for the ventilation design. Keywords: ventilation design, meteorology, portal pressure difference, wind loads, barometrical pressures, long tunnels 1. INTRODUCTION Meteorological conditions can have a significant impact on the ventilation design of tunnel systems. Especially in mountainous regions, if mountain ridges separate the tunnel portals, barometrical pressure differences between the portal areas may alter in a range of several hundred Pascal. Besides the barometrical pressure difference, two further meteorological effects have to be considered in the design of tunnel ventilation systems. Wind pressures lower the effectiveness of the ventilation system whenever the wind direction is opposite to the direction of air extraction. Further, differences in the temperatures inside and outside the tunnel induce thermic pressures. To get an idea of the barometrical pressure differences in relation to wind pressures, Table 1 lists both, barometrical portal pressure differences and wind pressures, for some Austrian tunnels. Within a range of several Pascal, the listed wind pressures are much lower than the barometrical pressure differences.

2 Table 1: Impact of wind on portal pressure differences for some Austrian road tunnels Name of the tunnel Length [m] 95 th -percentile of portal pressure differences [Pa] Barometrical Wind In total A10, Tauerntunnel 6, (3.6%) A10, Oswaldibergtunnel 4, (4.5%) 31.4 A12, Landecker Tunnel 6, (14.9%) 79.2 S16, Strenger Tunnel 5, (1.2%) REGULATIONS IN GUIDELINES The Austrian RVS [1] recommends to measure barometrical pressure, wind direction and wind speed at the (projected) portal position and if existent at the (projected) top of the shafts within a measurement period of several years. Depending on the risk level of the tunnel, either the 95 th -percentile or the 98 th -percentile is assessed for both, barometrical pressures and wind speeds. In case of lacking measurement data, as per RVS , appropriate meteorological analyses must determine the decisive meteorological impact. The regulations in the RVS focus on how to evaluate portal pressure differences resulting from wind loads. It does not provide a method regarding the evaluation of barometrical pressures, e.g. how to handle vertical temperature gradients and hence how to transfer pressure data from both portals onto a common altitude. The German RABT 2006 [3] as well focus on portal pressure differences resulting from wind loads. For the ventilation design, the 95-percentile of the wind component normal to the portal has to be considered. As per Swiss ASTRA [2] the 95 th -percentile out of period of at least one year has to be used within the ventilation design. Wind pressures and barometrical pressure differences have to be considered, each with the more unfavourable direction. Further, the pressure differences due to temperature differences inside and outside the tunnel have to be considered in the ventilation design. Neither RABT nor ASTRA describe a detailed method for the evaluation of barometrical portal pressure differences. 3. VARIATION OF PORTAL PRESSURE DIFFERENCES USING DIFFERENT METHODS Guidelines like the Austrian RVS, the Swiss ASTRA and the German RABT demand the consideration of barometrical and wind pressures by evaluating a meteorological portal pressure difference. There is no detailed method how to compute the relevant meteorological pressure difference defined. Table 2 lists portal pressure differences for the Arlbergtunnel in Austria that were calculated with different methods, which are all conform to the RVS and which still are used in different projects. The example shows the large margin in the evaluation of portal pressure differences in accordance with the RVS

3 Table 2: Variation of portal pressure differences for diverse evaluation methods, Arlbergtunnel (13,972 m excl. gallery), Austria 98 th -percentile of portal pressure differences [Pa] Direction-independent Direction-dependent All values Positive values only Transfer to portal altitude: h east h west h east h west p east - p west p west - p east The method direction-independent evaluates the 98 th -percentile of all hourly pressure values. The resulting portal pressure difference is used for both extraction directions (east to west and west to east). Within the direction-dependant method, portal pressure differences are evaluated separately. The portal pressure difference p east - p west is considered in the ventilation design, whenever the extraction direction is from west to east and v.v. Due to the varying air column, different altitudes lead to different barometrical pressures. Therefore, prior to the evaluation of barometrical pressure differences, barometrical pressures at both portals have to be transferred onto a common altitude. The transfer from one altitude to the other depends on the prevailing vertical temperature gradient at the respective portal position. This gradient is mostly unknown. It can be estimated by evaluation of data from different measurement sites (see below) or e.g. by considering the typical temperature gradient of K m -1 as per DIN ISO 2533 norm atmosphere. The RVS neither defines temperature gradients, nor the common altitude (e.g. altitude of the lower portal). Thus, varying methods are possible. Table 2 shows the spreading by transferring the measured barometrical pressure either to the altitude of the east portal (h east ) or to the altitude of the west portal (h west ). With consideration of an isentropic atmosphere with a linear temperature drop of K m -1, differences of up to 19 Pa occur for the Arlbergtunnel. Further potential for variations results from the consideration of all values either, or positive values only within the evaluation of the percentiles. The method positive values only restricts the data to positive values only, before the 98 th -percentile is calculated. Therefore, the evaluation of positive values only results in higher pressure differences than the method all values. The comparison in Table 2 reveals that the resulting portal pressure difference p west - p east varies in a wide range of 109 Pa and 217 Pa depending on the calculation method. 4. CALCULATION METHODS Up-to-date two general methods are common, to evaluate the typical meteorological conditions in the project area and to calculate the meteorological portal pressure difference. Either barometrical pressures, temperatures, wind direction and wind speed are measured at the (projected) portals, or the analysis of meteorological pressure differences is based on data of the local meteorological service. The Austrian meteorological service Zentralanstalt für Meteorologie und Geodynamik (ZAMG) for example delivers meteorological data for a representative year out of a period of ten years. Since the number of measurement stations of the ZAMG is limited, the nearest measurement station might be at a distance of several kilometers from the portal. Especially in mountainous regions, the local distance leads to uncertainties that are difficult to estimate.

4 On-site measurements at the (projected) portals avoid this problem. But in contrast to longterm measurements of the meteorological services, in the majority of projects on-site measurement periods of several years are not feasible. The portal pressure differences in Table 2 show a variance of up to 19 Pa depending on whether pressure data is transferred onto the altitude of the east portal or onto the altitude of the west portal. The reason for the variation is the necessity to approximate the vertical change of the atmosphere and thereby also the approximation of the vertical temperature gradient. A common approximation is the barometric formula in equation (1), that allows to transfer the barometrical pressure from one altitude to the other. As mentioned before, the vertical temperature gradient a in equation (1) is unknown. A typical approximation is to consider a temperature gradient of K m -1 according to DIN ISO = 1 h p h1 pressure at altitude h 1 [Pa] p h0 pressure at altitude h 0 [Pa] R universal gas constant for ideal gas [J mol -1 K -1 ] a vertical temperature gradient [K m -1 ] Δh vertical height between h 1 und h 0 [m] g gravitational acceleration [m s -2 ] M molar mass air [g mol -1 ] T h0 temperature at altitude h 0 [K] Equation (1) shows the barometric formula for a linear temperature trend in vertical direction of the atmosphere. The assumption of constant temperatures in vertical direction and thus of an isothermal atmosphere leads to the following equation: = (2) p h1 pressure at altitude h 1 [Pa] p h0 pressure at altitude h 0 [Pa] R universal gas constant for ideal gas [J mol -1 K -1 ] Δh vertical height between h 1 und h 0 [m] g gravitational acceleration [m s -2 ] M molar mass air [g mol -1 ] T temperature [K] Up-to-date, neither on site-measurements nor measurements of local meteorological services provide data that allow for a calculation of vertical temperature gradients at the tunnel portals. The consideration of two measurement sites at different altitudes allows for an approximation. In case of the Strenger Tunnel for example, the meteorological service ZAMG provided data of a measurement site in Landeck (809 m) and a second one in Ischgl/ Idalpe (2,312 m) to approximate a vertical temperature gradient at the east portal. For the west portal measurement data for St. Anton am Arlberg (1,300 m) and Galzig (2,079 m) were used. Figure 1 shows the measurement sites. The meteorological service chose them with respect to the local geography. (1)

5 Figure 1: Measurement sites ZAMG data (Google Earth) Figure 2 shows the vertical temperature gradients (hourly values in the course of one year) that were calculated with the data of each two measurement sites, compared to the vertical temperature gradients for the isothermal assumption (a = 0 K m -1 ) as well as for the temperature gradient according to DIN ISO 2533 (a = K m -1 ). a [K m -1 ] Course of one year Linear, a variabel, west portal Linear, a variabel, east portal Linear, a (DIN ISO 2533) Isothermal Figure 2: Vertical temperature gradient for different evaluation methods The data allows to determine the influence of the vertical temperature gradient on the portal pressure difference. Figure 3 and Figure 4 show each average hourly value for the pressure difference of the relevant year including a straight line for the 95 percentile.

6 Figure 3: Pressure difference west east Figure 4: Pressure difference east west Figure 5 shows the variance of the pressure difference between west east and east west west-east [Pa] east-west [Pa] Figure 5: Variance of the pressure differences Table 3 shows the results of different calculation methods for the Strenger Tunnel.

7 The calculation method is direction-dependent with consideration of either a varying vertical temperature gradient, a constant vertical temperature gradient of K m -1 according to DIN ISO 25 or an isothermal atmosphere. Table 3: Variation of portal pressure difference for diverse evaluation methods, Strenger Tunnel (5,851 m), Austria Vertical temperature gradient (a): 95 th -percentile of barometrical portal pressure differences [Pa], direction-dependent All values Positive values only p east - p west p west - p east p east - p west p west - p east h east h west h east h west h east h west h east h west a = variable* a = K m a= 0 K m -1 (isothermal) *: east portal: K m -1 a K m -1 and west portal K m -1 a K m -1 The comparison shows that the results differ in a range of 76 Pa (77% of the average value), from 60 Pa to 136 Pa. The major impact results from the direction-dependent evaluation (up to 64 Pa). The transfer from p east onto the altitude h west either, or p west onto the altitude h east results in a difference up to 29 Pa and the consideration of positive values only in the evaluation of the percentile leads to differences up to 30 Pa. A difference of up to 22 Pa occurs between the different models for the vertical temperature gradient. 5. CONCLUSION AND SUGGESTIONS Using the examples of the Arlbergtunnel and the Strenger Tunnel in Austria, calculations demonstrated the possible influence of divergent calculation methods on portal pressure differences. In case of the Arlbergtunnel, the different methods - all in accordance with the regulations of the Austrian RVS result in variations of up to 121%. The reason for the variation is a lack of regulation in the following regards: Common altitude for the pressure transfer Character of the vertical temperature gradient for the pressure transfer to a common altitude Direction-dependent or independent evaluation of portal pressure differences Consideration of all values or positive values only in the calculation of percentiles The regulation of the ASFiNAG PlaPB [1] considerably improved the situation, as it states the evaluation method more precisely. As per PlaPB the averaged measurement data of total pressures at one tunnel portal (altitude h 1 ) is transferred onto the altitude of the second tunnel portal (altitude h 2 ) with help of the barometric formula (cf. equation (2)). The temperature is used as the average temperature between the two portals. Up-to-date meteorological analysis base on on-site measurements or on measurement data of the meteorological services. On-site measurements mostly consist of data for only one year. Measurement data of the meteorological services often consider a representative year out of a period of ten years. This is an advantage, but on the other side, measurement sites of the meteorological services may be several kilometers away from the portal. To achieve a well-founded assumption about the meteorological portal pressure differences, a combination of on-site measurements and ZAMG-data seems to be the best solution. Hereby, the advantages of both methods are combined.

8 In general, on-site measurements should include wind direction, wind speed, barometrical pressure and temperature. To receive additional data about the vertical temperature gradient at the (projected) portals, on-site measurements should consider two measurement altitudes for the temperature: one at the altitude of the portal and a second one about 100 m above if possible. This allows to calculate the vertical temperature gradient for each hourly or halfhourly data point. Under consideration of the variable vertical temperature gradient, the barometric formula (equation (1), previous chapter) allows to transfer the measured pressure at the higher portal (hourly or half-hourly values) onto the altitude of the lower portal. The exact altitude of both portals (measurement sites) is essential to avoid errors in the pressure transfer from one altitude to the other. Subsequently, the respective percentile can be calculated. The consideration of positive values only in the percentile-evaluation will lead to a more conservative value. Seeing the uncertainties within the calculation process as well as a measurement precision of about 30 Pa for common pressure measuring instruments, a conservative approach for the meteorological impact on the ventilation system seems to be justified. If data of on-site measurements are not available for several years, a second evaluation ought to be done. The latter should base on measurement data of the meteorological service for the representative year. The comparison includes the aspect of time in the analysis of meteorological impacts. This way reduces the risk of a designing mistake in the ventilation system because of the meteorology and should be considered in the tunnel ventilation guidelines. A discussion about the variation of the wall friction between λ = or results for the m long Strenger Tunnel in a change of 3.2 Pa for 2 m/s, which is nothing compared with the meteorological issue. The variation of the meteorology results in a difference of four jet fans. We sometimes talk about the wrong screws. 6. REFERENCES [1] ASFiNAG, (January 2016) Technisches Planungshandbuch der ASFiNAG. PlaPB Tunnel Lüftung (TLü). Technische Richtlinie. Version [2] Eidgenössisches Departement für Umwelt, Verkehr, Energie und Kommunikation UVEK, (2008) Richtline. Lüftung der Strassentunnel. Systemwahl, Dimensionierung und Ausstattung. ASTRA Bundesamt für Strassen ASTRA. Bern. Version [3] Forschungsgesellschaft für Strassen- und Verkehrswesen, workgroup Verkehrsführung und Verkehrssicherheit, technical committee Ausstattung und Betrieb von Straßentunneln, (May 2006) Richtlinien für die Ausstattung und den Betrieb von Straßentunneln RABT. FGSV Verlag GmbH. Cologne. [4] Österreichische Forschungsgesellschaft Straße, Schiene, Verkehr, workgroup Tunnelbau, technical committee Betriebs- und Sicherheitseinrichtungen, (Juni 2014) Tunnel. Tunnelausrüstung. Belüftung. Grundlagen RVS Vienna. Version 1.