The problem regarding leakage is relatively new. This is due to that the requirements have changed significantly in the tunnel ventilation in recent years. Previously mainly transverse ventilation systems with linear extraction and supply air were installed to maintain air quality, today it is mostly pure exhaust systems with controllable flaps for concentrated smoke extraction in case of fire. The concentrated extraction leads to relatively large and suppressing the undesirable due to its leakage.
The principal objective of the research work is to create a comprehensive basis for a better understanding of the leakages in smoke exhaust ducts. The primary aim of the research is:
- A creation of fundamental knowledge in the field of leakages into smoke exhaust ducts.
- Investigation of the experience with leakage requirements and approaches in Switzerland and foreign countries.
- Quantification of leakages in existing road tunnels with smoke exhaust systems (in-site measurements).
- Development of a method to extrapolate the measurement data to arbitrary tunnels.
- Recommendations regarding prevention (e.g. reduction of leakages) and intervention (e.g. increasing ventilation capacity).
The research project is divided into four parts:
Part 1: Development of basics
The first part is on the one hand a literature review and on the other hand a theoretical and practical approache to leaks in exhaust air ducts. The literature already available will be collected, analysed and assessed and common practice as well.
Part 2: Measuring and data collection
The second part comprises primarily the preparation and execution of the measurements in the tunnel. In terms of a comprehensive and universal evaluation of the measurement results, the number and variety of tunnels should be high. The entire measurement campaign is ideally carried out at least in 7 tunnel systems (new and renovated) as part of acceptance tests. The choice of tunnel objects is to be defined and ev. adapted during the project. The measurements will be carried out by an accredited testing.
Part 3: Evaluation & Analysis
The analysis of the measurement results is carried out simultaneously with the measurements and mainly includes the following aspects:
- Collection of data from all measurements.
- Evaluating the data according to various criteria (age, construction, ...)
- Definition and quantification factors.
- Definition of one or more dimensionless leakage indicators for easy use in the ventilation design.
- Developing a method for easy determination of the effective leakage currents.
Part 4: Documentation
All aspects of the research are documented in a final report at the end of the research.
Between August 2007 and September 2009 a total of 16 leakage measurement campaigns in 10 different Swiss tunnels were carried out. In order to acquire a comprehensive set of data, the choice of tunnels reflected a wide range of types with respect to geometry, age, and construction method.
The measurements were carried out using the well-proven tracer gas method (SF6) with constant emissions.
The measured raw data consist of a set of 159 data-triplets. They are not explicitly suited to quantify the leakages; however they do identify interesting features such as the dependence between leakage and under-pressure and the relatively wide scatter of the results.
The reasons for this scattered behaviour could be attributed to uncertainty of the measurements, to construction irregularities of the exhaust ducts, or to material deformation of the duct itself.
Quantifying of the Leakages
A practical method has been developed enabling the sizing of the exhaust system taking into account the leakages into the exhaust duct. These are quantified using a well-known approach described in ISETH Mitteilung Nr. 39 (1978). The concept is to consider the leakage flow in a macroscopic manner, so as to describe the behaviour of the system mathematically, while certain parameters are then derived from the measurements. The values obtained by this approach must be considered as bandwidth.
With this method it is possible to calculate in a straightforward manner relevant quantities such as volume flow, under-pressure at the end of the exhaust duct, and ventilator power.
Recommendations / Further Steps
For an adaptation and/or continuation of the present research work, priority should be given to the following tasks: a) define guaranteed values and design values for the tightness of exhaust ducts and dampers, b) define specifications for exhaust flow and leakage flow measurements, c) carry out and analyse more measurements.
Quantification of leakages in tunnels.