Tunnel vision check combines with atmosphere monitoring

Chris Pick, of CODEL International Ltd, examines key issues relating to the hazards posed by air pollutants produced by traffic in the confined space of road tunnels and the need for effective and accurate monitoring

Road tunnels have become essential elements in the infrastructure of developed countries, most carrying an enormous amount of traffic. In the open air pollutants produced by traffic are safely dispersed, but in a confined space they can quickly become lethal.

Internal combustion engines operate by burning fuel, either petrol or diesel. Ideally, when oxidised, these fuels will result in the harmless release of carbon dioxide and water vapour. However, imperfections in the combustion process cause the release of other harmful products that are the source of pollution concerns: particulates, carbon monoxide and oxides of nitrogen.

Health and visibility
Fine particles of carbon, of about one micron diameter, are released by diesel engines. These are particularly harmful to the human respiratory system. They also remain suspended in the tunnel atmosphere, causing significant attenuation of light levels which becomes very obvious to the tunnel users. In serious pollution conditions, visibility within a tunnel can be reduced to 200 or 300 metres, presenting a danger to safe traffic flow.

Incomplete combustion in petrol engines results in the release of carbon monoxide, an extremely toxic gas. This is common when engines are cold and when under heavy load or acceleration. Under these conditions, tailpipe emissions of several hundred or even thousand parts per million (ppm) of CO are not uncommon. Prolonged exposure to levels above 100ppm are harmful, while levels of 4,000ppm are soon fatal.

Oxides of nitrogen are also an inevitable toxic by-product of the combustion process. In particular, NO² is extremely toxic; levels as low as 150ppm can be fatal while exposure to levels of 60ppm will produce irritation of the throat, with more serious respiratory, effects delayed for up to 48 hours.

Continuous monitoring
The aim of the ventilation system is to limit the presence of pollutants to within safe operating levels. There are typically four sets of measurements required to ensure correct ventilation control: particulates, CO, NO and air flow. The continuous monitoring of pollutant levels within road tunnels presents a unique set of problems for the instrument supplier and the tunnel operator. The continuous release of diesel fume particulates, combined with the acid gas release, in the form of oxides of nitrogen, soon coats the instruments with a sticky black deposit.

A further problem is one of access. It is often necessary to close a tunnel to perform instrument maintenance. Since often results in major inconvenience it is allowed only infrequently. As a result, instruments have to operate for long periods of time in a hostile environment without the need for maintenance.

Combined monitor
All these problems have been overcome with the latest type of monitoring equipment designed and manufactured by CODEL International which enables measurements of CO, NO and visibility to be combined into a single air quality monitor.

The monitor consists of a transceiver that projects visible and infrared beams to a reflector unit mounted 3 metres away. The reflected light is received by the transceiver and the specific absorption is measured to determine the visibility coefficient, carbon monoxide and nitric oxide concentrations within the path of the beams.

A high-powered, modulated LED is used for the visible light source while an infrared thermal source provides broad band infrared energy. Optical visibility is measured by a silicon photo-detector which determines the attenuation of the light beam along the instrument sight path, caused by the particulates in the tunnel atmosphere.

The infrared absorption due to CO and NO is measured using gas cell correlation technology. Sealed cells containing CO and NO are swept through the detector field of view, once per second, using a continuously rotating stepper motor. Measurements obtained with and without the presence of these sealed cells enables the absorption due to CO and NO in the instrument sight path to be determined. From this measurement of atmosphere concentration in ppm is obtained.

To measure air flow, a new monitoring technique has been developed by CODEL which combines the reliability of ultrasonic technology with the simplicity and operating advantages of the anemometers originally used for measuring air flow. And, because it uses ultrasound, there are no moving components, ensuring high reliability.

The number of sensors required and their location within a tunnel must be decided on a case-by-case basis. However, it is generally considered necessary to have a measurement station consisting of air quality and air flow measurements approximately every 500 metres along the tunnel. This is particularly the case in contraflow traffic arrangements where there is no net flow of air due to traffic movement. It is therefore necessary to access output data from monitoring stations distributed along the length of the tunnel. This data is then used to control the ventilation fans.

The tunnel atmosphere monitoring system developed by CODEL is modular, enabling the user's precise requirements to be satisfied with a minimum number of components, minimal tunnel cabling and minimal installation costs. With over 130 tunnels equipped with this system worldwide since its introduction in June 1999, the concept can be seen to be a clear success.



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