Gas Monitoring Systems for Tunnels

New Technology Gains Momentum
By Albert E. Ketler

The monitoring of toxic and flammable gases in tunnels is an important undertaking for tunnel management, as the safety and liability issues are profound. Furthermore, maintaining the gas monitoring system (GMS) in operational condition involves issues that seldom come to light until the wrong system is purchased, and the poorly specified performance, high maintenance costs and troublesome servicing issues come into focus.

Rel-Tek has pioneered in the development and manufacture of gas monitoring systems for tunnels and underground mining since 1979. New technology now enables longer-life sensors, higher speed telemetry, and – the big payback feature – fully automatic gas sensor calibration.

Carbon monoxide is the primary toxic gas to be monitored in tunnels, usually amounting to a CO sensor on the tunnel wall at 200- to 500-ft intervals. Employing electrochemical technology, these CO sensors are quite stable and have a long life (typically 8 to 10 years). This same sensor technology stems from underground mining, where CO monitoring has become the mainstay of conveyor belt fire detection since 1985.

Carbon dioxide sensors are often included in the sensor mix, as these reflect the hazards of poor ventilation. Internal combustion engines and fires replace ambient oxygen with the suffocate CO2. Although not considered toxic, a CO2 buildup can present breathing problems, and of course, CO2 is a good indicator of poor air quality. CO2 sensor technology is usually optical (non-dispersive infrared) and optical devices are more difficult to maintain in a wet and smoky tunnel environment. Rel-Tek supplied two gas monitoring systems for the Port Authority of Allegheny County’s Wabash and Berry Street HOV Airport Busway Tunnels in 1997, then predating the 2001 advent of the MagiKal automatic gas sensor calibration system.

Nitrogen dioxide, NO2, is another toxic gas that is sometimes monitored in tunnels. Produced from internal combustion engines, the highly toxic NO2 is a hazard worth monitoring. However, NO2 reacts quickly with moisture in the air, and accurate measurements are illusive. Consequently, NO2 sensors are not usually included in tunnel monitoring.

New requirements are afoot that require combustible gas monitoring on federally supported highway tunnel projects , Ref NFPA-502, 2011 edition, Annex G, being adopted for major projects. The increasing use of alternative fuel vehicles presents new hazards from combustible gases – predominantly CNG (typically 4,000 psi compressed methane with up to 10 percent ethane,) LNG (cryogenic methane,) LPG (liquid petroleum gas, propane) and HCNG (compressed hydrogen, alone or mixed with CNG.) Consequently, the rehab of old tunnels and new tunnel construction are including both toxic and combustible gas sensors, covering the growing risk of alt-fuel vehicle accidents with added hazards of fire and explosion. Catalytic bead is the preferred sensor technology here, as optical NDIR sensors overreact 20:1 to ethane, while not reacting at all to hydrogen.

Monitoring these gases requires a host of gas sensor types, toxic and combustibles. In addition, there is often a specification for monitoring smoke and fog, usually requiring open beam or enclosed optical obscuration detectors. Air velocity monitoring is also becoming of interest, controlling fans to provide sufficient ventilation, but not wasteful excesses.

Adding difficulty to the monitoring requirements is the need to service the sensors, particularly gas sensors that tend to drift with time and require monthly re-calibrations. Calibration of gas sensors used to be manual, requiring personnel to access the sensors, apply calibration gas from portable tanks, and make adjustments to keep the sensors accurate. Obviously, the stoppage and re-direction of traffic for these monthly episodes is not only troublesome for highway traffic, but also costly and risky to personnel who require signage, barrels and flagmen, while exposing personnel to injury.

New technology is offered by Rel-Tek that enables the gas sensors to be calibrated automatically. Computer techniques automatically apply NIST-certified calibration gas (cal-gas) to each sensor on a programmed calendar/clock basis, all without human intervention.

In 2007, the I-70 interstate highway tunnels in Wheeling, W.Va., were equipped with Rel-Tek CO monitoring systems including the powerful feature dubbed “MagiKal.” This same GMS product had been tested in the above-ground transit industry for more than a decade, monitoring CNG, LNG and LPG emissions in bus garages, and was recently applied to tunnels. The Wheeling systems were installed in January 2008, operating continuously for nearly four years, without a hitch. The West Virginia Department of Highways and the Wheeling Tunnel management are happy to avoid the monthly traffic stoppages and diversion of personnel normally required for gas monitoring.

Tunnels are particularly susceptible to gas sensor maintenance issues due to traffic disruptions, and, although scheduled in advance, sensor maintenance is never convenient, and usually diverts personnel from more important jobs. Also, the risk of injury to personnel in the dynamics of tunnel traffic is far greater than in a static garage or factory environment. MagiKal is a must-have feature for tunnels.

MagiKal can automatically calibrate virtually all types of gas sensors – usually a combination of toxic and combustible, and the mix can also including carbon dioxide and oxygen sensors. One GMS installed in 1996 at Sun Metro Transit in El Paso, Texas, uses three types of gas sensors – CNG, CO and O2 – and uses a custom cal-gas mixture to automatically calibrate more than 100 sensors, a combination of all three types using one cal-gas mixture.

Once a month, the monitoring computer activates the MagiKal process – normally late at night in light traffic – by alternately applying span and zero cal-gases to the sensors. Alarms are automatically disconnected and the previous sensor readings are continued for approximately 15 minutes, so there are no outward indications the calibration procedure is actually underway. Certified cal-gas is supplied from full-sized tanks located at mid span, each tank containing about 1,600 cubic meters (STP) of gas, usually sufficient for several years of calibrations. Cal-gas is cheap in this format, costing about $8 for each monthly calibration schedule, covering all gas sensors at once.

Auto-calibration data for each sensor is captured by the Millennia computer for each sensor and applied on completion of the process. Using 12-bit resolution, the sensor calibration accuracy is about +/-0.044 percent digitizing error, far better than possible when calibrating sensors manually, and without the substantial risk and cost for labor.

A GMS at Golden Empire Transit in Bakersfield, Calif., with 27 CNG sensors was installed in 1999 and upgraded with the MagiKal utility in 2009. After two years of experience using MagiKal, maintenance engineer Brian Nunn has reported that the system has been working well with very little maintenance.

New tunnel rehab or construction projects should include specifications and funds for a first-class GMS, including a full complement of sensors for toxic and combustible gas, smoke, temperature and air velocity, and attended to by a Millennia computer with HMI graphics, high-speed telemetry, unlimited data logging and alarm archiving, plus, of course, automatic gas sensor calibration. And, the monitoring speed should be quick with worst-case alarm response time not exceeding a few seconds.

For the Squirrel Hill Tunnel rehab project on Route 376 in Pittsburgh, Pa., previous specs showed CO sensors only. However, in light of the latest federal tunnel recommendations, the sensor complement has since been expanded to include other types of sensors to cover alt-fuel risks, while adding a high speed computer with automatic gas sensor calibration. Gas monitoring is a critical feature of all modern tunnels.

Tunnels under construction introduce an additional requirement of permissibility in the potentially explosive conditions while penetrating areas where oil, gas or coal may release combustible gases. Rel-Tek is the only GMS supplier approved by Cal-OSHA, as complying with its unique requirement for both OSHA (Class I, Division 1) and MSHA (DOL, Mine Safety and Health Administration) approvals.

Albert E. Ketler, P.E., is president of Rel-Tek Corp., Monroeville, Pa.