Tunnel Research Projects
The Memorial Tunnel Fire Ventilation Test Program
The Memorial Tunnel Fire Ventilation Test Program (MTFVTP) consisted of a series of full-scale fire tests conducted in an abandoned road tunnel. Various tunnel ventilation systems and configurations of such systems were operated to evaluate their respective smoke and temperature management capabilities. These tests generated a significant database relevant to the design and operation of road tunnel ventilation systems under fire emergency conditions. The need for such a program was identified by members of ASHRAE Technical Committee 5.9 Enclosed Vehicular Facilities (TC 5.9), which formulated the goals and scope of work the Phase I Report. That report recognized our society's increasing emphasis on life safety, but, as yet, the lack of a definitive and generally accepted consensus regarding the proper design and operation of road tunnel ventilation systems in a fire emergency. Further, as a result of significant decreases in vehicle engine emissions over the past three decades, the ventilation rates required to meet contaminant level criteria in road tunnels may not provide adequate ventilation capacity for a fire emergency. The Memorial Tunnel is located near Charleston, West Virginia. This is a two-lane, 2800-foot long, mountain tunnel having a 3.2 percent grade. In preparation for the MTFVTP, the tunnel was modified and instrumented to allow operation and evaluation with the following ventilation system configurations:
- Full Transverse Ventilation
- Partial Transverse Ventilation
- Partial Transverse Ventilation with Single Point Extraction
- Partial Transverse Ventilation with Oversized Exhaust Ports Point Supply and Point
- Exhaust Operation Natural Ventilation
- Longitudinal Ventilation with Jet Fans
The tunnel was equipped with instrumentation and recording equipment for data acquisition. Sensors measuring air velocity, temperature, carbon monoxide (CO), and carbon dioxide (CO2) were installed at various tunnel sections. Data from these sensors were recorded. Smoke generation and movement and the resulting effect on visibility was assessed using seven remote-controlled television cameras with associated recording equipment.
Ventilation system effectiveness in managing smoke and temperature movement was tested for the following fire sizes: 10, 20, 50, and 100 megawatts (MW). The heat release of a 20 MW fire is approximately equivalent to a bus or truck fire, and a 100 MW fire is equivalent to a flammable fuel spill feeding a pool approximately 480 square feet (sq ft) in area.
In addition to varying the fire size, systematic variations were made in airflow quantity, longitudinal air velocity near the fire, and fan response time for each ventilation system. Tests were also conducted to assess the impact of longitudinal air velocities on the effectiveness of a foam suppression system.
In the MTFVTP, various smoke management strategies and combinations of strategies were employed, including extraction, transport, control direction of movement, and dilution to achieve the goals of offsetting buoyancy and external atmospheric conditions and to prevent backlayering. A total of 98 tests were conducted.
The basic findings to date are summarized below:
- The ASHRAE criteria of 100 cubic feet minute per lane-foot (cfm/lf) of tunnel for emergency road tunnel ventilation has been used as a minimum design basis for many years. However, to date there has been no validation of that value. It is clear from the fire tests that while this criteria may be appropriate for some situations, it is not applicable in others, and is therefore limited as a general criteria. The Memorial Tunnel fire ventilation tests have shown that longitudinal airflow near a fire is equally important as extraction rate for temperature and smoke management. Therefore, specifying a ventilation rate for temperature and smoke management, solely on its extraction capabilities, is insufficient. Further, any criteria established for emergency ventilation should include the impact of tunnel physical characteristics and tunnel ventilation system.
- Longitudinal ventilation using jet fans was shown to be capable of managing smoke and heat resulting from heat releases up to 100 MW. The required longitudinal air velocity to prevent backlayering in the Memorial Tunnel was approximately 600 feet per minute (fpm) for a 100 MW fire. Since the longitudinal velocity generated by jet fans will manage temperature and smoke only on one side of the fire, to the detriment of smoke and temperature conditions on the opposite side, such systems should be applied only in road tunnels with uni-directional traffic flow.
- Jet fans positioned downstream of, and close to, the fire were subjected to temperatures high enough to cause failure. Accordingly, this condition needs to be considered in the system design and selection of emergency operational modes.
- Full transverse ventilation systems can be installed in single-zone or multi-zone configurations and can be operated in a balanced or unbalanced mode. Single-zone, balanced (equal flow rates for supply and exhaust air) full transverse systems indicated very limited smoke and temperature management capability. Ventilation-rates of 100 cfrr-l/lf exhaust capacity did not manage conditions resulting from heat release rates of 20 MW and greater, unless the system was operated in an unbalanced mode (reduced supply airflow). Single-zone full transverse systems operated in the unbalanced mode had improved temperature management capability. However, even 100 cfm/lf exhaust capacity provided only limited temperature and smoke management for a 20 MW heat release rate. Multiple zone full transverse systems have the inherent capability to manage smoke and temperature by creating longitudinal airflow.
- Partial transverse ventilation systems can be installed in single-zone or multi-zone configurations and can be operated in supply or exhaust mode. Single-zone partial transverse systems capable of only supplying air (no possible reversal of fans to exhaust air) were relatively ineffective in smoke or temperature management. Single-zone partial transverse systems which can be operated in the exhaust mode provided a degree of smoke and temperature management.
- Longitudinal airflow is a significant factor in the management of smoke and heat generated in a fire. Ventilation systems which effectively combine extraction and longitudinal airflow can significantly limit the spread of smoke and heat. Multiple-zone ventilation systems allow control over the direction and magnitude of longitudinal airflow, and can effectively manage smoke and temperatures in the tunnel. Two-zone partial transverse ventilation with 100 cfm/lf effectively managed 20 MW heat release rates.
- Single point extraction (SPE) is a ventilation system configuration capable of extracting large volumes of smoke from a specific location through large, controlled openings in a ceiling exhaust duct, thus preventing extensive migration of smoke. These openings range from 100 sq ft to 300 sq ft in size and are generally spaced on 300-foot centers along the tunnel. The SPE transverse type system effectively managed smoke and temperature conditions for a 20 MW fire with ventilation rates lower than 100 cfm/lf. SPE systems are applicable to bi-directional traffic flow, with a degree of dependency, however, on the location and spacing of the SPE openings. Smoke and heat being drawn from the fire to the SPE opening could pass over or possibly around stalled traffic and vehicle occupants.
- Oversized exhaust ports (OEP)are a modification to transverse type systems which provides smoke extraction capability in the immediate location of a fire. The concept consists of 30 sq ft oversized exhaust ports spaced approximately 30 feet apart (comparable to normal-sized exhaust port spacing) and designed to fully open when subjected to the heat of a fire. Significant improvement in temperature and smoke conditions were obtained using OEPs relative to the basic transverse ventilation system using conventional size exhaust ports. The OEP enhancement is also applicable to tunnels with bi-directional traffic.
- Natural ventilation resulted in extensive spread of heat and smoke upgrade of the fire. However, the effects of natural buoyancy are dependent on the fire size and the physical characteristics of the tunnel.
- Fan response time, the interval between the onset of a fire and ventilation system activation, should be minimized since hot smoke layers can spread quickly, e.g., up to 1600 to 1900 feet in the initial two minutes of a fire.
- The maximum temperature experienced at the inlet to the central fans (closest location to the fire approximately 700 feet) was 325 F for a 100 MW fire, 255 F for a 50 MW fire, and 225 F for a 20 MW fire.
- The restriction to visibility caused by smoke occurs more quickly than does a temperature high enough to be debilitating. Carbon monoxide (CO) levels near the roadway never exceeded the guidelines established for the Test Program.
- The effectiveness of the foam suppression system was not diminished by operation in strong longitudinal airflow. In the MTFVTP, the system was operated and tested in conditions with 800 fpm longitudinal air velocity.
- Adequate quantities of oxygen to support combustion were available from the tunnel air. The possible increase in fire intensity resulting from the initiation of ventilation did not outweigh the benefits.
Status
The full-scale testing program got underway in September 1993, following the completion of the tunnel retrofitting and the installation of all instrumentation and control systems. The last of 98 tests was completed in March 1995.
Based on the results of these tests, the use of "jet" fans ventilation system for road tunnels may be considered as a viable alternative.