Design considerations that allow for gaseous-fueled buses
By Robert R. Adams
Edited by David Hubbard
[NOTE: This is a condensed overview of only a few considerations addressed in a white paper by Robert R. Adams, executive vice president, Marathon Technical Services, Heidelberg, OT, Canada, at the request of the Clean Vehicle Education Foundation, Acworth, GA. His white paper in its entirety is available at www.cleanvehicle.org/technology.]
Over the past 10 years approximately 25 to 35 percent of transit and shuttle bus orders have been for CNG, LNG and hydrogen powered vehicles. One of the most significant costs in implementing gaseous-fueled vehicles can be the cost to retrofit maintenance garages initially designed for diesel and gasoline vehicles.
The extra cost to design and construct fuel-flexible facilities at the outset are relatively small and incremental, and give owners the freedom and flexibility to operate a fleet with varying fuel sources. Many features of a fuel-flexible facility do not represent additional cost items; they are simply rather adaptations of existing equipment or systems.
Marathon Technical Services offers this brief overview of the special considerations required to make bus garages safe for use with lighter-than-air, gaseous-fueled vehicles. Many of these recommendations also provide additional benefits including improved indoor air quality and enhanced personnel comfort and safety.
Conversely, several transit agencies have discovered that retrofitting a facility designed as diesel-only can be extremely expensive. In more than a few cases some determined beforehand a diesel garage retrofit to be cost prohibitive. Some older systems are not acceptable for gaseous fuels, such as a conventional ventilation system. A retrofit may require the complete replacement of the system including all ventilation units and ducting, which would be both costly and disruptive to normal operations. A fuel-flexible garage offers a number of personnel safety and comfort advantages including improved air quality and heating capability.
The prudent approach for the long term for facilities expected to last up to 50 years would be to follow the example set by one large transit agency that operated two functioning CNG garages. Unsure of its future CNG strategy, this agency hedged its bets by designing and constructing its next maintenance facility with fuel-flexible heating, ventilation and electrical systems, while deferring the actual CNG station and indoor fueling component for later construction until they were needed. By keeping its fuel options open the agency paid a small premium up front that would save time and money at a later date.
Another client of Marathon Technical Services recently completed a similar fuel-flexible design. This agency recently took delivery on an order for hybrid buses while protecting its CNG option from a future cost prohibitive retrofit and operational delays.
Marathon Technical Services strongly encourages any design consultant for a transit agency to work with someone with experience with gaseous-bus maintenance facilities to ensure that the plant and equipment are consistent with established precedents.
Before discussing the recommended building design features for CNG, LPG and GH2 buses, it is important to understand what makes these fuels different from gasoline or diesel. CNG, LPG and hydrogen are lighter than air and quickly rise and disperse on escape. Although lighter-than-air fuels have safety advantages, roofs and ceilings facilities must be designed without any unventilated pockets in the ceiling space that could trap gas.
Gasoline and diesel form a pool of liquid with a vapor layer above, and remain flammable or explosive until the leak is manually contained and cleaned up. Natural Gas has a very selective and narrow range of flammability. The mixture of gas in air that will support combustion between 5 and 15 percent natural gas in air by volume. Ratios outside of this range will not support combustion.
Flexible fuel garages must be able to quickly and automatically remove the risk using ventilation to dilute then exhaust any leaked gas.
CNG and GH2 both have an ignition temperature of around 900 to 1,200 degrees Fahrenheit. Where gasoline is approximately 500 to 800 degrees and diesel is less than 500 degrees, the relatively high ignition temperature for CNG and GH2 is an additional safety feature. To ensure a safe environment in the maintenance garage, the surface temperature of equipment that could contact a gas leak is usually limited to 750 degrees.
Structural considerations
Maintenance garages constructed in the 1970s featured T-shaped or waffle-shaped pre-cast concrete roof structures. In either case, this type construction is problematic since it creates pockets where a gas release could collect. Most single story maintenance garages constructed today use a flat roof with open web steel joists in combination with I-beams to support the roof. any accidental gas release can move freely to exhaust fans without pocketing.
Gable roof structures are very cost effective and are ideal for gaseous fuel buses. The tendency is for any leak to naturally channel toward the roof peak where it can be quickly and safely exhausted. An example of this type of structure would be Metropolitan Atlanta Rapid Transit Authority (MARTA) Garage on Perry Boulevard, built for CNG vehicles in 1996.
In the case of either the flat roof using open web steel joists or the gable roof structure, the only structural cost to make the structure CNG compatible would be to allow for additional ventilation units depending on the type of heating and ventilating equipment.
High ceilings are desirable in any transit garage to assist in ventilation. In gaseous fuel garages, high ceilings provide more space for any escaped gas to rise above potential ignition sources and move unobstructed toward the exhaust fans.
While there is no code requirement, Marathon Technical Services recommends the use of rubber roll-up doors equipped with breakaway rails, which are typically more reliable and energy efficient than sectional doors since they open and close more quickly than conventional sectional doors. Roll-up doors also should be equipped to automatically open quickly in the event of a gas leak.
Marathon Technical Services recommend deferring any installation of gas detection equipment until gaseous-fueled buses are in fact in service. There is no economy in installing this system ahead of time.
It is not acceptable to use any heater with a surface temperature greater than 750 degrees. The use of open-flame or high temperature radiant electric or gas heaters in a gaseous fuel bus garage is prohibited.
Marathon Technical Services recommends indirect heating units equipped with heat exchangers to allow heat recapture while providing 100 percent fresh air (no recirculation). The exhaust and intake would be physically separated and in opposite orientation to ensure no exhaust air is reintroduced into the building. This heated makeup air system is required whether the garage is fuel flexible or just diesel. BR