Metro Magazine

AUG 2014

Magazine serving the bus and rail transit & motorcoach operations since 1904

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BUS OPS AUGUST 2014 METRO-MAGAZINE.COM A8 Fast-fll Cascade or Buffer The following typical design will provide maximum reliability and performance for light and medium-duty vehicle applications: • NGV1 or CT5000 fast-fll type connectors (depending on fow requirements). • 3/8-inch or 3/4-inch conductive CNG hose rated at 5,000 PSIG with hose retractors and inline breakaways. • Coalescing flters for each line. • Mass fow meter and electronic display compatible with fuel management systems for each hose. • Electronically controlled priority fll (to control gas fow from compressor to storage) and sequencing (gas fow control from storage to vehicle); or electronically controlled buffer control valve panel. • Electronic temperature compensation adjusts vehicle fll pressures to compensate for ambient temperature changes and "heat of compression," the temperature rise in the ve- hicle tank during a fastfll operation. The system should be designed for a reference pressure of 3,600 PSIG. Station Reliability Considerations • Station electronic controls should be integrated to maximize safety and performance. Remote network monitoring should be provided to reduce downtime and repair costs. • If dedicated CNG vehicles are used, and if no other accept- able local fueling options exist, feets often include one additional (redundant) compressor to ensure that they can meet fueling needs with one compressor down for service. Similarly, station owners also often include standby power (generator) to provide power for half or more of their com- pressors in an extended power outage. Benefts Often Outweigh Costs Design and selection of a CNG fueling station is more involved than gasoline or diesel fueling stations. A properly sized and designed station will reliably serve a feet for many years, al- lowing the feet to comply with federal and local clean air man- dates, while enjoying the benefts of a relatively inexpensive domestic vehicle fuel. Purchasers should check to ensure the machines have been used successfully on CNG applications for several years. If you are using a lubricated compressor, it should utilize synthetic oil that will not vaporize. This oil can be effectively removed from the gas stream using a properly designed flter system. The compressor supplier must ensure that the syn- thetic oil is compatible with any equipment or oil that might be in the system. Air-cooled compressors have proven their durability and are the industry standard. Horizontally opposed designs are proving popular above 50 hp with their inherent balance (low vibration) and ease of ser- vice, and they can be used at very high horsepower applica- tions. Other confgurations for compressors less than 300 hp include V and W cylinder arrangements. A CNG compressor generates 90 to 100 dBA noise at three feet, so it may be necessary to provide an acoustical enclo- sure. A properly engineered enclosure is costly, but it is the most effective noise control on the site and facilitates installa- tion of equipment near property lines or buildings. In climates where temperatures drop below 30°F, enclosures and com- pressors can be equipped with heaters. Higher utility gas service pressure will reduce the compres- sor size, cost and horsepower. See if this is offered by the lo- cal gas utility. The higher the inlet pressure, the fewer stages and the lower horsepower is required to produce the same standard cubic feet per minute (scfm) of fow. For example, based on 100 scfm and 4,500 PSIG discharge pressure, 5 PSIG suction requires fve stages at 60 hp; 100 PSIG suction requires four stages at 36 hp; and 300 psig suction requires three stages at 26 hp. The U.S. CNG industry has standardized on 3,600 PSIG vehicle pressure, which implies dispensed pressures of up to 4,500 PSIG. Compressors should be rated for continuous operation with a discharge pressure of 4,500 PSIG. Most new CNG stations use electric motors to power the compressor rather than the natural gas engines sometimes used in the past. Electric motors are simple, compact, create no on-site emissions, and are very reliable. 3. Gas Storage Gas storage is used on fast-fll stations only. Several confgu- rations — small tubes, large tubes and spheres — are avail- able. Fleets can use cost as the main selection criterion. For cascade stations, storage should be set up in three banks. Storage vessels should have a minimum design pres- sure of 5,500 PSIG —the most common current storage de- sign pressure — and should be equipped with condensate drains. Generously sized storage will improve station perfor- mance and reliability. 4. Gas dispensers Gas dispensers vary widely depending on application; how- ever, the following are general guidelines: Time-fll • NGV1 time-fll type connectors. • 3/8-inch conductive CNG hose rated at 5,000 PSIG — the most common current rating — with hose retractors and inline breakaways. • Coalescing flters for each manifold. This heavy transit bus is flled using a large "5000 series" nozzle. It is strongly advised that heavy vehicles be ordered with both "series 5000" and "NGV1" receptacles. ROB ADAMS is the founder and principal of Marathon Corp. He can be reached at FLEET SOLUTIONS

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