Vehicle Safety for the Natural Gas Engine
Today, a wide variety of vehicles from passenger cars to pickup trucks to school busses operate on one of the most plentiful fuels available, compressed natural gas. There are some very good reasons for this.
Natural gas is one of the most environmentally friendly fuels available. When burned, it produces far fewer air pollutants than conventional gasoline - up to 90 percent less carbon monoxide and up to 85 percent less reactive hydrocarbons - and contains no particulates. Further, it produces far less carbon dioxide (a "greenhouse gas") than conventional fuels. This makes natural gas an ideally friendly fuel for the environment.
Natural Gas Vehicles - The Safe Alternative
Safety is probably among the chief concerns when adding NGVs to a fleet. Although gasoline and diesel seem safer because they have been around for a very long time, natural gas has many characteristics that actually make it safer than these more familiar fuels.
First of all, natural gas is lighter than air, so it will not pool on or near the ground. When accidentally released in an unconfined space, it quickly disperses in the atmosphere, away from the scene of an accident.
Natural gas also has a high ignition temperature, igniting at around 732°C (1,350°F) compared to between 204°C (400°F) and 482°C (900°F) for gasoline and diesel. What's more, natural gas has a narrower flammability range than other fuels including liquid petroleum gas (LPG). Natural gas is only flammable in concentrations above 5.3 percent in air. Gasoline, diesel and LPG, on the other hand, are flammable at much lower concentrations, above 0.5 percent to 2 percent.
The upper limit of the flammability range of natural gas at about 15 percent, is a bit higher than other fuels. Because natural gas disperses quickly it is highly unlikely it will collect in a flammable concentration in the open air.
Natural gas is a safe fuel for other reasons as well. Unlike gasoline or diesel, it is nontoxic, non-caustic and poses no threat to land or water in the case of a spill. Also, several studies relating to the safety of operating NGVs in tunnels, in parking garages and on ferries have shown that natural gas poses no greater hazard than conventional fuels.
But what if natural gas is released in a confined space? Won't it float to the ceiling in a garage, for example? That is a concern. Experts recommend, and regulations in some cases require, that shops servicing NGVs install a methane detector and ensure there are not areas in the service area where natural gas could collect in dangerous concentrations if released. Adequate ventilation is also important, but natural gas's lighter-than-air properties make it easier to ventilate than the heavier vapors of gasoline, diesel and LPG. Check with your gas company or contractor to be certain, but you shouldn't have to augment your existing air-handling system to service NGVs.
Products manufactured for the NGV industry - from under-hood components to fuel-pump nozzles - are governed by a variety of standards to ensure safety. For example, onboard components including pressure regulators, valves, fueling nozzles and connectors, etc., must be certified by International Approval Services, jointly sponsored by American Gas Association Labs and Canadian Gas Association Labs. The National Fire Protection Association (NFPA) has created a safety standard (NFPA 52) for equipment installation and fueling infrastructure. The Uniform Fire Code (Article 52, Standard No. 52-1) also addresses NGV safety issues relating to fueling stations, including fueling indoors. The Compressed Gas Association has numerous standards pertaining to cylinders.
Through several NGV Coalition committees, the industry has created a series of strict voluntary standards and continues to improve and add to these. Standards for fueling connectors and cylinders are already on the books (known as ANSI/NGV1 and ANSI/NGV2, respectively).
As a fleet operator, you needn't become a NGV standards expert, but you can rest assured the industry is working hard to address safety concerns and ensure NGVs perform as safe and safer than conventional vehicles. As you introduce NGVs to your fleet and need to know more about which safety standards apply to your operation, consult your gas company representative. Your gas company probably has several NGVs in its fleet and has worked with local officials on meeting codes. If your local fire marshal isn't familiar with NGVs, your gas company representative in many cases can put him or her in touch with someone who is.
Fuel for NGVs is stored in cylinders under pressure, typically at 20,685 or 24,820 kPa (3,000 or 3,600 psi). To the uninitiated this sounds hazardous just by definition. As one industry report notes, "There is no doubt that carrying a highly pressurized fuel onboard a vehicle represents a `high perceived risk' which simply does not occur on conventionally fueled vehicles." The NGV industry, to its credit, has made addressing any risks, perceived or real, a top priority, and this is particularly evident in the area of storage container (cylinder) safety.
In recent years the industry has developed strict voluntary standards for cylinders in conjunction with governmental bodies such as the American National Standards Institute (ANSI). In 1994, the National Highway Traffic Safety Administration (NHTSA) adopted a set of regulations (known as DOT 304) governing cylinder durability, strength and pressure relief. The regulations also set testing criteria and generally follow the industry's existing ANSI/NGV2 standard. In the future NHTSA will codify additional standards, including cylinder resistance to chemical corrosion.
Cylinders undergo a variety of rigorous tests. In the bonfire test, as its name suggests, a pressurized cylinder is subjected to flames of around 649°C (1,200°F) to determine its resistance to bursting. To pass the test, the cylinder must remain intact long enough to safely release its contents. A pressure-relief device, required by NHTSA rules, vents the contents when the cylinder reaches about 99°C (210°F) to 104°C (220°F), and the cylinder typically remains intact for another 100 degrees beyond that. Cylinders must also pass a pressure test that subjects them to 18,000 cycles of pressurization and depressurization.
Manufacturers also test cylinders for resistance to rough handling and impact by dropping them from a certain height, hitting them with a heavy weight or gouging them. The cylinders are then cycled the equivalent of a million miles of operation. In the gunfire test, manufacturers test the resistance of a cylinder to fragmentation by shooting it. Cylinders have also shown their durability in crash tests featured in informational videos produced by the NGV Coalition and others. In these tests, cylinders are mounted in a vehicle that is then dropped from several feet. The cylinders come through unscathed, even though the car is demolished.
Commenting on cylinder safety, one industry expert notes that the science of designing safe pressurized cylinders is by no means new, and the safety record is excellent. For example, some 1.4 million cylinders have been used by firemen for more than 15 years with only two catastrophic accidents, he notes, and those cylinders, pressurized to 31,025 kPa (4,500 psi), are not mounted in a vehicle but carried on a person's back.
When designing for safety, companies manufacturing fuel dispensers for compressed natural gas typically concentrate on two areas: hardware and what engineers call the "fill routine." For example, some dispenser valves are designed to shut off the flow of fuel if needed or vent the gas away from the dispenser in a closed-loop return system. Dispensers also have what is called "driveway protection." If someone drives away while fueling, a breakaway device on the dispenser ensures that the hose won't rupture or topple the dispenser. By the way, the breakaway force, according to code, for compressed natural gas dispensers is about 195 vs.1112 N (44 vs. 250 lbs) for gasoline pumps.
To guard against overfilling or any other kind of uncontrolled fill, dispensers are designed to monitor, either mechanically or electronically, whether the vehicle is receiving the amount of fuel it should. If the fill pressure rises beyond a predetermined level, the dispenser shuts off automatically. By the same token, if there's no pressure or a sudden loss in pressure - as in the case of an aborted fill or a hose being cut, - it also shuts off.
Natural gas fuel dispensers are designed to be as easy to use and as similar to conventional fuel dispensers as possible, with safety and convenience in mind. With a minimum of training, anyone who can pump gasoline can safely dispense natural gas.
When it comes to dealing with any fuel, it is important that technicians and vehicle drivers learn about safety. Technicians working with liquid fuels have learned to keep them contained, keep them off their skin, and keep them away from any source of ignition. They have learned, too, what to do in the event of a spill or leak.
In the same way, NGV technicians and drivers need to learn how to safely handle a gaseous fuel, natural gas. While working with a gaseous fuel that's under pressure may be no more inherently hazardous than working with a liquid fuel, it is different. To do their jobs safely, your technicians and drivers need to know what the differences are and how to work with them.
Unlike liquid fuels which are simply poured into a fuel tank, CNG fuel systems require a closed fuel dispensing system that allows the natural gas to flow into the fuel cylinder without leaking into the atmosphere. Such a system must use ANSI (American National Standards Institute) NGV1-standard fuel nozzles which will not allow the CNG to flow unless the nozzle is properly connected to the vehicle fuel inlet. Such a system will also comply with the NFPA (National Fire Protection Association) standards for fueling station construction and safety.
The typical commercial natural gas dispenser looks a lot like a gasoline pump. It has a hose that connects easily to the vehicle fuel inlet with a snap-type coupling device and a meter that indicates how much fuel has been pumped. Once the cylinders are full, the pump shuts off automatically.
Most stations can fill fuel cylinders on a "quick fill basis" or on a "timed fill" basis. With a "quick fill" system cylinders can be filled in about five minutes. Stations with "timed fill" systems may take longer, depending on the number of vehicles being fueled and the size or capacity of the fueling unit. Many fleets operate their own timed fill stations because this allows them to fill their vehicles unattended at night.
Home dispensers for natural gas are also available. These are about the size of a window air conditioner and can be hooked up directly to the natural gas supply of the home. Home dispensers normally use the "timed fill" principle so a vehicle can be fueled overnight.
|Series 60 Service Manual - 6SE483|
|Generated on 10-13-2008|