The INL liquefied natural gas research team is working with LNG industry groups and with the U.S. Department of Energy in the early stages of an effort to standardize the receptacles used with vehicle fuel tanks and the nozzles used at LNG refueling stations. The use of LNG as a vehicle fuel presents technological challenges at the filling station different from those associated with conventional fuels. LNG is a cryogenic liquid stored and transferred under pressure, with temperatures typically ranging from -260 to -200°F and pressures ranging from 25 to 250 psi.
In current use, the end of the hose at an LNG filling station is equipped with a special nozzle that mates with a special receptacle on the spout of the vehicle's fuel tank. The assembly includes some means to lock the nozzle to the receptacle for a hopefully leak-tight fit during filling, then unlock for removal of the nozzle with minimum fuel spillage and minimum risk of leakage from either the nozzle or the receptacle.
As the use of LNG in vehicles increases, the need to improve and standardize the associated nozzles and receptacles becomes more urgent. Currently at least three different nozzle/receptacle designs are commercially available: Parker, C.J. Carter (Formerly MOOG), and MVE. Each has advantages and disadvantages, strengths and weaknesses. None is ideal. None is compatible with either of the other two (that is, for example, the Parker nozzle will not mate with the Carter receptacle). The need for standardization lies in the eventual impracticability of having several incompatible configurations in use in the commercial sector.
At present, use of LNG in the field is limited mostly to demonstration vehicles that are part of larger fleets, with the LNG vehicles being refueled at their own home filling station. Upon entering the LNG market, the fleet owner selects one of the available configurations and equips the fleet's vehicles and filling station accordingly. This arrangement, though not ideal, is workable at that level of use, and is typical of an industry in its infant stage. However, as LNG use becomes more widespread to include private vehicles and travel far from the home filling station, the lack of standardization becomes an obstacle.
We look forward to the day when private and commercial LNG-powered vehicles operate and refuel with the same ease and convenience as gasoline- and diesel-powered vehicles do now. That will not happen until nozzles and receptacles are improved and standardized, so that drivers and attendants with minimal training can safely and easily refuel any LNG vehicle at any LNG filling station without the use of special protective clothing or equipment.
Performance requirements are of two types, the first type relating to design improvements and the second type relating to standardization for compatibility of nozzles and receptacles.
Design improvements would produce the following outcomes:
- Reduce/eliminate leaks during the filling operation
- Prevent the nozzle from freezing to the receptacle during filling. Such freezing occurs when moisture in the air condenses on the cold fittings as the very cold LNG passes through the fittings during filling. When this happens, the nozzle cannot be removed until some effort is made to thaw or break the ice.
- Eliminate risk of injury from splashbacks when the nozzle is removed from the receptacle. This phenomenon is typically caused by the release of pressure on the liquid or liquid/vapor mix between the nozzle and the check valve.
- Provide protection from spills and splashbacks to reduce the need for personal protective equipment (gloves, goggles, face shield, overalls, etc.) during the filling operation. An investigation of this issue might show that equipping the nozzles with splash guards and accordion tube shields will be sufficient to address this issue.
- Provide the nozzles with thermal shielding or insulation to permit safe operation without gloves while still protecting the hands from freeze burns. Provide design features that eliminate the need for the operator to place his hands near the discharge area of the nozzle when actuating the locking device.
- Reduce/eliminate leaks at the receptacle after the nozzle is removed. A receptacle design with a redundant check valve might be sufficient to address this issue. Usually the leak is small, releasing a small amount of methane vapor. However, a few times each year nationwide, incidents occur involving large hazardous leaks discharging cold liquid methane.
- Reduce/eliminate leaks from the nozzle after removal from the receptacle. Such leaks are usually caused by malfunction (poor seating or damaged seals) of the nozzle's engagement valve (the valve that opens on engagement with the receptacle and closes on disengagement). Even with the dispenser's pump shut off, cold liquid methane can shoot in a stream from such a leak, driven by the pressure in the hose.
- Prevent freeze lock-up of the swivel at the hose/nozzle connection. The accumulation of frost on the swivel joint during fill-up causes the joint to freeze, making disconnection and handling of the nozzle difficult.
- Incorporate design features that make the locking device easy to operate (not require great strength or grip).
- Keep purchase and maintenance costs reasonable.
- Eliminate the need for frequent repair or replacement of nozzle and receptacle parts.
The research and collaboration efforts would focus on the design improvements listed above, with standardization (compatibility) as a secondary goal. The standardization effort would consider several possible outcomes, including:
- A single standard design for nozzles and receptacles, with joint ownership of the design among the manufacturers.
- A single standard design for receptacles, and standard requirements for nozzles, leaving considerable latitude in nozzle design. Different nozzle designs meeting general standards would all be compatible with the single receptacle. This was the outcome in the recent standardization of nozzle/receptacle designs for compressed natural gas (CNG) vehicles.
- The use of adapters in lieu of standardization to accomplish compatibility, with various incompatible nozzle and receptacle designs meeting the general design requirements for safety, functionality, and ease of use.
The earlier the design is standardized, the less the existing manufacturers and customers have invested in the current designs, and the smaller the impact on the population of LNG vehicles and filling stations already in the field. (After standardization, older vehicles and filling stations will need to be either retrofitted or equipped with adapters). In the present situation, existing manufacturers have an interest in capturing the largest possible market share in competition with the other manufacturers, yet the market is too small for them to justify investing large amounts of private sector money in design improvement research. The standardization effort will be most successful if all the current manufacturers perceive that their best interest lies in participating in the collaboration.