As the fleet of LNG tank ships continues to grow, LNG’s prospects are looking robust in other transportation sectors too. As the drive towards good environmental stewardship continues to intensify, Liquefied Natural Gas (LNG) has been making great strides as the cleaner burning “fuel of choice” for shipping and transport.
In part two of this series we explore two types of couplings used in LNG transport, the myth of LNG as a highly dangerous fuel and its future expansion into other transportation sectors—and beyond.
By Sean Andersen, LNG Specialist, Dixon
Cryogenic dry disconnect couplings
The self-sealing valve design of cryogenic dry disconnect couplings is based on a style that has been used with fuel oil and chemicals for a half-century. Cryogenic dry disconnect couplings consist of a tank unit or adapter with a spring-loaded poppet and hose unit or coupler with a valve driven by an internal cam.
The coupling procedure is simple: align the rollers on the coupler with the notches on the adapter, push the coupler onto the adapter and then rotate past 100 degrees. This procedure locks the coupling together, creates a self-seal, and opens the internal valves for full flow with low pressure drop.
The dual poppet design shut-off mechanism seals liquids and gases safely behind the valve, thereby eliminating fugitive emissions as well as the danger of a spill of LNG fluid. While the operation of dry disconnect couplings is quite intuitive, it is important that proper personal protective equipment (PPE) be worn during the bunkering process, conforming to standards set by ISO/ DTS 18683:2015; standards established by bodies such as the International Code of Safety for Gas-Fueled Ships (IGF), plus organizations such as the Pipeline and Hazardous Materials Safety Administration (PHMSA), the Society for Gas as a Marine Fuel (SGMF), and the U.S. Coast Guard.
Cryogenic breakaway couplings
Cryogenic breakaway couplings use a closure mechanism similar to the dry cryogenic coupling, with an optimized poppet designed to close off and stop the liquid flow while containing the LNG in the hose and pipe.
The breakaway couplings have three external break bolts. In the case of axial tension, all of the bolts will take up the force corresponding to the break force on the hose. Nonaxial forces concentrate the tension forces more strongly on one bolt, so that the breakaway coupling reacts in a natural way to the reduction of the hose break forces.
Cryogenic breakaway couplings are available in two additional designs, depending on the layout of the bunkering System:
- Cable release cryogenic breakaway couplings are designed to minimize fuel spills or damage associated with inadvertent drive-away or pullaway incidents. They are activated by a cable that limits the stress that can be applied to the hose or to the loading equipment. A cable release guards against damage happening to loading arms or other piping and equipment, which might otherwise happen due to the force of the breaking bolts.
- Powered emergency release couplings (PERCs) incorporate the ability to actively and remotely release the coupling “ondemand” without causing any strain on the transfer system. The release of the coupling happens by injecting high-pressurized nitrogen into a dedicated chamber in the coupling body, resulting in instantaneous release of the PERC. The PERC will also activate and respond to stress on the loading system, thereby giving it both “active” and “passive” security and safety characteristics.
The PERC option is substantially more complex than other breakaway coupling options because of the power pack, activation equipment and other components that are necessary to accomplish PERC’s “on-demand” breakaway functionality. But for some operators, the added expense is worthwhile based on how their bunkering operations are set up, as well as the need for safety.
With all breakaway couplings, repair kits are available that enable the bunkering operation to be brought back online quickly – usually within an hour’s time. Commonly referred to as a “breaking bolt” and “seal kits,” they include new breaking bolts that are installed following draining of the line, replacing the seals, and then pushing the two hose assembly halves back together.
In fuel bunkering, the smaller the hose assembly, the easier it is to maneuver the powered systems due to their lighter weight and lower connection force requirements. Also affecting the flow rate is the choice of hose size and the breakaway coupling, which is why engineers consider both in tandem when designing their system.
To read the full article by Sean Andersen, please contact the Editor.