CNG and LNG Fuel System Line Cracking
Described in SAND2014-2342.
Tubing leak flow (cracking open tubing joint)
A natural gas release may occur during the purge of a vehicle fuel system as part of regular operational maintenance on a Compressed Natural Gas (CNG) or Liquefied Natural Gas (LNG) fueled vehicle.
Prior to starting maintenance, a technician purges the remaining natural gas by cracking a ½” tube fitting on the fuel system, on the vehicle side, at a height of 1.0 meter from the floor.
CNG Scenario Criteria:
This video is of a CNG line cracking where the storage volume is 3.3 liters (201 in3) and the storage pressure is 248 bar (3600 psia), which is an overall natural gas fuel system mass of 630 g.
Transient blow-downs were modeled as an isentropic expansion using NETFLOW (SAND2001-8422).
Mechanical Failure of a Thermally Activated PRD (worst case scenario, not necessarily credible)
Described in SAND2016-4534.
Full storage tank blowdown
In the event a Compressed Natural Gas (CNG) vehicle cylinder becomes engulfed in a flame, onboard storage cylinders are protected against excessive pressure buildup by a thermally triggered Pressure Relief Device (PRD) designed to fully open without the possibility for reseat in the event of activation.
Inadvertent actuation due to a mechanical failure would result in a rapid and uncontrollable decompression of all cylinder contents.
Advances such as the use of dual activated valves have been implemented to reduce the likelihood of unintended release, although there remains some nominal risk due to the potential for human error.
The Standards Development Organizations view such a release as a bounding event for hazard potential.
For this scenario, the entire contents of a 700 L (42,700 in3), fully pressurized (250 bar) (3600 psia) CNG cylinder at room temperature (294 K) was released into the NGV maintenance facility.
NETFLOW was used to model the transient blow-down.