Natural Gas Vehicles Release Characterization and Modeling

////Natural Gas Vehicles Release Characterization and Modeling
Natural Gas Vehicles Release Characterization and Modeling 2018-07-25T18:09:57+00:00

Project Scope

BeamsLeakThe overall project goal is to provide technical assistance to DOE, Clean Cities coordinators, Clean Cities stakeholders, and advanced technology end-users to address these technical challenges and bring advanced transportation technologies to market using Sandia’s depth in applied science and engineering.

Focus Areas

  1. Develop reduced-order engineering models of CNG/LNG /propane release behavior
  2. Evaluate key risk scenarios in order to develop best practices and code revisions
  3. Develop educational materials and inform code committees of risk mitigation strategies


  • Create a robust modeling capability that is capable of simulating current and future alternative fuels
  • Develop and communicate educational material for target audiences
  • Inform NFPA and ICC codes and standards with scientifically based results
Natural gas vehicle (NGV) leak scenarios inside maintenance facilities are investigated utilizing results of HAZOP studies. Currently we have results for heavy duty (large trucks or buses) inside a “typical” garage for this size of vehicle. The project incorporates both HAZOP studies and modeling of leaks.

  • Hazard identification and quantification
    • Conduct HAZOP study to provide a comprehensive list of credible hazard scenarios
  • Scenario modeling of credible releases
  • Development of best practices to mitigate hazards
  • Facility design guidance
  • Proposed changes to existing fire protection codes

Sideleak2.5 LNGWeep

Safety standards development for maintenance facilities of liquid and compressed gas fueled large-scale vehicles is required to ensure proper facility design and operation envelopes. Standard development organizations are utilizing risk-informed concepts to develop natural gas vehicle (NGV) codes and standards so that maintenance facilities meet acceptable risk levels. Phase I work summarizes existing NGV repair facility code requirements and highlights inconsistencies that need quantitative analysis into their effectiveness. A Hazardous and Operability study was performed to identify key scenarios of interest. Scenario analyses were performed using detailed simulations and modeling to estimate the overpressure hazards from HAZOP defined scenarios. The results from Phase I will be used to identify significant risk contributors at NGV maintenance facilities, and are expected to form the basis for follow-on quantitative risk analysis work to address specific code requirements and identify effective accident prevention and mitigation strategies.

Results for Phase I work provided the following observations:

  • Little sensitivity was observed for ventilation or roof supports due to the short durations of the releases relative to the ventilation rates and the propensity of the support structures to enhance mixing .
  • For the low-flow release scenarios that involved a dormant LNG blow-off or a CNG fuel system purge, the flammable masses, volumes, and extents were low, and the flammable regions disappeared shortly after the conclusion of the leaks. Moreover, predicted peak overpressures indicated there was no significant hazard expected.
  • For the larger release, the release plume quickly achieved a nearly steady flammable volume that extended from the release point at the vehicle up to the ceiling, before spreading across the ceiling.
  • No attempt to calculate local blast-wave pressures was performed, which could result in additional overpressures above those described here. However, for the low release cases, the relatively small volumes of the flammable regions mean that there is little opportunity for flame acceleration needed for blast-wave development.
beams-leak-sm Analyses in Support of Risk-Informed Natural Gas Vehicle Maintenance Facility Codes and Standards: Phase II, by Myra L. Blaylock, Chris LaFleur, Alice B. Muna, Brian D. Ehrhart, Sandia National Laboratories, March 2018. SAND2018-2945.
beams-leak-sm Analyses in Support of Risk-Informed Natural Gas Vehicle Maintenance Facility Codes and Standards: Phase I, by Isaac W. Ekoto, Myra L. Blaylock, Christine A. LaFleur, Jeffery L. LaChance, Douglas B. Horne, Sandia National Laboratories, March 2014. SAND2014-2342.
FV_Release_942_max “Analysis of a Full Scale Blowdown Due to a Mechanical Failure of a Pressure Relief Device in a Natural Gas Vehicle Maintenance Facility” by Myra Blaylock, Radoslav Bozinoski, and Isaac Ekoto. Sandia National Laboratories, May 2016. SAND2016-4534.
 SmallGarage_CrossVent2 Presentation at the NGV Technology Forum, October 18 and 19, 2016, in San Diego, California, by Myra Blaylock Sandia National Laboratories, 2016. SAND2016-10561 PE.
 SmallNoVentTubing Presentation at the NGV America Technology Committee Maintenance Facility Working Group, by Myra Blalock, Sandia National Laboratories, 2016. SAND2016-5842 PE.
risk-informed Risk-Informed LNG/CNG Maintenance Facility Codes and Standards, by Chris LaFleur, Myra Blaylock, Rad Bozinoski, Amanda Dodd, Ethan Hecht, Doug Horne, Alice Muña, Sandia National Laboratories, 2015. SAND2015-7361PE.
flow-chart-sm Guideline for Determining the Modifications Required for Adding Compressed Natural Gas and Liquefied Natural Gas Vehicles To Existing Maintenance Facilities, prepared by Douglas B Horne, P.E., Clean Vehicle Education Foundation, 2012.
Clean Cities
Natural Gas Vehicles for America
Maintenance Garage Description

Single volume building enclosure modeled:

30.5 m long (100’)
15.2 m wide (50’)
6.1 m tall (20’)
1:6 pitched roof

Option for horizontal support beams:

investigate accumulation of flammable mixture in discrete pockets
nine beams

Two air circulation vents:

one near the floor on a short building side-wall
second on the opposite side wall near the roof

Natural Gas Vehicle (NGV) modeled as a cuboid:

2.44 m (8’) tall and wide
7.31 m (24’) long


Figure 1. Schematic of the NGV maintenance facility used for the simulations. The roof had a 1:6 pitch and had layouts with and without 9 evenly spaced, horizontal supports. Two circulation vents were located on the smaller building side-walls, with one placed low and the other high to maximize room currents.

Figure 1. Schematic of the NGV maintenance facility used for the simulations. The roof had a 1:6 pitch and had layouts with and without 9 evenly spaced, horizontal supports. Two circulation vents were located on the smaller building side-walls, with one placed low and the other high to maximize room currents.


CNG and LNG Fuel System Line Cracking

Described in SAND2014-2342.

Tubing leak flow (cracking open tubing joint)

Scenario Basis:

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

Scenario Basis:

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.

Scenario Criteria:

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.

Myra Blaylock, Ph.D
Sandia National Laboratories
(925) 294-2775
Dr. A. Christine LaFleur, PE
Sandia National Laboratories
(505) 844-5425