Cng Cylinder Design and Safety

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<p>CNG Cylinders 101NG Transit Users Group MeetingLawrenceville, GA October 27, 2005</p> <p>Livio Gambone, P.Eng.</p> <p>Presentation TopicsCNG Fuel Properties Fuel Tank Technologies (pictures) Service Conditions Tank Design Considerations Testing of Fuel Tanks (videos/pictures) In-Service Performance (pictures) In-Service Inspection</p> <p>CNG Fuel Properties</p> <p>CNG Fuel ExperienceLarge-scale use since 1960s Some 3,500,000 CNG vehicles now in operation worldwideMostly in Italy, Argentina, Brazil, Pakistan, etc. as lower cost fuel Growing rapidly for transit operations in Europe as lower emission fuel</p> <p>Some 7,500 fill stations</p> <p>CNG Fuel Temperature EffectsTypically stored at 3,600 psi at 70FIf ambient temperature goes up or down, pressure also correspondingly goes up or down4567</p> <p>3600</p> <p>Pressure (psi)1805</p> <p>-40</p> <p>Temperature (F)</p> <p>70</p> <p>130</p> <p>CNG Fuel FillingDuring filling, gas heats up as it compresses in the tanks Typically, stations only fill to service pressure of 3,600 psiEnd up with 3,600 psi at some elevated temperature (say 100F) in the tanks As gas cools to ambient (say 70F), pressure of gas decreases End result is less gas - instead of having a fill of 3,600 psi at 70F, one has say 3,400 psi at 70F</p> <p>CNG Filling Full FillsTo prevent underfills 2 approaches:Tanks can be slowly filled to allow heat to dissipate Tanks can be pressured beyond service pressure, i.e. fill so that one gets higher pressure at a higher temperature, thus cooling to 3,600 psi at 70F Tanks actually designed to be filled up to 1.25 times service pressure (all qualification testing done at 1.25 times)</p> <p>Fuel Tank Technologies</p> <p>Fuel Tank Technologies4 basic types of tank designs Which design to use depends on need to reduce weight and how much can pay All designs have equivalent safety, as all meet requirements of same standards Design type can also determine how a tank may be handled, and how it may be filled</p> <p>Type 1 &amp; Type 2 Tank DesignsType 1 - All metal (aluminum or steel)Cheap but heavy</p> <p>Type 2 - Metal liner reinforced by composite wrap (glass or carbon fiber) around middle (hoop wrapped)Liner takes 50% and composite takes 50% of the stress caused by internal pressurization Less heavy, but more cost</p> <p>Type 3 Tank DesignMetal liner reinforced by composite wrap around entire tank (full wrapped)Liner takes small amount of the stress Light-weight, but expensive</p> <p>Type 4 Tank DesignPlastic gas-tight liner reinforced by composite wrap around entire tank (full wrapped)Entire strength of tank is composite reinforcement Light-weight, but expensive</p> <p>Service Conditions</p> <p>Service ConditionsRoad conditions present a very severe environment for pressure vesselsTemperature extremes (-40F to +185F in vehicles) Multiple fills (pressure changes) = fatigue cracking Exposure to road environments and cargo spillage Vibration Vehicle fires Collision</p> <p>Standards require tests or installation requirements for all these conditions</p> <p>Tank Design Considerations</p> <p>Tank Design ConsiderationsLimited to life of vehicleAlternative is overdesign to last forever</p> <p>Leak-Before-Break so that if tank stays in service beyond design life, and experiences excessive fill cycles, will only fail by leakage Fire protection provided by thermally-activated pressure relief device (PRD) protecting every tank</p> <p>Testing of Fuel Tanks</p> <p>CNG Tank StandardsAll CNG Vehicle Fuel Containers MUST meet the federal governments FMVSS 304 (49 CFR 571.304), Compressed Natural Gas Fuel Container Integrity. All CNG Vehicle Fuel Containers SHOULD meet ANSI/CSA NGV2, Basic Requirements for Compressed Natural Gas Vehicle Fuel Containers. This industry standard is more comprehensive and up-to-date than FMVSS 304.Courtesy H. Seiff, CVEF, CNG Cylinder Inspection Requirements, NG TUG, Anaheim, CA, Nov.18, 2004</p> <p>Performance TestingQualification tests required by standards to ensure tanks and components will perform safely when subject to automotive service conditions. Automotive OEM will perform additional tests to ensure the durability of the fuel storage system.</p> <p>Hydraulic Pressure Cycle TestingTest using water instead of gas (easier to pressure cycle) Failure mode must be leak, not rupture</p> <p>Low Temperature Pressure CyclingHydraulic pressure cycle test while the tank is chilled to -40F Tank then heated to 149F followed by more pressure cycle testing</p> <p>Drop Impact Testing</p> <p>Appearance of Impact Damage After Drop TestCarbon fiber composite Type 4 design Drop test performed with tank EMPTY (most severe condition) Difficult to visually detect</p> <p>Drop Test Failure During Pressure Cycle Testing</p> <p>At location of impact damage, tank bursts during pressure cycling = failure to meet test requirements</p> <p>Bonfire Test of Hydrogen Tankto assure gas will vent before cylinder ruptures when exposed to fire</p> <p>Environmental Exposure TestMultiple Type 3 tanks sitting in road salt bath and exposed to various concentrated solutions</p> <p>White pads contain battery acid, fertilizer solution, gasoline, etc.</p> <p>While exposed to solutions, tanks are also being pressure cycled with fluid to simulate filling and emptying</p> <p>Environmental Test FailureCracking of glass fiber by acid environments note that the acid passed through the protective coatings being evaluated in an attempt to protect the glass fibers</p> <p>CNG Permeation Test</p> <p>Type 4 tank inserted in sealed chamber to measure amount of CNG that permeates through plastic liner over time</p> <p>Gunfire Test of CNG Tank</p> <p>Damage Tolerance Gunfire TestType 3 composite tank First bullet made 75 mm cut in carbon fiber and exposed aluminum liner Second bullet caused the release of the tanks hydrogen gas</p> <p>Vibration of Vehicle Fuel System</p> <p>Hydraulic Crush Test (150,000 kgf)Type 3 steel tank Used hydraulic ram to attempt crush of pressurized tank Test ended at 150,000 kgf when reinforced concrete wall on opposite side of ram broke</p> <p>2 Ton Drop Impact on Pressurized Tanks</p> <p>In-Service Performance</p> <p>Abrasion DamageType 2 steel composite hoop wrap tank Tank dragged on road under vehicle after support strap broke No effect on burst strength</p> <p>Abrasion DamageType 4 composite tank Tank dragged on road under vehicle after support strap broke No effect on burst strength</p> <p>CNG Vehicle Collision Type 1 Steel TankVehicle impacted at 50 mph by gasoline vehicle Impact ruptured gasoline tank causing fire (note scorch marks on Type 1 tank Tank PRD vented the CNG</p> <p>Collision DamageType 4 composite tank The Civic was crushed to the B pillar behind the drivers seat The driver walked away. There was no leak or rupture of the natural gas fuel tank or system</p> <p>Collision DamageType 4 composite tank Tank was mounted on CNG bus roof The bus impacted a low overhang, collapsing the roof of the bus Tank exceeded minimum burst pressure with sustained damage</p> <p>Bus Overpass Impact Type 4 TanksTanks at full pressure Impact collapsed roof and caused severe abrasion to outside tanks Center tank punctured (1 dia. hole) releasing CNG without further incident</p> <p>CNG Bus Fire</p> <p>CNG Bus Fire Caused by Engine</p> <p>Type 2 tanks Note the tanks are intact PRDs activated and safely released gas</p> <p>In-Service Inspection</p> <p>Label Requirements (S7.4, FMVSS 304)</p> <p>Each CNG fuel container shall be permanently labeled with the information specified in paragraphs (a) through (h) of this section..</p> <p>(g) The statement: This container should be visually inspected after a motor vehicle accident or fire and at least every 36 months or 36,000 miles, whichever comes first, for damage and deterioration.Courtesy H. Seiff, CVEF, CNG Cylinder Inspection Requirements, NG TUG, Anaheim, CA, Nov.18, 2004</p> <p>Periodic In-Service Inspection Requirements (Sec. 4.1.4, NGV2)</p> <p>Each container shall be visually inspected at least every 36 months, or at the time of any re-installation, for external damage and deterioration.The inspection shall be performed by a qualified container inspector in accordance with (1) the manufacturers recommendations and (2) the inspection procedures provided in CGA pamphlet C-6.4</p> <p>Courtesy H. Seiff, CVEF, CNG Cylinder Inspection Requirements, NG TUG, Anaheim, CA, Nov.18, 2004</p> <p>Stress Corrosion Cracks in Glass Fiber Composite</p> <p>Caused by exposure to acid environments Susceptible glass fiber types no longer used in tank designs</p> <p>Impact DamageImpact damage on carbon fiber difficult to detect Obvious if tanks have been directly impacted in a collision Safest answer is to replace impacted tanks</p> <p>Liner Rust StainsType 2 design Composite wrap has hoop cracks allowing moisture through to surface of steel liner Rust from steel liner bleeding to the surface Hoop cracks in Type 2 designs do not affect composite strength, but may result in other problems</p> <p>Aluminum Corrosion DamageType 2 tanks with exposed aluminum heads Covered by steel end brackets = galvanic corrosion</p> <p>Composite Wrap Burn Damage</p> <p>Mounting Bracket Vibration DamageMounting brackets with insufficient rubber pad isolating steel from contact with glass fiber</p> <p>Mounting Bracket Vibration Damage</p> <p>Crack in glass fiber caused by impact of steel mounting bracket on surface</p> <p>Thank You!Contact Information:Livio Gambone, P.Eng. Manager, Vehicle Programs Powertech Labs Inc. 12388 88th Avenue Surrey, B.C. V3W 7R7 Email: livio.gambone@powertechlabs.com</p>