CNG Cylinders 101 NG Transit Users Group Meeting Lawrenceville, GA October 27, 2005 Livio Gambone, P.Eng.
Presentation Topics CNG Fuel Properties Fuel Tank Technologies (pictures) Service Conditions Tank Design Considerations Testing of Fuel Tanks (videos/pictures) In-Service Performance (pictures) In-Service Inspection
CNG Fuel Properties
CNG Fuel Experience Large-scale use since 1960 s Some 3,500,000 CNG vehicles now in operation worldwide Mostly in Italy, Argentina, Brazil, Pakistan, etc. as lower cost fuel Growing rapidly for transit operations in Europe as lower emission fuel Some 7,500 fill stations
CNG Fuel Temperature Effects Typically stored at 3,600 psi at 70ºF If ambient temperature goes up or down, pressure also correspondingly goes up or down 4567 3600 Pressure (psi) 1805-40 Temperature (F) 70 130
CNG Fuel Filling During filling, gas heats up as it compresses in the tanks Typically, stations only fill to service pressure of 3,600 psi End up with 3,600 psi at some elevated temperature (say 100ºF) in the tanks As gas cools to ambient (say 70ºF), pressure of gas decreases End result is less gas - instead of having a fill of 3,600 psi at 70ºF, one has say 3,400 psi at 70ºF
CNG Filling Full Fills To 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 70ºF Tanks actually designed to be filled up to 1.25 times service pressure (all qualification testing done at 1.25 times)
Fuel Tank Technologies
Fuel Tank Technologies 4 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
Type 1 & Type 2 Tank Designs Type 1 - All metal (aluminum or steel) Cheap but heavy 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
Type 3 Tank Design Metal liner reinforced by composite wrap around entire tank ( full wrapped ) Liner takes small amount of the stress Light-weight, but expensive
Type 4 Tank Design Plastic gas-tight liner reinforced by composite wrap around entire tank ( full wrapped ) Entire strength of tank is composite reinforcement Light-weight, but expensive
Service Conditions
Service Conditions Road conditions present a very severe environment for pressure vessels Temperature extremes (-40ºF to +185ºF in vehicles) Multiple fills (pressure changes) = fatigue cracking Exposure to road environments and cargo spillage Vibration Vehicle fires Collision Standards require tests or installation requirements for all these conditions
Tank Design Considerations
Tank Design Considerations Limited to life of vehicle Alternative is overdesign to last forever 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
Testing of Fuel Tanks
CNG Tank Standards All CNG Vehicle Fuel Containers MUST meet the federal government s 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
Performance Testing Qualification 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.
Hydraulic Pressure Cycle Testing Test using water instead of gas (easier to pressure cycle) Failure mode must be leak, not rupture
Low Temperature Pressure Cycling Hydraulic pressure cycle test while the tank is chilled to -40ºF Tank then heated to 149ºF followed by more pressure cycle testing
Drop Impact Testing
Appearance of Impact Damage After Drop Test Carbon fiber composite Type 4 design Drop test performed with tank EMPTY (most severe condition) Difficult to visually detect
Drop Test Failure During Pressure Cycle Testing At location of impact damage, tank bursts during pressure cycling = failure to meet test requirements
Bonfire Test of Hydrogen Tank to assure gas will vent before cylinder ruptures when exposed to fire
Environmental Exposure Test Multiple Type 3 tanks sitting in road salt bath and exposed to various concentrated solutions White pads contain battery acid, fertilizer solution, gasoline, etc. While exposed to solutions, tanks are also being pressure cycled with fluid to simulate filling and emptying
Environmental Test Failure Cracking 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
CNG Permeation Test Type 4 tank inserted in sealed chamber to measure amount of CNG that permeates through plastic liner over time
Gunfire Test of CNG Tank
Damage Tolerance Gunfire Test Type 3 composite tank First bullet made 75 mm cut in carbon fiber and exposed aluminum liner Second bullet caused the release of the tank s hydrogen gas
Vibration of Vehicle Fuel System
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
2 Ton Drop Impact on Pressurized Tanks
In-Service Performance
Abrasion Damage Type 2 steel - composite hoop wrap tank Tank dragged on road under vehicle after support strap broke No effect on burst strength
Abrasion Damage Type 4 composite tank Tank dragged on road under vehicle after support strap broke No effect on burst strength
CNG Vehicle Collision Type 1 Steel Tank Vehicle 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
Collision Damage Type 4 composite tank The Civic was crushed to the B pillar behind the driver s seat The driver walked away. There was no leak or rupture of the natural gas fuel tank or system
Collision Damage Type 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
Bus Overpass Impact Type 4 Tanks Tanks at full pressure Impact collapsed roof and caused severe abrasion to outside tanks Center tank punctured (1 dia. hole) releasing CNG without further incident
CNG Bus Fire
CNG Bus Fire Caused by Engine Type 2 tanks Note the tanks are intact PRDs activated and safely released gas
In-Service Inspection
Label Requirements (S7.4, FMVSS 304) Each CNG fuel container shall be permanently labeled with the information specified in paragraphs (a) through (h) of this section.. (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
Periodic In-Service Inspection Requirements (Sec. 4.1.4, NGV2) 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 manufacturer s recommendations and (2) the inspection procedures provided in CGA pamphlet C-6.4 Courtesy H. Seiff, CVEF, CNG Cylinder Inspection Requirements, NG TUG, Anaheim, CA, Nov.18, 2004
Stress Corrosion Cracks in Glass Fiber Composite Caused by exposure to acid environments Susceptible glass fiber types no longer used in tank designs
Impact Damage Impact damage on carbon fiber difficult to detect Obvious if tanks have been directly impacted in a collision Safest answer is to replace impacted tanks
Liner Rust Stains Type 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
Aluminum Corrosion Damage Type 2 tanks with exposed aluminum heads Covered by steel end brackets = galvanic corrosion
Composite Wrap Burn Damage
Mounting Bracket Vibration Damage Mounting brackets with insufficient rubber pad isolating steel from contact with glass fiber
Mounting Bracket Vibration Damage Crack in glass fiber caused by impact of steel mounting bracket on surface
Thank You! Contact Information: Livio Gambone, P.Eng. Manager, Vehicle Programs Powertech Labs Inc. 12388 88 th Avenue Surrey, B.C. V3W 7R7 Email: livio.gambone@powertechlabs.com