Cng Cylinder Design and Safety
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Transcript of Cng Cylinder Design and Safety
CNG Cylinders 101
NG Transit Users Group MeetingLawrenceville, GA
October 27, 2005
Livio Gambone, P.Eng.
Presentation Topics
CNG Fuel PropertiesFuel Tank Technologies (pictures)Service ConditionsTank Design ConsiderationsTesting 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 fuelGrowing 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 70ºFIf ambient temperature goes up or down, pressure also correspondingly goes up or down
1805
3600
4567
Pressure (psi)
Temperature (F)-40 70 130
CNG Fuel – Filling
During filling, gas heats up as it compresses in the tanksTypically, stations only fill to service pressure of 3,600 psi
End up with 3,600 psi at some elevated temperature (say 100ºF) in the tanksAs gas cools to ambient (say 70ºF), pressure of gas decreasesEnd 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 designsWhich design to use depends on need to reduce weight and how much can payAll designs have equivalent safety, as all meet requirements of same standardsDesign 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 pressurizationLess 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 stressLight-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 reinforcementLight-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 crackingExposure to road environments and cargo spillage VibrationVehicle firesCollision
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 tankFirst bullet made 75 mm cut in carbon fiber and exposed aluminum linerSecond 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 vehicleImpact ruptured gasoline tank causing fire (note scorch marks on Type 1 tankTank 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 tankTank was mounted on CNG bus roofThe bus impacted a low overhang, collapsing the roof of the busTank 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 tanksCenter 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 recommenda-tions 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 detectObvious if tanks have been directly impacted in a collision Safest answer is to replace impacted tanks
Liner Rust Stains
Type 2 designComposite 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 ProgramsPowertech Labs Inc.12388 – 88th AvenueSurrey, B.C. V3W 7R7
Email: [email protected]