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: livio.gambone@powertechlabs.com