New Generation Carbon Filament-Wound Composite Cylinders for SCBA

Filament-wound composite cylinders have been authorized and used for commercial high-pressure gas storage applications since 1976, when glass filament-reinforced and Kevlar® filament-reinforced aluminum design and construction was first authorized for production by the U.S. Department of Transportation. Over the years, more than three million composite cylinders have been produced, mainly from S-glass and Kevlar®-49, and used for a wide variety of applications, including self-contained breathing apparatus (SCBA) air cylinders for the fire service; home oxygen therapy; aircraft evacuation system/helicopter flotation system inflation; compressed natural gas fuel storage for buses, trucks, and automobiles; medical oxygen and specialty gas storage; industrial compressed gas storage; and various pneumatic systems.

Starting in 1994 in Europe and 1997 in the United States and Japan, new generation composite cylinders using carbon fibers received for the first time the necessary government approvals authorizing their commercial use. The industry produces approximately 300,000 composite cylinders per year. Significant changeover of many cylinder products from S-glass and Kevlar® to carbon design and construction is in process by users to achieve the significant weight savings at economical costs provided by carbon composites. Every day, thousands of firefighters, haz-mat teams, and other emergency responders use SCBA with composite material air cylinders to enter what otherwise would be an unsafe environment. The very lightweight carbon composite cylinder imparts physiological advantages to firefighters, including reduced air usage together with comfort and portability.

The following article documents the development of carbon composites for worldwide SCBA use; government authorizations; standards; designs; construction materials; qualification procedures; damage, abuse, and environmental resistance; and care, maintenance, and reinspection recommendations.

SAFETY REGULATIONS AND TECHNOLOGY

The safe transport and use of high-pressure gases (above 900 psi) are regulated by governments of most countries around the world. In the United States, there are an estimated 70 million U.S. DOT-regulated high-pressure cylinders (42 million steel, 25 million aluminum, and three million composite), 200 million refillable DOT-regulated cylinders (not high pressure), and two billion nonrefillable DOT regulated cylinders.1

With the large number of cylinders, there are literally billions of fillings and shipments of pressurized cylinders each year. One single incident can be catastrophic. The compressed gas industry has a very impressive safety record established over 85 years. The underlying reason for the impressive safety record is strict control by regulation of design and manufacture, control of use by training personnel in the procedures to ensure safety, and regular reinspection of high pressure cylinders. (1)

High-pressure steel cylinder manufacturing began in the United States in 1907. Since then, steel strength level and manufacturing were progressively improved to result in today’s high-performance products. Lightweight aluminum cylinders were introduced in 1972, followed by even lighter weight composites in 1975: S-glass/epoxy hoop wrapped and full wrapped seamless aluminum, and Kevlar®/epoxy full wrapped seamless aluminum. These first commercial DOT-authorized composite cylinders were a result of aerospace technology development and NASA’s space program technology utilization initiatives with industry and the DOT.2-4


The new generation carbon full wraps with internal seamless, thin-walled aluminum liners are the lightest weight composite cylinders available, having only 34 percent the weight of equivalent aluminum or steel cylinders (see Figure 1); they are 15 to 30 percent lighter than comparable S-glass and Kevlar® cylinders. Carbon composite cylinder authorizations by the governments were a direct result of the excellent use and application experience with S-glass and Kevlar® composite cylinders over 20 years and the aerospace database developed during 1986-1994 for very high-performance carbon filament-wound rocket motor cases, military aircraft life support and pneumatic power sources, strategic defense initiative and missile system applications, and launch vehicle and spacecraft pressurant tanks.5-7

Compared with S-glass and KevlarT, carbon full wraps have the highest strength and stiffness; lightest weight; excellent resistance to moisture, corrosive chemical attack, ultraviolet radiation, cyclic fatigue, and stress rupture; and good damage tolerance but poor durability, requiring an outer glass/ epoxy protective layer. A special attention area is the need for a barrier between the carbon composite and aluminum liner to prevent galvanic corrosion. Carbon cylinders are more costly because (1) the carbon fiber raw material is higher in cost than glass or Kevlar®, and (2) the additional protective covering of glass/epoxy must be applied to the outside surface.

CARBON COMPOSITE CYLINDER

AUTHORIZATIONS AND STANDARDS

The first government carbon cylinder authorization for commercial use was issued by the Health and Safety Executive (HSE) of the United Kingdom in January 1994. The lightweight cylinders are used primarily for firefighter SCBA air storage at 207 or 300 bar (3,000 or 4,350 psi). Since 1994, European re-gion authorizations have spread to include 19 countries (Austria, Belgium, Czech Re-public, Denmark, Finland, Germany, England, Hungary, Iceland, Ireland, Luxembourg, Netherlands, Norway, Poland, Portugal, Russia, Spain, Sweden, and Switzerland).


Luxfer received the first-ever government authorizations granted for commercial carbon cylinder production in the United States from the DOT in January 1997 and in Japan from MITI/KHK in September 1997. The immediate application was for life support air cylinders for fire service SCBA (see Figure 2) at 153 bar (2,216 psi), 207 bar (3,000 psi), and 310 bar (4,500 psi). Authorizations have been obtained throughout the Asia-Pacific region including Korea, Taiwan, China/Hong Kong, Australia, and New Zealand. Authorizations have been now granted in South America and Canada.

The primary European standard for carbon composites is the United Kingdom’s HSE-AL-FW2 and its derivatives used on the continent in the 18 other countries that have authorized carbon composites. The European Committee for Standardization (CEN) is developing a composite cylinder specification to harmonize the European requirements.

In the United States, the DOT standard required by regulation is DOT-CFFC and the manufacturer’s exemption (DOT-E 10915 for Luxfer Gas Cylinders).

Under United Nations sponsorship, the International Standards Organization (ISO) and delegates from many countries including the United States are developing a worldwide composite cylinder specification for adoption by the UN.

Focusing on U.S. DOT-CFFC requirements (for example), this standard establishes strict regulatory controls for size, service pressure, service life, inspection, authorized materials, design, manufacturing, and production lot qualification.

null

EXPERIMENTAL RESULTS

Figure 3 compares DOT-CFFC’s qualification requirements for carbon to DOT-FRP-1 (first draft issued in 1981) for glass and KevlarT composite cylinders. DOT-CFFC is a more difficult test series and requires a higher factor of safety and larger sample size for qualification tests and defines required residual burst pressures after test condition exposures.

All authorized DOT-CFFC must meet requirements as specified in Figure 3.

A wide range of extreme condition tests were performed to demonstrate the safety, reliability, damage tolerance, durability, and environmental resistance of carbon composite cylinders. The tests imposed conditions much more severe than specified in the regulations and standards (DOT, HSE, CEN) and were intended to simulate worst possible scenarios of SCBA air cylinder in-service use/abuse prior to obvious detection in normal reinspections. The objectives were to provide evidence ensuring safety in use and reliability in long-term service.

Reference burst tests were performed on undamaged cylinders to establish the baseline. Angle Iron Drop Tests involved dropping cylinders from a height of 3 m (10 feet) horizontally onto the angle corner to create localized damage. For Flaw Testing, local flaws were cut into the cylinder sidewall to 50 percent of composite thickness by length five times thickness [typical flaw dimensions for a 307 bar (4500 psi) cylinder were depth of 2.2 mm (0.086 in) and length of 11 mm (0.43 in)]. Multiple Drop Tests from a height of 1.2 m (4 feet) were performed in five different orientations and positions with cylinders half-filled with water. Extended Drop Tests consisted of 100 drops from 1.2 m (4 feet) to simulate SCBA “quick removal” by dropping. The Drag Tests consisted of dragging cylinders on an asphalt surface by truck and cable for 1.6 km (1.0 mile) at approximately 16.1 kph (10 mph) to simulate “worst-case” abrasion damage. Residual burst pressures were in accordance with specification requirements.

The effect of maximum allowable defect damage (MADD) on cylinder strength was evaluated. Verification testing involves machining off the MADD thickness from the outside surface of the cylinder, then subjecting the cylinder to 10,000 operating pressure cycles, 30 test pressure cycles, plus burst testing to determine residual strength. The results were no significant burst test strength reduction and the required 3.4 minimum burst factor of safety being retained in all cases. Typical MADD values are 0.70-0.90 mm (0.027-0.035-in) for cuts, scratches, gouges, and abrasion.

Three different submersion soak tests were performed in 10 to 15 percent H2S04 (sulfuric acid): 72 hours, 72 hours with cylinders at service pressure, and 72 hours with 10,000 service pressure cycles applied during exposure. There was no effect on cylinder burst pressure of the cylinders after exposure.

To demonstrate effectiveness of the galvanic protection barrier between the aluminum liner and overwrapped carbon/epoxy composite, accelerated five percent salt solution in water submersion corrosion tests with thermal and pressure cycling were performed. Three liner conditions were evaluated: no barrier, epoxy barrier, and polyurethane barrier. In burst testing after exposure, there was no strength degradation compared with unexposed cylinders. Post test examination showed that cylinder liners with polyurethane coating had no corrosion, with epoxy coating had minor (insignificant) corrosion, and with no coating exhibited some corrosion.

A series of tests were performed on carbon cylinders in accordance with MIL-STD-810: Humidity (Method 507.3, Procedure II); Fungus (Method 509.3, Procedure I); Salt Fog (Method 510.3, Procedure I); Sand and Dust (Method 510.3, Procedure I); and Rain (Method 506.3, Procedure I). Following the exposures, there was no evidence of unacceptable condition, damage, or degradation; in burst testing after exposure, there was no significant loss of strength. The required burst pressure level was achieved in each test.

A cylinder was exposed to below water freezing temperature and an ice shell with minimum thickness of 3 mm (1/8 in) formed all over the cylinder (see photo 2 on page TK). The cylinder was held at 140°C (140°F) for one hour then warmed to room temperature to melt the ice; this cycle was repeated 10 times with no deleterious effects or reduction in burst pressure compared with unexposed cylinders.

Cylinders were mounted in a rack and submerged in the North Sea for six months, with no carbon cylinder strength reduction.

Cylinders were subjected to (1) seven-day immersion in hot 76°C (170°F) chlorinated hydraulic fluid (SKYDROL); (2) immersion in 76°C (170°F) alkaline cleaner (MacDermid Formula 28) for one hour followed by 24-hour drip drain in air; (3) 24-hour immersion in 76°C (170°F) JP4 fuel followed by drip drain in air; and (4) 24-hour immersion in vodka, orange juice, and clear cola. In all cases, there was no evidence of structural degradation, deformation, resin softening, stickiness, or loosening of the composite material or reduction in burst pressure from the reference unexposed cylinder test results.

Tests were conducted in accordance with TSO – C69 and FAR 25.853 to demonstrate required self-extinguishing characteristics, low level of smoke release, and toxic gas release.

CARE, MAINTENANCE, AND REINSPECTION


Cylinder undergoing icing testing.

Carbon composite cylinders must be maintained and inspected regularly. DOT-CFFC DOT-E 10915 specifies a service life of 15 years, with a potential for service life extension approval if certain conditions are met. Each cylinder must be reinspected and hydrostatically retested every three years. References 8 and 9 provide guidelines for visual inspection and requalification.

References

1. C.T. Johnson, “High Pressure Gas Cylinder Safety,” Natural Gas Vehicle Coalition Symposium, June 1994.

2. E.E. Morris, “Improved Fireman’s Compressed Air Breathing System Pressure Vessel Development Program,” NASA Contract 9-12414, Report DRL-T-701, August 1973.

3. E.E. Morris, “Composite Pressure Vessels for Aerospace and Commercial Applications,” ASME Composites in Pressure Vessels and Piping Publication PVP-PB-021, September 1977.

4. E.E. Morris, “Filament-Wound Composite Pressure Vessels in Transportation Applications,” 28th National SAMPE Symposium, 28, April 1983.

5. E.E. Morris, “Lighter Weight Fiber/Metal Pressure Vessels Using Carbon Overwrap,” AIAA Paper 86-1504, 1986.

6. E.E. Morris, “Lower Cost, High-Performance Composite Fiber/Metal Tanks for Spacecraft,” AIAA Paper 86-1694, 1986.

7. E.E. Morris, “Advances in Composite Fiber/Metal Pressure Vessel Technology,” AIAA Paper 89-2643, 1989.

8. Guidelines for Visual Inspection & Requalification of Fiber Reinforced High Pressure Cylinders, Pamphlet CGA C-6.2-1996, Compressed Gas Association, Arlington, Virginia, 1996.

9. Luxfer’s SCBA Cylinder Visual Inspection Guide, Luxfer Gas Cylinders, Riverside, California, 1998.

Edgar E. Morris is general manager of the Composite Cylinder Division of Luxfer Gas Cylinders. He has worked in the advanced composites industry for 40 years in technical, marketing, and management positions. After graduating from Stanford, he worked in Aerojet-General’s Structural Materials Division for 13 years in filament-winding of rocket motor cases and pressure vessels. Morris has led the transfer of aerospace composite technology to mass production of composite high-pressure gas cylinders in the high-volume, low-cost commercial cylinder safety, transportation, medical, and industrial markets.

Lonnie W. Smith is engineering manager at Luxfer’s Composite Cylinder Division. He received his B.S. degree in mechanical engineering from the University of Massachusetts, Dartmouth, in 1981. He has worked in the composite structures design and analysis field since 1981, involved in both aerospace and commercial composite structures applications. He has worked in the composite pressure vessel field since 1990 and has been employed at Luxfer since 1996.

Jui-Chi Ko has worked at Luxfer Gas Cylinders in the Composite Cylinder Division as a project engineer since 1995. He received his B.S. degree at the University of California, Berkeley, and M.S. degree at the University of California, Los Angeles. Both degrees were in mechanical engineering. At Luxfer, he has been actively involved in the design and development of composite cylinders, including mechanical and environmental testing.


Bonfire test in progress

(Photos courtesy of Luxfer Gas Cylinders.)

LAFD SUV in the ocean

Los Angeles Firefighter Swept Into Ocean as Mudslides Hit CA

A Los Angeles Fire Department vehicle was pushed into the ocean as heavy rains sent debris across several roadways.

Grandville (MI) Fire Truck Hit While Responding to I-196 Rollover

Michigan State Police are reminding drivers to pull over for emergency vehicles after a fire truck was struck Wednesday night.