On September 15, 1999, the U.S. Department of Transportation (DOT) published notice RSPA-99-5143 (notice 99-11) stating that its files contain reports of 12 explosive failures of aluminum cylinders made of 6351 alloy. Eleven of the failures occurred while filling. Six of the failures were SCBA cylinders and three were SCUBA cylinders. Although some of the failures were from mechanical damage and over-pressurization, in six cases the cause for cylinder failure was due to a phenomenon known as Sustained Load Cracking (SLC).
On December 7, 1999, the National Institute for Industrial Safety and Health (NIOSH) issued its own notice based upon the DOT data. This report contained a seven-point advisory recommending frequent technical inspections of all SCBA cylinders manufactured from 6351-alloy.
Over the past 15 years, numerous warnings concerning older aluminum cylinders made from 6351-alloy have been issued to divers and firefighters. However, the most recent DOT and NIOSH notices have received particular attention, partly because of four recent 6351-alloy cylinder ruptures: a SCBA cylinders in Allegan, Michigan (September 1997) and SCUBA cylinders in Campbell River, British Columbia, Canada (April 1999), Riviera Beach, Florida (February 1998), and Key Largo, Florida (March 2000). The latest failure occurred after DOT and NIOSH issued their reports, bringing the total number of recorded ruptures in the Unites States to 13. Along with the ruptures mentioned in the DOT and NIOSH reports, there have been 6351-alloy SCUBA cylinder failures in Australia, Canada, New Zealand, and the Middle East, for a recorded worldwide total of 17. To keep this failure rate in perspective, it is estimated that various companies manufactured a total of 25.4 million 6351-alloy cylinders.
Along with the wave of concern over cylinder failures has come a great deal of misinformation spread by inaccurate reports on the Internet, in newspapers, and trade journals, as well as word-of-mouth rumors. In this article, I will strive to set the record straight, separate fact from fiction, and provide an overview of the important issues surrounding SLC and 6351-alloy used in aluminum SCUBA and SCBA cylinders and liners for certain composite cylinders.
Sustained Load Cracking
Sustained Load Cracking, a metallurgical anomaly, occasionally develops in high-pressure cylinders made from 6351 aluminum alloy. As the name implies, SLC usually occurs in cylinders that have remained filled for sustained periods. U.S. manufacturers began using 6351 when DOT approved aluminum alloy cylinders for service in 1971. Manufacturers included Luxfer USA, Walter Kidde, Norris Industries, and Kaiser Aluminum. Permits for Norris and Kaiser (SP6688 and SP6576) have expired and those cylinders are no longer approved for use in the U.S. but may be used in Canada under a grandfather clause. Walter Kidde continued using 6351-alloy until the end of its cylinder production in 1989.
Luxfer USA manufactured cylinders from 6351-alloy from 1972 through mid-1988. In June 1988, Luxfer USA ceased manufacturing cylinders from 6351-alloy. Since then Luxfer has used a proprietary version of 6061-alloy. No sustained-load cracks have been detected in the more than 19 million Luxfer cylinders manufactured from 6061-alloy.
The Catalina Tank Company began making aluminum cylinders in 1986 using alloy 6061-T6. Although this alloy is generally not subject to SLC, a very few Catalina cylinders have been found with minor thread area cracks attributed to stress corrosion cracking. Therefore, PSI, Inc. recommends that thread examinations, including a mirror-and-light protocol be performed on all aluminum cylinders.
Hoop wrapped SCBA cylinders were introduced in 1976. Wrapped with fiberglass and other fibers to increase sidewall strength, some of these cylinders were rated at 4500 psig service pressure. All hoop-wrapped cylinders were limited by DOT to a 15-year service life. Out of more than 70,000 hoop-wrapped cylinders manufactured, one 4500-psig cylinder ruptured in 1984. The cause was initially listed as Room Temperature Grain Boundary Creep. However, Dr. Henry Holroyd, an eminent metallurgist subsequently described the 6351-alloy behavior as SLC. As a result of this failure, and in response to a DOT safety notice, in 1985 Luxfer USA required that all its E7235-4500 psig hoop-wrapped cylinders be retrofitted with a steel collar to prevent ruptures, and Luxfer instituted an aggressive retrofit program. No cylinders thus retrofitted have ever failed. Unfortunately, a few owners of these cylinders failed to comply with the Luxfer requirement. In March 1993, a non-retrofitted cylinder failed with tragic consequences--and SLC was the terminal cause. The cylinder had also exceeded the DOT-mandated 15-year life by 2 years at the time of the failure. Another contributing factor was the fact that the cylinder had not been properly inspected.
In 1985, PSI, Inc. became the first dive industry advocate to focus on SLC detection using a mirror and light inspection protocol. Although cylinder visual inspectors were warned to examine crown and threads of SCUBA 3000 psig and SCBA 2216 psig cylinders, few technicians heeded the warning. Tests conducted by Luxfer USA suggested that cylinders with service pressure of 3000 psig or less were unlikely to rupture even if SLC developed. Inspectors trained by PSI, Inc. did find a number of cracked cylinders as well as some cylinders leaking from outer-crown cracks, before the first SCUBA cylinder ruptured in Australia in 1988. Subsequently, DOT, Luxfer USA, and PSI, Inc. stepped up efforts to educate both owners and inspectors of 6351 alloy cylinders about the importance of thorough technical inspections.
Evidence indicated that SLC develops slowly over a multi-year period. For example, detailed metallurgical examination of one ruptured SCUBA cylinder showed that the crack had been growing for 8 years or more and would have been visible for about 6 years to a trained inspector using the proper crack-assessment protocol. Had that cylinder been carefully inspected, it could have been removed from service long before the rupture occurred.
Trained Inspectors Are Essential
Although cylinder ruptures are extremely rare when one considers the many millions of all-aluminum and aluminum hoop-wrapped cylinders that were made from 6351-alloy, a number of cracked cylinders have been found over the years. It is therefore essential for each of these cylinders to receive a careful examination at least annually by a trained inspector. PSI, Inc. has trained more than 11,000 visual inspectors around the world for the fire fighting, diving, and hydro test industries. Yet, many potentially unsafe cylinders continue to be returned to service by un-trained technicians who cannot differentiate between a tap stop and a crack. They do not understand the difference between a valley and a fold, nor do they know how many threads may be penetrated before a cylinder should be rejected and condemned. Some inspectors do not have the minimum essential tools recommended by the National Bureau of Standards for detecting cracks.
The SLC problem is real and the potential for further explosive ruptures exists. That potential may be thwarted by thorough inspections conducted by properly trained, diligent inspectors. It cannot be overemphasized that the quality of inspection is far more important than the frequency of inspection. An improperly trained inspector can look at a cylinder numerous times without detecting SLC. An inspector must use a focused, bright light source and a magnifying mirror that fits into cylinder openings as small as ½ inch diameter. In addition, fill station operators must be alert to any sounds of leaking air during fills. If a leak is suspected, Luxfer further recommends that a soapy solution be sprayed onto the cylinder crown when the cylinder is half full and again when full. Any bubbling is a signal to stop charging, immediately discharge the cylinder, and examine the crown and threads for cracks.
Eddy Current Examination for Cracks
In collaboration with Luxfer Gas Cylinders, Flare Technology, Inc., developed a device using eddy current technology as an additional tool for testing thread integrity in 6351-alloy cylinders. Introduced in 1996 under the brand name Visual Plus, this eddy-current device contributed greatly to the detection of early, difficult-to-observe sustained load cracks. Visual Plus is now marketed by Advanced Inspection Technology. In 1999, Flare introduced a slightly different eddy-current model, which it markets under the name Visual Eddy. Engineering Inspection Systems produces a similar eddy current assessment tool called Simple Eddy.
When properly calibrated and operated, these eddy current devices help inspectors detect hard-to-see cracks in 6351-alloy cylinders. However, it is imperative that these devices be used in precise accord with instruction by properly trained inspectors. Sadly, this writer is aware of many instances in which the instruments have been improperly used. Eddy current devices can give "false positive" readings when improperly operated or when the cylinder thread areas being tested have been inadequately cleaned. For example, in January 2000, Luxfer reported that fully 50% of the cylinders returned because of alleged thread cracks were not cracked at all. Such false readings also occur frequently with newer Luxfer cylinders made of the company's proprietary 6061-alloy. For this reason, Luxfer announced in February 2000 that it now recommends that eddy-current devices be used only on 6351-alloy cylinders and not on its newer 6061-alloy cylinders. It is also worth noting that some operators require and charge a fee for eddy-current testing on not only aluminum cylinders, but also steel cylinders. Eddy-current devices were not designed to be used on steel cylinders, and there is no history of SLC in steel cylinders. However, steel cylinders are susceptible to other types of problems, including internal corrosion, that can lead to ruptures: therefore PSI, Inc. offers training course for inspecting both steel and aluminum cylinders.
It is estimated that some 2,000 eddy-current devices are now being used in North American dive stores, fire departments, and hydrostatic test facilities, and the number is growing. While eddy current technology can certainly be a valuable adjunct to visual inspections, in no way does it replace any of the standard visual inspection procedures.
Preventing Explosive Failures
Can SLC related ruptures be stopped? Yes, they can. The solution is a team effort by knowledgeable technicians and owners who handle, inspect, and use the cylinders. The same solution also applies to other types of damage that may weaken a high-pressure cylinder. By law, when a hydrostatic requalification test is conducted, the retester must perform a visual inspection. An initial inspection should be performed before the actual pressure test to determine whether the cylinder is in proper condition to test. After the pressure test, a further inspection of the threads should be made since the hydrostatic test tends to make cracks more visible. In keeping with long-standing industry practices and the recent Federal recommendations, PSI, Inc. recommends that trained inspectors visually inspect 6351-alloy SCBA and SCUBA cylinders at least annually.
Use of a Tank Inspection Pipe (TIP), a recently developed tool, is an excellent way to examine magnified, brightly lit threads in SCUBA and SCBA cylinders
The fill station operator and owner also have important roles to play in cylinder safety. At each fill, the operator determines that the cylinder is within the valid hydro test period and checks for evidence of a visual inspection within the past year. The operator should have confidence in the quality of the inspection based on the knowledge, and experience of the inspector. He must also check for evidence of leakage. Cylinder owners are responsible for ensuring that the people inspecting their cylinders are properly trained and qualified. If unexplained pressure loss occurs in a cylinder at any time, the cylinder should immediately be drained and taken to a skilled inspector for a complete examination before being filled again.
All individuals, businesses, and agencies that issue or distribute evidence of inspection stickers are obligated to ensure cylinders susceptible to SLC-i.e., those made of 6351-alloy-- receive a meticulous visual inspection before the sticker is applied to signify the cylinder is ready for continued use.
Resources and suppliers of equipment mentioned:
PSI, Inc. (inspection mirrors, lights, training, textbooks, and manuals)
6531 NE198th St., Kenmore, WA 98028
Luxfer Gas Cylinders (manufacturer, training manuals)
3016 Kansas Ave., Riverside, CA 92517
Catalina Cylinders (manufacturer)
7300 Anaconda Ave., Garden Grove, CA 92641
Flare Technology (Visual Eddy manufacturer, training)
2869 Old Higgins Road, Elk Grove Village, IL 60007
Advanced Inspection Technology (Visual Plus distributor, training)
14020 Eagle View Rd., Lake Matthews, CA 92570
Engineering Inspection Systems (Simple Eddy manufacturer)
Box 40, Coral Ridge Road, Brooks, KY 40169
Forest City Scuba (TIP manufacturer)
1894 Daimler Road, Rockford, IL 61112
Inspecting Cylinders (PSI, Inc.)
Scuba Reprint File (PSI, Inc.)
Luxfer Guide For Scuba Cylinders
Luxfer's Scba Cylinder Visual Inspector Guide
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