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 when the cylinders were being filled. Six of the failures
were SCBA cylinders used in life-support systems for firefighters, and
three were SCUBA cylinders used by divers. Although some of the
failures were variously attributed to mechanical damage and
over-pressurization, in six cases the cause for the cylinder failures was
attributed 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 20 years, numerous
other 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 6351-alloy cylinder ruptures: a SCBA cylinder 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). This latest failure occurred after
the DOT and NIOSH reports were issued, bringing the total number of
recorded ruptures in the Unites States to 13. In addition to
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 a total of 25.4 million
6351-alloy cylinders were manufactured by various companies.
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 3AL SCUBA and
SCBA cylinders and liners for certain composite cylinders.
Sustained Load Cracking (SLC)
in 6351 Aluminum Alloy cylinders
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 of
time. U.S. manufacturers began using 6351 when aluminum alloy
cylinders were approved by the DOT for U.S. service under various special
permits beginning 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 and began using a
proprietary version of 6061-alloy: since that time, no sustained-load
cracks have been detected in the more than 19 million Luxfer cylinders
manufactured from this 6061-alloy.
The Catalina Tank Company began
making 3AL 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 3AL 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 has 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 two 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 eight years or more and would have been visible
for about six 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 firefighting,
diving and hydrotest 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 ˝ 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, to discharge the cylinder
immediately and to 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 (AIT). 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 not only on
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 3AA 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 6351-alloy SCBA and SCUBA cylinders be visually
inspected at least annually by trained inspectors.
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 (FSO)
and owner also have important roles to play in cylinder safety.
At each fill, the FSO determines that the cylinder is within the
valid hydrotest period and checks for evidence of a visual inspection
within the past year. The FSO 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 leaking. 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.
Cylinder inspection tools are
available directly from PSI at:
PSI, Inc.
16932 Woodinville-Redmond Road, Suite A201
Woodinville, WA 98072
425.398.4300 email
psi@psicylinders.com
For Additional Reading:
INSPECTING CYLINDERS (PSI, Inc.)
SCUBA
REPRINT FILE (PSI, Inc.)
LUXFER
GUIDE FOR SCUBA CYLINDERS
LUXFER’S
SCBA CYLINDER VISUAL INSPECTOR GUIDE