SCBA Cylinder Failure
THERE IS A fundamental interest in the emergency services (fire, EMS, and police) in protecting the respiratory system of responders who are working in a hazardous environment Before the advent of the self-contained breathing apparatus, firefighters who challenged a burning structure used moistened face cloths, a crude hair-inmouth filter, or other simple equipment to protect their lungs from smoke.
The development of modern positivepressure SCBA borrowed technology from mine safety organizations and the naval sciences. Following World War II and the rapid advances made to support underwater warfare, SCBA became available as a civilian tool. This created a demand from professional and recreational divers tor both mass-produced equipment and the provision for regular, convenient, and safe refilling of the high-pressure SCBA tank.
Much has been written about the value of SCBA, the procedures for its use and maintenance, and cautions about how and why it fails. In 1987, the National Fire Protection Association approved two standards regarding the safe design, manufacture, and use of SCBA. NFPA 1500, although not addressing the specifics of SCBA. broadly standardizes procedures for its use. NFPA 1981 establishes a generic performance standard for SCBA as a system and for SCBA subcomponents.
Neither standard establishes an indepth reference for high-pressure SCBA air cylinders. Attention was focused on procedures and equipment components which were felt to be the weakest links in the SCBA ensemble. The pressurized cylinder of breathing gas, the “air tank,” by its appearance of solid construction and lack of publicity about documented failures, is regarded as the strongest link in the SCBA system. But these cylinders do have the potential for catastrophic failure.
Catastrophic failure of SCBA cylinders occurs most often during the fill cycle. The fill/use pressure gradient (see Figure 1, page 54) suggests that there are only two periods during filling or storing cylinders at which catastrophic failure may occur to cylinders that have a flaw in the shell. The first is during the recharging phase. It is at this time that rapid, high-pressure charging can cause severe stress to the flawed cylinder and lead to a classic failure scenario.
Knowing what goes into construction and testing of SCBAs is the first step to building a successful maintenance program.
The second period of the cycle at which failure can occur is during the storage of the pressurized cylinder before use. Although the cylinder pressure during this period has diminished to some extent following the peak pressure attained during filling, two situations may arise that will increase the cylinder energy level, thereby increasing internal pressure to potentially exceed the cylinder flaw burst pressure: (a) when the filled cylinder, having lost a measure of energy due to slow cooling in ambient air, is reheated either through direct heat impingement of a fire or exposure to direct thermal effects of the sun; (b) when, during prolonged storage of filled cylinders, internal and external corrosion lowers the cylinder flaw burst pressure below the stable pressure of the cylinder.
In any other possible use scenario, the working pressure of the cylinder will be below the cylinder flaw burst pressure. To keep that proportion intact, the in-service testing procedures detailed later must be adhered to.
Reported incidents of high-pressure breathing gas cylinder explosions, although not frequent, are not rare. An explosion and fire caused by the failure of an oxygen cylinder in a southern California fire house severely damaged a fire truck. A novice SCUBA diver painted three aluminum cylinders with epoxy and heat-cured the finish. One cylinder, weakened by the heat, exploded during refilling, destroying the filling facility. The remaining two cylinders failed identically during post-incident testing. In Freeport, Bahamas, a similar incident resulted in destruction of a facility of concrete and cement block construction. In Cozumel, Mexico, a filling facility was destroyed by an outof-date steel cylinder filled to just threequarters of its working capacity.
ORIGIN OF PRESSURIZED BREATHING GAS CYLINDERS
In 1868, a Frenchman by the name of Denayrouze patented the “Aerophore.” It was comprised of a rudimentary demand regulator (activated by a slight vacuum in the human respiratory system) and a storage tank holding 600-psi compressed air. A short time later, the Aerophore was presented to the French Academy of Sciences. Jules Verne, a member of the Academy at the time, was impressed by its possibilities and included the “Aerophore” in his popular work of science fiction, 20,000 Leagues Under the Sea.
Between 1868 and World War I, many elementary pressurized air containers of much greater capacity (in pressure and volume) were developed. In France during World War II, Jacques Cousteau developed an improved demand regulator and the first generation of modern high-pressure air cylinders, without seams or welds so as to minimize failure.
The modern cylinders have an ellipsoidal interior top and bottom to hold high pressure. They are made from steel or aluminum, or with a steel or aluminum core wrapped with synthetic fibers, which are then coated with resin. The steel or aluminum cylinders are made with alloys to balance strength with elasticity.
Since 1942, steel cylinders containing pressurized, nonflammable gas are stamped with the prefix, “3A.” These cylinders are thick-walled and very durable, but they are widely used as carbon dioxide containers and, as such, become hosts to carbonic acid (HCO), a corrosive formed by the combination of C and water. Newer standard “3AA” steel alloy cylinders have thinner walls and withstand corrosion well, but must be inspected visually each year.
SCBA CYLINDER FAILURE
The first three letters of the cylinder type specification indicate that the cylinder has been manufactured to specific Department of Transportation (DOT) requirements. ‘ITie next series of alphanumeric digits indicates the specification, special permit (SP), or exemption (E) to which the cylinder hies been manufactured. The last four numbers state the service pressure of the cylinder, the maximum working pressure permitted in the cylinder at 70°F. (See Figure 2, page 56.)
ALUMINUM CYLINDER MANUFACTURE
Aluminum tanks are generally made from an alloy, 6351, containing aluminum, magnesium, manganese, and silicon. This affords improved corrosion resistance, strength, and metal-working characteristics. These vessels require minimum care for safe use: a fresh water rinse after use, an annual visual inspection, and five-year hydrostatic test. Hydrostatic testing of the 6351AL vessels has little adverse effect upon cylinder integrity, and it is recommended more frequently when anomalies are suspected. Initial DOT approval of aluminum cylinders demands satisfaction of the 3AA steel tank requirements, as well as ultrasonic testing of raw material; 100,000 pressure changes under hydrostatic test conditions; and increased wall thickness and cylinder base design.
At the factory, the raw aluminum ingot that will become the cylinder is received and identified. The alloy is tested and the ingot prepared for the cold extrusion process by the application of a phosphate lubricant. Following the extrusion, the rough pressure vessel is cleaned and the metal is chemically treated, then machined to its final shape. Next the vessel cup is heated to its annealing temperature (aligning the molecular composition) and the top formed.
When catastrophically released, the energy of a fully charged SCBA cylinder becomes a potential source of great damage and personal injury. For this reason alone, SCBA users should be familiar with testing at each phase of manufacture and use.
The final forming process of aluminum cylinders uses one of two methods. The first method is two-phased: one downward press stroke to shape the entire cylinder bottom and a second folding activity to form the top of the cylinder. The second method is fourphased: the first, one-third downward press and retraction; the second, twothirds downward press and retraction; the third, a full downward press completing the shape of the lower portion of the cylinder; the fourth, the folding and spining of the top to form the neck.
The neck of the cylinder is then drilled and the cylinder prepared for hardening. The rough cylinder is next reheated and water quenched. The cylinders are held at higher than nominal atmospheric temperatures for twenty hours in a baking oven. Lastly, the neck of each cylinder is rebored and tapped to receive a valve.
A random sample of cylinders is selected for destructive tensile strength, pressure, and impact testing. The remaining finished cylinders are then hydrotested and identified with a serial number that traces the cylinder back to the raw ingot. Finally, the cylinder is visually inspected inside and out and affixed with a test-date stamp.
SCBA CYLINDER FAILURE
There is some suggestion that the two-phase forming process stresses the metal in the area of the upper third of the rough cylinder—the single down stroke and top fold may work the blank near the neck to excess. It is suggested that in 4,500 psi cylinders the flaws may appear earlier in the cylinder life (that is, following fewer fill/use cycles) than in cylinders of lower service pressure. As the number of fill/use cycles increases in lower-pressure cylinders, similar flaws may also appear. These flaws do not appear to be occurring in cylinders constructed using the fourphase forming process.
Inspection of aluminum cylinders and any aluminum-cored composite cylinders must include very close inspection of the top portion of the cylinder, 360 degrees around, for evidence of failure.
STEEL CYLINDER MANUFACTURE
Steel cylinders are made from a complex A151 -4130 alloy. The raw material arrives at the plant in the form of rolled steel. It is cut into more workable sheets and annealed at 1,300°F for 50 hours. After annealing, the steel stock is cooled slowly and cut again into smaller sheets, punch-pressed into 24-inch disks, randomly sampled and tested, and all disks assigned a batch number.
The disks are hydraulically pressed into the form of a cylinder without a neck. The cylinder is next drawn into a longer and narrower shape, litis process adversely affects metal characteristics, so it is reannealed, stripped of scale, and lubricated for the final shaping process.
Following the final draw, the cylinder is reheated and the neck formed. Another heating process raises the cylinder temperature to 1,650°F. It is then oilquenched and allowed to cool gradually. The neck is bored and tapped for the valve and the tank surface cleaned by blasting.
The cylinders are hydrostatically tested at five-thirds of their working pressure (3,400 psi nominally). Upon successful completion of this test, cylinders are thoroughly dried, visually inspected inside and out, assigned serial numbers, and stamped with the original test date.
COMPOSITE CYLINDER MANUFACTURE
Composition cylinders integrate the lightw eight and tensile strength of hightech fibers with a metallic core. A metallic cylinder is first fashioned using the methods described for steel or aluminum cylinder manufacture. The composite cylinder, however, is constructed with a much thinner core cylinder wall. Used without fiber/resin strengthening, this cylinder may w ell burst at pressures under 4,000 psi. The fiber/resin wrapping will increase normal burst to above 17,000 psi.
Around the core cylinder, the fiber is wrapped longitudinally and latitudinally. The wrapped core cylinder is then coated with a hardening compound which permanently marries all of the fiber wrappings. Tit is process transforms the end-for-end tensile strength of the individual fiber to omnidirectional strength of the hardened fabric.
All of these high-pressure cylinder types, in spite of the explicit and implicit strength characteristics, have vulnerabilities. Steel corrodes, aluminum fatigues, and composite cylinders suffer abrasion in addition to corrosion or fatigue of the core cylinder.
The storage capacity of SCBA cylinders varies among cylinders, but not widely in range. For simplification, approximate capacities are shown in Figure 3 and dimensions in Figure 4. Careful consideration should be given to the potential of the compressed gas contents of the cylinders. A fully charged, 72 Standard Cubic Foot (SCF) cylinder contains 1.2 million foot-pounds of potential kinetic energy. When catastrophically released, this energy becomes a potential source of great damage and personal injury. For this reason alone, SCBA users should be familiar with the testing of cylinders at each phase of manufacture and use.
A 3AA cylinder must undergo four tests during its lifetime. The first test is the hydrostatic elastic expansion test, and is nondestructive. The cylinder is placed into water and then water is placed inside the cylinder under pressure. The cylinder, very much like a rubber balloon, will expand as higher internal pressure forces the cylinder wall outward toward the lower pressure. When the designated test pressure has been reached, the internal pressure is reduced. The ballooned metal w all of the cylinder must return to its original dimensions (elasticity) within a factor of 10%. This is measured volumetrically. Expansion greater than 10% is considered to be plastic deformation and is cause for condemning and destroying the cylinder. No maintenance or reworking can negate the outcome of this test. A failure during this test is not catastrophic due to the incompressible characteristic of the water used to pressurize the cylinder. A small water release at failure will equalize pressure.
The second test is referred to as a flattening test, and is destructive. It is applied to selected samples of a cylinder batch at the final stage of manufacture. When the tank has been forcibly flattened by a hydraulic press, a close visual examination takes place to detect cracks—a test of ductility. Failure of this test fails the entire shipment.
The third test, also destructive, is an elongation or stretching test of an additional sample from the batch. A crosssection of the sample cylinder is taken before the test. A force is applied in an attempt to pull the metal apart. Two points of reference are noted. The first, yield strength, is that point at which the metal stretches beyond elasticity into plastic deformation —the point at which metal will not return to its normal shape and size. The second observation is when the metal ruptures. At this point, the tensile strength has been exceeded.
The fourth test, a non-destructive one, is a visual inspection of every surviving cylinder interior, either annually, whenever zero pressure differential is reached, or whenever any other potentially contaminating conditions are experienced. The valve is removed, a small light is inserted into the cylinder, and an inspector views all of the cylinder interior. A careful visual inspection also must be made of the cylinder exterior.
Another destructive test that might be employed is the “bonfire” test. In this test, the cylinder is suspended over a large quantity of kindling wood. The wood is ignited and, as the cylinder temperature rises, pressure builds up within the cylinder, leading eventually to its failure. In an example test, a 3AA cylinder failed at 4,050 psi and 880°F. At failure, a major portion of the cylinder, 11.6 pounds, traveled 120 feet. This evaluation process is flawed, since the extreme heat changes the molecular structure of the metal and fiber-resin, creating false burst-pressure outcome. But the physical findings are a clear warning to firefighters. During fires, fire-damaged pressurized cylinders may fail and cause serious injuries.
All 3AA cylinders are clearly marked with the working service pressure. When the cylinder is initially manufactured, the date of initial hydrotesting may bear a “plus” sign suffix. Within a five-year period from this first hydrostatic test date bearing the plus sign, the tank may be filled to 10% over its stated and stamped service pressure. For instance, a tank bearing “2250 + ” may be filled to 2,250 psi plus 10%, or plus 225 psi, to a total of 2,475 psi. However, this does not apply to composite cylinders, nor does it apply beyond the first five years of cylinder life.
The test pressure is ⅛ the service pressure —not ⅝ of the “plus” dated service pressure, but of the stamped service pressure. In the example above, the hydrostatic test pressure is 3,750 psi. The rupture disk, housed in the valve assembly, will hold a strength of approximately 90% to 100% of this test pressure.
A new cylinder has a theoretical burst pressure of a little more than two times the test pressure. In the example indicated above, the burst pressure of the 2,250 psi service cylinder is approximately 7,500 psi.
Some fiber-wrapped aluminum core cylinders (DOT-E 7277) operate at higher pressures (4,500 psi), storing 87 SCF of breathing air. The weight is approximately 19 lbs. The thickness of the aluminum cylinder and the composite wrapping are determined by a required minimum burst pressure of at least three times the working pressure and side burst pressure of three times the working pressure; composite cylinders must meet more tensile strength requirements. Typically, these cylinders burst at approximately 17,000 psi. Failure first occurs in the wrapping filament with the core vessel ballooning to rupture during the second phase.
SCBA CYLINDER FAILURE
DYNAMICS OF SUDDEN TANK FAILURE
Sudden failure of air-pressurized SCBA cylinders in air is dramatic, injurious to bystanders, and sometimes fatal. To demonstrate the effects upon surrounding structures and bystanders in the event of sudden cylinder failure, five actual failure tests were conducted in a 64 ft⅜ test chamber constructed of Winch, marinegrade steel. Four standard 72 SCF steel SCBA cylinders were tested horizontally at 14.7 psi (control), 1,100 psi, 1,500 psi, and 2,250 psi without any dependent supporting surface. This series of tests exposed the test chamber to direct cylinder impact forces accelerated over 24 inches and two to three atmospheres over pressure. A fifth test was conducted on a vertically positioned cylinder at 2,250 psi.
The test design utilized an instantaneous cylinder flawing technique employing flexible linear shape charges (FLSC) to deeply etch the test cylinder wall. The flawing technique allowed the varying of internal pressure, fill rate, cylinder temperature, and failure rate—without altering the molecular structure of the cylinder metal. Appended to this technique was a computer simulation of the strength and failure parameters of the cylinders and the test chamber. As the energy potential for the cylinder w -as increased, there was considerable interest in the integrity of the test chamber. The test chamber duplicated an air-cushioned filling chamber that has been recommended by some experts.
Each explosion test was recorded on movie film. Prior to beginning the formal test, the gauge used at the test site to verify the pressures contained within the cylinder was calibrated utilizing the breathing air compressor gauges. Cylinder pressure variances were judged constant and insignificant. The cylinder was then placed into the test chamber and stabilized approximately 24 inches from the bottom of the chamber. Once the cylinder was stabilized, a 12-inch segment of FLSC pyrotechnic material was placed along the longitudinal axis of the cylinder. As an initiator for the FLSC, approximately one ounce of C4 plastic explosive was affixed as a detonator with a thermal fuse.
The fuse was lit and detonated the C4 explosive. The C4 explosive simultaneously perforated the cylinder and detonated the FLSC. The FLSC deeply etched the side of the cylinder, as clearly indicated in photographs of the test. The etching had the same effect as a major internal or external flaw would have had upon strength characteristics of the cylinder without varying cylinder pressure or temperature.
The initial test used an uncharged cylinder at atmospheric pressure. The test explosion indicated the explosive charge would not mutilate or significantly alter the test cylinders. Inasmuch as there was no over-pressure contained in the cylinder, there was no cylinder failure except for the hole pierced in the top of the C4 initiating explosion. There was little movement of the test chamber. Since the test cylinder contained no pressure, the movement of the chamber was completely related to the explosive. It was not significant.
The second test was run at a cylinder pressure of 1,150 psi. The exploding cylinder dimpled the bottom of the chamber and created some superficial scoring on the internal walls. The chamber plenum was blown back by the blast of air released from the tank, and the impact of the tank upon the bottom of the chamber caused the chamber to spring off its blocks. There was some flexible, linearshaped charge debris on the chamber wall, and the tank valve was propelled horizontally, striking and scoring the chamber wall. While debris would have seriously injured exposed bystanders, there was only slight deformation of the test chamber.
The third test was conducted at a cylinder pressure of 1,500 psi. The cylinder valve stem was blown horizontally into the side of the chamber and left an obvious indentation. As in test number two, the cylinder striking the test chamber did not appreciably damage it, causing only a small additional amount of deformation. The cylinder opened more widely than the 1,150 psi cylinder. The chamber plenum was violently blown, but the chamber was uncompromised.
The fourth test was conducted at a cylinder pressure of 2,250 psi. On detonation, the cylinder was indescribably distorted and blown free of the test chamber. It remained in one piece and fell about 30 feet from the chamber. There was an obvious point on the chamber wall that had been struck by the sharp edge of the tank as it moved downward, but, again, there was no penetration of the wall of the test chamber.
ITie fifth test was conducted on a second cylinder with a pressure of 2,250 psi. The cylinder was stabilized on its end with its valve up. Hie explosive charge was placed near the valve stem. This charge was only C4 explosive with a detonator and fuse. No FLSC was used in this test.
The point of contact of the explosive with the cylinder was calculated to create a hole which would be similar to a hole left by complete failure of the threads in the valve stem. This would allow the valve to blow’ free, leaving approximately a Vs-inch-diameter hole through which the pressurized air would escape.
On detonation, the cylinder was driven violently downward by the rapidly venting compressed air. The cylinder bottom left a large impression on the bottom of the test chamber. Most of the chamber deformation appeared to be elastic and the recoil of the chamber bottom plate drove the cylinder backward against the rush of the venting gas. The cylinder rose vertically 50 feet, spun violently, and landed 30 feet from the chamber.
Upon reviewing the results of the five tests, it was clear that the cylinders had exploded in a predictable manner. The outcome of these explosions gave excellent indication that the test had accurately modeled previous actual cylinder explosions. Photographs of actual cylinder failures show remarkable similarity.
In all cases, the test chamber initially controlled the blast forces of the ruptured cylinders; however, in a realistic setting, any person in proximity of a failed cylinder will suffer serious injury or death.
Also observed was the value of a compressible medium (air) surrounding an exploding cylinder. Incompressible media surrounding the cylinder, such as water, are associated with far greater damage to the containing chamber and bystanders than is a medium such as air. The force of the explosion is transmitted by the incompressible water to all surfaces and seams of the test chamber.
CORROSION AND CARE
The University of Rhode Island, in two studies conducted in 1970 and in 1975, tested numerous steel and aluminum high-pressure air cylinders. The most marked results occurred with steel cylinders that were filled with large quantities of saltwater and fresh water and maintained at 104°F for 100 days. ‘Hie cylinders were reduced in wall thickness after this period of time such that cylinder burst would have occurred at the normal service pressure of 2,250 psi.
Extreme care should be taken to avoid contaminating the interior of any cylinders, but especially steel cylinders. Whenever such contamination occurs, or at least annually, every cylinder should be flushed, dried, and visually inspected. Care must be taken to examine the valve threads for damage as well. The outside surface of the cylinder should be carefully inspected for cracks, gouges, signs of heat impingement, distortion, or other signs of potential weakness.
SCBA CYLINDER FAILURE
The internal inspection should focus upon evidence of moisture, corrosion, and other substances, such as oil. If evidence of contamination is apparent, the cylinder should be hydrostatically tested, cleaned, and dried before being returned to service.
Care should be taken to select a vendor that has factory or agency authorization and has demonstrated experience in performing all highpressure cylinder testing and maintenance. Proper marking and record keeping are part of this competence. The maintenance and repair agency should utilize accredited and formalized procedures and exercise adequate quality control of all work performed.
Before using or tilling the cylinder, the cylinder valve should be opened briefly to blow out water or any other contaminant that may be in the valve. Otherwise, contamination of the tank or breathing apparatus may occur.
lilt the cylinder only with clean, dry air (see CGA Pamphlet G-7.1) from a compressor-storage cylinder system with proper filters and a maintenance program that includes frequent draining of the moisture separators. Air with moisture concentration greater than 25 parts per million should not be used. No part of the cylinder may be safely lubricated with oils or greases of any kind at any time. Use of the air reserve in water-use SCBA cylinders, except in any emergency, should be avoided. Use of this type of cylinder at atmospheric pressure may permit water to be forced throughout the regulator and into the tank. All used cylinders, regardless of pressure, should be refilled promptly to reduce the chances of condensation occurring within the tank.
Aluminum alloys in cylinders experience significantly less internal corrosion; however, they are more susceptible to external abuse and weakening abrasion. Aluminum-alloy cylinder valve threads must be clean, undamaged, and without corrosion. Because aluminum is softer than steel, valve removal and replacement requires greater care than required for steel tanks to avoid damage.
Federal law (U.S. DOT Hazardous Materials Regulations —49 CFR 173 34F749 CFR 173-302) requires that all compressed-air cylinders receive and pass a hydrostatic pressure test even five years. Composite cylinders must be hydrostatically tested every three years. The record of this test, cold-stamped on steel cylinders and labeled on all other cylinders, permits refilling of the cylinder to its original rated working pressure. Absence of evidence of this test requires that the cylinder be removed from service, without exception, until the test is performed.
The user is an integral factor in the SCBA incident prevention program. Proper use, regular inspection and tests, and understanding potential failure scenarios will help prevent catastrophic failures.
Evidence of the hydrostatic test must not be accepted as a substitute for routine visual tank inspection and maintenance, annually at the minimum. Corrosion-causing catastrophic failure or less catastrophic contamination may occur in little more than 30 days.
High-pressure air cylinders may be coated with a variety of external and internal coatings to improve corrosion resistance. Steel cylinders may be galvanized, covered with vinyl or epoxy, or painted; aluminum cylinders are either painted or anodized. Composition cylinders —fiber-resinwrapped steel or aluminum shells— are often painted, while some manufacturers color the resin used in the manufacturing process. All external and internal cylinder coatings pose a potential corrosion liability by hiding active corrosion taking place beneath. This potential should be considered during all inspections.
A major component in any highpressure air cylinder safety program is complete information management of the cylinder population. Records of purchase, use, repair, routine testing, routine maintenance, and inspection by cylinder serial number and signal of responsible party is mandatory.
Aging steel and aluminum highpressure breathing gas cylinders pose a significant explosion hazard unless regular inspection, testing, and proper maintenance procedures are rigorously adhered to. Newer aluminum and fiber-wrapped composite cylinder also may be explosion hazards if subjected to severe external forces or less visible heat exposure (as compared to heat that has charred the paint). Detailed external visual inspection and maintenance of exterior finish will make detection easier.
“Unique” field cylinder explosion incidents involving all three types of 3AA cylinders have been precisely replicated through advanced techniques that created cylinder flaws typical in the work environment.
The cylinder user is an integral factor in an incident prevention program. Knowledge of proper cylinder use, regular inspection of cylinder exteriors to detect flaws and to review’ inspection and test dates, and an understanding of potential failure scenarios will play a vital role in preventing predicted catastrophic failures.