Cylinder failures
Robert S. Frankle
Harry F. Wachob
Failure Analysis Associates, Inc.
Menlo Park, California
We have received several inquiries in response to the article “Acidic Fluid Leads to Air Cylinder Failure” (November 1996) from SCBA users seeking additional information about safe fiberglass cylinder operation.
Fiberglass under load and exposed to a corrosive environment can fail after a time due to stress-corrosion cracking [cracking under the combined effect of stress (load) and corrosion]. In a fiberglass SCBA cylinder, the load on the glass fibers comes primarily from the pressure in the cylinder. Exposing the loaded fibers to a corrosive environment (such as acid) can lead to stress-corrosion cracking of the glass fibers in the fiberglass composite material. The cracking continues until a sufficient number of fibers have failed and the internal cylinder pressure can no longer be contained. In stress-corrosion cracking, failure of an exposed cylinder can occur at a pressure below the operating pressure. The time to failure following exposure to acid depends on several factors, including the load magnitude and the concentration and type of acid. In the Humboldt (CA) Fire Protection District (HFPD) failure described in the article, the SCBA cylinder came in contact with a liquid aluminum cleaner containing hydrofluoric, sulfuric, and phosphoric acids. The SCBA cylinder burst six days after the exposure.
Currently, there is no proven, nondestructive inspection technique for detecting stress-corrosion cracking in a fiberglass SCBA. However, awareness of stress-corrosion cracking and its appearance should be included in visual examination of SCBA cylinders. Initial stress-corrosion cracks in the fiberglass may be very fine, localized, and difficult to see. Later, as more fibers fail, the cracking will be more extensive. When the fibers fail, they often lift up from the surface of the cylinder, which makes them easier to detect. Stress-corrosion cracking is characterized by fibers with flat fracture surfaces. The ends of the broken fibers are the same length, as if the fibers had been cut with a sharp knife.
Following initial exposure, the corrosive medium can penetrate and progressively degrade the fiberglass SCBA. No cleaning method that will remove all of the corrosive residue and arrest the damage caused by stress-corrosion cracking currently exists.
The HFPD failure points out the need for proper handling of SCBA cylinders at all times, even during seemingly benign operations, such as transportation and refilling. Fiberglass SCBA cylinders must be handled in a manner that will prevent exposure to corrosives, such as acids. In the Humboldt incident, the exposure was not associated with normal usage in firefighting operations. A bottle containing the acidic cleaning fluid broke, exposing the cylinder to the acid while it was being transported for refilling.
In Detroit, Michigan, a fiberglass SCBA cylinder was run over inadvertently by a fire truck during a firefighting operation. Following the operation, the visibly damaged SCBA carrier and harness assembly were removed from service. However, the SCBA cylinder involved in the incident was not removed from service, even though it likely was damaged, too. A few hours later, the cylinder burst while stored in a fire truck. This illustrates the need for procedures and practices that address safe maintenance and handling of cylinders that may have been damaged.
Many years of successful service demonstrate that SCBA fiberglass cylinder failures are rare. However, the cylinders are certainly not indestructible, as illustrated by the Humboldt and Detroit failures. Fortunately, these failures caused only property damage.
Stress-corrosion cracking and impact damage are recognized as important considerations in the design and operation of composite structures. However, there is a lack of scientific data pertaining to the stress-corrosion cracking behavior of fiberglass structures in general and fiberglass SCBA cylinders in particular. Therefore, we cannot fully and quantitatively answer all of the questions we have received concerning fiberglass SCBA cylinders.
Generally, fiberglass SCBA cylinders should be treated as critical pieces of firefighting equipment. Good practice includes handling and maintaining fiberglass SCBA cylinders to prevent damage from impact and exposure to corrosive environments. Regular, careful visual inspection of cylinders may help to detect conditions, such as stress corrosion cracking, that can cause failure. Any cylinder suspected of being damaged by a corrosive environment, impact, heat, flame, or some other means, should be depressurized as soon as safely possible, removed from service, and evaluated for structural integrity.
