self contained breathing apparatus (SCBA)
About[]
A self contained breathing apparatus, or SCBA, sometimes referred to as a Compressed Air Breathing Apparatus (CABA) or simply Breathing Apparatus (BA) is a device worn by rescue workers, firefighters, and others to provide breathable air in a hostile environment. When not used underwater, they are sometimes called industrial breathing sets. The term "self-contained" means that the breathing set is not dependent on a remote supply (e.g., through a long hose). If designed for use under water, it is called SCUBA ( self-contained underwater breathing apparatus).
An SCBA typically has three main components: a high-pressure tank (e.g., 2200 psi to 4500 psi), a pressure regulator, and an inhalation connection (mouthpiece, mouth mask or face mask), connected together and mounted to a carrying frame.
There are two kinds of SCBA: open circuit and closed circuit.
Air cylinders are made of aluminium, steel, or of a composite construction (usually carbon-fiber wrapped.) The composite cylinders are the lightest in weight and are therefore preferred by fire departments (UK: fire brigades), but they also have the shortest lifespan and must be taken out of service after 15 years. Air cylinders must be hydrostatically tested every 3 years for composite cylinders, and every 5 years for metal cylinders. During extended operations, empty air cylinders can be quickly replaced with fresh ones and then refilled from larger tanks in a cascade storage system or from an air compressor brought to the scene.
Cylinder regulations[]
- Regulations for cylinders are found under Title 49 of the Code of Federal Regulations, part 180, sections 201-215. A citation such as 49 CFR 180.205 directs the reader to Title 49, section 205 of part 180 of the Code.
- The proper value for a test pressure can be found in Table 1 of 49 CFR 180.209 or the applicable Special Permit stamped into the cylinder.
- The Code of Federal Regulations — 49 CFR 180.205(g)(4) — mandates that hydrostatic test equipment be verified within ±1 percent of the calibrated cylinder's pressure and corresponding expansion values.
- Multiple testing of cylinders is covered in 49 CFR 180.205(g)(3)(i). Department of Transportation approved testing facilities must display the Letter of *Authorization in the area where the testing is taking place, and all required Transportation Department and referenced Compressed Gas Association publications must be on hand at the testing site.
- Illustrations of typical equipment layout for hydrostatic testing can be found in the Compressed Gas Association's Publication C-1 and the Compressed Gas Association Handbook.
- Compressed Gas Association Pamphlet C-1.1 addresses personnel training and certification guidelines for hydro-test facilities.
Hydrostatic Testing[]
The hydrostatic test actually measures the elastic expansion of the metal and its ability to return to its original shape within 10% of the original volume. The water test is safe since it cannot compress or expand like air.
In order to protect the public and reduce the risk of high pressure cylinder failures, the US Department of Transportation (DOT) has Federal regulations (CFR section 49) for manufacturing, testing and transporting high pressure vessels. These CFR regulations are strictly followed by hydro testing facilities under the penalty of severe fines and imprisonment.
Since 1913, the Compressed Gas Association (CGA) has been developing and promoting safety standards and practices in the industrial and medical gas industry including guidelines for the inspections of high pressure cylinders.
Valves, Cylinder[]
Valves attach to the neck of the SCUBA cylinder using one of two types of straight screw threads. Screw threads date back to the third century. For a few centuries everyone did their own thing regarding screw threads. All U.S. cylinder valves are required by the Department of Transportation (DOT) to have an over-pressure release safety mechanism, called a burst disk assembly. The thickness of a copper disk in the assembly determines when it will rupture, releasing the cylinder pressure in a controlled manner. New DOT regulations require that during each five-year hydrostatic test, the burst disk must be verified as certified to release at the cylinder test pressure or up to 10% less. Dive or scuba shops recommends that the cylinder valve be overhauled and the burst disk replaced whenever the cylinder is hydro tested (i.e., every five years) and annually if the cylinder is known to have been overfilled. If a burst disk is ever removed from the valve, the disk must be discarded because reusing a disk can cause it to rupture prematurely.
References[]
- Compressed Gas Association
- DOT Cylinder Maintenance, Retest and Certification Requirements
- Cylinder Testing 101