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Improving Transportation of Dangerous Goods In the Air Mode
On July 7, 2008, PHMSA published in the Federal Register its latest effort toward improving the integrity of combination packages used in air transport to reduce incident rates. In this report Mr. Wybenga reviews the background on incidents, and the packagings ability to withstand the pressure differential, orientation and vibration, and the environment of air transport, as well as the Michigan State University Study.
On July 7, 2008, PHMSA published an Advanced Notice of Proposed Rulemaking (ANPRM) in the Federal Register (73FR38361), its latest effort toward improving the integrity of combination packages used in air transport to reduce incident rates. Comments must be received by September 5, 2008. This effort follows a meeting conducted by PHMSA held in June 2007 on this subject. (For the ANPRM, see HAZMAT Alert 2008-22 July 10 , 2008 on the HazMat Reference Gateway at www.hazmatship.com).

The following provides one perspective on the ANPRM; shippers and carriers are invited to share their thoughts by contacting the publisher at publisher1@verizon.net.

Background On Air Incidents.
Based on PHMSA data there are roughly between 400 and 500 air incidents involving declared hazmat shipments in combination packagings per year. The vast majority occur on cargo aircraft. Most failures are identified after the package has completed one flight. More than half of the incidents involve flammable liquids with paint or paint related materials being the most common substances involved. Inner packagings are usually plastic (approximately 30%) or glass bottles (approximately 28%), and metal cans (approximately 42%). Closure failure is the most common source of failure for inner packagings.

Ability to Withstand Pressure Differential
The ANPRM states, “The packagings of most concern to PHMSA and FAA are non-specification combination packagings that must be “capable” of meeting pressure differential requirements but are not required to be certified as meeting a specific performance test method..” The ANPRM goes on at length about the ability to withstand the pressure differential requirement. The non-specification packages mentioned include consumer commodity or ORM-D packages as well as limited quantity packages. The provisions for the pressure differential requirement are stated in section 173.27(c) and apply to non-specification and specification packagings (UN packagings).

The purpose of the pressure differential requirement is to prevent loss of contents in the event an aircraft experiences a loss of pressure. Such an event is rare among large commercial aircraft but smaller feeder aircraft used in package delivery fly at lower altitudes and do not maintain their cargo spaces in a pressurized condition. Exposure to pressures of about 60 kPa (i.e., 5.5 psi less than the 14.7 psi pressure at the time of package filling) is common.

Given PHMSA’s emphasis on the pressure differential requirement, one is compelled to ask what happens to a filled package when it undergoes an external drop in pressure. To a certain extent this depends on the package. A rigid glass bottle will only be affected at its closure. A low aircraft pressure on an upright, improperly sealed bottle would tend to draw out vapor from the bottle’s vapor space. Any release of contents would be limited to vapors. The lower pressure over the liquid would cause some liquid in the bottle to evaporate providing additional vapors for release. Somewhat similar effects are experienced by plastic bottles and metal cans except that the low outside pressure also tends to draw the flexible sides of these packages outward, possibly temporarily increasing the volume of the inner package until pressure is restored. In either case the loss of contents would normally be expected to be minimal. Greater loss of contents might be expected if the inner package was improperly oriented and liquid were against the closure but this would depend on physical properties of the liquid. More than likely a properly closed bottle, where cap and bottle are designed for each other and are suitable for retaining liquids at ground level in any orientation, is not likely to lose an appreciable amount of liquid in any orientation when subjected to only a pressure differential onboard an aircraft.
 
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