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Billing Code - PHMSA - Department of Transportation

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vapor is discharged unobstructed.<br />

This requirement will enhance safety by assisting in the proper function <strong>of</strong> the<br />

pressure release devices. Further, it will prevent unsafe conditions in transport if the liquid<br />

form <strong>of</strong> the gas were released through the pressure release device. For instance, if the gas<br />

released is a Division 2.1 (flammable) gas or a Division 2.3 (poisonous) gas, it will reduce<br />

the opportunity for the released liquid form <strong>of</strong> the gas to create an asphyxiant environment<br />

through the rapid displacement <strong>of</strong> the normal concentration <strong>of</strong> oxygen in the breathable<br />

atmosphere. Finally, it will reduce the potential <strong>of</strong> released cryogenic oxygen in liquid<br />

form to create an oxygen enriched atmosphere that may explosively combust.<br />

Section 173.318<br />

Section 173.318 provides the requirements for cryogenic liquids contained in cargo<br />

tanks. Specifically, the HMR requires each cargo tank containing cryogenic liquids be<br />

equipped with one or more pressure relief devices.<br />

<strong>PHMSA</strong> is revising § 173.318, “Cryogenic Liquids in Cargo Tanks” to include a<br />

requirement consistent with the UN Model Regulations that all pressure relief device inlets<br />

must under maximum filling conditions be situated in the vapor space <strong>of</strong> the closed<br />

cryogenic receptacle and the devices be so arranged as to ensure that the escaping vapor is<br />

discharged unobstructed.<br />

This requirement will enhance safety by assisting in the proper function <strong>of</strong> the<br />

pressure release devices. Further, it will prevent unsafe conditions in transport if the liquid<br />

form <strong>of</strong> the gas were released through the pressure release device. For instance, if the gas<br />

released is a Division 2.1 (flammable) gas or a Division 2.3 (poisonous) gas, it would<br />

reduce the opportunity for the released liquid form <strong>of</strong> the gas to create an asphyxiant<br />

99

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