LPG Odorants in Rail Tank Cars

But in what appears to be a regulatory lapse, the federal odorant standards overseen by the Department of Transportation’s Federal Railroad Administration (FRA) apply only to LPG rail shipments in cargo and portable tank containers; there is no comparable regulation for LPG transported in railroad tank cars.  In a safety advisory issued April 2012, the FRA said it was reviewing the “situation to determine if further action is warranted.”  Also, in the absence of a regulation, FRA’s safety advisory listed measures that operators who ship LPG in rail tank cars should take to ensure that odorant is added to LPG rail tank cars in sufficient quantities and that the condition of the car does not result in odorant fade.

Ethyl Mercaptan

Hazardous materials regulations (HMRs) at 49 CFR 173.315(b)(1) provide that odorizing LPG shipments in cargo and portable tanks with   1.0 pounds of ethyl mercaptan per 10,000 gallons of LPG, or the equivalent, is an acceptable form of odorization.   That section also provides an exception from the odorization requirement if odorization would be “harmful in the use or further processing of the [LPG], or if odorization will serve no useful purpose as a warning agent in such use or further processing.”  Essentially, this exception applies to LPG being transported to industrial end-users. 

The above requirements are mirrored in OSHA standards regarding the storage and handling of LPG (29 CFR 1910.110(b)(1).  In addition to the federal regulations, the National Fire Protection Association (NFPA) has established odorization standards that also largely mirror the federal requirement.  The FRA notes that it is standard industry practice to exceed the established regulatory minimum of ethyl mercaptan to combat the effects of odorant fade should a release of material occur.

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Causes

Odorant fade can generally be traced to two causes–oxidation and/or the condition of the container.  Oxidation can cause odorant fade when the presence of rust in a tank car, or the subsequent formation of rust over time, as a result of the presence of oxygen and moisture, decreases the amount of odorant that is in the LPG due to a chemical reaction between the odorant and the oxidized (rusted) surface.  The presence of rust causes mercaptans to oxidize into other compounds that have a different odor and lower intensity.
Next, the condition of the LPG container itself can potentially cause odorant fade.  An odorant can adsorb onto the metal surface of the container or even potentially be absorbed into the metal surface itself.  This process is most likely to occur when the container is new and has not previously contained odorized LPG.  It can also occur when the inside of the container has been left open to the air while the container is out of service or after it has been cleaned and purged.

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Safety Measures

In an effort to ensure that LPG in railroad tank cars contains a sufficient level of odorant through the entire transportation cycle, FRA’s safety advisory recommends that:

•              Facilities that load, offer, receive, or offload railroad tank cars containing LPG review their procedures to ensure they are adequate to address odorant fade and its various potential causes and that those procedures ensure that tank car shipments of odorized LPG are odorized to meet applicable regulatory and industry requirements and maintain sufficient levels of odorant through the entire transportation cycle.  Such procedures should ensure that quantitative testing methods are used to measure the amount of odorant in LPG.
•              Facilities that load odorized LPG into railroad tank cars have adequate procedures in place to identify if a tank car received for loading of odorized LPG has been out of LPG product service for any extended length of time, is coming from a tank car repair or cleaning facility, or has been subjected to any condition that could lead to corrosion of the tank.
•              Facilities that load odorized LPG into railroad tank cars inspect, to the degree possible, railcars they receive for signs of oxidation or corrosion.
•              Facilities that load odorized LPG into tank cars take any other corrective actions needed to ensure sufficient levels of odorization remain in the shipment through the entire transportation cycle, such as increasing the amount of odorant injected into the LPG, if necessary.

FRA’s safety advisory was published in the April 13, 2012, FR.