Propane Transfer Fugitive Emissions

This page last reviewed on November 14, 2014

Why Are Fugitive Propane Emissions an Air Quality Concern?

Propane is a volatile organic compound and ozone precursor.  Ozone is a criteria pollutant, and a major component of photochemical smog.  Exposure to ozone can result in damage to the respiratory tract, exacerbation of asthma symptoms, and accelerated deterioration of certain materials such as rubber and plastic.


Propane is a naturally occurring carbon based fuel that is extracted during crude oil refining and natural gas processing.  When exposed to normal atmospheric conditions propane exists naturally as a vapor.  However, for the sake of efficient storage, transport and transfer, propane is generally compressed into a liquid.  Prior to reaching its final point of combustion, the same gallon of propane is generally transferred several times.  During the transfer process, fugitive emissions of raw propane are released to the atmosphere through Fixed Maximum Liquid Level Gauges (FMLLG) and the disconnection of hose-end nozzles.

An Example of the Propane Transfer Process

FLLG MotorfuelThe vast majority of propane tanks and cylinders are equipped with an FMLLG.  The FMLLG is a brass vent valve threaded into the vapor space of a propane tank that transfer technicians use as the primary indicator for determining when a tank is full.  In general, "full" means that the liquid level inside the receiving tank has reached 80 percent of volumetric water capacity.  Figures 1 and 2 illustrate one type of FMLLG and one of severalForklift Cylinder Gauges potential applications.  When opened, FMLLGs are a significant source of fugitive propane emissions.

During the propane transfer process, transfer technicians open the receiving tank's FMLLG, which allows raw propane vapor to vent to the atmosphere.  The FMLLG is typically left open throughout the duration of the transfer process.  After opening the FMLLG, the flow of propane from the source tank is started and allowed to continue until the liquid level in the receiving tank reaches the FMLLG's dip tube.  The dip tube is typically a hollow brass rod calibrated for length that descends to a point within the tank corresponding to 80 percent of volumetric water capacity.  Once the liquid level reaches the dip tube, liquid propane begins to discharge from the FMLLG and the tank is considered full.  At the first sign of liquid phase emissions, transfer technicians immediately close the FMLLG and stop the flow of propane.  Additional propane is released to the atmosphere when transfer technicians disconnect the source tank's hose-end nozzle from the receiving tank's fill or service valve.  Please see the flow chart (Figure 3) and the "Propane Transfer Process Emission Video" for additional insight into propane transfer emissions.

Transfer process flow chart

Propane Transfer Process Emission Video

Contact the California Air Resource Board

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