Matters of degree
The National Fire Protection Association (NFPA) recommends that enough odorant be added to propane so it can be detected at one-fifth (20%) of the lower limit needed for an explosion. This works out to be about a pound of ethyl mercaptan per 10,000 gallons of propane. As a precaution, some gas companies add an extra 50% of the odorant.
The intent is to provide a warning long before propane reaches a dangerous level in the air. Sounds simple and straightforward. But in reality, the level of mercaptan in propane used by consumers can vary widely.
Propane is stored as a liquid and used as a gas. When converted into a vapor, propane expands by about 270 times in volume. A large amount of propane can be compressed and stored as a liquid in a relatively small tank.
Ethyl mercaptan and propane have different boiling points and vapor pressures. Mercaptan has a boiling point of 95° F (35° C) and is less volatile than propane, which boils at -44° F (-42° C).
When a tank is filled or refilled, propane vaporizes much more readily than mercaptan. The first gas drawn off the gas will have the least odorant, and the concentration of mercaptan in the liquid portion of propane gradually increases as the fuel is consumed. There can be more than a 30-fold difference in the concentration of mercaptan in a full and nearly empty tank of propane.
This process is repeated over and over as propane is pumped from larger to smaller tanks down the distribution chain. When buying or refilling a tank of propane, you have no way of knowing whether the gas is odorized to the level in NFPA guidelines.
Simply adding more mercaptan to propane is not a solution. Over-odorization results in a high number of false leak reports, and doesn’t address issues of odor fade and impaired olfaction.
The NFPA odorization guideline applies to propane when it is produced, not when it is used by consumers. The standard does not ensure that propane remains odorized throughout the distribution system until it is used as fuel.
Back to Propane and Odor Fade