“Switch Loading”: Responsible for up to 80% of static-initiated explosions
At this time of year, when the air gets cooler and drier, we are often reacquainted with the annoyances of static electricity. When we walk across the carpet in our socks, we can generate a static charge that gives us a surprising “shock” when we grab a doorknob or hug a loved one. But static charges can also initiate devastating fires and explosions when a spark occurs in an explosive environment.
Some people may be surprised to learn that liquids flowing through pipes can also generate static charges. This is especially important for non-conductive liquids such as fuels and other petroleum products. Static charges arise from the flow and movement of the non-conductive liquid when it comes in contact with other materials such as pipes, pumps, filters, etc.
Armstrong recently investigated a tanker truck explosion that occurred as a result of “switch loading”. Switch loading is the process of loading diesel fuel into a tank that had previously contained gasoline and still contained gasoline vapors. According to ASTM, switch loading or splash filling (or both) account for 80% of static-initiated explosions.
The trailers of fuel tanker trucks typically contain multiple separated compartments for transporting fuel. Normally, gasoline fuel tanks are “too rich” to ignite and diesel fuel tanks are “too lean” to ignite. But in an intermediate scenario, such as during switch loading, it is possible to generate an explosive environment in the tank where the fuel-to-air concentration is between the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL) for the system. When a static discharge occurs inside the tank, the spark can ignite the fuel with destructive results.
Numerous precautions are typically taken to minimize the generation of static discharges when filling a fuel tank, such as minimizing splashing, limiting flow speeds, and allowing adequate charge “relaxation time” after passing through a filter. For more information, refer to the standard published by ASTM: D4865 – Standard Guide for Generation and Dissipation of Static Electricity in Petroleum Fuel Systems.