Ever needed to temporarily “change” the use of a flammable/combustible liquid storage or process tank? The tank is designed to NFPA 30 for flammable and combustible liquids so there is no risk in this “change”?!?!? WRONG…SWITCH LOADING is a MAJOR RISK that facilities who store, handle, or process flammable liquids need to understand, but often do not. The lesser vapor pressure product works with the higher vapor pressure product to generate the conditions for an explosion inside the container! Add the fact that the product entering the tank is a “nonconductive” liquid and we have a recipe for disaster.
The best known “switch loading” accident occurred in OK in 2007. The cost of the accident, including emergency response, environmental remediation, evacuation, lost product, property damage, and claims, was $2,357,483. There were no injuries or fatalities. Nearby residents were evacuated, and schools were closed for 2 days.The National Transportation Safety Board determined that the probable cause of the storage tank explosion and fire was ignition of a flammable fuel-air mixture within the tank by a static electricity discharge due to the improper manner in which the company conducted tank operations. The safety issues identified during the investigation of this accident are as follows:
- Tank operations, including switch loading, at the tank farm.
- The adequacy of emergency planning and emergency response.
- The adequacy of federal regulations and industry standards for emergency planning.
The following incident is a PERFECT example of why an MOC is needed when different products are stored in tanks, REGARDLESS if both the materials are flammable/combustible; just a difference in VAPOR PRESSURES between the products can be a set up for disaster. “Switch loading” refers to filling a tank, which previously contained a high or intermediate vapor pressure product, with a low-vapor-pressure product. “Vapor pressure” is the pressure exerted by a gas in equilibrium with a liquid at a given temperature. High-vapor-pressure products include aviation gasoline, motor gasoline, and naphtha. Intermediate vapor pressure products include toluene, xylene, benzene, and jet fuel (JP-4, Jet B). Low vapor-pressure products include heating oil, kerosene, and diesel fuel. At normal ambient temperatures, when a tank is filled with a high vapor pressure product, such as gasoline, the vapor space above the surface of the liquid EXCEES the LEL (too rich) to be flammable. When a tank is being filled with a low vapor pressure product, such as diesel, and there are NO residual vapors in the tank, the vapor pressure of the diesel is TOO LOW to make the vapor-air mixture above the surface of the liquid flammable unless the ambient temperature is at or above the flash point of the product. Diesel fuel has a FP of around 140° F – 165° F, well above what could be considered “ambient temperatures”. So changing the material from a high VP (lower FP) material to one that has a LOWER VP (higher FP), we have the makings of a “switch loading” accident. NOTE: Most switch loading accident occur in transportation containers, but as evident in the accident report below, it CAN HAPPEN in any container!
On April 7, 2003, at about 8:55 p.m., central daylight time, an 80,000-barrel storage tank at a tank farm in Oklahoma, exploded and burned as it was being filled with diesel. The tank, designated tank 11, had previously contained gasoline, which had been removed from the tank earlier in the day. The tank contained between 7,397 and 7,600 barrels of diesel at the time of the explosion. The resulting fire burned for about 21 hours and damaged two other storage tanks in the area. Personnel had previously conducted switch loading operations in tank 11 without incident. For the accident switch load, the diesel temperature (66° F) and the ambient temperature (52° F) were well below the flash point of the diesel (162° F). The flash point of the diesel was 162° F; the distillation curve showed it to be a “typical” diesel; the electrical conductivity was less than 1 pS/m; the water content was 65 parts per million by weight.
For combustion to occur in air with a hydrocarbon fuel, two conditions must be met:
- an ignition source must be present, and
- the fuel vapor concentration must be between the lower and upper flammability limits for that particular hydrocarbon vapor.
Outside this range, the vapor-air mixture is either too lean or too rich to be ignited. The flammability is dependent on the liquid’s vapor pressure and the environment in which it is handled. At the time of the accident, a switch loading operation was being conducted by loading diesel into a tank that had previously contained gasoline. Because of the hazards involved in this type of operation, both the National Fire Protection Association (NFPA) and the API have specific guidelines for dealing with the switching of product in a tank.16 Both organizations recommend precautions if the vapor space in a tank is at or above the lower flammable limit because of the previously stored product and if the tank is to be filled with a low-vapor-pressure, low-conductivity (static accumulating) product. These precautions include minimizing static generation, preventing charge accumulation, avoiding spark discharge, and controlling the environment inside the tank. Specific recommendations are suggested to minimize generation of static charge; these include
- avoiding splash filling and upward spraying,
- limiting the initial fill line and discharge velocities,
- avoiding loose or floating ungrounded conductive objects (such as gauge floats or sample cans) in the tank,
- avoiding pumping or flowing hydrocarbons with dispersed water or solids, and
- limiting the maximum fill rate.
For floating roof tanks, these precautions apply until the roof is floating, after which the liquid surface is at the same electrical potential as the floating roof, and the vapor space has been reduced. Factors that contribute to generating a flammable environment include
- the types of products involved,
- the amount of time the floating roof is landed with product in the tank, and
- the time the tank has for ventilation
Click Here to read the FULL NTSB Report (HIGHLY RECOMMENDED).