Hazards of Contamination – Case Study of a Explosion Resulting from Switch Loading

Scott Davis*1, Peter Hinze1, and Kees van Wingerden2 1 GexCon US, Bethesda, Maryland, USA 2 GexCon AS, Bergen, Norway *Presenter Email: [email protected]

Abstract When a tanker truck is drained of a Class I liquid (e.g., gasoline) and refilled with a similar liquid, there is no flammable mixture on the surface of the rising fluid because Class I fluids create mixtures at their surface that are too fuel rich to burn. Similarly, if a tanker truck is drained of Class II liquid (e.g., diesel) and refilled with a Class II liquid, there is no flammable mixture created because Class II liquids do not produce ignitable mixtures at ordinary temperatures. An extremely hazardous condition occurs, however, if the liquids are switch loaded, which is when a Class II or III liquid is loaded into a tank vehicle that previously contained a Class I liquid. Under these conditions, the atmosphere above the fluid surface can become ignitable. An explosion can occur if an electrostatic spark occurs from the (poorly conducting) fluid surface to another object, igniting the flammable mixture in the headspace of the tank. This paper describes a recent incident involving an explosion during the filling of a tank of a transport truck with . This particular tank was previously filled with gasoline and was not completely emptied of the gasoline prior to the switch loading. After only filling one-quarter of the programmed load, an electrostatic spark ignited the flammable mixture within the head space and caused significant damage to the truck and neighboring tanks. The study will address issues and lessons learned regarding spark promotion with the tank, reduction of static generation by the fluid being filled into the tank, and elimination of the flammable mixture within the tank prior to switch loading.

Background The incident involved the loading of petroleum transport tank trailer. The tanker has five compartments, which are designated numbers 1-5, with the number 1 compartment located near the tractor and the number 5 compartment located at the rear of the tanker. The total load capacity is 9200 gallons. Compartment 1 held 3000 gallons of fuel, compartment 2 held 1500 gallons, compartment number 3 held 1200 gallons, compartment number 4 held 1000 gallons, and compartment number 5 held 2500 gallons of fuel. Compartments numbers 4 & 5 were separated by a double bulkhead and compartment 5 was also equipped with internal baffles, which consist of intermediate walls with openings that allow fuel to shift within the compartment. Each compartment was fitted with a 1/8” stainless steel ground wire, commonly referred to as a “static diffuser cable.” The ends of the static diffuser cables were crimped or looped to attach to welds at the top and bottom of each compartment, resulting in the cables running vertically from the top to the bottom of each compartment. Loading rates for diesel at the loading rack are automatically controlled by a computer system such that they start at 0 gallons per minute and ramp up to 150 gallons per minute for the first 20 seconds (or 50 gallons) of the loading. Subsequently, the rate increases to a maximum of 600 gallons per minute, which are within compliance with API RP 1004 (i.e., maximum filling rate of 900 gallons per minute). For loading operations, the tank trailer user manual specifically warns: “Always clean a tank before loading a different type of product. When a tank trailer is emptied of fuel, a mixture of vapor and air remains that is within the flammable range. Refilling the trailer with a type of product other than that which was just emptied may detonate that mixture and cause an explosion, which may cause extensive property damage and/or serious personal injury, including death.”

Events leading up to the incident A static dissipater is added to the ultra-low sulfur diesel processed at the refinery. The treated diesels samples from the product rundown stream are tested every day for conductivity. The target electrical conductivity range after the static dissipater addition was 50-150 picoSiemens per meter (pS/m). Diesel product samples taken during the weeks prior to the subject incident indicate the presence of the static dissipater additive at varying levels, some of which were above targeted conductivity levels and some of which were below. It was determined (after the incident) that the chemical injection pump was not functioning properly. A work order for the additive pump had been issued; however, no sampling records exist for the product loaded into the incident trailer. On the day prior to the incident, the driver picked up and made three fuel deliveries. The last load consisted of 9109 gallons of unleaded gasoline, which means that every compartment was nearly filled with unleaded gasoline. He finished loading his trailer at approximately 3:51 PM and delivered this load to a service station that same afternoon. Records indicate that there was confusion as to the exact quantity of gasoline unloaded at the third and final delivery, and it was reported that there was a quantity of remaining liquid gasoline in all compartments of his tank trailer prior to loading on the morning of the accident at issue. Day of the incident On the day of the incident, an explosion and fire occurred at approximately 7:39 AM. The temperature was approximately 43 degrees Fahrenheit and the wind was blowing approximately 6 mph. The explosion and fire occurred in the tank trailer while the driver was loading it with ultra-low sulfur diesel fuel. Surveillance footage of the incident shows that the driver pulled his tank trailer into the lane at the loading rack and performed the following as he hooked up the trailer: (1) connected the gasoline and diesel loading arms to the truck; (2) attached the Scully grounding and overfill protection to the truck; (3) connected the individual compartment vents to the vent rail system; and (4) attached the truck rack’s vapor recovery hose to the truck’s vapor recovery system. While the Scully grounding and overfill protection as well as the vapor recovery system should have been made prior to connecting the loading arms for vapor containment, this appears not to have been a cause of the incident. Table 1 below shows the order of filling and the volume filled for these compartments. The driver first loaded gasoline in compartment 2, then ultra-low sulfur diesel in compartments 1, 3 and 4. The explosion occurred while the driver was loading ultra-low sulfur diesel into the last compartment 5. The activity log indicated that the total load of diesel was into 5865 gallons. The total amount of diesel loaded in compartments 1, 3 and 4 was 5170 gallons, which leaves 695 gallons loaded into compartment 5 at the time of the incident. A witness indicated that immediately prior to the accident the driver answered a telephone call on his cellphone device utilizing an earpiece. When the driver turned to take the telephone call the ignition occurred. Figure 1 and Figure 2 show images of the fire originating at compartment 5 and progressing towards the front of the truck (incident truck is in the lane behind the visible truck).

Table 1: Events related to filling and explosion on the day of the explosion

Time Activity

7:20 AM Filling of compartment 2 (1500 gal) commences - 1490 gallons of gasoline

7:25 AM Filling of compartment 1 (3000 gal) commences - 2990 gallons of diesel

7:31 AM Filling of compartment 3 (1200 gal) commences - 1190 gallons of diesel

7:34 AM Filling of compartment 4 (1000 gal) commences - 990 gallons of diesel

7:37 AM Filling of compartment 5 (2500 gal) commences - 2490 gallons of diesel

7:39 AM After approximately 700 gallons had been loaded, an explosion and fire occurred originating in compartment 5. This initiated the loading rack’s flame activated foam system

7:48 AM The fire at the truck rack was completely extinguished, including in all compartments of the incident tank trailer.

Figure 1: Incident truck in the lane behind the visible truck. Lower image is explosion initiating at the rear of the truck in compartment 5.

Figure 2: Explosion progressing from compartment 5 and engulfing all compartments. Inspections Inspection confirmed the trailer had a total nominal capacity of 9200 gallons, a maximum allowable pressure of 3.3 psig, maximum load rate of 1300 gallons per minute and unloading rate of 190 gallons per minute. The external body of the tank trailer was deformed at locations that appeared to be consistent with the damaged internal baffles (surge heads) and bulkheads (see Figure 3 and Figure 4). The manhole cover for compartment 4 was completely blown off the trailer (see Figure 3). The internal baffles had a manway in the center and two drain holes (orientated in the 6 and 12 o’clock position relative to the manway). The internal baffles (surge heads) were all directionally deformed in the direction from the rear of the truck towards the front (i.e., from compartment 5 towards 1), as shown in Figure 5. Internal inspection of the interior confirmed that all of the internal baffles and bulkheads were deformed in the direction from the rear of the trailer to the front (e.g., from compartment 5 to compartment 1 as shown in Figure 6). The height of the bottom outlet and spray deflector was approximately 2 inches (see Figure 7).

Figure 3: External damage to truck trailer body near compartments 3 (1200 gal) and 4 (1000 gal). Note manhole cover blown off on compartment 4.

Figure 4: External damage to truck trailer body near compartments 1 (3000 gal) and 2 (1500 gal)

Figure 5: Damaged internal double bulkhead. View from compartment 4 looking back to compartment 5.

Figure 6: Damaged internal bulkheads and baffles

Figure 7: Bottom outlet and spray deflector of compartment 5 Electrical continuity was also verified from the ground terminal on the two Scully outlets on the truck to various locations: Pos. 1 – hose tray, Pos. 2 – manhole, Pos. 3 – ground cable, Pos. 4 – internal baffle and Pos. 5 – internal bulkhead. The results can be seen in Table 2. Table 2: Electrical Continuity Measurements (Ohms)

Analysis

Ignition of Ignition of liquid hydrocarbons occurs when the is in the vapor state. For a hydrocarbon to ignite, its vapor concentration in air (or another oxidizer) must be within a very specific range. This range is called the fuel’s flammability limits, and is typically referred to at its limits as the lower flammability limit (LFL) and the upper flammability limit (UFL). A fuel vapor cloud with a concentration outside of this range will be impossible to ignite; if the concentration of fuel is too low the mixture is referred to as being too fuel lean (i.e., below LFL), and if it is too high it is too fuel rich (i.e., above UFL). For multi-component fuels, such as gasoline and diesel fuel, the characterization of flammability limits is necessarily more complex. Gasoline and diesel are fuels with hundreds of different species. In practice, gasoline transport in tanker is inherently safe because the concentration of fuel vapors is always over the UFL; that is, the concentration of gasoline vapors in the headspace of a tanker truck, or the fuel tank of a passenger vehicle, is always too rich to ignite. This is because gasoline’s flash point—the lowest temperature at which a flammable vapor mixture may be formed above a liquid surface—is extremely low, typically below -40° F. This is especially true for gasoline sold during the colder months (typically after September 15 for many areas in the US), which has an even higher vapor pressure resulting in vapor concentrations further above the UFL. Conversely, transport of diesel fuels is safe because the vapor pressure is much lower. So much lower that the headspace of a tank trailer transporting diesel contains a concentration of fuel vapor that is too lean to ignite. For reference, the flash point of diesel fuel is typically over 140° F. However, when diesel and gasoline are mixed, a hazardous condition may arise, as the flash point of the mixture passes through typical ambient temperatures. If there is a quantity of gasoline in a large vessel and diesel fuel is added, the concentration of vapor in the headspace air will change from being too rich to support combustion, to being within the flammability limits, to, ultimately, being too lean to ignite as the quantity of diesel fuel is increased. If there is a competent ignition source when the vapor concentration is within the flammability limits, then an explosion can occur. Due to the high volatility of gasoline, especially winter blends, only a small fraction of gasoline contamination of the diesel is required (see testing section) in order for the vapors above the contaminated diesel to transition to within the flammable range. In particular, gasoline sold during the colder months (typically after September 15) has a higher vapor pressure and, therefore, represents an even greater hazard as it more easily reaches a flammable concentration when mixed with diesel fuel.

Flash point testing The flash point of a liquid is the lowest temperature at which sufficient vapor is evolved to form a flammable mixture with air at standard atmospheric pressure. The sample is heated to the desired temperature and a test flame or other ignition source is then introduced over the surface of the sample and it is noted whether or not ignition occurs. The Pensky-Martins Closed Cup technique was chosen to replicate the condition of liquids and vapors being contained in a vessel. Flash-point tests were conducted in order to determine the minimum amount of gasoline required in diesel to maintain the sufficient vapor being evolved to form a flammable mixture.

Table 3: Sample flashpoint results for diesel/gasoline mixtures at 45°F (8°C)

The temperature at the facility at the time of the incident was approximately 45°F (~8°C). The temperature of the diesel being pumped into the incident tank trailer was recorded at approximately 87°F. Therefore, the likely liquid temperature on the day of the incident was between 45°F and 87°F. Both diesel and unleaded gasoline (winter blend) were purchased and it was determined that approximately 4% gasoline (by weight) was required for mixtures at 45°F (see Table 3) and approximately 0.5% gasoline (by weight) was required for mixtures at 87°F.

Switch loading Switch loading is the practice of reloading a tanker compartment which previously carried a high volatility distillate fuel (e.g., gasoline) with a lower volatility distillate fuel (e.g., diesel fuel). Switch loading has been identified and recognized as a potentially hazardous practice by the National Fire Protection Agency (NFPA). In addition, API Recommended Practice 2003 indicates that in most cases, static-related fires within tank truck compartments have involved either the loading of an intermediate vapor pressure product or switch loading. In NFPA 385, Standard for Tank Vehicles for Flammable and Combustible Liquids, section B.5 of Appendix B is entitled Switch Loading, and it describes the circumstances under which switch loading may be hazardous. More specifically:

 When diesel fuel is loaded into a tank that had previously contained gasoline, there may be a flammable vapor mixture at the surface, which may be ignited by a competent ignition source, such as a static discharge. For switch loading, NFPA 385, section 9.1.10 states the following:

 To prevent a hazard from a change in the flash points of liquids, no cargo tank, or any compartment thereof, that has been utilized for Class I liquid shall be loaded with Class II or Class III liquid until such tank or compartment and all piping, pumps, meters, and hose connected thereto have been completely drained. Gasoline is a Class I liquid (flash point below 100°F), and diesel fuel is a Class II liquid (flash point above 100°F). In addition, NFPA 385 discusses loading procedures that present minimal to no risk of ignition. More specifically:

 Refilling a tank that had previously contained gasoline with gasoline again is not generally a hazardous action, because the vapor mixture at the surface remains above the flammable range and cannot be ignited

 If diesel fuel is reloaded into a tank that had previously contained diesel, there is no danger of creating a flammable mixture, as the vapor concentration will always be below the LFL. API RP 2003 further warns against contamination from inadequate flushing of product lines and other equipment before another product is introduced (such as may occur with switch loading).1 Similarly, the Owner’s/Operator’s Manual includes the instruction to “[a]lways clean a tank before loading a different type of product.” Finally, the Loading Rack Rules and Regulations states that “[t]ruck drivers are responsible for emptying all drainable product before loading into any compartment.” Switch loading hazards are exacerbated when a quantity of remaining liquid gasoline is present in the compartment prior to filling with diesel, because even small amounts of contamination by gasoline can result in flammable vapor concentrations at the liquid surface.

Static build-up during fuel loading Static electricity is typically generated when two dissimilar substances are in intimate contact and in relative motion. When petroleum-based fuels flow through hoses, pipes, or fuel filters, a charge separation occurs; the liquid and the piping (or container) become oppositely charged electrically. Factors that influence the rate of charge generation are the surface area where the fuel and the piping and/or container are in contact, and the rate at which the fuel is flowing. The higher the surface area and the greater the fuel flow rate, the more rapidly static charge is generated. Charge in liquid fuels in a grounded container will dissipate at a rate that varies according to the conductivity of the fuel. A static dissipater additive does not eliminate generation of static electricity. The static dissipater additive makes the fuel – in this case ULSD – more conductive, which allows the static generated to move more freely to ground. It does not eliminate or diminish the generation of static electricity in the fuel, it reduces accumulation of static electricity and potential for static discharges.

1 API RP 2003, p. 11. Various codes and standards recommend ways to reduce the probability of static discharge igniting a flammable cloud. These recommendations typically focus on several different strategies: 1. Reduced fuel flow rates during initial fill, 2. Reduced maximum flow rates, 3. Inclusion of bonding wires inside the compartments to be filled, and 4. Inclusion of a static dissipater additive, These standards and recommendations will be discussed below. ASTM D975 The American Society for Testing and Materials (ASTM) has a standard that enumerates the specifications for diesel fuel , ASTM D975 Standard Specification for Diesel Fuel Oils. This standard states the following2: Accumulation of static charge occurs when a hydrocarbon liquid flows with respect to another surface. The electrical conductivity requirement of 25 pS/m minimum at temperature of delivery shall apply when the transfer conditions in Table 2 exist for the delivery into a mobile transport container (for example, tanker trucks, railcars, and barges). Table 2 of the standard shows that, for delivery of fuel into tank truck compartments, with a maximum pipe diameter of 0.1023 m (4.028 in), the fuel velocity must be 4.9 m/s or greater for the 25 pS/m requirement to apply. The maximum flow rate from the fuel rack on the day of the incident into the tank trailer was about 439 gal/min, which is a flow velocity of 3.372 m/s through a pipe with an internal diameter of 4.026 inches. Hence, since the fuel flow velocity was less than the 4.9 m/s specified in the table, the 25 pS/m conductivity requirement does not apply for this situation. The API Recommended Practice 2003 has a similar specification for the maximum flow rate, where the value is determined by 0.5/d m/s, where d is the diameter of the inlet pipe in meters. For a pipe of 4.026 inches (0.1023 m), this value for maximum fill velocity is 4.889 m/s. ASTM D975 further indicates that ULSD fuels are susceptible to very low electrical conductivity. In addition it states in the appendix: Fuel handlers should not be lulled into a false sense of security if the fuel meets or exceeds the minimum conductivity requirement. Improved fuel conductivity will accelerate the dissipation of electric charge but not eliminate the risks associated with handling combustible or flammable fuels. Fuel handlers should be aware of the increased static electricity production when diesel fuels are filtered through fine-mesh strainers and filters. Fuel handlers are encouraged to use industry-recommended safety practices to minimize the risk associated with handling fuel. One such safe operating practice

2 ASTM D975-12, Section 8.1. recommends lower maximum flowrates upon initial loading procedures. Loading operations involving “switch-loading” of tanker trucks and other vessels pose increased risks. For those fuel transporters that practice switch loading of fuels without container cleaning and purging after hauling high or intermediate fuels or solvents, risks are involved with that practice. Switch loading should be discouraged because of the difficulty in ensuring removal of all residual vapor-producing materials. API Recommended Practice 2003 The American Petroleum Institute (API), which represents all aspects of the U.S. petroleum industry, publishes API Recommended Practice 2003, which has guidelines for tanker truck filling. Table 1a is entitled Summary of Precautions for Tank Truck Loading. This table states the following: INITIAL LOADING—Top loading down spouts and bottom loading outlets should be equipped with spray deflectors. Splash filling should be avoided. The liquid velocity in the fill line should be limited to about 1 m/s (3 ft/s) until the outlet is submerged to prevent spraying and to minimize surface turbulence… As these guidelines show, for an initial filling rate that is low enough (liquid velocity about 1 m/sec or 3.28 ft/sec), the development of static electricity will be mitigated. In this case, the starting fill rate for the rack is 150 gal/minute, which, when passing through a pipe with an internal diameter of 4.026 inches, is approximately 1.15 m/sec. The standard further states: Switch loading brings potentially high risks. Experience shows that many static related incidents have occurred during switch loading. Switch loading is the most frequently cited cause of static incidents when handling bulk hydrocarbons. Loading heating , diesel fuel or lubricating base oil with low electrical conductivity into a tank that previously contained gasoline is an example of switch loading. Even when the compartment appears free from standing liquid from the previous load the tank or compartment can contain a flammable mixture. Residual vapors from the previous high or intermediate vapor pressure cargo in an "empty" tank mixed with air can be in the explosive range. Then, static electricity can accumulate when loading a low conductivity, low vapor pressure product. In the wrong set of circumstances there can be a static discharge and ignition.

Analysis The explosion occurred when the last compartment (compartment 5) was filled with approximately 700 gallons of ultra low sulfur diesel. Review of the evidence suggests that the explosion originated within compartment 5 with a static discharge being the most likely ignition source. Due the malfunctioning chemical pump, it appears that adequate levels of the static dissipater additive were not present in the diesel fuel during loading. This increased the likelihood of static accumulation during the filling, which ultimately resulted in a static discharge within the compartment. Another key causative factor at issue in this incident was the presence of gasoline within compartment 5 prior to filling with the diesel fuel. The lack of static dissipater additive cannot explain the presence of a combustible environment inside the tanker. Rather, there was a required ratio of diesel fuel quantity to gasoline quantity for there to be a flammable mixture inside the compartment. Further investigation, testing and analysis confirms that there was a significant quantity of gasoline in compartment 5 when the driver went to load on the date of the incident. Experiments have also determined that the significant quantity of gasoline, which remained in compartment 5 of the tanker truck, was a significant causal factor in the fire/explosion. Lastly, the operator of the tank trailer had no knowledge of the term “switch loading” or any risks associated with switch-loading prior to the date of the accident. It is therefore extremely important that operators and drivers are explicitly trained on safety practices regarding this hazard

Conclusion An explosion and fire occurred at the refinery while the driver was loading ultra low sulfur diesel into compartment 5 of his tank trailer. The explosion initiated in compartment 5, after approximately 700 gallons of diesel had been loaded into the tank. The explosion deformed the internal baffles and bulkheads towards the front of the tank trailer, and blew off the manhole cover to compartment 4. The were several contributory factors to the incident: 1. Switch loading in compartment 5 creating a flammable vapor space on top of the liquid during the filling. 2. Contamination of the diesel by a quantity of remaining gasoline present during the switch loading. The contamination by gasoline increased the vapor pressure of the mixture so that a flammable vapor space was present even after approximately 700 gallons of diesel had been loaded into the tank. 3. Reduced conductivity of the diesel due to failure of the chemical pump adding static dissipater, increasing the likelihood of static accumulation during filling, which resulted in a electrostatic discharge ignition of the flammable mixture. The investigation also concluded that there was inadequate training of the drivers regarding the safety hazards associated with switch loading.