Electrostatic Charges Pose Risks, Challenges at Petroleum Terminals
Sep 1, 1999 12:00 PM
THE RISK of fire or explosion is always present at even the best maintained and managed petroleum loading racks. Operators need to be involved in a constant process of risk reduction and elimination.
One of the biggest risks is static electricity buildup during the loading process. A couple of recent static electricity incidents damaged loading racks in California belonging to Chevron Products Company.
An analysis of those incidents was presented by Al Mosser, Chevron senior standards engineer, and Terry Hartman, fire protection/process safety, Chevron Research and Technology Company. They spoke June 15 during the Independent Liquid Terminals Association (ILTA) annual meeting in Houston, Texas.
Each of the incidents occurred during switch loading of low sulfur diesel. The static ignitions happened at the same terminal but at different loading racks. No injuries resulted, but the racks were moderately damaged. The tank trailers were significantly damaged.
Chevron officials were unable to determine a specific cause for the recent incidents, but they found a number of contributing factors. Mosser and Hartman suggested a number of changes for loading racks and cargo tanks.
Hartman explained that four conditions must be present for an electrostatic ignition. A means of generating an electrostatic charge must exist, and there must be a way of accumulating a charge capable of producing an incendiary spark. A spark gap is needed, as well as an ignitable vapor-air mixture.
Preventive Measures A number of steps can be taken at tank truck loading racks to control or eliminate the conditions that can lead to electrostatic ignition. Risk control starts with preventing static charges from being generated. Static charges are generated in a number of ways in loading operations-fluid flowing through a pipe, fluid flowing through a filter, or splash loading.
First and foremost, operators should avoid splash filling. Inlet velocities should be kept below three feet per second until the inlet nozzle is submerged. Product residence time from the filters to the cargo tank should be greater than 30 seconds. Adequate bonding or grounding is essential. There must be no loose conductive objects or spark promoters inside cargo tank compartments. Tanks should be allowed to sit for at least a minute after filling is completed before being gauged or sampled.
In one of the recent incidents described during the presentation, the pumps were running at 1,280 gallons per minute, while the filters had a lower flow rating. Filter size was 25 microns, and there was no written procedure to monitor filter condition. Shifting to the cargo tank, Hartman suggested that the current splash deflector design may be inadequate.
Charge Accumulation Electrostatic charges accumulate for a number of reasons, one of the most important being low conductivity of a material. Highly refined products are poor conductors, which makes them good accumulators. Low sulfur diesel is so low in conductivity that a static charge won't readily dissipate no matter how well the tank is grounded. Ungrounded, conductive objects also accumulate charges.
Residence time is a factor in charge accumulation. The new API 2003 standard recommends up to 100 seconds residence time from the filters to the cargo tank. In the recent incidents, static charges may have accumulated on isolated or loose conductive objects in the tanks.
Chevron also found three possible areas where spark gaps may have existed in the cargo tanks-lip of the baffle opening, mechanical push rod that connects the internal valve to the vapor vent, or a welding rod that had been left inside.
Flammable vapor-air mixtures can occur for a couple of reasons at a petroleum loading rack. It could be that product is being handled at temperatures that are close to or above the flash point.
An ignitable mixture can be achieved during switch loading, especially when a low-vapor-pressure product is loaded into a tank that previously contained a high- or intermediate-pressure product. In the Chevron incidents, the trailers previously had hauled gasoline and were being loaded with diesel.
Options Studied Chevron has evaluated a number of factors in its effort to eliminate the risk factors. Mosser and Hartman have considered removing filters, lowering flow rates, developing new procedures for monitoring filter condition, using inert gases to limit oxygen concentration, and eliminating switch loading.
"We've taken some steps already, but we believe more study is needed," Mosser said. "Drastic measures, such as requiring dedicated trucks, would have a significant impact on the petroleum industry. We are taking a closer look at filter systems, and we are evaluating injection of conductivity additives and increased residence time.
"On the tank trailers, we want to work with the industry to ensure adequate splash deflector design and to develop an inspection program for deflectors and internal bonding cables. Baffle openings should be reviewed from a design perspective. New tanks need to be inspected to ensure that debris, such as welding rods, is removed before the vehicles are put into service."
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