Extended Tank Probes Alleviate Problems with Overfill Spills
Sep 1, 1998 12:00 PM
All tank truck probes used for overfill protection during bottom loading should reach far enough into the vessel to trigger terminal rack shutdown before a spill can occur, said Al Mosser, senior standards engineer for marketing operations services of Chevron Products Company.
Mosser discussed product spills caused by tank truck overfilling at the 1998 annual Independent Liquid Terminals Association International Operating Conference and Trade Show in Houston. He is scheduled to discuss the subject at the National Tank Truck Carriers Cargo Tank Maintenance Seminar and Equipment Show, Oct 19-21 in Chicago.
"Sixty-gallon outage is needed in every tank compartment to assure that shutdown can occur within four to six seconds," he said. "This is taking into consideration that loading flow rates commonly are in the range of 600 to 900 gallons per minute."
A 4,400-gallon tank compartment should have a probe that extends about four inches into the tank while the probe in a 1,250-gallon tank should protrude at least eight inches into the tank. "Note that most standard length overfill probes aren't long enough," he said. "Longer probes should be specified."
Truck inspections should focus on verifying the proper installation and operation of the probes. Wet testing of each probe with the truck's equipment plugged in to either a test box or a rack fully tests the system. Also each probe should be measured to ensure that it is installed deep enough to provide the 60-gallon outage. Tank capacity charts, available from tank manufacturers, may be needed to properly calculate the outage.
Industry Standards Mosser recommended that tank truck owners install adequate probes, and that the industry raise its standards regarding overfill protection.
"Many tanks do not have overfill probes installed deep enough, especially on smaller compartments," he said. "Many companies think that their systems are fail-safe, but overfills still happen because of inadequate equipment and errors by operators who are usually drivers," he said.
In addition to tank equipment improvement, he listed three other ways to prevent overfilling: * Terminal loading racks should have a redundant valve in conjunction with an independent shutoff system. * Drivers should be trained in appropriate loading techniques. * All incidents and near-incidents should be reported and reviewed.
"The industry practice at terminal loading racks is to have the overfill prevention system signal the loading riser control valve to close," Mosser said. "But, when the cause of the overfill is the failure of this valve to properly close, a classic Catch-22 happens. The solution is to have a truly independent shutoff system-a redundant valve."
A further problem involved in overfilling tanks rests with the operator. Sixty percent of overfill incidents result from errors made by people who are filling the tank. Three of the most common errors occur when the operator sets too large a preset,loads on top of product that is already present in the tank, or loads a second batch of product into the same compartment.
"Regardless of the success of our behavioral safety program, we can never guarantee that operators won't make mistakes, so we must have very reliable shutoff systems to fill the gap of human error," Mosser said.
Avoiding Problems Reporting all incidents and near misses allows companies to review what problems were caused and determine ways to avoid them in the future. "If an overfill probe gets wet, something went wrong," he said. "The thing to do is to address all the root causes."
Numerous overfills occur each year. Although few serious incidents have happened in recent years, the statistics don't erase the danger. "Since fuel and oxygen are present when the product spills out of the tank, the only missing item for a potential disaster is an ignition source. That could be provided by an adjacent truck engine," he said.
Mosser pointed out that "static" overfill prevention systems are not self-checking. "If the truck's controller does not pass the overfill signal to the loading rack, no overfill protection is in force," he said.
Drivers can test static systems by pushing the test button on the tank truck's on-board controller while watching the rack overfill unit's light go from green-to-red and return from red-to-green after the button is released. "This verifies that the on-board controller is properly passing the overfill signal to the rack," he said. "If a unit fails this test, the tank should not be loaded until repairs are completed and the test properly completed."
Systems Use Some systems use an electronic dummy or terminator. If the tank truck has a by-passed or shorted-out probe, it incorrectly gives a green light when the situation is not safe.
Chevron has developed a 'potential' overfill detection system, which uses a green-to-red warning light that responds during loading. The system does not respond when the driver has completed the procedure and is disconnecting. Overfill prevention systems may register alarms when truck probes are not wet, but a break has occurred in the system's circuitry. These can be considered false alarms.
"The real challenge is to properly reset the system after determining what caused the problem - an actual wet probe or just a break in the circuit," he said. "Terminals typically get about 15 alarms daily, the overwhelming type being a circuitry break, not wet probes. These are far too many to shut down the entire loading rack, or even one lane. What is required is for the driver to follow exact loading procedures and for someone to replace worn truck sockets and damaged rack cords and plugs. Then, about one to three alarms per week can be expected, which is an acceptable level."
Fire codes and industry standards should be updated in order to prevent spills. "The codes, standards, and recommendations are far too general," he said. "More specific requirements are necessary. None of these deal with a failed open riser control valve."
Terminal operators should review and adjust control valves' needle valves, check and adjust overfill cord lengths, verify and modify control system programming, and increase preset slow flow stop quantity to 100 gallons.
In addition, the terminal should minimize tank safety valve closing time to within four to six seconds. Either install actuators on each riser's manual isolation valve, or shutdown all pumps and tank safety valves, check and secure rack overfill unit's bypass switches, and periodically test rack overfill unit operation.
Finally, he advised shippers and carriers to work together and with other groups in the industry to raise the standards, and to encourage terminal operators to take action to prevent overfills.
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