Overfill Concerns Reviewed at Meeting
Feb 1, 1999 12:00 PM
Maintaining overfill prevention equipment saves money and enhances safety, said Mark Adams of Scully Signal Company. He discussed the benefits of a preventive maintenance program for overfill prevention equipment at the NTTC meeting.
"The addition of a preventive maintenance inspection program for overfill prevention equipment can increase the equipment's longevity and performance," he said. "If the preventive maintenance inspection program is done in conjunction with regularly timed vehicle inspections, cost of vehicle downtime is minimized."
Premature failure of a vehicle's overfill prevention system is, in most cases, the result of corrosion. Questionable wiring integrity or worn socket connections are easily spotted during visual inspections and require minimum time to repair. The inspection should include removing the sensor holder covers and socket faceplates. Examination should focus on the areas of connection for corrosion, which usually is manifested in the form of a white powder or green coloration on the connector.
Bad wiring connections due to corrosion, when found early, can be repaired by simply removing corroded connections and resplicing. When corrosion is not addressed in a timely fashion, the corrosion can spread throughout the system wiring, thus reducing the operational integrity of the overfill prevention system, as well as creating a situation that could prematurely fail the system components.
"Corrosion must be removed to reduce the risk of it spreading throughout the overfill prevention system's wiring," he said.
Relative changes in temperature will create humidity that can cause corrosion. Mating of the metal strands within the wiring and the metal contact pins also can cause corrosion.
One way to reduce corrosive situations is to insulate the electrical connections. If the amount of air around an electrical connection is restricted to a nominal amount, humidity levels and electrolysis effects on corrosion growth are reduced.
Corrosion at the wiring connections to the sensors adds resistance and often will result in symptoms of intermittent operation that are difficult to diagnose. Symptoms may come and go or even change, based on air temperature and relative humidity. Readings with voltmeters will vary day to day. The overfill sensor may be corroded to the point of preventing the sensor from drawing adequate power. Or, the sensor may return a signal to a control device, whether it is a rack monitor or on-board compatibility module.
Testing and verification while the sensor is isolated from the system are the best way to ensure that properly working sensors are not replaced unnecessarily.
When overfill system problems are suspected, it is recommended that all electrical connections be inspected for possible corrosion prior to the actual testing of the system.
The best way to connect the wiring to the sensors is to solder the connections. However, it may not be practical in all cases. Use silicone to cover the connections. Electrical tape works well, but petroleum vapors will diminish the integrity of the tape. Wire nuts with metal inserts are not recommended. The inserts can cut the wire strands, which can prematurely fail the electrical connection.
Corrosion buildup within the sensor holder can be removed by scraping the residue out of the sensor holder and wiping the area clean with a cloth dampened with transmission fluid. When corrosion build-up is found, it is usually a sign of large accumulations of moisture. Moisture can leak into the sensor holder through breaks in the cable jacket or tubing, poor conduit compression fitting installations, defective or worn compression fittings, cracked or worn sensor holder cover O-rings, or improperly fastened sensor-holder covers.
The sensor-holder cover O-ring should be checked for cracks or breaks by gently stretching the O-ring and examining it. The cable outer casing or conduit tubing should be inspected for breaks or cracks that would provide a moisture path into the sensor holders. Defective or loose compression fittings should be replaced. The use of silicone around the cable where it passes into the compression fitting will allow for a more resilient watertight connection.
The vehicle socket is the point where proper wiring contact, integrity, and connection are most critical. If the contact faceplate of the socket is heavily soiled or worn, a proper response will not be given to the loading rack monitor. If the wiring connections to the vehicle socket are corroded or have heavy oxidation on them, sensors won't operate properly, and the trailers can't be loaded.
The condition of the contacts on the faceplate should be bright and shiny. They need to be free from dirt, oil, grease, corrosion, and other forms of debris that may act as insulators. Socket caps are available and should be used to cover the faceplate when the socket is not being used. Socket faceplate gaskets should be inspected to ensure they are not cracked or broken. Drain screws and drain holes should be inspected for proper orientation.
The drain screw should be attached in the top of the socket housing, and the drain hole should be located in the bottom. It is important that the drain hole be free and clear of anything that can prevent moisture from draining out of the socket housing. Any built-up corrosion should be scraped free and then wiped away with a cloth dampened with transmission fluid.
J-slots are another critical inspection area on the socket. Wear in the j-slots is caused by electrolytic activity between the plug guide pins and the socket housing material. J-slots can eventually wear and become oversized, resulting in loose-plug-to-socket connections and poor plug and socket alignment. Putting nonconductive grease into the j-slots minimizes the electrolysis action by acting as an insulator between the socket housing and the plug guide pins.
Ground wires and connections carry current. Corrosive action will limit the current allowed to pass. Corrosion around and under the ground bolt must be cleaned away as well as any corrosion on the ground bolt itself. Once the frame and ground bolt have been properly cleaned, tighten the ground bolt onto the vehicle and use some grease to cover the area where it meets the frame. This will limit the amount of moisture and air that can get between the ground bolt and the frame, thereby reducing the possibility of corrosion forming.
On-board compatibility units should be carefully inspected. Premature failures of compatibility modules are due to corrosion nearly 80% of the time, according to Adams. Intermittent current through the compatibility modules can create conditions that may blow internally installed fuses that are not replaceable. When this happens, the compatibility modules have to be replaced. Cover gaskets on the compatibility units need to be checked for integrity. Defective gaskets should be replaced when identified.
Overtorquing of cover bolts may warp covers, also allowing a moisture path. Always use an even torque when tightening the bolts. Cover bolt tightening sequence is important. Improper sequencing can warp the cover, allowing a moisture path into the housing. An X pattern working from the inner bolts to the outer bolts is recommended. All corrosion should be removed, and the corroded area should be cleaned of any residue.
Terminal block connections on the compatibility modules should be coated with a nonconductive grease after all power, sensor, and socket connections have been made. Precoating could cause a poor electrical connection. Housing compression fittings should be checked for tightness, not only at the point of threading into the housing, but with regard to cable compression. If the cable can be easily pulled out of the compression fitting, try tightening the compression collar. If the collar will not tighten enough to properly secure the cable, replace the compression fitting itself.
Avoid the use of bare copper wire. Copper wire oxidizes very quickly and will cause intermittent problems within the overfill prevention system. If you find bare copper wiring when inspecting or working on overfill prevention equipment, replacement is recommended. Tinned copper wire is recommended because it has a slower oxidation rate and ensures a sound electrical connection. Route wires and cables through insulated holes and openings to avoid chaffing.
To avoid petroleum product spills at terminal racks, overfill protection probes must descend far enough into the tank to trigger shutdown, said Al Mosser, senior standards engineer for Chevron Products Company. The threat of a catastrophic explosion lies at the heart of spills.
"Fuel, oxygen, and an ignition source are all that you need," he said, citing the dangers of an engine running in an adjacent rack or other sparking possibilities.
Coupled with the need for proper equipment is driver training that focuses on ways to avoid spills and the suitable response when they do occur. About 60% of reported overfillsinvolve driver error, said Mosser.
Mosser recommended that if an overfill takes place, the driver should drain the entire tank, because incompatible product may have entered other compartments.
Avoiding spills should be the goal, which is why Mosser champions the use of extended overfill probes. Sixty-gallon outage is needed in every tank compartment to assure that shutdown can occur within four to six seconds. 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. "Standard length overfill probes aren't long enough," he said. "Longer probes should be specified and then tested for accuracy."
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