Feb 1, 2007 12:00 PM
AS TANK truck carriers concentrate on transporting ultra low sulfur diesel (ULSD) — and coping with the rules for maintaining the product at no more than 15 parts per million (ppm) of sulfur content — Bill Harris of Heil Tank International has some advice.
“Strict loading and unloading procedures and/or dedicated compartments will minimize risk of ULSD contamination,” he said at the National Tank Truck Carriers 2006 Cargo Tank Maintenance Seminar October 16-18 in Nashville, Tennessee.
The Environmental Protection Agency (EPA) set the 15 ppm standard that required refiners and fuel importers to start producing the fuel by June 1, 2006. The rule went into effect October 15, 2006, for the product sold at retail stations. ULSD fuel is mandatory for highway and virtually all off-road vehicles by 2010, and by 2014, all highway non-road, locomotive, and marine diesel fuel will be ULSD.
Tank truck carriers have voiced concerns about maintaining the product purity and whether the situation would call for ULSD-dedicated trailers in order to avoid contamination.
Harris said that if tank trailers are drained properly, they should be able to haul ULSD after containing other fuel products and still meet the 15 ppm requirement. “We try to design a petroleum trailer to drain completely and as quickly as possible to minimize the unloading time,” Harris added.
Heil has determined that trough-bottom and double-taper designs are most effective for complete unloading, and recommends the trough design for timely unloading.
“A trough is nothing more than a manifold that allows fuel to flow through 3-inch drain holes along the longitudinal axis of that compartment into the manifold of the sump and the piping,” Harris said. “You also can have a double-taper trailer, and that's typically four-to-six inches of drop from either the front or the rear head to the middle of the trailer, give or take a few inches.”
Harris also pointed out that safe and efficient unloading of tank trailers depends on features, such as how the manifold area is designed, piping and valving, pumps and meters, overfill protection systems, and interlocks. “That really drives how efficient, safe, and complex that loading system and unloading system is,” he said, adding, “It's really independent of the tank bottom design.”
For the tank to drain completely, three physical conditions must be present: the force of gravity acting on the liquid (force=mass x acceleration); the coefficient of friction/surface area within the compartments; and the discharge area (sump and piping).
“If you've got that same amount of force we talked about coming down on the liquid and it's going through a six-inch hole versus a four-inch hole, obviously the liquid going through the six-inch hole is going to download quicker. Of course this assumes there are no flat spots or traps in the tank, valves, piping, etc.”
In 2005, Heil conducted a test on a 9,200-gallon tank trailer with five compartments and unloaded it until the sight glass was empty. By testing two of the compartments, (one 2,600-gallon and another 1,200-gallon) the company determined that 80 fluid ounces of product were retained in the first compartment and 38 fluid ounces in the other one. After the tests were completed, Heil determined the resultant ppm of ULSD was at 14.85 ppm, within the 15 ppm limit, Harris said.
He pointed out that residual fuel comes from the sides of the tank, the emergency valve, and low spots due to terrain conditions where the trailer is parked. However, he added that trailers with troughs can overcome 23 inches of incline front to rear or 36 inches of decline rear to front, and still allow complete drainage.
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