Keeping ULSD Low

July 1, 2006
June has arrived, and with it, the first mandated requirements to deliver ultra low sulfur diesel.

JUNE has arrived, and with it, the first mandated requirements to deliver ultra low sulfur diesel.

As we are all well aware, sulfur as a contaminant in diesel fuel for highway use cannot exceed 15 ppm beginning June 2006. What does this mean in terms of trailer design and compartmentation?

Heil Trailer International performed a trial test on a 9,200-gallon five-compartment trailer. The front compartment was the largest (2600 gallons nominal), and the third compartment the smallest (1200 gallons nominal).

After simulating a quick unloading procedure, the front compartment was left with a retained volume of 79.828 fluid oz (a little over ½ gallon). If the retained volume had been HSD (high-sulfur diesel) containing 3,400 ppm of sulfur and that same compartment was later loaded with 2,494 gallons of ULSD containing 14 ppm of sulfur, this mixture would yield 14.85 ppm of sulfur or 99% of the allowed 15 ppm.

The third (smallest) compartment was left with a heel volume of 37.76 fluid oz of HSD. If the compartment was later reloaded with 1,270 gallons of ULSD, this mixture would yield the same 14.85 ppm of sulfur or 99% of the allowed 15 ppm.

To determine the residuals in the test above, the trailer was drained to a point where the sight glass was empty. Once the emergency valve was closed, the trailer was left in this condition for 30 minutes. The emergency valve was then re-opened and the residual fuel measured. The product collected after the tank was “emptied” comes from the walls of the tank and is time dependent. The more time allowed, the more product drains from the walls and can be offloaded.

The concept of a “drain dry” trailer is virtually impossible without an outside system being applied to the trailer to achieve this end result. Drainage is a matter of physics and is dependent upon the force acting on the liquid, surface area (or the material's coefficient of friction), and area of discharge or piping openings.

The results highlight the importance of a petroleum trailer's loading and unloading procedures. If the shipper does not use dedicated compartments for ULSD, the drivers must factor time into their deliveries to accommodate complete drainage, as well as ensuring that they position the trailer at the offload site so that the terrain and designed pitch of the trailer work in harmony. As long as terrain slope favors drainage, complete unloading is dependent on time. In their test above, Heil elevated the front of the tank another 10 inches. Drainage results improved by less than an ounce as measured over the same period of time.

In a separate unloading test, Heil compared the drainage characteristics of a double tapered compartment with a straight bottom compartment fitted with a trough. The test vessel consisted of two 2,500-gallon compartments: one compartment had a Heil standard trough welded to the bottom, while the other compartment was a Heil tapered bottom. Both compartments were welded together and left uncovered, giving researchers a bird's-eye view of the drainage test.

Each compartment was filled with exactly 2,250 gallons of water pumped through a meter to determine the exact capacity.

The troughed compartment had ten 3" diameter drain holes in the bottom of the vessel that permitted the payload to drain into the 9' long trough. These holes added an additional 70 square inches of drainage capability.

The double tapered compartment was rolled with a taper (approximately 7¼" of drop over the length of the compartment) corresponding to a deep drop double tapered trailer with 13" of drop, more than twice the industry standard.

Both emergency valves were opened simultaneously, and the water drained at approximately 375 gallons per minute, dependent upon the force acting on the liquid (or the liquid's height above the ground in this case), the surface area the liquid flows over (which constitutes a skin friction force acting on the water), and the area of the discharge valve and piping openings the liquid flows through. During the test, Heil controlled these three variables and ensured each was indistinguishable between the two vessels.

Near the end of the test, the damming affect the surge baffle had on the water was noted in the double taper compartment. Because the water must flow through a single opening in the surge head at the 6 o'clock position, turbulent flow was created, which slows the final stage of drainage on that compartment. This has been labeled “gurgle time” by tank truck operators. “Gurgle time” is defined as the final seconds of product drainage that is slightly restricted by the opening in the surge heads.

This restriction or damming affect is eliminated on compartments with drain troughs because the product's flow path bypasses the surge heads. The water flow remains laminar or non-turbulent as it travels through the ten drain holes in the bottom of the vessel into the trough. This permits a troughed compartment to actually drain slightly quicker in the final seconds of unloading.

In this test, both compartments emptied in approximately six minutes (within five seconds of each other). The troughed compartment actually drained slightly quicker during the test due to the “gurgle time” the tapered compartment experienced. Since compartment design did not afford an advantage, the test once again highlighted the importance of unloading procedures.

Many fuel distributors have evaluated the risks of contamination and have implemented procedures whereby the last load to a HSD customer will be a load of ULSD (at no additional charge). This has the effect of rinsing the tank prior to loading ULSD destined to a service station or truck stop.

Ultimately, the distributor must use a procedure that ensures the compartment completely drains (trailer positioned and given time to completely unload) or is “rinsed” to prevent cross contamination of HSD and ULSD.