Forward Surge on a Partially Loaded Smooth-Bore Tank
On a clear afternoon a driver is descending a 5 percent grade on a two-lane off-ramp posted 25 mph. The tractor-trailer is a 6,500-gallon smooth-bore cargo tank (no internal baffles) carrying 3,575 gallons of a liquid with a density of 6.5 lb/gal, about 55 percent of capacity. The driver entered the ramp at 40 mph and applies the service brakes firmly as the ramp curves right. The liquid surges forward; the steer axle feels light and the steering response changes, then the trailer pushes the tractor as the liquid runs rearward when the driver releases the brake. The tractor’s steer axle is rated 12,000 lb and was at 10,300 lb before the ramp. The driver has 300 feet of clear space ahead and a guardrail on the outside of the curve.
1. Explain why this partially loaded smooth-bore tank is more dangerous than the same tank either full or empty, and what the forward surge does to steering and braking.
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A smooth-bore tank at 55 percent has a large free surface. When the driver brakes, the liquid continues forward and piles against the front head, adding dynamic load to the steer axle and lifting load off the drive axles. The steer axle was already at 10,300 lb of a 12,000-lb rating; a forward surge of only about 1,700 lb reaches the rating, and a larger surge exceeds it. The front tires can become overloaded while the drives lose traction. When the driver releases the brake, the liquid runs rearward and pushes the tractor, which can cause the trailer to shove the tractor into the curve. A full tank has little free surface and an empty tank has little mass; the partial load is the worst case. Baffles divide the liquid and dissipate energy, but this tank has none.
2. What is the safest immediate sequence for the driver, and why is a hard brake application or abrupt steering correction the wrong move?
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The driver should stop the hard braking and use steady, light-to-moderate brake pressure, slowing before the curve rather than in it. If the tractor has an engine retarder, the driver can use it to hold speed smoothly, but should not increase throttle to pull the surge rearward. Steering should be smooth and gradual; a sudden correction while the steer axle is light can cause a loss of control or a rollover. The driver should not use the trailer hand valve, which can set up a jackknife. If speed is still too high, the driver should use the full lane, look through the curve, and be ready to stop after the curve, not jerk the wheel toward the guardrail. The correct sequence is to reduce speed before the ramp, brake progressively, and let the liquid settle between brake applications.
3. What operational changes should be made before this tank is taken down the same ramp again?
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Before the next trip, the load should be changed so the tank is either full or empty, or the compartments being used should be filled to the top, leaving no large free surface. If partial loads are routine, the operation should use a baffled tank or a tank with smaller compartments. The driver should plan the route to avoid steep ramps with curves, or enter them at or below the posted speed, and should allow extra following distance. The company should review the load chart and axle weights for the 55 percent case, because the steer axle is close to its rating even before surge. Training should cover smooth brake application and recognition of light steering as a surge warning. Repeating the same ramp at 40 mph with the same partial load is not acceptable; the fix is load configuration, speed, and route, not driver reaction alone.