Vacuum Collapse During Gravity Unloading of Diesel
At a farm fuel depot a driver is gravity-unloading 1,800 gallons of diesel from a 4,000-gallon DOT-406 cargo tank into an aboveground storage tank. The cargo tank’s normal vent is located on top of the tank and is fitted with a flame arrester; the driver notices the vent screen is packed with mud and insect debris from the last rain. The unloading valve is open, the hose is connected, and the tank begins to deliver slowly. After about 400 gallons the driver hears a sharp pop and the tank shell starts to make oil-canning sounds; the hose begins to flatten. The depot’s receiving tank vent is open, and the ambient temperature is 95°F. The driver has a 3-inch emergency shutoff valve at the tank and a wooden plug kit. The cargo tank’s pressure-relief valve is on the same manway as the blocked vent.
1. What is happening to the cargo tank, and why is the blocked vent dangerous even though the tank has a pressure-relief valve?
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The tank is being pulled into a vacuum. As diesel drains by gravity, the liquid level falls and the vapor space above it must be replaced by air. The normal vent is blocked by mud and insect debris, so air cannot enter. The pressure-relief valve may be a pressure-relief device only, or its vacuum breaker may be stuck or on the same clogged manway; it is not a substitute for an open vent. The oil-canning sound and the flattening hose are signs that the shell is being stressed. A cargo tank is not designed for vacuum; the shell can permanently dimple or crack. The 95°F temperature makes any volatile vapor in the tank more expansive, but the main problem is the blocked vent during draining.
2. What immediate actions should the driver take to stop the damage and protect people?
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The driver should close the tank’s emergency shutoff valve and stop the flow immediately. The driver should not open the manway or apply compressed air to the tank; either can cause a sudden pressure change or release vapor. The driver should move people away, eliminate ignition sources, and notify the supervisor and the depot. If the vacuum must be relieved, it should be done slowly through the vent or vacuum breaker after the blockage is cleared, with proper PPE and from the upwind side. The driver should not reconnect the hose and resume unloading. If the shell is dimpled, the tank should be taken out of service and inspected before any further loading or unloading.
3. What inspection and unloading-procedure changes would prevent a recurrence?
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The pre-trip inspection should include the normal vent, flame arrester, vacuum breaker, and all vent screens; the driver should look for mud, insect nests, ice, or paint. The vent should be checked as part of the loading and unloading checklist, not just at the terminal. The driver should confirm the receiving tank is vented and that the hose is not collapsed. Unloading procedures should require the vent to be open and the flow rate to be controlled. The carrier should add this event to the maintenance record and inspect the tank for shell damage. The driver should be trained to treat oil-canning as an emergency stop, not as a normal noise.