3.3 Loading & Unloading: Valves, Bonding, Vapor Recovery & Attendance

Transfers are when the tank is open to the outside world: valves are open, hoses are connected, vapors are moving, and product is flowing fast. The federal knowledge list for the endorsement ends with operating practices and procedures (49 CFR 383.119(i)), and for most tank drivers, loading and unloading is where those practices matter most.

Valve and Closure Position

The AAMVA manual (8.1.1) says to check the intake, discharge, and cut-off valves and make sure the valves are in the correct position before loading, unloading, or moving the vehicle. It also says never drive a tank vehicle with open valves or manhole covers (8.1.2).

For hazardous materials, 49 CFR 177.834(j) requires that before driving a cargo tank motor vehicle containing hazmat, all manhole closures are closed and secured and all valves and closures in liquid discharge systems are closed and free of leaks.

Top Loading vs. Bottom Loading

Many petroleum terminals use bottom loading with standardized couplers (API RP 1004). Whatever the method, follow the site's procedure and use fall protection wherever you work on top of a tank.

Static Electricity: Bonding and Grounding

Moving liquid through pipes, hoses, and filters can build static charge. A spark where flammable vapor meets air can start a fire.

• Bonding connects two conductive objects, such as the cargo tank and the fill pipe, so they are at the same electrical potential and a spark cannot jump between them.
• Grounding connects an object to earth so charge drains away.

The federal rule for flammable liquids (49 CFR 177.837(c))

• When a cargo tank is loaded through an open filling hole, a bond wire must connect the tank shell to the stationary system piping or connected steel framing. The connection must be made before any filling hole is opened and must stay in place until after the last filling hole has been closed.
• When a tank is unloaded by suction through an open filling hole, electrical continuity must be maintained from cargo tank to receiving tank.
• Bonding or grounding is not required when loading or unloading through a vapor-tight top or bottom connection with no release of vapor where a spark could occur. Contact must be made before flow starts and not broken until flow is complete.

Terminals and carriers often require a bonding or grounding connection at every transfer anyway, sometimes with an interlock that stops the pump until the connection is verified. Follow the stricter rule. The test-ready habit: connect bonding or grounding first, disconnect it last.

Engine Off for Flammable Liquids

Under 49 CFR 177.837(a), Class 3 flammable liquids may not be loaded into or unloaded from a cargo tank motor vehicle while the engine is running, unless the engine is used to operate a pump. A diesel engine may be left running during transfer if the outside temperature is at or below 10°F.

Vapor Recovery

Liquid entering a tank pushes out the air and vapor that were inside it.

• At bulk terminals, the vapor hose carries those vapors to the terminal's vapor recovery or vapor destruction system.
• At gasoline stations, delivery systems called Stage I vapor recovery return vapors from the station's underground tank back into the cargo tank as fuel drops.

The AAMVA manual lists vapor recovery kits as special purpose equipment you must check before a trip. Leaking vapor hoses or stuck vapor valves defeat the system and create fire risk.

Overfill Protection

Bottom-loading tanks commonly have overfill sensors in each compartment connected to the rack by a cable. If product reaches a sensor, the rack stops flow. Never bypass the sensor cable or override a shutdown; find out why it tripped. Top-loading drivers rely on meters and watching the level, which is one more reason to stay attentive.

Loading Rate Limits

To prevent rupture in a loading or unloading accident, 49 CFR 173.33(b)(4) says the loading or unloading rate must be no more than the rate on the cargo tank specification plate. If no rate is marked, pressure in the tank may not exceed 130% of MAWP.

Vacuum: The Tank That Collapses Inward

Pumping liquid out of a closed tank without letting air or vapor in lowers the pressure inside. The outside air, at about 14.7 psi at sea level, then pushes on a very large shell area. A 9,000-gallon tank has well over 100,000 square inches of surface, so even a few psi of vacuum means hundreds of thousands of pounds pressing inward, enough to crush a thin shell.

49 CFR 178.345-10 requires cargo tanks to have pressure and vacuum relief sized to prevent rupture or collapse from loading, unloading, or heating and cooling of the lading. The driver's job:

• Keep vents and vacuum relief devices clear and working (the manual says keep vents clear so they work correctly).
• Open the proper vent, or connect the vapor return line, before starting to unload.
• Never block a vent to stop a drip.

Attendance (49 CFR 177.834(i))

For hazardous materials cargo tanks:

• Loading: the tank must be attended by a qualified person at all times while it is being loaded. The person responsible for loading is responsible for making sure it is attended.
• Unloading: the motor carrier must ensure the tank is attended by a qualified person at all times during unloading.

A person attends the transfer if, throughout the process, the person is within 25 feet of the cargo tank. During unloading, that person must be alert and have an unobstructed view of the cargo tank and delivery hose to the maximum extent practicable. The regulation also allows remote monitoring by cameras or instrumentation systems that meet detailed equipment requirements.

A person is qualified if the person knows the nature of the hazardous material, has been instructed on emergency procedures, and, except for remote monitors and certain hose inspectors, is authorized to move the cargo tank and has the means to do so.

For combustible liquids such as many diesel deliveries, 49 CFR 177.837(d) allows the attendant to stay within 150 feet of the cargo tank and 25 feet of the delivery hose, observing both the cargo tank and receiving container at least once every 5 minutes during unloading that takes more than 5 minutes.

In practice, do not walk into the building, sit distracted in the cab, or hand the job to someone who cannot move the truck.

Past-Due Retests

A specification cargo tank that is past due for a required retest or inspection may not be filled and offered for transportation until the test is done (49 CFR 180.407(a)(1)). A tank filled before the due date is not affected. Check the test markings before loading (Section 6.2).

Common Test Traps

• "When should the bonding cable come off?" After the last filling hole is closed or the flow is complete, never while product is moving.
• "Where must the attendant be?" Within 25 feet, alert, with an unobstructed view.
• "Why did the tank collapse during unloading?" The tank was not vented, so outside air pressure crushed it.