2.3 Bulkheads, Multi-Compartment Tanks & Weight Distribution
Key Takeaways
- The AAMVA manual says some liquid tanks are divided into several smaller tanks by bulkheads.
- When loading and unloading compartments, pay attention to weight distribution and do not put too much weight on the front or rear of the vehicle.
- A partially filled compartment still surges, so bulkheads shorten surge but do not remove it.
- Too much weight up front causes hard steering and can overload the steering or drive axles; too much at the rear makes steering light and cuts drive traction.
- 49 CFR 173.33 prohibits loading hazardous materials together in the same cargo tank motor vehicle if mixing them would create an unsafe condition.
Compartmented tanks let one trailer carry several products or make several deliveries. They also give the driver a job a single-compartment tank never does: keeping the load balanced as compartments fill and empty. The AAMVA manual (8.2.3) makes that the point of its bulkhead section.
What a Bulkhead Is
The manual says some liquid tanks are divided into several smaller tanks by bulkheads. A bulkhead is a solid, liquid-tight wall. Unlike a baffle, it has no holes, so the product in one compartment stays there.
| Feature | Bulkhead | Baffle |
|---|---|---|
| Construction | Solid wall | Wall with holes |
| Divides tank into | Separate compartments | Sections of one compartment |
| Different products | Possible, one per compartment | No |
| Effect on surge | Limits surge to each compartment | Slows front-to-back surge |
Compartments can also contain baffles, and each compartment usually has its own manhole, vent, and outlet valve.
Compartments and Surge
A bulkhead shortens the distance liquid can travel, which helps. But a partially filled compartment still surges, and side-to-side surge in each compartment is not stopped by a bulkhead. After a delivery that leaves one compartment partly full, drive as you would any partial load.
Weight Distribution: The Driver's Responsibility
The manual's instruction is direct: when loading and unloading the smaller tanks, the driver must pay attention to weight distribution. Don't put too much weight on the front or rear of the vehicle.
The general cargo section of the manual explains what goes wrong:
| Problem | Cause | Effect |
|---|---|---|
| Too much weight on the steering axle | Too much product toward the front | Hard steering; can damage the steering axle and tires |
| Steering axle too light | Weight shifted too far to the rear | Too light to steer safely |
| Too little weight on drive axles | Load concentrated away from the drive axles | Poor traction; drive wheels spin easily, especially in bad weather |
| Any axle group over its limit | Poor compartment plan | Illegal axle weight (20,000 lbs single, 34,000 lbs tandem on the Interstate) |
How compartment location affects axle loads on a tractor-tank trailer
- Product in front compartments, ahead of the trailer axles, puts more weight on the kingpin. That weight is carried by the tractor's drive axles and steering axle.
- Product in rear compartments, over or near the trailer axles, is carried mostly by the trailer axles.
So a load concentrated in the front can overload the tractor axles and make steering hard. A load concentrated at the rear can overload the trailer axles while leaving less weight on the drive and steering axles, which means less traction and lighter steering.
Planning Loading and Unloading
Multi-stop deliveries change the balance at every stop. A good plan:
- Start with the carrier's loading chart or instructions for that trailer. Many specify which compartments to fill for common load sizes.
- Check both gross and axle weights. Being under 80,000 lbs gross does not guarantee each axle group is legal.
- Think about the vehicle after each stop. Choose an unloading order that avoids leaving a heavy front or heavy rear for the next leg.
- Remember partial compartments surge. A plan that leaves several compartments partly full makes the rest of the trip harder to drive.
- Use a scale when in doubt, especially with dense products or unusual splits.
Keeping Products Separate
Compartments only work if products stay where they belong.
- Incompatible hazardous materials: 49 CFR 173.33(a)(2) says two or more materials may not be loaded or accepted for transportation in the same cargo tank motor vehicle if mixing them would create an unsafe condition, such as an explosion, fire, excessive pressure or heat, or toxic vapors.
- Tank and residue compatibility: 49 CFR 173.33(b) says a hazardous material may not be loaded into a cargo tank if it could react dangerously with the tank material or combine with residue in the tank to create an unsafe condition.
- Separation between compartments: where products must never mix, carriers and shippers may require double bulkheads, two walls with a vented space between them, so a leak shows up outside the tank instead of in the next compartment.
- Fuel grades: a small amount of gasoline delivered into a diesel tank can lower the diesel's flash point and create a fire hazard. Drivers confirm the product, compartment, and receiving tank before each drop, using the color and marking codes their company and terminals use.
- Vapor lines: compartments often share vapor recovery piping. Follow the connection order your terminal and carrier require (Section 3.3).
Before You Move
After loading or a partial delivery:
- Confirm every compartment's valves and manhole covers are closed; never drive with open valves or manhole covers (AAMVA 8.1.2).
- Check that the compartments you intended to fill or empty are the ones that were.
- Note which compartments are partly full so you can expect surge.
Common Test Traps
- "Bulkheads and baffles both have holes." Only baffles do.
- "Any legal gross weight is balanced." Axle weights and the front-to-rear split still matter.
- "Rear-heavy improves steering." A steering axle that is too light is unsafe to steer.
How are bulkheads different from baffles?
What does the AAMVA manual say a driver must watch when loading and unloading the smaller tanks of a compartmented tank?
A driver empties the front compartments first and leaves only the rearmost compartment full. Which handling problem does the AAMVA manual associate with weight shifted too far to the rear?