Cargo tanks are permanently built onto the vehicle and are never taken off to be filled or emptied. Packaging that is attached only temporarily, or filled on the ground and then hoisted aboard, is a portable tank.
Because the liquid has to squeeze through the openings in each baffle, it cannot slam as hard toward the front when you brake or toward the rear when you accelerate. Baffles do not make the truck feel heavier, and they do little against side-to-side surge.
MAWP is defined as the most pressure permitted at the highest point of a cargo tank sitting in its normal operating position. Running a tank above that rating risks leaks or even a rupture.
Tip Use the top as the reference point, because the weight of the liquid itself adds extra pressure lower in the tank.
Gasoline does not need a high-pressure tank, so it usually rides in an MC-306 or its newer version, the DOT-406, both made for liquids at very low pressure (about 3 to 4 psi). MC-307/DOT-407 tanks handle chemicals at somewhat higher pressure, and MC-312/DOT-412 tanks are made for corrosives.
An ISO portable tank sits in a steel frame about 20 feet long, built to standard container dimensions so cranes and container equipment can lift and secure it like any other shipping container.
With no baffles to break up the flow, the liquid in a smooth bore tank surges hard toward the front when you stop and toward the rear when you pull away. Those surges are at their worst during starts and stops, so that is when you must be most careful.
A portable tank can be lifted off, so it is filled and emptied apart from the vehicle and only placed aboard for transport. A cargo tank, in contrast, stays mounted throughout, and portable tanks still need ID numbers like other bulk packaging.
Sanitation rules bar baffles in tanks that haul food products such as milk, because the partitions make the interior very hard to clean completely. Dense or expanding liquids are handled by loading less and leaving outage, not by doing without baffles.
Baffles slow the liquid as it moves toward the front or rear, so a baffled tank surges less from front to back than a smooth bore tank when you brake or speed up. They do not do much to stop side-to-side surge.
Baffles are bulkheads with holes in them: the liquid can still move from one section to the next, but more slowly, which weakens forward-and-backward surge. A tank split by solid walls into sealed compartments is a different design.
A bulkhead is a solid wall that turns one tank into several smaller ones, so you must watch how the weight is spread when filling or emptying them. A baffle is a partition with openings: liquid still passes through it, but its rush toward the front and rear is slowed.
Baffles are partitions with openings that slow the liquid as it moves toward the front or rear of the tank, so forward-and-backward surge is weaker than in an unbaffled tank. Side-to-side surge can still occur, however.
Dividing the tank with bulkheads creates smaller sections, which leaves the liquid less room to rush toward the front or back. That cuts down forward-and-back surge, so the cargo is less likely to shove the vehicle ahead when you brake.
Baffles are crosswise walls with openings that slow the liquid as it rushes toward the front or rear of the tank. They do little to keep it from sloshing sideways, so side-to-side surge, which can lead to a rollover, still occurs.
A cargo tank never comes off its vehicle: it is permanent bulk packaging, and product goes in and out while the tank stays mounted. A container removed for filling or emptying is a portable tank instead.
Bulkheads seal each compartment off from the next, so no liquid moves between them and one tank can carry different products at the same time. Baffles are different: they have openings, so liquid can still flow through them.
"Smooth bore" means there is nothing inside the tank, no baffles or bulkheads, to slow the liquid down, which is why people also call it an unbaffled tank. Food products such as milk are usually hauled this way because these tanks are easier to clean, but their cargo surges strongly.
Baffles break up the forward and backward rush of the liquid, which is why baffled tanks have weaker front-to-back surge. They are not built to block movement from side to side, so that surge can still build up, especially in turns, and can roll the vehicle over.
DOT-406 is the current specification for cargo tanks that haul gasoline and similar fuels, replacing the older MC-306, and a typical tank trailer of this type carries roughly 9,000 gallons.
DOT-406 tanks are low-pressure vessels, the usual choice for gasoline and other fuels, and their maximum allowable working pressure is just 4 psi. Much higher ratings belong to other tank types, such as those built for compressed gases like propane.
Tankers that haul food, such as milk trucks, usually have no baffles, because sanitation rules forbid interior fittings that would make the tank hard to clean. With nothing inside to slow the liquid, these smooth bore tanks can surge powerfully toward the front and rear.
This kind of portable tank is a metal tank built into a steel framework about 20 feet long, the same length as a standard 20-foot shipping container, so it can travel by truck, rail or ship.
A portable tank is designed to be filled and emptied while it sits off the vehicle, and it goes onto a truck only for transport. A cargo tank, by contrast, stays fixed to the vehicle while it is loaded and unloaded.
With nothing inside the tank to slow it, the liquid in a smooth bore tanker surges hard toward the front or back whenever your speed changes, and that is most noticeable as you pull away or brake to a stop. Gentle starts and gradual stops help keep the surge under control.
A bulkhead is a sealed wall between compartments, so cargo in one section cannot reach the next. Baffles are different because they have holes that let some of the liquid move through.
A smooth bore tank has no baffles at all, so the liquid can rush the whole length of the tank each time the truck speeds up or slows down. The resulting front-to-back surge is very strong, which is why starts and stops demand extra care.
Baffles contain holes, so liquid can still flow from one section to another, but they reduce the front-to-back surge that builds up when the vehicle speeds up or brakes. Bulkheads, by contrast, are solid and seal each compartment off completely.
Inside a smooth bore tank there are no baffles or bulkheads to slow the liquid down, so it surges forward and back with great force. As you start moving it rushes to the rear, and as you brake it rushes to the front, shoving the truck both times; that is why starting and stopping demand the most care.
Every question in this set has its own page with the correct answer and an explanation.
