Liquid Surge, Outage and Safe Handling of Tank Loads
Domain 3 tests liquid surge and outage - the two ideas that separate tanker theory from general CDL material. Expect definitions, "which tank surges most" questions, and simple outage arithmetic.
Surge
- Surge is the movement of liquid inside a partly filled tank. It pushes the vehicle forward when you brake, sideways in a turn, and can rock the tractor-trailer.
- The worst surge is in a partly filled tank with a large free surface; a completely full tank has no free surface and a nearly empty one has little mass to move.
- Baffles and compartments reduce surge but do not remove it.
- Manage it with smooth starts, gentle braking, early downshifts and lower speed in curves.
Outage
- Outage is the empty space left in a tank for expansion. It is a volume, not a dipstick reading.
- Fill only to the outage limit; product expands as it warms.
- Outage also protects axle and bridge weight limits: a dense product can be legal by volume and overweight on the scale.
Worked example
An 8,000-gallon tank needs 1.5% outage. 8,000 x 0.015 = 120 gallons, so the maximum load is 7,880 gallons. If you planned 7,930 gallons, then 7,930 + 120 = 8,050 - over tank capacity - so the load must be cut to 7,880. The rule: capacity minus outage equals maximum load.
Traps that cost marks
- Choosing a full tank as the worst surge case.
- Believing baffles eliminate surge.
- Computing outage on the load instead of the tank capacity.