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Liquid Surge

Liquid surge results from movement of the liquid in partially filled tanks. This movement can have bad effects on handling. For example, when coming to a stop, the liquid will surge back and forth. When the wave hits the end of the tank, it tends to push the truck in the direction the wave is moving. If the truck is on a slippery surface such as ice, the wave can shove a stopped truck out into an intersection. The driver of a liquid tanker must be very familiar with the handling of the vehicle.

Bulkheads:

Some liquid tanks are divided into several smaller tanks by bulkheads. When loading and unloading the smaller tanks, the driver must pay attention to weight distribution. Do not put too much weight on the front or rear of the vehicle.

Baffled tanks:

Baffled liquid tanks have bulkheads in them with holes that let the liquid flow through. The baffles help to control the forward and backward liquid surge. Side-to-side surge can still occur. This can cause a roll over.

Unbaffled tanks:

Unbaffled liquid tankers (sometimes called “smooth bore” tanks) have nothing inside to slow down the flow of the liquid. Therefore, forward-and-back surge is very strong. Unbaffled tanks are usually those that transport food products (e.g., milk). (Sanitation regulations forbid the use of baffles because of the difficulty in cleaning the inside of the tank.) Be extremely cautious (slow and careful) in driving smooth bore tanks, especially when starting and stopping.

How Much to Load

Never load a cargo tank totally full. Liquids expand as they warm and you must leave room for the expanding liquid. This is called “outage.” Since different liquids expand by different amounts, they require different amounts of outage. You must know the outage requirement when hauling liquids in bulk.

A full tank of dense liquid (such as some acids) may exceed legal weight limits. For that reason, you often may only partially fill tanks with heavy liquids. The amount of liquid to load into a tank depends on:

  • The amount the liquid will expand in transit.
  • The weight of the liquid.
  • Legal weight limits.

8.3 Safe Driving Rules

To drive tank vehicles safely, you must follow all safe driving rules:

  • Drive smoothly - Because of the high center of gravity and the surge of the liquid, you must start, slow down and stop very smoothly. Also, make smooth turns and lane changes.
  • Braking - If you must make a quick stop to avoid an accident, use controlled or stab braking. If you do not remember how to stop using these methods, review Section 2.13. Also, remember that if you steer quickly while braking, your vehicle may roll over.
  • Curves - Slow down before curves, then accelerate slightly through the curve. The posted speed for a curve may be too fast for a tank vehicle.
  • Stopping distance - Keep in mind how much space you need to stop your vehicle. Remember that wet roads double the normal stopping distance. Empty tank vehicles may take longer to stop than full ones.
  • Skids - Do not over steer, over accelerate or over brake. If you do, your vehicle may skid. On tank trailers, if your drive wheels or trailer wheels begin to skid, your vehicle may jackknife. When any vehicle starts to skid, you must take action to restore traction to the wheels.

Test Your Knowledge

  • How are bulkheads different than baffles?
  • Should a tank vehicle take curves, on-ramps, or off-ramps at the posted speed limits?
  • How are smooth bore tankers different to drive than those with baffles?
  • What three things determine how much liquid you can load?
  • What is outage?
  • What is the minimum tread depth for front tires?
  • For other tires?

Study section 8.2 if you can't answer all of these questions.

Make sure you fully understand the meaning of "liquid surge" as this will likely show up on the written exam.
The written exam may ask about this. You should understand that liquid will surge back and forth and on a hard stop, the liquid may surge forward hard enough to push your vehicle forward.
You need to have a clear understanding of what "bulkheads" are and how they differ from "baffles" which are discussed below.
Make sure you fully understand the difference between baffles and bulkheads. This is very likely to show up on the written exam.

There are a couple key points that show up regularly on the written exam from this paragraph:

  • There is nothing inside an unbaffled tank to slow down the flow of the liquid.
  • Unbaffled tanks are usually associated with food grade tanks as sanitation regulations forbid the use of baffles due to the difficulty in cleaning the tank.
It's important to be familiar with the term "outage" as this shows up on the written exam frequently. Remember that tanks should never be completely filled as liquids can expand due to temperature changes.
Remember: As with most other types of commercial trucks, empty tankers will take longer to stop than those which are fully loaded.

Baffle:

A partition or separator within a liquid tank, used to inhibit the flow of fluids within the tank. During acceleration, turning, and braking, a large liquid-filled tank may produce unexpected forces on the vehicle due to the inertia of liquids.

Bulkhead:

A strong wall-like structure placed at the front of a flatbed trailer (or on the rear of the tractor) used to protect the driver against shifting cargo during a front-end collision. May also refer to any separator within a dry or liquid trailer (also called a baffle for liquid trailers) used to partition the load.

HOS:

Hours Of Service

HOS refers to the logbook hours of service regulations.

Review Questions - Click On The Picture To Begin...

What is a Bulkhead?
  • A device that is placed on the top of liquid or gas and compresses it as low as possible to decrease the center of gravity
  • Used to hold all liquid or gas to one side of the tank
  • Used to elevate liquid to a higher center of gravity
  • A divider inside a tank to section off liquid or gas

Quote From The CDL Manual:

Some liquid tanks are divided into several smaller tanks by bulkheads. When loading and unloading the smaller tanks, the driver must pay attention to weight distribution. Do not put too much weight on the front or rear of the vehicle.

Next
Which of these tanker trucks are the least likely to have baffles installed
  • A tank carrying automotive gasoline
  • A tank carrying milk
  • A tank filled with propane
  • A tank filled with oil

Quote From The CDL Manual:

Unbaffled liquid tankers (sometimes called "smooth bore" tanks) have nothing inside to slow down the flow of the liquid. Therefore, forward-and-back surge is very strong. Unbaffled tanks are usually those that transport food products (e.g., milk). (Sanitation regulations forbid the use of baffles because of the difficulty in cleaning the inside of the tank.) Be extremely cautious (slow and careful) in driving smooth bore tanks, especially when starting and stopping.

TruckingTruth's Advice:

Baffles make cleaning the inside of a tank vehicle very difficult. Since most food grade tankers have to be completely clean and sanitary before each load, it is very rare for food grade tankers to have baffles.

Prev
Next
Why don't most food grade tankers come equipped with baffles?
  • Food products are generally light enough that the entire tank can be filled, thus, limiting any liquid surge
  • To save room in the tank, most shippers of liquid food products prefer to forgo baffles so more product can be loaded
  • Food grade tankers are required to be fitted with baffles or bulkheads
  • Sanitation regulations forbid the use of baffles because of the difficulty in cleaning the inside of the tank

Quote From The CDL Manual:

Sanitation regulations forbid the use of baffles because of the difficulty in cleaning the inside of the tank.

TruckingTruth's Advice:

Anytime you see a food grade tanker, it is safe to assume there are no baffles installed. Not only is that important for you to know when pulling a food grade tanker, but you can also drive differently around other vehicles that are pulling food grade tankers (give them more room).

Prev
Next
What type of tanker trailers are normally unbaffled?
  • Food grade tankers
  • All tanker trucks are equipped with baffles
  • Tankers designed to haul non-liquid products
  • Fuel tankers

Quote From The CDL Manual:

Unbaffled liquid tankers (sometimes called "smooth bore" tanks) have nothing inside to slow down the flow of the liquid. Therefore, forward-and-back surge is very strong. Unbaffled tanks are usually those that transport food products (e.g., milk). (Sanitation regulations forbid the use of baffles because of the difficulty in cleaning the inside of the tank.) Be extremely cautious (slow and careful) in driving smooth bore tanks, especially when starting and stopping.

Prev
Next
What are unbaffled tanks?
  • There are barriers in the tank with holes in them to slow down and restrict surging liquid
  • There are no barriers inside to slow down or stop liquid from surging
  • Liquid is held in place by multiple barriers, restricting liquid surge
  • Small tanks which are loaded and secured onto flatbed trailers

Quote From The CDL Manual:

Unbaffled liquid tankers (sometimes called "smooth bore" tanks) have nothing inside to slow down the flow of the liquid. Therefore, forward-and-back surge is very strong. Unbaffled tanks are usually those that transport food products (e.g., milk). (Sanitation regulations forbid the use of baffles because of the difficulty in cleaning the inside of the tank.) Be extremely cautious (slow and careful) in driving smooth bore tanks, especially when starting and stopping.

TruckingTruth's Advice:

Unbaffled tankers are much more dangerous to drive, especially when the tanks aren't completely full, as liquid can surge and slosh around in any direction hindering vehicle control.

Prev
Next
While driving a liquid tanker without Anti-Lock Brakes, all of the following statements are true, except:
  • All of these answers are true
  • Steer quickly and swerve around a hazard while braking at the same time
  • Use controlled braking to avoid an accident
  • Use stab braking to avoid an accident

Quote From The CDL Manual:

To drive tank vehicles safely, you must follow all safe driving rules:

Braking – If you must make a quick stop to avoid an accident, use controlled or stab braking. If you do not remember how to stop using these methods, review Section 2.13. Also, remember that if you steer quickly while braking, your vehicle may roll over.

Prev
Next
What term describes the following statement: Liquids expand as they warm and you must leave room for the expanding liquid.
  • None of these answers are correct
  • Contraction
  • Frontage
  • Outage

Quote From The CDL Manual:

Liquids expand as they warm and you must leave room for the expanding liquid. This is called "outage."

Prev
Next
While driving a liquid tanker around a curve, you should:
  • Always drive the posted speed for a curve
  • Understand that the posted speed for a curve may be too fast for a tank vehicle
  • Be confident that your lower center of gravity will reduce the risk of a rollover
  • Use quick steering movements to keep liquids from collecting on one side

Quote From The CDL Manual:

Curves: Slow down before curves, then accelerate slightly through the curve. The posted speed for a curve may be too fast for a tank vehicle.

TruckingTruth's Advice:

All maneuvers in a tanker truck should be made slower and smoother than other vehicle types.

Prev
Next
Unbaffled liquid tankers are sometimes called:
  • Unrestricted tanks
  • Smooth bore tanks
  • Free flow tanks
  • Selective range tanks

Quote From The CDL Manual:

Unbaffled liquid tankers (sometimes called "smooth bore" tanks) have nothing inside to slow down the flow of the liquid. Therefore, forward-and-back surge is very strong. Unbaffled tanks are usually those that transport food products (e.g., milk). (Sanitation regulations forbid the use of baffles because of the difficulty in cleaning the inside of the tank.) Be extremely cautious (slow and careful) in driving smooth bore tanks, especially when starting and stopping.

Prev
Next
When loading a tank, you should:
  • Use disposable baffles and barriers when hauling food grade tankers
  • Never load a cargo tank so that it's totally full
  • Expect liquid to evaporate and lighten your gross weight over time
  • Always load a cargo tank with "filler" so that it's completely filled

Quote From The CDL Manual:

Never load a cargo tank totally full. Liquids expand as they warm and you must leave room for the expanding liquid. This is called "outage." Since different liquids expand by different amounts, they require different amounts of outage. You must know the outage requirement when hauling liquids in bulk.

A full tank of dense liquid (such as some acids) may exceed legal weight limits. For that reason, you often may only partially fill tanks with heavy liquids. The amount of liquid to load into a tank depends on:

  • The amount the liquid will expand in transit.
  • The weight of the liquid.
  • Legal weight limits.
Prev
Next
How much do liquids expand as they warm?
  • different liquids expand by different amounts
  • None of these answers are correct
  • Liquids expand at 1kg/m per 10 degree increase
  • Most liquids do not expand when heated

Quote From The CDL Manual:

Never load a cargo tank totally full. Liquids expand as they warm and you must leave room for the expanding liquid. This is called "outage." Since different liquids expand by different amounts, they require different amounts of outage. You must know the outage requirement when hauling liquids in bulk.

Prev
Next
All of the following statements about Liquid Surge are correct, except:
  • When the surge hits the end of the tank, it tends to push the truck in the opposite direction the wave is moving
  • If the truck is on a slippery surface such as ice, the wave can shove a stopped truck out into an intersection
  • Results from movement of the liquid in partially filled tanks
  • When coming to a stop, the liquid will surge back and forth

Quote From The CDL Manual:

Liquid surge results from movement of the liquid in partially filled tanks. This movement can have bad effects on handling. For example, when coming to a stop, the liquid will surge back and forth. When the wave hits the end of the tank, it tends to push the truck in the direction the wave is moving. If the truck is on a slippery surface such as ice, the wave can shove a stopped truck out into an intersection. The driver of a liquid tanker must be very familiar with the handling of the vehicle.

Prev
Next
Which statement about liquid surge is true?
  • Liquid surge will not affect handling and will only affect braking or acceleration
  • Liquid surge only occurs during acceleration or stopping and does not occur during turns
  • When liquid surge hits the end of the tank, it tends to push the truck in the direction the wave is moving
  • On tanker vehicles, brakes are designed to hold the vehicle in place when liquid surges forward

Quote From The CDL Manual:

Liquid surge results from movement of the liquid in partially filled tanks. This movement can have bad effects on handling. For example, when coming to a stop, the liquid will surge back and forth. When the wave hits the end of the tank, it tends to push the truck in the direction the wave is moving. If the truck is on a slippery surface such as ice, the wave can shove a stopped truck out into an intersection. The driver of a liquid tanker must be very familiar with the handling of the vehicle.

Prev
Next
How are bulkheads different than baffles?
  • Bulkheads are only allowed in food-grade tankers where baffles can be installed in any type of tanker
  • Bulkheads are only used in gas tankers and baffles are only used in liquid tankers
  • Bulkheads are solid barriers in a tank while baffles are barriers with holes in them, allowing liquid to flow through
  • Baffles are solid barriers in a tanks while bulkheads are barriers with holes in them, allowing liquid to flow through

Quote From The CDL Manual:

Bulkheads: Some liquid tanks are divided into several smaller tanks by bulkheads. When loading and unloading the smaller tanks, the driver must pay attention to weight distribution. Do not put too much weight on the front or rear of the vehicle.


Baffled tanks: Baffled liquid tanks have bulkheads in them with holes that let the liquid flow through. The baffles help to control the forward and backward liquid surge. Side-to-side surge can still occur. This can cause a roll over.

Prev
Next
Regarding stopping distance while driving a tanker, which statement is false?
  • Liquid surge may force your truck forward after you have already come to a complete stop
  • Wet roads double the normal stopping distance
  • All of these statements are true
  • Fully loaded tankers take longer to stop than empty ones

Quote From The CDL Manual:

Stopping distance - Keep in mind how much space you need to stop your vehicle. Remember that wet roads double the normal stopping distance. Empty tank vehicles may take longer to stop than full ones.

Prev
Next
Baffled tanks are:
  • Tanks without any bulkheads
  • Used only for gas products
  • Designed to contain several solid bulkheads
  • Liquid tanks with several bulkheads containing holes that liquid can flow through

Quote From The CDL Manual:

Baffled liquid tanks have bulkheads in them with holes that let the liquid flow through. The baffles help to control the forward and backward liquid surge. Side-to-side surge can still occur. This can cause a roll over.

Prev
Next
Why do liquid tankers need to be driven more smoothly than most other vehicles?
  • While stopping, liquid will slosh back and forth
  • Quick acceleration will cause unnecessary sloshing and control difficulties
  • All of these are correct
  • During turns, liquid will surge to the side of the trailer, increasing the chance of a rollover accident

Quote From The CDL Manual:

In order to drive tank vehicles safely, remember:

  • Drive smoothly - Because of the high center of gravity and the surge of the liquid, you must start, slow down and stop very smoothly. Also, make smooth turns and lane changes.
  • Braking - If you must make a quick stop to avoid an accident, use controlled or stab braking. Also, remember that if you steer quickly while braking, your vehicle may roll over.
  • Curves - Slow down before curves, then accelerate slightly through the curve. The posted speed for a curve may be too fast for a tank vehicle.
  • Stopping distance - Keep in mind how much space you need to stop your vehicle. Remember that wet roads double the normal stopping distance. Empty tank vehicles may take longer to stop than full ones.
  • Skids - Do not over steer, over accelerate or over brake. If you do, your vehicle may skid. On tank trailers, if your drive wheels or trailer wheels begin to skid, your vehicle may jackknife. When any vehicle starts to skid, you must take action to restore traction to the wheels.
Prev
Next
What is Liquid Surge?
  • If the tank is punctured, the liquid will rush out of the opening
  • When liquid naturally swirls inside of a tank
  • None of these answers are correct
  • Results from movement of the liquid in partially filled tanks

Quote From The CDL Manual:

Liquid surge results from movement of the liquid in partially filled tanks. This movement can have bad effects on handling. For example, when coming to a stop, the liquid will surge back and forth. When the wave hits the end of the tank, it tends to push the truck in the direction the wave is moving. If the truck is on a slippery surface such as ice, the wave can shove a stopped truck out into an intersection. The driver of a liquid tanker must be very familiar with the handling of the vehicle.

TruckingTruth's Advice:

Liquid surge can result from nearly any conceivable movement. Steering left, right, accelerating, or slowing down can all cause liquid to surge. Even after stopping a liquid filled tanker, the liquid will surge back, then forward again, possibly forcing your truck to "jump" forward once more. That's why it's good practice to stay further back from vehicles, even when stopped.

Prev
Next
The amount of liquid to load into a tank depends on:
  • The amount the liquid will expand in transit (outage)
  • All of these should be taken into consideration when loading liquid into a tank
  • Legal weight limits
  • The weight of the liquid

Quote From The CDL Manual:

A full tank of dense liquid (such as some acids) may exceed legal weight limits. For that reason, you often may only partially fill tanks with heavy liquids. The amount of liquid to load into a tank depends on:

  • The amount the liquid will expand in transit.
  • The weight of the liquid.
  • Legal weight limits.
Prev
Next
What is Outage?
  • When liquids condense as they warm
  • When liquids condense as they cool
  • When liquids expand as they warm
  • When liquids expand as they cool

Quote From The CDL Manual:

Never load a cargo tank totally full. Liquids expand as they warm and you must leave room for the expanding liquid. This is called "outage." Since different liquids expand by different amounts, they require different amounts of outage. You must know the outage requirement when hauling liquids in bulk.

A full tank of dense liquid (such as some acids) may exceed legal weight limits. For that reason, you often may only partially fill tanks with heavy liquids. The amount of liquid to load into a tank depends on:

  • The amount the liquid will expand in transit.
  • The weight of the liquid.
  • Legal weight limits.
Prev
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