Boil Over and Slop Over

Question 1

A ‘boil over’ is most likely to occur in a tank fire containing what product?

Answer 1

Crude oil (which is processed into a range of products, including Gasoline and Petroleum). There is usually a layering of different products contained within the oil.

Question 2

What is the function of a ‘bund’?

Answer 2

Bund walls (constructed of concrete or earth similar to dam walls) surrounding flammable liquid tanks are designed to contain the spill should a leak occur. They are designed to hold 100% of tank’s contents, or the contents of the largest tank in the bund.

Question 3

Explain the process which can lead to a ‘boil over’ in a storage tank

Answer 3

A boil happens over a prolonged period of time, usually takes many hours to occur. A layer of water is located at bottom of tank, mixed with sludge and waste. When there is a fire, the thermal layer travels at a rate of 0.3m - 1m an hour until it reaches the bottom of the tank. The water is instantly vapourised (water to steam ratio 1700:1) and with no where else to go, it forces it’s way back up the tank through the unburnt product, causing a piston effect.

Question 4

Explain what could cause a ‘slop over’ to occur

Answer 4

If any pockets of water (for example from ill directed cooling lines) interspersed within crude oil during a fire, they will be instantly heated to boiling point and expand into steam.
The force of this expansion causes frothing of the top layers of the product, which can flow down the sides of the tank and into the bunded area.

Question 5

Identify two (2) types of installed systems that are associated with crude oil tanks

Answer 5

Foam pourers:
Used to combat the rim and seal the fire

Monitors:
Used for full surface fires and bund fires

Question 6

Important points in regards to boil over/slop over (11)

Answer 6

• Protect exposures
• Avoid entering bunded areas
• Cool sides of tank to maintain integrity
• Monitor progression of heat layer
• Do not apply water to fire
• Do not apply foam until sufficient concentrate is on scene
• Use aerial appliances and monitors
• Direct foam into vortexes
• Difficult to direct foam from ground
• Use installed systems where possible
• Seek expert advice