Nebosh

1. Q5. Give the meaning of the following terms:
 Upper Flammable Limit (UFL); (2)
 Lower Flammable Limit (LFL); (2)
 Flashpoint; (2)
 Highly Flammable Liquids (HFL); (2)
Upper Flammable Limit (UFL)
The Upper Flammable Limit (UFL) pinpoints the richest flammable mixture. The maximum
ratio of flammable gas or vapor in the air above which explosion cannot take place because the
ratio of the gas to oxygen is too high. Also known as Upper Explosive Limit (UEL).
Lower Flammable Limit (LFL)
The Lower Flammability Limit (LFL) pinpoints the minimum mixture able to sustain a flame
(blaze). The combination of substance and air lacks sufficient fuel (substance) to burn. This is
sometimes called the Lower Explosive Limit (LEL).
Flashpoint
Flash point is the temperature to which a liquid must be heated before it will produce a
flammable mixture of vapor in air that will ignite when an ignition source is applied. Flammable
liquids have a flashpoint between 21 and 55°C while, highly flammable liquids have a flashpoint
below 21°C.
Highly Flammable Liquids (HFL)
Highly flammable liquids are those that have closed cup flashpoints below 100°F (37°C) and
vapor pressure not exceeding 40 Psi (276 k Pa) (2.76 bar) at 100°F (37°C). Flammable liquids
can give off large volumes of flammable vapors at room temperature. These vapor’s, when
mixed with air (an ignition source) can explode, often violently. Examples of common highly
flammable liquids include petrol, paint thinners etc.
Flammable liquids are referred as Class 1 liquids.
• Class IA liquids - flashpoints below 73°F (22.8°C) and boiling points below 100°F
(37.8°C).
• Class IB liquids - flashpoints below 73°F (22.8°C) and boiling points at or above 100°F
(37.8°C).
• Class IC liquids – flashpoints at or above 73°F (22.8°C) and below 100°F (37.8°C).
Q3.
(a) Outline the term Flash Point. (3)
(b) List the Hazards associated with LNG (4)
(a) Flash Point
Flash point is the lowest temperature at which a liquid can form a flammable mixture in air near
the surface of the liquid. Lower the flash point, greater the fire hazard. For Example, Gasoline
has a flash point of -40C and is more flammable than Ethylene Glycol which has a flash point

2. of 111C. A closely related and less common term is Fire Point. It is the temperature at which
the flame becomes self-sustained so as to continue burning the liquid (at the flash point, the
flame does not need to be sustained). The fire point is usually a few degrees above the flash
point. The flashpoints of some common laboratory solvents are:
• Ethanol +12C
• Toluene +4C
• Acetone -19C
• Diethyl Ether -45C
(b) Hazards associated with LNG
Hazards associated with LNG are
• Explosion
• Vapor Cloud
• Rapid Phase Transition
• Sloshing
• Freezing Liquid
Explosion
An explosion happens when a substance rapidly changes its chemical state. LNG tanks store the
liquid at an extremely low temperature, about -256°F (-160°C), so no pressure is required to
maintain its liquid state. Since LNG is stored at atmospheric pressure not pressurized, a cracker
puncture on the container will not create a direct explosion.
Vapor Cloud
As LNG leaves a temperature organized container, it begins to warm up, returning the liquid to a
gas. Initially, the gas is colder and heavier than the surrounding air. It creates a fog, vapor cloud
above the released liquid. As the gas warms up, it mixes with the surrounding air and begins to
disperse. The vapor cloud will only ignite if it encounters an ignition source while concentrated
within its flammability range.
Rapid Phase Transition
LNG floats when released on water being less dense than water and vaporizes. If large volume of
LNG is released on water, it may vaporize too quickly causing a Rapid Phase Transition (RPT).
An RPT can only occur if there is mixing between the LNG and water. Other liquids with
different temperatures and boiling points can create incidents when they come in contact with
each other.
Sloshing
Carrying LNG in partially filled tanks could lead to sloshing, a violent motion of the fluid.
Sloshing could lead to an increased high pressure of LNG on the tank walls, especially in harsh
wave environment.
Freezing Liquid
LNG is a cryogenic liquid stored and transported at approximately -160ºC. If LNG is released,

3. A cryogenic liquid can cause freeze burns and eye damage. All facility personnel must wear
gloves, face masks and other protective clothing as a protection from the freezing liquid when
entering potentially hazardous areas.
Q1.
(a) Identify the key stages of a workplace risk assessment. (5)
(b) Outline the meaning of ‘As Low As Reasonably Practicable’ (ALARP). (3)
(a) Identify the key stages of a workplace risk assessment.
Key stages of a workplace risk assessment
A risk assessment is not about generating huge amounts of paperwork, but rather about
identifying sensible measures to control the risks in a workplace. For example, Heavy Liquefied
Petroleum Gas (LPG) cylinders can cause muscular strain when they are handled manually OR if
the cylinder is damaged causing gas to leak which is then exploded. If that leak occurs in a store
room or similar enclosed space, it could result in an explosion that could destroy the building and
kill or injure anyone nearby.
How to assess the risks in your workplace
• Identify the hazards.
• Level of risk involved (very low, low, medium, high, very high).
• Classify the risks and decide precautions.
• Record your significant findings.
• Review your assessment and update if necessary.
(b) Outline the meaning of ‘As Low as Reasonably Practicable’ (ALARP)
ALARP stands for "as low as reasonably practicable", is a term used in health and safety to
represent that satisfactory attention is taken in respect to risk, risk control and risk reduction. The
ALARP principle is the concept of "reasonably practicable", involves weighing a risk against the
trouble, time and money needed to control it. ALARP is not inflexible and consequently can be
challenging because it requires employers conclusion. It is therefore for employers to ensure that
their chosen design or design concept reduce risks "as low as reasonably practicable’.