Flashpoint,Firepoint and spontaneous ignition

Question 1

Define flashpoint

Answer 1

the liquid temperature at which application
of an ignition source will cause a flame to
flash across the surface of the liquid.
the flame will NOT be sustained

Question 2

define fire point

Answer 2

the lowest temperature at which the
rate of supply of fuel vapours (by evaporation)
can sustain the flame

Question 3

define spontaneous ignition temperature

Answer 3

This is the lowest temperature at which the substance will ignite spontaneously, that is the substance will burn without the application of a flame or other ignition source. This is sometimes referred to as the auto-ignition temperature

Question 4

The
type of chemical reaction which occurs in the
flame is known as a chain reaction. describe chain reaction

Answer 4

Once a flame has been established and flammable
vapours are rising from the fuel surface, heat and
what are called chain carriers are produced where
the flame reactions are occurring. A proportion of
these will pass into the next layer of gas and start
the oxidation and heat release processes there,
rather as in a relay race. Chain carriers are believed
to be atoms or fragments of molecules known as
free radicals which are extremely reactive

Question 5

What are FREE Radicals

Answer 5

Chain carriers are believed
to be atoms or fragments of molecules known as
free radicals which are extremely reactive

Question 6

Before we discuss flames, it is useful to define two

types of gas flow

Answer 6

laminar flow and turbulant flow

Question 7

What is laminar flow and give an example of laminar flow flame

Answer 7

is steady flow in which
two particles starting at any given point follow the
same path. Particles never cross each other’s paths,
so the particle paths are bunched together like
uncooked spaghetti in a packet. At any given time,
the velocities of all particles on one path are the
same, but the velocities of particles in different
paths might be different. Laminar flow is associated
with slow flow over smooth surfaces.

Question 8

Describe turbulant flow

Answer 8

there are random
changes in velocity and direction of the flow,
although the flow as a whole is moving in a definite
direction. If we consider wind blowing down
a street on a windy day, leaves and litter may be
blown up, down, across and around, revealing
local changes in the flow, but the general direction
of the wind is still down the street. Turbulent flow
tends to occur in fast flows over rough surfaces
and around obstacles.

Question 9

Premixed flames

Answer 9

occur when a fuel is well-mixed
with an oxidant, e.g., 10% methane mixed with air.
For ignition to occur, energy must be supplied to
the system in the form of a spark or small flame. A
self-sustaining flame will then be established
around the ignition source and propagate outwards
in all directions.

Question 10

The figures quoted for limits of flammability may
vary as there are a number of factors which may
slightly alter the value:

Answer 10

pressure, temperature,
dimensions of the test apparatus, direction of
flame propagation and moisture content of the
mixture all have some effect. (In general, the limits
widen with rise in temperature.)

Question 11

stoichiometric mixture.

Answer 11

optimum mixture
in which there is just sufficient fuel to use up all
the oxygen.

Question 12

burning

velocity

Answer 12

For each mixture of fuel and air between the flammability
limits, there is a characteristic burning
velocity at which a premixed flame will propagate
through a stationary gas

Question 13

what are diffusion flames

Answer 13

Diffusion flames occur at the interface where fuel
vapour and air meet. Unlike premixed flames, the
fuel vapour and the oxidant are separate prior to
burning.