The learning objectives of this section
are to familiarize all personnel
involved in tank atmosphere control with
the use of flammability composition
diagram
understand the upper and lower explosive
limits
be fully aware of the company policy
regarding tank atmosphere handling
from the previous subjects we saw that
measuring oxygen content we use the
server max OA 262
for measuring hydrocarbon gases by
percent of volume the instrument Rican
17 HC is used
and an explosive meter Rican GP 204 is
used for measuring combustible gas below
lel in air
finally the delay good multi gas
detector is used for measuring low
concentrations of toxic gas
the effects illustrated on the diagrams
in this training module should only be
regarded as a guide to the principals
involved and not used for deciding on
acceptable gasps compositions in
practical cases
the example flammable diagram is for
hydrocarbon gas above crude oil where
the uel is eleven percent and the lel is
1.5%
the left side of the diagram vertical
axis gives the hydrocarbon gas value
the bottom line of the diagram
horizontal axis represents the oxygen
content from zero to twenty one percent
note that twenty one percent oxygen
represents one hundred percent clean air
and as previously mentioned 10.8 % x
value of oxygen is the minimum oxygen
content to be present to ignite the
mixture
the upper limit of the range is known as
the uel upper explosive limit also known
as the UF l upper flammable limit this
level means the hydrocarbon
concentration above which that is
insufficient air to support and
propagate combustion
the mixture said to be too rich
the lower limit of this strange is known
as the lel lower explosive limit also
known as the LF L lower flammable limit
this level means the hydrocarbon
concentration below which that is
insufficient hydrocarbon gas to support
and propagate combustion
the mixture is said to be too lean
in this example a cargo
tank has been water washed and is going
to be made ready for entering the
following measurements have been taken
hydrocarbon equal to fourteen percent by
volume oxygen equal to two percent by
volume
the plot shows the current situation at
Point ay
to avoid the flammable area a lioness
trona from point B keeping well clear of
the flammable area
in this example the ship is applying
with the flue gas plant check the oxygen
content in the supplied inert gas
under no circumstances must the oxygen
content in the supplied in earth gasps
exceed five percent by volume
in this example the oxygen content of
the supplied inert gas is three percent
by volume Point C
if we start venting at point a with
air a lion would extend from point A
straight to point B through the
flammable area
avoid this at all costs
additionally we have to supply more
inert gas to decrease the oxygen content
before any air venting can commence
as in hurting continues a line would
extend from point A to point c as long
as the oxygen content in the supplied
inert gas is three percent by volume
continue to measure the tank atmosphere
after some time this will result in the
chosen line being crossed
in this example the reading is
hydrocarbon equals two-point-eight
percent by volume and oxygen equals
two-point-eight percent by volume point
d
act point continue to measure
at Point D the inerting is to be stopped
and the air venting starts
all measurements taken from now on we'll
follow a straight line from Point D to
point B
in this example the lower explosive
limit lel is given as 1.5% by volume
it is of extreme importance to continue
to measure the hydrocarbon content with
the rican 17 HC interferometer at less
than 1.5% by volume before the rican GP
204 explosive meter is used
when you mesh act point deep
when you measure below the lel using the
rican 17 HC replace this indicator with
the rican GP 204 continue to measure
hydrocarbon gas and oxygen until the
indicators show 0% hydrocarbon by lel
and twenty one percent oxygen only then
is the cargo tank ready for entering
remember to keep venting with air and
take frequent measurements
also remember to issue the permits
needed