Catatan Ujian II Safety
Inerting
Inerting is the process of adding an inert gas to a combustible
mixture to reduce the concentration of oxygen below the limiting
oxygen concentration (LOC)
The inert gas is usually nitrogen or carbon dioxide, although
steam is sometimes used. For many gases the LOC is approximately
10%, and for many dusts it is approximately 8%.
Inerting begins with an initial purge of the vessel with inert
gas to bring the oxygen concentration down to safe concentrations.
A commonly used control point is 4% below the LOC, that is, 6%
oxygen if the LOC is 10%.
Ideally this system should include an automatic inert gas
addition feature to control the oxygen concentration below the
LOC.
This control system should have an analyzer to continuously
monitor the oxygen concentration in relationship to the LOC and a
controlled inert gas feed system to add inert gas when the oxygen
concentration approaches the LOC.
More frequently, however, the inerting system consists only of a
regulator designed to maintain a fixed positive inert pressure in
the vapor space; this ensures that inert gas is always flowing out
of the vessel rather than air flowing in.
The analyzer system, however, results in a significant savings
in inert gas usage while improving safety.
Vacuum Purging
Vacuum purging is the most common inerting procedure for
vessels.
This procedure is not used for large storage vessels because
they are usually not designed for vacuums and usually can withstand
a pressure of only a few inches of water.
The objective of the following calculation is to determine the
number of cycles required to achieve the desired oxygen
concentration.
Because the vessel is pressurized with pure nitrogen, the number
of moles of oxygen remains constant during pressurization, whereas
the mole fraction decreases. During depressurization, the
composition of the gas within the vessel remains constant, but the
total number of moles is reduced. Thus the oxygen mole fraction
remains unchanged.
One practical advantage of pressure purging versus vacuum
purging is the potential for cycle time reductions.
The pressurization process is much more rapid compared to the
relatively slow process of developing a vacuum. Also, the capacity
of vacuum systems decreases significantly as the absolute vacuum is
decreased. Pressure purging, however, uses more inert gas.
Therefore the best purging process is selected based on cost and
performance.
Combined Pressure-Vacuum Purging
In this case the beginning of the cycle is defined as the end of
the initial pressurization. If the initial oxygen mole fraction is
0.21, the oxygen mole fraction at the end of this initial
pressurization is given by
Pressure purging is faster because the pressure differentials
are greater; however, it uses more inert gas than vacuum purging.
Vacuum purging uses less inert gas because the oxygen concentration
is reduced primarily by vacuum. When combining vacuum and pressure
purging, less nitrogen is used compared to pressure purging,
especially if the initial cycle is a vacuum cycle.
Static Electricity
A common ignition source within chemical plants is sparks
resulting from static charge buildup and sudden discharge.
Static electricity is perhaps the most elusive of ignition
sources.
prevent the accumulation of static charge
Static charge buildup is a result of physically separating a
poor conductor from a good conductor or another poor conductor.
When different materials touch each other, the electrons move
across the interface from one surface to the other.
Upon separation, more of the electrons remain on one surface
than on the other; one material becomes positively charged and the
other negatively charged.
If both the materials are good conductors, the charge buildup as
a result of separation is small because the electrons are able to
scurry between the surfaces.
If, however, one or both of the materials are insulators or poor
conductors, electrons are not as mobile and are trapped on one of
the surfaces, and the magnitude of the charge is much greater.
Common industrial examples are pumping a nonconductive liquid
through a pipe, mixing immiscible liquids, pneumatically conveying
solids, and leaking steam that contacts an ungrounded
conductor.
any charge accumulation exceeding 0.1 mJ is considered
dangerous.
Static charges of this magnitude are easy to generate; the
static buildup created by walking across a carpet averages about 20
mJ and exceeds several thousand volts.
To prevent these ignitions, one must understand (1) how charges
accumulate on objects, (2) how charges discharge by means of charge
transfer, and (3) how to estimate the resulting energy discharged
in relation to the minimum ignition energy (MIE) of the explosive
environment.
Charge Accumulation
