Indian Institute of Technology Roorkee
Lecture 28
Designs to Prevent Fire & Explosion: Inerting
& Purging
Welcome to the new lecture that is the design
to prevent fire and explosions, so up till
now in the different modules we have studied
different type of fires and explosion, we
have classified them, we have find out that
what are the causes for those fires whether
it is a positive sense or in a negative sense.
Now, since we all agree upon that these fires
and explosion are extremely destructive in
nature, so we must design the system through
which we can prevent the hazard of fire and
explosion.
So, in these this particular chapter we will
discuss all the aspect that how we can prevent
the fire and explosion though theoretically
we know we all agree upon that if you remove
any arm of a fire triangle then definitely
we can extinguish the fire, but sometimes
it is not at all feasible. So how we can design
the system in such a way that we can prevent
the hazard of fire and explosion? So, in this
particular chapter we will discuss this type
of things in different modules.
So, in this particular module we are going
to study the design to prevent the fire explosion
systems, how we can design it, different type
of inerting, vacuum purging, pressure purging,
combined pressure and vacuum purging, vacuum
and pressure purging with impure nitrogen,
sweep through purging and siphon purging.
Now, first of all let us have a discussion
about how to prevent the fire and explosion.
Usually a twofold strategy is used to limit
the potential damage from fires and explosion.
One is the prevent the initiation of fire
and explosion obviously this is one of the
foremost requirement and the second is that
minimize the damage after a fire or explosion
has occurred. For any fire or combustion explosion
to occur, three conditions must be met.
Please recall the fire triangle that fuel,
source of ignition and supply of Oxygen and
after this to sustain the fire net chemical
reaction that obviously in terms of combustion.
So, if we wish to make the system safer and
if we cannot avoid the generation of fire,
then we must to design the system accordingly.
The rule of thumb says if any of three condition
of fire triangle is eliminated the triangle
is broken and it is impossible for a fire
or combustion and explosion to result or to
sustain. So, this is the theoretical aspect,
so while designing the prevention methodology
for fire and explosion we must keep this thing
in our mind.
Now, question arises that how we can design
the system? So there are seven broad spectrums
through which we can go ahead with designing
aspect. Inerting, static electricity, controlling
static electricity, ventilation, explosion-proof
equipment and instruments, sprinkler system
and miscellaneous design features for preventing
the reference. So we will discuss all the
aspect in due course of time in various models.
So, let us have first thing that is Inerting.
Inerting is a process of adding an inert gas
to a combustible mixture to reduce the concentration
of oxygen below the limiting oxygen concentration
or minimum oxygen concentration. So that the
required stoichiometric demand is depleted,
and flammable mixture or inflammable mixture
does not catch fire. The inert gas is usually
Nitrogen or Carbon dioxide, although steam
is sometimes used, but not in a common fashion.
For many gases the MOC is approximately 10
percent and for many dusts it is approximately
8 percent.
An inerting system is required to maintain
an inert atmosphere in the vapor space above
the liquid. Suppose, I am working here in
a pool of say hexane and just because of its
vapor pressure certain quantity of vapors
has been generated and just above the pool
of liquid surface. So, by virtue of my system
design I am not in a position to cordon off
oxygen or supply of air. Then inerting is
a useful way (to inert) to minimize the impact
of fire hazard.
We can introduce certain inert gases so that
the minimum oxygen concentration or limiting
oxygen concentration is depleted. So ideally
this type of system should include an automatic
inert gas addition feature to control the
oxygen concentration below the limiting oxygen
concentration.
Now this control system should have an analyzer
to continuously monitor the oxygen concentration
in relationship to the LOC and controlled
inert gas feed system to add inert gas when
the oxygen concentration approaches the limiting
oxygen concentration. So that as soon as it
approaches to LOC then it should activate
and introduce the inert, so that the whatever
oxygen is there it should come down (to the
level) below the level of limiting oxygen
concentration.
However, the inerting system consists only
a (regular) regulator design to maintain a
fixed positive inert pressure in the vapor
space. Now, this ensures that inert gas is
always flowing out of the vessel rather than
air flowing in. So that the impact of air
be minimized. The analyzer system however,
results in a significant saving in inert uses
without sacrificing safety. Now, thing is
that whenever you are introducing inert gas
then definitely certain quantum of money is
involved because ultimately you are going
to procure Nitrogen or Argon whatever inert
gas is being used and you must have a pumping
system through which you can introduce the
inert gas to the system.
So, if you are having a proper analyzer system
then definitely you need not to bother the
continuous supply of inert gas to the system
because ultimately whenever there is a heat
system then if you are having the unnecessary
inert gas over there then it will attract
its certain heat value. So that is why you
may have a significant saving if you adopt
the analyzer system over there.
Now, how we can perform the inerting? This
is the big question. So there are several
purging methodology, purging is one of the
ways through which you can introduce the (inerting
matter material) inerting system. So, there
are several purging methods used to initially
reduce the oxygen concentration to the low
set point. There are 6 different ways through
which you can perform these purging operation,
vacuum purging, pressure purging, combine
vacuum and pressure purging, vacuum and pressure
purging with impure nitrogen, Sweep through
purging and siphon purging.
So, let have first let us have a look of vacuum
purging. Now, vacuum purging is one of the
most common inerting procedure for vessels.
It is quite simple only thing is that it is
highly energy intensive. Now, this process
procedure is not used for large storage vessels
because they are usually not designed for
vacuum and usually can withstand a pressure
for only a few inches of water. So if you
are willing to adopt this protocol for a large
hall or a large vessel then obviously you
can create the vacuum inside, but it will
be a costly affair. So, it is usually applicable
for a small vessel, small pressure vessel,
etc.
Now, there are three steps involved in vacuum
purging. One is drawing a vacuum on the vessel
until the desired vacuum is reached. You need
to calculate a priori that how much vacuum
is desired to have inerting in the particular
system. Relieve the vacuum with an inert gas
such as Nitrogen or a Carbon dioxide to atmospheric
pressure and then if you have not achieved
the desired result, so by analyzing the LOC
or minimum oxygen concentration, then you
need to repeat the step number 1 and 2 until
the desired oxidant concentration is reached.
So, first thing is that suppose you are having
the vessel you need to draw the vacuum and
once it is achieved then you can introduce
because of the pressure difference you can
easily introduce the inert gas inside and
if it is not then definitely again you need
to repeat the vacuum system to this vessel
and then again you need to introduce the things
accordingly till you achieve the minimum oxygen
concentration.
We can understand this particular aspect with
this small plot here at the x-axis we are
having the time domain and at y-axis the pressure
domain.
Now here the PH is the initial high pressure
this one and PL is the initial low pressure
or desired vacuum pressure. Y naught is initial
oxidant concentration and Y1 is the final
target oxidant concentration, so first thing
is that you are having certain quantity of
oxygen in a system specially suppose you are
willing to have an introduction of inert gas
to a vessel. Then initially you are having
Y naught concentration of Oxygen.
Then you reduce the pressure until it reaches
to PL that is the lower pressure limit, here
you maintain the pressure and you introduce
the desired oxygen concentration. Suppose
you have achieved the desired Oxygen concentration
by introducing the inert material and then
you raise the pressure so that it can acquire
the desired pressure to maintain the reaction
mechanism.
So, here you have a Y1 that is the final oxygen
concentration and suppose if you are unable
to have this LOC requirement then you need
to repeat this system for again and again.
So, here in this particular plot the things
are repeated for twice so until it reaches
the final desired concentration of oxidant.
Now this is, you can calculate this through
small mathematical formula that is concentration
after J purge cycles suppose in the previous
case we have to studied 2 purge cycles, concentration
after J purge cycles vacuum and relief is
given by Y j is equal to Y naught into nL
upon n H to the power j equal to Y naught
P L upon P H to the power j. So, you can easily
calculate that what will be the final target
oxidant concentration under the system in
question.
So, the total moles of inert gas added for
each cycle is constant and you can have a
look through this previous figure, this figure.
So, for j cycle the total inert gas is given
by delta n, delta n represent the change in
number of moles of Nitrogen, Nitrogen in this
case is introduced as the as an inert gas
is equal to j P H minus P L into V upon R
g T, T is the temperature of the system, R
g is the universal gas constant and V is the
volume. So, by this way you can calculate
that how many cycles are required, how many
moles are needed to, how many moles of inert
is needed to inert the system, the pressure
system, pressure vessel system inside the
reactor.
Now let us have look of pressure purging.
Now, vessel can be pressure purged by addition
of inert gas under pressure. Sometimes it
performs action like sweep through purging
so after the added gas is diffused throughout
the vessel, it is vented to the atmosphere
usually down to atmospheric pressure. So,
more than one pressure cycle may be necessary
to reduce the oxygen content to the desired
concentration.
Now here again we are having small plot. Here
we are having a high pressure and a low-pressure
zone. Y naught is the initial concentration
of oxidant so we increase the pressure of
the system and then we introduce the inert
gas to it so that it maintains that Y1 that
is the concentration of oxidant in the vessel
and then we lower down it again then we have
the same type same concentration and then
again we raise the pressure to PH and then
again reintroduce the inert system and maintain
the Y2 concentration of the oxidant and then
by this way we can have the Y2 that is the
number of moles of oxygen in the system. And
usually in all systems we used to keep the
number of moles of oxygen constant, throughout
constant. So, in this way we are having the
two pressure purge cycles.
Again, we are having the mathematical representation
for calculating the number of moles after
j cycle. Y j is equal to Y naught into n L
upon n H to the power j is equal to Y naught
P L upon P H to the power j and the total
number of moles of inert Nitrogen in the case
is added delta n Nitrogen is equal to j into
P H minus P L into V upon R g T.
So, vessel is initially it is PL and pressurized
using the source of pure Nitrogen at PH, nL
is the total numbers of moles at atmospheric
pressure at low pressure, then nH the total
number of moles under pressure that is high
pressure. So, by this way you can calculate
the number of moles needed in terms of inert
gas to make the pressure inerting.
Now, there are so many advantages and disadvantages
associated with pressure purging. Now, one
practical advantage of pressure purging versus
vacuum purging is that potential of cycle
time reduction. Now, the pressurization process
is much more rapid compared to the relatively
slow process of developing a vacuum. Also,
the capacity of vacuum system decreases significantly
as absolute vacuum is decreased. Pressure
purging however, uses more inert gas compared
to the vacuum one, so the best purging process
is selected usually based on the cost and
performance.
So, if you are willing to inert the hydrocarbon
piping system then definitely the pressure
purging is most advantageous because it will
be very difficult to have a vacuum purging
for a pipeline having a distance of say 1000
kilometer or 500 kilometer it will be very
difficult. So, in some cases both pressure
and vacuum purging are available and are used
simultaneously to purge a vessel.
In that particular case, we need to go ahead
with the combined pressure vacuum purging
system because sometimes it is a need of time.
So, the computational procedure depends on
whether the vessel is first evacuated or pressurized.
Now, purging cycle for pressure is the first
purge then purging cycle two for evacuate
that is the first purge.
So, this is the comparative diagram here we
are having the pressure first purging and
here we are having the vacuum first purging.
Here we are having the initial pressure rate
say P naught then it raised up to PH then
into introduction of certain quantity of inert
material, inert gas and then the pressure
lowered down to PL and then it is a repeated
cycle. The same thing here we are lowering
down the pressure to have certain concentration
of oxygen and then it is raised then again,
the cycle is repeated to maintain the appropriate
level of oxidant within the system.
Now, question arises that vacuum whether we
have to choose the vacuum or pressure and
which one? Now, pressure purging is faster
because pressure differentials are greater.
So obviously if we analyze the things in terms
of cost, in terms of ease of operation, pressure
purging is bit faster. Vacuum purging uses
less inert gas than the pressure purging so
if you are not having a consideration of cost
of inert gas then definitely you can go ahead
with the pressure purging and if it is the
consideration then you have you may think
the option of vacuum purging. So, combination
of these two gain benefits of both especially
if initial cycle is vacuum cycle. So that
you can utilize the aspect of pressure differential.
Sometimes we may have to use the vacuum and
a pressure purging with impure Nitrogen. So,
previous equation which we have discussed,
developed for the vacuum and pressure purging
apply to the case of pure nitrogen only. Many
of the Nitrogen separation processes available
today do not provide pure Nitrogen they typically
provide the Nitrogen in the range of 98 percent
plus minus range.
Now, see the question is that if we go ahead
with the highest purity then the cost of Nitrogen
production or a cost of inert production would
be on the higher side. So, we need to optimize
the thing. So Sometimes it is available say
98 percent 95 percent pure and if it is permissible
prior to thinking of the concentration of
Nitrogen and prior to thinking of out the
other impurities present in the Nitrogen string.
Then it is always advisable to use such kind
of a Nitrogen to reduce the cost of the system.
So, assume that the Nitrogen contains oxygen
with a constant mole fraction of Y oxy then
Y j is equal to Y j minus 1 P L upon P H plus
Y oxygen into 1 minus P L upon P H. So, this
gives you that what would be the mole concentration
of Oxygen in due course of time.
Now, next is the sweep through purging. Now,
usually it is a very common phenomenon and
it is a very beneficial phenomenon for keeping
the low concentration of Oxygen within a vessel.
So usually it adds purge gas into a vessel
at one opening and withdraws the mixed gas
from the vessel to the atmosphere or scrubber
from another opening.
It is a commonly used process when vessel
or equipment is not rated for pressure or
vacuum. The purge gas is added and withdrawn
at atmospheric pressure. Sometimes say because
of inherent inability you may not be able
to have the vacuum system or a pressurized
system with in a vessel may be because of
mechanical problem, may be the vessel is not
designed to withstand such a high or a low
pressure then this particular technique is
extremely useful.
Now, usually what we used to do in, let us
have this is my vessel so from one end we
used to draw the vacuum and simultaneously
we use to introduce a certain quantity of
inert gas over here and continuously we used
to draw the mixed gas from that stream. So,
once because of because we have to maintain
a certain pressure difference, so we draw
the vacuum first stage then we close it 
then we introduce certain quantity of inert
gas and continuously we draw the mixed gas
from this front. One time in one phase we
are having the advantage, another phase we
are having a certain disadvantage.
The disadvantage is that whatever mixture
is coming out from this we need to treat this
mixture appropriately. It may a form inflammable
mixture, so it may create a problem at the
exit port. Sometimes it may have some toxic
material then again, we need to go for the
scrubbing system. So, we need to be very careful
whenever we are adopting this type of a purging
scheme so purging results usually are defined
by assuming perfect mixing within the vessel
specially applicable whenever we are designing
the sweep through purging.
Now, we have to maintain the things at a constant
temperature and a constant pressure. Under
these conditions the mass or volumetric flow
rate of exit stream is equal to the inlet
stream. So that we can maintain the properties
of the system appropriately.
Now, here is one of the mathematical relations
through which you can analyze the volumetric
flow rate. The volumetric quantity of inert
gas require to reduce the oxidant concentration
from C 1 to C 2 let us have Q v 2 and it is
determined using this particular equation
Q v 2 is equal to V ln C1 minus C 2 upon C
2 minus C naught where C is the concentration
of oxidant within the vessel, C naught is
the inlet oxidant concentration, V is the
vessel volume, Q v is the volumetric flow
rate and T is the time required.
Now, purging results are defined by assuming
perfect mixing within the vessel. Constant
temperature and a constant pressure. So, under
these conditions the mass or a volumetric
flow rate for the exit stream is equal to
the inlet stream. So, these are certain things
we need to keep in mind.
Another purging method is the Siphon purging.
Now sweep-through purging process requires
large quantity of nitrogen. Because you need
to fill the entire mass of I mean entire volume
of the vessel with the nitrogen. Now, this
could be expensive when purging a large storage
vessel. So, Siphon purging is used to minimize
this type of purging expenses. Now, it is
a very simple and quite effective. The Siphon
purging processes starts by filling the vessel
with the liquid and sometimes with water or
any liquid compatible with the product.
Now, then the purge gas is subsequently added
to the vapor space of the vessel and liquid
is drained from the vessel simultaneously.
So, it is just like this that you are having
this vessel. You filled it 
with liquid and then you introduce the inert
gas and simultaneously you withdraw the filled
liquid or a water in this case so that you
can inert. Because there will be no room for
air to entrap so that 
the concentration of oxygen will remain low
as desired. The volume of purge gas is equal
to the volume of the vessel and rate of purging
is equivalent to the volumetric rate of liquid
discharge.
So, when using the siphon purging process
it may be desirable to first fill the vessel
with 
the liquid then use sweep-through purging
process to remove oxygen from residual head
space. Now, by using this method the oxygen
concentration is decreased to low concentration
with only a small added expense for the additional
sweep-through purging. So, these things should
be in our mind.
So, in 
this particular module we have discussed a
first phase of design to prevent 
the fire and explosion and 
a primarily we discussed about the 3 different
purging methodology. Vacuum purging, Pressure
purging, combined vacuum and a pressure purging,
Siphon purging and Sweep-through purging.
So, in 
the next module we will discuss the other
purging methodology and other methods through
which we can prevent the disaster of a fire
and explosion.
You can always see the various references
those who are enlisted over here for your
further reading. Thank you.
