DESIGNS TO
PREVENT FIRE &
EXPLOSION
LECTURE 11
Eliminate Ignition Sources

Fire or Flames
 Furnaces and Boilers
 Flares
 Welding
 Sparks from Tools
 Spread from Other Areas

Matches and Lighters

Typical Control






Spacing and Layout
Spacing and Layout
Work Procedures
Work Procedures
Sewer Design, Diking, Weed
Control, Housekeeping
Procedures
Eliminate Ignition Sources

Hot Surfaces



Hot Pipes and Equipment
Automotive Equipment
Electrical





Typical Control



Sparks from Switches
Static Sparks
………………………
Lightning
Handheld Electrical Equipment
Spacing
Procedures
Typical Control




Area Classification
Grounding, Inerting,
Relaxation
Geometry, Snuffing
Procedures
What else can be done?





Inerting
Controlling static electricity
Explosion-proof equipment & instruments
Ventilation
Sprinkler systems
Inerting




Process of adding inert gas to combustible mixture to reduce
concentration of oxygen below limiting oxygen concentration
(LOC)
Inert gas- nitrogen, carbon dioxide, steam(sometimes)
Inerting begins with initial purge of vessel with inert gas to
bring oxygen concentration down to safe concentrations
Commonly used control point=4% below LOC~6% oxygen if
LOC is10%
Methods of inerting






Vacuum purging
Pressure purging
Combined pressure-vacuum purging
Vacuum & pressure purging with impure nitrogen
Sweep-through purging
Siphon purging
Vacuum purging



Not used for large storage vessels because they are not
designed for vacuums
Reactor~designed for full vacuum(-760 mm Hg gauge OR 0.0
mm Hg absolute)
Steps in vacuum purging:
 Drawing vacuum until desired vacuum is reached
 Relieving vacuum with inert gas~N2 or CO2
 Repeat steps 1 & 2 above until desired oxidant
concentration is reached

Concentration after j purge cycles, vacuum and relief is given
by:
j
 nL 
 PL 
yj  y 
 y 

 nH 
 PH 






y0=initial oxidant concentration
yj=final target oxidant concentration
PH=initial pressure
PL=vacuum pressure
nH=number of moles at PH
nL=number of moles at PL
j

Total moles of inert gas added for each cycle is constant. For j
cycles, the total inert gas is given by:
nN2
V
 j  PH  PL 
RgT
Example 7.1
Use a vacuum purging technique to reduce the oxygen
concentration withing a 1000-gal vessel to 1 ppm. Determine
the number of purges required and total nitrogen used. The
temperature is 75 degrees F, and the vessel is originally
charged with air under ambient conditions. A vacuum pump is
used that reaches 20 mm Hg absolute, and the vacuum is
subsequently relieved with pure nitrogen until the pressure
returns to 1 atm absolute
Pressure purging


Vessels can be pressure-purged by adding inert gas under
pressure
After the added gas is diffused throughout the vessel, it is
vented to the atmosphere~usually down to atmospheric
pressure
j
 nL 
 PL 
yj  y 
 y 

n
P
 H
 H
nN2




j
V
 j  PH  PL 
RgT
Vessel is initially at PL and is pressurized using a source of
pure nitrogen at PH
nL=total moles at atmospheric pressure (low pressure)
nH=total moles under pressure (high pressure)
Initial concentration of oxidant (yo) is computed after the
vessel is pressurized (1st pressurized state)
Example 7.2

Use a pressure purging technique to reduce the oxygen
concentration in the same vessel discussed in Example 7.1.
Determine the number of purges required to reduce the oxygen
concentration to 1 ppm using pure nitrogen at a pressure of 80
psig and at a temperature of 75 degrees F. Also, determine the
total nitrogen required
Combined pressure purging


Purging cycles for a pressure-first purge (Fig 7.3)
Purging cycles for evacuate-first purge (Fig 7.4)
j
 nL 
 PL 
yj  y 
 y 

 nH 
 PH 
j
Vacuum and pressure purging with
impure nitrogen



Previous equation only applies for pure nitrogen
Nitrogen 98%+ range
Remaining impurities=oxygen
 PL
y j  y j 1 
 PH

 PL 
  yoxy 1 


 PH 
Advantages & disadvantages



Pressure purging is faster because pressure differentials are
greater. However uses more gas than vacuum purging
Vacuum purging uses less inert gas because oxygen
concentration is reduced primarily by vacuum
Combined pressure-vacuum purging~less nitrogen is used
compared to pressure purging, especially if the initial cycle is a
vacuum cycle
Sweep through purging



Adds purge gas into a vessel at one opening and withdraws the
mixed gas from the vessel to the atmosphere from another
opening
Commonly used when vessel not rated for pressure or vacuum
Purge gas is added and withdrawn at atmospheric pressure
 C1  C0 
Qvt  V ln 

 C2  C0 





V=vessel volume
C0=inlet oxidant concentration
Qv=volumetric flow rate
t=time
Reduce oxidant concentration from C1 to C2
Example 7.3
A storage vessel contains 100% air by volume and must be
inerted with nitrogen until the oxygen concentration is below
1.25% by volume. The vessel volume is 1000ft3. how much
nitrogen must be added:


assuming nitrogen contains 0.01% oxygen
If it is pure nitrogen
Siphon purging




Sweep-through process requires large quantities of
nitrogen~expensive
Siphon purging is used to minimize this type of purging
expense
Starts by filling vessel with liquid-water or any liquid
compatible with product
Purge gas is added to the vapor space of the vessel as the
liquid is drained from vessel
Static Electricity



Sparks resulting from static charge buildup (involving at
least one poor conductor) and sudden discharge
Household Example: walking across a rug and grabbing
a door knob
Industrial Example: Pumping nonconductive liquid
through a pipe then subsequent grounding of the
container
Dangerous energy near flammable vapors
Static buildup by walking across carpet
0.1 mJ
20 mJ
Double-Layer Charging

Streaming Current



The flow of electricity produced by transferring
electrons from one surface to another by a flowing
fluid or solid
The larger the pipe / the faster the flow, the larger the
current
Relaxation Time


The time for a charge to dissipate by leakage
The lower the conductivity / the higher the dielectric
constant, the longer the time
Controlling
Static Electricity

Reduce rate of charge generation


Increase the rate of charge relaxation


Reduce flow rates
Relaxation tanks after filters, enlarged section of pipe
before entering tanks
Use bonding and grounding to prevent discharge
Controlling
Static Electricity
GROUNDING
BONDING
Explosion Proof Equipment

All electrical devices are inherent ignition sources

If flammable materials might be present at times in an
area, it is designated XP (Explosion Proof Required)

Explosion-proof housing (or intrinsically-safe
equipment) is required
Area Classification

National
Electrical Code
(NEC) defines
area
classifications
as a function of
the nature and
degree of
process
hazards present
Class I
Flammable gases/vapors present
Class II
Combustible dusts present
Class III
Combustible dusts present but not
likely in suspension
Group A
Acetylene
Group B
Hydrogen, ethylene
Group C
CO, H2S
Group D
Butane, ethane
Division 1
Flammable concentrations normally
present
Division 2
Flammable materials are normally in
closed systems
VENTILATION

Open-Air Plants


Average wind velocities are often high enough to safely
dilute volatile chemical leaks
Plants Inside Buildings

Local ventilation



Purge boxes
‘Elephant trunks’
Dilution ventilation (1 ft3/min/ft2 of floor area)

When many small points of possible leaks exist
Sprinkler system types




Antifreeze sprinkler system
 A wet pipe system that contains an antifreeze solution and
that is connected to water supply
Deluge sprinkler system
 Open sprinklers and an empty line that is connected to
water supply line through a valve that is opened upon
detection of heat or flammable material
Dry pipe sprinkler system
 A system filled with nitrogen or air under pressure. When
the sprinkler is opened by heat, the system is depressurized,
allowing water to flow into the system and out the open
sprinkler
Wet pipe sprinkler system
 A system containing water that discharges through the
opened sprinklers via heat
Summary


Though they can often be reduced in
magnitude or even sometimes designed out,
many of the hazards that can lead to
fires/explosions are unavoidable
Eliminating at least one side of the Fire
Triangle represents the best chance for
avoiding fires and explosions