1. CONCEPTS TO PREVENT FIRES & EXPLOSION
CHAPTER 7

2. Objective
Examine the designs to prevent fires and explosion and compare the advantages and disadvantages of each designs.

3. The specific topics include:
Inerting
Controlling static electricity
Explosion-proof equipment & instruments
Ventilation
Sprinkler systems

4. 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
Example: 6% oxygen if LOC is10%

5. Methods of inerting Vacuum purging Pressure purging
Combined pressure-vacuum purging
Vacuum & pressure purging with impure nitrogen
Sweep-through purging
Siphon purging

6. Vacuum purging
Not used for large storage vessels because they are not designed for vacuums and usually can withstand a pressure of only a few inches of water
Reactor however are designed for full vacuum(-760 mm Hg gauge OR 0.0 mm Hg absolute)
Vacuum purging is a common procedure for reactors
Steps in vacuum purging:
Drawing vacuum until desired vacuum is reached
Relieving vacuum with inert gas~N2 or CO2 to atmospheric pressure
Repeat steps 1 & 2 above until desired oxidant concentration is reached

7. Concentration after j purge cycles, vacuum and relief is given by:
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

8. Total moles of inert gas added for each cycle is constant
Total moles of inert gas added for each cycle is constant. For j cycles, the total inert gas is given by:

9. Example 7.1
Use a vacuum purging technique to reduce the oxygen concentration within 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

10. 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

11. 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)

12. One practical advantage of pressure purging vs vacuum purging is the potential for cycle time reductions
The pressurization 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

13. 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

14. Combined pressure-vacum purging
Purging cycles for a pressure-first purge (Fig 7.3)
Purging cycles for evacuate-first purge (Fig 7.4)

15. Vacuum and pressure purging with impure nitrogen
Previous equation only applies for pure nitrogen
Nitrogen 98%+ range

16. 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

17. 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
V=vessel volume
C0=inlet oxidant concentration
Qv=volumetric flow rate
t=time
Reduce oxidant concentration from C1 to C2

18. 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

19. 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

20. 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
0.1 mJ
Static buildup by walking across carpet
20 mJ

21. Double-Layer Charging
Streaming Current
The flow of electricity produced by transferring electrons from one surface to another by a flowing fluid or solid
When a liquid or solid flows through a pipe (metal or glass), an electrostatic charge develops on the streaming material
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

22. Controlling Static Electricity
Reduce rate of charge generation
Reduce flow rates
Increase the rate of charge relaxation
Relaxation tanks after filters, enlarged section of pipe before entering tanks

23. Use bonding and grounding to prevent discharge
The voltage difference between two conductive materials is reduced to zero by bonding the two materials, that is by bonding one end of a conducting wire to one of the materials and bonding the other end to the second material
Bonding and grounding reduces the voltage of an entire system to ground level or zero voltage
This also eliminates the charge buildup between various parts of a system, eliminating the potential for static sparks

24. Controlling Static Electricity
GROUNDING
BONDING

25. 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

26. 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

27. VENTILATION Open-Air Plants Plants Inside Buildings
Average wind velocities are often high enough to safely dilute volatile chemical leaks
Plants Inside Buildings
Local ventilation
Purge boxes
‘Elephant trunks’
Dilution ventilation (³ 1 ft3/min/ft2 of floor area)
When many small points of possible leaks exist

28. 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

29. 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