CONCEPTS TO PREVENT FIRES & EXPLOSION CHAPTER 7

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

The specific topics include: 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 Example: 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 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

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

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:

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

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

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)

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

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

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

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 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 0.1 mJ Static buildup by walking across carpet 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 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

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

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

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 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

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