DESIGNS TO PREVENT FIRES AND EXPLOSIONSINERTING AND PURGING

Y. Alper Gül

METHODS TO PREVENT FIRES AND EXPLOSIONS

• Inerting• Vacuum Purging

To avoid potential damage from fire and explosion:

• Prevent the initiation of the fire or explosion

• Minimize damage after a fire or explosion has occured

Methods

• Inerting• Use of flammability diagrams• Controlling static electricity• Ventilation• Explosion-proof equipments and instruments• Sprinkler sytems• Early design features

3 conditions must be met for fire or combustion explosion:

1. Combustible material2. Oxygen3. Ignition source

If any of the three conditions is eliminated – impossible for a fire or combustion explosion to result!

INERTING

AIM: Reducing the concentration of oxygen below the limiting oxygen concentration, LOC.

HOW: with adding an inert gas to a combustible mixture.

INERTING

• For many gasses the LOC is approximately 10%

• Control point for the purging is oxygen concentration 4% below the LOC, 6%

• Inerting gasses– Nitrogen– Carbon dioxide– Steam (not too comon)

INTERTING

Inerting Systems:• Generally automatically works

• Should have an analyzer to monitor oxygen concentration continuously

• Should have closed loop control system with resonable set points like 6% low and 8% high

INERTING

• There are several purging methods used to initially reduce the oxygen concentration to the low set point:– Vacuum purging– Pressure purging– Combined pressure-vacuum purging– Vacuum and pressure purging with impure nitrogen– Sweep through purging– Siphon purging

VACUUM PURGING

• Common procedure for reactors since they are designed for full vacuum; -760mm Hg gauge

• Most common inerting procedure for vessels• Not suitable for large storage vessels because:– They are not designed for vacuums– They can withstand a pressure of only a few inches

of water

VACUUM PURGING

Procedure for Vacuum Purging:1. Drawing a vacuum on the vessel till the set

point reached2. Relieving the vacuum with an inert gas;

nitrogen and carbon dioxide3. Repeating step (1) and (2) until the desired

oxidant concentration is reached

VACUUM PURGING

VACUUM PURGING:RECOMENDATION

• A pressure gage or manometer should be provided at the purge gas inlet and outlet to indicate equipment conditions– Positive or negative pressure in excees of safe limits should be avoided

• Combustible gas and oxygen analyzers should be used to monitor the concentrations of them– When combustibles are removed, the maximum flammable vapor

concentration should be less then 25% of the lower explosive limit in air– If vapor exceed safe levels, inert gas should be introduced, and purging

resumed.

VACUUM PURGINGCalculations with asuming ideal gas behavior to determine the number of cycles to reach desired oxygen concentration:

The number of mo les of oxidant for the low&high pressure, & for first cycle:

After vacuum relieved with pure nitrogen, the moles of oxidant are stay same as in vacuum state. To find its concentration:

and

If the vacuum and inert relief process is repeated, the concentration after the

second purge is:

The concentration after «j» purge cycles is given by:

VACUUM PURGINGThe total moles of nitrogen added for each cycle is constant. For #j cycles total nitrogen is given by

Example Problem

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 F, and the vessel is originally charged with

air under ambient conditions. A vacuum pump is used that

reraches 20mm Hg absolute, and the vacuum is subsequently

relieved with pure nitrogen until the pressure returns to 1 atm

absolute.

Number of purges = j = 3.37. Four (4) purge cycles are required to reduce the oxygen concentration to 1 ppm

Calculation of the total nitrogen:

REFERENCES1. FM Global, Loss Prevention Data Sheet 7-59, Inerting and

Purging of Tanks, Process Vessels, and Equipment, 20002. Crowl, Daniel A., and Joseph F. Louvar. Chemical Process

Safety Fundamentals with Applications. 2nd Edition. N.J: Prentice Hall, 2002