CRITICAL OXYGEN CONTENT

LIMITING OXYGEN CONCENTRATION (LOC) OR MAXIMUM OXYGEN CONCENTRATION (MOC) IS THE CONCENTRATION OF OXIDANT BELOW WHICH A DEFLAGRATION CAN NOT OCCUR IN A SPECIFIED MIXTURE. THERE IS MAXIMUM OXYGEN CONCENTRATION (MOC) REQUIRED TO PROPAGATE A FLAME. THIS IS ESPECIALLY USEFUL BECAUSE EXPLOSIONS AND FIRES ARE PREVENTABLE BY REDUCING THE OXYGEN CONCENTRATION REGARDLESS OF THE CONCENTRATION OF THE FUEL.

PURGING AND INERTING (OR BLANKETING) PROCESS VESSELS AND EQUIPMENT ARE TWO COMMON YET DISTINCTIVE PRACTICES TO CONTROL CONCENTRATIONS OF OXYGEN THEREBY REDUCING FIRE AND EXPLOSION HAZARDS. PURGING USUALLY REFERS TO THE SHORT-TERM ADDITION OF AN INERT GAS (eg., NITROGEN OR CARBON DI-OXIDE) TO A TANK, PROCESS VESSEL OR OTHER PIECE OF PROCESS EQUIPMENT THAT CONTAINS FLAMMABLE VAPORS OR GASES TO RENDER THE SPACE NON-IGNITABLE FOR SPECIFIC TIME PERIOD (SAY, DURING A MAINTENANCE OUTAGE). IN CONTRAST, INERTING(OR BLANKETING) IS THE LONG-TERM MAINTENANCE OF A INERT ATMOSPHERE IN THE VAPOR SPACE OF A CONTAINER OR VESSEL DURING OPERATION.

THE GOAL OF PURGING AND INERTING(OR BLANKETING) IS TO REDUCE A VESSEL'S OXYGEN CONCENTRATION BELOW THE LOC/MOC. THE LOCs/MOCs FOR COMMON FLAMMABLE GASES AND VAPORS USING NITROGEN OR CARBON DI-OXIDE DILUENT ARE LISTED IN THE BELOW TABLE :-

SUBSTANCE	LOC FOR N₂/AIR O₂, VOLUME%	LOC FOR CO₂/AIR O₂, VOLUME%
METHANE	12	-
ETHANE	11	-
PROPANE	11.5	-
BENZENE	11.4	-
TOLUENE	9.5	-
GASOLINE	12	-
KEROSENE	10	-
CARBON MONOXIDE	5.5	-
CARBON DI-SULFIDE	5	7.5
HYDROGEN	5	5.2
HYDROGEN SULFIDE	7.5	-
VINYL CHLORIDE	13.4	-
CYCLOPROPANE	11.5	14
NATURAL GAS(PITTSBURGH)	12	14.5
n-BUTYL CHLORIDE	14	-
ACETONE	11.5	14
ETHANOL	10.5	13
METHYL ETHER	10.5	13
METHYL ETHYL KETONE	11	13.5

SOURCE : NFPA 69

A SAFETY MARGIN MUST BE MAINTAINED BETWEEN THE MOC (OR LOC) AND THE NORMAL WORKING CONCENTRATION IN THE SYSTEM. CONSERVATIVE CONTROL TYPICALLY USES 2 - 4 PERCENTAGE POINTS BELOW THE LOC. THAT IS, IF THE LOC OF ETHANOL USING N₂ AS DILUENT IS 10.5%, THE CONTROL POINT WOULD BE 6.5% - 8.5%.

ALTHOUGH NITROGEN OR CARBON DI-OXIDE ARE THE MOST COMMON INERTING GASES, STEAM IS SOMETIMES USED. IF SO, IT MUST BE SUPPLIED AT A RATE SUFFICIENT TO MAINTAIN THE VESSEL TEMPERATURE AT 160^OF OR HIGHER AND CARE MUST BE TAKEN SO THAT CONDENSATION BY COOLING DOES NOT DRAW IN ATMOSPHERIC AIR OR COLLAPSE THE VESSEL BY IMPLOSION. THIS TEMPERATURE IS EXPERIENCE BASED AND IS CONSIDERED SUFFICIENT TO PREVENT CONDENSATION OF THE STEAM WHICH WOULD HINDER THE PROTECTION TO THE VESSEL FROM THE STEAM PURGE.

BELOW THE MOC(OR LOC), THE REACTION CANNOT GENERATE ENOUGH ENERGY TO HEAT THE ENTIRE MIXTURE OF GASES ( INCLUDING THE INERT GAS ) TO THE EXTENT REQUIRED FOR THE SELF PROPAGATION OF THE FLAME.

LOC OR MOC HAS THE UNITS OF %O₂ IN AIR PLUS FUEL. IF THE EXPERIMENTAL DATA ARE NOT AVAILABLE, THE MOC OR LOC IS ESTIMATED USING THE STOICHIOMETRY OF THE COMBUSTION REACTION AND LFL ( LOWER FLAMMABILITY LIMIT ). THE PROCEDURE WORKS FOR MANY HYDROCARBONS. THE PROCEDURE IS GIVEN BELOW : -

LOC = MOLES O₂/ ( MOLES FUEL + MOLES AIR )

=[ MOLES FUEL/ (MOLES FUEL + MOLES AIR) ] * ( MOLES O₂ / MOLES FUEL )

= LFL * ( MOLES O₂ / MOLES FUEL )

SINCE LFL = MOLES FUEL / ( MOLES FUEL + MOLES AIR )

FROM STOICHIOMETRY, ONE CAN FIND OUT MOLES OF O₂ PER MOLE OF FUEL.

HENCE WE CAN FIND OUT THE MOC OF GAS/VAPOR IF WE KNOW THE LFL OF THE

GAS/VAPOR AND STOICHIOMETRY OF ITS COMBUSTION REACTION.

LFL IS LOWER FLAMMABILITY LIMIT

21/05/2006 18:39:42