Handbook of Solvents - George Wypych - ChemTech - Ventech!

1060 George Wypych
presence of ignition. The leanest mixture at which this will occur is called the lower flammable
limit. The richest mixture at which it will occur is the upper flammable limit. The percent
of vapor mixture between the lower and upper limits is known as the flammable range.
The temperature limits of flammability can be determined in an air/vapor mixture
above a flammable liquid in a closed vessel. 48 The temperature in vessel is varied until a
minimum temperature is reached at which the flame will propagate away from the ignition
source. A glass vessel is equipped with an ignition device, a magnetic stirrer, a clamping devices,
and a safety glass window. The initial temperature of determination is estimated from
closed-cup flash point measurement. If the flash point is below 38 o C, the initial temperature
should be 8 o C below flash point temperature. If the flash point is between 38 and 96 o C, the
starting temperature should be at least 14 o C below the flash point. If the flash point temperature
is above 96 o C then the initial temperature should be 22 to 44 o C below the flash point
temperature. Selecting higher initial temperature may result in explosion. The lower temperature
limit of flammability is obtained from the test which can be used to determine
guidelines for the safe handling of solvents in closed process and storage vessels.
The concentration limits of flammability are determined using another method. 49 The
method is limited to atmospheric pressure and temperature of 150 o C. Equipment is similar
to that used in the previous method. A uniform mixture of vapor and air is ignited and flame
propagation from ignition source is noted. The concentration of flammable components is
varied until a composition is found which is capable to propagate flame.
15.1.17 FLASH POINT
A variety of apparatus such as the small scale closed tester, 50 the tag open cup, 51 the tag
closed tester, 52 the Cleveland open cup, 53 the Pensky-Martens closed cup, 54 and the equilibrium
method 55 are used to determine the flash point. The selection of method is based
suggestions included in separate standard. 56
The small scale closed tester 50 is a metal cup with a thermometer fitted below the bottom
of the internal chamber with a hinge mounted cover having filling orifice. The sample is
introduced to the cup and the cup is maintained at a constant temperature by means of temperature
controller. After a specific time, a test flame is applied for 2.5 s and an observation
is made whether or not flash has occurred. If flash did not occur the cup is cleaned, a new
sample is introduced and the temperature is increased by 5 o C. The measurements are repeated
until the flash point is determined with accuracy of 1 o C.
The tag open cup 51 is a larger unit equipped with water-glycol bath for temperature
control or a solid carbon dioxide-acetone bath for lower flash points. A much larger sample
is used with this equipment and the temperature is gradually increased at a rate of 1 o C/min.
A taper flame is passed for 1sin1minintervals until the flash point is detected. The tag
closed tester 52 can use either a manual or an automated procedure. A sample volume of 50
ml is used. Either a gas flame or an electric ignitor is used. In the automated mode, the
equipment is programmed to perform the standard procedure.
The Cleveland open cup 53 is placed on a heated plate which increases temperature at a
rate of 5-6 o C/min. This method can also be automated. The method is designed for testing
petroleum products which are viscous and have flash point above 79 o C. The
Pensky-Martens 54 closed cup tester was also designed for petroleum products but for those
with flash points from 40 to 360 o C. This apparatus has its own heating source, stirrer, and
cover by which it differs from Cleveland cup. It can be either manual or automated. The
equilibrium method 55 uses either a modified tag close cup or the Pensky-Martens apparatus.
The modification intended to keep the vapor/air temperature in equilibrium with the liquid
temperature. The method is limited to the temperature range from 0 to 110 o C.