Equilibrium test flames

For rapid determination of closed cup flash points between 5 and 105 °C, rapid equilibrium method, ISO 3679-1983, ASTM D 3941-1980, ASTM D 3828-1981), the Setaflash tester is recommended (Fig. 3.15). An aluminium block is heated to 3 °C below the expected flash point then 2 cm of the sample is injected into the sample hollow by a syringe. After 60 s, ignition is attempted by a 3.5-mm long test flame with a momentary opening of the sliding cover on the sample hollow. With repeated trials, the flash point is bracketed at first within 5 then within 1 C, using fresh sample portions in each test. For calculation of the closed cup flash point, the above barometric correction is again used. 

Figure 6. A test of the equilibrium of Hs + OH — HsO + H in six fuel-rich Ht-On-Ng flames. Experimental points are based on OH fluorescence data and Na-Li data for H-atom at 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 msec, with temperatures increasing slightly with time in each flame.

We would expect the radical balance reactions (6) through (16) to be equilibrated at all points in these flames. Tests of the equilibration have been made, as for example in Figure 6, by evaluating the equilibrium concentration ratios using experimentally measured concentration values. Since O, H, S, and H2S concentrations were not measured directly we can indirectly evaluate the equilibration of the radical balance process by using reactions that are sums of the above listed processes. Four such reactions are listed below with an indication of a combination of reactions (6) through (16) that is chemically equivalent. 

Factors affected by the rate of pressure increase in a given space and for a given amount of energy include the pressure reached at equilibrium per unit amount of substance, and the reaction rate (or deflagration rate). The improved time/pressure test method is considered to be the test method that satisfies both constraints. In the case of having a clear flame front like KCIO 3 /cellulose mixtures, the deflagration rate can be obtained by the method discussed above. 

These tests cover the methods of determination of the minimum temperature at which vapors in equilibrium with liquid solvent are sufficiently concentrated to form flammable mixtures with air at atmospheric pressure and concentration limits of chemicals. Flammable (explosive) limits are the percent levels, volume by volume, of a flammable vapor or gas mixed in air between which the propagation of a flame or an explosion will occur upon the... 

Depending on the viscosity of liquid and its expected flash point range, one of the above methods is chosen as described in detail elsewhere. It should be additionally noted that if the flash point method uses continuous heating, it is not suitable for testing mixtures of flammable substances because their vapor concentrations are not representative of equilibrium eonditions. One of the weaknesses of flash point analysis is that the flame is well above the liquid surface therefore full vapor concentration is not attained. Many cases exist where a flash point cannot be detected but the material does form flammable mixtures. Before a method is chosen and a data interpretation made full information on the test procedure should be studied in detail and the proper authorities should be eonsulted to define safe practices for a particular material. 

A test specimen of size 6 x 18 in. is employed. A specimen thickness of lin. or less is desirable. Prior to the test, the specimens must be dried for 24hr at MO F and then conditioned to equilibrium at 73°F and 50 percent relative humidity. The specimen is mounted in the specimen holder so that it is inclined at 30° to a 12 x 18 in. vertical radiant panel maintained at 1238°F. A small pilot flame is positioned at the top of the specimen to ignite the developing volatile gases. Once started, the flame front progresses downward. The time for the flame to reach each of the 3-in. marks on the specimen holder is recorded. Observations such as dripping of the burning specimen are also recorded. 

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