Flashpoint measurement

For this reason a thorough analysis of the level of error of flashpoint measurements is required. It is interesting to note that this situation has been criticised by a lot of authors quoted in the following pages and that ail of them have recommended a world-wide experimental approach. Moreover, they all suggested the need for an estimation method while waiting for a standardisation in tests. This standardisation has not happened yet and will be less likely to since there is no apparatus that is better than the rest and it would imply ignoring the manufacturers commercial interests. 

Flashpoints (the reason for the plurality is explained in paragraph 1.3) have the advantage of being linked to the boiling point, the pressure and the lower explosive limit. This is the reason why flashpoints are such important parameters in the evaluation of the inflammability risk of a liquid or a solid. The measurement of flashpoints implies the existence of an ignition flame for the gaseous mixture. Nevertheless, contact of a suitable substance-air mixture with a hot surface can be sufficient to start the combustion of the mixture. The autoignition temperature is the parameter that determines the possibility that an inflammable material will combust in contact with a hot substance without the presence of a flame. 

This paragraph is closely linked to the study of flashpoints. In order to be able to get an overall view on uses of lower explosive limits, one will need to refer to this paragraph and the next. To the user, LEL Is characterised by the high level of experimental error that goes with its measurement. But, for a purely mathematical reason, this level of error will not affect the use of this parameter greatly. It is thus undeniably useful and will be helpful in the difficult task of measuring flashpoints. 

Flashpoint is the temperature at which an inflammable liquid builds enough vapour so that this, together with air, forms an inflammable mixture in the presence of an igniting flame.The inflammation has to be very brief when this parameter is measured. If the combustion lasts for longer than five seconds, this temperature is defined as fire point. Fire point is never used because it is really difficult to obtain an accurate value. Flashpoint is the most important parameter in fire hazard. It plays an essential role in the determination of risk criteria related to the inflammability of a substance. 

The situation just described creates real confusion in the experimental data of flashpoints and seriously complicates their use in the evaluation of the risk level of a given substance. Nevertheless, how measured flashpoints depend on the type of apparatus is known. 

It is certainly this lack of clarity which leads to the current situation. Flashpoints are usually given without any mention of either the open or closed cup aspects or the make of apparatus. Amongst the thousand or so organic substances listed in Part Two, more than one hundred of them mention oc and cc , which enables comparisons to be made. Nevertheless, a study of the data indicates that the difference between experimental values can reach 56°C for the same substance (for instance, butadiene). It happens quite often that for flashpoints lower values oc than cc values for the same substance are found.The nature of the sources of the level of measurement error in flashpoints can easily be guessed at. 

The high value of the standard deviation shows how the situation changes from one substance to the next. The experiment with thousands of measurements accumulated by the author s students shows that when the Cleveland s apparatus is used for flashpoint oc and Setaflash for cc , the difference between flashpoints is between 7 and 15°C, depending on the substance. When there is such a difference between two experimental values of a flashpoint, it can then, without too much risk of error, be put down to the cup effect. 

This effect has not been found in the source measurements carried out by the same or different operators in the same or different laboratories, with the same make of apparatus. None of the levels of error of measurement ever relates to an experimental value of flashpoint. This is incredible since over the past few years techniques of statistical control of testing and experimental planning have been developed which are now compulsory for some activities under the ISO 9000 quality standard. 

In order to estimate part of this level of error, data from the author s laboratory was used where the student calculated flashpoints of varied substances using the Cleveland apparatus oc and Setaflash cc that work below 70-80°C. Every year about thirty groups of two students analyse flashpoints of about ten substances. Measurements are thus repeated many times by each group, which enables estimations of standard deviations within a group intragroup standard deviation) to be made and also between groups (intergroup standard deviation). 

This long assessment of the analysis of the level of error of measurement that goes with flashpoint will be completed later (see para 1.3.7) by considering the effect of impurities that can be found in substances at their flashpoint. Nevertheless, it is sufficient to prove that it is not possible to have any confidence in the data of flash-points that can be found in the technical literature, especially when the safety expert has unique data only. To the author s knowledge, there were not until now... 

One can take as an example the flashpoint method used in the author s laboratory. The measured flashpoints were the subject of repetitions, which gave an accurate confidence interval which was relatively narrow. Therefore these figures can be used with confidence. The following is obtained. 

The author gives an exampie of a study concerning a mixture of ethanol, toluene and ethyl acetate. The case is presented in the form of a Scheffe plan for which choice of compound quantities are not optimised to obtain a good matrix as shown in the matrix of effects correiation there is no point repetition in the middle of the matrix, which thus exciudes the quantification of the level of error of measurement that can only be estimated by the residual standard deviation of the regression. Finaliy, the author uses flashpoints of pure substances from partial experimental data. The available data give 9 to IS C for ethanol (the author 12.8), 2 to 9°C for toluene (5.56) and -4 to -2°C for ethyl acetate. 

This last inflammability parameter presents problems. After stating its definition it will be seen that measuring autoignition temperature proves to be a difficult exercise because its measurement is sensitive to the experimental conditions, even more sensitive than for flashpoints. Worse, this parameter seems to be controlled by kinetic factors far more complex to master than the thermodynamic factors that probably control flashpoints (in fact it is a liquid/vapour equilibrium). So whilst the influence of the nature of the cup metal on a flashpoint has never been demonstrated, this demonstration was easily made with autoignition temperatures. 

The liquid temperature (Tl) corresponding to Xl is measured for practical purposes in two apparatuses known as either the closed or open cup flashpoint test, e.g. ASTM D56 and D1310. These are illustrated in Figure 6.3. The surface concentration (Xs) will be shown to be a unique function of temperature for a pure liquid fuel. This temperature is known as the saturation temperature, denoting the state of thermodynamic equilibrium... 

The well-known flashpoint is a measure of the flammability of liquids. 

Explosion of a reactor consequent upon taking a flash point (which refers to air) as indicating safety limits in pure oxygen is reported [22]. However, measurement of flashpoints with an enclosed apparatus generally showed little change in flash point with increasing oxygen concentration [23],... 

The flashpoint is a measure of the ease of ignition of a liquid. It is the lowest temperature at which the material will ignite from an open flame. The flashpoint is a function of the vapor pressure and the flammability limits of the material. It is measured in standard apparatus, following standard procedures (ASTM D92 and ASTM D93). Both open- and closed-cup apparatus is used. Closed-cup flashpoints are lower than open cup, and the type of apparatus used should be stated clearly when reporting measurements. Flashpoints are given in Sax s Handbook of Hazardous Materials (Lewis, 2004). The flashpoints of many volatile materials are below normal ambient temperature, for example, ether —45°C, gasoline —43°C (open cup). 

Usually, flashpoint is measured in air. Open cup methods may overestimate the flashpoint for liquids containing multiple components because of the loss of more volatile components during testing. Nevertheless, open cup testers can provide flashpoint for situations of open vessels and spills. In contrasts, closed cup techniques prevent the loss of volatile components by keeping the sample enclosed until the ignition source is introduced, and therefore, closed cup data are more conservative than and generally preferred to open cup data. 

For LTL tests, ASTM El232 recommends that a 5-L glass flask equipped with a magnetic stirrer bar be used, which should be placed in a thermally insulated chamber. An electrical arc or fuse wire is used as the ignition source. The equilibrium closed bomb test method has been successfully used to measure flashpoint in air and in oxygen under atmospheric, vacuum, and high pressures. 

The flashpoint of a flammable liquid is the lowest temperature of the liquid at which its vapour forms an ignitable mixture with air. It gives a measure of the risk of formation of explosive or ignitable mixtures when the liquid escapes from its packing. A flammable liquid cannot be ignited so long as its temperature remains below the flashpoint. IMO Gen. Intro. 6.1... 

Operating at temperatures below the flashpoint of a liquid will avoid the formation of a flammable atmosphere as the vapour concentration will be below the lower flammable limit. It is also possible to work at temperatures where vapour concentrations exceed the upper flammable limit, but in this case it is necessary to pass through the flammable region when the plant is started up and shut down. Additional measures such as inert gas blanketing may be neees-sary at these times. When safety is based on temperature control a margin of at least 5°C should be allowed between the maximum operating temperature and the flashpoint of the liquid. 

Reduction may also be achieved by the reduction of exposure time to a hazard, such as would be the case when managing the exposure time of persons involved in the transportation, storage, use and disposal of chemicals such as solvents. The Workplace Exposure Limit (WEL) assigned to acetone, for example, is 500 ppm in an 8-hour period, thus reducing exposure to the prescribed limit ensures that persons are kept free from ill health arising from the hazardous chemical. Acetone is, however, a chemical with a low flashpoint so even a small amount could present a fire and explosion risk and thus reducing the volume is also a fire control measure. 

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