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Spray ignition testing of hydraulic fluids - options and uncertainties

Phillips, W.D.

Spray ignition testing is widely recognised as an important part of the assessment of the fire-resistance of a hydraulic fluid. It attempts to simulate the behaviour of a fluid escaping under pressure from a line in the vicinity of an ignition source. Various methods have been developed over the years that have used different test conditions in terms of fluid pressures and temperatures, orifice sizes and shapes and ignition sources. It has been seen above that in most specifications the fire resistance of fluids is normally measured by performance in a range of tests. This is because it is very difficult to predict the conditions under which the fluid will be used and hence the hazards to which it will be subjected. However, although spray ignition is a very important hazard for hydraulic fluids, results of the current tests cannot easily be related to service conditions and need a further benchmark or reference point to define the level of performance required. Unlike past spray tests, which normally evaluated just one parameter, e.g persistence of burning, current procedures are able to assess several combustion characteristics, including heat released, smoke and toxic gas production. To date the focus has been on ignitability/heat released as this is the primary concern. Smoke production is a secondary feature and is unlikely to receive serious attention until satisfactory precision of the heat release tests is established. Although both the widely used spray ignition tests are major improvements on past procedures, neither is perfect! The 'Buxton' test requires additional precision studies if it is to be used for international standardisation, and some agreed way of relating the classification to the hazard level would be advantageous. However, it has the advantage of being a multi-site test and appears to correlate well with the ranking of fluids under other test conditions. It is normally used in conjunction with other fire test procedures. By contrast, the FM procedure is a single site test and therefore cannot offer the full precision required for international standardisation. Confidence in the repeatability of the method is also currently lacking in view of the absence of any published data. Since the results depend on three components, viz. heat of combustion, critical heat flux and heat release, and as two of these have no precision data, the interpretation of the results should be handled with care. No attempt has yet been made to study the effect of air temperature and humidity, but as these parameters are more difficult to control in the FM test, they may have a significant effect on results.