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Flammability Tests on Hot Surface for Several Hydraulic Fluids
2018/12/20
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European directives [2-4, 7] and other documents[1, 5, 6, 8] emphasis the necessity of reducing the flammability risk when using industrial fluids(hydraulic fluids, lubricants, processing fluids like those used in steel treatment and cutting etc.), especially in explosive atmosphere. Thus, “in particular, where fluids are used, machinery must be designed and constructed for use without risks due to filling, use, recovery or draining.” [7] Risk assessment implies a complex analysis of design, equipment, procedures and operators. Thus, the same document [7] underlines that “machinery must be designed and constructed to avoid all risk of fire or overheating posed by the machinery itself or by gases, liquids, dust, vapours or other substances produced or used by the machinery.”
Both manufacturers and users ask for tests that could certify fluid flammability characteristics, preferring ISO or ASTM standards [1, 10, 20, 21]. Many documents, including EU Directives, give recommendations to use standardised tests for estimating flammability of fluids [1-4, 6-8, 14-16]. The evaluation of fire resistance of a hydraulic fluid cannot be done by one test only and the aspects of fire resistance have to be pointed out by several tests, including those simulating on small scale the worst scenario that could happen in real applications using hydraulic fluids. Many of these tests give a result as “pass” or “not pass” [8, 20, 21]. The fluid that passed a particular test or, better, a set of tests, is included in recommendations or approvals, but these ones are specific to regional or national reglementations.
TESTING PROCEDURE
The tests were done with the help of an original equipment (Fig. 1) [27] allowing a dedicated soft assistance in order to protect the operator and to sustain reproducibility, according to the standard SR EN ISO 20823:2004 Petroleum and related products. Determination of the flammability characteristics of fluids in contact with hot surfaces- Manifold ignition test. This test simulates an accident or the hazardous event when a fluid drops on a hot surfaces: 10 ml of fluid is dropped during 40…60 seconds on a manifold kept heated at a constant temperature, from a distance of 300±5 mm above the manifold surface. For each temperature and fluid there were done 3 tests. The highest temperature, for which the fluid does not burn or ignite, was established is the same “verdict” was obtained for all the three tests. All the temperature values given in this study have the accuracy given in Figure 2. The equipment is controlled and assisted by a PC with a dedicated soft in order to protect the operator from being near the heated zone. Figure 2 presents the display of the soft.
There were tested the following grades of hydraulic oils HLP 68 X-Oil [25], HFC Prista [23], Shell Irus Fluid DR 46 [22], a rapeseed oil (obtained after a dewaxing process) [9] and an emulsion 5% MHE 46 in water as recommended by Prista producer [24].There were identified distinct behaviours of these fluids under the test conditions.
Shell Irus Fluid DR 46 is a tri-aryl phosphate ester fire resistant hydraulic fluid. It contains carefully selected additives to give superior oxidation and hydrolytic stability. Shell Irus Fluid DR 46 should be used in hydraulic systems operating in close proximity to potential ignition sources. This includes equipment such as die-casting machines, billet loaders, electric arc furnaces, forging presses and others operating in fire hazard situations.
PRISTA HFC is a fully synthetic fire resistant waterglycol based hydraulic fluid blended with an additive package to improve the anti-wear properties and corrosion protection of the finished product [23].
HLP 68 X-Oil [25] is an optimized alloyed hydraulic oil with a high performance level and a broad field of industrial application. It especially distinguishes with good viscosity-temperature behaviour, high ageing stability and reliable corrosion protection.
Additives provide an excellent wear protection under extreme loads, too. The behaviour against sealing materials is neutral.
Prista MHE-40 is used as 5% working fluid in oil-inwater emulsion for hydraulic systems with high risk of flammability [24]. The tests were done on the fully mineral oil and for the emulsion 5% MHE 40(vol.) in water.
RESULTS AND DISCUSSION
Analysing the recorded films of the tests (Figs. 3-10),the authors noticed the followings: there were stages when the fluid only evaporates or change structure without ignition, these being useful in establishing the time response of fire/security sensors.
The hydraulic fluids HFC Prista, Shell Irus Fluid DR 46 and the emulsion 5%MHE Prista in water does not burn even for the highest tested temperature (700°C±5°C), a temperature also included as imposed for hydraulic fluids with the best behaviour under the conditions of EN ISO 20823:2003. The other two tested fluids burn. The rapeseed oil has 551°C the highest temperature at which it does not burn for repeated test (at least three) and HLP 68 X-Oil has for the same parameter the value of 500°C.
The behaviour of this rapeseed oil (dewaxed grade) under the testing conditions imposed by SR EN ISO 20823:2003 could be grouped in the following ranges, characterised by temperatures for which the fluid behaviour is the same :
1. a temperature range for which there are repeatedly obtained the same results when testing the fluid on hot manifold (200…551?C, the fluid does not burn);
2. a temperature range for which the test results is randomly different (in one test the fluid does not burns, but in the following one it is burning and so on): 551…557?C; in practice it could be included in the range for which the fluid burns and the use of the fluid in this range is strongly not recommended;
3. The temperature range for which the fluid burns, θ > 560?C.
Any test is irrelevant for the temperature range 552…562?C for the dewaxed rapeseed oil because the difference (10°C) is the same to the allowance range (±5°C).
The flammability risk could be substantially reduced by using emulsions as that one obtained from 5% vol. MHE 40 in water. The authors noticed that this emulsion does not burn on the surface heated at 700°C, but the mineral oil – 100% MHE 40 Prista does burn at a much lower temperature of 450°C and it is very sensitive to the surface quality. The authors also noticed that the test done at this temperature of 450°C gives inconsistent results. From 9 tests, during 6 ones, the fluid ignited and burnt when it was dropped on the clean surface of the heated manifold.
When the test was done on the same manifold, but dirty from previous tests, the temperature of ignition of the same fluid was even lower: 415°C. This is a conclusion that could be the subject of a further investigation, as in practice many surfaces could be far for being clean due to the technological process or, worse, due to the “leak” of operators’ responsibility or an inadequate maintenance.
The designer has two possibilities for reducing the flammability risk: to use a fluid that does not burn or to change the design in order to have a better protection against hazardous events that could cause fluid ignition. Of course, the first solution is better especially when the equipment works in a particular environment, including mining, metallurgy, glass industry etc. The designer has to select the hydraulic fluid from families like the synthetic, mineral or that of emulsions. The synthetic ones have some advantages, but they are still expensive. The engineer has to balance the advantages and disadvantages of each group. For instance, the emulsions could be much less expensive, but they have to be circulated in systems (piping, pumps etc.) exhibiting a good corrosion resistance or, at least, an acceptable resistance for a particular application.
CONCLUSIONS
There is no test ensuring a high level of safety for fire resistance but a particular set of tests, selected after a well-documented risk assessment could give a better solution.
The determination of fluid flammability on hot surfaces imposes particular solutions for improving the security of the designed system, including fluid selection, avoiding scenarios with hot surfaces near piping and hoses etc.
The list of hydraulic fluids possible to be selected and the tests that these fluids have to pass, will have to be known and set even in the design stage of the equipment. It is also important to analyse similar accidents related to the real applications in order to notice possible improvements in equipment,process and environment control and for workers’training.
These analyses may be useful for designers in order to better assess the risk and to estimate costs of different solutions implying different grades of hydraulic fluids.
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