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Assessing risk in hydraulic fluid fires

Hydraulic fluid fire
A range of fire resistant hydraulic fluid may help prevent plant fires

The range of hydraulic fluid fires across the Australian mining industry has long been a concern for mining operators and plant owners. Recent machinery fires have resulted from damaged hydraulic hose fittings, inappropriate location of hoses near hot surfaces or hydraulic oil-soaked lagging being ignited by hot components.

Dr Stuart Jagger (a leading fire prevention expert) says ‘hydraulic and related fluids are used in a wide range of industrial situations where prevention of fire is critical. Of particular concern are systems used in tunnelling and mining. A number of incidents have occurred in which mineral oil-based fluids have provided a major fuel input to large fires.’

Dr Jagger (Et al.) used the example, the fire on the Kaprun funicular railway.1 The industry response has been to develop fire-resistant (FR) fluids, usually by either adding water to the oil to create an oil/water emulsion that is used as the hydraulic fluid, or by engineering chemically fire-resistant fluids.

The use of fire suppression equipment, with conventional mineral hydraulic oils, is also an option that has been used and may be available in some industries.

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Dr Jagger believes that fire situations may arise from a variety of causes. For example, fire may result from fluid released as a high-pressure spray or as a static pool. He says that there is no one standard scientific test which can reflect the full range of hazard scenarios.

In his article ‘Assessing Hydraulic Fluid Fire Resistance,’ Dr Jagger (Et Al) presents a range of findings of a comprehensive and specially adapted test program on a range of generic fluid types commonly encountered in the United Kingdom. The objective is to better quantify the fire resistance of specific hydraulic fluids. Further, it explores the use of these test results in a risk assessment-based fluid selection process which may have application for mining industry scenarios.

Application of risk assessment for Hydraulic Fluid Fires

Dr Jagger says that three types of information are required for a generalized risk assessment:

  • The probability that a release may occur and the release characteristics;
  • The likelihood that this will ignite;
  • The consequences of fluid combustion including the resulting temperatures, smoke obscuration and toxic gas concentrations.

He says that the difficulty often encountered is the lack of data for many of the inputs. The results from laboratory tests have the potential to alleviate some of these problems, but it is not always clear how they can be used.

Dr Jagger says ‘The probability and characteristics of a release are not fire issues, but traditionally these have been specified by reference to historical data and use of simple engineering calculations respectively. ‘

The specification of the probability of ignition is the most difficult area.

“In a full risk assessment, the machine and its operational environment might be examined in detail to derive a full inventory of potential ignition sources.”

“This would include the location and temperature of any hot surface, the presence of lagged pipes or the existence of unsealed electrical equipment, for example”

Thus, in normal operation, if surface temperatures were below the hot manifold ignition temperature, then that source may be discounted in all but fault conditions.

Dr Stuart Jagger.

Similarly, if naked flames are not allowed in the machine operating environment, the possibility of open flame ignition might be discounted, or at least reduced to a low level.

As in many risk assessment situations, an expert judgment must be invoked. Thus, a probability of ignition might be assigned by considering both test data and the machine environment together.

Consider for example, hot surface ignition. A scheme might be developed in the following form:

  • For a test hot manifold temperature Te. (If normal temperatures on the machine exceed this, the ignition probability is 1).
  • Down to normal operating temperatures below this, the ignition probability might be assigned from a sliding scale down to zero at some fraction of Te, say 0.5 or 0.75 Te.
  • Some additional allowance would need to be made for the possibility of a fault condition.

The consequences of fluid combustion are more easily incorporated within the framework of a risk assessment. Thus the tests provide explicit measurements of parameters such as heat release rate, smoke or CO production.

The rate of smoke propagation and loss of visibility due to smoke inside a tunnel or machinery space may be computed from a knowledge of the rates of heat release and smoke evolution as illustrated in a full risk assessment on hydraulic systems carried out by Thyer.4

In many, cases a full risk assessment may not be justified. In this situation it may be appropriate to apply risk index methods, such as those described by McBride.3 In these methods, a parameter variously described as a “risk score” or “risk value”, S, is computed according to the equation:

A number of attributes, for example, smoke production, ignition temperature, n, are chosen to characterize each situation based on fire scenarios or loss statistics and weights assigned to them through an assessment of probabilities. The possible accident scenarios may be chosen and wj and rj aligned with the likelihood of occurrence and consequences.

In these schemes, numerical values are introduced for wj and rj using normalized scales. In this case, the likelihood of occurrence might be assessed using a scale from 0 to 3 with 0 being not credible, 1 unlikely, 2 medium probability and 3 highly likely.

Dr Jagger (et al.) paper provides a range of guidance on the assessment of hydraulic fluids for fire propagation in machinery. It also may assist mining industry risk assessment on the application and use of fluids in mitigating effects of fires on plant and equipment.

Miners, particularly underground miners, should examine opportunities of selecting a fire-resistant hydraulic oil over a mineral hydraulic oil. A range of products are now available that can be considered in an overall risk assessment process.

References

  1. http://en.wikipedia.org/wiki/Kaprun_disaster
  2. S. Jagger, A. Nicol and A. Thyer. 2002. “A Comprehensive Approach to the Assessment of Fire-resistant Hydraulic Fluids.” HSL Internal Report, FR/02/05.
  3. M. McBride. 2003. “Review of Fire Risk-Assessment Methodologies and Applications.” Draft HSL Internal Report.
  4. A. Thyer. 1998. “Risk Assessment for the Copenhagen Metro Construction Group BM.” HSL Internal Report FS/98/09.
  5. S. Jagger, A. Nicol, A. Thyer, “Assessing Hydraulic Fluid Fire Resistance” published online in Machinery Lubrication [Accessed 19 July 2019]

Dr. Stuart Jagger

Dr. Jagger works at the Health and Safety Laboratory where he has been Head of the Fire Safety Section for some 16 years. He thus leads a team of some 10 scientists responsible for identifying, quantifying and mitigating the hazards of fire in the workplace. The Health and Safety Laboratory forms the research agency of the Health and Safety Executive which is the United Kingdom body which regulates and inspects for all aspects of health and safety in the workplace.

During his time at HSL Stuart has participated in a number of high profile fire investigations including that at King’s Cross underground Station in 1987, where Stuart was responsible for the first successful use of computational fluid dynamics in a major catastrophic fire investigation and subsequent presentation at a Court of Inquiry. Stuart has also participated and led a number of European wide fire-based research projects on topics such as warehouse and tunnel fires and the assessment of mathematical models used in hazard analysis. Stuart has also acted as consultant to the US NFPA on standards governing the storage of LNG.

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