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Hydraulic Fluids & Equipment –
Fire Hazards


Hydraulic systems are widely used in industry in manufacturing and process machinery and in mobile equipment. They use pressurised liquid to transmit mechanical force, power or motion to the various parts of equipment and machines. Some of the more common mobile applications of fluid power include actuating automotive power steering and power brakes, controlling the activities of earth moving equipment, and multiplying force in materials handling equipment such as forklift trucks. The industrial uses of fluid power are many. It controls material handling equipment, powers machine tools and actuates automatic production line operations. It is used to bore, mill, shear and grind. It powers a wide variety of hydraulic presses that form, straighten, bend, extrude and stretch materials. It can control a delicate mechanical operation, or it can move loads of many tonnes.

Typical applications of hydraulic systems include:

  • Foundries – tilting furnaces, mould and core making machines, furnace doors, etc

  • Die-casting

  • General metal working – hydraulic presses, machine tools, shears, riveters, etc

  • Glass manufacturing – feeder and forming machines, shears, beam furnaces, etc

  • Plastic goods manufacturing – injection and blow moulding processes.

  • Aircraft flight simulators



All hydraulic fluids in common use today are combustible. The fire hazard is as severe for a small free-standing system containing up to 40 litres of fluid as it is for a large central system containing 4000 litres or more of fluid.

Any amount of pressurised fluid escaping through a ruptured hose, gasket or pipe joint may atomise forming fine droplets in air. If the fluid ignites, a serious torch fire will develop. The torch fire could then easily become the ignition source for any combustible materials that are exposed to the flame.

Hydraulic systems can operate at immense pressures e.g. up to 690 bar (10,000 psi). The pressure inside an aerosol can, which by design produces an atomised liquid, is rarely more than 3 bar. Atomised hydraulic fluid can travel up to 12m from the source of the leak. Any hot surface within this range e.g. heated or molten metal, electric heaters, open flames, welding arcs, some high intensity discharge (HID) lamps etc is a potential ignition source that could result in the severe torch fire.

Early hydraulic equipment used water as a power and motion conductor, but water is a poor lubricant and corrodes machinery. Mineral oils soon replaced water. Oils perform well as hydraulic fluids. They lubricate well and actually help preserve machinery. Oils can be custom-blended for different uses, they are inexpensive and are readily available.

Traditional petroleum-based (mineral) hydraulic oils have flash points in the range of 150°C to 320°C and auto-ignition temperatures in the range of 260°C to 400°C.  They are thus classed as combustible liquids. Most fluid power systems are designed to operate in the 45°C to 60°C range, thus the inherent fire hazard may be considered by the layman to be small.

The key to preventing and controlling any fire is to consider the fire triangle and to break at least one side of it. To prevent a fire involving hydraulic fluids either contain the hydraulic fluid within the various pipes or systems, or eliminate ignition sources in the vicinity. Both are difficult to do. A further alternative is to use ‘less flammable’ hydraulic fluids in place of mineral oil. Assuming that prevention measures fail then a hydraulic oil fire can be controlled by shutting off the flow of hydraulic oil to the fire – and then by extinguishing any residual fires.

The major considerations in hydraulic fluid system fire prevention and control are:

  • Hydraulic fluid type

  • Piping system design, operation and maintenance

  • System interlock design

  • Housekeeping

  • Fire protection

General Precautions:

Whenever possible, locate hydraulic systems and equipment in cut-off areas. Either drain those areas to a safe location or provide curbs/bunds around the equipment to retain spilled fluid.

Provide at least one manual emergency shutdown device for each system that can be actuated from a clearly marked, accessible location that is remote from all hydraulic systems. The shut-down device must be accessible at all times and not be located in or exposed by the hazard as the atomised hydraulic fluid (and the resultant flame) can travel 12m or so from the source of the leak.

Consider the following when designing hydraulic systems:

  • Avoid combustible piping or hoses.

  • Use moveable pipe connections instead of hose whenever possible. If hose is used it should be a metallic armoured type or it should be enclosed in a second, larger, flexible metallic hose.

  • Design all piping and connections to resist damage from vibration and mechanical injury.

  • Provide piping, hose and connections that are suitable for the pressures involved and the type of hydraulic fluid that is used.

  • Arrange system controls to shut down the system when abnormally low or high pressure, low reservoir level or excess flow occurs.

NOTE: many hydraulic systems are equipped with low reservoir level interlocks. These are designed to prevent the pump from being damaged by running dry and are set at 10% to 20% of the tank level. Thus 80% to 90% of the contents of the fluid tank would have to be lost before this interlock operates to shut down the system. It is very good practice and often a quick and easy fix to install a second level sensor and interlock at 80% to 90% full. In that way a relatively small quantity of fluid will be lost to a leak before the system shuts down automatically.

  • Keep all hydraulically operated equipment and surrounding areas clean and free of fluid residue and combustible materials.

  • Maintain and inspect all hydraulic systems regularly.

  • Provide suitable portable fire extinguishers for the hydraulically operated equipment and surrounding building areas.

  • Do not cut or weld in any area where hydraulic fluids are used until the area is free of all oil deposits and the system is shut down and de-pressurised.

  • Locate potential sources of ignition, such as electrical switchgear, electric motors, hot surfaces and open flames away from hydraulic equipment – or locate the hydraulic equipment away from the potential ignition sources. Shield any equipment that cannot be located remotely.

  • For large central systems, and whenever practical, use detachment or shielding to protect electrical equipment from possible fires.

  • Emergency plans should recognise the hazards of the hydraulically operated equipment and should include provisions for coping with a hydraulic fluid fire.



In response to the fire hazards associated with petroleum based fluids, less flammable hydraulic fluids (often referred to as ‘fire resistant’) have been developed. Based on the definition of flammable liquids (flash point) very few hydraulic fluids, including petroleum based fluids, are flammable. However, the less flammable hydraulic fluids are NOT fire resistive. All will burn under certain conditions; however, if less flammable fluids are properly maintained, they are much more difficult to ignite.

There are four basic types of less flammable hydraulic fluids in general use. They are:

  • High Water Base Fluids (HF-A). These contain 90% or more water. Additives to the water are general synthetics or soluble oils. These fluids have viscosities approaching that of water and are used mainly in light duty applications – pressures below 7 bar – using special pumps. Recommended temperature limits are 4°C to 50°C. High water base fluids are compatible with most types of seals and gaskets, except those made from cork, paper, leather and synthetic fibres.

  • Water-in-Oil Emulsions (HF-B). Consist of 35% to 50% water in mineral oil, with a small amount of emulsifying agents, rust inhibitors and anti-wear additives. The water is dispersed in fine droplets in the continuous oil phase. Recommended temperature limits are -10°C to 65°C. At temperatures above 65°C a periodic addition of water is normally required. Either excessive heating or cooling can upset emulsion stability. Loss of water from emulsions tends to reduce viscosity and increase flammability. Water-in-oil emulsions are compatible with most types of seals and gaskets, except those made from cork, paper, leather and synthetic fibres.

  • Water-Glycol Fluids (HF-C) normally consist of 35% to 50% water for fire resistance, ethylene or propylene glycol to improve low temperature properties and additives for proper viscosity and resistance to corrosion, wear and bacteria. Recommended temperature limits are -18°C to 65°C with normal operation at 50°C to 65°C. At temperatures above this range, the rate of water evaporation is such that additional makeup water is frequently required. As water evaporates, the viscosity and flammability of the fluid increases. Operation at hazardously low water concentration is difficult. Water-glycol fluids are compatible with most types of seals and gaskets used with mineral oils, but are incompatible with certain types of cork, paper, leather and synthetic fibre materials.

  • Synthetic Fluids (HF-D). Most are one of four types: phosphate esters, chlorinated hydrocarbons, blends of phosphate esters and chlorinated hydrocarbons and fluids containing other compositions. Most of these have relatively high specific gravities (heavier than water). Because of the high density, particles do not settle out as easily, making good filtration necessary. Recommended temperature limits are -7°C to 93°C. Synthetic fluids are not compatible with natural rubber or neoprene seals or hoses. These should be replaced with fluorocarbon, silicone, butyl rubber, Teflon or nylon materials. Synthetic fluids may also attack metal protective paints, lacquers and electrical wiring insulation.

Ideally new hydraulic systems will be specified to use less flammable hydraulic fluids – though this may not always be possible. Conversion of existing plant to use less flammable fluids may also be a valid recommendation. However, this may not be an easy fix. Under no circumstances should this be considered without prior and detailed consultation with the manufacturers of the plant and the fluid supplier. In many cases this will not be achievable without significant modifications to the hydraulic fluid system, the cost of which may be prohibitive. Changes to seals, gaskets, packings, electrical wiring, painted surfaces and other equipment may be needed. In addition, it should be determined how the new fluid will affect system pump, valve and actuator performance.

12 months after retrofitting a system with a less flammable fluid that is likely to be more corrosive than the existing fluid, all vessels should be inspected internally to determine the corrosion rate, if any and the time interval to the next inspection or the remaining life of the vessel. In general fluids containing water are likely to be more corrosive than those which do not. Vessels that cannot be inspected internally could be surveyed using ultrasonic techniques. However, these will not be effective unless baseline readings are taken before the retrofit is completed.


Hydraulic system vessels and piping should be designed to contain the system pressure under all possible operating conditions.

Systems should be protected against over pressure with at least two independent methods. One should be a relief valve or rupture disk located where each pump discharges.

Where possible avoid the use of threaded connections unless they are assembled and prepared according to written procedures that will assure sound and leak-free joints.

Brace/support piping and other components to avoid stressing joints, especially if tubing or threaded pipe is involved. Protect piping from impact damage caused by vehicles, machine parts or other external loads.

Thoroughly train equipment operators in normal procedures; emphasise procedures needed to control machine pressure, stress and vibration. Regularly drill operators in emergency operating procedures, particularly rapid shutdown and depressurising, spill control and perhaps fire control. Reinforce rules for housekeeping and leak detection and control.

Make periodic inspections to prevent leaks. Operators may perform daily and shift inspections including:

  • Inspect all joints daily before start-up or at least once during each shift. Look for cracks in tubing or threaded pipe connections. Inspect hoses for cracks, abrasions or other signs of damage.

  • Inspect and test relief valves and other over-pressure devices and inspect and maintain pumps, valves and seals in accordance with the manufacturer’s recommendations. Look for leaks daily and repair them promptly.

  • Maintenance staff should also perform routine inspections for leaks every week and more frequently if adverse conditions such as chaffed hoses or leaking joints are found.

  • Ensure that worn or damaged hose and piping is replaced with material suitable for the service. Pressure and temperature ratings and fluid compatibility are important.

  • At facilities where there are several hydraulic systems try to standardise fittings that are used. Many different ‘standard’ connectors, O-rings and other fittings are available. Some cannot be identified visually, even when placed next to fittings made to a different standard. Mixed fittings can often be assembled, but most ‘hybrid’ connections will leak.

  • Do not reuse O rings or other seals.



  • Water-glycol, water-oil emulsions and high water base fluids should be checked for viscosity, water content and acidity at three to six monthly intervals – more frequently if operated at high temperatures. Distilled or treated water should be added when necessary as well as other additives, in accordance with manufacturer’s instructions.

  • Synthetic fluids or blends should be checked for viscosity, acidity, particulate matter and oil or water content at least annually. When necessary the fluid should be filtered or reclaimed in accordance with manufacturer’s instructions.



As a minimum plant controls should include the provision of emergency manual shut-off switches to shut down the hydraulic oil pumps and flow from accumulators on all equipment using petroleum based and synthetic fluids in the event of a leak or fire condition. These should be located remotely in a suitable and readily accessible position that is not exposed by the leak or fire and should be suitably marked. It should be ensured that all operatives are correctly instructed and trained in emergency procedures. Shut off switches may be dedicated to individual machines or to groups of machines that are located in the same fire area.

Recommendations for an automatic means of shutdown should normally apply to systems using petroleum based hydraulic fluids with individual reservoirs of a capacity of 500 litres or above. Some insurance companies may require automatic shutdowns for individual systems of smaller capacities and for smaller capacity systems that are grouped together in the same fire area. Some insurance companies base the requirement for automatic shutdowns on the magnitude of the loss estimate assuming no shutdown and for unattended systems if the improvements are cost justified.

The aim of these interlocks is to minimise fluid supply to a rupture. This could either prevent a fire from starting or starve a hydraulic oil fire of fuel. They may be applied to most systems:

  • Oil pump interlock to shut down the pump in the event of abnormally low system pressure (a leak will result in a drop in pressure).

  • Oil pump interlock to shut down the fluid system in the event of excess flow (the flowrate in the pipework will increase when the pump tries to make up the fluid that is being lost to a leak).

  • Both of these interlocks may require time delays to prevent nuisance trips when the system starts or during initial system fill or during loading cycles when there is little resistance to machine movement. Such interlocks may be designed to respond to different flows and pressures at various points in a large system.

  • An oil pump interlock with an oil reservoir low level switch.

As stated earlier a low level interlock is often installed as standard to prevent the pump and hydraulic powered equipment from being damaged by running dry. These low level interlocks usually set at 10% to 20% of the tank level, thus 80% to 90% of the contents of the fluid tank would have to be lost before this interlock operates to shut down the system. It is very good practice and often a quick and easy fix to install a second level sensor and interlock at 80% to 90% full. In that way a relatively small quantity of fluid will be lost to a leak before the system shuts down automatically.

Other system shutdown interlocks may be useful to limit fluid supply to a fire. These include one or more of:

  • An interlock with sprinkler system water flow through the use of a water flow switch.

  • In interlock with a fire detection system covering the machines. Smoke detection may be too sensitive giving unwanted alarms. Excess temperature or rate-of-rise temperature detection above the equipment may be more appropriate.

  • A fusible link shut-off valve on the hydraulic oil lines, prior to any flexible connections.



Maintaining effective standards of housekeeping around hydraulic oil systems is paramount and should be subject to frequent formal inspections. Even with excellent maintenance, hydraulic fluid leaks are difficult to avoid. The following recommendations apply: -

  • Provide drip trays under seals and joints where leakage cannot be eliminated. Use curbs or pits or other suitable arrangement to contain spills in areas where spills are likely.

  • Fully depressurise, isolate, lock out and, if possible, drain any portion of the system which is to be opened (e.g. for maintenance).

  • Promptly remove and replace any thermal or electrical insulation that becomes soaked with fluid.

  • Clean up all spills promptly and ensure all resulting fluid residue is removed. Leakage from high pressure lines may atomise and spread far from the leak. The use of inert oil absorbent granules is preferable to the use of rags, sawdust etc.

  • Minimise storage or use of combustibles such as cardboard boxes, pallets etc near hydraulic systems. Never allow rags or other combustible waste materials to accumulate.

  • Welding, cutting and other hot work procedures in the vicinity of hydraulic oils/equipment etc should be avoided if at all possible, by design and should not be carried out in the vicinity of pressurised (operating) hydraulic equipment.  If hot work is unavoidable then it should the subject of a rigorous and properly enforced hot work permit system that applies to own personnel and contractors. All oil deposits should be cleaned up and the hydraulic system depressurised before the commencement of work is authorised. An absolute minimum 60 minute post-work continuous fire watch period should be maintained before the hot work task is considered complete. The hydraulic equipment should not be re-energised before the hot work task has been completed.


Adequate portable fire extinguishers of suitable sizes and types should be provided and located where they will be accessible in an emergency situation. Ideally plant personnel will be trained to use them. The fire extinguishers will not be capable of tackling a jet fire involving atomised oil, but they may be of use in extinguishing small residual fires involving combustible materials that have been ignited by the initial jet fire.

Automatic sprinkler protection or other forms of active fire protection may be required. This is often dictated by cost benefit and practicality. Sprinklers could be installed throughout the facility or spot protection could be provided over and for at least 15m in all directions from the hydraulic equipment and hydraulic fluid piping.

Enclosing equipment in fire resistant (minimum 1 hour fire rating) rooms and compartments may be required in major facilities or where loss estimates from a hydraulic oil fire are very high. Providing active fire protection inside small enclosures (e.g. water-mist, carbon dioxide, foam flooding etc.) may be a more attractive proposition than installing sprinklers in a large area. Clean-agent gaseous extinguishing systems (halocarbons e.g. FM200 or inert gases e.g. Inergen) are not suitable for this application.



Provide the following comments in reports when describing important hydraulic equipment:

  • Type of hydraulic fluid used. State whether it is a traditional petroleum based fluid or a ‘less flammable’ alternative. Very often this information will only be available by viewing the material safety data sheet (MSDS), which all sites should have. Sites that have several hydraulic systems may use a number of different hydraulic fluids. A good question to ask is whether or not the fluid is ‘as installed’ or has been retrofitted.

  • System capacity and operating pressure.

  • Arrangement and location of remote accessible manual shut-offs.

  • Any methods and arrangements of automatic shutdowns. How and when are they tested.

  • Operator training.

  • Housekeeping.

  • Condition of equipment and hoses.

  • Combustibles in and around the area that could be exposed by a hydraulic oil fire (i.e. that could contribute to a fire spreading over a wider area or resulting in significant business interruption).


FURTHER REFERENCE - this addresses use of hydraulic fluids in mines, but the same principles apply above ground. - this is a research paper looking at ignition of hydraulic fluids (petroleum based and ‘less flammable’ fluids) by spark/flame and hot surfaces.

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