Oct 1999

Hydraulic Fluids

Many hydraulic systems utilize combustible oil under high pressure to transmit power or motion. Hydraulic fluids are used in die casting and plastic molding machines; mining equipment; machine tools, such as drill presses, milling machines, grinders, boring mills, shapers, saws, broaching machines, punch presses, shears and riveters; and in hydraulic couplings, torque converters, elevators, lift trucks, heat-treating furnace-door operations and melting-furnace tilting units.

Pressurized oil in hydraulic systems presents a considerable fire hazard, particularly in processes where ignition sources are constantly present, as in plastics forming, die casting, automatic welding and melting, and heat-treating of metals. Atomization of the fluid greatly increases the ease of ignition. Escaping hydraulic oil has caused many severe fires, particularly where building contents or construction were combustible and sprinkler protection was lacking.

Fire Characteristics

When hydraulic oil is released under pressure, the usual result is an atomized spray or mist of oil droplets that may extend as far as 40 ft. from the break. The flammable oil spray can be ignited readily by hot surfaces, such as heated or molten metal, electric heaters, open flames or welding arcs. The resulting fire usually is torch-like with a very high heat release rate.

Automatic sprinkler discharge can protect the building structure and prevent involvement of other nearby combustibles, but the torch-like fire presents little burning surface for cooling or wetting. The high heat release can continue, opening many sprinklers, until the oil discharge is stopped.

Causes of Oil Release

High-pressure pipe with welded and screwed joints, steel and copper tubing, and metal-reinforced rubber hose are used to distribute oil to the various units at pressures ranging as high as 10,000 psi. Failure of piping, particularly at the threaded sections, failure of valves and gaskets or fittings, pulling out of copper and steel tubing from fittings, and rupture of flexible hose have been the principal causes of oil release from the system. Lack of adequate supports or anchorage, allowing vibration or movement of piping, has been a factor in some of these failures. Repeated flexing and abrasion of rubber hose against other hose or machine parts have created weak spots, which eventually resulted in rupture. Tubing under pressure has released oil when accidentally cut by welding torches or other equipment.


1. Use of hydraulic fluids with low fire hazard potential is encouraged. Such fluids include water-glycol, halogenated-hydrocarbon, phosphate-ester, or water-oil-emulsion types.

These less flammable hydraulic fluids have been developed to replace petroleum-based oils in all types of hydraulic systems. These fluids still exhibit some degree of combustibility (i.e., if sprayed onto very hot surfaces, a flaring fire can occur) but are significantly less hazardous from a fire protection standpoint.

Loss experience indicates that properly maintained systems with less flammable hydraulic fluids greatly reduce the extent of damage in a fire as compared to systems with petroleum-based oils. Incidents involving less hazardous hydraulic fluids have resulted in relatively minor fires causing little damage. In one instance a utility indicated that a 1-in. diameter leak occurred in a control oil system with fluid sprayed onto a 1,000°F surface. A small fire resulted that was easily extinguished with a light water spray. Operators were able to isolate the line with no property damage and a one-hour delay in startup. Plant personnel estimated that if mineral oil were involved under the same conditions, a severe fire would have occurred with no possibility of the operators accessing the area to isolate the leak. This would have resulted in major fire damage and an extended outage.

A. New Installations:

1. Before the purchase of new hydraulic equipment, the manufacturers should be consulted so that equipment suitable for use with less flammable hydraulic fluids can be obtained.

2. Pumps, seals, gaskets, packings and other system components should be suitable for the fluid to be used.

B. Conversions of machines to use less flammable fluids:

1. Equipment and fluid manufacturers should be consulted for proper conversion procedures.

2. Before the changeover to a new fluid, a record should be made of normal operating conditions, including temperatures, pump inlet and outlet pressures, and flow rates, to determine if equipment is operating properly before and after conversion.

3. The old fluid should be completely drained from the system. Do not mix fluids of different types, or of the same type from different suppliers.

4. Seals, gaskets, packing, filters, valves should be changed to help prevent leakage (as recommended by the fluid supplier).

5. Paints that are not compatible with the new fluid should be removed from the reservoir and other internal parts of the system.

6. The system should be filled with the new fluid and the system operated for several hours and then drained to completely remove the old fluid and particulate matter. The system should then be refilled with the new fluid.

7. The newly converted system should be closely checked for its operation to see that the pump, filters, heat exchanger and seals are functioning properly, that pump suction vacuum is not excessive, and that proper fluid viscosity and stability is being maintained.

2. Provide an automatically actuated means for shutting down the oil pump and shutting off flow from accumulators for hydraulic systems with individual reservoirs containing more than 100 gal. of petroleum-based hydraulic oil. Shutdown may be accomplished through the use of an automatic sprinkler waterflow switch, a heat detector rated at least 50°F above the highest anticipated operating environment temperature located directly above the hydraulically operated equipment, or an oil reservoir liquid level switch electrically interlocked with the power supply to the oil pump. The liquid level switch arrangement should only be used where the hydraulic oil system release can be limited to 25 gal. or less.

3. Flexible connectors and hoses should be avoided. Where hose must be used for flexible connections, it should be steel reinforced, designed for the hydraulic fluid being used, and capable of withstanding four times the maximum working pressure. Hose couplings and fittings and minimum bending radii should be in accordance with the hose manufacturer’s instructions. Hose should not rub against other objects as a result of machine movement, vibration, or pressure surges.

4. Automatic sprinkler protection should be provided over and at least 20 ft. beyond the hydraulic equipment. Complete sprinkler protection is needed if combustible construction or occupancy extends beyond this area. Sprinklers also should be provided for shielded areas, such as pits beneath equipment or areas under platforms. Wet systems using high temperature rated sprinklers are preferred. The sprinkler demand should be in accordance with NFPA standards or the authority having jurisdiction.