Hydraulic System Fires: A Case Study with Investigation Tips

Overview

Several years ago, a serious fire broke out in a food processing plant in North Carolina, when a high pressure hydraulic line broke, allowing its contents to spray out into the plant. The fluid sprayed onto a nearby industrial cooker. The open-flame burners on the cooker instantly ignited the hydraulic oil, causing a hot torch-like fire. The fire melted a natural gas regulator, releasing gas and adding fuel to the already severe fire. Twenty-five people were killed, and the plant was permanently shut down.

Cause

As with many accidents, several factors in the case came together at just the wrong moment, resulting in disaster. First, the open-flame design of the cooker gave the hydraulic oil an ignition source. A guard or shield around the burner could have prevented this disaster.

Second, high-pressure hydraulic lines were located near the open flame cooker. The plant should have been designed so that the potentially hazardous hydraulic lines were as far as possible away from such an obvious ignition source. Better yet, it may have been possible to replace the hydraulics with some other less hazardous device.

Lastly, a fire-resistant hydraulic fluid was not in use. Instead, they used a flammable petroleum based fluid. Petroleum-based hydraulic fluid is usually difficult to ignite. However, when a high-pressure hydraulic line breaks, the fluid atomizes into a fine mist that can spray up to fifty feet. An open flame, a hot surface, or even a spark can instantly ignite the mist, resulting in a sudden, hot, torch-like fire. Fire-resistant hydraulic fluids are widely available, and they have been proven much safer.

Fire-Resistant Hydraulic Oil

The dangers of petroleum based hydraulic fluid have been recognized for many years. Petroleum based hydraulic oil mist ignites readily and releases a great deal of heat. In addition, it can spread a flame away from the source of ignition and onto other flammables. This property is known as propagation. Due to these dangers, many different manufacturers have worked to develop an acceptable, but fire-resistant, substitute. These fluids are not completely fireproof, but they are much safer than regular hydraulic oil. They will burn in the presence of a strong constant ignition source, but they will not propagate a flame away from its source. When the ignition source is removed, fire resistant hydraulic fluids go out.

Unfortunately, fire resistant hydraulic fluids are not perfect. Petroleum based hydraulic fluid is superior in high-speed or high-pressure applications because it lubricates moving parts better than fire resistant substitutes. This results in less wear and tear on machinery and lower repair or replacement costs. Petroleum based fluid is also more tolerant of high temperatures and cheaper.

Converting a hydraulic system from petroleum based fluid to fire-resistant fluid requires some work. Fire-resistant fluids can damage some types of pipe seals and flexible tubing, as well as oil-compatible paint. For this reason, all seals should be replaced during the changeover process. Tanks should be stripped and repainted. Many fire resistant hydraulic fluids are mostly made up of water. They gradually lose their water content through evaporation, especially under high temperature. When the water evaporates, the fluid loses viscosity and its fire resistant properties are diminished. Water must be continually added to these fluids to make up for the evaporation loss.

Sometimes it is necessary to use petroleum based hydraulic fluid in areas at high risk for fires. Here is a safety checklist of fire prevention tips.

1. Is the area equipped with automatic sprinklers? Do they work?
(American National Standards Institute NFPA/JIC T2.24.1-1990 -Hydraulic Fluid Power-Systems Standard for Stationary Industrial Machinery July 1991)

2. Does the system have an automatic shutoff switch or a manual shutoff located somewhere away from the immediate area? Hydraulic oil fires usually happen so fast that there is no time to shut off the pumps before the area must be evacuated. Automatic or remote shutoff switches can rob the fire of its fuel supply.
(Ryan, Kenneth E., "Fire Hazards of Hydraulic Fluids," Professional Safety, June, 1984) pp. 34-36.)

3. Are there fire extinguishers readily available in case of small fires? The extinguishers should be in good working order and suited for class B fires. A small hose with an approved nozzle should also be present.
("Sudden Fiery Gusher," Factory Mutual Record, (September-October 1969) pp 9-11.)

4. Is it possible to avoid threaded pipe connections in the hydraulic system? These connections are the most likely place for a failure to occur. Instead, weld the connections together.
(Factory Mutual Engineering Corp., Loss Prevention Data 7-98 Hydraulic Fluids February 1981.)

5. Are the hydraulics isolated as much as possible from ignition sources such as flames, sparks, hot surfaces, or metal castings?
(Fitch, J. C. and E. C. Fitch, "Safety Considerations in Fluid Power Products -Part 2: Design Aspects," The Basic Fluid Power Research Journal, Volume 12, No. 3, (1979), pp. 233-236.)

6. Is flexible hose armored or enclosed in a second tube to catch escaping fluid?
(How Fire-Resistant Fluids are Used Today," Applied Hydraulics & Pneumatics Volume 12, (November 1959) pp. 67-73)

7. Is there a plan for regular inspection of the whole hydraulic system? The system should be inspected weekly for signs of wear, weakness or abrasion.
(Factory Mutual Engineering Corp., Loss Prevention Data 7-98 Hydraulic Fluids February 1981.)

Reference:

National Fire Protection Association Fire Protection Handbook. Sixteenth edition, 1986 Section 10 chapter 4.