How to prevent hydraulic oil fires
Piping system at risk of bursting
A piping system normally consists of steel pipes, high-pressure hoses and non-pressurised return pipes and hoses. Piping systems for hydraulic oil are exposed to the risk of bursting for various reasons. Consider, for example, the following real-life examples:
- The dust extraction hose of a grinding machine was lying on hydraulic hoses. Sparks ignited the dust extraction hose from the inside. The hose burned through and set fire to the high-pressure hoses. Subsequently, the hoses burst and the spraying oil set fire to the entire hall. The hall and its contents were destroyed in the fire.
- The last slab was not quite straight when it was transferred onto a roller table. The slab struck the roller table, which was on hoisting cylinders. The resulting pressure shock burst a rusty steel pipe. A pin hole leakage from the steel pipe caused oil to jet onto a steel plate, before being deflected onto a hot slab. This lit a blowpipe flame which reached the wall and then the ceiling of the hall, before advancing to another wall and further downwards. The narrow flame set fire to cable trays on both sides of the hall.
- Due to vibration, a steel pipe wore through and broke. The resulting oil spray hit the chimney of a waking beam furnace and caught fire. Adjacent pump cables burned, breaking in two. However, oil continued to flow because the large cylinders were in an upright position. The damage remained relatively minor, since the majority of heat escaped through a staircase out of the furnace pit.
Pumps should be stopped only when safe to do so
- A hot band machine had a ½” high-pressure hose about 20 cm long. A pressed fitting detached, causing oil to discharge onto a hot coil and ignite. The hydraulic system of the hot band machine was shared with the main rolling line, with no separate emergency circuit. The pumps could be stopped only when to do so was safe for the rolling line. For that reason, the emergency stop was not pushed before the strip on the rolling line was fully coiled.
- Due to a malfunction, a hot billet was lifted away from a hot extrusion press, after which it fell onto high-pressure hoses. The flames blocked access to the hydraulic cellar, where the stop button was located. At high risk, an operator managed to enter the cellar and stop the pumps.
- A hot rolled strip passed the guide of a down coiler and cut through the hydraulic hoses of a mandrel. As the operator fled the scene, he managed to open the valve of the four water cannons pre-aimed towards the down coiler. The hand valve was installed in a risk zone, only 10 metres away from the coiler. The damage remained minor since the fire was put out almost instantly.
- A small gap had been left between a casting platform and an embedded slag pot. Hot metal ran through the gap onto the hydraulic pipes of the mould oscillator, leading to a fierce fire which destroyed all cables, hoses, and other equipment under the casting platform.
Hot oil burst the pipe
- For maintenance purposes, a manual valve in hydraulic piping had been closed off. When the continuous casting line was restarted, another automated valve shut off. This meant that oil remained in the closed part of the pipe. As the oil heated, it expanded and caused the pipe to burst. The pressure then lowered, which in turn made the automated valve open. This allowed oil to flow out of the burst pipe. The resulting fire destroyed the instrumentation and other parts of the machine.
- During hot rolling, the front-end of a billet split, forming a ‘crocodile’. It deviated from the rolling track and then hit and broke the coupling of a guide cylinder. The oil fire spread onto the control room’s door. The operators managed to exit through a back door serving as an emergency exit. The fire also spread under the roller table, where the hydraulic pack with its 200-litre pressure accumulators were located. When the suction hoses burned through, the full contents of the 1,000-litre tank discharged into the sump and continued to burn there.
- The most affected items were damaged very rapidly after the outbreak of the fire, the burning non-pressurised oil having no major impact on the overall damage. Production would have been interrupted for much longer if the control room equipment had been destroyed, but luckily this did not happen.
As mentioned above, the most simple and reliable way to prevent a hydraulic oil fire is to replace mineral oil with non-combustible fluid. The risk of oil fire can be eliminated by using electric or pneumatic drives instead of hydraulic drives. Some fire risks are also related to alternative technologies, but not risks of fierce and uncontrolled fires.
Fluids considered non-combustible consist of water-based solutions, including less than 20% of glycol or oil. With water-based emulsions, however, it must be understood that the residues of such fluids are combustible when their water content has evaporated. Control of fluid leakages is important for several reasons: fire safety, the functionality of the machines and soil protection.
Consider the following issues
- Hydraulic systems, which are equipped with pressure accumulators, must be provided with block and bleed valves to bring an instant halt to the flow of oil from the accumulators to the fire upon activation of an emergency shutdown.
- Hydraulic pipes and hoses must be installed in safe places and should be screened off from potential ignition sources such as hot surfaces. A pin-hole leakage can also be deflected from a steel surface. All exposed high-pressure hoses should be installed in pyro-jackets, which prevent oil from spraying around if the pressure hose bursts.
- In so far as possible, piping should be made of steel pipes rather than hoses. Hoses should only be used for achieving the required flexibility rather than because they are easy to install.
- In humid and corrosive locations, stainless steel pipes are recommended.
- Pipes should be fastened properly to prevent wear due to vibration.
- The area around hydraulic hoses should always be kept clear of litter and other combustibles. Naturally, this also applies to the hydraulic pump room.
- Hydraulic pipes and hoses shall not be kept pressurised when not needed. In some applications hydraulic power is used only for setup motion and after that the piping can be non-pressurised.
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Published in Risk Consulting magazine in 2013, article updated in 2022 by Eero Kankare