How to prevent hydraulic oil fires
Hydraulic oils are not generally considered a serious fire hazard, because they have high ignition temperatures. This fact can lead, however, to a false sense of security, since spraying hydraulic oil will burn just as fiercely as other hydrocarbons.
Hydraulic fluid has been a factor in many fires and is often responsible for markedly increasing the extent of fire damage. Because hydraulic systems are highly pressurised, flames from a hydraulic oil fire can spread over dozens of metres. On factory premises, such a conflagration will unavoidably reach cables or other combustible material, which will catch fire and be rapidly destroyed.
It is therefore essential to assess the risks related to hydraulic oils to ensure sufficient protection methods. In fact, the risk of an oil fire can be completely eliminated by shifting to non-combustible hydraulic fluids or adopting electrically or pneumatically operated equipment.
Replacing mineral oil with non-combustible fluid
Since mineral oil has several technical benefits – not to mention its price – it is used in most hydraulic systems. Equipment manufacturers have so far chosen non-combustible fluids only for certain special applications.
When purchasing new systems, it is advisable to agree the preferred fluid in advance with the manufacturer. It is difficult to change the hydraulic fluid type at a later stage because the manufacturers’ warranty conditions require that the fluid specified by the manufacturer be used. If this happens to be mineral oil, switching to another fluid falls under the user’s responsibility and may void the warranty.
How damage occurs in the metal industry
Hydraulic oil fires most commonly occur when oil sprays towards a hot surface. This can happen when a hot slab, liquid metal or hot slag drops onto hoses, which ignites the oil. A fire can also start due to an oil hose or pipe failure, with oil being sprayed onto a hot slab or another hot surface.
In most cases, oil ignites as soon as a leak occurs. In a recent loss case, oil from a system sprayed over a production line for a protracted period. After the leak was detected, the line was stopped, and employees went to inspect the burst pipe. During the inspection, the oil vapour ignited and the resulting explosion caused serious injuries and property damage. In this case, the ignition was caused by an overheated bearing, and indeed, a broken bearing is statistically one of the most typical ignition sources of mineral oil fires.
Attention to the pump room
When anticipating loss scenarios, it should be noted that a hydraulic system consists of three main elements: the hydraulic pack, the piping system, and the drives. Sometimes pumps and hydraulic drives are installed in the same place, as in the case of metal working and plastic injection-moulding machines.
If an oil fire causes immediate damage to pump cables, the pump will stop even if the operator does not activate an emergency stop. When pressure accumulators are emptied or isolated, the oil fire will cease to escalate intensively.
In other cases, pumps and hydraulic drives are located at a distance from each other. Pumps will then continue to fuel the flames until the flow is stopped either by closing the line in question or switching off the pumps. In the worst-case-scenario, the only stop button is located in the hydraulic room, which might be inaccessible due to the fire. In such cases, the oil flow will not stop before the tank is emptied or the low-level switch activates.
In addition, stopping the system can take time. Such a fire typically leads to the collapse of the roof surrounding the fire area.
A system-wide emergency shutdown may cause technical problems
In the case of large central systems, it should be noted that a system-wide emergency shutdown may cause technical problems and even damage elsewhere in the system. Each sector should therefore have its own separate set of emergency stop valves. An oil fire can also break out in the hydraulics room due to a broken pump or cable fire. An oil fire in an unprotected pump room will destroy all equipment extremely rapidly.
Normally, the pumps are equipped with suction hoses or rubber bellows, which will burn and break at the very early stages of a fire. After the pumps stop, the fire will not die out since the discharged oil continues to burn.
The time taken to repair a hydraulics room after a fire depends, to a large extent, on the equipment in question. If large servo valves need to be replaced, as much as three to four months may be required before the equipment is restored. Renewing standard equipment normally takes only one, or at the most, two months. It is easy to protect hydraulic pump rooms with water sprinklers. In less critical locations, it may be sufficient to ensure control of the pump temperature and tank fluid level alongside the use of a fire alarm system.
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