How to prevent hydraulic oil fires
Hydraulic oils are not generally considered a serious fire hazard, because they have high ignition temperatures. This amounts, 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, 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 in order 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 is under the user’s responsibility.
How damage occurs in the metal industry
Hydraulic oil fires most commonly occur when oil spray and a hot slab meet. This can occur 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 an inflammable 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 halted and employees went to investigate the accident location. However, the oil vapour ignited only then, the resulting explosion causing major physical injuries and property damage. In this case, the ignition was caused by an overheated bearing. Indeed, a broken bearing is one of the most typical ignition sources of mineral oil fires.
Attention to the pump room
When anticipating damage scenarios, it should be noted that a hydraulic system consists of three main elements: the pump unit, the piping system and the drives. Sometimes pumps and hydraulic drives are located in the same place, as in the case of metal working and plastic injection-moulding machines.
An oil fire will cause immediate damage to pump cables, which will stop the pump even if the operator does not activate an emergency shutdown. 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 feed more oil into the fire seat, 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 hydraulics room, which might be inaccessible due to the fire. In such cases, stopping the system could take time. Such a fire typically leads to the collapse of the roof of 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, from a broken pump or cable fire. An oil fire in an unprotected pump room will destroy all equipment in the blink of an eye.
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 continue as the discharged oil burns.
To a great extent, the time taken to repair a hydraulics room after a fire depends 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 restarted. 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 use of a fire alarm system.
Piping system at risk of bursting
A piping system regularly consists of steel pipes, high-pressure hoses and non-pressurised return pipes and hoses. Experience has shown that a piping system for hydraulic oil is exposed to a 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 inside. The hose burned through the high-pressure hoses and ignited the oil spraying from them. Due to the burning oil, the hall and all its machinery were destroyed.
The last slab was not quite straight when it was transferred onto a roller table. The slab struck the hoisting cylinders of the roller table, the resulting shock bursting one of the steel pipes, which in turn struck the hoisting cylinders and broke. 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 up another wall and back down, igniting the 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. Nearby 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 and hall.
Pumps could be stopped only when safe to do so
A hot band machine had a ½” high-pressure hose of 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. However, the damage remained local.
Due to a malfunction, a hot billet was lift away from an extrusion press, after which it fell onto high-pressure hoses. The flames were on the point of blocking access to the hydraulic cellar, where the stop button was located. However, 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 at the down coiler. The hand valve was clearly located in the danger zone, only 10 metres from the coiler. The damage remained minor, since the flames were not directed at dangerous locations.
A small gap had been left between a casting platform and a small slag pot. Hot metal ran through the gap onto the hydraulic pipes of the die systems, leading to a fierce fire which destroyed all cables, hoses and other equipment under the casting platform.
Hot oil bursted 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 equipment 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 the guide hydraulics. 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 unit with its 200-litre pressure accumulators were located. When the suction hoses burned through and severed, the full contents of the 1,000-litre tank were discharged into the sump and continued to burn there.
Most of the damage arose 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, 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 related to also such 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 solutions, however, it must be understood that if such fluids leak onto the floor, their water content will evaporate and the remaining substance might be combustible. Control of fluid leakages is important for several reasons: fire safety, the functionality of the machines and soil protection.
Consider the following issues
If, in spite of everything, a combustible hydraulic fluid is used, at least the following issues must be considered:
- Hydraulic systems, which are equipped with pressure accumulators, must be provided with block and bleed valves in order to bring an instant halt to the flow of oil from the accumulators to the fire, upon activation of the 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 hose itself is broken.
- In so far as possible, piping should consist 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 so as to prevent wear due to vibration.
- The hydraulic system must include a relief valve, which allows the pressure to be released back into the tank in the event of an external pressure shock. For instance, if a cylinder piston is subject to a shock, the pressure will rise abruptly, possibly breaking hoses or pipes.
- The area around hydraulic hoses should always be kept clear of litter and other combustible objects. Naturally, this also applies to the hydraulic pump room.