The personnel and especially those responsible for the maintenance of the factory should take time to consider how much an unexpected outage will cost, how it can be avoided and what to do if such an event occurs.
Significance of maintenance, initial extinguishing and protection level
An accident was narrowly avoided at a powder factory when the grease in the bearing support of an air exhaust fan caught fire. However, the situation was detected quickly and brought under control through prompt initial fire extinguishing. The fire could have resulted in an explosion and a long interruption in powder production.
Nothing out of the ordinary was detected during the normal inspection round carried out in the production facilities of the powder factory. However, a moment later a control room security camera revealed dust in the tower room. Upon inspection, it was found that the bearing support of the air exhaust fan was on fire and the flames were already at the door. Initial fire extinguishing was carried out with a powder extinguisher and the fire brigade was called. The fire was then doused with water, which cooled the bearing housing, extinguishing the fire. Upon its arrival, the fire brigade secured the site and ventilated the facilities.
What happened?
It seems that the axle of the air exhaust fan had broken by the bearing housing, after which the ends of the axle had rubbed against one another. As a result of friction, the bearing overheated and the vaseline in the bearing housing caught fire. This theory is supported by the fact that the inner circle of the bearing was intact and there was molten metal inside the bearing support. Another possibility is that the bearing had jammed, causing both overheating and the breaking of the axle, as well as igniting of the vaseline in the bearing housing. A dust puff (in the production facility) caused due to sudden overpressure towards to drying chamber was detected from the contol room. The air content and pressure control in the tower dryer is based on controlling the balance between the pressure blower and the air exhaust fan. In an operational situation, there is slight negative pressure in the tower dryer and the dust collector cyclones, controlled using the adjustment valve on the pressure side of the air exhaust fan. The small fire load and high standard of hygiene in the facilities prevented the fire from spreading.
What might have happened?
The development of a fire or dust explosion in the production facility itself was possible since the sudden rise in pressure opened the explosion relief panels in the drying chamber and the dust floating around in the drying chamber entered the production facility. Another risk of a dust explosion existed in the area of the cyclones and the drying chamber, which were connected to the air exhaust fan by a suction duct. In the drying process, the task of the air exhaust fan is to suck the drying air from the drying chamber through the dust collector cyclones, creating a high risk of a dust explosion in this equipment. This is also indicated by the fact that their operation was disrupted due to the sudden damage which occurred. Dust explosions have arisen in similar production equipment, progressing from minor ignitions into explosions both in and outside the pipes if the fire load is great enough. An explosion would have meant an interruption in production affecting the dryer and the capacity of the entire drying line, causing substantial difficulties and requiring the reorganisation of production.
Elements of loss prevention
Loss prevention comprises several elements, each of which play their own role. An expert and skilled operation and maintenance personnel are required, as well as initial fire extinguishing equipment and skills, a protection level(a fire alarm or automatic fire extinguishing system) taking account of the speed at which a fire develops), as well as sufficient extinguishing force (factory fire brigade or extinguishing team + municipal fire brigade). Since initial fire extinguishing must often be carried out with water in addition to powder extinguishers, fire hydrants are needed. Maintenance should be preventive and based on measurements. For example, condition monitoring and the lubrication of bearings in machinery are important because, once broken, the smallest of bearings can bring a major production chain to a standstill. The latest monitoring equipment can be used for measuring any ultrasound vibrations caused by a roller bearing, for detecting incipient damage at an early stage and planning a managed maintenance outage of the equipment. Losses can be prevented, e.g. with the following measures:
- Production facilities must be kept clean and tidy and the fire load low. Thus, the risk of explosion and of fire spreading will diminish.
- The explosive parts of production machinery must be protected and urgent releases of pressure into outdoor air enabled.
- Parts of the production process which include a fire load, such as drying chambers and fluidised beds, should be equipped with a water spray system.
- Production facilities must be protected with an automatic fire alarm. The so-called aspirating smoke detection system is higly recommended one.
- It shall be ensured that the critical parts of the process are controlled and monitored through automation.
- The main engines and blowers of the equipment must be subject to regular measurement and preventive maintenance. Continuous follow-up of condition monitoring is highly recommended with respect to critical equipment.
- Initial fire extinguishing equipment, including fire extinguishers and hydrants, must be kept up-to-date.
- The fire service must be familiarized with the plant. The production plant’s personnel, extinguishing team or factory fire brigade should practice fire fighting together with the municipal fire brigade.
Further information:
Ville Valta, If
Pentti Kautto, Inspecta
