Dairy industry fires and explosions: prevention, protection and mitigation
30 June 2017
There are more fires than explosions in milk dryers. Traditionally, these units produced a basic whole milk or skim milk powder, although customised formula and ingredients are now more common. The milk powder was prone to sticking to the walls of the chamber, while stalactites on the vortex of cyclones and nozzle build-up was not uncommon. Cleaning was not as thorough then, when only producing single products, especially in cyclones.
Cleaning of plant frequently involved manual access due to blockages and wall build, although automated CIP is now used more in dryer plant. The increase in the use of filter bags, although not acceptable for all products, is more popular to reduce exhaust emissions which has increased the risk of fire and explosions. The risks involved in the use of internal and or external fluidbed is a big talking point at the moment with the increased use of fines returns highlighting the need for an improved isolation policy.
Fires and explosions in the European dairy industry are becoming an increasing problem: An analysis of 116 incidents in the milk powder processing industry showed that five of these incidents experienced an explosion, with another three explosion-like effects without any damage that could be traced to pressure effects of an explosion. The remaining incidents were fires only, with damage ranging from medium to total loss of the drying unit.
More than 90 % of these incidents could be traced directly back to self-heating processes within the drying installation. The five to eight explosions were found to have been a consequence of the fire - not the other way round.
Another analysis looked at 240 incidents that occurred in spray driers in the food industry from 1953 to 1993. 20 incidents were reported to have experienced an explosion while 210 were fires only, and here too It was believed that fire was the initiator of the explosion. The result of the statistical analysis defined that further steps are needed for a new approach.
In the Irish dairy industry between 1980 and 1987, 12 incidents were reported involving fires in spray drying plants, and in five of these cases explosions were reported. In the UK dairy industry from 1972 to 1982, seven explosions were reported in spray drying plants.
By studying the plant details, internal mechanical and/or outside introduced ignition sources were ruled out. So the problem can be solved if an appropriate method can be found to detect heating before it turns into a fire. And if a fire does develop, a range of protection systems are needed to mitigate its effect.
In spray driers the fuel (milk powder) and oxygen cannot be removed, but the ignition source could be controlled. Operating experience shows that the primary source of ignition for fires or dust explosions in drying installations for milk, or in secondary installations such as filters and dust precipitators, are smouldering spots or self-igniting milk products.
Up to the present time, early-warning fire detection in such facilities consisted of installed temperature measurement devices, infra-red sensors or sniffing lines through which the smell of fires could be detected. The frequency of damaging events, however, shows that these devices are not sensitive enough to recognise small smouldering spots on time.
Early-warning fire recognition through CO detection
An early recognition of smouldering fires at an initial stage is possible through inspection of the exhaust air from drying installations for the presence of carbon monoxide, a gas which is the product of the thermic decomposition of milk products. Due to high volumes of air, the CO would be diluted so the system had to be very sensitive. Also it had to use differential readings between the inlet (reference) air which can be excessively CO loaded from boilers, trucks, forklift etc. and the exhaust air (sample) which would add anything burning inside the drying process.
Infrared gas analysis
With NDIR, the non-dispersive infra-red absorber, a measuring principle is made available, which is suitable for detecting traces of carbon monoxide. NDIR CO gas analysers, with a measuring range of 0 to 10 ppm, have been tested in the area of emission control and under harsh conditions, and can be considered as reliable for this type of measuring task.
An early-warning fire detection system consists of the following main components:
1. gas sampling probe
2. sample gas preparation
The use of an early-warning fire detection system as the primary safety measure in spray drying systems in the milk industry, requires a high degree of reliability and availability. An unambiguous and quick recognition of smouldering fires must be guaranteed, while false alarms should be kept to a minimum.
The locations at which gas samples are taken and the place of installation must be carefully selected at the time of installation. The location for the gas sampling probe in the exhaust gas canal must be chosen in such a way that the sum of all exhaust gas flows is measured, if at all possible, at the same time. Because the exhaust gases are loaded with dust and have a high dew point temperature, a dust and water removal system is required at the sampling point and over the gas sample line.
The reaction time of the system is a cumulative combination of the time during which the air is present in the system, the residence time in the gas sampling lines and the response characteristics of the analyser. To avoid a false alarm caused by a sudden rise in CO content in the intake air (for instance as a result of heavy traffic), the transit time in the sampling lines must be balanced. The gas sampling lines to the analyser should be kept as short as possible, in order to avoid unnecessary delay times. This proved to be an insurmountable task when considering direct fired dryers. Since infrared gas analysers are very sensitive to contamination, the gas samples must be carefully pre-conditioned, by removing humidity and product residues.
Detection of a smouldering fire
During the test phase of an early-warning fire detection system, it was concluded that an unusually steep increase in CO content in the exhaust air can best be used to trigger a pre-alarm. With such an advance warning, the operator would have sufficient time to localise and remove the smouldering spot, without having to accept long-term shutdowns.
The initiation of a forced shutdown, or the activation of fire extinguishing installations, should occur automatically, after a certain individually determined threshold has been exceeded.
Next generation CO early fire prevention systems
In the past, effective CO systems had false signal potential based on environmental conditions outside of the dryer. Now, a new generation of CO monitoring systems are available that are designed to compensate for internal and external environmental and flow conditions. To do this, all CO fluctuations are measured at the same point in time so the effects of the dryer process on the sample are taken into account.
About the author
Declan Barry is the Owner of ATEX Explosion Hazards Ltd, which specialises in explosion protection for food and beverage sector companies in the UK and Ireland.