Spark detection and extinguishment
January 1, 2010
by Ulrich Diener
Formation of sparks, pockets of ambers and low-speed detonations or fires often occur in mills, especially in feed mills. As a result, the number of injuries occurring in the grain industry due to explosions is increasing.
Sparks in hammermills are caused by contaminations, while pockets of ambers in presses are caused by hot material, friction or crushing.
Pockets of ambers can also occur due to negligence during grain intake — all it takes is a lit cigarette being thrown away in the intake hopper to set the stage for disaster. Therefore, the safety monitoring of the elevator at the grain intake is highly recommended.
Knowing about the danger of sparks and embers implies the responsibility to take preventive measures (extinguishment of the sparks) immediately.
For organic products as they are transported in a mill, pure water or water dust are the most efficient means of extinguishment. If other products such as powder or foam are used, this will require shutdown of the plant while cleaning. A simple material outlet by means of a quick exclusion gate is possible but not very popular with respect to environmental protection considerations.
Depending on the construction of the plant, two different sensor types for spark detection are available. They include: Pure infrared sensors. They work in complete darkness (conveyor pipes, etc.) and recognize sparks or pockets of ambers by temperature changes in the product flow. Day light sensors (mixed light). These sensors operate with ambient light as well, including ambient light caused by checking doors in roller mills, decharging hoppers, press outlets, etc. The construction of this sensor is quite complicated, because it has to be able to compensate and refine ambient light reliably.
High temperature sensor.
The product temperature at press outlets can be up to 150 degrees C, which is much too high for a normal spark sensor. For this special application, fibreoptic technology-based sensors are available. The control unit can be mounted at a distance of up to 3 meters from the hot product stream. For the high temperature sensors, the same mounting possibilities as for standard sensors are available.
Mounting of spark detectors.
Depending on the construction of the pipes or the buckets as well as the diameter and the kind of stored product, the following devices can be supplied: quick mounting brackets which can be integrated from the outside without fixing mounting parts inside of the pipe/buckets; quick mounting brackets as mentioned above but with zone separation; purge air mounting devices as a quick mounting unit or for welded fixing if material is very dusty or sticky — these devices are permanently flushed with a little air; and welded fixings.
For mounting, a special tool can be supplied.
The signals of the spark sensors are evaluated by the spark sensor control, which switches within seconds to initiate the necessary functions. The spark detection control consoles have modular construction and can optionally be extended at any time. The amount of the registered sparks are quantified in the control console and can initiate further functions like extraction or extinguishment, depending on the amount of sparks registered. In general, the control consoles are equipped with emergency power supply (battery buffer), which is laid out in such a way that the connected peripheral parts (spark detection, extinguishment devices, valves, etc.) can be operated for a duration of at least 18 hours should power failure occur. The control console checks every single sensor for correct functioning as well as all wires. Every irregularity is displayed and registered in the internal record. All extinguishment devices such as valves and pressure reduction systems, if used, are checked.
The control consoles have an internal record for registration of all events. Every adjustment, operation, check, etc., is registered with time and date.
The logbook is secured by multi-step password protection so that easy cancellation of the entries is not possible
Depending on the construction of the bucket or the pipe, two, three or more spark sensors are used. Special mounting kits are available which enable the integration in a closed pipe system easily from the outside. After the signal analysis, either the extinguishment or the extraction device is activated.
In a hammermill, there is a continuous generation of sparks, of which between 90% and 99% can be classified as nonhazardous. It’s the dangerous sparks that have to be abolished.
It is important to detect these sparks in the aspiration and in the outlet of the hammermill and equip this part with a connected extinguishment device.
New tests show that the best possible safety can be achieved when an additional extinguishment nozzle is integrated in the hammermill. Preventive steps must be taken to ensure that smoldering parts in the hammermill do not cause a further extinguishment via aspiration or outlet.”
In general, it is best to use an extinguishing device with water. Every additional detergent or dry powder requires an expensive cleaning of the system, which inevitably causes a lengthy shutdown of the plant.
Correctly designed spark detection and extinguishment means destroying sparks but allowing the production process to run as usual.
If a larger extinguishing event with different water applications should occur, it is recommended that you dry the system with maize or grain peel.
It is generally recommended to only use the minimum amount of water needed to safely destroy the sparks. It is better to use different nozzles with short water intervals than only one nozzle with a larger amount of water.
In a cube press, the aspiration and product outlet must be equipped with an extinguishment device.
The monitoring of the product stream is made after the chilling, because within the chiller there is no danger. If the aspiration press and chiller are separate, both have to be equipped with spark detection sensors.
As with a hammermill, it is recommended that you integrate an additional extinguishment nozzle in the press in order to destroy smoldering parts immediately.
Different extinguishment systems, such as powder, water, foam and inert gas, are available. In flour mills and feed mills with running production, most systems are based on water in order to prevent the contamination of the material. Also, difficult cleanings during the production process can be avoided.
It is important to use pipe diameters large enough to get a lot of water with constant pressure in a short time. If the water pressure is not sufficient, the usage of a water increasing system is necessary. Perhaps water pressure bottles can be used, but the problem with using them is the production process has to be stopped until the bottles have been replaced.
The same quick mounting set used for the spark sensors can be used for the mounting of the extinguishing nozzles.
If in particular cases it is not possible to extinguish the detected sparks and smoldering parts, the exclusion into closed buckets can be taken into account. For the exclusion, new, quick closing gates are used which can divert the product stream within 300 milliseconds. Spark detection in flour and feed mills is essential. In many countries, these systems are required by the regulations of the employer’s liability insurance associations. Where regulations do not exist, the plant owner should voluntarily think about this investment for a spark detection system. Every explosion or fire which can be prevented can avoid human tragedy and material loss.
Ulrich Diener of Laupen, Switzerland-based Agromatic can be reached at email@example.com.