Design protection for explosive hazardous environments in the food industry

17 December 2009

The food and beverage industry should, some may think, not be an area requiring too much thought when looking at classifying areas for explosion risk, after all, it is not the chemical industry where there are many so called dangerous materials which can catch fire or even explode. Bread making, the snack food industry, chocolate and many more processes to the layman seem quite innocuous and produce foods we all are very familiar with. Surely no danger?

Design protection for explosive hazardous environments in the food industry

However modern food factories are employing processes which are frequently seen within the chemical field such as materials handling, powder processing, solvent extraction and bulk production. It is in this high volume production of finely powdered materials where lies a recipe for disaster, unless proper risk assessment is carried out and the appropriate safe design incorporated within the system. Fine organic powders in the right proportions within an ordinary air atmosphere can explode with devastating consequences given an ignition source.
Many new food products use additives which are finely ground and the trend for instantising in the snack food market often involves ultra fine ingredients. Ultra fine powdered materials represent enhanced explosion risks. In the food industry there are many examples of finely divided powders (flour, coffee, tea, spices etc) and as all processes require electrical power with the associated motors and electrical switchgear the combination produces the perfect recipe for an explosion to occur in a normal atmosphere. Moving foodstuffs around, for example by conveyors, creates dust - these fine powder particles form a potentially explosive dust cloud. There have been many recorded instances of industrial explosions where the initiating explosion was not the most serious event but just the cause for a violent disturbance of general factory area dust which has ignited in a so called secondary dust explosion . These can be extremely violent, occur in large work areas or volumes and can be extremely serious.

Keeping work areas very clean in the food industry is not only for hygiene purposes but essential to reduce the potential for these secondary dust explosions.

Methods of explosion protection
Explosion venting is the most cost effective method and often selected where the explosion is relatively mild and not generating high pressures quickly. Basically the explosion is allowed to occur and quickly vented to a safe area by means of a rupture panel or quick opening door fitted in an appropriate part of the process. One problem is designating a safe area to vent to. Even venting to a safe outside area can be a problem as the explosion vent must be well away from pedestrian areas and not be impeded. Should an explosion occur the vent panel and duct channels a violent flame front which is dangerous. Replacement of the vent panel is required and other mechanical damage may occur so a full system check is required after any event. The big advantage is low capital cost and a relatively simple design.
Explosion suppression is suitable when the process would generate a relatively mild explosion but when venting to a safe external area is not possible. The process is for example in the middle of a factory. Here the explosion is allowed to occur and then immediately damped or quenched using inert gas or special inert powder. Like venting the equipment to be protected must be able to withstand small overpressures and any open inlet pipes/ ducts protected by quick acting slam shut valves. As pressure is used to detect the explosion, accidental system overpressure without any explosion will trigger the system to fire and contaminate the food material batch being processed. This is a disadvantage and something which should be taken into account in the initial system selection. Regular maintenance of the suppression system is required to reduce problems. In cost terms however the system is still quite cost effective and one stage up from simple explosion venting.
Inert gas systems utilise an inert gas (usually nitrogen) atmosphere for the whole process and can be very effective particularly in batch operations such as mixing. The inert gas blanket has to be continuously monitored for oxygen content and topped up as required. There is therefore a running cost in N². The system requires good oxygen monitoring and controls but is safe as it ensures that the food process is always operating outside the danger zone for an explosion to occur. It ensures no fuel ( oxygen ) is available to sustain an explosion. Nitrogen leaks from the process to outside may occur and gradually make the atmosphere dangerous to operators.
Explosion containment is where the whole of the operating system is designed to withstand the maximum pressure of any explosion usually about 8barg. Equipment is designed to be shock pressure resistant to 10barg hence standard designs of mixers, dryers, mills etc all have to be to this increased standard.
All interconnected equipment has to be able to withstand the overpressure so ducting and vessels such as filters need to be designed to 10barg shock pressure standards. The boundaries of the system also have to be protected so any inlets/ outlets will need to have special valves to stop any explosion exiting. Rotary valves and quick acting slam shut valves on air inlets/outlets are used all designed to this 10barg standard.
Naturally this has a cost implication particularly when stainless steel and exotic materials are used. This method is now becoming more popular within the food industry as it is recognised as one of the most safe solutions. Most equipment suppliers will have special designs for Shock Pressure Resistance often referred to as PSR designs and companies such as Hosokawa Micron Ltd have the overall experience to design fully integrated, explosion protected systems that offer engineering safe solutions with full compliance testing and documentation.
Below are details of a typical installation that demonstrates how Hosokawa Micron Ltd have worked with a customer to develop the right ATEX compliant solution for production of a food product.
Food industry case study
ATEX/DSEAR legislation compliancy, production flexibility and operational mobility were key selection criteria when Chaucer Foods Ltd. began researching size reduction equipment capable of milling croutons and baked snack foods down to a fine crumb.
After extensive research and product trials, the UK based manufacturer of freeze dried fruits, croutons, snacks and various bread based ingredients purchased from Hosokawa Micron Ltd, a PSR 10, a unique 10 barg shock-pressure resistant design of Hosokawa Rietz Angle Disintegrator and bulk packing station. The system is designed and manufactured to meet stringent GMP requirements within a sanitary design and is ATEX compliant having an internal zoning classification to ATEX 20 for the safe size reduction of potentially explosive and combustible products.
The Hosokawa Rietz Angle Disintegrator is designed for delumping, pulverising and grinding to produce uniform particle sized end product. The machine’s simple screen change operation and infinitely variable mill speed that allows Chaucer Foods to select different crumb sizes to suit requirements. Construction of the Angle Disintegrator facilitates quick product change and easy access to screens and hammers for easy cleaning. Following successful completion of trials at Hosokawa’s Runcorn test facility Chaucer Foods Ltd placed a order which included the disintegrator, bulk packing station, control panels and dust collection unit and the installation and commissioning of the whole system in accordance with comply with current ATEX/DSEAR regulations.
Chaucer Foods UK Technical and Development Manager, Ian Pomfret , has been impressed with the system , which has proved to be trouble free and comments on the valuable co-operation with Hosokawa in this very significant project.

Pomfret said: " The Hosokawa Technical Team have worked exceedingly well with our in house team, not only have they resolved our complex process requirements but have brought valuable and constructive suggestions to the table. Our new milling system is in line with Chaucer Food’s ISO 14001 accreditation, with the system giving us greater raw material conversion efficiency and creating a new product offering to add to our portfolio".