The science of safe handling
17 December 2009
A safety warning issued in April this year by the US Chemical Safety Board found that 281 fires and explosions in the USA killed 119 workers and injured 718 others since 1995. The majority of these incidents took place in mineral processing, food, chemical or pharmaceutical manufacturing units.
The science of safe handling
Two specific areas were identified: flame propagation within the conveying, processing or storage system and explosion through the disturbance of accumulated dust build-up in the surrounding external environment.
For instance, otherwise minor sources of ignition (or even draughts through open doors/windows!) can lead to disturbance of a dust layer, which then results in a major explosion as the very fine (typically less than 500 micrometers in diameter) particles mix with air and are ignited by a source such as electrostatic discharge, friction, hot surfaces or sparks from machinery or electrical equipment.
Clearly, it can be seen that material handling systems must achieve an inherently safe level of performance for the type of product. However, good housekeeping and the minimisation of dust leakage also have an important part to play in maintaining plant safety. It is a vital partnership between plant operator and equipment supplier.
So what factors can contribute to safe product handling in pneumatic conveying systems? Of course, there are many, but in regard to rotary valves, the key areas are seen as full compliance with ATEX directives, safe and simple Clean In Place design to ensure that product compliance meets international standards, adherence to Food Machinery Directives, design features to cut leakage, and active safety devices to reduce or eliminate mechanical failure leading to product contamination.
This is especially important where pharmaceutical or organic food and non-organic food products are processed through the same system. Cross-contamination can mean drastically reduced quality assessment and lower sale prices as well as the ever-present threat of product recall. It is not enough to rely on reactive strategies; to compete effectively –and profitably- in a modern market, manufacturers must consider the basic design of all key elements of their process plant and look to an holistic approach to safety.
Industrial producers are acutely aware of the dangers of flame propagation and explosion when handling fine powders, especially when handling fine metallic powders, organic-based materials such as grain, sugar, coal, sawdust, milk powder, cereals, flour and certain types of chemical. The ATEX 94/9/EC Directive was introduced to address these potential problems by setting agreed standards for the design, construction, testing and ongoing maintenance of rotary valves. When necessary, Independent assessment is provided by an approved Notified Body, which tests valve types and issues Examination Certificates for rotary valves which meet the Directive’s stringent requirements.
Where a valve is used for explosion/flame containment, it is classified as an Autonomous Protective System. Again, each valve must be accompanied by an EC Type Examination certificate, issued by an approved Notified Body. This will identify any limitations for use, including the class of dusts allowed to be handled, internal clearances, blade thickness and pressure limits. It is the responsibility of the buyer to check with the supplier that their application is satisfied by these conditions. When supplied for this purpose, all Rota Val ATEX valves are individually hydrostatically pressure tested to prove the integrity of individual components as part of their ATEX production procedures.
A normal requirement of most Notified Bodies in issuing their certificate is that all internal components must be permanently fixed to avoid the possibility of items becoming loose or displaced, and so opening up a flame path. This precludes the use of adjustable blades or replaceable blades and screwed fixings, as these could lead to an increase in rotor to housing clearances and so to a failure to meet the ATEX requirements. However Rota Val do have approved design for the inclusion of two adjustable scraper blades when necessary, and when supported by the requisite number of fixed vanes, to deal with products that build-up hard deposits on the internal surfaces.
Remember, it is essential that the end user ensures that clearances stay within the certified limits during the life of the valve. This is particularly important when handling abrasive products. Rota Val have taken design steps to ensure that housing and rotor structures significantly reduce the potential for damage when the valve is being stripped down or rebuilt after cleaning. For example, the provision of two underslung support rails and tapered housings ensure that rotors can be withdrawn easily, with minimal effort, and can be replaced without contacting the housing. A removable saddle provides additional support for the rotor when it has been freed from the external bearings and rotor housings. Such features are important to minimise the risk to operator safety and component damage which will probably invalidate any ATEX certificate for use for flame containment.
So, given a valve which is expressly designed, manufactured and thoroughly tested to international standards to meet essential safety requirements, what other considerations are there?
It has already been established that leakage of fine dust from a process can contribute to an explosion. Where the product being handled actually poses a serious threat to operator health, there are already rigorous Occupational Exposure Levels which require very low leakage, differential pressures or even complete enclosure of product process.
However, it is obviously good practice to minimise leakage for any type of product. It saves money, boosts productivity, enhances profits, cuts waste, reduces environmental contamination and improves the operators’ attitude and performance. Those are compelling reasons to build in product containment at the system design stage.
Basic design and experienced selection of components is the key here. Rota Val rotary valves all use outboard bearings, which allows precise matching of a wide range of sealing materials to the needs of the application without major housing modification. Seals can be FDA-approved for food, high temperature seals, abrasive-resistant, air purged or pressurised to contain any potential product escape, and can also be designed to be self-purging with fluid or air for in-place cleaning. Finally, they may be equipped with leakage monitors to shut down the system if any failure occurs.
As the product is not in close contact with the bearings, these resist wear better, making these valves more economical to operate over long periods. The overall effect is one of low leakage and low maintenance to meet customer’s needs at an acceptable price for the application, whether ultra-high safety for pharmaceutical products or robust longevity for minerals handling. Careful design and long experience of basic structural needs has resulted in a very wide range of valves. The vast majority of these will be acceptable for standard applications, but the genius behind them is their ready ability to be modified easily to meet unusual process requirements at an economic cost.
Having dealt effectively with inherent safety, performance to specification, reduced maintenance and product waste, Rota Val also considered pro-active safety measures.
Product contamination is a nightmare for any manufacturer. As described above, it can have a major impact on productivity and profitability. Rota Val have designed a new system which will stop a rotary valve instantly if it detects rotor to housing contact, substantially eliminating valve damage and consequent contamination by metal particles.
Designated the RotaSafe RM (Rotor Monitor), it may be attached to any Rota Val rotary valve. Contact between the rotor and valve housing can result from rotor deflection due to product build-up (glazing) on contact surfaces, ingress of ‘tramp’ material in the working clearances, bearing failure or excessive temperature or pressure. Incorrect assembly and poor maintenance can also cause rotor contact with the housing.
The system prevents valve material shedding into and therefore contaminating the product being handled and protects valve components from serious damage. Prevention of damage to the internals of ATEX Autonomous Protection valves is vitally important to avoid invalidation of the ATEX certification through an increase in rotor clearances. The detection circuit is Intrinsically Safe and therefore suitable for all Zones. Circuit settings are adjustable to eliminate ‘nuisance’ tripping.
Finally, there is a new addition to the Machinery regulations relating to "The supplementary essential Health and Safety requirements relating to the design and construction of foodstuffs machinery and machinery for cosmetics or pharmaceutical products". This specifies that , amongst other requirements, all surfaces and mechanical joints should be easily accessed for cleaning, and that all guard fixings must be held captive. Once again, Rota Val are at the forefront of meeting regulations through innovative design in ensuring the new standards are fully met.
Ian Blackmore commented: "Our continuing efforts to meet customer needs – even before they knew they had them- are clearly evidenced by our Fast-Clean Rotary Valves, Fast-Clean Blowing Seals, Hypergienic and ATEX-certified ranges. Together, these valves offer significant performance benefits, cut production costs and fully meet international safety regulations. In the light of the USA study and others in Europe, I sincerely believe that we lead in contributing to matters of safety by our dedication to quality of design, manufacture, complete adherence to standards and thorough understanding of the industry built up over the last forty years".
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