ATEX Backdraught Break Rotary Valves and Screw Feeders
Explosion isolation is achieved by a protective system, which prevents an explosion pressure wave and a flame or only a flame from propagating via connecting pipes or ducts into other parts of apparatus or plant areas. Systems providing complete isolation by operation of the isolation device(s) prevent the propagation of the flame as well as pressure effects. Systems providing partial isolation only isolate the flame propagation. This distinction is important for practical application, because it is not necessary in all cases to achieve a complete isolation of flame and pressure. In some cases it is sufficient to achieve only flame isolation.
Passive isolation systems do not require the addition of detection and control and indicating equipment to function. They operate from the pressure created by the explosive pressure wave / front.
Active isolation systems require detection and control and indicating equipment to function. Detection systems are systems usually based on optical or pressure sensors.
It is very important for explosion isolation systems that a detailed analysis of all relevant characteristics and conditions is made. This analysis can include (as a minimum) the following:
- Specification of the physical characteristics of the connected equipment where the isolation device shall be installed, e.g.:
- type of connected equipment (pipe, belt, screw etc),
- strength of connected equipment,
- layout of connected equipment including length, bends, junctions,
- cross sections including changes in cross sections,
- presence of internal obstructions;
- Specification of the relevant process conditions:
- area classification (zones),
- max/min temperature, min/max pressure,
- type of conveyed product: bulk, dense phase conveying, lean air/dust mixtures, gaseous mixtures,
- flow rates and directions,
- specific product properties (such as sticky material, moisture content, abrasive, corrosive, toxic, tendency for product built-up);
- Specification of the explosion characteristics of the products involved, taking into account the influence of the process conditions for example pressure, temperature, etc.
- Definition of the protection methods of the equipment to which the transportation system/pipe is interconnected;
- Identification of potential ignition source location(s);
- Definition of the protection objectives:
- stop flame and burning particles from an explosion,
- stop pressure wave;
Based upon the above elements, together with the specifications of the isolation system(s), the appropriate explosion isolation system can be chosen.
The distance between the protected equipment and the isolation system shall be such that (i) the isolation system has operated before arrival of the pressure wave and/or flame; (ii) inadmissible flame accelerations leading to unacceptable high pressures are prevented.
ATEX explosion break / backdraught and rotary valves are installed into the ductwork / piping system to prevent an explosion / flame front from travelling from one vessel/pipe to another, or to an area where personnel may be present.
For instance they may be required between a dust collector or receiver vessel which is piped to a storage tank / hopper etc. The other vessel may contain a large amount of flammable / explosive dust and it is imperative that a flame front is prevented from reaching the other vessel.
An ATEX screwfeeder also provides an explosion break between one vessel and another, again preventing the flame front from possibly igniting a larger fuel / product source.