Explosions can occur in a closed system if the distribution of combustible dusts with particle sizes <0.5 mm produces potentially explosive atmosphere in a mixture as soon as a specific concentration is exceeded. The need to avoid such explosions places high demands not only on safety equipment manufacturers but also on plant owners.
These demands were specified in detail with the introduction of a standard for explosion protection. This process began with EN 50014 and has been governed since 2003 by 94/9/EC (Atex 95 Directive) for the manufacturers of devices and safety systems for conventional applications in hazardous areas or 99/92/EC (Atex 137) for the owners of electrical facilities containing potentially explosive atmosphere.
Hazardous areas are classified as Zones 20 to 22, based on an incidence which is documented as being "continuous", "for short periods" or "not likely to occur" according to the probability of explosive atmosphere developing due to flammable dusts. This classification into hazardous areas takes account of the specific properties of the dusts concerned, like ignition temperature, ignition energy and maximum explosion pres-sure, as well as the subdivision into dust explosion classes St 1 to St 3, including the maximum rate of pressure rise (Kst value) in bar·m/s:
· St 1 > 0 to 200 bar·m/s
· St 2 > 200 to 300 bar·m/s
· St 3 > 300 bar·m/s
Double-valve systems comprise a combination of two shut-off valves belonging to the GS series (pressure shock resistant and flameproof), a special surge drum and an EC type-examination tested valve control function. This function must ensure that one valve is always shut, since flame tightness is only guaranteed if the valve disc is closed and the seal seat intact.
To guarantee the necessary operating reliability in this regard, a special pneumatic locking mechanism has been developed; in combination with a precision-adjustable limit switch unit it makes sure that the closed state of each shut-off valve is easily recognised and only one valve can be opened safely. The locking mechanism operates independently of the PLC's electrical signals, so that no special safety measures are required for the electrical control system. Although safety-oriented control could certainly be implemented as an alternative, the cost of realising an equivalent level of functional reliability would be disproportionately high.
The shut-off valves in the standard DKZ 103 GS series are detonation puncture-proof against explosions of combustible gases whose detonation behaviour does not exceed that of propane gas; they are also detonation puncture-proof against organic particles in dust explosion classes St 1 and St 2 as well as metallic particles in dust explosion class St 3, providing their detonation behaviour does not exceed that of aluminium particles with a Kst value of 500 bar·m/s. The other limits of application are defined as an explosion pressure shock resistance of 10 bar and a nominal range of DN 50 to DN 500.
Compared to conventional rotary valves, the emphasis here is on suitability for gas and St 3 particles. Despite this considerable flexibility, the standard series is unfortunately unable to meet all practical demands. The following conditions are therefore specified: maximum temperature +150 °C, explosion pressure shock resistance 13 bar, maximum Kst value 800 bar·m/s. These shortcomings provided the inspiration for the DKZ 103 GS-St 3 series, which was specially developed for metal particles not exceeding the explosion behaviour of aluminium particles with a maximum Kst value of 1000 bar·m/s. The other limits of applicability here are as follows: maximum explosion pressure shock resistance 14 bar, maximum temperature +150 °C, nominal range DN 50 to DN 200.
It was a customer need that gave rise to the DKZ 103 GS-0.3 series up to 200 °C. In this case, the 200 °C temperature caused the biggest problems. The explosion pressure was reduced to 0.2 bar by blow-out discs. The nominal range extends from DN 50 to DN 250. The seals normally have a conductive or discharge-capable design inside the shut-off valves. These qualities are determined by the carbon black and in the past were not FDA compliant. In the meantime, special, conductive qualities in conformity with FDA requirements are available. Particles with a minimum ignition energy greater than 3 mJ are an exception here: the use of non-conductive seals is permitted. White qualities in conformity with FDA can consequently also be implemented.
The qualitative design of seals depends, amongst other things, on the operating conditions of the safety systems. The technology has become established over the years as a result of its successful deployment, and it has often proved to be a better, i.e. the only, alternative to mechanical devices like rotary valves, especially in low-pressure or vacuum applications.
Discharge from filters and air classifiers are typical uses. Particularly when designing lock systems for vacuum equipment, this task can only rarely be satisfactorily taken care of by conventional rotary valves. Tests by several users have shown that the separation rate can be significantly improved – due to the reduced air leakage – when using double dust traps, i.e. a timed lock.
The migration from rotary valves to double dust traps can thus normally be completed within a very short period of less than a year. With some products, it is important to simulate almost continuous discharge. Flaps with a cycle of 12 to 14 strokes/min are required here to withstand the extreme operational demands.