Safety Systems

Andreas Reusch, Warex Valve GmbH, describes explosion protection systems for use in bulk materials handling plants.

Explosions can occur in closed systems if a potentially explosive atmosphere comes about in a rnixture once a specific concentration is exceeded. This usually occurs through the distribution of combustible dusts with grain sizes of ~0.5 mm. High demands are placed not only on safety device manufacturers but also on plant owners to avoid such explosions. The demands have been specified with the introduction of a standardisation for explosion protection, which began with EN 500 14 and, since 2003, has been stipulated by Guidelines 94/9/EC (ATEX 95) for manufacturers of devices and safety systerns for conventional application in potentially explosive areas, i.e. 99/92/EG (ATEX 137) for owners of electrical facilities with potentially explosive atmospheres.

Explosive areas are assigned into zones 20 to 22 according to the probability of explosive atmosphere development due to flammable dusts. The designation is based on an incidence that is documented as being "constant," "occasional“ or "seldom." The substance-specific properties of the dusts, like ignition temperature, ignition energy and maximum explosion pressure should be taken into account in the assessment of potentially explosive areas. # Additionally, the subdivision into the appropriate St 1 to St 3 dust explosion classes should be considered, including the maximum rate of pressure rise (K* values) 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 of the GS series (shock pressure resistant and fiame tight proofed), an appropriate surge drum, as well as an EC type-examination tested valve activation.
This activation should ensure that one valve is always shut, since flame tightness is only guaranteed when the valve disk is closed and the seal seat intact.

Pneumatic locking device for functional security
To ensure the requisite functional security in this regard, a special pneumatic locking mechanism has been developed that, in combination with a preusion-adjustment end swirch unit (switch hysteresis lower than l", i.e. 2"). assures that the closed state of the respective shut-off valve is easily recognised and that only one valve can be opened safely. This locking mechanism operates independently of the electrical signals of the PLC, thereby rendering a safety-related consideration of the electrical control system unnecessary.

A safety-relevant control could certainly be irnplemented as an alternative - the expenditure for realising a comparable functional assurance, however, would be disproportionately higher. The shut-off valves of 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 explosions. They are alsodetonation puncture-proof against organic particles of dust explosion classes St 1 and St 2, as well as metallic particles of dust explosion class St 3, whose detonation behaviour does not exceed that of aluminium particles with a K„ value of 500 bar . m/s.
The further limits of applicationare characterised by an explosion shock pressure resistance of 10 bar and a nominal range of DN 50 to DN 500. Compared to conventionally used rotary valves, ernphasis is placed here on suitability for gas and for St 3 particles. Despite this applicability, which is certainly quite universal, the standard series is unfortunately not up toeverv ~ractical demand. Hence one of the rereauisites I. reads: temperature up to 150 C, explosion shock pressure resistance 13 bar, suitable for K, value from up to 800 bar . m/s. This is what gave rise to the DKZ 103 GS-St 3 series, which was especially developed for metal particles not exceeding the expIosion behaviour of aluminium particles with K,, values of up to 1000 bar . m/s.
The further limits of applicability in this respect are: explosion shock pressure resistance up to 14 bar, temperature up to 150 "C, nominal range DN 50 to DN 200. In conformity with FDA A customer requirementgave rise to the development of the series DKZ 103 GS-0.3 up to 200 "C. Here, the temperature of 200 'C was problematic. The explosion pressure was reduced to 0.2 bar by blow-out disks. The nominal range begins here at DN 50 and ends at DN 250. The seals are normally made in a conductible (i.e. dischargeable) design inside the shut-off valves. These qualities are determined by the conductive sootand in the past were not in conformity with FDA. In the rneantime, special qualities are available that are conductible and in conformity with FDA.
One particularity concerns particles with a minimum ignition energy greater than 3 mJ. The implernentation of non- conductible seals is permitted here. Consequently, white qualities in conformity with FDA can also be implemented here. The qualitative design of seals depends, among other things, on the operating conditions of these safety systems. The technology has asserted itself over the years through successful implementation, and hs oben proven to be the better solution vis-3-vis mechanical devices such as, for example, rotary valves, especidy in the low or under pressure area. A frequently arising case in operation concerns the discharge from filters and air classifiers. In particular it is when implementing lock systems in the field of vacuums that this task is not always to be solved satisfactorily with conventional rotary valves. Tests by different Users have shown that the separation rate when using double dust traps, in a timed lock - due to lower air leakage, can be clearly improved. In this way one can also reckon withthe subsequent conversion from rotary valves to double dust traps within very short running periods of less than one year.
For some products it is important to simulate a practically continuous discharge. Here, flaps with cycles of 12 to 14 strokesJmin are exposed to the greatest operational demands.

Decisive facts

For users
Douple dust traps have the following advantageous unique features:
• Shock pressure resistance, flame tight proofed up to 14 bar.
• Suitible for K, value up to 100 bar. m/s.
• Applicable up to 200 °C.
• Applicable also for gases.
• Reduced air leakage in operations.
• No air leakage when closed

Safety system
• Type-examination testef by Dekra-Exam GmbH, with autonomous locking mechanism, no safety related PLC required.
• Also for metal particcles, for K, value up to 1000 bar. m/s.