2. Rotary valves are commonly fitted at the bottom of vessels handling powdered products, where they serve to control the flow rate of material from the vessel. In addition, they have long been recommended by HSE as explosion chokes where explosible dusts are handled, to prevent an explosion propagating from one vessel to another.
3. Until recently, there has been little information published on the design of such valves for use as explosion chokes. Two recent incidents investigated by FCG inspectors noted that valves with rubber tipped blades had failed to prevent passage of an explosion. Where such valves are fitted the intended level of plant protection may not be achieved.
4. To function successfully as an explosion choke, a rotary valve must be strong enough to withstand an explosion without significant deformation. Rubber edged blades will bend under the influence of a pressure wave and are unlikely to be satisfactory.
5. In addition the valve must prevent the passage of burning particles from one side to another. In the same way that flameproofed enclosures for use in hazardous areas have maximum specified gaps, it has been shown that similar considerations apply to barriers for dust explosions.
6. For rotary valves, the important dimensions are:
1) the clearance between the blades and the casing (the gap width, W);
2) The thickness of the blade tip (the gap length, L); and
3) the minimum number of blades acting as a barrier at any position of the rotor (N).
See the Appendix for diagrams.
7. Recent research published by Ciba-Geigy Central Engineering Department has investigated these parameters for a range of explosible dusts. The dust properties do affect the critical dimensions but for most organic materials, this effect is of little practical importance.
8. The results have been expressed as a nomograph, but for simplicity, combinations of dimensions shown to be satisfactory for cornstarch are given below.
N gap length (L) gap width (W)
6 3 mm 0.9 mm
4 3 mm 0.4 mm
4 10 mm 1.1 mm
2 10 mm 0.2 mm
9. Where rotary valves are recommended under HSW Act s.2 or Factories Act 1961 s.31, the user should either obtain assurance from the manufacturer that the valves have been shown in tests to perform satisfactorily as explosion chokes for the material concerned, or should specify a valve that conforms broadly with the dimensions in para 8.
10. Where rubber tipped blades are used it is particularly important that the user can provide evidence that they will be effective.
24 October 1990
DISC NO: FOD1C\EDITORS\J267\26.9.90
NEW REF: J:\EDITORS\CA1\J267SE90.SAM