Page 571 - Dust Explosions in the Process Industries

538 Dust Explosions in the Process Industries

Prior to initiation of the fully automated test, the 1 m3 vessel is partially evacuated to
compensate for the supply of air during the dust injection process, which constitutes the
first step of the automatic test sequence. When all the explosible dust has been injected
into the test vessel and atmospheric pressure restored, the explosible dust cloud is ignited
after a predetermined delay. The pressure detector of the suppression system under test
has been preset at a given trigger level PA, and when this explosion pressure is reached,
suppressant injection starts. The efficacy of the suppression is reflected by the magni-
tude of the peak pressure Prd.
By varying the trigger level PA of the pressure detector and the K,, value of the dust,
the efficacy of the specific suppression system under test can be assessed for a range of
explosible cloud conditions.
The standard test method is unsuitable for predicting the performance of suppression sys-
tems if the industrial enclosure to be protected has one or more of the following features:
Vessel aspect ratio greater than 2: 1.
Partially vented vessels.
Container fitted with fixed or mobile apparatus that could impede the distribution of
suppressant.
Operating pressures and temperatures substantially higher or lower than normal atmo-
spheric conditions.
High levels of turbulence or dust or powder throughput.
Vessel volumes substantially greater or smaller than those used in the efficacy test.
Figure 7.63 shows a test arrangement relevant for testing suppression of volumes
large; than 1 m3. Design methodsfor systems for suppression ofvolumes of up to 250
m3 is discussed in Section 1.4.7.

larger thai 1 rn3 '(kourtesy ;f Fike Corporation,
United States).