Page 136 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 109

25 weight% NH4H2P04,35 weight% NaC1, and 80 weight% CaCO, were required to sup-
press flame propagation.
As already mentioned, the new powdered suppressant Dessicarb (>98.5% NaHCO,)
so far has proven the most effective agent for suppressing aluminum dust explosions.
Furthermore, this material is suitable for use even in the food industry. It is soluble in
water and can therefore be removed effectively by water only. Recently, superheated
steam (water at >180°C) has also been used as a nonpolluting suppressant. Further, more
recent works on automatic suppression are reviewed in Section 9.3.7.6 in Chapter 9.

1.4.8
CONTROL AND lNTERLOCKlNG SYSTEMS TO PREVENT AND
MITIGATE DUST EXPLOSIONS IN INTEGRATED PROCESS PLANTS

1.4.8.3
Overview

The subject has been discussed in two papers by Faber (1985, 1989b). A wide range of
sensors for automatic measurement of a number of physical and chemical process vari-
ables are in use. Microprocessor technology has made it simple to utilize the signals from
the sensors for control and interlocking purposes in a variety of ways.
The variety of process variables measured or detected includes
@ Rotational speed, position, and translatory motion of mechanical objects, level of
dusts and powders in silos, filter hoppers, and so forth.
0 Temperaturein powder and dust deposits,bearings and electricalmotors, and gas flows.
0 Gas pressure in process equipment and connecting ducts.
0 Concentration of specific components in gases, such as oxygen in inert atmospheres
and carbon monoxide in the case of self-heating.
0 Presence of flames and hot gases.
0 Concentration of dust suspended in a gas.
0 Simple, digital quantities, such as whether an explosion vent door has opened or
remains closed.
A comprehensive account of physical and chemical principles used for measuring such
quantities and instruments using these principles has been given by Bentley (1988).
Faber (1989b) mentioned three objectives for monitoring process variables:

0 Normal process control.
e Warning in case of abnormal process conditions.
0 Triggering and control of measures for mitigating hazardous process conditions, such
as dust explosions.

1.4.8.2
A Practical Example

Faber (1985,1989b) used the plant for grinding and drying of coal shown in Figures 1.128
and 1.129as an example. Such plants produce the fuel for pulverized-coalfired power plants.