The electrostatic hazard

The presence of static can be detected by instruments that measure the voltage or field strength, or, more crudely, by a gas discharge lamp which will glow if held in the field, or by the use of a wide band radio receiver and listening for the craekling sounds produeed by the field. In severe cases the sparking can be heard and seen. 

Precautions (BS 5958 Code of Practice for control of undesirable static electricity) 

Whenever possible, the hazard should be avoided by preventing the build-up of electrostatic charges. This can be done in the case of conducting material by earthing. The resistance to earth should be not more than 1 megohm, 

It is more difficult to remove the charges from insulating materials. In some cases this may be done by modifying the material to improve its conductivity. For example, flammable liquid fuels can be doped with additive and carbon black can be added to rubber during manufacture to reduce the resistivity of the material. Where this is undesirable, or uneconomic, it may be possible to carry out the process in a humid atmosphere where the water vapour acts as a leakage path to earth. This method is not suitable, however for water-repellent materials. 

Limitation of energy is a method used in electrostatic spraying where a high impedance is inserted in the HV circuit to reduce the potential earth fault current and thus the spark discharge energy to a sufficiently low value to avoid ignition. 

---

The next task in considering the electrostatic hazards associated with a fluidized bed is to consider the specific conditions required for the ignition of suspended dust and powders. It is crucial to cover this subject because... 

Modification of the Test to Measure the Electrostatic Hazard Under Normal Handl ing Conditions , ERDE Rept 22/R/56 (May 1956) 19) R.M.H. Wyatt et al,... 

In " Introduction of Ref 35, t>p 2- 3 tc is stated that spark (See its definition and distinction from arc given on pp 4 -5 and under item Vin of this section) sensitivity testing can provide an important measure of the electrostatic. hazards associated with the handling of primary expls The threshold (or minimum) energy required for initiation is of particular concern. The usual approach.to determine this minimum energy is to discharge a capacitor thru a spark gap in or neat the expl and... 

W. M. Bustin and W. G. Dukek, Electrostatic Hazards in the Petroleum Industry, ]ohn. Wiley Sons, Inc., Chichester, Sussex, P019 lUD, UK, 1983. 

Performing an electrostatic hazard review of the whole plant and all the processes whenever powders and flammable solvents are used... 

Figure 7-78A. Electrical charge induction in tank shell. By permission, Bustln Dukek, Electrostatic Hazards in the Petroieum industry, Research Studies Press, Somerset, England [64].

Bustin, W. M. and Dukek, V/. G., Electrostatic Hazards in the Petroleum Industry, Research Studies Press, Ltd., England, 1983. 

An electrostatic dust ignition can occur when the discharge releases sufficient thermal energy within a sufficiently short period of time and small volume to ignite the suspended dust. Electrostatic ignition is complicated by the fact that there are a number of distinct ESD mechanisms important in electrostatic hazards and hazard abatement (Glor, 1988). 

Here, C0bject is the capacitance and V the voltage between the two metal conductors. The capacitance of typical chargeable components depends on their size, shape, and distance from a grounded surface. For purposes of electrostatic hazard assessment, it is usually convenient to rely upon measured data like that in Table 6. 

Figure 11. Model for electrostatic hazards associated with the accumulation of charged powders, based on Glor (1988).

Exhaust gases leaving the kiln pass through highly efficient air pollution control devices such as baghouse filters or electrostatic precipitators. The high temperatures required to make cement destroy 99.99% or more of the organic hazardous wastes. The content of hydrocarbons... 

Another type of testing, that of electrostatic sensitivity, has been demonstrated in some cases to be more properly a delicate test of the ignitability of the material under localized thermal stress which correlates best to the friction sensitivity of the system under test rather than to electrostatic hazards. On the other hand, electrostatic tests done in the supposed atmosphere above a propellant mixer were reduced in absolute value by more than an order of magnitude when the ammonium perchlorate dust actually present was introduced in the test since this altered the potential path for spark discharge within the system. 

---