Page 570 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

536 Applied Process Design for Chemical and Petrochemical Plants

an arcing condition. These latter can certainly ignite dusts Figures 7-78A and 7-78B illustrate that when a charged
and vapors, but usually design efforts are made to prop- fuel enters a metal tank, it attracts a charge of equal mag-
erly insulate to prevent occurrences. Electrical codes aid nitude but opposite sign to the inside surface of the tank
in this design (see Chapter 14, Volume 3). shell. At the same time a charge of the same magnitude
The use of intrinsically safe electrical and instrumen- and sign as the charge in the fuel is repelled to the out-
tation equipment in appropriately designed environ- side surface of the tank. Note that Bustin [64] states the
ments can guard against many electrically related dis- choice of signs on the fuel is arbitrary.
charges. Reference should be made to authoritative If the tank is grounded, the repelled charge is neutral-
books on this subject. ized; hence, the tank stays at zero voltage (see Figure 7-
Static electricity is caused by the contact and separation 78B). The charge from this neutralization current is equal
of a good conductor material from a poor or nonconduc- in magnitude and sign to the charge carried into the tank
tor material or the separation of two nonconductor ( or by the liquid. The ammeter is exactly equal to the "stream-
poor) materials. Static electricity is the accumulation of ing" current entering the tank [64].
bound charges of the same sign and are prevented from Inside the tank a voltage difference exists between the
reuniting quickly with charges of the opposite sign. This negative charge on the shell and the positive charge in
electrostatic phenomena is often characterized by the the liquid. For a grounded tank, the voltage is zero at the
presence of high potential but small currents or charge shell. Note that grounding a closed metal tank has no
quantities [63]. effect on the voltage difference between the two parts in
When two objects/particles separate after being in con-
the tank. Grounding a metal tank does not alter the risk
tact ( equal charges), one particle loses electrons and of an electrostatic spark being generated within, but it
becomes positively charged while the other gains elec-
trons and becomes negatively charged. does eliminate the possibility of an external spark dis-
Low humidity allows the resistance of insulating sur- charge from the tank to ground.
faces to increase to a very high level, and this allows elec- Static electricity is classed as (a) spark discharges and
trostatic charge separation and accumulation to occur (b) corona discharges. The spark is a quick, instantaneous
[64]. Static electricity is usually present in some degree in release of charge across an air gap from one "electrode
many industrial situations, but ignitions caused by static source" to another. The corona is a discharge that branch-
discharge are preventable. The charging process arises at es in a diffuse manner, spreading over a large area of a
an interface between dissimilar materials, that is, between poor conductor or ending in space [64]. The current is
hydrocarbons and metal or hydrocarbon and water [64]. weaker (less) from a corona than a spark [64]. For a flam-
The charge separation process occurs al the molecular mable mixture to ignite, the electrical discharge must
level, but does not occur while the materials are in con- release sufficient minimum energy to allow ignition to
tact. When the charges are separated by moving the mate- take place, and this minimum energy varies between flam-
rials apart, the voltage potential rises. In a pipeline there mable hydrocarbons and between dusts.
is a "streaming current" established by charges off the To avoid electrostatic discharging or even charging,
inner pipe wall being carried in the fluid by the flow. the following list of conditions suggested by Haase [63]
In storage or process tanks, a charge generation can should be considered:
occur if a liquid enters above the liquid surface by the
spraying or splashing of the liquid and a charged mist may • electrostatic grounding of all conducting surfaces
form [64] and the bulk liquid will become charged. • increasing the conductivity of the materials

+

+ - - +
++++++ ++++++
+- +++++ -+ +++++
++ ++
- +
+ '---,+-....,.+-..,..+-+,.......�

Figure 7-78A. Electrical charge induction in tank shell. By permis- Figure 7-788. Electrical induced positive charge, grounded tank. By
sion, Bustin & Dukek, Electrostatic Hazards in the Petroleum Indus- permission, Bustin & Dukek, Electrostatic Hazards in the Petroleum
try; Research Studies Press, Somerset, England [64]. Industry; Research Studies Press, Somerset, England [64].