Dust Accumulation

There are three primary causes of dust accumulation on electrodes:
• Inadequate rapping system
• Stickydust
• Operation at temperatures below the dew point level
The usual cause for buildup of dust on the collection plates or discharge wires is failure of
the rapping system or an inadequate rapping system. The rapping system must provide
sufficient force to dislodge the dust without damaging the ESP or causing excessive reentrainment.
The failure of one or two isolated rappers does not usually degrade ESP performance
significantly. The failure of an entire rapper control system or all the rappers in one
field, however, can cause a noticeable decrease in ESP performance, particularly with
high-resistivity dust. Therefore, rapper operation should be checked at least once per day,
or perhaps even once per shift. A convenient time to make this check is during routine T-R
set readings.
Rapper operation may be difficult to check on some ESPs because the time periods
between rapper activation can range from 1 to 8 hours on the outlet field. One method of
checking rapper operation involves installing a maintenance-check cycle that allows a
check of all rappers in 2 to 5 minutes by following a simple rapping pattern. The cycle is
activated by plant personnel, who interrupt the normal rapping cycle and note any rappers
that fail to operate. After the check cycle, the rappers resume their normal operation.
Maintenance of rapper operation is important to optimum ESP performance.
Excessive dust buildup also may result from sticky dusts or operation at gas dew point
conditions. In some cases, the dusts may be removed by increasing the temperature, but in
many cases the ESP must be entered and washed out. If sticky particulates are expected
(such as tars and asphalts), a wet-wall ESP is usually used because problems can occur
when large quantities of sticky particles enter a dry ESP.
Sticky particulates can also become a problem when the flue gas temperature falls below
the dew point level. Although acid dew point is usually of greater concern in most applications,
moisture dew point is important, too. When moisture dew point conditions are
reached, liquid droplets tend to form that can bind the particulate to the plate and wire.
These conditions also accelerate corrosion. Carryover of water droplets or excessive moisture
can also cause this problem (e.g., improper atomization of water in spray cooling of
the gas or failure of a waterwall or economizer tube in a boiler). In some instances the dust
layer that has built up can be removed by increasing the intensity and frequency of the rapping
while raising the temperature to "dry out" the dust layer. In most cases, however, it is necessary to shutdown the unit and wash out or "chisel out" the buildup to clean the plates.
Localized problems can occur where inleakage causes localized decreases in gas temperature.
In an operating ESP, differences in the V-I curves can be used to evaluate if a dust buildup
problem exists. Buildup of material on the discharge electrodes often means an increase in
voltage to maintain a given operating current. The effect of dust buildup on discharge electrodes
is usually equivalent to increasing the effective wire diameter. Since the corona
starting voltage is strongly a function of wire diameter, the corona starting voltage tends to
increase and the whole V-I curve tends to shift to the right (see Figure ). Sparking tends
to occur at about the same voltage as it does without dust buildup, unless resistivity is
high. This effect on corona starting voltage is usually more pronounced when straight
wires are uniformly coated with a heavy dust, and less pronounced on barbed wires and
rigid electrodes or when the dust layer is not uniform. Barbed wires and rigid electrodes
tend to keep the "points" relatively clean and to maintain a small effective wire diameter
and, therefore, a low corona starting voltage. Nevertheless, a higher voltage would still be
required to spread the corona discharge over the wire when dust buildup occurs. Thus,
buildup on the discharge electrodes would still be characterized by a higher voltage to
maintain a given current level.

V-I curve for a field with excessive wire

buildup