Typical High, Normal and Low Resistivity Curves

Evaluating the current and spark rate trends from the inlet to the outlet fields provides
a means of evaluating the general resistivity conditions. Moderate dust resistivity conditions,
under which ESPs work very well, are indicated by low secondary currents in
the inlet field and progressively higher values going toward the outlet. Spark rates
under moderate resistivity are moderate in the inlet fields and decrease to essentially
zero in the outlet field. High resistivity conditions are indicated by low secondary currents
in all of the fields coupled with very high spark rates. Conversely, low resistivity
has very high currents and low spark rates in all the fields.

Figure 1 shows the typical trend lines for moderate (normal) and high resistivity
dusts. As the resistivity goes from moderate to high, the currents decrease dramatically
in all of the fields. This is due to the suppressing effect caused by the strong electrostatic
field created on the dust layer, and to increased electrical sparking. The
decrease in currents is most noticeable in the outlet fields which previously had relatively
high currents. Spark rates increase dramatically during high resistivity. Often
most of the fields will hit the spark rate limits programmed in by the plant operators.
Once the spark rate limit is sensed by the automatic voltage controllers, it no longer
attempts to drive up the voltage. This causes a reduction in the operating voltages of
these fields. The overall impact on the opacity is substantially increased emissions. In
some cases, puffing again occurs during rapping. This is due to reduced capability of
the precipitator fields to collect the slight quantities of particles released during rapping
of high resistivity dust.

Figure 2 shows the typical trend lines for moderate (normal) and low resistivity
dusts in a four-field ESP. The moderate resistivity dust shows a steady increase of current
from the first field to the fourth field, while the secondary current increases rapidly
for all fields when the dust exhibits low resistivity. This effect is especially
noticeable in the inlet fields which previously had the lowest currents. This increase in
current is due simply to the fact that the dust layer’s electrostatic field is too weak to
significantly impede the charging field created by the discharged electrodes. At low
resistivity, the spark rates are generally very low or zero. The voltages in all of the
fields are a little lower than normal since the automatic voltage controllers sense that
the power supply is at its current limit; therefore, the controller does not attempt to
drive the voltage up any further. While the low resistivity conditions persist, there can
be frequent and severe puffs (opacity increase) which occur after each collection plate
rapper activates.

Figure 1. Typical T-R set plots - high resistivity versus moderate (normal)

resistivity

Figure 2. Typical T-R set plots - low resistivity versus moderate (normal)

resistivity

Using the current, voltage, and spark rate plots is a very good way to use readily available
information to evaluate the impossible-to-directly monitor but nevertheless
important resistivity conditions. It is possible to differentiate between problems
caused by mechanical faults in a single field (such as insulator leakage) and resistivity
conditions which inherently affect all of the fields in varying degrees. However, these
trend lines are not a perfect analysis tool for evaluating resistivity. A few precipitators
never display typical electrical trend lines since they have undersized T-R sets, undersized
fields, improperly set automatic voltage controllers, or severe mechanical problems
affecting most of the fields.