Some ESPs operate for 10 to 15 years without experiencing a single wire breakage.
Whereas others experience severe wire breakage problems causing one or more sections to
be out of service nearly every day of operation. Much time and effort have been expended to determine the causes of wire breakage. One of the advantages of rigid-frame and rigid electrode
ESPs is their use of shorter wires or no wires at all. Although most new ESPs
have either rigid frames or rigid electrodes, and some weighted-wire systems have been
retrofitted to rigid electrodes, the most common ESP in service today is still the weighted wire.
Wires usually fail in one of three areas: at the top of the wire, at the bottom of the wire,
and wherever misalignment or slack wires reduce the clearance between the wire and
plate. Wire failure may be due to electrical erosion, mechanical erosion, corrosion, or
some combination of these. When wire failures occur, they usually short-out the field
where they are located. In some cases, they may short-out an adjacent field as well. Thus,
the failure of one wire can cause the loss of particle collection in an entire field or bus section.
In some smaller ESP applications, this can represent one-third to one-half of the
charging/collecting area and thus substantially limit ESP performance. One of the advantages
of higher sectionalization is that wire failure is confined to smaller areas so overall
ESP performance does not suffer as much. Some ESPs are designed to meet emission
standards with some percentage of the ESP de-energized, whereas others may not have
any margin to cover downtime. Because they receive and remove the greatest percentage
of particulate matter, inlet fields are usually more important to ESP operation than outlet
fields.
Electrical erosion is caused by excessive sparking. Sparking usually occurs at points
where there is close clearance within a field due to a warped plate, misaligned guidance
frames, or bowed wires. The maximum operating voltage is usually limited by these close
tolerance areas because the spark-over voltage depends on the distance between the wire
and the plate. The smaller the distance between the wire and plate, the lower the sparkover
voltage. Under normal circumstances random sparking does little damage to the ESP.
During sparking, most of the power supplied to energize the field is directed to the location
of the spark, and the voltage field around the remaining wires collapses. The considerable
quantity of energy available during the spark is usually sufficient to vaporize a
small quantity of metal. When sparking continues to occur at the same location, the wire
usually "necks down" because of electrical erosion until it is unable to withstand the tension
and breaks. Misalignment of the discharge electrodes relative to the plates increases
the potential for broken wires, decreases the operating voltage and current because of
sparking, and decreases the performance potential of that field in the ESP.
Although the breakage of wires at the top and bottom where the wire passes through the
field can be aggravated by misalignment, the distortion of the electrical field at the edges
of the plate tends to be the cause of breakage. This distortion of the field, which occurs
where the wire passes the end of the plate, tends to promote sparking and gradual electrical
erosion of the wires.
Design faults and the failure to maintain alignment generally contribute to mechanical
erosion (or wear) of the wire. In some designs, the lower guide frame guides the wires or
their weight hooks (not the weights themselves) into alignment with the plates. When
alignment is good, the guide frame or grid allows the wires or weight hooks to float freely
within their respective openings. When the position of the wire guide frame shifts, however,
the wire or weight hook rubs the wire frame within the particulate-laden gas stream.
Failures of this type usually result from a combination of mechanical and electrical erosion.
Corrosion may also contribute to this failure. Microsparking action between the guide frame and the wire or weight hook apparently causes the electrical erosion. The
same type of failure also can occur in some rigid frame designs where the wires ride in the
frame.
Another mechanical failure that sometimes occurs involves crossed wires. When replacing
a wire, maintenance personnel must make sure that the replacement wire does not
cross another wire. Eventually, the resulting wearing action breaks one or both wires. If
one of the wires does survive, it is usually worn down enough to promote greater sparking
at the point of contact until it finally does break. Any wires that are found to be exceptionally
long and slack should be replaced; they should not be crossed with another wire to
achieve the desired length.
Corrosion of the wires can also lead to wire failures. Corrosion, an electrochemical reaction,
can occur for several reasons, the most common being acid dew point.When the rate
of corrosion is slow and generally spread throughout the ESP, it may not lead to a single
wire failure for 5 to 10 years.When the rate of corrosion is high because of long periods of
operating the ESP below the acid dew point, failures are frequent. In these cases the corrosion
problem is more likely to be a localized one (e.g., in places where cooling of the gas
stream occurs, such as inleakage points and the walls of the ESP). Corrosion-related wire
failures can also be aggravated by startup-shutdown procedures that allow the gas streams
to pass through the dew point many times. Facilities have mainly experienced wire breakage
problems during the initial process shakedown period when the process operation may
not be continuous. Once steady operation has been achieved, wire breakage problems tend
to diminish at most plants.
Wire crimping is another cause of wire failure. Crimps usually occur at the top and bottom
of the wires where they attach to the upper wire frame or bottle weight; however, a
crimp may occur at any point along the wire. Because a crimp creates a residual stress
point, all three mechanisms (electrical erosion, mechanical erosion, and corrosion) may be
at work in this situation. A crimp can:
1. Distort the electric field along the wire and promote sparking;
2. Mechanically weaken the wire and make it thinner;
3. Subject the wire to a stress corrosion failure (materials under stress tend to corrode
more rapidly than those not under stress).
Wire failure should not be a severe maintenance problem or operating limitation in a welldesigned
ESP. Excessive wire failures are usually a symptom of a more fundamental problem.
Plant personnel should maintain records of wire failure locations. Although ESP performance
will generally not suffer with up to approximately 10% of the wires removed,
these records should be maintained to help avoid a condition in which entire gas lanes may
be de-energized. Improved sectionalization helps to minimize the effect of a broken wire
on ESP performance, but performance usually begins to suffer when a large percentage of
the ESP fields are de-energized.
Whereas others experience severe wire breakage problems causing one or more sections to
be out of service nearly every day of operation. Much time and effort have been expended to determine the causes of wire breakage. One of the advantages of rigid-frame and rigid electrode
ESPs is their use of shorter wires or no wires at all. Although most new ESPs
have either rigid frames or rigid electrodes, and some weighted-wire systems have been
retrofitted to rigid electrodes, the most common ESP in service today is still the weighted wire.
Wires usually fail in one of three areas: at the top of the wire, at the bottom of the wire,
and wherever misalignment or slack wires reduce the clearance between the wire and
plate. Wire failure may be due to electrical erosion, mechanical erosion, corrosion, or
some combination of these. When wire failures occur, they usually short-out the field
where they are located. In some cases, they may short-out an adjacent field as well. Thus,
the failure of one wire can cause the loss of particle collection in an entire field or bus section.
In some smaller ESP applications, this can represent one-third to one-half of the
charging/collecting area and thus substantially limit ESP performance. One of the advantages
of higher sectionalization is that wire failure is confined to smaller areas so overall
ESP performance does not suffer as much. Some ESPs are designed to meet emission
standards with some percentage of the ESP de-energized, whereas others may not have
any margin to cover downtime. Because they receive and remove the greatest percentage
of particulate matter, inlet fields are usually more important to ESP operation than outlet
fields.
Electrical erosion is caused by excessive sparking. Sparking usually occurs at points
where there is close clearance within a field due to a warped plate, misaligned guidance
frames, or bowed wires. The maximum operating voltage is usually limited by these close
tolerance areas because the spark-over voltage depends on the distance between the wire
and the plate. The smaller the distance between the wire and plate, the lower the sparkover
voltage. Under normal circumstances random sparking does little damage to the ESP.
During sparking, most of the power supplied to energize the field is directed to the location
of the spark, and the voltage field around the remaining wires collapses. The considerable
quantity of energy available during the spark is usually sufficient to vaporize a
small quantity of metal. When sparking continues to occur at the same location, the wire
usually "necks down" because of electrical erosion until it is unable to withstand the tension
and breaks. Misalignment of the discharge electrodes relative to the plates increases
the potential for broken wires, decreases the operating voltage and current because of
sparking, and decreases the performance potential of that field in the ESP.
Although the breakage of wires at the top and bottom where the wire passes through the
field can be aggravated by misalignment, the distortion of the electrical field at the edges
of the plate tends to be the cause of breakage. This distortion of the field, which occurs
where the wire passes the end of the plate, tends to promote sparking and gradual electrical
erosion of the wires.
Design faults and the failure to maintain alignment generally contribute to mechanical
erosion (or wear) of the wire. In some designs, the lower guide frame guides the wires or
their weight hooks (not the weights themselves) into alignment with the plates. When
alignment is good, the guide frame or grid allows the wires or weight hooks to float freely
within their respective openings. When the position of the wire guide frame shifts, however,
the wire or weight hook rubs the wire frame within the particulate-laden gas stream.
Failures of this type usually result from a combination of mechanical and electrical erosion.
Corrosion may also contribute to this failure. Microsparking action between the guide frame and the wire or weight hook apparently causes the electrical erosion. The
same type of failure also can occur in some rigid frame designs where the wires ride in the
frame.
Another mechanical failure that sometimes occurs involves crossed wires. When replacing
a wire, maintenance personnel must make sure that the replacement wire does not
cross another wire. Eventually, the resulting wearing action breaks one or both wires. If
one of the wires does survive, it is usually worn down enough to promote greater sparking
at the point of contact until it finally does break. Any wires that are found to be exceptionally
long and slack should be replaced; they should not be crossed with another wire to
achieve the desired length.
Corrosion of the wires can also lead to wire failures. Corrosion, an electrochemical reaction,
can occur for several reasons, the most common being acid dew point.When the rate
of corrosion is slow and generally spread throughout the ESP, it may not lead to a single
wire failure for 5 to 10 years.When the rate of corrosion is high because of long periods of
operating the ESP below the acid dew point, failures are frequent. In these cases the corrosion
problem is more likely to be a localized one (e.g., in places where cooling of the gas
stream occurs, such as inleakage points and the walls of the ESP). Corrosion-related wire
failures can also be aggravated by startup-shutdown procedures that allow the gas streams
to pass through the dew point many times. Facilities have mainly experienced wire breakage
problems during the initial process shakedown period when the process operation may
not be continuous. Once steady operation has been achieved, wire breakage problems tend
to diminish at most plants.
Wire crimping is another cause of wire failure. Crimps usually occur at the top and bottom
of the wires where they attach to the upper wire frame or bottle weight; however, a
crimp may occur at any point along the wire. Because a crimp creates a residual stress
point, all three mechanisms (electrical erosion, mechanical erosion, and corrosion) may be
at work in this situation. A crimp can:
1. Distort the electric field along the wire and promote sparking;
2. Mechanically weaken the wire and make it thinner;
3. Subject the wire to a stress corrosion failure (materials under stress tend to corrode
more rapidly than those not under stress).
Wire failure should not be a severe maintenance problem or operating limitation in a welldesigned
ESP. Excessive wire failures are usually a symptom of a more fundamental problem.
Plant personnel should maintain records of wire failure locations. Although ESP performance
will generally not suffer with up to approximately 10% of the wires removed,
these records should be maintained to help avoid a condition in which entire gas lanes may
be de-energized. Improved sectionalization helps to minimize the effect of a broken wire
on ESP performance, but performance usually begins to suffer when a large percentage of
the ESP fields are de-energized.
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