To determine if a baghouse is operating properly and to aid in troubleshooting when failures
occur, the operator must monitor certain operating parameters. Routine monitoring of key
parameters, either on a continuous or periodic basis, is imperative for performance evaluation
and problem diagnosis. An adequate baseline must be developed to determine when future
changes in performance occur. Some typical parameters that are monitored are: inlet and outlet
gas temperature (only on units operated above ambient temperature), pressure drop, opacity,
and gas velocity. In addition to these parameters that can be routinely measured, it can be
important to periodically evaluate the chemical composition of the gas stream, including moisture,
acid dew point, and particle loading and size distribution. The following describes how
the above parameters affect performance and the techniques used to measure each. In addition,
there is also some common auxiliary equipment that should be monitored or periodically
checked. These include receiver air pressure, bag tension, fan amperage, and high hopper
level.
Gas Temperature
Gas temperature is important because fabrics are designed to operate within a given range. Exceedances of these fabric temperature limits, even for short periods of time, can weaken or damage the bags. Exposure of the fabric to temperatures above the maximum limits can cause immediate failure due to loss of strength or elongation from melting. Minimum temperatures are related to the
dew point temperature of the gas stream. Operation of the baghouse below these dew point
temperatures can result in moisture or acid condensation and cause bag blinding or chemical
attack of the fabric. Condensation problems are one of the major causes of bag failures.
Temperature measurements are also used to indicate inleakage into the gas stream. Temperature
drops across baghouses can range from 1 to 2 degrees on small units to up to 25
degrees on large baghouses (EPA 1984). The facility must establish an acceptable or normal
operating range. If this range is exceeded, it indicates that a problem is occurring and
needs to be addressed.
To measure temperature, a thermocouple with digital, analog, or strip-chart display is
used. The temperature signals are often tied to an alarm limit indicator to notify the operator
of trouble. Temperature measurements are generally made at the inlet and outlet of the
unit with the inlet being the primary focus.
Pressure Drop
Baghouses are designed to operate within a certain pressure drop range, based on a specific
gas volumetric flow rate. Within this range during normal operation, the pressure
drop fluctuates with the cyclic cleaning process. The average baghouse pressure drop
gradually increases as the filter cake builds on the bags and then takes a step decrease
immediately after the compartment has been cleaned. The pressure drop across the baghouse gives an indication of the resistance to gas flow (drag) and the effectiveness of the
cleaning system. Changes in pressure drop (either gradual or especially sudden) can indicate
the need for maintenance. In addition, changes in the shape of the cleaning cycle pressure
drop curve (i.e. pressure drop vs. cleaning cycle time) can also indicate the need for
maintenance or change in system operation.
At a minimum the pressure differential across the baghouse should be continuously
recorded by the operator. Static pressure taps are connected to a transmitter/recorder so
that the differential can be monitored preferably from a central control room. The most
common problem with measuring the pressure drop is plugging of the static tap lines. The
pressure sensors should be shielded from direct impact of the dirty gas stream, and a
means to clean the lines should also be installed.
Opacity
Opacity is a measurement of the amount of light scattering that occurs because of the particles
in a gas stream. Although opacity is not a direct measurement of particle concentration,
it is a very good indicator of the amount of dust leaving the baghouse, and thus
provides a performance measure. Once a unit is operating at normal conditions, the opacity
value for the system should be maintained within a narrow range. A continued elevated
opacity level indicates operating problems, such as bag failures. The opacity monitor (also
called transmissometer) can be used to identify the problem area. For multicompartment
baghouses, each compartment can be isolated to identify the compartment where problems
are occurring.
There are a number of vendors who sell continuous opacity monitoring systems. Many of
these monitors are double-pass opacity monitors where the light source is on one side of
the stack while the reflector is on the other side of the stack. Continuous opacity monitoring
systems provide continuous feedback on a real-time basis and for set averaging periods.
Coupled with a strip-chart or data acquisition system, they provide excellent trend
information on baghouse operation. See Jahnke (1993) for more information on this topic.
Some facilities use broken bag detectors that give a relative indication of the dust loading
leaving the baghouse. Broken bag detectors are single-pass opacity monitors where the
light source is on one side of the stack and the detector is on the other side (there is no
reflector). These are less expensive than double-pass opacity monitors and don’t meet the
EPA performance specifications for opacity monitors.
Gas Volumetric Flow Rate
baghouses are designed to accommodate a range of gas flows. If
gas flow rates increase, the operating pressure drop and air-to-cloth ratio will increase.
This in effect means that the baghouse has to work harder and the bag life can be shortened
due to more frequent cleaning and high particle velocity.
Presently most sources do not continuously measure gas flow rates. Gas flow rates are
generally only measured during emission compliance testing or when there is a perceived
problem.Manual pitot tube traverses are normally used to measure gas flow . Because of new technologies and regulations,
some of the larger sources are beginning to install continuous flow measurement
systems. Multipoint pitot devices are being used to continuously measure gas velocity.These devices generally consist of two tubes (in the same structure) with two sets of holes;
one to sense the impact pressure and the other to measure static pressure. These devices
must be calibrated to the individual stacks where they are installed.
Composition of Flue Gas
Baghouses are designed based on the composition of the flue gas they treat. Important flue
gas parameters are moisture level, acid dew point, particle size, and concentration. If the
operating temperature falls below the condensation point, either during startup/shutdown
or normal operation, blinding of the bags can occur. Similarly, if the temperature falls
below the acid dew point, there is a substantial risk of corrosion. These parameters are
generally only measured during a diagnostic test or emission compliance stack test. However,
it is important to identify both of these minimum temperature points and have operating
procedures for startup/shutdown that minimize the condensation potential. Particle
size distribution and loading must be considered during design and also during operation;
however, within certain limits (± 10 to 20%) changes in these parameters do not seriously
affect baghouse efficiency (EPA 1984). Unless there is a defined problem such as bag
blinding or abrasion from particles these parameters are rarely measured.
Typical monitoring devices are listed in Table.
occur, the operator must monitor certain operating parameters. Routine monitoring of key
parameters, either on a continuous or periodic basis, is imperative for performance evaluation
and problem diagnosis. An adequate baseline must be developed to determine when future
changes in performance occur. Some typical parameters that are monitored are: inlet and outlet
gas temperature (only on units operated above ambient temperature), pressure drop, opacity,
and gas velocity. In addition to these parameters that can be routinely measured, it can be
important to periodically evaluate the chemical composition of the gas stream, including moisture,
acid dew point, and particle loading and size distribution. The following describes how
the above parameters affect performance and the techniques used to measure each. In addition,
there is also some common auxiliary equipment that should be monitored or periodically
checked. These include receiver air pressure, bag tension, fan amperage, and high hopper
level.
Gas Temperature
Gas temperature is important because fabrics are designed to operate within a given range. Exceedances of these fabric temperature limits, even for short periods of time, can weaken or damage the bags. Exposure of the fabric to temperatures above the maximum limits can cause immediate failure due to loss of strength or elongation from melting. Minimum temperatures are related to the
dew point temperature of the gas stream. Operation of the baghouse below these dew point
temperatures can result in moisture or acid condensation and cause bag blinding or chemical
attack of the fabric. Condensation problems are one of the major causes of bag failures.
Temperature measurements are also used to indicate inleakage into the gas stream. Temperature
drops across baghouses can range from 1 to 2 degrees on small units to up to 25
degrees on large baghouses (EPA 1984). The facility must establish an acceptable or normal
operating range. If this range is exceeded, it indicates that a problem is occurring and
needs to be addressed.
To measure temperature, a thermocouple with digital, analog, or strip-chart display is
used. The temperature signals are often tied to an alarm limit indicator to notify the operator
of trouble. Temperature measurements are generally made at the inlet and outlet of the
unit with the inlet being the primary focus.
Pressure Drop
Baghouses are designed to operate within a certain pressure drop range, based on a specific
gas volumetric flow rate. Within this range during normal operation, the pressure
drop fluctuates with the cyclic cleaning process. The average baghouse pressure drop
gradually increases as the filter cake builds on the bags and then takes a step decrease
immediately after the compartment has been cleaned. The pressure drop across the baghouse gives an indication of the resistance to gas flow (drag) and the effectiveness of the
cleaning system. Changes in pressure drop (either gradual or especially sudden) can indicate
the need for maintenance. In addition, changes in the shape of the cleaning cycle pressure
drop curve (i.e. pressure drop vs. cleaning cycle time) can also indicate the need for
maintenance or change in system operation.
At a minimum the pressure differential across the baghouse should be continuously
recorded by the operator. Static pressure taps are connected to a transmitter/recorder so
that the differential can be monitored preferably from a central control room. The most
common problem with measuring the pressure drop is plugging of the static tap lines. The
pressure sensors should be shielded from direct impact of the dirty gas stream, and a
means to clean the lines should also be installed.
Opacity
Opacity is a measurement of the amount of light scattering that occurs because of the particles
in a gas stream. Although opacity is not a direct measurement of particle concentration,
it is a very good indicator of the amount of dust leaving the baghouse, and thus
provides a performance measure. Once a unit is operating at normal conditions, the opacity
value for the system should be maintained within a narrow range. A continued elevated
opacity level indicates operating problems, such as bag failures. The opacity monitor (also
called transmissometer) can be used to identify the problem area. For multicompartment
baghouses, each compartment can be isolated to identify the compartment where problems
are occurring.
There are a number of vendors who sell continuous opacity monitoring systems. Many of
these monitors are double-pass opacity monitors where the light source is on one side of
the stack while the reflector is on the other side of the stack. Continuous opacity monitoring
systems provide continuous feedback on a real-time basis and for set averaging periods.
Coupled with a strip-chart or data acquisition system, they provide excellent trend
information on baghouse operation. See Jahnke (1993) for more information on this topic.
Some facilities use broken bag detectors that give a relative indication of the dust loading
leaving the baghouse. Broken bag detectors are single-pass opacity monitors where the
light source is on one side of the stack and the detector is on the other side (there is no
reflector). These are less expensive than double-pass opacity monitors and don’t meet the
EPA performance specifications for opacity monitors.
Gas Volumetric Flow Rate
baghouses are designed to accommodate a range of gas flows. If
gas flow rates increase, the operating pressure drop and air-to-cloth ratio will increase.
This in effect means that the baghouse has to work harder and the bag life can be shortened
due to more frequent cleaning and high particle velocity.
Presently most sources do not continuously measure gas flow rates. Gas flow rates are
generally only measured during emission compliance testing or when there is a perceived
problem.Manual pitot tube traverses are normally used to measure gas flow . Because of new technologies and regulations,
some of the larger sources are beginning to install continuous flow measurement
systems. Multipoint pitot devices are being used to continuously measure gas velocity.These devices generally consist of two tubes (in the same structure) with two sets of holes;
one to sense the impact pressure and the other to measure static pressure. These devices
must be calibrated to the individual stacks where they are installed.
Composition of Flue Gas
Baghouses are designed based on the composition of the flue gas they treat. Important flue
gas parameters are moisture level, acid dew point, particle size, and concentration. If the
operating temperature falls below the condensation point, either during startup/shutdown
or normal operation, blinding of the bags can occur. Similarly, if the temperature falls
below the acid dew point, there is a substantial risk of corrosion. These parameters are
generally only measured during a diagnostic test or emission compliance stack test. However,
it is important to identify both of these minimum temperature points and have operating
procedures for startup/shutdown that minimize the condensation potential. Particle
size distribution and loading must be considered during design and also during operation;
however, within certain limits (± 10 to 20%) changes in these parameters do not seriously
affect baghouse efficiency (EPA 1984). Unless there is a defined problem such as bag
blinding or abrasion from particles these parameters are rarely measured.
Typical monitoring devices are listed in Table.
Typical baghouse monitoring devices
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