With increased local and global attention being
given to the control of air pollution, containment of nuisance and process
dust in all industrial applications is becoming increasingly important.
This calls for the proper design, installation, operation, and
maintenance of dust collection equipment. Since its inception, the
fabric-style dust collector (baghouse) has offered companies the ability to
effectively capture airborne particulate from an air stream. Whether toxic
or not, containment of particulate is necessary to provide a healthy and
clean work environment.
To ensure that bag house equipment functions as designed, an inspection
schedule is recommended, as well as the timely repair and replacement of
damaged or malfunctioning equipment. A routine inspection and maintenance
program will positively impact the equipment's performance and life. The
following is an overview of procedures that can be used to tailor such a
41. Inspection/Maintenance Program A typical program consists of a
schedule for periodic inspections that are performed on a daily, weekly,
monthly, semi-annual, and annual basis. When a baghouse is not periodically
inspected, the effectiveness of its operation can be adversely affected.
Subsequently, the bag house may not meet the outlet emissions specified on
an EPA operating permit.
Figure 1 provides a sample inspection log for baghouses. Logs such as
this may be used as is or modified to fit a specific installation. In either
case, they cover the most important items that should be monitored to
maintain an effective and efficient dust collection system. An added benefit
is the development of an accurate history of operation, should questionable
performance be experienced at a later date.
42. Pressure Drop Pressure drop, or differential pressure, is the amount
of static resistance experienced when operating a positive or negative
pressure bag house. The pressure drop is typically measured across the
filter bags in inches of water column (in. w.c.). Pressure drop is a good
indicator of the amount of dust that has collected on the filter media, and,
if continually monitored and logged, the condition of the bags themselves.
New filter bags have the lowest pressure drop because of the inherent
permeability of the media. As the bags develop a dust cake, some particulate
embed themselves into the interstices of the filter media, and the pressure
drop will increase accordingly. It is the filtering of the airstream through
this accumulated dust cake that provides high efficiency collection of fine
particulate. In fact, the highest efficiency a dust collector can offer is
just before the cleaning mechanism is initiated. However, high differential
pressures can cause bleed thru or blinding of the filter media. Therefore,
it is suggested not to exceed the manufacturer's recommended operating
Keeping a daily log of a baghouse's differential pressure from the time
the filter media is new will provide the opportunity to diagnose problems
that may occur (i.e. an increase in dust emissions, reduced ventilation air
at the dust source, shortened bag life, etc.). Following an initial
seasoning or conditioning period of the filter bags, the pressure drop
should stabilize into a consistent operating range relative to the cleaning
cycle, application, and style of equipment. Therefore, at subsequent bag
changes, this operating range can be predicted. Deviation from this
historical level will alert an operator to investigate the cause of such an
43. Cleaning System Any method used by the equipment to dislodge
accumulated dust cake from the filter media is its cleaning system. These
systems may be reverse air, shaker, or pulse clean. Regardless of the style
of cleaning, it is imperative that this system function properly at all
times. Without an effective cleaning system, dust will continue to build on
the bags. The result will be an increased pressure drop and reduced volume
of ventilation air at the pick-up points. Further, airstream velocities
within the ductwork will decrease and cause drop-out of dust in the ducts.
This may choke the entire system and render it ineffective.
As indicated on the sample inspection logs, cleaning systems require more
than just periodic monitoring. All components of the system should be
regularly inspected and corrections be made in a timely manner. Besides the
items noted on the logs, refer to the original equipment manufacturer's
installation & maintenance manual to include other items specific to their
44. Hopper Discharge The hopper on a baghouse is not to be used for
storage of the collected product, unless originally designed to do so.
Storing material in a hopper can lead to bridging of the dust, or it may set
up as a solid mass, thus requiring considerable labor and down time to
correct the problem. Material buildup, if not discovered in time, can fill a
hopper to its inlet and plug the unit. The hopper should be emptied on a
routine basis. Further, with low-density materials, the airstream may sweep
the dust into the bag section and ruin filter bags and clog the dust
It is strongly recommended that, whatever method is used for material
discharge (rotary valve, screw conveyor/pneumatic conveyor, etc.), it should
be inspected frequently. This inspection also should occur at shutdown and
45. Visible Emissions Any particulate that can be seen discharging from
the exhaust stack is considered visible emissions. These emissions are an
indication that there is a breach in a seal or a broken (torn) filter bag.
In either case, the leak must be found and corrected immediately. Not only
will the emission cause a health concern and damage the property outside the
plant, monetary fines imposed by the local, state, and federal environmental
agencies also may result. In addition, a fan located downstream of the
collector can be damaged from abrasion or become imbalanced if this
condition is not corrected quickly.
The exhaust from the dust collector should be continually monitored and
checked off in the inspection log. Besides visual inspections, one may
consider incorporating a broken bag detector into the clean air ductwork.
Should a bag begin to fail, or there be a leak in the bag seal, the
particles that bypass the media will be detected. Typically, these detectors
use triboelectric or scattered light technologies. These devices can be
wired to an alarm horn, siren, or flashing light bag-house.
46. Exhaust Fan In a dust collection system, an exhaust fan is needed to
accelerate ventilation air from the point of pick-up, through the ductwork
and baghouse filter media, and out the exhaust stack. A fan is selected to
accommodate each application with respect to volume (ACFM, or actual cu ft
of gas per minute) and pressure drop throughout the system. This pressure
drop is calculated by evaluating the static resistance of the bag house, all
ductwork, and pick-up points/hoods.
Should an exhaust fan experience loose or worn belts or an imbalanced
impeller, it will not exhaust the volume of air it was originally designed
to handle. Without adequate ventilation air, a dust collection system will
not operate effectively. Therefore, thorough fan inspections are to be
performed on a semi-annual basis. However, any time unusual vibration,
squealing, or other obvious variances from standard operation is observed,
the original manufacturer should be contacted for evaluation and comment.
47. Filter Media The most important item in a baghouse is the filter
media, because it allows for the accumulation and support of a dust cake.
This dust cake is what provides high filtering efficiencies during
operation. Periodic inspections of the filter bags is mandatory. Inspect the
clean air side of the bag houses for leaks and the bags for tears. Should
pressure drop within a dust collector become extremely high, relative to
historical data, the cause may be excessive dust cake or blinding of the
Excessive dust cake is evident when visually inspecting the filter bags
(when the dust collector is presumed to be clean) and finding them covered
with a layer of the collected dust. Should this occur, one could suspect
that the cleaning system is not functioning properly. However, if the dust
cake has hardened to the bags and will not dislodge easily, the most
probable cause is moisture in the baghouse. Moisture in a dust collector may
have resulted from dew point excursions, high moisture content in the
process gas or in the compressed air supply, or a leak in the collector or
ductwork that allowed water to enter the dust collector.
The other obvious cause of high differential pressure may have been
caused by the blinding of the filter bags. Blinding can occur from improper
start-up conditioning of the filter bags following the previous bag change.
48. Structural Integrity The structural integrity of equipment can not
only affect its performance, but also cause health and housekeeping
concerns, as well as reduce equipment life. An overall inspection should be
done annually. Inspect all welds, joints, and flange seals, as well. Any
leaks in the collector must be sealed either mechanically or by using
silicone caulking. In a negative pressure system, a breach in a seal or weld
will introduce ambient air into the collector. With this air, moisture and
contaminants can find their way into the collector. In a positive pressure
system, dust will blow out of the collector causing housekeeping problems
and a potential health hazard to employees exposed to the dust.
Look for the obvious. Check the structural support members for signs of
fatigue and excessive corrosion. Be certain that all fasteners are in place
and tightly secured, especially on the ladder and access platform. Replace
any missing bolts, clean and reweld any cross bracing or gussets that may
have cracked welds. Look closely at the filter's external walls for
corrosion or signs of bowing. Clean and repaint where necessary. Repair any
holes that may have developed in the dust collector walls or hoppers.
49. Auxiliary Equipment Aside from the baghouse itself, a thorough
inspection of any system will include a check of all miscellaneous
complimentary equipment. Some of these items may include the exhaust fan,
rotary airlock valve, screw conveyor, inlet and/or outlet dampers, etc. It
is very important that any ancillary equipment be added to the inspection
410. Ductwork Another important component in a ventilation system is the
ductwork. If the particulate does not have an opportunity to reach the bag
house, the dust collector will not be able to perform its function. Standard
practices suggest a minimum airstream velocity within any duct of 3,500 fpm,
and between 4,000 and 4,500 fpm for heavier dusts, such as sand. Should the
dust travel at lower than adequate velocities, it will tend to settle and
accumulate in the ducts and choke the system. This restriction of flow will
increase the pressure drop in the system as well as the energy required to
induce the air to move. The result will be reduced ventilation air at the
pick-up points. Periodically inspect the entire length of the duct work for
Start-up procedures Proper start-up procedures will help extend the life
of new filter media in a dust collector. What is generally accepted as
"start-up" procedures is the process designed to intentionally develop a
dust cake on the bags. This is referred to as seasoning, or conditioning,
the filter media.
Seasoning of a collector's filter bags is one of the most important
procedures that a company can perform. In a fabric filter dust collector,
the filter media is used to support a dust cake. A dust cake is the porous
layer of collected particulate that develops during the conditioning period
of new collector bags and following each cleaning cycle. The process can be
accelerated in many installations by introducing a precoat material, such as
agricultural lime, into the system. Commercial precoats also are available.
Following installation of the filter bags and inspection of the related
auxiliary equipment, the exhaust fan can be started. However, it is
extremely important that the new filter bags are not exposed to the full
volume (ACFM) of the fan.
First, close the fan damper (or inlet dampers) to one-half open until the
monitoring gauge reads about 50% to 65% of the manufacturer's recommended
maximum flange-to-flange differential drop. At roughly 75% of the
manufacturer's recommended differential pressure, the cleaning system can be
initiated. Normal operation and periodic cleaning will bring the pressure
drop to a calculable and historically stable level.
Depending on the application, development of this differential pressure
may take a number of hours or even days. This is necessary to ensure that
the new filter media is exposed to low filtering velocities of dust-laden
air. Reducing the volume decreases the airstream's velocity (air-to-cloth
ratio), thus protecting the virgin bags from a high velocity impingement of
dust. Should the bags be exposed to the fan's full volume, fine particles
may embed themselves into the inner fibers of the bags and begin blinding
condition. This also can damage the fibers of the media, reducing the life
of the bags.
With this information as a guide, a maintenance program can be developed
for any dust collection system. However, this is not offered as an
all-inclusive list. Each piece of equipment and application is different,
and each has its own unique components and features. Those unique
characteristics should be accounted for in the maintenance program.