Page 172 - Root Cause Failure Analysis

160 Root Cause Failure Analysis

Cleaning
As previously indicated, most bag-type filters require a precoating of particulates
before they can effectively remove airborne contaminates. However, particles can
completely block airflow if the filter material becomes overloaded. Therefore, the pri-
mary operating criterion is to maintain the efficiency of the filter media by controlling
the cleaning frequency.

Most systems use a time-sequence to control the cleaning frequency. If the particulate
load entering the baghouse is constant, this approach would be valid. However, the
incoming load generally changes constantly. As a result, the straight time-sequence
methodology does not provide the most efficient mode of operation.
Operators should monitor the differential-pressure gauges that measure the total pres-
sure drop across the filter media. When the differential pressure reaches the maximum
recommended level (data provided by the vendor), the operator should override any
automatic timer controls and initiate the cleaning sequence.

Inspecting and Replacing Filter Media
Filter media used in dust-collection systems are prone to damage and abrasive wear.
Therefore, regular inspection and replacement is needed to ensure continuous, long-
term performance. Any damaged, tom, or improperly sealed bags should be removed
and replaced.

A common problem associated with baghouses is improper installation of filter
media. Therefore, it is important to follow the instructions provided by the vendor. If
the filter bags are not properly installed and sealed, overall efficiency and effective-
ness are significantly reduced.

CYCLONE SEPARATORS
A widely used type of dust-collection equipment is the cyclone separator. A “cyclone”
essentially is a settling chamber in which gravitational acceleration is replaced by cen-
trifugal acceleration. Dust-laden air or gas enters a cylindrical or conical chamber tan-
gentially at one or more points and leaves through a central opening. The dust
particles, by virtue of their inertia, tend to move toward the outside separator wall from
which they are led into a receiver. Under common operating conditions, the centrifugal
separating force or acceleration may range from five times gravity in very large diame-
ter, low-resistance cyclones to 2,500 times gravity in very small, high-resistance units.

Within the range of their performance capabilities, cyclones are one of the least expen-
sive dust-collection systems. Their major limitation is that, unless very small units are
used, efficiency is low for particles smaller than five microns. Although cyclones may
be used to collect particles larger than 200 microns, gravity-settling chambers or sim-
ple inertial separators usually are satisfactory and less subject to abrasion.