TM 5-810-15
determined from the average of these two mea-
particulate emissions. A beam of light is projected
surements. The velocity of the flue gas stream is
across the flue gas stream. A measurement detector
determined from the difference between the mea-
registers variations in the light transmittance caused
surements. An input signal from the plant barome-
by the amount of particulate in the flue gas.
f. Data acquisition systems. Data acquisition
ter can be provided so that flow rate can be
calculated in standard cubic feet per minute. Auto-
systems (DAS) typically consist of personal
matic zero checks of ultrasonic flow monitors are
computers (PC). A typical system includes a central
conducted by sending successive pulses in the same
processing unit (CPU), hard disk drive, a floppy
direction. Span checks are conducted by again
disk drive, a keyboard, a cathode ray tube (CRT)
firing successive pulses in the same direction, but
or TV screen and a printer. Serial ports and
with a time delay between the pulses which
required software are included to accept the input
represents a specific flue gas flow velocity.
signals from the monitoring equipment. The hard
(2) Thermal flow rate monitors. Thermal
disk drive provides magnetic storage of data and
flow monitors depend on temperature
allows quick access for rapid calculation. The
measurements and thermal properties of the flue
floppy disk drive allows storage of years of
gas. There are two types of thermal measurement.
historical data in more than one remote location
One type measures the temperature difference
which decreases the risk of loosing this information
between two similar resistance temperature devices
while at the same time provides rapid regeneration
(RTD), one is heated at a constant rate and the
of past reports. The printer provides hard copy of
other is unheated. The temperature difference will
all data while the keyboard and CRT allow
be a function of the velocity of the flue gas. The
operator interface. The DAS performs several
higher the velocity, the greater the cooling effect,
tasks. Signals from the monitors must be
and hence the smaller the temperature difference.
interpreted and stored. This data is stored in the
The other type of thermal probe varies the current
form of ASCII files. A continuous readout of
to the heated element as necessary to maintain a
emissions in the required measurement units is
constant temperature difference. The higher the
produced. The DAS performs monitor calibration
velocity of the flue gas, the greater the heat rate
errors and bias adjustments. Missing data
required to maintain the temperature differential.
procedures are also computed and recorded by the
Zero and span checks of these devices require their
DAS. Required reports are also generated by the
removal from service. Techniques for conducting
DAS.
g. Regulatory requirements. The regulations in-
automatic daily calibration drift tests have not yet
been developed.
clude several specific equipment requirements.
(3) Differential pressure flow monitors.
These include span values, calibration capabilities,
Differential pressure flow monitors use the pitot
calibration error limits, relative accuracy, bias
tube principle to measure the flow. A pitot tube is
limits, calibration gas quality and cycle response
a device which measures both the static pressure
time.
and the impact pressure created by the flue gas.
(1) Proper monitor location for specific
The square root of the difference in these two
installations is essential. The final location must be
pressures is a function of the gas velocity. Types
representative of total emissions, must pass the
include single point and across-the-duct averaging.
relative accuracy (RA) test and must meet point/
One version of the averaging pitot probe has a
path requirements as outlined in the regulations.
diamond shaped cross-section and multiple impact
Location has to provide representative flow over all
and static pressure taps along the length of the
operating conditions. This requires that the velocity
probe. Standard differential pressure transmitters
at sample point be representative of the average
are used to sense the difference between the static
velocity over the cross section. Emission rate in
and total pressure. These devices are simple and
terms of lb/MB must reflect actual emissions.
use standard pressure transmitters. In high partic-
Monitor location must also represent actual
ulate applications, a purge system may be needed
pollutant concentration. Location has to minimize
to keep the pitot pressure taps clear. Zero checks
the effects of condensation, fouling and other
are accomplished by pneumatically connecting the
adverse conditions. Tests are also required to
two sides of the pressure transmitter. These checks
determine the acceptability of the location and to
can easily be automated for daily zero drift checks.
also determine the number and location of flow
Span checks can be performed by using a water
monitor points.
manometer. This type of span procedure is more
(2) There are specific reporting requirements
difficult to automate.
that have to be addressed. Notification must be
e. Opacity monitors. Opacity monitors use the
given to governing federal, state and local agencies
principle of transmissometry to indicate the level of
prior to certification and recertification tests. A
4-2
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