Chemical Engineering January 2017 - 58

ter filters, temperature control, vacuum
pump, and sample bottles that
ensure that the sampling system
and procedures do not negatively
interfere with (and thus invalidate)
sample collection.
Functions of a sampling system
Sampling systems for flares typically
have the following six functions:
1. To take a representative sample
that is based upon the specific
needs of the application
FIGURE 2. Proper sampling location and stream
conditioning can help to ensure the best results
(for instance, those that are not accounted
for in the early design of a
sampling system), automatic sampling
may in fact not allow for a true
representation of the process emissions.
In these cases, manual sample-collection
capabilities can help to
address such variability.
For example, randomly sampling
when the temperature has dropped
at night may show differences in
samples compared to those drawn
at some automatic interval. Manual
operation also allows for
repeated
samples to account and correct for
special circumstances that may have
inappropriately influenced the sample
collection (making it not representative
of the source). For example, if a
process has a new blending agent,
achieving consistent operation may
take several attempts to get the desired
mixture. Thus, several manually
drawn samples may be needed in
order to verify correct measures.
Automatic sampling. On the other
hand automatic sampling systems
(which require no operator involvement)
can look at non-standard
flare events and quantify pollutants
such as NOx, SO2 and other chemicals.
Such systems allow operators
to set up a system for sample
collection to coincide with a timed
schedule or anticipated events. In
addition, automatic sampling systems
meet the regulatory requirements
of 40 CFR 50.4 [4], in terms
of capturing regular timed samples
for SO2 sampling in a defined time
period. Appendix A of those federal
regulations [4] define the acceptable
requirements for sample probe, absorber,
moisture trap, flow control
and measurement, particulate-mat58
2.
To condition and treat a sample so
that it can be used with an analyzer.
This may include removing
solids, moisture, or providing temperature
control
3. To switch sample streams in order
to get multiple reference samples
of a process
4. To handle caustic, hazardous, and
extreme environmental conditions
that could adversely impact the
operator or environment when attempting
to extract the sample
5. To allow transport of the sample
for analysis
6. To allow for a mechanism to dispose
of the sample
As noted, sampling systems typically
are of a time-based, flow-based, or
volume-based scheme, with samples
triggered off of events associated
with these schemes. These
schemes can be single action or a
combination of events.
* Time-based sampling will attempt
to fill a cylinder over a pre-determined
amount of time, although
the sample period within any given
time-based sampling can vary.
Time-based sampling systems
are usually used when there is a
continuous flowrate or the material
composition is thought to be
constant
* Flow-based sampling is designed
to take samples in proportion to
the flowrate. These systems have
sample rates that are dependent
on the flowrate and may increase
or decrease in relation to flowrate,
unlike time-based systems
* Event-based sampling systems
take samples when a specific
event triggers the operation; these
may include an overpressure condition,
changes in density, opacity
or some other monitored variable
Many may argue: Why not just use
a continuous analyzer to accomplish
sampling requirements? As noted
earlier, flares typically handle upset
conditions. Therefore, under normal
conditions, the flowrates, pressures
or caustic concentrations may be
measurable in accordance with design
conditions - but when compared
to an upset condition where
flowrates, pressures, and so on are
very high, accurate measurement
may not be attainable.
Providing a mechanism to analyze
the stream continuously is very
costly and technically challenging.
In particular, most analyzers cannot
be calibrated to handle the high
flow conditions while still operating
at the very low range and staying
in calibration. This dynamic range
condition of most flares challenges
many instrument manufacturers.
Manual grab-sampling systems help
to address these challenges. However,
not all grab-sampling systems
on the market today are able to
comply with MACT, LDAR and
NESHAP standards.
Choosing a sampling system
Historically, manual grab-sampling
systems were constructed with an
open tap (two-way valve) or Strahman-style
valve. This approach effectively
provides no vent capture
or fast loop to prevent operators
from taking a bad sample or exposing
the sample to the environment.
By comparison, today's improved
sampling systems that comply with
the regulatory standards address
this earlier design deficiency as part
of their modern design. Specifically,
effective sampling systems that ensure
compliance to MACT, LDAR,
and NESHAP requirements have
a closed-loop design and a closed
vent. Meanwhile, for operators, the
following industry best practices can
help to ensure the most appropriate
sample collection, to ensure regulatory
compliance:
* Ensure there is no condensation
or other issues that could interfere
with the sample
* Understand material compatibility
to the process stream to ensure
that the system is inert to the
process
* Ensure an adequate purge system
to remove residual sample material
from the system
* Understand remote locations in
terms of how sample collection will
tie into the process line
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Chemical Engineering January 2017

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