Basic Concept
and Construction of SCS
Sample
conditioning systems are comprised of all the components necessary to extract a
representative sample and to condition the sample for measurement by the
analyzer. The design of the total sample conditioning system must be engineered
for each specific application. The sample conditioning system assembly tends to
be complex (actually small chemical processes with control systems) and is
often the least reliable segment of an analyzer system. It is important that
all factors that influence an overall system and the operation of an analyzer
be given thorough consideration. Such factors as unknown process conditions,
process upsets, and contaminants can lead to poor designs.
Primary
Conditioning System
Installation
of primary conditioning systems at the sample tap typically allows for more
reliable operation of extremely difficult samples that cannot be transported
easily to a remote sample conditioning system. Remote pressure let-down and
vaporizer regulator stations are another type of primary conditioning system.
These stations must be mounted near the sample tap to flash liquid samples and
reduce lag times by controlling the sample pressure before transport to the
analyzer location. Insulation of a heated enclosure may be required on these
installations if the sample dewpoint temperature cannot be maintained.
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Sample Probe
Sample probes
should be used because they provide a more representative sample due to the
higher flow rates away from process pipe walls.
Note: Process
lines smaller than 2 in. usually do not require a sample probe. If a probe is
required for small diameter lines, the line may be swaged up to 3 in. or larger
with a spool piece to allow a probe to be inserted.
Sample probes
significantly reduce wall contaminates and particulates and act as a first
stage of filtration and conditioning. Special design considerations are
required when process velocities are excessive (which may require thickwalled
probes and in high temperature applications). Corrosive service or high
temperatures may require special alloys or materials. Flanged thermowells are
often used as sample probes by merely cutting the end off at a 45° angle. The
probe is positioned 180 degrees from the process flow to minimize particulate
matter in the extracted sample.
Transport Line
In some
installations, where the analyzer and sample system must be located at a
distance from the sample point, the sample transport time must be considered.
The time necessary to transport the sample to the analyzer can in some cases
contribute the largest share to the overall system dead-time or turnaround
time. Turnaround time is defined as the total time of sample system lag,
dead-time, and the analyzer response time. A convenient means for reducing this
transport lag is the use of a fast loop between the process equipment, an
intermediate location (commonly just outside the analyzer shelter), and a low
pressure return point in the process where the sample can be returned. Sample
transport time, or lag time, is a function of the sample line length and
diameter, the absolute pressure in the line, and the sample flow rate. Sample
transport time is simply the total volume of the sample line divided by the
average flow rate in the line. The lag time can be calculated with the aid of
the following equations:
SAMPLE CONDITIONING SYSTEM Design
The functions
performed by the sample conditioning system as the interface between the
process and the process analyzer include the following:
- Taking and delivering a representative
sample from the process.
- Transporting the sample from the sample tap
to the analyzer and from the analyzer to the waste disposal and/or process
return point.
- Conditioning the sample by adjusting the
pressure, temperature, flow rate, filtering, and phase maintenance of the
sample as required to make the sample compatible to the process analyzer.
- Sample-stream switching and calibration
switching into the analyzer.
- Design incorporates capability for ease of
maintenance, cleaning, or (when needed) flushing the entire system.
The complete
stream composition of all the components and contaminants must be considered.
Some contaminants, such as solids or entrained liquids in a gaseous stream, may
have to be removed by the sample system. The process conditions and range of
all components during normal and especially abnormal conditions (such as
startup, shutdown, rate change, and so forth) must be considered for the
analyzer application and sample system design.
Analyzer System Package Photo
Referenced Documents
- API RECOMMENDED PRACTICE 555 SECOND EDITION, NOVEMBER 2001 ANSI/API RP 555-2001
- Process Stream Analysis (Analyzer Sample Systems), A Review of Basic Design Consideration. (Emerson)
