Sampling Systems Part A Flashcards
List the 5 functions of a sampling system.
1) Extracts sample from the process
2) Transports the extracted sample to the analyzer
3) Conditions the sample to match analyzer’s spec
4) Provides a facility to introduce calibration samples to the analyzer
5) Handles excess sample and waste from the analyzer
Reliability is critical for analyzer systems used for process control. Based on research, what percentage
of analyzer system problems are attributable to the sampling system (i.e. rather than the analyzer)?
80%
What are two advantages of the bypass-return fast loop type of transport?
1) Takes care of excess sample from a bypass stream to loop it back into the process
2) Transport samples over long distances
3) Decreases lag time as it brings in larger flow rates
Explain the difference between in situ, at-line, and distant from line sensors. Which are considered
extractive sensors?
In-situ: sensor interacts directly with the proces stream
At-line: sensor is installed in a bypass near the process line. Can be extracted.
Distant from line: sampling system extracts a sample from the process and transports it to the analyzer. Can be extractive.
What are the advantages of in situ sensors? What are the disadvantages?
Adv: instant response, process testing occurs directly at the source, simpler and cheaper to implement as no conditioning is required
Dis: sensor must be rugged enough to withstand the process, harder to service as exposure to the process is present
What are the advantages of extractive sampling? What are the disadvantages?
Adv: sample can be conditioned to meet analyzer spec, sensor is more accessible for maintenance
Dis: sample transport and conditioning cause lag time, the sample might not be representative
What is the advantage of using a sample probe rather than a sample tap?
1) Prevent solids from entering the sample
2) Extract sample from the well mixed centre of the pipe
When would you use a multiple entry (multi-port) probe?
Single point probes do not produce a representative sample for large diameter pipes, stacks, or vessels. A multi port probe is used to achieve a composite sample.
What is isokinetic sampling? When would isokinetic sampling be used? Is the probe port open to the
process flow or away from the flow?
Isokinetic sampling is used to analyze solids in a process stream. The probe needs to be open to process flow to ensure heavier particles can be drawn in.
What is the purpose of sample conditioning?
Prepare the sample of a process stream for analysis according to analyzer specifications. Conditioning components adjust flow, temperature, pressure, and filter samples.
When is a wire mesh strainer used?
When large particles such as rust and scale need to be filtered.
When is a guard filter used?
When small particles are present in the sample
Knock-out pots are used to remove ________ _______ from ________ samples.
Water droplets, wet
When can cylindrical bypass filters be used?
When a fast loop transport system is used. This design allows for self-cleaning of the analyzer
What are two ways to remove water vapour from samples?
1) Refrigerated condenser
2) Permeation dryer
When should packed-tube absorbers be used? What are they used for?
When a sample is corrosive and damaging to the analyzer sensor. They are used to react with the interfering chemical.
When should spargers be used? What are they used for?
Work as scrubbers and remove unwanted chemicals
Clean samples contain ____ levels of solids, while dirty samples contain ______ levels
of solids
Low, high
What constitutes whether a sample is clean or dirty?
Analyzer specifications
Why would an analyzer that normally analyzes clean samples still have a guard filter?
To protect the analyzer from unexpected dust, smoke or storms.
What are two examples of dirty service sample systems?
1) Stack gas analysis for pollution control
2) Flue gas analysis for combustion control
T/F A sampling handling system must ensure the sample meets the “clean” specification 90% of
the time.
False, must be all the time even under unusual conditions
What is blow-back?
A technique used to keep probes that are susceptible to plugging clean as they are exposed to a heavy load of solids.
