Key points
TL;DR Summary: Improving Process Analyser Accuracy
Maintaining process analyser accuracy is vital for safe and efficient plant operations. However, several common issues in sampling system design and maintenance can compromise readings. These include incorrect tap placement, probe sizing, field station pressure, deadlegs and cross-stream contamination.
Fast loops, proper temperature and pressure control, and sample conditioning techniques like coalescers are essential. Educating technicians and working with experienced partners helps prevent sample degradation and ensures reliable data.
Improving Sampling System Analyser Reliability
With the right knowledge, technicians can avoid the eight most common problems that impact the accuracy of process stream readings.
Maintaining process analyser accuracy is crucial to keeping a plant operating at peak performance, but the exactness of these systems depends on a host of external factors that are not always possible to control.
From initial design to long-term maintenance, plant engineers and technicians must contend with these highly sensitive systems and anticipate challenges to their proper operation (Figure 1).
Failure of one component in the system could mean process stream readings will no longer be accurate or timely, preventing technicians from reporting potential problems until it is too late. When failure occurs, samples may no longer accurately represent the current state of process fluids, and they may change chemical compositions in unpredictable ways.
Beyond component failure, systems that are poorly designed may cause problems with excessive time delay between sample capture and analysis, resulting in lost product or the need for costly reprocessing.
Similarly, if the system’s temperature, pressure, flow, and/or state are not tailored to the specific process fluids, the system can become overwhelmed, putting employees at risk of exposure to potentially dangerous operating conditions.
Though the number of potential process analyser problems can vary as much as the components in the system, most malfunctioning sample conditioning systems share common points of failure. To help improve consistency and reliability, consider eight common sample conditioning system challenges and how to solve them.
“Failure of one component in the system could mean process stream readings will no longer be accurate or timely, preventing technicians from reporting potential problems until it is too late.”
Improper Tap Location
Identifying where to place a tap within the system is not always a simple proposition. To prevent time delay, engineers should avoid placing taps on low-flow sections of the process pipe because it can take longer for changes in process chemicals to appear in the post-sampling analysis.
Additionally, mixing volumes like tower bottoms, tanks, or drums can also delay analysis. To ensure taps are in the most effective position, engineers should choose locations where process fluids are well blended, downstream from induced turbulence from a pump discharge, flow orifice, or piping elbow.
Probe Misuse
In general, probes should be long enough to reach the middle third of the process pipe to ensure representative samples (Figure 2). However, sometimes probes are too long and are affected by resonant vibration, so it’s important to size your probe properly.
Diameter also matters because capturing too much process fluid can delay analysis. When possible, engineers should specify probes that fit specific fluid systems.
Incorrect Field Station Designs
Gas samples require a different approach than liquids. System designers should reduce pressures to between 10 to 15 psig, or even 5 psig, if possible, using analyser field stations. Otherwise, time delay occurs between collection and analysis as high-pressure gases move more slowly through the system.
In addition, lower pressures prevent condensation that can compromise the sample. Installing pressure regulators at field stations helps move the gases more quickly to the analyser, allowing more accurate results.
Skipping a Fast Loop
Using fast loops or bypasses when vapourising liquid samples can prevent fluids from moving too slowly to the analysers, resulting in delays that can compromise the sample’s accuracy. Without a fast loop, a significant volume of slow-moving liquid may be upstream of the regulator, which could lead to many hours of time delays. Fast loops preserve the accuracy of the sample and offer actionable data to ensure the system is working properly (Figure 3).
Creating Deadlegs
Any tee or cross in the system creates deadlegs (Figure 4) which, left unaddressed, will compromise sample accuracy. With a deadleg, residual molecules from older samples will contaminate the current sample unless all ports are flowing.
Examples of deadlegs include gauges or switches, pressure relief valves, or lab sample points. Wherever possible, technicians should identify any deadlegs in the direct line to the analyser and remove them.
Cross-Stream Contamination
A single block or three-way switching valve is not a sufficient barrier between calibration fluid and the sample. With this setup, calibration fluids could leak across the valve seat into the sample and compromise its accuracy. A better configuration is the double-block-and-bleed setup. In this scenario, any potential leaks will bleed into a vent line and avoid sample contamination.
Improper Temperatures and Pressures effect Process Analyser Accuracy
Preventing phase changes during the sampling process is essential to accurate analysis. Mixed-phased samples do not represent the fluid or gas being analysed and may be incompatible with the analyser.
Software programs are available that will generate phase diagrams for a given chemical composition, which will help to ensure the sample remains in one phase during collection and analysis.
Generally, technicians should keep gases above the dew point so they do not condense and liquid samples below their bubble point to prevent lighter components from boiling off.
“Using fast loops or bypasses when vapourising liquid samples can prevent fluids from moving too slowly to the analysers, resulting in delays that can compromise the sample’s accuracy.”
Improper Sampling Conditioning
When possible, remove liquids from gas samples. Failure to do so could damage the analyser and compromise the analysis. Technicians can remove large droplets in a gas sample with gravity, using either a knockout pot or a fallback probe.
An alternative is through inertia with a kinetic separator or cyclone. For fine droplets suspended as an aerosol, coalescers are effective. These two separation methods must be used in combination because neither one will remove all liquids on their own.
With a coalescer, high flow rates may push fine droplets through the coalescer’s elements and will not drip out as expected, so maintaining the correct flow is key. Finally, as the sample leaves the coalescer, it will be near the dew point and threatening to condense again.
Raising the temperature or lowering the pressure, which can be done with a needle valve, will prevent condensation and avoid compromising the sample.
The Bottom Line
Educating technicians about how to identify and address threats to process analyser accuracy and prevent sample contamination is critical to successful maintenance (Figure 5). Work with a supplier that can offer resources on how and what to look for to ensure long-term fluid system performance.
FAQs: Common Process Analyser Accuracy Challenges
What causes inaccurate process analyser readings?
Common causes include poor sampling design tap placement probe misuse deadlegs and improper temperature or pressure control
Why is tap location important?
Taps in low-flow areas or mixing volumes can delay sample response and reduce representativeness
How should probes be sized?
Probes should reach the middle third of the pipe without being too long to avoid vibration or excessive volume capture
What is a fast loop and why is it useful?
A fast loop circulates fluid quickly to the analyser reducing time delays and improving sample accuracy
What are deadlegs in sampling systems?
Deadlegs are stagnant areas where old samples can contaminate new ones and should be eliminated
How can you avoid cross-stream contamination?
Use double-block-and-bleed valve setups to prevent calibration fluids from leaking into process samples
Why are pressure and temperature control important?
They prevent phase changes in samples which can make analysis inaccurate or damage the analyser
How do you condition a gas sample properly?
Use separation methods like knockout pots kinetic separators or coalescers and maintain proper flow to prevent condensation or carryover
