It Adds Up: Too Much Delay Hurts Analyzer Results
by Jeff Hopkins, on 6/27/18 8:45 AM
Four critical areas where samples can take too much time moving along
"In a process analyzer sampling system, there is always a delay between the moment you grab the sample and the time you obtain a reading. This time delay may be longer than you think. Underestimating it can lead to inferior process control."
There is always a delay between the moment you grab a sample and the time you get a reading from your process analyzer. You might assume your time delay is one minute, which is the industry benchmark. But what if your assumption is way off? Your analyzer readings may no longer be relevant.
So let's go through four common causes of time delay and how you can deal with them.
- Delay in the probe
- Delay in sample transport (including the field station and transport lines)
- Delay in sampling conditioning (including stream switching)
- Delay in the analyzer
Delay in the probe
The larger the volume of a sample probe, the more delay you'll get. The probe should be long enough to reach to the middle third of the process line diameter, where the stream moves the fastest. But it's shouldn't be any longer – or wider – than necessary.
The tap location is a related issue. If you put the probe near a low-flow section of the process pipe, you'll need to wait longer for any change in the process chemicals to show up. So put the tap upstream of such sources of in-process delay, including drums, tanks, dead legs and stagnant lines.
Delay in sample transport
In the sample transport areas, expect delays due to:
- Remote Sample Tap Locations —The further a sample has to travel for analysis, the longer the delay. So put the tap as close to the analyzer as possible. For longer transport lines, consider using a fast loop to accelerate flow.
- Line Length and Diameter —The further the sample has to move and the larger the internal volume of the transport lines, the longer the delay.
- Low Pressure in a Liquid Sample Transport Line —For liquid samples, the tap location should provide enough pressure to deliver the sample through the transport lines or fast loop without a pump.
- High Pressure in a Gas Sample Transport Line —With a gas, the higher the pressure, the slower the flow. To speed up flow, lower the pressure.
Understanding Time Delay - Four Main Areas to Inspect (Technical Paper)
Delay in Sample Conditioning Systems
You’ll find delays in the following sample conditioning areas:
- Unpurged Tee-Pieces Causing Dead Legs —A dead leg is an unpurged side volume that allows molecules into and out of the flowing system media. Common dead legs include connection points for gauges, purge and bleed valves, calibration manifolds, and lab sampling points. You’ll need to purge these areas before performing sample analyses, which means more delay. Relocating a dead leg is sometimes the simplest solution.
- Adsorption of Samples on Tube Walls and Filters —When a sample touches the walls of tubing or any other solid surface, a few of molecules stick. When you’re working on a parts-per-million analysis, this loss from adsorption (or gain from desorption) can be significant. With a liquid sample, worry only when you’re measuring less than 1 ppm. For gas samples, build in sufficient wait times between switching sources to allow for the previous gas molecules to clear.
- High Internal Component Volumes — To get a representative sample and accurate analyzer readings, the entire volume of every device in the flow path needs to be purged. Minimize the size of these components when possible.
Industrial Sampling Systems, the definitive reference guide by expert Tony Waters (book excerpt)
Delay in the Analyzer
Time delay also occurs in the final analysis at the analyzer due to:
- Discontinuous Analyzer Response Times — Some analyzers take more time than others due to processes that take place inside. For example, a colorimeter needs to develop its measured color before completing an analysis, and a gas chromatograph needs to separate its measured components before analyzing them.
- Continuous Analyzer Response Times — Some analyzers run continuously, but even these do not provide an immediate result.
- Manual System Operator Response Times — When manually managing the sample analysis process, be sure to factor in some time for the operator to notice and respond to necessary system adjustments.
If you are serious about reducing time delay, we have a three-page technical paper you can download for free. Swagelok also offers sampling systems training courses where you can learn how to precisely calculate time delay and take corrective actions. When you tap into Swagelok's expertise, you won't have to wait around to measure good results.
Swagelok Grab Sampling Systems Application Guide (Catalog)
More like this:
- Tony Waters Wrote the Book on Industrial Sampling Systems (blog article)
- Technical Webinar: Pre-Engineered Subsystems (PDF download & webinar replay page)
- More Swagelok technical articles on analytical instrumentation (section of this site)