Grab Sampling Four Common Grab Sampling Deficiencies and How to Fix Them
Grab sampling is an important function in industrial fluid systems, but a few common problems can negatively impact efficiency, representativeness, and operator safety.
Grab sampling is an important practice in industrial fluid system operation in numerous process-driven industries around the world. Obtaining timely, accurate samples that are reflective of your real process conditions can help validate product quality and ensure end-use products are up to target specifications and customer demands. Safety must also be considered – following the right procedures, using the correct equipment, and building well-designed, safe grab sampling systems can help reduce the risk for technicians, especially when working with toxic or flammable process liquids/gases (Figure 1).
Not all grab sampling systems are fully optimised for accuracy and safety, and even well-designed systems often have areas with opportunities for improvement, whether via using higher-quality components or following well-established best practices. In fact, many of the most common areas that can negatively influence accuracy and safety in a grab sampling application are the most easily fixed. This article will explore some of these common problems and how to address them.
1. Improper Sample Identification
The proper identification of specific grab sampling points throughout your fluid system is a good place to begin the optimisation process. Plants often place tags to identify equipment such as vessels, heat exchangers, control valves, pumps, and more. There is no reason the same should not be done with grab sampling locations.
However, precise labeling is not always seen in the field. It is not uncommon to find grab sampling locations simply labeled as ‘sample point,’ while specifics about the process, the type of container to use, and the sample to be drawn may only be identifiable by a knowledgeable operator. Without a means for other operators or technicians to identify the sample point without consulting system schematics or tracing the pipework to the nearest process equipment, issues can arise. For example, it is much easier to make the mistake of intermixing different samples when containers (cylinders, bottles, or bags) are not dedicated to specific grab sample points. Depending on the chemical composition of the process fluids, intermixing samples or using improper containers can create an unavoidable safety risk.
Instead, fluid system operators can benefit by developing a specific naming convention for grab sample points throughout their system, allowing any worker to easily identify each location (Figure 2).
To create an asset register that correlates with each sample point, the following information should be included:
- 1. Location of sample points
- 2. The sample’s composition and fluid state
- 3. Pressure and temperature
- 4. Container type
- 5. Potential hazards
- 6. Spares and part numbers
- 7. Frequency of sampling
- 8. Flush time for representative sampling
2. Unoptimised Sampling Locations
Operational efficiency should be considered when determining sampling locations, but this can often go overlooked.
For example, it is not uncommon to find grab sampling systems that have been installed over a control valve. This can lead to two issues. First, when the control valve is closed, flow is cut off to the system, which can compromise samples and be detrimental to accuracy. Second, when the control valve is fully open, no pressure drop will occur, and the flow to the sample system will be unknown – it may not even be flowing at all.
Instead, sample points should generally be positioned over a pump, before and after a process stream, or in end-product lines. This allows operators to efficiently draw reliable samples without the need to consider the control valve position. Easy accessibility for operators should also be considered. Finally, wherever possible, the consolidation of individual sample points can help reduce the risk of mistakenly taking a sample from the wrong point.
3. Poor Sample Quality
Maintaining the quality of your sample is one of the most important criteria for it to remain an accurate representation of your process. Proper methods of drawing the sample from the process are critical here, but in many plants, it is common to observe sample points where samples are drawn directly from the nozzle.
This practice can lead to a few issues or inefficiencies and potential quality compromises. For instance, in gas samples, additional volume within the nozzle can result in a delayed sample response time due to high-pressure compressed gas volume in the pipe nozzle volume. Depending on the system, this issue can cause hour-long delays in achieving a representative analysis of the sample. Additionally, nozzles do not provide any sort of filtration functionality, which is a necessity in systems in which condensates can further compromise sample quality and accuracy.
For these reasons, a probe should be used when extracting a sample from the main process. Probes avoid condensate particles and eliminate additional volume (see Figure 3) that can result in poor sample quality. Additionally, probes help ensure that samples are drawn from one-third of the inner process pipe wall to the center of the process line; this specificity contributes to greater representativeness, as the middle of process streams are better mixed and representative compared to around the edges where the flow is slow. For gas samples, pressure should be reduced immediately after the process interface valve to reduce compressed gas volume. This helps reduce sample response time as well as the risk involved in transporting high-pressure gas through the plant.
Sample probes can be installed in existing sample supply nozzles in most process lines as long as there is a suitable end connection. For liquid sampling, probes should ideally be installed in the lower part of a vertical process line to ensure that gas in the lines will not travel down the probe. For gas samples, probes should be installed on the top of a horizontal process line. In general, probes should not be installed on a bend; this helps to avoid grabbing a separated sample and reduces the likelihood of contamination.
4. Suboptimal Sample Handling
Finally, the methods by which an operator extracts and handles samples are critically important to maintain both sample representativeness and user safety. But once again, there are some common problems that can arise in many plants.
One example includes high-pressure liquid grab sampling that can cause spillovers and splashing of the process fluid. This is not only wasteful but can be a significant safety risk for the technician. It may also be in violation of environmental regulations. Another example is the use of an open bottle for transport of a liquid sample back to the lab. Open bottles are not only a spillage risk but will inevitably lead to sample inaccuracy – light components in the sample will evaporate or fractionate above their dew point if they are not maintained under a specific pressure and temperature. Carrying an open bottle also exposes the technician to potentially dangerous fumes.
Fixed volume liquid sampling can reduce each of these risks and can also be particularly beneficial in high-pressure applications. Fixed-volume sampling can help ensure repeatable, accurate sample volumes, and the grab sampling system can be calibrated to leave 20 % of the volume in the container free, enabling thermal expansion of the liquid.
Better Sampling, Better Outcomes
Awareness of these common grab sampling mistakes, and the practical knowledge of how to avoid them, can help operators and technicians safely draw reliable samples. Addressing these common challenges, as well as working with a reliable vendor that can apply its knowledge of grab sampling to your specific system, can lead to higher quality end products and satisfied customers.