November 2024
Special Focus—Process Controls, Instrumentation and Automation
Reduce costs by following grab sampling best practices
Accurate grab sampling practices are necessary to validate the quality of chemical products with precise compositions. Operators must follow proper procedures to avoid compromising the sample. Otherwise, the result may be an inaccurate analysis of their product, leading to quality lapses and potentially costing a facility thousands of dollars in revenue.
Accurate grab sampling practices are necessary to validate the quality of chemical products with precise compositions. Grab sampling involves safely capturing a small representative portion of system media from a process line or pipeline for offline laboratory analysis (FIG. 1). Operators must follow proper procedures to avoid compromising the sample. Otherwise, the result may be an inaccurate analysis of their product, leading to quality lapses and potentially costing a facility thousands of dollars in revenue.
FIG. 1. A technician fills a grab sampling cylinder with gas for offline analysis. Grab sampling is often a cost-effective and efficient way to determine whether products are up to specification by capturing representative samples from the main process.
Depending on the system media’s temperature, process phase, consistency, pressure, chemical makeup and other factors, operators can follow several approaches to draw accurate, high-quality samples from a process line. Grab sampling is one of the most cost-effective and common sampling methods, and it can be trusted for accuracy and representativeness when following best practices.
Grab sampling plays a key role in quality control, process control, compliance with regulations and verifying the performance of process analyzers. Representative grab sampling often depends on sound sampling system design practices, and the use of the correct collection vessel to maintain the sample’s composition. Therefore, it is important to get both variables right.
Determining the right sampling vessel. Sample representativeness is highly influenced by container type. For example, placing a captured sample in an open bottle for transport may result in contamination. In addition, some chemicals will even evaporate or fractionate if they are not maintained under a certain pressure.
Determining the container type necessary for a sample is the first decision that must be made before selecting a grab sampling system. Both the system pressure requirements and the sample phase influence this decision. Cylinders can contain pressure and can be used for gas or liquid samples, while bottles, which are not airtight, cannot contain pressure and are used only for liquid samples.
When pressure is not an issue, bottle containers can help simplify the sample collection and transportation of the liquid samples (FIG. 2). Bottles can be filled directly from the process at atmospheric pressure and safely transported without the risk of spillage or evaporation, especially when using a bottle with a self-sealing septum cap.
FIG. 2. Grab sampling bottles should only be used for liquid samples, and when atmospheric pressure will not influence the representativeness of a sample.
Bottle systems can be used in a number of liquid applications in which the process fluid is not at risk of fractionating or evaporating when stored at atmospheric pressure. This precaution allows designers to use less-expensive glass laboratory bottles for samples. Additionally, the user can immediately evaluate the sample quality visually.
Meanwhile, cylinder containers must be used when it is necessary to capture a gas or liquid under certain pressure conditions from the system (FIG. 3). Importantly, it is not possible to capture a gas sample safely or accurately in a bottle—cylinders must always be used. Sample cylinders are usually.
FIG. 3. Grab sampling cylinders are appropriate to use when a sample must remain under the exact conditions as the main process for analysis. Gas samples must always be collected in a cylinder.
made with seamless tubing for consistent wall thickness, capacity and size. The smooth internal neck transition eliminates trapped fluid and makes it easy to clean in the lab and reuse in the field, thereby reducing costs.
Sample cylinders protect the integrity of the sample throughout transport, preventing the evaporation or fractionation of chemicals. They also present the opportunity to use various grab sampling system configurations that capture gases in the sealed cylinder.
Determining the optimum grab sampling system design. After determining the appropriate transport vessel, several other key criteria must be considered when designing the grab sampling system. Any given fluid system comprises a number of different variables. Each of those can influence your sampling practices in one way or another. Some of the most important criteria that will impact your design include:
- Pressure: To ensure safe operation, it is important that designers do not exceed the maximum rated pressure of a grab sampling system. Using a rupture disc or relief valve can also enhance safety when sampling chemicals that can rapidly expand and pressurize due to temperature changes.
- Temperature: Likewise, a grab sampling system’s maximum rated temperature should not be exceeded to best protect the integrity of the system’s seats and seals. Ensuring temperatures do not go below the system’s minimum temperature rating is also important. The correct temperatures are important to keep the fluid flowing at a sufficient rate for timely analysis. When the supply temperature exceeds 140°F (60°C), designers may want to consider slightly cooling the sample to reduce the risk of burns for operators retrieving samples, while considering that too much cooling may affect sample representativeness.
- Chemical hazards: Many chemical processes being sampled can pose a risk to operators exposed to the gas or liquid being drawn. The samples may also pose an environmental hazard. For these reasons, it is important that grab sampling systems are leak-free. Choosing high-quality components for connection points can contribute to leak tightness, as can selecting a prefabricated grab sampling system that has already been assembled by a supplier (FIG. 4).
FIG. 4. Prefabricated grab sampling system panels feature high-quality components, including leak-tight fitting connections and sound designs meant to minimize risks related to system media, temperature, pressure and other variables.
- Purging needs: Certain chemicals have the potential to contaminate or corrode grab sampling lines if they are not thoroughly flushed from the system after the sample has been obtained. In such cases, designers should consider the addition of a purge setup, which will help remove residual process fluid from the lines.
- Material compatibility: The materials used in a grab sampling system must be compatible with the process fluid and the operating environment. Standard 316 stainless steel is appropriate for many general applications, but some system requirements may dictate the use of more robust materials, such as alloys 400 and C-276, to ensure compatibility and mitigate corrosion potential throughout the system.
The most efficient grab sampling system design is often a closed-loop system that sees system media continuously circulating through the cylinder while the operator captures a sample. A closed-loop system pulls samples from a positive-pressure process and returns them back to the process at a lower-pressure location—for example, upstream of a pump—using the differential pressure to drive the fluid through the sample system. This circuit creates a path for the sample to flow from the process to the grab sampling station and then back to the process, or to a vent or flare during sample retrieval.
Closed-loop systems can additionally reduce the need for purging, as the grab sampling system essentially becomes an extension of the main process system. When opening the grab sampling system inlet valve, process fluid flows through the system tubing and the sample cylinder before flowing out via a hose to the outlet port. Any older process fluid remaining in the short inlet line will quickly move through the closed-loop path and return to the process as the cylinder fills. When the sample vessel is ready for removal, the operator can simply close the cylinder’s inlet and outlet valves and turn the system to vent, isolating the supply/return lines and allowing fill lines to vent. Finally, the operator closes the inlet valve to halt flow and removes the cylinder for lab analysis. The fluid in the cylinder remains under the same process conditions that existed at the time of the sample—except for temperature—and will be a reliable representation of the process.
Successful sampling. Designed correctly, a grab sampling system offers an effective way to safely collect samples from a pipeline, tank or process vessel to be transported to a lab for offline analysis. Through careful planning, system designers can select the proper cylinder or bottle type for the sample and configure other system variables to maintain sample integrity. By following the above established best practices, system designers, installers and operators can realize success in any application.
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