…also known as spot sampling, laboratory sampling, field sampling, or sometimes just sampling—involves the collection of a sample fluid in a
pipeline, tank, or system. The sample is then analyzed to help operators validate process conditions, evaluate products for environmental emissions
according to local regulations, and detect whether the product is up to customer specifications.
Importantly, grab sampling is a process that must be performed in accordance with established best practices and with reliable equipment. If
performed incorrectly, the integrity of the sample can be compromised, offering operators an inaccurate analysis of their processes that may
negatively influence decision-making. This white paper will detail grab sampling best practices for both gaseous and liquid fluid system media, the
characteristics of well-designed and easily operable grab sampling systems, and how to reliably achieve grab sampling accuracy.
Customizing grab sampling systems allows for safe, efficient sample capture while meeting your specific plant application needs. The panels are available in a variety of configurations and advanced features.
An Introduction to Grab Sampling
To the naked eye, the inner workings of an industrial fluid system are not always easy to discern. This is why sampling practices have long been deployed to grant operators an accurate picture of process conditions and system fluid.
Grab sampling is a process by which a sample is extracted from a fluid system for remote laboratory analysis. Sampling panels can be installed at convenient locations throughout a facility, including near storage containers, on long transport lines, on process lines, at flare locations, and others—wherever you need to draw fluid for analysis. And while online analyzers have become increasingly popular for their ability to analyze process fluid in real time, traditional grab sampling still has distinct advantages.
Generally, grab sampling systems are:
More economical than online analyzers
Easier to install and maintain than analyzers, which must be housed in analyzer shelters
Easy to install closer to process lines
Able to allow all sample analysis to be performed in a centralized laboratory
Helpful for validating the results of an online analyzer
By adhering to best practices, grab sampling can grant you a clear picture of true process conditions and equips you with the right information to make adjustments as needed. Let’s investigate how to achieve successful grab sampling in your plant.
Plant floor space is limited.
Budget is limited.
Process involves hazardous materials, and human exposure should be limited.
Fluid system is large, and samples must be taken from multiple points.
Sample purity is a high priority.
Time delay must be kept to a minimum.
Process rate of change is very slow.
Maintaining sample temperature is a high priority.
Field atmospheric conditions are variable.
The process being sampled has a heavy particulate load.
Grab sampling and online analyzers are two common ways to monitor process conditions. Depending on your needs, one method may be more beneficial than the other.
Best Practices for Successful Sampling
Ideally, your sample is an exact chemical replica of the fluid currently flowing through your system. But since grab sampling is a manual process, external factors can cause your sample to misrepresent true process conditions.
In grab sampling, the goal is to minimize those opportunities as much as possible. While individual considerations may be required based on the uniqueness of your process and operations, representative sampling typically adheres to a few primary best practices.
Accurate analysis depends on capturing a sample representative of your true process conditions. Achieving this level of accuracy starts with properly extracting your sample from the process tube or pipe.
In most systems, it is considered a best practice to use a probe to draw samples from the middle third of your process line. Probes are typically made from metal and are inserted into a nozzle at the process tap location. The probe extends into the process fluid where sample fluid enters the proboscis, allowing the probe to withdraw a continuous flow for analysis.
As shown, the probe acts as a filter that helps exclude quantities of solid particulate, pipe scale, and entrained liquid drops from the extracted sample. Depending on your
operating conditions and the number of particles in the process stream, the probe’s filtering capabilities can lead to significant savings stemming from extended filter life.
A probe’s filtering properties are the result of particle momentum. Inertia keeps the particles in a process stream moving downstream instead of making a sharp turn into
the proboscis. The separation is most effective when the particles are heavier or denser than the process fluid. By sampling from the center of the process stream, and by
filtering out potential contaminants, a probe can help you extract more representative samples than a nozzle connection on its own.
Probes are not always ideal for grab sampling. If your process stream does not contain solids, for example, the probe’s filtering capabilities provide little additional value. If cost is a concern, it may be more beneficial to omit probes from your system. Additionally, in certain applications, omitting some solids and liquids from the sample may negatively influence your sample’s representativeness.
Samples Should Be Timely
Successful sampling involves understanding when the sample was taken. It is important to minimize the time it takes for your sample to travel from the process line to your sample container, helping reduce the opportunity for external factors to interfere with the sample. To minimize transport time, a probe can once again provide benefits. By sampling from the center of a process line, where stream velocity is
the highest, a probe can speed up your response time. Next, high-quality containers can help preserve the state of your sample. Quality containers cannot, however, maintain precise temperature levels over long periods. If this is a concern, it may be better to consider online analysis.
In general, it is important to remember that one of an online analyzer’s biggest advantages is its ability to analyze sample conditions in real time—often within a minute from being taken from the main process. This is advantageous for delicate system fluid that may quickly change phase or otherwise become compromised after being drawn from the process. In grab sampling applications, it may take significantly longer for a sample drawn from a panel to be analyzed in the lab. If your process conditions change quickly, an online analyzer may be advantageous. This allows you to make rapid changes to your process controls as necessary, based on the online analyzer’s results. If your process fluid’s rate of change is slow and does not require rapid control measures, grab sampling can be ideal.
Samples Should Be Pure
Keeping your sample free of contamination is important. Unwanted particulates can lead to inaccurate or misrepresentative analysis in the laboratory. Sound sampling system design can help contribute to higher rates of sample purity. A few design considerations should include:
Avoiding dead legs.
For example, when designing the sampling location, it is important to avoid dead legs upstream of the sampling container. Dead legs are spaces that can trap old sample material and can occur at tees, pressure gauges, temperature indicators, or other areas. The trapped material can bleed into new samples, compromising their accuracy.
Dead legs do not behave in a predictable manner but generally become more problematic as the ratio of length to diameter increases. Lower flow in the analytical line also increases the degree of the dead leg’s negative impact on your sample. As such, it is best to move the dead leg downstream of the sample bottle or cylinder in order to minimize the potential for contamination.
Allowing for adequate flushing.
Dead volume trapped in your transport line and sampling system must be flushed before a sample is drawn. Regularly flushing your transport line helps keep it clean, reducing the potential for sample contamination. Continuous flow can also help reduce flush time.
Allowing for efficient purging.
Fully purging your sampling panel between uses helps reduce the potential for any lingering material to compromise the next sample. For that reason, it is important for the sampling system to have a separate purging option or sufficient flow to fully purge the old sample to a vent or a disposal system. Additionally, industry standards recommend cleaning and drying sampling cylinders before reuse to reduce the
potential for contamination by old sample particulates.
Samples Should Be Stored in the Proper Container
Proper sample containment—including your choice of sampling container as well as the methods by which an operator extracts and handles samples—is also highly important to ensure sampling success.
Your choice of sample container is dictated by the phase composition and behavior of your sample. Dramatic pressure or temperature changes can influence the behavior of your sample and may alter its composition. It is essential to take these factors into your consideration:
Drops in temperature or pressure increases will lead heavier components to condense out of gas samples before lighter ones
These kinds of phase changes will alter the composition of your sample, making it unrepresentative of true process conditions. It is important to maintain the sample at process conditions when possible, and your container choice can help you do it.
Bottles, typically made of glass or sometimes polyethylene, are most appropriate for nonvolatile liquids or liquids with a maximum vapor pressure of 14.7 psi at ambient conditions. Bottles are not pressure-containing and should not be considered for process fluids that must be kept at a certain pressure to avoid phase change. Bottles are, however, more cost-effective, making them compelling when other factors do not dictate the use of a pressure-containing cylinder.
Cylinders should be used for volatile liquid and gas sampling. Cylinders can keep your sample at pressure for long periods of time, helping maintain representativeness throughout the cylinder’s journey from the sampling point to the laboratory. They are also a good choice if your samples are toxic, minimizing exposure and risk to users.
When using cylinders, it is also important to consider the materials of construction, which must be compatible with the system fluid. 316 stainless steel is a good option for general industrial applications, though higher-performing alloys may be a necessity when sampling particularly aggressive chemicals.
Samples Should Be Handled Properly
Because grab sampling is an inherently manual process, operators should follow established best practices when handling samples from their collection point throughout their journey to the laboratory. In even the most well-designed grab sampling systems, human error has the potential to lead to contamination or, worse, operator injury.
Sampling a liquid from a high-pressure source can lead to 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.
Using an open bottle for transport of a liquid sample back to the lab is 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.
Proper training among your operators can help establish a baseline of knowledge. Established best practices should be followed at every sampling point.
What to Look for in Your Sampling System
Grab sampling can be a highly effective way to monitor your process conditions. But because it inherently introduces the potential for human error, it can be beneficial to look for features in your grab sampling panels and systems that can help minimize the risk, improve usability, and maximize safety.
While your specific application may not call for all of the following design features, they can provide significant benefits when and where they are appropriate.
Designed for Your Process
Since gas and volatile liquid sampling have unique considerations when compared to liquid sampling, it is advantageous to install systems designed specifically for those processes.
For example, a closed-looped sampling system that pulls from a process and returns back to the process at a lower pressure location (e.g., upstream of a pump) by using differential pressure to drive the fluid through the system can be ideal for sampling gases or volatile liquids. Samples are deposited into durable, pressure-rated metal sample cylinders that prevent the sample from escaping, protecting operators.
Comparatively, in applications where the process liquid is not at risk of fractionating when stored at atmospheric pressure, a sampling system may dispense the liquid into a non-pressure-containing bottle with a self-sealing septum cap.
Well-designed grab sampling panels also improve ease of use and reduce the potential for error. For example, a panel designed for gas grab sampling can help ensure proper cylinder orientation for top-down sample collection.
Geared Valve Assemblies
Obtaining samples, venting, flushing, and purging are all accomplished by operating a series of different valves in a grab sampling system. Traditionally, these valves would each need to be activated independently, in the appropriate sequence, to properly draw a representative sample.
Today, geared valve assemblies are available that are designed to actuate each valve in the appropriate sequence, helping the operator to more easily control fluid routing through the panel and into the container. This design feature helps reduce the likelihood of error by preventing valves from being actuated out of sequence, thereby reducing the potential for cross-contamination.
Continuous Flow Capabilities
A sampling system that has been designed for continuous flow has the advantage of decreasing flush times when transport lines are long. Additionally, it helps maintain process conditions and helps keep transport lines fresh and ready for sample capture. Continuous flow can also help keep a viscous sample from solidifying in the transport line.
Fixed Volume Functionality
Fixed volume samplers prevent overfilling of sample bottles by premeasuring a specific amount of fluid prior to dispensing into the bottle. This feature can help improve operator safety and reduce waste.
Some grab sampling panels are available with an engraved, customizable instruction placard. This can help unfamiliar operators perform the process of drawing a sample more confidently. Sampling panels can house documentation, including drawings, test records, spare parts lists, and other paperwork to reduce guesswork when it is time to service the unit.
Grab sampling panels inherently come into regular contact with potentially corrosive fluids or gases, and for that reason, must be constructed with materials that resist degradation over the long term. 316 stainless steel construction for wetted materials can reduce potential for corrosion in many general sampling applications. 304 stainless steel construction for nonwetted materials can contribute to long-term durability. Selecting the appropriate alloy combinations can help maximize performance while optimizing cost.
Improve Your Grab Sampling Practices Today
Standardizing your selection of grab sampling panels can also benefit your overall operations. Particularly in large facilities, there are often many locations where grab sampling is regularly performed. Using standardized grab sampling panel designs for all of your sampling points can help eliminate potential confusion and promote a consistent process at every location.
Interested in exploring grab sampling options further? Swagelok offers fully customizable grab sampling systems and sample cylinders that allow for safe, efficient sample capture while meeting your specific plant application needs. With a variety of available configurations and advanced features, we offer ideal sampling panel options that are safe, intuitive, easy to maintain, and can be ordered as a single part number.
We also have a variety of resources available for you to hone your grab sampling skills: