Optimizing grab sample quality and safety

July 14, 2022
Maximizing profitability during large-scale facility construction requires a uniquely designed system.

Building large-scale industrial facilities like chemical refineries, wastewater treatment plants, manufacturing sites, or oil and gas platforms is complicated. Industrial construction often includes pre-approved schematics of fluid systems that are then built by subcontractors on-site, which does not allow contractors to fine-tune the system as it is built. Leaks of potentially harmful fluids not only put the operators at risk but can also lead to costly environmental damage. In addition, correcting any potential shortcomings that exist because of improper system design could require taking the system offline for an unknown period of time. Such downtime can be damaging to a plant’s operations, not only costing money in the short term but potentially harming a facility’s competitiveness over the long term.

For these reasons, careful planning of industrial fluid systems should be at the forefront of any industrial facility construction. Though every fluid system is unique to a facility’s specific circumstances, there are some universal considerations that must be examined before a system is built to avoid issues in the future.

Establishing effective grab sampling systems

Grab sampling is a process by which a sample is extracted from a fluid system for remote laboratory analysis (see Figure 1). Plant operators use sampling to ensure the fluid meets critical quality standards throughout a facility. Common grab sampling points include near storage containers, on long transport lines, and on process lines at flare locations. In addition, samples may be analyzed before and after a commodity is transferred to determine the value of the cargo, particularly if there is a custody change.

It is especially important to pay close attention to the construction of grab sampling systems when building large-scale facilities. The key to avoiding simple grab sampling system errors is to build them using criteria specific to each facility. Standardized approaches may save time in the short term but may lead to unforeseen problems down the road.

Figure 1. With grab sampling, operators extract a sample from a fluid system and take it away for remote laboratory analysis. Images © 2022 Swagelok Company

Advancements in grab sampling systems have helped make the process simpler, safer, and more repeatable. For plants that have been following the same grab sampling process for years, it is worth considering making improvements that could drive greater sampling efficiency and accuracy.

For example, grab sampling panel designs must account for flushing and purging to ensure that transport lines are clean and free of contaminants. Flushing the lines may take an excessive amount of time if the design is not optimized, especially for longer transport lines. Also, it is important that purge gases not be introduced back into the system where they could potentially contaminate or otherwise impact the quality of the process fluid. Modern panels incorporate safeguards to ensure purge gases stay within the confines of the sampling system.

Elsewhere, some new sampling panels have been designed with heightened user convenience in mind. Obtaining samples, venting, flushing, and purging are all accomplished by operating a series of different valves. Today, some geared valve assemblies are designed to activate the necessary valves in the proper sequence, helping the operator more easily control fluid routing through the panel. Additionally, technicians can more easily isolate gauges when performing maintenance. Geared valve assemblies also help minimize the chance of operator error by preventing valves from being activated out of sequence.

The importance of proper grab sampling design

Given the importance of accurate sampling on overall plant operations, it is important to minimize the external factors that can affect samples and possibly taint the results. To be useful, samples should reliably reflect the quality or true process conditions of the systems. That is why it is vital to design grab sampling systems properly, with accurate analyses in mind. Even seemingly simple variables in a system’s design can have an outsized effect on sample quality and therefore negatively impact the results. Ultimately, your system design should ensure your samples are:

  1. Timely: The more time it takes for a sample to reach the sample container from the process line, the more opportunity there is for contamination and degradation to take place. During the design process, this calculation must be taken into account and adjusted accordingly to keep those times as minimal as possible.
  2. Representative: Samples should be representative of the process fluid in the system at any given time. Unnecessary variation can render the eventual analysis moot and can be mitigated through well-designed systems.
  3. Compatible: As the system is constructed, care should be taken to choose system materials that are compatible with the sample being taken. Selecting compatible materials will help reduce the amount of maintenance and repairs necessary over the life of the system.

Additionally, proper design of sampling systems can help prevent waste and minimize unnecessary danger to the operator and the environment.

To ensure the grab sampling system is built to provide samples that are timely, representative, and compatible, it is helpful to enlist the support of experienced advisors to help in the following areas:

Selecting the appropriate sampling vessel: Choosing the appropriate sample container during the design stage—one that can handle the process pressures and temperatures without leaking—will help enhance safety for operators. Choosing between sample cylinders or bottles is one of the first decisions to be made for liquid grab sampling and is largely dictated by the type of sample being collected. Sample volatility and toxicity may necessitate the use of sample cylinders. Sample bottles can be effective for collecting nonvolatile, nontoxic liquids.

Assisting with probe selection: To achieve the optimal level of accuracy, it is important to extract the sample from the process tube or pipe from the ideal location. Bringing in an external advisor from a trusted supplier to identify the best location can make the process easier. They may also be able to give advice about which probe size is appropriate for the application in question. Finally, advisors can help by performing specific calculations to establish the system design, including the right tubing lengths and component diameters. Having this assistance at the beginning of the design process will prevent problems from occurring once the system is operational.

Determining grab sample location and grab sample panel placement: To ensure the most accurate sampling results, grab sample points should be located on the process lines at strategic points. In addition, the grab sample panel should be placed in an easily accessible area so operators can collect samples safely. It should not be too high or difficult to reach. For maximal performance, operators should be able to safely retrieve samples and transport them to the laboratory for offline analysis.

Conducting temperature and pressure calculations: Standardized system designs often overlook the need to make specific calculations to maintain sample quality. Specifically, temperature and pressure calculations like the Joule-Thomson (JT) calculations should be conducted to predict the temperature effects of pressure changes. These parameters allow operators to monitor the sample and ensure it does not change phase during the sampling process. For example, if pressure drops, which could cause the gas to cool past its dew point, liquid will form and change the composition of the sample. This can prevent an accurate analysis from being done at the off-site laboratory, skewing how the facility maintains its system.

Conducting purge-time calculations: It is essential to ensure that sample nozzles, probes, transport lines, and panels be purged completely between uses. Otherwise, residual fluids may compromise the next sample. Operators must be able to perform the proper calculations to determine how long the system must be purged to ensure a fresh sample the next time the equipment is used. There are several other calculations an advisor can assist with, including time-delay calculations and phase-change calculations for different fluid mixtures.

Before doing any large-scale industrial construction, proper heed should be given to the design of the fluid and grab sampling systems. Finding suppliers who can advise on best practices for the layout of the system can be a significant help. The earlier the advisors are brought into the project—including during facility construction—the more easily they can make the complex process run smoother and ensure the completed fluid system will follow best practices for quality, safety, and profitability.

This story originally appeared in the July 2022 issue of Plant Services. Subscribe to Plant Services here.

About the Author: Matt Dixon

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