Sampling gases and volatile liquids: essential strategies

Following gas sampling best practices can improve product output and reduce fugitive emissions.

Sampling various industrial process fluids during a chemical plant’s operation is crucial. It allows operators to decide whether adjustments need to be made to maintain high-quality end products. Additionally, regular testing provides insights into whether the analysis tools are working and can help plants remain safe and compliant with regulations governing accidental emissions. Finally, regular sampling allows operators to control chemical costs by reducing leaks and validate process conditions.

Technicians who sample fluids frequently understand the process comes with challenges, and the measure of success is how well they navigate them. A set of overall best practices can streamline sampling procedures and provide technicians with the guidance they need to collect good samples every time.

The essentials of proper sampling

If gases and volatile liquids undergo phase changes, it can lead to poor samples that do not accurately reflect the real-world conditions. There are four rules to follow to collect proper samples:

1. The sample must represent the process: Use probes to draw samples from the middle of the process pipe and avoid phase changes during sample transportation.
2. The sample must be timely: Minimise transport time from the draw point to the laboratory to help ensure process conditions are accurately represented.
3. The sample must be pure: Avoid dead legs upstream of the sample container and allow for adequate purging and flushing of the sampling system to minimise the potential for contamination.
4. Sampling must be safe: Grab sampling inherently involves a human operator interacting with process fluids, which often can be harmful if direct contact occurs.

Following best practices and using well-designed sampling systems is essential — not just for sample accuracy, but also to operator safety.

Enabling safety in sampling is about more than just the operator: it also relates to the environment and helping to prevent sampling operations from adversely affecting a plant’s surroundings. A closed-loop sampling system can help reduce the possibility of fugitive emissions.

Reducing fugitive emissions through closed-loop sampling

Fugitive emissions happen anywhere leaks occur. To protect the environment and workers, more regulations than ever govern the release of fugitive emissions, and it is incumbent on chemical plants to work hard to eliminate them as much as possible.

Fugitive emissions can happen during fluid sampling procedures, depending on how skilled the technician is. One potential cause of these leaks is the use of poorly constructed sampling systems or building them with substandard components. Fortunately, employing closed-loop sampling systems can help simplify the sampling process and significantly reduce fugitive emissions.

Closed-loop systems draw in process fluid and flow it through the sampling point where a portion of the sample is collected in a sealed container (a cylinder or bottle). The system then returns the fluid to the main process. Because they can eliminate waste by returning fluid to the main process, and because they can shield both operators and the environment from exposure to the fluid, closed-loop systems often represent the best method to help operators reduce emissions and maintain safe working environments.

Below are a few features to look for when selecting a closed-loop system.

System constructed from leak-tight components

A sampling system’s quality depends on the quality of its components. For example, low emission (low-E) valves, which demonstrate through rigorous testing their ability to minimise fugitive emissions, may be incorporated into your sampling system’s design to minimise the potential for unwanted leaks.

Certified low-E valves and other components are becoming increasingly common across industries and in various fluid system applications as emissions regulations become stricter.

System designed and assembled to minimise potential leak points

Even the highest-quality valve or fitting has the potential to leak. However, by designing systems with as few connection points as possible, you can reduce the potential for unwanted leaks and resulting emissions. If leaks do occur where high-quality components have been used, this is typically a result of improper installation practices.

Be sure that your grab sampling systems have been assembled and tested by trained specialists to avoid these issues.

Suitable sample drawing technology

The point at which the technician draws the sample into a bottle or cylinder container represents another point where leaks can occur. There are a few technologies that can help eliminate them. In a non-volatile liquid closed-loop system where samples are collected in bottles, the fluid is commonly dispensed via a needle that pierces a rubber septum. Ideally, as the needle is withdrawn, the rubber septum reforms a complete seal.

In gas or volatile liquid sampling systems where cylinders are used, cylinders that feature easily operable quick connects will allow for efficient and safe connecting and disconnecting from the sampling point.

Well-designed, closed-loop grab sampling systems can limit emissions by preventing exposure of process fluid to the environment. They can also reduce risk to sampling technicians by keeping process fluids fully contained. The right supplier can provide easy-to-use panels in standard, configurable designs that can be reproduced across a facility, not only minimising emissions and risk, but simplifying operator training and the potential for errors.

Figure 1: To produce the most accurate samples of gases and volatile liquids, technicians should deploy sampling cylinders rather than unpressurised sample bottles.

Choosing the right cylinder

Importantly, minimising fugitive emissions and enabling safety for operators during sampling activities requires careful consideration of the vessels that will be used to capture and transport samples.

Operators should avoid sample phase changes between the process stream and the lab by keeping gases and volatile liquids pressurised in a sample cylinder instead of less expensive, unpressurised sample bottles (Figure 1). Additionally, cylinders offer more effective protection for technicians and the environment, particularly if the gas is toxic.

While most sample cylinders are constructed using seamless tubing, offer consistent wall thickness, and have similar size and capacity, differences do exist. Consider the following features when selecting a cylinder:

• Easily operable quick connects, allowing for efficient and safe connecting and disconnecting from your sampling point.
• A smooth internal neck transition, which can help eliminate trapped fluid and make cylinders easy to clean and reuse.
• Overpressure protection, which can provide higher levels of operator safety: rupture and relief valves are available in a tee or may be integral to the cylinder isolation valves.
• Proper material composition and finish, as special alloys or materials may be required depending upon the gas or volatile liquid being sampled.
• Incorporated bypass lines, which can be beneficial for purging toxic sample remnants and enhancing technician safety: if spillage occurs when the cylinder is disconnected the spill is composed of purge fluid rather than the toxic sample.
• Durable design and construction: sample cylinders must often be transported considerable distances for laboratory analysis.
   

Working with a reputable supplier will provide the support necessary to choose the right cylinder for specific applications.

Figure 2: Technicians who are sampling volatile liquids should fill the cylinder from the bottom, which forces out trapped gas through an outage tube.

How to fill a cylinder

Hold the cylinder vertically as the filling takes place. Volatile liquids should fill the cylinder from the bottom to the top, which forces trapped gas in the cylinder out during filling, usually through an outage tube (Figure 2). Outage tubes also keep the cylinder from overfilling, which can cause rupture if the temperature changes precipitously.

For gases, technicians should fill cylinders from the top down, which prevents condensation build-up by pushing it out the bottom of the cylinder.

Make sure cylinders are properly maintained

All industrial equipment risks deterioration though repeated use, and cylinders are no exception. Damaged or malfunctioning cylinders can put technicians at risk and make the sample process less accurate. That is why technicians must perform regular, proactive sample cylinder maintenance. Sample cylinders should be pressure-tested and inspected annually and recertified every five years.

In between the annual inspections, occasional damage may occur. Four common issues technicians should be aware of are:

• Leakage across key components, including valves and quick connects
• Corrosion resulting from compatibility issues with the sample
• Internal surface conditions, absorption
• Improperly installed components

Reducing the chance of human error during sampling

No matter how carefully technicians handle grab sampling, there is always the possibility they will make a mistake. Thankfully, it is possible to limit them via the following best practices

Documentation

Each grab sampling panel should have an engraved placard on it to provide technicians with instructions on how to use the panel. By providing detailed instructions, technicians should be able to draw a sample even if they have never used the panel before.

Explaining how to order replacement parts takes the guesswork out of maintenance.

Simplification

From the initial design to eventual operation, grab sampling systems should be as simple as possible to operate. A well-designed, intuitive system will enable technicians to do proper sample pulls every time.

Figure 3: The more skilled the technician, the less likely it is that fugitive emissions will occur from leaks during the sampling process.

Additionally, installing geared valve assemblies forces technicians to activate the valves in the proper sequence. Such a setup improves collection safety because it minimizes the opportunities for errors. Geared valve assemblies may also control the purge function, preventing purge fluids from accidentally being introduced into the process.

Automation

To eliminate the potential for human error entirely, designers may choose to automate sample collection. Eliminating all human involvement in sampling may cost more upfront, but it may pay for itself in error reduction. To achieve automation, grab sampling panels would typically be fitted with valve actuators and sensors for feedback, all controlled by a PLC. Be careful about installing wiring in hazardous areas.

Properly designed grab sampling systems should boost confidence about the overall consistency and quality of end products, which allows customers to have confidence in what they receive from the plant. Following established best practices throughout a facility will ensure high standards are met throughout the process.