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The Fluid Systems Engineering and Management Blog

by Swagelok Northern California

Choosing Among Fast Loop Gas Sampling Options: The Basics and Beyond

by Morgan Zealear, on 1/14/21 8:45 AM

fast loop gas sampling

If you’re considering installing a fast loop gas sampling system for a new process or upgrading an existing system to provide a more consistent and reliable process, it’s worthwhile to take a holistic design approach. Time invested upfront helps ensure you’ll choose the fast loop gas sampling system designed to provide years of reliable service. I’ll start with the basics, then expand the discussion to cover other factors that influence design. 

Why a Fast Loop Gas Sampling System?

For sampling many non-toxic gases, a single line is all that’s needed. However, in refineries just about every gas sampling process is inherently hazardous or toxic. Cal/OSHA and BAAQMD regulations are forcing refineries to reduce flaring. For those situations, fast loop gas sampling is the only option. The system transports the gas from the source to and through a sampling cylinder with the return back to the source. For this process, a well-designed fast loop gas sampling system eliminates leakage to the atmosphere and ensures a reliable sampling process. 

In comparison to a single line system, fast loop sampling offers added efficiencies. Often the sampling station is at a distance from the source. And while distance is a factor in obtaining a consistently representative sample, fast loop gas sampling transports the gas sample to the sampling station at a higher speed, reducing sampling time. To contend with longer distances, fast loop gas sampling systems use larger diameter tubing to achieve a faster flow and minimize pressure drop.

Factoring Transport Line Distance Into the Design

The distance between the gas supply and sampling station can be a factor when designing the sampling process. Gas pressure at the source, flow rate, tubing diameter, tubing volume between supply, and sampling station, sampling cylinder volume all need to be considered in determining the time required to obtain a representative gas sample from the supply. You can make it easy on yourself by relying on an experienced fast loop gas sampling vendor to incorporate those factors into the design. 

Fast Loop Gas Sampling Design: Pressure and Temperature

To ensure you’re getting a consistently reliable gas sample and maintaining sample integrity as the cylinder is transported between the sampling station and analyzer, you’ll need to consider several other factors when designing a fast loop gas sampling system. 

Gas sampling cylinder volume is depending on the volume required by the sampling station. You’ll want to select the minimum cylinder size that delivers the required volume to the analyzer. That will also simplify the sampling process and cylinder transport logistics. 

Gas sampling cylinder volume

If the temperature of the sample gas is significantly higher than the atmospheric temperature, cooling in the cylinder will gradually diminish gas pressure and volume. Depending on the time required to transport the cylinder between the sampling station and analyzer, that may factor into the size of the sample volume and cylinder required. 

If the temperature of the sample gas is significantly lower than the atmospheric temperature, the gas pressure in the cylinder will increase during transport to the analyzer. Placing the sample cylinder in a cooler may be required to deliver a representative sample. Alternatively, a relief valve in the cylinder can be used to avoid the dangers of increased pressure during transport.

In addition to the sample cylinder volume, the interior cylinder surface can affect sample quality. The type of gas being sampled will determine if your sampling cylinders require any special coatings such as Dursan®, Silcolloy®, or SilcoNert® to reduce adsorption and absorption of the gas into the metallic surface during transport to the analyzer station.  

Fast Loop Gas Sampling: To Purge or Not

You also have the option to purge the sampling system before or after taking a gas sample. If you’re sampling non-toxic gases, it’s doubtful you’ll need the purge function. However, if you’re sampling gases that are toxic or contain condensible hydrocarbons, you’ll want to include the purge function in the design. 

Fast Loop Gas Sampling

Fast loop gas sampling design for purging the fill line after taking a sample. 

Typically nitrogen is the purge gas introduced to flush the fill lines in the opposite direction of filling. The purge process requires a purge-compatible sample cylinder (shown in the diagram above) to avoid purging the cylinder of the captured sample gas. 

Fast Loop Gas Sampling Systems: Focus on Design and Fabrication

Fast loop gas sampling systems are designed to provide high-flow through transport lines to reduce sample time, consistently deliver representative samples, and maintain sample integrity. Achieving these design and operational goals requires an in-depth understanding of process conditions, gas properties, and the reason for the analysis. While the basic design of fast loop gas sampling systems is well understood, it’s the many other variables of a specific sampling process that require expertise in selecting the components and tailoring the final design to optimize the process. 

For decades, Swagelok has been working closely with Bay Area refineries to design and fabricate fast loop gas sampling systems. Our Field Engineers consult on-site to assess the specific sampling requirements. We design the system, calculating gas flow rates, selecting tubing size, pressure gauge, flowmeter, valves, and cylinder type, and fabricate the system in our local facilities, following ISO 9001 quality standards. 

Prior to delivery, Swagelok fully tests for system integrity and gas flow to ensure optimal performance. All fast loop gas sampling systems are backed by the Swagelok Lifetime Warranty and supported by Swagelok service representatives.

To find out more about how Swagelok Northern California can help greater effieciency and reliability to your gas sampling processes by providing expert consultation, desing, and fabrication, contact our team today by calling 510-933-6200.


Morgan Zealear-1Morgan Zealear | Product Engineer – Assembly Services

Morgan holds a B.S. in Mechanical Engineering from the University of California at Santa Barbara. He is certified in Section IX, Grab Sample Panel Configuration, and Mechanical Efficiency Program Specification (API 682). He is also well-versed in B31.3 Process Piping Code. Before joining Swagelok Northern California, he was a Manufacturing Engineer at Sierra Instruments, primarily focused on capillary thermal meters for the semiconductor industry (ASML).

Topics:Assembly ServicesGrab Sample SystemsMission Critical

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