Best Practices for Sampling Gases and Volatile Liquids

Matt Dixon, Senior Principal Design Engineer, Swagelok

Optimising your chemical processing costs and maintaining high-quality product outputs require regular capture of representative process fluids for laboratory analysis. Grab sampling—also called spot sampling, field sampling, or just sampling—helps validate process conditions and verify that the product you are making meets internal or customer-based specifications. Grab sampling can also help verify readings of online analysers, an increasingly popular way to obtain real-time insights into your process.

Gas grab sampling presents a unique set of challenges that operators must overcome to in order to obtain an accurate portrait of true process conditions. Read on as we explore these challenges and how to overcome them with the right techniques.

How to Maintain Precise Process Conditions with Gas Sample Cylinders

Gases and volatile liquids must be collected and maintained at temperatures and pressures that inhibit phase changes, which can alter the samples composition.

Basic Rules for Sampling

Grab sampling helps validate process conditions and ensure a quality end product. Keep these rules in mind for your grab sampling process:

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. Minimizing transport time from the draw point to the laboratory helps 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 minimize the potential for contamination.

Consider a process fluid that has a gas dissolved into it. If temperatures increase and pressures decrease, the dissolved gas may boil out of the sample. Or consider a gas sample where temperatures decrease and pressure increases, which may cause liquid to condense and drop out of the sample. In each of these scenarios, the sample composition is fundamentally changed and, therefore, is no longer representative of process conditions.

For these reasons, it is necessary to use sample cylinders (as opposed to less-expensive, unpressurized sample bottles) to collect your gas or volatile liquid in order to maintain the correct phase and preserve sample representativeness. Cylinders are also highly effective at protecting sample technicians and the environment from fumes or emissions if the gas is toxic.

How to Choose an Appropriate Cylinder

Most sample cylinders are made with seamless tubing for consistent wall thickness, size, and capacity, but there are some variables to consider based upon your specific sampling needs. You can work with your cylinder supplier to choose the appropriate type. A few features to consider seeking out when choosing cylinders include:

• 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 makes cylinders easy to clean and reuse
• 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 enhance technician safety. A bypass line allows the purge fluid to flow through the quick connects, ensuring that 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
  
  

How to Properly Fill a Cylinder

In most instances, it is most appropriate to fill a sample cylinder in a vertical orientation. There are a few reasons why.

If sampling a volatile liquid, it is best to fill the cylinder from the bottom to the top. This allows any trapped gas that may be in the cylinder to be flushed out of the top as it is filled, typically through an outage tube. An outage tube also allows the cylinder to maintain some amount of vapor space—a cylinder that is completely filled could potentially rupture in the event of an unexpected change in temperature. Conversely, when sampling gases, cylinders should be filled from the top down. This allows any condensation that may have formed in the line to be flushed out through the bottom.

How to Minimize the Potential for Human Error

By necessity, grab sampling involves a technician physically drawing a sample from the process—this inherently introduces the potential for human error into your process. As such, a volatile liquid or gas sampling system that limits this potential can contribute to the safest and most accurate possible grab sampling process.

For example, a grab sampling panel specifically designed for gas sampling can ensure proper cylinder orientation and top-down sample collection at every use, as noted earlier. This eliminates the potential for an individual technician to improperly insert or draw the sample. The same can be said for a panel designed specifically for volatile liquid sampling.

Geared valve assemblies, which ensure that all valves required to collect a sample are being opened in the correct sequence, can also contribute to fool proof sample collection by eliminating the need for the operator to open and close those same valves manually. A purge function can also be incorporated into a geared valve assembly, ensuring that purge fluids cannot be reintroduced into the main process due to human error.

Finally, in large facilities, there are often many locations where grab sampling is regularly performed. In order to eliminate inconsistencies, selecting a standardized grab sampling panel design for every sampling point can help eliminate potential confusion and promote a consistent process at every location.

Ultimately, grab sampling should be a process that gives you greater confidence that you are providing a high-quality, up-to-spec product for your customers. Choosing easy-to-use, well-designed grab sampling systems and following established best practices throughout your facility can contribute to that goal.

Interested in learning more about how to enhance grab sampling practices in your facility? Swagelok’s specialists can help you evaluate your grab sampling needs and assist you in choosing a system that is right for you.