Why a sample probe matters?

To the uninitiated a probe may seem silly. You may have even asked yourself questions at some point like, “why am I placing a pipe into another pipe?” or “why isn’t a connection to a tap point sufficient?”. You may have even sarcastically thought, “aren’t we re-inventing the wheel here?” However, a probe is so much more than “Just good practice”. It is an instrumental tool in an analyzer engineer’s toolbox that not only helps to produce a more reliable measurement but also reduces the overall maintenance for an analyzer.

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This may seem counterintuitive but fluids in a pipe actually move at different speeds depending on how far the fluid is away from the pipe wall. On the pipe wall, a fluid will essentially be stationary. The velocity of the fluid will gradually increase until it reaches the bulk fluid’s velocity. This point is known as a fluid’s boundary layer. Even a turbulent flow that lacks well defined flow will have a layer along the pipe that essentially has no velocity. Because fluid in the center of the process pipe is moving faster, a sample taken here will yield a faster response than one from the process pipe wall.

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However, a probe can do more than just produce a more responsive sample. It can also help remove particles without even employing a filter. In fact, a rule of thumb passed around industry circles is that a probe can reduce suspended particles by 80%. While the exact benefits of a probe will vary depending on the process, the more massive particles will have a higher inertia compared to gas molecules. Because of this, particles tend to continue moving in the same direction as the bulk fluid, rather than changing directions and entering the sample probe.

Some specialty probes can even help to selectively remove specific chemical species from the sample fluid. Take Applied Analytics’ DEMISTER Probe as an example. This probe is commonly used in conjunction with a TLG-837 air demand analyzer to provide feedback for a refinery’s Claus unit. In this process,

H2S is reacted with SO2 to form elemental sulfur which is removed with the aid of sulfur condensers. However, the same property that makes it easy to remove sulfur also makes it challenging for staff to maintain traditional sampling systems and analyzers at this location in the plant.

The temperature that this reaction occurs at is close to sulfur’s freezing point of 112.8 °C. If a cold spot appears on the probe and sampling system, the entire probe can rapidly become fouled. Even probe designs that incorporate heat tracing and insulation do not resolve this problem entirely. These systems are designed for ideal ambient conditions, not the cold, wet, and windy weather that plants can often face in reality.

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The DEMISTER probe works by incorporating a cold finger at the center of the probe. This causes the temperature inside the sample to drop below the dewpoint of elemental sulfur. This will selectively remove the elemental sulfur through condensation while keeping H2S and SO2 present in the stream for analysis. If you’re curious to learn more about this system, you can read about it here on our website.

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So, the next time you’re faced with a challenging analyzer design, don’t forget to think about how the probe can help solve your problem.