Dec-2019

Sampling system design

Correct probe orientation is a critical consideration in the design of a sampling system for process streams.

RANDY RIEKEN
Swagelok Company

Article Summary

In analytical sampling systems, probes are a common method used to extract samples from the middle of a process stream. They are typically inserted into a nozzle at the process tap location to extract samples for analysis.

The probe is a metal, glass, or ceramic proboscis that extends into the process fluid (see Figure 1). Process fluid enters the proboscis and the probe withdraws a sample for analysis. Alternatively, a nozzle can be used without a probe to deliver samples to the analyser. But, for most systems, probe sample delivery offers advantages compared to nozzle sample delivery, including inherent filtering, faster analyses, and better samples.

For reliable sampling, it is important for the probe to be correctly oriented. A probe acts as a filter in an analytical instrumentation system, excluding dust, pipe scale, and entrained liquid drops from the extracted sample. Its filtering effect occurs as particles in a process stream move downstream due to their momentum, rather than taking a sharp turn into the mouth of the probe (see Figure 2). If a probe is oriented improperly, operators may encounter numerous problems, including particle uptake, analysis time delay, and other factors that can negatively impact efficient analyses and operation. More importantly, samples obtained via an improperly oriented probe may be inaccurate, leading to poorer quality products and potential safety issues.

Additionally, probes vary in their design, and it is important for operators to know when, where, and how to use each type to ensure accurate and timely sample analysis. For example, one of the most commonly used types of probe has angle-cut ends and typically requires the entry port to be pointed downstream — but not always. Other probes have square-cut ends and therefore no preferable orientation within the system flow. Both designs are shown in Figure 3.

A good knowledge of probes is essential for sampling system operators. The following article will review common probe designs in detail, describing unique challenges operators should know about and what precautions they should take to ensure safe and reliable operation. It will explore probe sampling in a variety of environments and applications, as well as demonstrate the proper placement of nozzles to ensure sampling system accuracy. In addition, it will highlight considerations for when one should and should not use a probe for sampling. Appendices 1 and 2 at the end of this article provide guidelines on sampling accuracy with proper nozzle placement and when to use a probe for sampling.

Orienting angle-cut probes
Angle-cut probes are the standard choice in many fluid processing applications, especially in applications where the entry port faces downstream. They are durable, inexpensive, and generally effective. As Figure 3b shows, angle-cut probes typically feature 45° or 30° angles. The angle creates an oval-shaped entry point that is slightly larger than the pipe bore. Therefore, this type of probe is less likely to become blocked or clogged, making it ideal for process streams where solid particles or liquid droplets could contaminate the sample.

Because of the asymmetric cut, however, this probe must be properly oriented to the system flow. Typically, it should be oriented downstream, with some exceptions. No matter the orientation, it is good practice to engrave the desired probe orientation on the flange or valve to help installers.

Figure 4 is a simplified illustration of the velocity vectors experienced by a particle that is denser than the process fluid. Shown here, the particle has more momentum than the fluid and tends to travel at an axial velocity (UA) equal to the velocity of the process fluid in the pipe. The sample flows into the probe and creates a radial velocity (UR) that helps to pull the particle toward the entry port. The resultant particle trajectory (UP) veers toward the probe at an angle determined by the relative magnitude of the two velocities. The angle cut allows a veering particle to miss the probe entry port, while a square-cut probe is more likely to capture those particles along the UP trajectory.

Figure 2 also demonstrates how the probe’s UR draws particles in. This velocity can be controlled by adjusting the internal diameter of the probe. A wide-bore probe helps to minimise particle uptake, but additional purge time will be required. It is important for operators to account for any time delay this purge may cause.

Orienting square-cut and other probes
There are several common applications where angle-cut probes may be less than ideal, and a square-cut probe may be better suited. A few of them may include:
-    Liquid sampling at its bubble point temperature
-    Vapour sampling at its dew point temperature
-    Sampling at very low process pressure
-    Sampling an upward process flow
-    Sampling a gas containing very fine solids
-    Sampling for laboratory analysis.

In the first three examples, pressure variations in the process fluid when it streams past an obstruction — a probe, for instance — can create a challenge. Here, the stream pressure tends to slightly increase as the fluid contacts the probe and drops slightly in the probe’s wake. Because of this fluctuation, pressure within an angle-cut probe with an upstream entry point tends to become higher than the pressure in the surrounding fluid. Conversely, when the angle-cut probe entry point faces downstream, pressure within the probe is slightly lower than the surrounding fluid. Here, a square-cut probe can help yield a smaller pressure reduction in the sampled fluid — and may be preferred to using an angle-cut probe pointing downstream.

Each example listed above has unique considerations that operators must contemplate when deciding on proper orientation. The rest of this article will explore each in detail.

Sampling a liquid at its bubble point
Even the slightest pressure drop experienced by an angle-cut probe facing downstream can be enough to vaporise some liquid when sampling from a liquid at its bubble point. These conditions can impact the accuracy of a sample.

In this situation, the liquid in the probe is at a lower pressure than the surrounding fluid. Therefore, the liquid starts to bubble and cool slightly, resulting in a two-phase mixture that is neither stable nor representative. It will therefore result in an inaccurate analyser reading.

Since the bubble point occurs inside the probe, it is not visible and the operator may not notice this inaccuracy when it occurs. The vapour may then recondense in the transport line as the liquid cools, further impacting sample accuracy. The sample will have a variable composition and will generate erratic results.

If the stream is clean, an operator may reverse the angle-cut probe so the entry port faces upstream. This will cause a pressure increase in the probe, preventing bubbling. This is a convenient fix but can only be done in a clean stream — if the stream is dirty, the reversed angle-cut probe will collect particles, rendering an unacceptable sample.