Correctly Orient Probes in Sampling Systems

Selecting the right probe design by itself isn’t enough to ensure good results

By Dean Slejko, Swagelok Company

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Most analytical sampling systems use probes to extract samples from the middle of a process stream for analysis. The probe, a proboscis-like tube, sucks up process fluid and delivers it to an analyzer. How you orient the probe has implications on sample and particle uptake, analysis time delay, sample conditions and a variety of other factors that may slow down or otherwise compromise analysis.


Figure 1 shows two typical probes that differ in their sample entry port location. The simplest probe (a in the figure) has a square-cut end; it’s symmetrical and has no preferred orientation to the flow. The second probe (b) has an angle-cut end, forming an entry port that usually points downstream.

It’s important to know when and where to use each of these probe designs to ensure timely, accurate sample analysis. Below, we’ll review each design in more detail, noting the difficulties present and precautions to take when sampling from a variety of environments.

ANGLE-CUT PROBES
The most common probe used in process-plant sampling systems features an end that typically is cut 45° or 30° from normal. The angle cut results in an oval entry port that’s slightly larger than the pipe bore and, therefore, a little less likely to get blocked. Because the probe is asymmetric, it must be oriented properly to the flow. In most process applications, the entry port should point downstream. However, as we’ll discuss, some applications favor the port facing the flow. Consider engraving the desired probe orientation on the flange or valve to assist installers.

When its entry port points downstream, an angle-cut probe is very effective at keeping solid particles or liquid droplets out of the extracted sample. Figure 2 is a simplified illustration of the velocity vectors experienced by a particle that’s denser than the process fluid. 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 flow into the probe creates a radial velocity (uR) that tends 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; a square-cut probe is more likely to capture those particles along the uP trajectory.

Figure 2 also demonstrates how the probe’s intake velocity (uR) draws particles in. You can control this velocity by adjusting the internal diameter of the probe. To minimize particle uptake, choose a wide-bore probe. However, be sure to account for any time delay the wider-bore orifice may cause due to the additional purge time that it will need.

Angle-cut pipe probes are effective, durable and inexpensive. They now are the de facto standard at process plants and are very common, especially with the entry port facing downstream.

SPECIAL SITUATIONS
While an angle-cut probe with a downstream-facing entry port usually is a good choice, a few applications work better with the probe orientation reversed or with a square-cut probe. So, let’s look at some sampling situations that may require special care:

• a liquid at its bubble point temperature;
• a vapor at its dew point temperature;
• a very low process pressure;
• an upward process flow;
• a gas containing very fine solids; and
• for laboratory analysis.

The first three relate to pressure variations in a process fluid when it streams past an obstruction such as a probe. The stream pressure increases slightly as the fluid comes into contact with the probe and drops slightly in the wake of the probe. Therefore, the pressure within an angle-cut probe that has its entry port pointing upstream tends to be slightly higher than the surrounding fluid’s pressure while the pressure within a probe that has its entry port pointing downstream is slightly lower than the surrounding fluid’s pressure.

A square-cut probe also yields a slight reduction of pressure in the sample — but far less than that of an angle-cut probe. Because of this, a square-cut probe occasionally is preferred.

Sampling a liquid at its bubble point. The slight pressure drop experienced in an angle-cut probe facing downstream may suffice to vaporize some of the liquid. Because the liquid in the probe is at a lower pressure than the surrounding fluid, it starts to bubble and cool slightly. The result is a two-phase mixture that’s neither stable nor representative — and that, therefore, subverts analysis accuracy.

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  • <p>Once again, Thank you for the 'refresher'</p>

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