Analyzers / Design & Simulation / Reaction & Synthesis

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

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.

The occurrence of bubbling may not be evident. You can’t see it. And the vapor may recondense in the transport line as the liquid cools. Alas, your sample now will have a variable composition and yield erratic results, particularly if you’re measuring light components.

Sometimes, there’s a quick fix. If the stream is clean, reverse your probe so the entry port faces upstream. Pressure will increase in the probe, which will stop the bubbling. However, remember your probe will collect particles in this orientation. If the process liquid is dirty, an upstream-facing port won’t work.

You instead may have to experiment with a square-cut probe, which will maintain a more even internal pressure and minimize bubbling. It may help to locate the nozzle for the probe in the bottom of the line. This way, the probe reaches up into the flow and encourages any bubbles to rise into the process stream and away from the probe. Make the probe wide enough to realize an internal liquid velocity of about 0.1 m/s so the bubbles can escape.

A better solution — if feasible — is to find a location with more pressure to prevent bubbling at the probe. If no process pump is available, try moving the sampling tap to a lower elevation in the same line. Liquid pressure increases at lower levels and the higher pressure may suffice to stop the liquid from boiling.

Sampling a vapor at its dew point. For such an application, the angle-cut probe is very effective with its entry port pointing downstream. The lower pressure in the probe precludes the possibility of condensation within the probe and may vaporize any fine liquid drops present in the process.

However, this might not suit gas pipeline operators, which sometimes want to sample only the hydrocarbon vapor from a line containing condensed hydrocarbon liquids. Obtaining a meaningful vapor sample from this two-phase process stream is notoriously difficult.

Using an angle-cut probe pointing downstream isn’t an option, as the pressure drop inside the probe would vaporize some of the liquid droplets and immediately spoil the sample for analysis. Instead, opt for a square-cut probe, as it minimizes the pressure drop and liquid content in the extracted sample. Subsequent sample conditioning is necessary; it hopefully will remove any remaining liquid while ensuring the sample temperature and pressure don’t change. It’s a difficult operation, typically done manually, because the slightest temperature or pressure change causes the entrained liquid to evaporate or some of the vapor to condense, irretrievably ruining the sample.

To remove the liquid phase completely without changing sample temperature and pressure, use a special probe that houses a coalescer element or membrane filter in the body of the probe.

In less critical applications, an angle-cut probe with the entry port facing upstream also is an option. Because this orientation slightly increases pressure in the probe, liquids won’t vaporize into the extracted sample gas. However, the pressure rise may induce some minor local condensation. This arrangement may improve aerosol removal and liquid drainage, particularly if the sample extraction velocity is low. Figure 3 illustrates the concept: fast moving droplets impinge onto the exposed inner surface of the probe and coalesce to form larger drops that drip back into the pipeline from the sharp tip of the probe.

Sampling at a very low process pressure. The small pressure drop caused by an angle-cut probe facing downstream isn’t welcome when you’re sampling from a process stream that’s already at very low pressure. The probe reduces the pressure available to drive the sample to the analyzer location and on to its disposal point, thereby increasing the time delay for analysis.

However, if you’re running a fast loop back to process, you can compensate for the lost pressure at the sample takeoff probe by using the same kind of probe in your return tap. The pressure loss will be the same, so the full differential pressure is available across the fast loop.

If your sample is clean, point the sample takeoff probe upstream and the sample return probe downstream. This arrangement enhances the differential pressure between the sampling taps. However, remember it also will increase the number of particles in the fast-loop flow.

Sampling an upward process flow. It’s often a good practice to locate a sampling probe in a vertical process line flowing upward. If you install an angle-cut probe with its entry port pointing up, you may need to worry about large particles and debris falling against the process flow. If they do, the probe tip will scoop them up and become blocked. To avoid that risk, a straight-cut probe may be a better choice. Or, perhaps less conveniently, you can use an angle-cut probe in a 45° nozzle and align the end parallel to the flow.

Sampling a gas containing very fine solids. In most process applications, an angle-cut probe with its entry port facing downstream is very effective because any particles present are too heavy to follow the fluid flow as it turns into the entry port. However, this advantage may be lost when sampling from a gas stream containing particles that are fine enough to stay with the gas as it swirls around the probe. Eddy currents on the downstream side of the probe will carry those particles into the angled portion of the probe. For these types of samples, a square-cut probe is a better choice, as the fine particles will have less of a chance of entering the probe.

Sampling for laboratory analysis. Taps for laboratory samples often use angle-cut probes with their entry ports facing upstream. This is the opposite orientation to most probes for continuous process analysis and reflects the laboratory preference for the sample to include any solid particles and liquid droplets carried in the process stream. Laboratory sampling frequently must adhere rigorously to strict industry-standard procedures. Note, however, that a difference in probe orientation may explain a difference in analytical measurements.

MAKE THE RIGHT CHOICE
Proper probe location and orientation are best determined by process stream conditions and application needs. Angle-cut probes will be your design of choice in most cases. However, you must pay attention to the direction the angle cut faces to account for natural pressure fluctuations on the downstream side of the probe, as well as particles within the process stream. Occasionally, use of an angle-cut probe is ill advised. Instead, you’ll want to opt for a square-cut probe. Ultimately, the right probe design and orientation will help you extract a representative and timely sample to ensure accurate analytical measurements.

CHECK OUT THE ENTIRE SERIES
This is the final article in a three-part series. The first article focused on nozzles — “Properly Position Sampling Nozzles.” The second discussed when to use a probe — “Should You Use a Probe for Sampling?” All three articles are adapted from “Industrial Sampling Systems: Reliable Design and Maintenance for Process Analyzers,” a process sampling textbook authored by Tony Waters and published in 2013 by Swagelok Company. For more information, visit www.industrial-sampling-systems.com


DEAN SLEJKO is product manager, analytical products, for Swagelok Company, Solon, Ohio. E-mail him at dean.slejko@swagelok.com.