What products are involved?
SRD - Inline Density and Viscosity Meter
What is the purpose of this article?
To give an overview of the SRV NPT 3/4”-14 variant, installation guidelines, instructions, special considerations and available accessories.
1. IntroductionSRD is Rheonics sensor for inline density and viscosity measurement. Read more on SRD here.
The NPT 3/4”-14 connection is Rheonics most commonly used design for SRD. In most cases, it is the most price-efficient option with the shortest lead time and the largest selection of accessories.
NPT threaded connections are very common in industrial applications. It is used both in low and high-pressure environments, up to 500 bar / 7500 psi. This type of connection allows for an easy process integration using some of Rheonics accessories or a thread in a pipe or vessel.
This threaded connection requires the use of a Teflon band or compound. This helps to ensure a sealed connection and avoids cold welds (the process connection adaptor may get stuck with the sensor, leading to severe damage).
Figure 1: SRD NPT dimensions.
Specifications | |
Sensor | |
Order code | SRD-X1-34N |
Connection type | NPT |
Thread size | 3/4" |
Threads per inch (TPI) | 14 |
Wrench for installation | 32mm (1-1/4") |
CAD files | |
2. General Installation GuidelinesFollow the next installation guidelines to ensure the best possible performance of the SRD sensor.
i. Sensing area immersed in fluid
ii. Avoid dead zones or deposits
iii. Recommended tip orientation in perpendicular installation
iv. Thermal balance for high accuracy
2.1 Submersion
The SRD has a sensing element shown inside the red section in Figure 2. The sensor measures what is within a certain area called sensing area, so this should always be fully submerged in the fluid. This can be a problem when on a pipe, the flow rate is low and the pipe isn’t usually full, or when the sensor is installed too far out of the main pipe (like in a long standpipe) where fluid is not flowing but is almost static. Notice that a clearance of at least 12 mm is required between the sensor tip and a pipe wall or any other obstruction.
Generally to ensure proper immersion in perpendicular orientation with the fluid, the SRD-X1-34N is installed with the WOL-34NS or WOL-34NL. For parallel (elbow installations) the flow cell IFC-34N-SRD is recommended.
Figure 2: SRD sensing zone.
2.2 Stagnation/dead zone
For accurate measurements stagnations and dead zones must be avoided. These are defined as any space, inside the sensing area (red zone) explained above, where fluid or solids can accumulate and deposit, or where there isn’t a good fluid exchange, which means that old fluid is not being replaced by new fluid as it flows through the line.
2.3 Orientation dependency
SRD has a sensing tip that is recommended to be aligned with the fluid flow direction to avoid creating recirculation zones around it. This is only relevant for perpendicular installations. Read more on this topic, here.
Figure 3: SRD sensing tip orientation.
2.4 Thermal balance
It's important to consider that the short version of the SRD (e.g. SRD-X1-34N) is not good to maintain accuracy on its density and viscosity readings over a large temperature mismatch between ambient and fluid. It is suitable for high accuracy application only when ambient conditions (at probe back end) and fluid temperature are at similar (within 15 °C difference). A bigger temperature mismatch between front (fluid) and back (body) of resonator causes a frequency shift that influences readings. For applications with large differences in ambient and fluid temperature, you need the long insertion probe version of SRD so both ends of the resonator are at the same temperature. Learn more on this here: Maintaining the SRD's temperature balance for high density accuracy
2.5 Additional information for installation:
Flow
For Newtonian fluids, the flow rate does not affect the measurements, so SRD should measure the same viscosity and density values in static and moving states of a fluid at the same temperature. For non-Newtonian fluids, the flow rate does matter and viscosity readings may differ between static and moving conditions. Density reading won't be affected by the type of fluid (Newtonian or non-Newtonian) or its state (static or dynamic) as long as the fluid uniformity is the same.
For processes with non-constant flow rates, the recommendation is to install the SRD in a section in the pipeline with the most consistent flow rate to have steady reading values.
Flow rate is also relevant to ensure the full submersion of the SRD sensing element into the fluid. For a process with a low flow rate, pipes may not be full of fluid at all points, so SRD should then be placed in a section where the pipe is usually full of fluid (i.e. after a pump). Installing the sensor upwards can also solve the issue but should not be considered for fluids with solids or that can create deposits around the sensing element.
In all cases, the SRD’s sensing element should be submerged in the fluid. It’s best to avoid installations with long standpipes (i.e. using long weldolet or tee), since that may lead to poor fluid transfer resulting in measurements that do not reflect the true state of fluid or worse high noise measurements.
Fluid type
Rheonics SRD requires a correct installation for accurate, consistent and repeatable readings. Regarding the fluid types compatible with the probe, consider the following:
- Avoid bubbly fluids or high percentage of bubbles or air in fluid. Bubbles can add too much noise on the SRD readings. Seek for Rheonics Support to evaluate if filters can be applied to minimize the effect.
- High speed mixing applications in tank may affect SRD's repetability and accuracy. It is generally recommended to install the SRD in lines where the flow direction is well defined.
For more information review this article.
3. Installation common issuesFor accurate, repeatable and reproducible readings of the SRD it is important to maintain the sensing element immersed and without stagnation zones that can lead to deposits around the sensing area.
Below some examples are listed where these concepts are not achieved leading to potential issues for the SRD density and viscosity readings.
3.1 Potential issues- NPT Tee connection
Potential issue explanation: A common mistake with installations of the SRD-X1-34N is using an off-the-shelf NPT Tee shown in Figure 5. An installation of the SRD mounted vertically or horizontally in a Tee port can create stagnant and dead zones. Also, the SRD tip may pass through the tee and the sensing area (shown in Figure 2) can be obstructed and exposed to uneven inner walls that can create changes in the fluid’s flow lines (increasing turbulence) as in Figure 6.
Figure 5: Standard NPT Tee [6]
Figure 6: Installation in standard NPT Tee with potential issues
Recommendation: In these cases, the Rheonics IFC-34N-SRD is recommended to ensure the flow surrounds the sensing element properly and stagnant zones are removed as in Figure 7.
Figure 7: Correct installation in Rheonics NPT flow cell
3.2 Potential issues - NPT connection adapter
Potential issue explanation: At times, users may want to adapt the 3/4” NPT connection of the SRD-X1 to a different flange, like Tri-Clamp. They would use an adapter similar to the one shown in Figure 8. Mounting the SRD probe in these adapters can lead to both incomplete immersion and stagnation zones prone to fouling or deposits at the base of the sensing element which will affect the measurements (see Figure 9). This scenario may not affect applications for low-viscosity fluids like ink but could be a significant problem for high-viscosity fluids like dough, adhesives, glue, and others.
Figure 8: Standard NPT to Tri-Clamp adapter [7]
Recommendation: The client should order the SRD with the required process connection to directly mount the probe inline without the need for additional adapters (Figure 9, installation to the right). If Tri-Clamp is required visit the SRD-X3. If custom flanges are required visit the SRD-X2.
Figure 9: Installation recommendation with NPT adapters
3. Process connection installation instructions3.1 Connection steps
Optional: Wrap a sealing compound (e.g. Teflon tape) around the sensor NPT thread 1.5 to 3 times in a clockwise direction, to ensure a leak-free seal for some fluids.
i. Place the sensor in the female threaded accessory.
ii. Turn by hand the sensor NPT thread until hand tightened.
iii. Use a wrench (32mm size or 1-1/4") to give the sensor from 1 up to 3 more full turns.
In total, according to ASME Standards, at least 3.5 and at maximum 6 full turns need to be ensured for a correct NPT installation. The sensor shouldn't be completely tightened.
3.2 Specific Process Connection installation instructions
In line: Ensure the sensing element is exposed to fluid, and avoid stagnation and dead zones. Consider placing the sensor upwards or parallel to the fluid for small flow rates that can’t fully surround the sensing element.
In tank: Place the sensor at the bottom, wall or lid of the tank as long as the sensing element can be fully submerged in the fluid and dead zones are avoided. Usually, it is useful if the sensor is placed at a tank’s height that has always fluid. Rheonics TMA-34N accessory is a tank mount adapter used for NPT SR-sensors, that should be considered in tank installations. This is detailed sections below.
Others - Thread flange: SRD NPT sensor can be mounted on a flange thread of the same size (3/4"-14). The flange needs to have an inner diameter larger than 26mm on the process side, the material should be either 304 or 316L and the height can't exceed 25mm. If the flange has a bigger height, an FPC-type (long insertion) sensor is needed.
Once again, in these and any installation cases, stagnation zones around the sensing element should be avoided. For thread flanges, the height of the standpipe of the connection is relevant as shown in the next figure.
4. Accessories available4.1 Weldolets
a. WOL-34NS
Weldolet for 2” pipes ANSI standard (60.3 OD) that ensures maximum exposition of the sensing element to the fluid. Also suitable for SRV NPT 3/4” -14. Refer to the WOL-34NS page for more information.
Figure 11: WOL-34NS
b. WOL-34NL
Weldolet for 2.5” and larger pipes, also suitable for tank installations, that ensures maximum exposition of the sensing element to the fluid. Also suitable for SRV NPT 3/4” -14. Refer to the WOL-34NL page for more information.
Figure 12: WOL-34NL
4.2 Flow cell - IFC-34N-SRD
Suitable only for SRD NPT 3/4”-14 and recommended for inline installation with nominal diameters of DN5 to DN25. Connecting threads for this accessory are NPT 3/4” -14. (Another variant is available for the SRV NPT).
For a diameter larger than DN25 (1”), we recommend using a bypass. Then, the flow cell can be connected via flexible hoses or fixed piping. Installing the sensor in a bypass is a common solution for process lines that require intense cleaning through means of pigging.
Read more about this on its accessories page.
Figure 13: IFC-34N-SRD
4.3 Tank mount adapter - TMA-34N
Suitable for SRV and SRD only in their NPT 3/4"-14 variant, this accessory is recommended for open vessels or tanks with a lid. It is composed of a small cell, wherein the sensor is installed, and a 3/4" pipe fixed at the top and with a variable length so the sensor can go as far as needed.
Read more about this on the accessories page or in its support article here.
Figure 14: TMA-34N with SRD.
References[1]: https://es.rheonics.com/products/inline-density-meter-srd/
[2]: https://rheonics.com/product-accessories/
[3]: https://rheonics.com/product-accessories/wol-34ns/
[4]: https://rheonics.com/product-accessories/wol-34nl-accessory-page/
[5]: https://www.asme.org/codes-standards/find-codes-standards/b1-20-1-pipe-threads-general-purpose-inch
[6]: https://www.mcmaster.com/products/tees/low-pressure-stainless-steel-threaded-pipe-fittings-9/