Accurately measuring levels in vessels is important in many aspects—safety, efficiency, reliability and availability to name a few. Mechanical level measurement devices historically have been failure prone and have required regular maintenance.Guided wave radar (GWR) technology avoids the problems with associated with mechanical devices, but is affected by obstructions in the vessel such as agitators, heat exchangers and other structures. In a Control Engineering Europe article, Ensuring reliability and accuracy with GWR, Emerson’s Thomas Rundqvist describes how a separate bypass chamber combined with guided wave radar level measurement can be used successfully for these vessels.
…isolates the radar transmitter for maintenance purposes, which is desirable for applications involving high temperatures/pressures or hazardous liquids. A further advantage is that if there is turbulence in the vessel the chamber acts as a stilling well, providing a calmer, cleaner surface and helping to increase measurement reliability and robustness.
Sizing the chamber is important to accurately measure the level inside the vessel. Some factors affecting accuracy include:
…the chamber diameter, the size of the process connections between the chamber and the vessel, and the ambient conditions.
Differences in temperature, vapor condensation and liquid stratification can also cause differences in level between the chamber and vessel. Insulation and lagging can help to prevent these differences.
Thomas provides tips on how to size the chamber correctly.
The chamber length is specified to accommodate the desired measurement span. The upper and lower portions of the chambers should be designed to accommodate the upper and lower blind zones of the GWR. The upper blind zone is the minimum measurement distance between the upper reference point and the product surface. At the end of the probe the measuring range is reduced by the lower blind zone. The blind zones vary depending on the probe type and fluid dielectric properties.
Larger ones may be needed for fluids that may outgas, to avoid GWR probe grounding and to prevent fluid stagnation in the chamber.
Combining the guided wave radar instrument with the chamber can simplify the installation and avoid these issues.
Thomas also provides considerations for the radar and probe selection [hyperlink added].
For most applications, single lead probes are the best choice. They are less susceptible to build-up and are more tolerant of coating than twin or coaxial probes.
For liquefied gases, such as LNG, that has very low dielectric values but is a clean fluid benefit from coaxial probes. Steam applications:
…such as feedwater heaters and boilers, when the pressure is greater than 27.6 bar, the dielectric of the steam vapour will impact the level accuracy. Here special probes are required that compensate for this situation which may require a longer chamber top end.
To simplify the commissioning process, ensure the correct sizing and combinations of chamber and radar, and support the need for faster installation and reliable measurements immediately, combined chamber and GWR packages should be sought. These need to incorporate a correctly sized chamber, GWR and probe as a complete, ready-to-install unit, helping to avoid many of the pitfalls described in this article.
Read the article for more on these tips and see the technical note, Combining Guided Wave Radar with Rosemount 9901 Chambers for help in sizing the chambers for your applications.
You can also connect and interact with other level measurement professionals in the Level group in the Emerson Exchange 365 community.