Written by Emerson’s Marianne Williams along with Jörg Kempf, the article describes the case where a separate instrument chamber is required. Choosing the right chamber helps speed up installation, increase measurement accuracy and overall reliability.
The authors note:
…many vessels have restrictions that preclude this, including agitators, heat exchangers and other internal structures. To overcome this, instruments are usually installed in a chamber mounted on the outside of the vessel.
To improve accuracy:
…it is important that the level within the chamber replicates as closely as possible the level inside the vessel. Factors that may affect the accuracy of the level measurements taken in the chamber include the chamber diameter, size of the process connections between the chamber and the vessel, and the ambient conditions. As a general rule the use of larger diameter chambers and effective insulation will help to avoid many of the issues described below.
Factors that can affect measurement accuracy include outgassing from pressure drops, probe grounding, temperature differences between chamber and vessel, and vapor condensation. For outgassing, a wider chamber reduces the effects on the liquid level. For the issue of probe grounding:
Narrow chambers are more susceptible to the probe touching (grounding) or getting close to the wall of the chamber, especially as the length increases. Rigid probes are preferred in narrow chambers, but these must be installed carefully to avoid bending. It is recommended that a centering disc is used to prevent the probe from contacting the pipe wall when the probe is longer than 1.5 m. If flexible probes are used, provision must be made to pull the probe taut so that it does not touch the wall.
Chamber insulation can help minimize temperature differences with the vessel. Finally, it is important to design the chamber to have good fluid circulation:
…to prevent stagnant fluids and deposition in the chamber. To accomplish this, it is important to minimize restrictions between the vessel and the chamber, by using large-diameter connections and short piping.
When it comes to the selection of the guided wave radar probe, the authors note:
For most applications, single-lead probes are the best choice. When a GWR transmitter is used in a chamber, the microwave signals are guided and contained within the chamber.
Some advantages and disadvantages are described:
Single-lead probes are less susceptible to build-up and are more tolerant of coating than twin or coaxial probes. In very low-dielectric but clean fluids, such as liquefied gases, (for example LNG), a coaxial probe may be used.
The authors close by noting that some GWR suppliers can supply the chamber and the GWR transmitter to avoid some of the complexities cited above. They note that Emerson [hyperlink added]:
…offers a combined 9901 chamber and GWR solution designed to meet industry safety standards.
Guided wave radar level measurement is but one of many level measurement technologies. You might find the table in Rosemount Level Measurement guide a helpful resource in selecting the right level measurement technology for your application.