Guided Wave Radar (GWR) transmitters
Best Practices for Guided Wave Radar Transmitters in Chambers
Factors Affecting Measurement Accuracy
If the pressure in the system drops, out-gassing may occur. This causes gas bubbles to push the surface level artificially higher (Figure 2). This effect may be exaggerated in a narrow chamber. A wider chamber is less restrictive and out-gassing has less effect on the liquid level.
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.
The temperature of the fluid in a chamber may be different from the temperature of the fluid in the process vessel, causing the volume within to expand or contract and so changing the level. Insulation can help to prevent this (Figure 1a).
Condensation of vapor can result in the build-up of additional fluid in the chamber. This is especially common with light hydrocarbon vapors, where condensate stratifies on top of the measured fluid. Again, effective thermal insulation can help to prevent this.
With all chamber installations, good fluid circulation is essential 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. Insulation and heat tracing will minimize temperature changes and prevent condensation, freezing, or solidification of fluids. In applications where fluid or high vapor flowrates will be introduced into the top connection, a small restriction may be needed to prevent any disturbance to the probe.
Existing and New Chambers
When replacing a traditional level measurement device such as a displacer, there may already be a chamber in place which may be suitable for a GWR transmitter. Care should be taken to ensure compatibility with the GWR instrument chosen. Common errors include an incorrectly sized probe which either touches the bottom of the chamber or is too short to measure properly at low levels (see below for more about probe length).
The existing chamber should be checked for signs of corrosion, stress or fatigue, particularly around the welds. Vibration and temperature cycling can create dangerous cracks, especially in chambers that may have been in place for many years.
For new chambers, check that the manufacturer uses only certified and traceable materials, and that the chamber is manufactured and designed to national standards such as ASME or PED. Poor chamber construction can be a source of frustration for the instrument installer: typical issues include snagging of the centering disk, and chamber discontinuities causing noisy signals.
A further consideration is that the chamber process connections and instrument connection must be sized to match the vessel and instrument connections respectively. The location of the bottom connection will determine the chamber style (side and side, or side and bottom process connections). The chamber centre to centre (CC) dimension is critical and must match the process vessel CC. Once the total chamber length is determined, it is important to verify that there is sufficient head and ground clearance above and below the chamber, allowing for the instrument and drain (Figure 3).
The chamber length is specified to accommodate the desired measurement span. The upper and lower portions of the chamber should be designed to accommodate the upper and lower transition zones of the GWR. The upper transition 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 similarly reduced by the lower transition zone. The sizes of the transition zones vary depending on the probe type and fluid dielectric properties.
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