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Ensuring reliability and accuracy with GWR

06 September 2016

Thomas Rundqvist explains how combined chamber/guided wave radar units provide a pre-assembled and ready-to-install solution that can simplify level instrumentation commissioning.

Reliable and accurate measurements are essential when attempting to optimise a process and maximise the output from existing equipment/infrastructure. Mechanical level devices, which are prone to failures and require regular maintenance, are increasingly being replaced by guided wave radar (GWR) which is helping to improve the accuracy and reliability of level measurements, while also reducing maintenance costs.

GWR can be applied to a wide range of applications and is usually installed directly into the storage vessel. However, some vessels have restrictions including industrial agitators, heat exchangers and other structures situated within the vessel that can prevent direct installation. By installing the radar in a separate chamber that is attached to the vessel, these issues can be bypassed.

This also 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.

Measurement accuracy
When a chamber is used, the level inside it must replicate the level within the vessel as closely as possible in order for the GWR to provide accurate measurements. However, there are certain factors relating to the chamber that can affect measurement accuracy. These include the chamber diameter, the size of the process connections between the chamber and the vessel, and the ambient conditions.

The temperature of the fluid in a chamber can be different from the temperature of the fluid in the process vessel. This can cause the volume within to expand or contract which will change its representation of the level. The use of insulation/lagging around the chamber can help to prevent this effect.

Another consideration is condensation of vapours. This can result in the build-up of additional fluids in the chamber that are not present in the vessel, leading to an inaccurate level measurement. This is most common with light end hydrocarbon vapours where the fluid stratifies on top of the measured fluid. Again, effective insulation and lagging of the chamber can help to prevent this effect.

Correct chamber sizing
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.

Chambers are available in a range of diameters. The smallest are usually 2in in diameter, and these offer the most cost-effective solution. Larger diameters are available and are usually deployed where an application is susceptible to the following issues:

Outgassing: If the pressure in the system drops, the result could be outgassing. The pressure drop causes gas bubbles to push the surface level artificially higher, which leads to the measurements being unreliable. In chambers with a smaller diameter, the effect of this can be exaggerated. Larger diameter chambers, however, are less restrictive, so the effect of outgassing on the liquid level is reduced.

Probe grounding: It is vital that the probe should be prevented from touching the chamber wall, or even getting close to it. The preferred option in narrow chambers is rigid probes, but care needs to be taken with installation, so that bending is prevented. Centering discs must be fitted when the probe is longer than 1.5m, so that the probe does not contact the chamber wall.

Fluid circulation: In order to prevent stagnant fluids and deposition in the chamber it is important to minimise any restrictions between the vessel and the chamber, and to use both larger diameter connections and short process connection piping distances. Insulation and heat tracing will prevent freezing or solidification of fluids. In applications where fluid or forceful vapours will be introduced into the top connection, a small restriction may be needed to prevent any disturbance on the probe.

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.

Radar and probe selection
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. In very low dielectric but clean fluids, such as liquefied gases (for example LNG) a coaxial probe may be used. In 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. The chamber and GWR must also be compatible with suitable fittings that match those of the vessel and selected GWR. Any errors can make installation difficult or even impossible.

A GWR and a chamber can be bought separately and then fitted together, but there are drawbacks to this solution from the user’s point of view. Assembling a GWR and a chamber separately is time-consuming, and requires the right skills to do the job correctly. There could also be issues if the two items are being bought from different suppliers, involving separate documentation, which could, in turn, mean longer lead times. Testing has to be carried out once the GWR is fitted into the chamber, and there is always a chance that the sizing of the chamber and the probe could prove incompatible.

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.

Thomas Rundqvist is a marketing manager at Emerson Process Management.

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