Nuclear Level Measurement
Nuclear (gamma) level measurement technology has been applied effectively for measuring liquids and solids for more than 30 years. A typical system consists of a gamma ray source, a detector and microprocessor. The gamma source is normally mounted outside the vessel, and emits energy through the walls and contents, collimated towards the detector, which is mounted on the opposite side. The gamma energy emitted reaches the detector when the vessel is empty. As the process level rises in the vessel, energy reaching the detector will decrease in an inversely proportional relationship to the level. A computer processes the detector signal and transmits the process variable as 4-20 mA analog or digital fieldbus signal.
Gamma technologies are an attractive option to users because they can usually be mounted external to the tank or vessel, allowing for installation and maintenance without modifications to the tank or interrupting the process. With pressurized or exotic alloy vessels, this can be a major advantage since adding openings to accommodate a sensor may not be practical.
|Multiple gamma sensors, mounted outside the tank, help see through buildup on tank walls for a true level reading.|
Gamma sensors have capabilities that make them particularly suited for applications that other technologies find difficult:
Solid or liquid contents not a factor;
Immune to internal obstructions;
Wide range of process temperatures;
Chemical characteristics not critical;
Unaffected by turbulence, flow variations; and,
Reads through mist, foam and heavy vapor density.
Technologies such as radar, ultrasonic and differential pressure are all affected to some extent by foam or changes in gas density, giving errors in critical level measurements. A gamma system can easily and economically compensate for these effects with the addition of a second point detector which is coupled to the processor. This is illustrated in the externally mounted sources graphic. The upper sensor provides a reference beam through the air space, providing an accurate picture of changes in internal density that might affect the actual liquid level reading. An auto calibration feature built into the signal processor enables the system to nullify the effects of any build-up on the vessel walls, giving a true level measurement of the process.
Lower radioactivity required
Gamma based technology still requires licensing, however the safety requirements for plant and personnel have become less stringent as necessary radiation levels have decreased.
Historically, detector technology consisted of Geiger-Müller tubes or ion chambers, necessitating high energy gamma sources to function. These radiation sources were problematic in that they required specialized licensing and safety precautions. In recent years, detector sensitivity has increased dramatically with the use of scintillation materials such as sodium iodide (NaI), and plastic (PVT) crystals. The disadvantage of these materials is that they are rigid, heavy, and limited to individual lengths of 15 ft (4.6 m). However, newer flexible detector technology using lightweight scintillating fill-fluid or fiber-optic bundles encased in liquidtight armored sheathing have overcome these problems. The flexibility of the detector enables it to be contoured around horizontal or spherical vessels or parts of the vessels where space is limited, ensuring complete coverage over the desired measurement range.
The maximum temperature for scintillation fill-fluid sensors is 80
These improvements in detector sensitivity have led to the introduction of RLL (radiation low-level) source designs, which use a fraction of the isotope activity required to make the same measurement with an older sensor. Installations previously requiring cesium 137 or cobalt 60 sources can now operate with activity levels more than 90% lower and still give greater degrees of accuracy and repeatability while maintaining a 15 year average working life. RLL technology utilizes up to a total of 0.033 GBq of cesium source, compared to some conventional density gamma gauges using sources with a hundred times more activity to make the same measurement. In some cases these RLL sources do not require licensed personal to install, commission or move them. Ongoing maintenance costs are also lower as the RLL does not require shutter checks.
For complicated applications
Complex level measurements which have proved challenging for traditional technologies are easily and economically performed by gamma gages. For example, how do you determine the density profile and levels when there are multiple phase products in the same vessel? The device illustrated in the moving interface graphic depicts a system consisting of a low-energy gamma emitting source, a detector, retraction mechanism and microprocessor. The mounting of the source and detector depends on the vessel shape and size. The illustration shows both the source and detector mounted in the vessel in sealed wells.
|Moving source/sensor combinations can detect and measure any number of stratified products in a vessel.|
The source and detector are raised and lowered in tandem in their respective wells through the density phases. Attenuation of the gamma energy is inversely proportional to density; therefore as the source and detector move through process layers, the detector output will change accordingly. The detector converts gamma energy to an electrical signal, which is passed to the microprocessor where the actual densities are calculated via proprietary algorithms.
By using non-contact technology, no components are wetted to the process, making it ideal for processes with harsh conditions, such as high temperatures, high pressures, corrosive, abrasive, or toxic products. Typically, the measurement is repeatable to a process density of
While gamma level sensors may have been considered, historically and traditionally, a technology to use only when all others have failed, current product design offerings have made it far more attractive and easier to use in a wide variety of mainstream applications.
|Drew Cheshire is general manager, Ronan Measurements Division. Reach him at firstname.lastname@example.org .|