Managing interference radiation in nucleonic level measurements
In this paper we will identify common sources of interference radiation, and explain how they affect nucleonic measurements. Also we will elaborate how Berthold with its highly sophisticated RID feature helps plant operators to maintain a reliable and repeatable measurement.
Mehret Dieter, Mehret Dieter and Thomas Schmidt
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Radiometric measurements for industrial processes have been around for many years. They are a mainstay in making the most difficult level, density and bulk flow measurements. Nuclear measurement gauges work where no other technology will. They give excellent performance under hostile and rugged conditions. High temperatures, pressures and other difficult industrial processes usually pose no problem for a nuclear measurement. Typical measurement tasks include level measurement in reactors, any kind of vessels or tanks, density measurement, phase separation levels in oil separators or measurement of moisture content. Also, they can be used as contactless limit switch.
A. What is a Radiometric Measurement?
Nuclear measurement gauges operate on a simple yet sophisticated concept – the principle of attenuation. A typical radiometric measurement consists of
• A source that emits γ-radiation, produced from a nuclear radioisotope
• A vessel or container with process material under investigation
• A detector capable of detecting γ-radiation.
If there is no or little material in the pathway of the radiation beam, the radiation intensity will remain strong. If there is something in the pathway of the beam, its strength will be attenuated. The amount of radiation detected by the detector can be used to calculate the desired process value. This principle applies to virtually any nuclear measurement.
Nuclear measurement technology is highly reproducible. Using the laws of physics and statistics as well as sophisticated software, the success of making any nuclear- based measurement is almost 100 percent. Considering the benefits of a totally non-contacting and non-intrusive technology, nuclear measurement technology becomes the number one method for the most difficult and challenging process measurement applications.
1) Radiation Sources
There are many known natural and artificial isotopes, not all of them are used for radiometric measurement. In industrial applications only, a few nuclides are really used for measurement purposes. The radioactive isotope is usually placed in a rugged, steel-jacketed, lead housing for maximum safety. The housing shields the radiation, emitted from a radioactive isotope, except in the direction where it is supposed to travel. Using a small collimated aperture in the shielding, the beam can be projected at various angles into the pipe or vessel. This warrantees a high quality of measurement with minimal exposure of personnel to radiation. Basically, the ALARA (As Low As Reasonably Achievable) principle for maximum work safety applies on everything that has to do with nuclear isotopes.
The radiation detector contains a crystal made from a special polymer material or an inorganic crystal, like doped sodium iodide – the so-called scintillator. The scintillator converts the incoming gamma particles into flashes of visible light. The crystal is optically coupled to a photo multiplier tube, which converts light into electrical pulses. While the vacuum photomultiplier has been used successfully for decades, nowadays silicon photomultipliers (SiPM) are also available and have made their way to being used in industrial detectors.
Figure 4 shows schematically how a detector works. When the radioactive beam strikes the crystal, after having passed through the walls of the vessel, pipe and the process itself, each gamma photon in the beam generate a light flash, resulting in thousands of subsequent light pulses that are recorded by the photomultiplier tube. Each light pulse is converted into electrical pulses by the photomultiplier. After digitising of the signal, these pulses are counted to determine a so-called count rate, which is typically expressed as counts per second (cps) or frequency (Hz). The intelligence that distinguishes between various measurement tasks (i.e. level or density) with the designated media, is implemented in the transmitter or control unit. The count rate is used to deduct a process related signal which can be used for a display, an analogue current output or bus connections into a DCS or PLC.
The detector measures any γ-radiation arriving in the scintillator, without distinction of “useable count rate” deriving from the source or natural background radiation from the environment. We will learn later how interference radiation coming from weld inspection but also changes in natural background radiation, etc. can be handled.
a) Point Detectors
Detectors with a small scintillator are called point detectors. They often employ a small cylinder as scintillator, e.g. 50 mm diameter and 50 mm in height. They are typically used for density applications but also for level switch or continuous level measurements. Depending on the measurement task other scintillator sizes may be used. Due to the small sensitive volume of a point detector, the effect of background radiation is small. Additionally, point detectors can be easily equipped with a lead collimator to further suppress sensitivity to background radiation.
b) Rod Detectors
In some cases, it is beneficial to have the scintillator covering a longer range, this is called a rod detector. Typically, in level measurements either source or detector span the whole measuring range. Their length can be up to 8 m. The main benefit of a rod detector is its lower cost compared to a rod source. Albeit, the rod source would be the technologically superior system. The gamma radiation, which a rod detector is able to detect, is influenced by the geometry of the radiation array.
However, as rod detectors are typically not shielded (and shielding would diminish the cost advantage) they are much more sensitive for changes in natural background radiation making this effect dominant to most other errors. Especially considering that fluctuations of ±15% through accumulation of Radon-222 and its decay products, e.g. after rain, are possible.
Nuclear gauges work with the principal of attenuation. Basically, every matter interacts with γ-radiation and has an attenuating effect. Under process control perspective, this is not only the media to be measured, it is also the steel walls of the vessel, potential inside construction, insulation, framework, etc.
In every measurement it is necessary to manage the statistical and systematical errors, by applying stochastic methodologies. Besides that there are other error sources, that cannot be handled without additional provisions.
4) Temperature and Aging Effects
Can be reduced by applying top notch compensation methods. Sophisticated algorithms and methods independently measuring the sensitivity of a detector by comparing the signal to a known reference can be used to compensate for these effects. Such an automatic gain control or high voltage control should be included into the measurement system. For example, the algorithms used by Berthold Technologies rely on a spectral analysis of either the radiation received from used primary radioisotope or – even more sophisticated – of cosmic radiation.
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