Reliable measurement solutions for the oil and gas sector are not a matter of course. Process conditions prevailing there are anything but ideal and thus pose great challenges for measurement technology, whether density or level measurements. Extreme harsh conditions such as high temperatures and pressures and aggressive mediums or multiple layers complicate the measurement and lead to a failure of the commonly used and popular technologies.
Where other measuring methods reach their limits, the use of radiometric measurements is unrivalled. The term radiometry can be derived from the two words ‘radius’ and ‘metrica’, the Latin words for ‘ray’ and ‘measure’. Therefore, radiometry basically means ‘measuring with radiation’ and that is exactly what happens. Radiometric measurements for industrial processes have been around for several decades. They are an important mainstay in performing the most difficult and critical level and density measurements and can even be used as limit switches.
Figure 1. Typical arrangement of a radiometric level measurement.
Principle of attenuation
A typical radiometric measurement consists basically of two components: a radioactive source and a detector, each mounted on the opposite side of the relevant vessel (see Figure 1). Thereby, the measurement is based on a simple yet sophisticated concept: the principle of attenuation. The source emits gamma radiation, which penetrates the vessel as well as its contents and is afterwards detected by the opposite placed detector. If there is no material in the radiation path, the intensity of the radiation remains almost the same, neglecting the attenuation by the vessel itself. However, if the radiation must penetrate matter, it is attenuated, whereby the attenuation increases with increasing density of the penetrated material. This means that the density of the material in the vessel can be determined from the radiation reaching the detector and a corresponding calibration. As this principle applies to almost every nuclear measurement, it can be used to determine not only the density of the material in the vessel, but also its filling level or to perform a limit switch task.
Typically, the radioisotopes Cobalt 60 (Co-60) or Cesium 137 (Cs-137), both high-energy gamma emitters, are used as sources for industrial applications. To avoid any unnecessary radiation exposure, the source activities used are designed to be as small as possible and application specific. The radioactive material is safely sealed and usually placed in a rugged, steel-jacketed, lead housing as a shield, which guarantees safe handling. The housing shields the radioactive beam except in the direction where it is supposed to travel. With the help of a small, resealable opening, the radiation outlet can be shut-off, thus safe and easy transport and installation of the shield is possible until the measurement is put into operation. Depending on performance requirements and economic aspects, customised point or rod sources are available. Shield construction and the use of high-density materials provide appropriate shielding and thus minimise the exposure of personnel to radiation. Besides the warrantee of a high-quality measurement, the ALARA (As Low As Reasonably Achievable) principle for maximum work safety applies to everything that has to do with nuclear isotopes.
Detectors used for radiometric measurements contain a small crystal made from a polymer material or sodium iodine, which is optically coupled to a photomultiplier tube. While the vacuum photomultiplier has been used successfully for decades, nowadays silicon photomultipliers (SiPM) are equally available and are widely used in industrial detectors. Since they are scintillation counters, the detectors work according to the following principle: when the radioactive beam strikes the crystal, after having passed through the vessel walls and the process material itself, each gamma photon in the beam generates a light flash that is internally amplified to thousands of flashes. These are then recorded by the photomultiplier tube and converted into an electrical signal, which can be used for a display or an analog output into a distributed control system (DCS) or programmable logic controller (PLC). Thereby, the desired measuring range of the vessel filling level, e. g. 0 – 1.000 mm, is transformed into a current signal of mostly 4 –20 mA. Due to the design of the detector and the available materials, the scintillation detectors are very robust and provide reliable and repeatable results even under extreme measuring conditions. There is a patented method for detector stabilisation available, which allows for reliable compensation of temperature influences and ageing effects, leading to excellent measuring performance over many years. The high sensitivity of the detectors is also a reason for the necessity of only low source activities and associated low dose rates or radiation exposure. To select the right detector for every application and every requirement, different detector types with different scintillator sizes (point or rod detectors) are available on the market.
Figure 2. Detectors for long measurement ranges mounted on a vessel.
Customised solutions for level measurement which ideally comply with the existing requirements may be achieved by using various detectors and sources. Depending on the measurement geometry, accuracy requirements and economic aspects, the perfect combination is selected. Radiometric based continuous level measurement systems are used to monitor measuring ranges from a few millimeters up to several dozen meters (Figure 2).
Level control, not only of single-phase media
Often more than one layer has to be monitored in a vessel. Due to separation processes, caused by different densities or chemical compositions, for example, several layers can make level measurement more difficult, especially if the levels of each individual phase are of interest. For the measurement of more than two phases or of interfaces that are not clearly defined, e.g. emulsions, a more complex measurement system is needed. Depending on the process, a transition phase changing in height or only very small differences in density of the individual phases must be expected and build-up on the vessel walls or installations inside the vessel are also not uncommon in separation processes. With multiphase level measurement systems, separating layers are reliably monitored and the dependent process can thus be optimally controlled. Nowadays quite a few different measuring systems are available. One example of such a system consists of several density measurements installed on different heights with detectors mounted on the vessel outside and sources inserted into a closed dip pipe inside the vessel to create a density profile of the vessel contents (see Figure 3). Using sophisticated algorithms, Berthold’s multiphase level measurement system, the so-called EmulsionSENS, can not only output the measured density values but can also determine the filling levels of each individual layer. The reliability of this system is also characterised by the fact that level determination is still possible even if one or even more detectors fail. Thus, the separation process can always be monitored.
Figure 3. Multiphase level measurement system and a resulting density profile.
Non-destructive testing as a challenge
Even the most robust measurement has its challenges. For radiation-based measurements, this is interference radiation in industrial complexes. Interference radiation is caused by non-destructive testing, which are typically weld or vessel integrity inspections and are basically performed on a daily schedule. A low-energy radiation source, such as Iridium-192, will likely be used. Such interferences can cause a significant increase in the count rate detected by the detector and therefore a misinterpretation of the supposedly measured level (Figure 4). Since neither the timing nor the impact of interference radiation is predictable, radiometric measurements require a reliable solution for dealing with such events.
Over time, the philosophy, and the technology for dealing with interference radiation, has changed and become increasingly sophisticated. Where at the beginning a manual notification to the control room about a pending weld inspection was necessary, a separate detector was later installed, which measured and signalled only the external radiation. Some manufacturers still rely on this solution today, whereas other manufacturers now use sophisticated algorithms instead of an additional detector. In this case, the detection of interference radiation leads to a freezing of the measured value or, even better, to a discrimination of the interference. These features ensure a continuous process flow, avoid unplanned shutdowns, and thus provide real added value when using radiometry. Berthold detectors are available with such features, known as XIP (X-Ray Interference Protection) and RID (Radiation Interference Discrimination). With appropriate precautions, even non-destructive tests involving external radiation therefore pose no problem for radiometric measuring systems.
Figure 4. Impact of interference radiation.
Radiometric measurements are usually the very last option used for level measurements. Of course, the use of radioactivity implicates certain expenses, such as official permits. However, with the appropriate precautions and compliance with a few regulations, this technology offers many advantages that should not be neglected. Radiometric measurement technology is highly reproducible and reliable. Using the laws of physics and statistics as well as sophisticated software, nuclear based measurements are extremely successful. Considering the benefits of a totally non-contacting and non-intrusive technology, nuclear measurement technology is a suitable method for the most difficult and challenging process measurement applications. The components are not exposed to harsh process conditions and thus high temperatures or high pressure is not problematic and there is no risk of hazardous material leaks. The systems are characterised by their wear- and maintenance-free operation and, in addition, a one-time calibration during the initial commissioning is usually sufficient. The systems can be easily retrofitted to existing tanks; the necessary components are simply added. The operating costs of the systems are low and thousands of dollars can be saved each year in reduced downtime and equipment and maintenance costs. Since nuclear measurement gauges have no moving parts or components inside vessels that need replacement, they offer accurate and repeatable measurements in the most rugged and hostile process environments.
Radiometric measurements are an excellent solution for critical level or level switch measurements where contacting or intrusive-type equipment will not work. They are attractive in today’s process measurement landscape due to their simple technical concept. Radiometric systems offer a reliable and efficient solution for demanding measurement tasks: starting with simple level measurements on storage tanks, through level switch measurements on critical processes to multi-phase monitoring. Making processes safe and saving money at the same time is possible with a radiometric level measurement.
Written by Sabrina Nees, Berthold Technologies, Germany.
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/06052021/radiometric-measurements/