Nuclear or nucleonic level measurement devices can be employed in both point and continuous level detection applications. They are usually applied in fields where all other level measurement techniques fail to work owing to their capability of working with hazardous situations. In this method, energy needed for level measurement is emitted arbitrarily by radio isotopes in the form of bursts.
The major source utilized in nucleonic level controls includes gamma radiations. These are electromagnetic radiations which exhibit almost identical behavior to that of microwaves and light waves. However, they have comparatively higher energy and shorter wavelength owing to which these radiations are competent enough to break through the walls of process vessel and material. The field strength of these gamma radiations is determined by a sensor mounted on the other end of the vessel, which ultimately detects the level of process material in the vessel. “Different radioisotopes are used, based on the penetrating power needed to “see” the process within the vessel. With single point gauges the radiation provides a simple on/off switching function, whereas with continuous level measurement the percentage of transmission decreases as the level increases.”1
Key features of nuclear level sensors include:
- Nuclear level measurement, also referred to as gamma level measurement is the technique which has been in use for level detection of fluids and solids for last 30 years.
- Nuclear level gauges find their use essentially with highly hazardous, contaminated, corrosive, volatile, and carcinogenic process materials.
- “Because the nuclear gage “sees” through tank walls, it can be installed and modified while the process is running–without expensive down time or chance accidental release.”2
- Prior to installation of nuclear level sensors, one must acquire a license from NRC i.e. Nuclear Regulatory Commission to provide an assurance that the installed system will be secure and not cause any harm to surroundings.
Nuclear Level Measurement System
A nuclear level measurement system basically includes following components:
- A source of gamma radiations
- A continuous detector and
- A microprocessor
In general, the gamma source emitting radiations is located external to the process vessel. These radiations passes through the vessel walls and material gets accumulated towards the detector. The detector is installed on the other side of the vessel. In case, the vessel contains no contents, the transmitted gamma rays arrive at the detector. As soon as the level of the contents increases in the vessel, there is a decrease in the amount of gamma rays getting to the detector. Thus, this gamma energy drops in an inverse proportion to the process level. “A computer processes the detector signal and transmits the process variable as 4-20 mA analog or digital fieldbus signal. Multiple gamma sensors, mounted outside the tank, help see through buildup on tank walls for a true level reading.”3
Major characteristics of gamma radiations which make them useful over other technologies for level measurement applications are listed below:
- They can detect both solid as well as liquid levels.
- They are resistant to obstructions in the process vessel.
- They offer an extensive range of operating temperature.
- Their chemical properties are not very much crucial.
- Their performance does not get affected by factors like surface turbulence or changes in flow etc.
- They can measure level in applications involving mist, foams and intense vapors too.
On the other hand, the readings of other level measurement techniques like radar, ultrasonic etc. give errors when exposed to foams or variations in gas density. Actually, in gamma level sensor designs, compensation can be provided for these errors by attaching a second point detector to the microprocessor.
Following are the major advantages associated with the use of nucleonic method of level measurement:
- Since nucleonic radiations are capable of penetrating through the vessel, no vessel penetration is required in nuclear level gauges. They can be easily installed outside the vessel or tank for measurement purpose. This enables us to carry out maintenance actions without causing any alterations or interruptions to the process or tank. For this reason, various applications like pressurized or exotic alloy vessels prefer to employ nucleonic transmitters than any other level transmitter.
- These devices are capable of withstanding intense levels of pressure and temperature. Besides, they prove helpful in level control of hazardous and corrosive materials contained in the process vessel.
- “Placing the source and / or detector in wells within the vessel can reduce source sizes. An extension of this is to use a moving source within the vessel; this facilitates the unique ability to combine density profiling with accurate tracking of a moving interface.”4
Some drawbacks involved with nucleonic level gauges include:
- Implementation of nuclear level detection method involves high cost as compared to other standard methods. Apart from the initial installation cost, there are some hidden costs associated with licensing and regular surveying procedures.
- The nucleonic level sensors after being used must be discarded in an appropriate manner via properly authorized associations meant for this purpose only. This procedure also proves to be very costly.