Nuclear level transmitters basic principle
Nuclear type transmitter can be used in continuous as well as point level measurement its basic principle is Nuclear devices comprise a shielded radioisotope source attached to one side of a vessel or pipe and a detector placed on the opposite side. Gamma rays are emitted from the source and are focused to travel through the tank wall, the medium in the tank, and the far tank wall through to the detector. Nuclear level switches use radioisotope sources sized to provide measurable radiation at the detector when no product material is present between source and detector. It can be used Noncontact and nonintrusive level measurement of liquids and solids.
Nuclear level transmitters use the same radioisotope sources, but respond to the total absorption of gamma rays as they pass from the source to detector. The amount of radiation reaching the detector is inversely proportional to the amount of material in the vessel.
Although the word “nuclear” sometimes causes concern, the industry has sustained an excellent safety record over the course of the last 30 years or more.
Main features of Nuclear level transmitters
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.”
- 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.
Applications: Non contact and non intrusive level measurement of liquids and solids.
Range :- External units have ranges from 1 in. (25 mm) to 23 ft (7 m) for single stationary units, higher for multiple units, and up to 50 ft (15 m) for motorized units. Traversing back scatter units mounted in dry wells can detect up to a range of 150 ft (45 m).
Radiation Exposure :-One roentgen is received during one hour spent within one meter of a one-curie (1-Ci)radiation source. A subject receives a dose of one rem (roentgen equivalent man) when exposed to one roentgen in any time period. General public allowable limits are 2 mrem/hr and 100 mrem/year. The dose for an occupational worker is 5 rems/year.
Temperature: – External detectors are suitable for − 40 to 160 ° F ( − 40 to 70 ° C) ambient conditions. Units can be provided with heaters for lower ambient temperatures and with air/water cooling for higher ambient temperatures. Traversing back scatter units can operate up to 250 ° F (121 ° C)
Price :-Assuming a 10-ft (3-m) diameter vessel with 0.25 in. (6 mm) wall thickness and 1.0-in. (25-mm) insulation, the following costs can be expected: on–off alarm, $2240 ;continuous level transmitter $8000 and up.
Nuclear level transmitter advantages and disadvantages
Advantage of Nuclear level transmitter :-
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.”
- It can be used for both point and continuous level measurements in both liquids and solids, as well as interface.
Disadvantage of Nuclear level transmitter :-
- 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.
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