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Chemical Industry

Application of High-temperature Liquid Level Switch in Urea Synthesis Reactor

As a fertilizer, urea plays a very important role in agricultural production and is an indispensable raw material in agricultural production. Urea was first produced industrially by the hydration of calcium cyanamide but the easy availability of ammonia led to the development of ammonia/carbon dioxide technology. This is a two-step process where the ammonia and carbon dioxide react to form ammonium carbamate which is then dehydrated to urea. To effectively control and monitor urea production, it is necessary to monitor and measure various liquid levels, especially the liquid level measurement of urea synthesis reactor, because the ambient is under both high temperature and high pressure (around 200℃ and 3Mp). Besides, the working condition of urea synthesis reactor is under strong corrosive and explosive hazards. Explosion accidents of urea synthesis reactor are common at home and abroad. In recent years, there were two urea synthesis reactor explosion accidents in Shandong and Hebei. According to the analysis, the causes of the explosion are due to lining corrosion leakage, main reason is laminate stress corrosion cracking and original manufacturing defects causes laminate opening to crack, resulting in "bumping" explosions. To prevent the accident, requires that the liquid level measuring instrument not only has high temperature and high pressure resistance, but also has strong corrosion resistance and a high explosion-proof grade.

There are various liquid level switches, including float level indicator, capacitive liquid level switch and tuning fork level switch, and how to select an optimal level switch for urea synthesis reactor.

The structure of float level indicator is mainly based on the principle of buoyancy and static magnetic field. The position of magnetic floating ball is affected by buoyancy: the changing liquid level leads to the change of its position. The magnetic floating ball acts with the sensor (reed switch) to generate switch signal. However, the float level indicator is not suitable for the high pressure and high temperature conditions due to its measurement low reliability, and complex cleaning due to the non-detachable mechanical structure.

For capacitive liquid level switch, the probe is made of insulating material PTFE, when is mounted in the medium with temperature close to 200 ℃, its pressure capacity is weak and is far beyond the scope that the probe can support. Under high temperature, the molecular gap of probe insulation material becomes larger, and urea medium may permeate into the insulating sleeve, which easy cause false indication.

For tuning fork liquid level switch, piezoelectric devices are utilized to achieve vibration drive and detection. Once the resonant vibrating fork comes in contact with the measured medium, the vibration frequency of forks will substantially decrease. The output signal from the piezoelectric detection device will decrease accordingly. Then an integrated circuit is designed to analyze the signal from the piezoelectric device and output a switch signal as a result. With strong disturbance resistance ability, tuning fork liquid level switch is not influenced by foam, bubbles, viscosity, vibration and other liquid characteristics. In terms of temperature resistance, the standard type of Jiwei Ring-11 Liquid Level Switch can withstand 150 ℃, and the highest temperature resistance can reach up to 250 ℃ by adding high-temperature extension tube.

Ring-11 High-temperature Liquid Level Switch has been successfully applied in urea synthesis reactor is because it has lots of advantages as the following:

1. According to the  working conditions, Ring-11 High-temperature Liquid Level Switch can withstand 250 ℃ high temperature by adding high temperature extension tube, and the temperature resistance range is wider.

2. 316L stainless steel material, air tight insulating sleeve and high protection level make Ring-11 High-temperature Liquid Level Switch be able to be used in occasions with high pressure and strong corrosion.

In the production of urea, the raw materials such as the aqueous solution of CO, NH ₃, ammonium carbamate and urea, ammonium carbonate solution, dilute ammonia water and a small amount of cyanic acid formed in the production process are corrosive, especially when both of temperature (130 ℃ ~ 200 ℃) and pressure are high.

Ring-11 High-temperature Liquid Level Switch can withstand high pressure and has strong corrosion resistance. Besides, it also provides ECTFE, PFA coating and other options for users to choose, and the protection level is up to IP66 / IP67. 

3. A variety of signal output mode options, with compact fork body, being more stable and reliable.
Ring-11 High-temperature Liquid Level Switch has three signal output modes: relay, two-wire and Namur. The length of fork body is only 40mm, which is especially suitable for liquid level measurement in narrow space such as pipeline. High sensitivity due to the precisely pre-adjusted harmonic resonant frequency of the fork, the lowest measurable medium density can be as low as 0.5g/cm³. 

4. Dual explosion-proof certifications with high explosion-proof level.
Ring-11 has passed the certification of intrinsic safety (Ex ia IIC T6 Ga) and explosion-proof (Ex d IIC T6 Gb), which is very suitable for use in the corrosive and explosive environment of urea synthesis reactor.

The above characteristics of Ring-11 High-temperature Liquid Level Switch provide reliable guarantee for stable and safe operation of the instrument, making it very suitable for application in urea synthesis reactor. 
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