Instrument Selection of Liquid Level Control in Steam Generator Recently, Ring-21 Compact Liquid Level Switch has been widely used in steam generator to measure high temperature water level to prevent liquid overflow or dry burning. A steam generator is a form of low water-content boiler, similar to a flash steam boiler. The usual construction is as a spiral coil of water-tube, arranged as a single, or monotube, coil. Circulation is once-through and pumped under pressure, as a forced-circulation boiler. The steam generator is mainly used in shopping malls, hotels, clothing factories, restaurants, bean-product factories and other places to provide necessary dry steam for steamers, steam irons and steam ironing machines. Due to the operating conditions of high pressure and temperature, strict regulations have been formulated for the selection of materials, design calculation, manufacture and inspection. Because even a small steam generator has the potential danger of explosion, the most likely cause of accidents is the inaccurate monitoring of the liquid level in the generator, it is very easy to cause liquid high-level overflow or dry burning. Therefore, the selection of liquid level instrument is particularly important. The most common liquid level instruments include: float level indicator, capacitive liquid level switch, photoelectric liquid level switch, tuning fork liquid level switch and etc. 1、The main working principle of float level indicator is based on Archimedes Buoyancy Principle. Float rises and falls as the fluid level in a vessel changes. Due to magnetic effect, as the float moves, the proximal reed switches in the Float Level Indicator are magnified by the permanent magnet in the float and change the status from “open” to “closed”. Float level indicator has applied widely in steam generator due to its low price and simple structure. However, the poor measurement accuracy and frequent maintenance cannot ensure reliable level measurement. 2、Capacitive liquid level switch is mainly based on the principle of capacitance induction. When medium reaches the height of measuring electrode, the capacitance changes. Because dielectric constant is easily affected by temperature and humidity, it is difficult to calibrate and leads to poor accuracy measurement, besides, maintenance and replacement also frequently occur. 3、Photoelectric liquid level switch is based on infrared detection, using the principle of refraction and reflection of light. The disadvantages of photoelectric liquid level switch are obvious, it is easy to be polluted by the medium on site. However, liquid in steam generator is not completely pure, when temperature drops, a layer of condensation on the inner wall of the tank tends to form, which makes the light reflection scattered and cannot be measured accurately. 4、For tuning fork liquid level switch, piezoelectric devices are utilized to achieve vibration drive and

www.jiweimeter.com 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, not influenced by foam, bubbles, viscosity, vibration and other liquid characteristics. Ring-21 High-temperature Compact Liquid Level Switch is based on the design principle...