Instrument Selection of Liquid Level Control in Steam Generator
Recently, Ring-21 Compact Liquid Level Switch has been widely used in steam generators 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 instruments 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, 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 generators 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 the medium reaches the height of the 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 a 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 switches, 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, 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 of detecting changing vibration frequency when fork body immerses in medium. Compared with other domestic brand solid-liquid level switches, Ring-21 can withstand 150 ℃ high temperature and 40 bar process pressure, and has higher reliability, which fully meets the requirements of a steam generator. In addition, Ring-21 High-temperature Compact Liquid Level Switch also has the following characteristics:
● Compact and portable, with overall length 160.5mm, maximum diameter 31.5mm and the fork body only 38mm long.
● Particularly suitable for pipelines and other applications within constricted space.
● Strong disturbance resistant ability, not influenced by foam, bubbles, viscosity, vibration or other liquid characteristics.
● Highly reliable, safe and easy to operate, with great flexibility.
● Economical and practical, high cost-effective ratio.
After the preliminary trial, clients were satisfied with the performance of Jiwei Ring-21 High-temperature Compact Liquid Level Switch and purchased in batch for steam generator application, its stable and reliable performance were favored by the operators.