Why Coated Materials Should Not Be Used for Threaded Connections in Tuning Fork Level Switches

Tuning fork level switches are control switches specifically designed for level measurement in containers, tanks, and various pipelines. The working principle is based on the change in the frequency of the fork’s vibration caused by damping. The vibration of the tuning fork is analyzed using the vibration mechanism of a rigid cantilever beam structure. The installation methods of tuning fork level switches typically include flange, threaded, and clamp connections. When measuring highly corrosive liquids, the switch probe’s wetted parts are often coated to enhance the sensor’s corrosion resistance. However, when a threaded connection is chosen, coating the wetted parts to enhance corrosion resistance is not recommended. Below, we will discuss the characteristics of threaded connections and corrosion coatings, and their impact on the switch probe.

1. Characteristics of Threaded Connections

The thread profile typically has a triangular or trapezoidal shape with sharp teeth along the circumference. The connection is made by rotating the internal and external threads, which provides a fixed connection. Threaded connections can be disassembled and reassembled by rotating the threaded fasteners, making them removable and convenient for maintenance. The compact design of threaded connections saves space and results in a more compact overall structure. However, when tightened, threaded connections generate significant axial force. Additionally, due to manufacturing tolerances, threads can have gaps, which can be problematic for applications requiring coatings, as these gaps may be too large. Threaded connections are generally unsuitable for high-pressure environments.

2. Characteristics of Coating for Corrosion Resistance

Corrosion-resistant coatings are applied to metal surfaces to isolate them from surrounding media, preventing corrosion on pipes or equipment. The primary mechanism of corrosion-resistant coatings is to form a protective layer on the metal surface that prevents corrosive liquids and oxygen from contacting the metal, thus preventing corrosion. This protective layer helps shield the base material from direct contact with the corrosive medium, achieving effective corrosion resistance. Coatings exhibit excellent resistance to acids, alkalis, salts, and chemical solvents, significantly improving the metal’s durability. They can also resist the effects of chlorides and chloride-containing environments, allowing the metal to work in harsh conditions involving high temperatures, pressures, and chemical media. The surface of the coating has excellent self-cleaning properties and is not easily affected by water or oil. Even a small amount of dirt can be wiped away easily or washed off with water. To ensure a good bond between the coating and the base material, the surface typically needs to undergo strict pre-treatment, such as rust removal, degreasing, and phosphating. If the pre-treatment is not thorough, the adhesion of the coating and its corrosion resistance will be significantly reduced. At pipe connections (e.g., joints), corrosion-resistant coatings are challenging to apply effectively, leading to issues such as incomplete coating adhesion or gaps in the coating, which affects overall performance.

3. Impact of Coating Material on Threaded Connection Performance

Generally, the type and thickness of the coating material can influence the strength and sealing ability of the threaded connection. On the one hand, the coating can increase the hardness of the threaded surface, enhancing connection strength. On the other hand, excessive coating thickness may lead to larger gaps in the thread engagement, affecting the sealability. Therefore, when selecting a coating material, its impact on both strength and sealability must be considered. Additionally, compatibility between the coating material and the threaded material is crucial. Some coatings may react chemically with the thread material, leading to delamination or performance degradation. Coating corrosion resistance typically forms a thin film on the metal surface, which may increase the diameter of the threads and cause the connection to become too tight or too loose. A tight fit may make it difficult to screw the threads together, potentially damaging the threads, while a loose fit can result in poor sealing and the risk of leakage. The structure of threaded connections is complex, with many small thread grooves and gaps, making the application of coatings relatively difficult. When coating threaded connections for corrosion resistance, it is crucial to ensure the coating is applied evenly across the entire threaded surface and does not clog the thread grooves or affect thread engagement.

4. Impact of Coating on Switch Performance

When a tuning fork level switch uses a threaded connection, the coating on the thread surface will introduce a layer of corrosion-resistant plastic material. When the level switch is installed by screwing the thread into the tank or container, a certain thickness of the coating material is present between the external thread surface of the switch probe and the internal thread surface of the tank’s mounting seat. This creates a damping buffer zone, which disrupts the rigid connection necessary for the tuning fork probe’s vibration. This results in energy loss in the vibration, weakening the probe’s oscillations and reducing the switch’s reliability. Furthermore, when the threaded connection with a coating is tightened, the sharp teeth of the threads can scrape off the coating, causing the corrosion-resistant layer to fail. Gaps may form between the threads, which could lead to leakage of the material inside the tank, potentially causing safety hazards. Therefore, when a threaded connection is used for tuning fork level switches, the use of coated materials for the wetted parts is not recommended.

Conclusion

The use of corrosion-resistant coatings is unsuitable for threaded connections primarily because it disrupts the rigid connection required for the tuning fork probe’s vibration, violating the cantilever beam vibration mechanism. This leads to energy loss and weakened probe vibration, ultimately compromising the reliability of the switch. Additionally, the coating is prone to peeling off at the threaded connection, creating gaps that increase the risk of material leakage and jeopardize safe production. In practical applications, it is essential to choose the appropriate corrosion protection method and connection type based on specific requirements and operating conditions to ensure the integrity, sealing, and durability of the connection.