When vessels and silos contain solid materials such as cement, grains, wood chips, and plastic pellets, it is vital to prevent overfills, as the consequences can be extremely serious. Falling materials have the potential to cause injuries or even fatalities. In addition, product can be wasted, production time lost, environmental fines incurred, and the vessel or silo badly damaged.
To minimize these risks, it is important to install an overfill prevention system, in which point level switches typically provide high-level alarms. A number of different point level switch technologies can be used to monitor the level of solids. The choice of which switch to implement depends on the size and space constraints of the vessel or silo, and the nature of the materials and process conditions involved. Here is a look at the different options.
Vibrating Fork Switches
Vibrating fork switches have two prongs that are immersed into the vessel or silo. An internal piezo-electric crystal causes the prongs to vibrate at their natural frequency when in free air. This frequency varies when the prongs are immersed into the material, enabling the presence or absence of the material to be identified. Variation in frequency is detected by the switch’s electronics, and the output state is then changed.
With no moving parts to wear or stick, vibrating fork switches are highly reliable and have low maintenance requirements. They are also compact and, therefore, ideal for installation in vessels or silos with limited space. These switches are also great for applications where high sensitivity is required, and perform well with low bulk density, fine-grained, and fine-powdered products. They can withstand high mechanical loads due to their short extension length, while devices with wetted parts made from corrosion-resistant stainless steel are suitable for hygienic applications.
Vibrating Rod Switches
Similar to vibrating fork switches, a vibrating rod switch uses piezo-electric vibration technology to energize the rod and keep it vibrating at its natural frequency in free air. The vibration is dampened when the electronics detect material covering the rod and this initiates the switching of the output relay.
Vibrating rod technology is unaffected by dust and has good resistance to caking and clogging. The single rod design eliminates the possibility of material build-up causing blocking or bridging. A compact design makes vibrating rod switches suitable for vessels and silos with space constraints, and they perform well with fine-grained and powdered products. For hygienic applications, switches with a stainless steel construction are available. As with vibrating fork technology, vibrating rod switches have low maintenance requirements.
When installed in a vessel or silo, a capacitance switch forms a capacitor to detect whether it is covered by the material. A probe acts as one plate of the capacitor and the vessel wall (or reference electrode in a non-metallic vessel) acts as the other plate. As the product level rises, the air normally surrounding the probe is displaced by material that has a different dielectric constant. A change in the value of the capacitor takes place because the dielectric between the plates has changed. The switch detects this change and converts it into a relay actuation or a proportional output signal.
The capacitance switch is a good all-round technology, suitable for use with most bulk materials, regardless of particle size. They are designed for low dielectric media and can tolerate a variety of challenging process conditions, including heavy loads and high temperatures and pressures.
The operation of paddle switches is based on a small internal electric motor continuously rotating a paddle. In free air, the paddle rotates freely at the full speed of the motor. However, when the paddle is impeded by rising material, the rotation will be slowed or stopped entirely, causing a microswitch to actuate an alarm signal.
This simple electromechanical measurement principle and the robustness of the paddle design makes these switches suitable for extreme process conditions, such as in high temperatures of up to 1,100°C, heavy loads, and dust. This makes paddle switches a popular low-cost option for small process vessels and most bulk solids.
Lovisa Rogestedt is a level products solutions engineer at Emerson.