If you have vessels and silos containing solids materials such as cement, grains, wood chips and plastic pellets, it is vital to prevent overfills. Should an overfill result in a spill, the consequences can be extremely serious, with falling materials having 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. These switches can also be used for low-level alarms – to prevent run-dry situations, which can disrupt a process or even stop production – or to simply indicate that a vessel or silo is either full or empty.
There are a number of different point level switch technologies that can be used to monitor the level of solids, but there is no ‘one size fits all’ solution. 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.
Let’s take a look at how the different point level switch technologies operate, and the applications they are best suited to.
Vibrating Fork Switches
Two prongs are immersed into the vessel or silo, and an internal piezo-electric crystal causes them to vibrate at their natural frequency when in free air. This frequency varies when the prongs are immersed into the material, thereby 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 extremely low maintenance requirements. They are also compact and therefore ideal for installation in vessels or silos with limited space. These switches are ideal for applications where high sensitivity is required, and perform well with low bulk density, fine-grained and fine-powdered products. In addition, 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 use in 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. When the level falls and the rod again has no material covering it, the vibration restarts and the relay will switch back.
Vibrating rod technology is unaffected by the dust created during filling cycles, and has good resistance to caking and clogging. Stainless steel construction makes these switches suitable for hygienic applications. The single rod design eliminates the possibility of material build-up causing blocking or bridging, which can affect the performance of vibrating fork switches. 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. As with vibrating fork technology, vibrating rod switches can withstand high mechanical loads and have extremely low maintenance requirements.
A capacitance switch forms a capacitor when installed in a vessel or silo, 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.
Capacitance switches can tolerate a variety of challenging process conditions, including variable density, low dielectric values, high temperatures and high pressures. This makes it a good all-round technology, suitable for use with most bulk materials, regardless of particle size. These switches are widely used in food and beverage, chemical, plastics, sand, cement and mining applications where there is a risk of coating or high levels of vibration.
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 the presence of the rising material, the rotation will be slowed or stopped entirely, causing a microswitch to actuate an alarm signal. As soon as the paddle has stopped rotating, power to the motor is cut. When the material level falls, the microswitch restarts the motor, enabling the paddle to begin rotating freely again.
This simple electromechanical measurement principle and the robustness of the paddle design makes these switches suitable for extreme process conditions, such as high pressures and temperatures of up to 1100° C. This makes paddle switches a popular low-cost option for small process vessels and most bulk solids, and a widely applied solution in food and beverage, plastics and chemicals applications involving granular, pelletized, and powdered dry products.
Emerson provides a complete portfolio of point level switch technologies, each of which is well proven in enabling organizations across many industries to solve some of their most difficult solids level measurement challenges. To read about some of our many success stories, please visit http://images.go.emersonprocess.com/Web/EmersonProcessManagement/%7B54315007-60f2-4c41-b24d-0bb6aa9f0951%7D_Solids_switch_application_flyers_00803-0100-2500_Rev_AA.pdf.