FEATURE FLOW & LEVEL CONTROL SELECTING THE RIGHT LEVEL SWITCH FOR SOLIDS


switch’s electronics, and the output state of the switch is then changed. With no moving parts to wear or stick,


vibrating fork technology is reliable and has low maintenance requirements. Vibrating fork switches are compact in design and suit vessels or silos with limited space. They are suitable for applications where high sensitivity is required, and they perform well with low bulk density, fine-grained and fine- powdered products. In addition, vibrating fork switches 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, such as in the food industry.


Point level switches play a vital role in increasing safety and optimising filling and emptying cycles in solids storage and handling. Felicia Malai, project engineer at Emerson, explains the differences between the technologies, and the benefits of each


A


cross a broad range of process and manufacturing industries,


there are many applications that require bulk solids and powders to be contained in process vessels or storage silos. These applications involve various materials, including coal, cement, fly ash, lime powder, sugar, fertilisers, grains, cereals, animal feed, flour, wood chips and plastic pellets. Accurate and reliable level measurement and control is essential in these applications, not only to better track the materials and optimise inventory management, but also for safety purposes. Limit detection is required in most


vessels and silos to prevent overfilling and spills, the consequences of which can be extremely serious. First and foremost, materials and objects falling from height put people at risk and can cause injuries and even fatalities. In addition, materials and production time can be lost, environmental fines can be incurred and a costly clean-up operation required, and the vessel or silo can suffer extensive structural damage, resulting in a loss of valuable storage capacity. It is therefore essential for plant owners


and operators to minimise risk by installing an overfill prevention system (OPS). Within an OPS, point level switches


8 MAY 2020 | PROCESS & CONTROL


are typically used to provide high-level alarms. They can also be used for low-level alarms – to prevent run- dry situations, which can disrupt a process or even stop production – and to


simply indicate that a vessel or


silo is either full or empty. Various point level switch


technologies are well proven in solids


and powder applications but, due to the diverse nature of the materials and process conditions involved, there is no ‘one size fits all’ solution. The choice of


which switch to implement is typically based on the size and space constraints of the vessel or silo, as well as the application conditions. The following sections explain the operating principles


of different point level switch technologies, and the applications to which they are best suited.


VIBRATING FORK SWITCHES A vibrating fork switch operates using the concept of a tuning fork. 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 then varies when the prongs are immersed into the material – the denser the medium, the lower the frequency will be – thereby providing an effective means of identifying the presence or absence of the material. Variation in frequency is detected by the


Emerson’s range of Rosemount solids switches (above)


In applications involving light, fine-grained and powdered materials where high measurement sensitivity is required, vibrating fork switches provide a cost-effective solution (left)


VIBRATING ROD SWITCHES Similar to vibrating fork switches, a vibrating rod switch uses piezo-electric vibration technology to energise the rod and keep it vibrating at its natural frequency when there is no material covering it. The vibration is dampened as the level in the vessel or silo rises, and when the electronics detect material covering the rod, this initiates the switching of the output relay after a built- in programmable time delay. As the level descends and the rod again has no material covering it, the vibration restarts and the relay will switch back. Robust design and high sensitivity make this an all-purpose limit switch. The technology is unaffected by the dust that can be created during filling cycles, and has good


resistance to media that


are susceptible to caking and clogging. Stainless steel


construction makes these switches


suitable for hygienic applications. The single rod design eliminates the


Vibrating rod switches made of food-grade stainless steel provide high chemical resistance


possibility of material build-up causing blocking or bridging, which can sometimes affect the performance of vibrating fork technology. Because of their compact design, vibrating rod switches are a good choice for vessels


and silos with limited space, and they are also a suitable solution for fine-grained and powdered products. As with vibrating fork technology, vibrating rod switches can withstand high mechanical loads and are reliable with low maintenance requirements.


CAPACITANCE SWITCHES The measurement principle of a capacitance switch is based on forming a capacitor when the switch is installed in a


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