FLO


FLOW & LEVLEVEL CONTROL


FEATURE


vessel or silo to detect whether it is cover ed by thematerial. 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 level of the product rises, the air normally surr ounding the probe is


displaced bymaterial that has a different dielectric constant. A change in the value of the capacitor takes place because the dielectric betwe en the plates has


changed. The switch detects this change and converts it i nto a relay actuat ion or a proportional output signal.


A capacitor is a cost-effective option that c an tolerate challenging process conditions. These include variable


density, low dielectric values (from 1.5), high temperatures (up to 500˚C) and hi gh pressures (up to 363psi, 25 bar). This makes capacitance switches a good all-round technology, suitable for us e with most bulk materia ls, regardless of particle size. They are particularly suitable in food and beverage,


chemical, plastics, sand, cement and mi ning applications where there is a risk of coat ing or h igh levels of vibration.


PAD


PADDLE SW CHESWIITCHES A paddle-type level


switch c an be easily mount ed through the wall


Capacitance switches are suitable for use with materials that have dielectric values as low as 1.5 and are designed to prevent material build-up in applications where there is a risk of coating


Capacit


of a vessel or si lo and positioned at the point where limit detection is required. The operation o f paddle switches is bas ed on a small internal electricmotor continuously rotating a paddle. In the absence ofmaterial, the paddle c an rotate freely at the


wh en the paddle is motor. However, full spe ed of the


risingmaterial, the presence of the impeded by the


rotat ion will be


The robust design of paddle switches makes them able to withstand heavy loads and


extreme temperatures of up to 1100° C (right)


slowed or stopped entirely, causing a


microswitch to actuate an alarmsignal. As soon as the


paddle has st opped rotating, power to themotor is cut.Wh en thematerial level falls, t hemicroswitch restarts the motor, enabl ing the paddle to begin


This simple electromechanical rotat ing freely again.


measurement principle and the robustness of the paddle desi gnmakes these switches suitable for extreme process conditions, s uch as high pressures and temperatures of up to 1100° C. Thismakes paddle switches a popular low-cost opt ion for small process vessels andmost bulk solids , and is us ed in food and beverage, plastics and chemicals applications involvi ng granular, pelletised, and powder ed dry products.


Various point level switch technologies c an provide limit detect ion to prevent overfills and run-dry situations in solids applications, but, because of the diversity of materials and challenging


process characteristics involv ed in many industries, there i s no universal solution. It is there select t he switch that


ismost suitab le fo r fore important to


the application in question. Expert support fro ma trusted automat ion technolog y


supplier c an he lp inmaking the right choice.


Emerson www.


wo ww.emerson.com/RosemountSolidsLeve l


Maintenance Free Non-Contacting Solids Flow Detection


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 W: www.pulsar-pm.c w..pulsar pulsar


ar-pm.com PROCESS & CONTROL | MAY 2020 9


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