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Home | Reference & Education | Science Limit Switch boxes are used for position feed-back signalling of the end positions for on-off applications to monitor the actual valve positions in the process plants. This is required for safe operation of automatic controlled processes carried out by PLC-systems etc.etc. Solenoid Valve Solenoid Valves are used for on-off applications where the valves are operated from fully closed to fully open position and no mid positions are required. The solenoid valves are available for many different Control voltages and ingress protection grades as for example IP65, IP67, IP68, NEMA 4 and NEMA6 and also for Hazardous areas where Explosion proof according to ATEX or NEMA 7 are required. Positioner for pneumatic actuators Positioners are used for Positioning, Control or Modulating applications where a defined valve position anywhere between fully open and fully closed valve is required. The positioning can be controlled by an Analogue input Signal 4-20mA or by a Digital input signal. The Positioners are used for all kinds of modulating such as flow control, temperature control, pressure control or mixture control in process plants. Positioners are available for use in Normal Zone and in Hazardous Zone where ATEX or NEMA 7 approvals are required. Smart positioners are used for applications where closed control loops, diagnostics or bus-communication is required. Limit Switches The Electric Actuators are often equipped with potential free limit switches for signalling to PLC controllers where the motor voltage of the actuator is not possible to use as feed back signal. Local Control For Local Control of the electric actuators a control unit is required, such units have a key selector switch for Local/Remote selection and a 3-position control knob for Open-Stop-Close control. Positioner for electric actuators For modulating applications with Electric Actuator a positioner is required. The positioners are available in most various designs, starting with the simplest for a 4-20 mA input signal only. If required there are optional feed-back signal transmitters available with or without relays. A basic rule is that the higher the resolution of the positioning and the feed back signal, the higher the price is. The best positioners for electric quarter turn actuators can perform 450 points of resolution for a 90° turning angle. These positioners are equipped with dynamic braking control features and also adaptive control by continuously monitoring and compensating for actuator backlash, motor coast and load changes to eliminate positioner dead band. Moreover these positioners have built in advanced safety features as stall detection and duty cycle protection for the actuator to ensure that no overloading occurs. Mounting Kit To assemble the actuators, pneumatic or electric, and the Valves there are in most cases need for Mounting Kits or Adaptor Kits. This is one of the most critical parts in the whole Valve and Actuator package since the performance of the package will never be better than the performance of the weakest link in the chain from the Actuator to the Valve. The machining tolerances of the mounting kit are of highest importance since too much play or poor alignment will affect the function and service life of the valve-actuator package negative. A basic advice is to avoid saving the money on the mounting details, it will cost you a lot more in problems such as lower process product quality, unplanned stand stills and shorter service life than you ever can save on these parts. For modulating duty, the recommendation is to use reinforced and play free couplings. Quality Actuator manufacturers are aware of this and therefore supply their own Mounting Kits for all different valves and On-Off or Modulating duty. This means several thousands of different Mounting kits, since there is until yet no fully accepted standard for the Valve top connections among the different valve manufacturers. Mounting The mounting of actuator on valve should only be carried out by professionals. The sizing and the required function of the actuator must be checked to make sure that it is correctly sized and equipped to suit the actual valve applications requirements. The mounting kit must have the correct fit. It is important to set the valve and the actuator in same end position before mounting them together. Finally the tightening of the screws with the correct torque, test cycling and setting of the signalling unit is carried out. This all together gives a long and reliable function of the actuated valve. Article Source: http://www.articlewheel.com
Greger Klen is writing about pneumatic and electric actuators and solenoid valves.
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