2012년 현재 하고 있는 일 설명

출처: Nicholas Sheble

 http://www.isa.org/InTechTemplate.cfm?Section=Article_Index1&template=/ContentManagement/ContentDisplay.cfm&ContentID=43794

Mechanically to the valve

The next component in the final control subsystem, if applicable, is the actuator. The actuator receives the conditioned signal and changes it to some form of mechanical energy or motion.

Typical devices used as actuators include solenoids, pneumatic valve positioners, AC and DC motors, stepper motors, hydraulic motors, and hydraulic pistons. Many control valves include a pneumatic valve positioner.

A valve positioner is a device used to increase or decrease the air pressure (from the I/P) operating the control valve actuator. Positioners usually mount to the control valve actuator and connect mechanically to the valve stem for position indication.

A positioner is a type of air relay, which acts to overcome hysteresis, packing box friction, and effects of pressure drop across the valve. It assures exact positioning of the valve stem and provides finer control. There are many types of positioners. The basic principles of operation are similar for all types.

The instrument pressure (from an I/P, for example) acts on the input module, which controls the flapper-nozzle system of the relay. Supply pressure applies to the relay and the output pressure of the relay goes to the control valve actuator.

Most positioners can set up and function for direct or reverse action. For a direct-acting positioner, increasing the instrument pressure causes the input module to pivot the beam. The beam pivots the flapper and restricts the nozzle. The nozzle pressure increases and causes the relay assembly to increase output pressure to the actuator.

With a direct-acting actuator, the increased pressure moves the actuator stem downward. The positioner connects mechanically to the stem of the valve. Stem movement feeds back to the beam by means of a feedback lever and range spring, which causes the flapper to pivot slightly away from the nozzle to prevent further increase in relay output pressure.

Note that some positioners accept a milliamp input and include an integral I/P transducer.

The last component in the final control subsystem is the final control element. Let's look at control valves (Other final control elements include servo valves, heaters, conveyors, auger feeds, and hopper gates.).

There are many different types, sizes, and applications for control valves. Selecting the correct control valve for a specific application is crucial to proper system performance. Under sizing and over sizing are common problems.

There are many valuable resources available to assist with proper selection, not the least of which is a control valve sales engineer. Here's a typical control valve.

The pneumatic signal from the positioner (or I/P if a positioner is not used) applies directly to the actuator. For this control valve, the air enters above the diaphragm and pushes against spring pressure to close the valve. The valve fully closes when the plug seats tightly against the seat ring.

As air pressure decreases, the spring pressure causes the diaphragm, stem, and plug to move upward, opening the valve. This means a loss of pressure would cause the valve to open. This is a fail-open valve.

Different configurations of air inlet, spring location, and valve seat arrangement result in different fail positions and determine whether the valve is direct- or reverse-acting. For example, this same valve, with the plug below the seat ring (reverse-seated), would open with increased air pressure and would fail closed on loss of air pressure.

So, all components in the final control subsystem must be configured correctly for the system to work properly. The fail-safe positions must be correct for the application, and the action must produce the desired results. These configurations must be properly documented and utilized during calibration, loop checks, or troubleshooting.