tailieunhanh - CONTROL VALVE HANDBOOK Episode 1 Part 5

Tham khảo tài liệu 'control valve handbook episode 1 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chapter 4 Control Valve Accessories This chapter offers information on digital valve controllers analog positioners boosters and other control valve accessories. Positioners Pneumatically operated valves depend on a positioner to take an input signal from a process controller and convert it to valve travel. These instruments are available in three configurations 1. Pneumatic Positioners A pneumatic signal usually 3-15 psig is supplied to the positioner. The positioner translates this to a required valve position and supplies the valve actuator with the required air pressure to move the valve to the correct position. 2. Analog I P Positioner This positioner performs the same function as the one above but uses electrical cur rent usually 4-20 mA instead of air as the input signal. 3. Digital Controller Although this instrument functions very much as the Analog I P described above it differs in that the electronic signal conversion is digital rather than analog. The digital products cover three categories. Digital Non-Communicating A current signal 4-20 mA is supplied to the positioner which both powers the electronics and controls the output. HART This is the same as the digital non-communicating but is also capable of two-way digital communication over the same wires used for the analog signal. Fieldbus This type receives digitally based signals and positions the valve using digital electronic circuitry coupled to mechanical components. An all-digital control signal re- 67 Chapter 4. Control Valve Accessories Figure 4-1. Modern Control Valves Utilizing Digital Valve Controllers places the analog control signal. Additionally two-way digital communication is possible over the same wires. Fieldbus technologies benefit the end user by enabling improved control architecture product capability and reduced wiring. Use of a single integrated analog I P positioner or digital controller figure 4-1 instead of a combination of pneumatic positioner and transducer two instruments