CWQAP - PCST (1) - Pages 10 PDF - Objectives and Principles of Operation

Objectives 4.1 Identify Components, Principles of Operation and Repair of Transducers. 4.2 Explain Principles of Operation, Testing and Replacement of Solenoids. 4.3 Explain the Testing and Replacement of Transducers. 4.4 Identify Actuator and Positioner Components. 4.5 Define Piston Actuator P...

Objectives 4.1 Identify Components, Principles of Operation and Repair of Transducers. 4.2 Explain Principles of Operation, Testing and Replacement of Solenoids. 4.3 Explain the Testing and Replacement of Transducers. 4.4 Identify Actuator and Positioner Components. 4.5 Define Piston Actuator Principles 4.6 Define Pressure Element Principles. 4.7 Diagnose Pressure Loop Problems. 4.8 Explain the Function of Switches. 4.9 Troubleshoot and Repair Electronic Transmitters. 4.10 Explain Calibration of Electronic Transmitters. 4.11 Explain Calibration Switches. 4.12 Explain Function of Pressure Regulators and Signal Booster. 4.13 Repair of Pneumatic Controllers and Relays. 4.14 Troubleshooting of Pneumatic Control Loop & Relays. 4.15 Explain Calibration of valve Positioners 4.16 Explain Repair of Switches. 4.17 Define Resistance Temperature Detector Principles. 4.18 Define Thermocouple Principles. 4.19 Troubleshoot RTD and Thermocouples. 40 Objective 4.1 Identify Components, Principles of Operation and Repair of Transducers 4.3 Explain the Testing and Replacement of Transducers. I/P Transducer The I/P transducer converts the signal to what is required for the final control element and sends it as an output. For example, 4–20 mA may be converted into 3–15 psi, I/P (current to pressure). In the process industries, the I/P transducer takes a current input, usually a 4–20 mA signal, and converts it to a pressure output, a 3–15 psi signal. The current signal sent into the I/P transducer comes from a controller or distributed control system (DCS) that tries to keep a process variable at the set point value. The I/P transducer output typically goes to a pneumatic actuator controlling a valve, which moves the measured variable closer to the set point. The Honeywell model 870022 transducer is common in the plants. The major parts of the Honeywell transducer are listed below and shown figures. 41 zero adjust knob: is used to set the transducer zero during calibration. A locknut locks the adjustment. span adjust: sets the span during calibration. A screw locks the span adjustment. terminal block: is where field wires connect to the transducer. force coil: produces a force proportional to the input current. feedback module: produces a force that opposes the force generated by the force coil. Pilot valve: supplies the pneumatic 3–15 psi output signal. Restrictor: reduces the air flow through the nozzle. valve port: is the pneumatic output connection. supply port: connects to the air supply. Operation This transducer is a single-acting, force-balancing, final control interface element. Figure below shows a functional block diagram of the transducer. The air-handling capacity of the transducer is determined by the one or two pneumatic stages. The single-stage model has a low air-handling capacity and is used to transmit pneumatic signals to a relay or to a valve positioner. The two-stage model uses a high capacity relay that can actuate a pneumatic valve directly. When calibrating, make sure that the two-stage model transducer output is connected to a capacity of 50 cu. in. or more. Smaller capacities may cause cycling of the transducer. 42 Transducer can be direct or reverse acting. In direct acting transducers, an increasing input current signal provides an increasing output air signal. In reverse-acting transducers, an increasing input current signal provides a decreasing output air signal. To convert the transducer to reverse action, reverse the input signal wires. Testing and Replacement of an I/P transducer 1.If you remove a transducer in the field, get a work permit first, and tell operations so that the relevant control valve can be bypassed. 2.Before removing a transducer, you must safely isolate it. 3.The controller output signal to the transducer must be disconnected at a nearby junction box or in the control room. 4.The instrument air supply to the transducer must be shut off. 5.The transducer can now be safely removed. 6.Replace with a new one. 7.Connect pneumatic lines. 8.Power up 9.Ask operations to make process in line. Questions 1.The I/P transducer takes a 4–20 mA input signal and converts it to what output signal? 2.The transducer is a single-acting, force balancing, final control _________element? 3.What must you do to convert the transducer from direct-acting to reverse-acting? 4. What is the signal range of transmitter output? 5. What do you do if one transducer gives error and not possible to eliminate it? 43 Objective 4.2 Explain Principles of Operation, Testing and Replacement of Solenoids. Solenoids A solenoid is an electromagnetic device that can be used to convert magnetism into mechanical movement. This movement can then be attached to many other mechanical pieces to make up a control element. The most common use of a solenoid in oil and gas industry is a solenoid operated valve. These usually control the flow of instrument air or lube oil. 44 45