Process Instrumentation, part 2: Control Loops and the Control ValveOutlineTypical Control LoopElements of Control LoopElements of Control LoopElements of Control LoopRegulatory Control ValveValve Trim Inherent CharacteristicsValve Trim Sizing: Flow Coefficient vs. Valve PositionValve Selection ExampleSystem Response based on Pump/ Piping SystemInstalled CharacteristicSlide Number 13Review of Controller TerminologyRelating this to our Control Loop:Controller TerminologyController TerminologyEvolution of ControllersEvolution of Controllers Emerson’s DeltaV System – current state of the TechnologyWiring Systems Connect Transmitters to DCS – at the Instrument End:Wiring Systems Connect Transmitters to DCS – at a Marshalling Cabinet:Wiring Systems Connect Transmitters to DCS – in the Controller Cabinet:Wiring Systems Connect DCS to Transducers – at Marshalling Cabinet:Regulatory Control ValveOutput Signals from Control System to Control SolenoidsInstalled Field Devices: Ball Valve w/ ActuatorDeltaV & Foundation FieldbusReferencesProcess Instrumentation, part 2:Control Loops and the Control ValveCM4120Unit Operations LabJanuary 20101Outline What is a Control Loop? A look at Regulatory Control Valves PID Controllers and terminology Instrument Connections to a DistributedControl System2Typical Control Loop3All elements of a loop have same loop numberElements of Control LoopInput side:TE → Element to measure temp RTD vs. T/CTT → Transmitter sends signalDashed line - signal transmission line4Elements of Control LoopController: TIC → Temperature Indicating Controller → Shared Display, Analog signal5Elements of Control LoopOutput side: TV → Valve to regulate steam flow TY → Transducer converts electric signal to pneumatic Solid line w/ dashes is pneumatic signal line F.C. is Fail position6Regulatory Control Valve7Trim setdesirable to have flow linearly proportional to valve position for good controlActuator(F.O. or F.C.?)Valve Trim Inherent Characteristics8Quick Openingsafety by-pass typeLarge flow response when valve starts opening ismore important than linear responseEqual Percentage~ 80% of all control valvesprovides linear response to valve positionLinearused when majority of system pressure drop isdue to valve positionValve Trim Sizing: Flow Coefficient vs. Valve Position00.510 20406080100Stem Position (% Open)f(x)=%QOLinear9By definition:for Cv = 1,1 gpm flow w/1 psi pressure dropacross valveValve Selection ExampleC.W.FT10“Control flow of reflux to distillation column”Determine pressure drop:@ design flow@ expected min/max flowSystem Response based on Pump/ Piping System11Design Range of flow is 100 to 200 gpm:Increase in valve opening → less ΔP across valve, but w/ increased line losses and decreased total available head from pump05101520250 50 100 150 200Flow Rate (GPM)Pressure Drop (psi)Line LossesPump HeadValve ΔPInstalled Characteristic12Size the valve trim, then select valve characteristicw/ the most linear response:…use Equal Percent Characteristic valve to achievea linear Installed Characteristic0501001502000 20406080100Stem Position (% Open)Installed Flow Rate (GPM)Linear Valve=% Valve“PID” Controllers13Review of Controller TerminologyProcess Variable (PV) = Measured variable of interest, in EUSetpoint (SP) = Desired value of the PV, in EUOutput (OP) = Controller output, 0-100%Error = Difference between Setpoint and PV14Relating this to our Control Loop:15Process VariableOutputSetpointController TerminologyPID control Dynamic equation that is used to match the controller’s response to a measured disturbance. Goal is to minimize disturbance and return to setpoint Equation is “tuned” to match process response using up to 3 tuning constants16Controller TerminologyTuning Constants:Proportional term – Adjusts output proportional to the error, GainIntegral term – Added to output based on error existing over time, ResetDerivative term – Additional adjustment to output based on rate of change of error, Rate17Evolution of Controllers1930’s – Pneumatic Controllers• air pressure w/ flappers, bellows, and valves adjust valve position based on measured process variable for P, PI, later PID control1950’s – Electronic Controllers• transistors, resistors, and capacitors for P, PI, PID control• capable of remote installation1960’s – Mainframe Computer Control• Refineries were typical users• Alarming capability and supervisory control• Single point of failure, no user-friendly graphical interface18Evolution of ControllersLate 1970’s – Distributed Control Systems (DCS)• Networked computers distributed thru plant• Pre-configured controllers• Data archival capabilities• Included an operator console• Hardware is proprietaryLate 1990’s – DSC’s built on commodity hardware platforms• Better scalability• Affordable• Interactive graphical interface19Emerson’s DeltaV System –current state of the Technology20PID controlDiscrete logic controlSignal conversionsAlarmingFuzzy control, etc.are continuously executed by the “MD” controllerWiring Systems Connect Transmittersto DCS – at the Instrument End:21Wiring to field junction cabinetRTD or T/C headTemperature transmittersWiring from transmitter to temp measuring elementLevel transmitterWiring Systems Connect Transmittersto DCS – at a Marshalling Cabinet:22Single pairs from field devices8 pr. Cables to controller cabinet8-pr. cables run from Field Junction Box (Marshalling Cabinet) to Distributed Control SystemWiring Systems Connect Transmittersto DCS – in the Controller Cabinet:23DeltaV “MD” controller8 pr. cables from field junction cabinetPower-limiting Zener barriersI/O cards2ndI/O chassis w/4-20 mA Output cardsWiring Systems Connect DCS to Transducers – at Marshalling Cabinet:24Wire prs. to transducersCurrent to pneumatic transducersAir lines to control valves8-pr. cable from controller cabinetRegulatory Control Valve25Air line from I/P transducerActuator w/ positionerControl valveBlock valvesBypass valveOutput Signals from Control Systemto Control Solenoids268-pr. cable from controller cabinetSolenoids for 2-position air-actuated ball valvesAir lines to ball valvesWire prs. to solenoidsInstalled Field Devices: Ball Valve w/ Actuator27Air line from solenoidActuatorBall valve bodyProcess lineDeltaV & Foundation Fieldbus28(4) mass flows, (4) densities, and (4) RTD temps (3)
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