KMJ32104/4 Process Dynamics & Control PDF

KMJ32104/4 Process Dynamics & Control CONTROL SYSTEM INSTRUMENTATION o Transducer, Transmitter, Final control elements, Sensor, Transmission lines, Calculation of size of control valve Dr. Norzilah b...

KMJ32104/4 Process Dynamics & Control CONTROL SYSTEM INSTRUMENTATION o Transducer, Transmitter, Final control elements, Sensor, Transmission lines, Calculation of size of control valve Dr. Norzilah binti Abdul Halif MBM 5, Materials Department, Taman Muhibbah KMJ32104/4|NAH|2024/2025 1 [email protected] Process Control: Key Concepts Revisited KMJ32104/4|NAH|2024/2025 2 Process Control: Key Concepts Revisited KMJ32104/4|NAH|2024/2025 3 Process Control: Key Concepts Revisited KMJ32104/4|NAH|2024/2025 4  Instrumentation in Process Control o The use of various devices and instruments to measure, monitor, and control different variables in a process system. Sensors/ Final Control Indicators/ Converter Transmitter Controllers Transducers Element Displays A device that A device that A device that Components Visual To receive translates a converts one converts a that physically representation input, to mechanical type of signal reading from a adjust the s of process perform a signal into an into another sensor or process conditions mathematical electrical type of signal. transducer into conditions, like Often located in function with signal. Example, a a standard opening or control rooms, the input, and Example: Inside converter may signal and closing valves, to provide to produce an a capacitance convert transmits that based on operators with output signal. pressure current into signal to a commands real-time device, a voltage or an monitor or from the monitoring of transducer analog signal controller. controller. system converts into a digital P transmitters, performance. changes in signal. Flow pressure into a transmitters, T proportional transmitters, change in Level capacitance. transmitters 5 KMJ32104/4|NAH|2024/2025  Sensor, Transmitter & Transducer Measurement range (span) Performance and Cost. Reliability Materials of construction Invasive or non invasive KMJ32104/4|NAH|2024/2025 6 Thermocouple Float activated Level Sensor Orifice Pressure Gauge Liquid Column KMJ32104/4|NAH|2024/2025 7  Sensor transmitter / Transmitter The output signal from a A standard temperature transmitter with a sensor transmitter (or 4–20 mA output signal might be adjusted transmitter) must be compatible with the input so that the input range of a platinum Linear transmitter range of the controller that resistance element (the sensor) is receives the signal. 50–150ºC. Generally designed to be direct-acting, Km > 0: the output signal increases as Km = transmitter steady-state the measured variable increases. gain. Most commercial transmitters have an adjustable input range. Indication that a malfunction Power supply of transmitter fails Relation between the temperature transmitter input and output KMJ32104/4|NAH|2024/2025 8  Sensor transmitter / Transmitter The transfer function for transmitter: Gain Km changes when the span is changed but is invariant to changes in the zero. o First-order transfer function between the actual value y and the measured value ym o m = The measurement time constant (the time the measuring element takes to partially sense and react to a change in the system) KMJ32104/4|NAH|2024/2025 9  Final Control Element o Element that manipulates the process variable For most chemical and petroleum processes, the final control element adjusts the flow rate of a material (solid, liquid, gas or multiphase) which indirectly adjusts the rates of material and energy transfer to and from the process. Variable Speed Pump -for abrasive liquids Control Valve - more widely used such as slurries. KMJ32104/4|NAH|2024/2025 10  Final Control Element: Control Valve Valve body Contains a variable orifice that adjusts the flow rate. Valve stem o The position determines the valve opening and consequently, the flow rate. 3. Compresses the o Can be attached to a plug, ball, disk, or gate spring Valve seat o Consists of a protective material (typically metal or soft polymer) inserted around the orifice. o To provide a tight shutoff and to increase the life of the valve when corrosive or solid materials pass through 2. As this signal , the pressure it. on the diaphragm Actuator spring o Provides the force for opening and closing the valve.  o Usually a pneumatically powered device, but electric and hydraulic powered actuators are also available 4. Pulling the stem out Air –to – open (A-O) Air –to – close (A-C) fail-close (F-C) fail-open (F-O) [1. From the 5. Opening the valve controller, further Pneumatic signal] KMJ32104/4|NAH|2024/2025 11  Control Valve: Valve Positioners A type of mechanical or digital feedback controller that senses the actual stem position x, compares it to the desired position and adjusts the pneumatic control signal to the valve. 12 KMJ32104/4|NAH|2024/2025  Control Valve: Dynamic Behaviour The dynamic behavior of the control valve (and valve positioner) can be approximated by a simplified first-order transfer function between the signal to the control valve p and the manipulated flow rate u: Fast dynamics o The valve time constant v is typically much smaller than the largest process time constant p, that is,  v ≪ p. o The model parameters also depended on the size and direction of the step signal applied to the valve. o Larger size control valves can have much slower dynamics. 13 KMJ32104/4|NAH|2024/2025  Control Valve: Sizing Valve sizing equation: Turbulent, #Non-flashing liquid passing through a control valve q Volumetric flow rate P Pressure drop across the valve gs Specific gravity of the liquid. 0  1 valve stem position. =1 Fully open f( ) valve characteristic Cv valve coefficient, determines the size and capacity of the control valve #A process fluid (like oil, water, or a chemical) used within the system for its primary function (e.g., heat N units conversion factor transfer, lubrication, or as a reactant) that does not serve any cleaning or purging role would be considered a non-flushing liquid. 14 KMJ32104/4|NAH|2024/2025 Control Valve: Sizing Rangeability R* Design conditions To ensure that the flow rate can be Control valve is half open (f (𝓁) = 0.5) adjusted over a wide range of values. To compensate for process disturbances. 15 KMJ32104/4|NAH|2024/2025 Control Valve Characteristics (Trim) For a fixed pressure drop across the valve: 16 KMJ32104/4|NAH|2024/2025  Selection of an Equal Percentage Trim Equal percentage trim: A specific type of valve design that provides a consistent percentage change in flow for each qual increment of valve stem movement. Step 1: Plot the pump characteristic curve and ΔPs. Step 3: Calculate q as a The difference between these function of 𝓁 and ΔPv from two curves is ΔPv. Step 1. Step 2: Calculate Cv for the design A Plot of q vs. 𝓁 should be conditions, qd and ΔPvd. reasonably linear in the operating region of interest (or at least around the design flow rate). If it is not very linear, adjust Cv and repeat. 17 KMJ32104/4|NAH|2024/2025