Instrumentation (and ) Fall 1393 Bonab University
Principles and Basic Definitions A process = a set of interrelated tasks that, together, transform inputs into outputs These tasks may be carried out by: people, nature, or machines using resources Engineering process must be considered in the context of the agents carrying out the tasks, and the resource attributes involved Reference: اصول و روشهای کنترل صنعتی : سید علی اکبر صفوی 1388 Start with an example: Oil refinery 2
Example: Oil refinery Crude oil (primary input) pumped to pre-heating boiler (to boiling point) Sent to distillation column Final products are separated: Heavy fuel oil (Naphta) Gas oil, etc. Operational units: Oil Tanks Furnace/boiler Distillation/fractionating column Input material: Crude oil Fuel Air 3
system duties Basic principles: Safe operation Stabilizing production rate Output products quality Example: In crude oil distillation, rate and quality are inter-related Main tasks of control system: Displaying status of process using variable measurements Changing the process variables in order to improve the conditions 4
Overview of a process control problem: Part of Oil refinery Consider the boiler part Solving the process control problem: Plant Engineer (PE) Engineer (CE) They discuss find the solution Fuel enters: Rate: F i Temp: T i (have fluctuation) Boiler output: Rate: F o Temp: T o Goal: Irrespective of crude oil condition and fluctuations, output oil temp = T* Boilere temp max = T m 5 (fuel temp and fuel tank pressure is variable)
Finding an effective solution The 3 control objectives are evident: Safety Output product quality Product rate (different crude oils boiling temp will be different) Finding the solution: Stepwise PE responsible for the whole process PE CE discuss issues: 6 Stage-1:introductory notes: CE: Goals? PE: Oil gets to column at T* (oil type changes every 2-3 days T* change) also T m CE: OK, so from 2 output parameters T is defined by the column, and we care more about F o
Finding an effective solution PE: Yes CE: So, control goal = regulate output T, and since every 2-3 days set-point changes servoing as well? PE: Yes CE: What input variable is under your control? PE: Air flow, Fuel* flow CE: So, other inputs like crude oil flow & its temp are disturbances? PE:Yes CE: Is there another important process variable that I should know? PE: Yes, fuel tank pressure (P t ), the heat energy it brings to boiler (λ f ) CE: What instrumentation? What actuators are available? PE: Thermocouple (T i, T); flowmeter (F i, Q f ), control valve on fuel, a pyrometer to measure surface temp for boiler, and alarm 7
Finding an effective solution - Stage-2: (modeling) Stage-2: (modeling) CE: Do you have a model of the process? PE: No, but the operator knows the behavior well. We have tried Manual control not very satisfactory: The strip chart recorder shows a sample output temp This is response to step change in F i Lots of oscillation around the set-point The control is not fast and accurate CE: Any idea why the problem exists? PE: we believe it s human accuracy and speed limit Although a good prediction of disturbance input The same problem exist with other disturbances: Fuel pressure Fuel Temp. 8
Stage-3: scheme CE: Let s start with a simple feedback loop: Measures output temp s fuel flow We ll use a PID controller (with T d =0) The performance is shown Less oscillation Slower Stage-3: (Choice/evaluation of control scheme) PE: Although the performance is a bit better than Manual: For a long time the crude oil enters the boiler with inappropriate temp. For a few hours output quality is affected 9
Stage-3: scheme CE: I think we can sense the disturbance input : Proposing a feed-forward controller Counter-acting (reducing) its effect before entering the process Figure shows the detail of the scheme Stage-4: (design of controller) CE: with this scheme: Good compensation at the beginning But since there is no sampling from output Temp: Gradually, an offset forms Seems that we can combine feedback scheme with it 10
Stage-4: controller design Combined Feedforward & feedback The decision on what command to send to the valve: Comes from TC & FC The performance is better than the previous ones PE: there is still a problem: Oscillations in the output temp. Part of that may be due to fuel tank pressure change CE: so, the controller sends the command to the valve, but: Assumes the tank pressure is constant Pressure change fuel flow changes Let s put a controller on the fuel flow 11
Stage-4: controller design The final controller: With an inner-loop control The previous controller s output: Is set-point for the inner-loop controller 12 The gradual improvement in the schemes is clear Compare performance from the simple controllers Visualize a case (with tank pressure change) Input flow change output temp change FFC has partly compensated the effect If still temperature in the output changes TC compensates it Correct command is given to FC regardless of pressure in the tank
variables Input variables Can independently affect the system Those who are readily available: control variables Output variables Information about the inner states State variables The minimum variables that completely define the inner states (detectors) Variables: Measureable online Non-measureable (at least not with enough frequency, say chromatography) 13
The hardware needed for the control system Sensors Measuring elements: Temp, pressure, liquid level, flow lers System heart: the only intelligent element With hydraulic, pneumatic, electric signals Transmitters Sensors to controller ler Final elements (actuators) The control command is applied to system with these elements (valves, fans, pumps, ) Other hardware elements D/A, A/D 14 Transforming signals (say, pneumatic)
Open-loop & Feed-forward schemes 15
Example: Complexity & Inter-dependency of variables A led Flow & Temperature system: D: Disturbance C: Cold H: Hot T: Temperature L: Level Cold Water (and its control loop) Adjusts level (and hence flow) But affects temperature as well Hot water (controller) Adjusts temperature But affects level as well 16
A useful summarizing video Loop tuning 17