Instrumentation (and Process Control)

Fall 1393

Bonab University

Process

Control

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: 1388سید علی اکبر صفوی : اصول و روشهای کنترل صنعتی

• Start with an example: Oil refinery

2

Process

Control

Example: Oil refinery

3

Process

Control

• Crude oil (primary input) pumped to pre-heatingboiler (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

Control system duties

4

Process

Control

• 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

Overview of a process control problem: Part of Oil refinery

5

Process

Control

• Consider the boiler part

• Solving the process control problem:• Plant Engineer (PE) – Control Engineer (CE)

• They discuss find the solution

• Fuel enters:• Rate: Fi

• Temp: Ti (have fluctuation)

• Boiler output:• Rate: Fo

• Temp: To

• Goal:• Irrespective of crude oil condition and

fluctuations, output oil temp = T*

• Boilere temp max = Tm

• (fuel temp and fuel tank pressure is variable)

Finding an effective solution

6

Process

Control

• 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:

• Stage-1:introductory notes:

• CE: Goals?

• PE: Oil gets to column at T* (oil type changes every 2-3 days T* change) – also Tm

• CE: OK, so from 2 output parameters T is defined by the column, and we care more about Fo

Finding an effective solution

7

Process

Control

• PE: Yes

• CE: So, control goal = regulate output T, and since every 2-3 days set-point changes servoingas 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 (Pt), the heat energy it brings to boiler (λf)

• CE: What instrumentation? What actuators are available?

• PE: Thermocouple (Ti, T); flowmeter (Fi, Qf), control valve on fuel, a pyrometer to measure surface temp for boiler, and alarm

Finding an effective solution - Stage-2: (modeling)

8

Process

Control

• 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 Fi

• 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.

Stage-3: Control scheme

9

Process

Control

• CE: Let’s start with a simple feedback loop:• Measures output temp

• Controls fuel flow

• We’ll use a PID controller (with Td=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

Stage-3: Control scheme

10

Process

Control

• 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 feedbackscheme with it

Stage-4: controller design

11

Process

Control

• 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

Stage-4: controller design

12

Process

Control

• The final controller:• With an inner-loop control

• The previous controller’s output:

• Is set-point for the inner-loop controller

• 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

Process variables

13

Process

Control

• 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)

The hardware needed for the control system

14

Process

Control

• Sensors• Measuring elements: Temp, pressure, liquid

level, flow…

• Controllers• System heart: the only intelligent element

• With hydraulic, pneumatic, electric signals

• Transmitters• Sensors to controller

• Controller

• Final elements (actuators)• The control command is applied to system with

these elements (valves, fans, pumps,…)

• Other hardware elements• D/A, A/D

• Transforming signals (say, pneumatic)

Open-loop & Feed-forward schemes

15

Process

Control

Example: Complexity & Inter-dependency of variables

16

Process

Control

• A Controlled 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

A useful summarizing video

• Loop tuning

17

Process

Control