Overview of Control System Design ERT 321: Process Control & Dynamics Miss Anis Atikah Ahmad Tel:...

Overview of Control System Design

ERT 321: Process Control & DynamicsMiss Anis Atikah Ahmad

Tel: 04-9764160Email: anis [email protected]

OUTLINE

1. Steps in Control System Design2. The Influence of Process Design on Process

Control3. Degrees of Freedom for Process Control4. Selection of Controlled, Manipulated &

Measured Variables5. Process Safety & Process Control

1. Steps in Control System Design

Formulate the control objectives.

Select controlled, manipulated, and measured variables.

Choose the control strategy (multiloop control vs. multivariable control) and the control structure (e.g., pairing of controlled and manipulated variables).

Specify controller settings.

The Influence of Process Design on Process Control

• Traditionally, process design and control system design have been separate engineering activities.

• Thus in the traditional approach, control system design is not initiated until after the plant design is well underway and major pieces of equipment may even have been ordered.


• A more desirable approach is to consider process dynamics and control issues early in the plant design.

• This interaction between design and control has become especially important for modern processing plants, which tend to have a large degree of material and energy integration and tight performance specifications.


• Now consider two specific examples of how process design affects process dynamics and control.

• Since the rise in fuel prices, there has been considerable interest in reducing energy costs of distillation trains by heat integration or thermal coupling of two or more columns.


• This figure compares a conventional distillation system with a heat-integrated scheme

• Heat integration reduces energy costs by allowing the overhead stream from Column 1 to be used as the heating medium in the reboiler for Column 2.

• However, this column configuration is more difficult to control for two reasons.


• The process is more highly interacting because process upsets in one column affect the other column.

• The heat integration configuration has one less manipulated variable available for process control because the reboiler heat duty for Column 2 can no longer be independently manipulated.




EXAMPLE:

• Two alternative temperature control schemes for a jacketed batch reactor are shown as follows.

• Discuss the relative merits of the two strategies from a process operation perspective.




• The first figure has a serious disadvantage that the coolant circulation rate varies and thus the corresponding time delay for the coolant loop also varies.

• When the time delay varies with the manipulated variable, a nonlinear oscillation can develop.

• If the reactor temperature increases, the controller increases the coolant flow rate, which reduces the time delay and causes a sharp temperature decrease.

• When the reactor temperature is too low, the controller reduces the coolant flow rate, which increases the time delay and results in a slow response.

• This nonlinear cycle tends to be repeated.




• This control problem can be solved by making a sample equipment design change, namely, by adding a recirculation pump.

• The recirculation rate and process time delay are kept constant and thus are independent of the flow rate of fresh cooling water.

• The nonlinear oscillations are eliminated.

Degrees of Freedom for Process Control

• The degrees of freedom NF is the number or process variables that must be specified in order to be able to determine the remaining process variables.

• If a dynamic model of the process is available, NF can be determined from

the relation:

Where NV is the total number of process variables, and

NE is the number of independent equations.

EVF NNN

Degrees of Freedom for Process Control• For process control applications, it is very important to determine the

maximum number of process variables that can be independently controlled, that is, to determine the control degrees of freedom, NFC

• In order to make a clear distinction between NF and NFC, we will refer to NF as the model degrees of freedom and NFC as the control degrees of freedom.

• Note that NF and NFC are related by the following equation,

DFCF NNN

Definition:The control degrees of freedom, NFC, is the number of process variables (e.g., temperatures, levels, flow rates, compositions) that can be independently controlled.

is the number of disturbance variables

Degrees of Freedom for Process Control

• General Rule.

For many practical control problems, the control degrees of freedom NFC is equal to the number of independent material and energy streams that can be manipulated.

Degrees of Freedom for Process Control: Example 1

Determine NF and NFC for the steam-heated, stirred-tank system modelled by the following equations:

Assume that only the steam pressure Ps can be manipulated.

TTAhTTAhdt

dTCm

TTAhTTwCdt

dTmC

PfT

wppwsssw

ww

wppi

ss


In order to calculate NF, we need to determine NV and NE

The dynamic model in the equations contains three equations (NE = 3) and six process variables (NV = 6): Ts, Ps, w, Ti, T, and Tw

[1]

[2]

[3]

Since thus, NF = 6 – 3 = 3.


TTAhTTAhdt

dTCm

TTAhTTwCdt

dTmC

PfT

wppwsssw

ww

wppi

ss

EVF NNN


If the feed temperature, Ti and mass flow rate, w are considerable to be disturbance variables, ND = 2

Since

Thus NFC = 1

It would be reasonable to use this single degree of freedom to control temperature, T by manipulating steam pressure, Ps


DFCF NNN

23 DFFC NNN


A conventional distillation column with a single feed stream & two product streams is shown in the following figure. Determine the control degrees of freedom, NFC and identify the process variables that can be manipulated and controlled in typical control problems.Since

Thus NFC = 1

temperature, T by manipulating steam pressure, Ps


DFCF NNN

23 DFFC NNN


For a typical distillation column, five variables can be manipulated: product flowrates, B and D, reflux ratio, R, coolant flow rate qc, and heating medium flow rate qh . Thus, according to general rule, Thus NFC =5

23 DFFC NNN


Five output variables could be selected as controlled variables: xD, xB , hB, hD

and P.

23 DFFC NNN

Degrees of Freedom for Process Control: Effect of Feedback Control

Case 1. The set point is constant, or only adjusted manually on an infrequent basis.

For this situation, ysp is considered to be a parameter instead of a variable.

Introduction of the control law adds one equation but no new variables because u and y are already included in the process model.

Thus, NE increases by one, NV is unchanged, and NF and NFC decrease by one.

t

Ic dtteteKtu

0

**1

tytyte msp

DFCF NNN EVF NNN

Degrees of Freedom for Process Control: Effect of Feedback Control

Case 2. The set point is adjusted frequently by a higher level controller.

The set point is now considered to be a variable. Consequently, the introduction of the control law adds one new equation and one new variable, ysp.

From the equations,

NF and NFC do not change.

The importance of this conclusion will be more apparent when cascade control is considered.

DFCF NNN EVF NNN

Selection of Controlled, Manipulated & Measured Variables

1. Selection of controlled variables

Guideline 1.All variables that are not self-regulating (output variable that exhibits an unbounded response after sustained disturbance) must be controlled.

Guideline 2.Choose output variables that must be kept within equipment and operating constraints (e.g., temperatures, pressures, and compositions) – originate from safety, environmental & operating requirements.


Guideline 3.Select output variables that are a direct measure of product quality (e.g., composition) or that strongly affect it (e.g., temperature or pressure).

Guideline 4.Choose output variables that seriously interact with other controlled variables. Eg: steam pressure header that supplies steam to downstream unit. If the supply pressure is not well regulated, it can act as a significant disturbance to the downstream units.

Guideline 5.Choose output variables that have favorable dynamic and static characteristics.


2. Selection of manipulated variables

Guideline 6Select inputs that have large effects on controlled variables. Eg: if a distillation column has a reflux ratio of 5, it is much easier to control reflux drum by manipulating the reflux flow rather than the distillate flowrate.

Guideline 7Choose inputs that rapidly affect the controlled variables. Rapid effect will reduce any time delay and time constant will be small.


2. Selection of manipulated variables

Guideline 8The manipulated variables should affect the controlled variables directly ratherthan indirectly. Eg: it is preferable to throttle the steam flow to heat exchanger rather than the condensate flow.

Guideline 9Avoid recycling of disturbance. It is preferable not to manipulate an inlet stream or recycle stream, because disturbance tend to be propagated forward or recycled backto the process. This can be avoided by manipulating a utility stream to absorb disturbancesor an exit stream that allows the disturbances to be passed downstream, provided that theexit stream changes do not unduly upset downstream process units.


2. Selection of measured variables

Guideline 10.Reliable, accurate measurements are essential for good control.

Guideline 11.Select measurement points that have an adequate degree of sensitivity.

Guideline 12.Select measurement points that minimize time delays and time constants.

Process Safety & Process Control

Process safety is considered at various stages in the lifetime of a process:

1. An initial safety analysis is performed during the preliminary process design.

2. A very thorough safety review is conducted during the final stage of the process design using techniques such as hazard and operability (HAZOP) studies, failure mode and effect analysis (FMEA), and fault tree analysis.

3. After plant operation begins, HAZOP studies are conducted on a periodic basis in order to identify and eliminate potential hazards.

4. Many companies require that any proposed plant change or change in operating conditions require formal approval via a Management of Change process that considers the potential impact of the change on the safety, environment, and health of the workers and the nearby communities. - Proposed changes may require governmental approval,

Process Safety & Process Control

Process safety is considered at various stages in the lifetime of a process (cont.)

5. After a serious accident or plant “incident”, a thorough review is conducted to determine its cause and to assess responsibility.

Process Safety & Process ControlThe role of the basic process control system

Multiple Protection Layers:

Emergency shutdown (ESD) Safety Interlock System (SIS)Relief devices,

dikes



• The protection layers are shown in the order of activation that occurs as a plant incident develops.

• In the inner layer, the process design itself provides the first level of protection.

• The next two layers consist of the basic process control system (BPCS) augmented with two levels of alarms and operator supervision or intervention.

• An alarm indicates that a measurement has exceeded its specified limits and may require operator action.



• The fourth layer consists of a safety interlock system (SIS) that is also referred to as a safety instrumented system or as an emergency shutdown (ESD) system.

• The SIS automatically takes corrective action when the process and BPCS layers are unable to handle an emergency. For example, the SIS could automatically turn off the reactant pumps after a high temperature alarm occurs for a chemical reactor.



• Relief devices such as rupture discs and relief valves provide physical protection by venting a gas or vapor if over-pressurization occurs.

• As a last resort, dikes are located around process units and storage tanks to contain liquid spills and protect the surrounding community from the spill.

• Emergency response plans are used to address emergency situations and to inform the community.

Process Safety & Process ControlProcess alarms

Types of alarms:• Type 1 Alarm: Equipment status alarm.

Indicates equipment status, for example, whether a pump is on or off, or whether a motor is running or stopped.

• Type 2 Alarm: Abnormal measurement alarm.Indicates that a measurement is outside of specified limits.

• Type 3 Alarm: An alarm switch without its own sensor. These alarms are directly activated by the process, rather than by a sensor signal.


•In Type 2 alarm system, the flow sensor/ transmitter (FT) signal is sent to both a flow controller (FC) and a flow switch (FSL – Flow Switch Low).

•When the measurement is below the specified low limit, the flow switch sends a signal to an alarm which activates an annunciator in the control room (FAL – Flow Alarm Low).

•For Type 3 alarm system, the flow switch is self actuated & thus does not require a signal from a flow sensor/transmitter.


Types of alarms:• Type 4 Alarm: An alarm switch with its own sensor

A type 4 alarm system has its own sensor that serves as a backup in case the regular sensor fails.

• Type 5 Alarm: Automatic Shutdown or Startup System.These important and widely used in Safety Interlock Systems.

Process Safety & Process ControlSafety Interlock System (SIS)

• It is important that the SIS function independently of the basic process control system (BPCS); otherwise, emergency protection will be unavailable during periods when the BPCS is not operating (e.g. due to a malfunction or power failure).

• Thus the SIS should be physically separated from the BPCS and have its own sensors and actuators.


•For the liquid storage system, the liquid level must stay above a minimum value in order to avoid pump damage such as cavitation.

•If the level drops below the specified limit, the low level switch (LSL) triggers both an alarm (LAL) and a solenoid (S), which acts as a reply and turns the pump off.


• For the gas storage system, the solenoid operated valve is normally closed.

•But if the pressure of the hydrocarbon gas in the storage tank exceeds a specified limit, the high pressure switch (PSH) activates an alarm (PAH) and causes the valve to open fully, thus reducing the pressure in the tank.

• For interlock and other safety systems, a switch can be replaced by a transmitter if the measurement is required (more reliable).

Date post:	16-Dec-2015
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Overview of Control System Design ERT 321: Process Control & Dynamics Miss Anis Atikah Ahmad Tel:...

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