- Email: [email protected]
Process Control Engineering Trends
Copyright 16> IFAC Information Control in Manufacturing, Nancy - Metz, France, 1998
PROCESS CONTROL ENGINEERING TRENDS Dominique GALARA Electricite de France - Research and Development Department, Power Plant Control Branch 6, quai Watier, BP 49, 78401 Chatou, Cedex, FRANCE, eMail : [email protected] Jean Pierre HENNEBICQ Electricite de France - Generation and Transmission Division, Hydro and Fossil fuel plant Operation 34-40, rue Henri RegnauIt, PARIS LA DEFENSE, FRANCE.
Summary : The aim of this paper is to highlight some trends in the field of process control engineering to improve competitiveness of process control for new applications, maintenance of applications or retrofits. Rationale scientific foundations based on system approach should be a significant contribution to improve process control engineering of the future. We focus our interest in the definition of perennial process control specifications, whatever the implementation into offthe-shelves control system should be. Copyright © 1998 IFAC
INTRODUCTION
The aim of this paper is to point out some trends in the field of process control engineering, in particular to highlight the definitions we gave for "process" and "control system".
This paper is an attempt to pOint out productivity deposits to improve process control engineering . The aim of this paper is to focus on process control engineering.
Currently the notions of "process" and of "control system" are standing on the know how of engineering companies, the skill of engineers and the return of experience.
The first difficulty we have to cope with , is the lack of universal definition for "process", due to the variety of process from the hardware point of view. For example, it is rather easy to distinguish manufacturing and power plant hardware processes.
For new engineers starting in the field of process control, it is not trivial to make reference to an explicit life cycle for control system.
Now, if we adopt the point of view of engineering methodology, it becomes difficult to justify this variety. Currently company made engineering methods exit and are used but aren't they instances of a generic process engineering methodology?
Control system are defined, in general implicitly, as programming schemes and mimics from mechanical studies (Piping & instrumentation diagram and textual process operation description), as summarized on figure 1.
We should not define "process" with reference to the process hardware, but with respect to the set of transformations of material and energy into product. Each transformation could be supported by a large variety of process hardware. The second difficulty we have to cope with is the lack of definition for "control system". Each industrial area has its own definition, in general control system technology-driven . On the market, the number of control system vendors is decreasing, as a consequence the new control systems are usable in different types of process industries.
Control system driven Npf"Hentltions
Figure 1: From mechanics down to control .system.
We should not define "control system" with reference to the technology of control system , but with respect to the monitoring and control processing and the monitoring and control documents, control systems should have to support.
Defining a control system as programming schemes and mimics should not be a problem if they are perennial. Unfortunately, technology evolution is very fast, control systems disappear due to the
11
«Monopoly» between suppliers: Last but not the least, control system obsolescence is reached before 10 years. Problems occur for process control and control system maintenance.
PROCESS CONTROL - - - - - - - -------- - - -
~
studies
control system - > specification
process dependent control system in dependant
control system design
-
WORLD OF Material MA TERIAL~ ; & ENERGY Energy
process dependent control system dependant
Control operators are in the "world" of information. N. Wiener said "information is information not material nor energy". The distinction between data and information is not trivial. G. Bateson [1] said "a data is a track of an event, an information is a difference generating a difference". From figure 4, it appears that a control system is depending on the share of monitoring and control activities between control operators and the control system.
-
Process monitoring and control activites
~
proc:ess monitoring & 'controlllCtlvitles :
monitoring & :> control document .specifications
->
monitoring &
_> control proc:esslng __> specifications
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. -'-~~-'- '- ' -- ~ ' product
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,--. >
Mechanical
-> _ _ _ _ _.-Waste
studies '
Process
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----.Iwasl'e
A control system should be is in the "world" of data, processing data coming from and going to the process and operators. The interface with the process are data from/to transmitters and actuators. The interface with the operators is a set of monitoring and control documents.
Process control is a set of process monitoring and control activities, shared between control operators and a control system.
>
~>
Process
A process should be is in the "world" of material and energy, transforming material and energy into product and waste.
Before defining perennial speCifications of any control system. We point out that a control system is a part of a process control.
&.
- -.'I. - -product -
Figure 4: Control system within process control.
TOWARDS PERENNIAL CONTROL SYSTEM SPECIFICATION
Energy
_
- -- -- -- -- -- -- -- -- -- --=-- -- - -- - - - - -- -y
system Implementation
The main points of interest should be: • for new applications, to explicit and to be able to select the requested specifications of any control system, • to achieve perennial specifications whatever the control system, in which they will be implemented, is, • to integrate explicitly the evolutions of the specifications due to process modifications or to control system upgrades, • for retrofit, to reuse the specifications for implementation into upgraded or new control systems.
WORLD OF MATERIAL & ENERGY
monitonng & I documents supportBd by a control system CONTROL - monifoi'ing & COilfrOI -: SYSTEM processing supported by a control system : - -- -- -- - -- -- - .. -- -- ~ -- -~ - - - -- - -- - -- -- -~
WORLD OF DATA
Figure 2: Life cycle of a control system.
Material
A ____________ _ _
- - - . - - - - - - - - - - - - - - - - - - - -c:O.lri,- - - - - -
-CO~ ~
activities supportBd by control operators A
One way to improve control systems should be to achieve explicit perennial and reusable control system specifications implementable into any future control system, whatever the technology evolution should be, as summarized on figure 2. Mechanical
monitoring & contrOl
WORLD OF INFORMATION
- >
control ~ontrol system ._ > , ,. system specification / design
process dependant control sysbHn independant
Figure 3: Process control. A control system should support a subset of the process monitoring and control activities, figure 4.
_..>
control system Implementation
process dependant control sysbHn dependant
Figure 5: Control system life cycle.
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activities is depending on the structure of the process. We should represent the structure of a process as an ordered network of active (pumps, valves .. . ) and passive (pipes, tanks ... ) components. We also have to identify the transmitters and actuators necessary for monitoring and to control activities. For normal process operation, the structure of a process is stable.
Process-engineers should establish the process monitoring and control activities taking into account process operation, experience feedback and requested performances (safety, availability). On the basis of the process monitoring and control activities, process-engineers should study and optimize the share of monitoring and control activities between control operators and a control system.
=----=-~~ -
=- - : : ( f - -
Once monitoring and control activities devoted to a control system should be identified; processengineers should describe perennial monitoring and control processing specifications, monitoring and control document specifications (as human machine interface), taking into account process operation constraints (safety , availability, performance).
-
----i<:~ ,., -= "'~--
We point out that the monitoring and control document speCifications should be completed and validated with control operators.
Figure 6: Scheme of the structure of a process.
The transformations are depending on the steadystates and transients of the process operation .
For engineering companies, anticipating the control system through engineering studies is important. The objective of reducing the cost and improving the quality of control engineering should be satisfied through explicit and standardized studies standing on reusable library of generic elements.
For steady-state process operation, we should define a set of activities to monitor and to control an ordered network of synchronic (Walliser [5]) transformations of material and energy within a structure.
PROCESS MONITORING AND CONTROL ACTIVITY REPRESENTATION from control point of view.
For transient process operation, we should define a set of activities to monitor and to control an ordered network of diachronic (Walliser [5]) transformations of material and energy within a structure.
In this section, the intention is to highlight what should be a representation of the process monitoring and control activities.
For process control, the point of interest is not in the structure of the process, but in the definition of the monitoring and control activities of the steady-state and transient transformations of a process.
We point out that there is a rupture between the world of mechanics and the world of process control. Any representation is directed by an intention, see H. Putmann [4]. The intention of control engineering is to represent process monitoring and control activities. To achieve this, process engineers need to know what to control, unfortunately this description is carried out by mechanical engineering, with another intention. We have to cope with two representations understandable in these two worlds. Here is a difficu Ity .
Process monitoring and control activities are depending on characteristics allowing to monitor and to control the steady-state and transient process operation . For the characteristics, P. Delattre [2] distinguishes qualitative and quantitative properties. Each characteristic is a unique qualitative alphanumerical identifier. Each characteristic is dedicated to a unique and unambiguous property of the structure or the transformations. Each characteristic can be completed with a quantitative value (a data and its measurement reference).
To define the process monitoring and control activities, we should used system approach, in particular from P. Delattre [2] suggesting the definition of a system as three interleaved representations : the structure, the transformations and the behavior.
Characteristics should be used to monitor and to control the structure and the related transformations of material and energy, such as : • The status and the state of the active
The structure of the process monitoring and control
13
•
• • •
components of the structure of the process, The status and the state of redundant components and circuits of the structure of the process, The status and the state of material and energy in the components and circuits, The status and the state of the transformations of material and energy, The status and the state of material and energy balances.
•
•
• •
We point out that these characteristics must be understandable in the worlds of plant operation (control operators ... ) and engineering (mechanics, process control).
•
These characteristics are chosen to be, on one hand , explicitly related to the physical characteristics of the structure and of the transformation of material and energy, and on the other hand , to be easily identified in the process monitoring and control activities.
operating such as chemical, thermal, hydraulic .. . Values of the characteristics of the structure and of the transformations, allowing the identification of the steady-state, Reconfiguration of the structure (components and circuits) to maintain the availability of the transformations, Upstream and down stream conditions of the transformations, Color of the graphic symbols of the structure supporting the transformations, Semiotic ; on a structure scheme components should be identified by names (tank, exchanger, boiler. .. ), instead on a steady-state schemes components should be identified by activities (tanking, cooling water, boiling water. .. ) representing the transformations in progress.
For the representation of the steady-state transformations of material and energy, we should propose a suggestive representation. For example, to value the characteristics on steady-sate transformation schemes. On figure 7 is represented a feed water conditioning steady-state transformation . .':>
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Figure 8: Steady-states of a process operation.
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For normal operating , in a steady state, the activated transformations are synchronic together, it means we have steady-state relationships between the characteristics of the different transformations.
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For complex processes, it may be difficult to manage the complexity of a process on a single scheme. To cope with this complexity, we should propose different layers of representation.
----
Figure 7: Feed water conditioning steady-state transformation scheme.
For complex processes we could have a synopsis of the process represented as a network of icons. Each icon should summarize a part of the process. Under each icon we should have the detailed part of the process.
The boarders between the transformations of material and energy should be defined with reference to principles of equilibrium within the different domains (electrical, mechanical, hydraulic, thermal, chemicaL .. ).
In this case we should have two layers of representation of the process ; the higher layer is the synopsis, the lower layer is a detailed partition of the process.
Figure 7 should be an attempt to represent a steadystate transformation scheme. The difference with a structure scheme should be the identification and the characterization of the transformations of material and energy , for example : • Identifiers of the transformations of material and energy , • Domain in which the transformations are
We point out that these two layers are not representing a hierarchical system , they only represent different aggregations of a single an unique process .
14
operation" steady state. In the fourth step, "in operation" steady-state, we should have monitoring and control activities to maintain the transformation in a steady state, as far as some incident can be recovered within the structure (redundant components, and circuits), and preventing the environment against accident. In the fifth step, we should have monitoring and control activities to shutdown the transformation on material and energy .
-
,
In the sixth step, we should have monitoring and control activities to remove residual material and energy to clean the structure.
.
.
Figure 9: From process steady-state operation down to transformation steady-state operation.
In the seventh step, we should have monitoring and control activities to restore the "transformation " in the "out of operation" steady state.
A process steady-state operation is "time independent". To complete the interpretation of P. Delattre [2] work, we have to introduce the behavior of the process ; the transient process operation. For normal operating, a transient process operation is the evolution from one steady-state process operation to another steady-state process operation . The transformations of material and energy are diachronic, some should move to "in operation" some others to "out of operation". It means we have relationships between the characteristics and the steady-state process operation. For normal operation , most of transformations should have two steady states: "out of operation" and "in operation". Some others should have more than two; the principle we describe should be the same.
Figure 10: Monitoring and control activities to monitor and control a transformation.
For normal operation , most of transformations should be monitored and controlled in four steps, to move from "out of operation" steady-state to "in operation" steady state and vice versa, as suggested on figure 10.
The monitoring and control activities should be encapsulated into blocks corresponding to the different steps. Each monitoring and control activity block should contain a graph of serial/parallel monitoring and control statements
In the first step, "out of operation" steady state, we should have monitoring activities to check the integrity and the availability of the structure and of up-stream material and energy .
For each step, the monitoring and control statements are described in a textual way. For each monitoring statement, we should have a verb describing the activity (verify ... ), a characteristic identifier (the temperature of ... ) and the aim of the monitoring (is lower than ... ) For each control statement, we should have a verb describing the control activity (open , maintain ... ), a characteristic identifier on which the control is applied (the valve, the level in the tank .. . ). These statements should take profit of the return of experience, from process operation .
In the second step, we should have monitoring and control activities to configure the structure (for example the circuit alignment to prepare the introduction of material and energy) and to check the upstream and downstream pre-conditions and interlocks allowing to start the transformation . In the third step, we should have monitoring and control activities to launch the transformation on material and energy and to achieve the nominal "in
The monitoring and control activities should also include, for example :
15
•
•
Requirements for the measurements and actuations (accuracy, performance, availability, and safety), Requests and reports for and from the upstream and down-stream pre-conditions and interlocks allowing to network the transformations.
energetic state and minimizing the impact on the environment. To improve the quality and to reduce the costs, the monitoring and control activities should be written with a monitoring and control statement block language. This language should allow to build blocks and to reuse standardized blocks available in a library. For the language we should distinguish different types of statements, for example related to : • Process operation (to monitor and to control in order to produce), • Structure test (to monitor and to control the ability of the structure to support the operations), • Transformation test (to monitor and to control the efficiency of the transformations), • Maintenance (to monitor and to control preventive maintenance), • Management (to monitor and to control process efficiency).
From a complete process operation, we should identify the different transformations within the different steady-states and the transients between the steady-states, in particular the pre-conditions and the interlocks between the transformation . On this basis, we should describe the ordered network of steady-state and transient monitoring and control activity blocks, with respect to the steadystate and transient process transformations. Process
Process
steady Proc... state tr'llnsient
Transformations"
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steady state
2
Process Process transient 2103
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This language should allow to describe and to validate by simulation the definition of the monitoring and control activities.
TURBlNNG STEAM
GENERATING
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It should be also interesting for process-engineers to optimize the ordering and the coordination of the different types of monitoring and control activities to optimize the start-up and shut-down of the process.
EYOIution
Last but not the least, plant management should be interested to prepare the start-up, the shut-down and the non scheduled activity in order to optimize the monitoring and control activities to save time. The interest of the definition of the monitoring and control activities is two folds: • Anticipate the behavior of the process monitoring and control before specifying the control system, in particular for critical situations, • Explicit the behavior of the process monitoring and control for diagnosis purposes.
Figure 11 : Monitoring and control activities to monitor and to control process steady-state and transient transformations,
The monitoring and control activities blocks should contain sets of monitoring and control statements to maintain the transformations in the predefined steady-states, to master the transients between the steady-states and to recover a predefined set of incidents and accidents of the structure and of the transformations.
SHARE OF MONITOmNG AND CONTROL ACTIVITIES BETWEEN OPERATORS AND A CONTROL SYSTEM.
Once the monitoring and control activities are defined and validated, these activities should be distributed between control operators and a control system.
An incident is a failure occurring on a component of the structure or on a transformation but the integrity of the process is maintained. In general an incident can be recovered by acting on the structure or moving towards a safe lower energetic state, without impact on the environment.
Here is another difficulty to share monitoring and control activities processing between operators and a control system.
An accident is a failure occurring on a component of the structure or on a transformation but the integrity of the process is not maintained. We can recover accidents, which have been studied by acting on the structure, to move the process towards a safe lower
Currently, in general, the share of monitoring and control activities between control operators and control systems is implicit standing on the company
16
know-how, the skill of engineers and the return of experience.
maintenance and technical management activities. In the allocation of monitoring and control activities to a control system , requirements should be included for Human machine interface, for example safety, availability and performances of monitoring measurements and control actuations, alarm presentation ...
There is not a rigorous method to share and to optimize the activity processing , between control operators and a control system , taking into account control operator and control system performances, such as activity duration, work load , serial and parallel activity management capacity, activity planning, availability, safety, cost.
Let us make the assumption that the monitoring and control activities should be shared between operators and a control system.
Nevertheless, monitoring and control activities could be devoted to control operators when the monitoring activities have to access to local indicators and the control activities to manual actuators.
The set of monitoring and control statements devoted to a control system should be translated into speCifications of the : • monitoring and control processing, • monitoring and control documents as human machine interface. These specifications should be standardized and written with ad hoc function block languages.
The allocation of monitoring and control activities to control operators should also take into account the desired level of automation and the related definition of the control team and the distribution of the activities between the operators (unit chief operator, process part operators, shift operators .. . ).
SPECIFICATIONS OF THE MONITORING AND CONTROL PROCESSING.
Monitoring and control activities should be devoted to a control system when the monitoring and control activity performances (accuracy, availability, safety) are too high or repetitive for control operators (activities related to protection, control loops and surveillance). - . - -- -
stioay' state 1
_.. -
-
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The specifications should be control system supplier independent.
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Figure 13 : SpeCifications of the monitoring and control processing and documents of a transformation.
Figure 12: share of monitoring and control activity between control operator and system.
The speCifications should structured with respect to the structure of the process; a network of transformations.
A monitoring and control statement block language. should be an interesting tool for process-engineers. They could analyze and optimize meaningful and rational share of activities between control operators and a control system , taking into account estimated activities duration (from return of experience of unit operation) , response time of the process and performances of control operators and control systems. Activity processing should not be restricted to control but should also include periodic tests,
The description of the speCifications to monitor and to control transformations should be organized into different layers. The first layer should dedicated measurements and actuations.
17
to
the
The specifications of the measurements should be described as : • requirements for performances (engineering unit, accuracy, update, safety, availability, failure reporting), • requirements for monitoring and control processing interfaces (data inputs and outputs networked with up-stream and down-stream processing) , • requirements for operator interfaces (types of interface, locations of interface, data access from the interfaces, safety and availability of interfaces) . These requirements should be a set of attributes encapsulated into measurement specification blocks (MSS). The specifications of the actuations should be described as : • requirements for performances (such as type of actuator, power supply, type of power interface, instrumentation, response time, safety, availability, failure reporting), • requirements for monitoring and control processing interfaces (data inputs and outputs networked with up-stream and down-stream processing), • requirements for operator interfaces (types of interface, locations of interface, data access attached the interface, safety and availability of interface ). These requirements should be a set of attributes encapsulated into actuation specification blocks (ASS).
• •
• • •
processing Identifier, Inputs (from measurements and actuations, from other monitoring and control processing , from control operators) , Outputs (to actuation, to other monitoring and control processing, to control operators), Performances (such as accuracy, safety, availability), Requests and reports from up-stream and downstream transformation monitoring and control processing . Reque.ts and Npotta from c::ontrol operatoB
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Monitoring and controf docunwm apecif"te:ltions
Figure 14: monitoring and control processing of a transformation.
We point out that the layers of the specifications are not a hierarchy of power, it is a hierarchy of organization where the data must flow between each processing to achieve and autonomous sub system able to monitor and to control each transformation, see figure 14.
The second layer should be reflex processing to monitor and to control the structure and characteristics of the transformations, such as : • Protection controller : controlling physical characteristics of transformations via actuation and with respect to thresholds, • Measurement monitoring : measuring structure and transformation characteristics calculated from one or several measurement, • Open loop controller : controlling redundant components and circuits with respect to requests from monitoring and control processing or operators, • Closed loop controller : controlling physical characteristics of transformations via modulated actuations and with respect set-points, • Sequence control : controlling on/off actuations, open and closed loop controllers,
We can refer to Laborit [3] "each cell, each organ, each system control nothing, but each cell, each organ, each system, receive information to know what it has to do, to co-operate to the operation of the overall system. Each cell, each organ, each system send information to the rest of the overall system to request needs to be satisfied to continue to operate in good conditions . . . This double circulation of information is fundamental to understand .. .to achieve an autonomous overall system" . The speCifications of the monitoring and control processing should be detailed, with a function block language, as networks of Elementary Function Slocks (EFS) and Application SpeCification Slock (ASS) , such as Measurement and actuation specification blocks.
The third layer should be processing to monitor the status and to control the state of the transformations. For the speCifications of the second and third layers, process-engineers should identify for each monitoring and control processing :
Process engineers should also define constraints,
18
attached to the elementary and application specification blocks such as safety, availability and time constraints, for example blocks processing order, constraints of implementation of blocks (in the same I&C device, or not) ...
For the specifications of the overall process, taking into account the level of automation we should have process speCification blocks networked to the transformations speCification blocks.
Requests and reports from control operators
.. .- -- ~ .. ~-- .
Requests ."d reporl$ from control operators
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................................ :~ ...~..........................~ ...?':-................................ ,
Monitoring and control document speeirtcations
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Figure 15: Functional Specification Diagram of the control processing "to control the pumps".
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Figure16: Process monitoring and control processing.
The monitoring and control requirement diagrams should be considered as perennial requirements implementable into any I&C devices.
We should paraphrase Laboritt [3] each specification block should receive data to know what it has to do in order to co-operate with the other speCification block to achieve an autonomous processing system, able to help control operators to monitor and to control the steady-states and the transients of the process operation.
We point out that the International Electrotechnical Commission is working on the standardization of elementary function blocks and should also propose standardized rules to build application speCification blocks.
A complete simulation of the overall monitoring and control processing should be interesting to validate: • The share of activities between control operators and the control system, • The behavior of the monitoring and the control processing, • The measurement and actuation of the monitoring and the control processing, • The data requests and reports of the control operators.
It should be possible to validate by simulation the speCifications of the monitoring and control processing for each transformations. Different types of simulation should be possible. The monitoring and control processing should be connected to a simulation of the measurements and actuations to take into account a predefined and realistic set of failures . The monitoring and control processing should be connected to a simulation of the measurements and actuations and of the process. The quality of the simulation is depending on the quality of the model. We point out that for new processes it should be difficult to anticipate the behavior of this process. But for well-known process it should be possible to set up a simulation of the process.
SPECIFICATIONS OF THE MONITORING AND CONTROL DOCUMENTS. These documents should support data exchanged between control operators and the monitoring and control system; it should be user friendly . The definition of speCifications for monitoring and control documents is not trivial. Ergonomic monitoring and control documentation should be a set of documents easy to understand and to use.
The speCifications of the monitoring and control processing of the transformations should be organized into reusable monitoring and control processing specification blocks.
19
Monitoring
oneS....-
engineering, and during the commissioning where they should be adapted to the real life process monitoring and control. ~
opeciIigIio".
For the process and transformations synopsis, we should have two complementary representations ; a global one representing the process and detailed representations of the transformations.
Monitoring .nd control processing SpKifiClltions
On the process synopsis we should have : • The process should be represented as a network of graphic symbols summarizing the main structure supporting the main transformations, • The graphic symbols should be dynamically colored with respect to the state and status of the structure supporting the transformations, • Monitoring data should summarize the main balances of material and energy, • Monitoring data should summarize the status of the structure supporting the transformations, • Control data should be usable to control the state of the process.
Figure 17 : Specifications of the process monitoring and control documents.
To monitor and to control a process, control operators need different types of documents: •
•
•
Transient monitoring and control documents, to start and to shut-down the process for normal operation, Process and transformation synopsis : • To monitor the normal transients of the process and of the transformations, • To monitor and to control the steady-states of the process and of the transformations, • To monitor emergency transients of the process and the transformations for abnormal operation . Transient monitoring and control documents, to move the process towards a safe energetic state in case of abnormal operation (incidents, accidents), requiring emergency monitoring and control activities.
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For the normal transient monitoring and control documents, control operators should use the ordered graphs of serial/parallel statements to monitor and to control the transients from cold shutdown steadystate up to nominal operation steady-state. These graphs of serial/parallel statements should be issued from the share of monitoring and control activities between control operators and the control system.
remote transmitters
rwmot. actuators
Figure 18 : Process synopsis.
On the transformation synopsis we should have : • The transformations as a network of graphic symbols representing the components and the circuit of the structure supporting the transformations, • The graphic symbols dynamically colored, with respect to the state and status of the components, the circuit and the transformations. • Monitoring data representing characteristics of the components, circuits, transformations and balances of material and energy. • Alarm handling alerting control operators to overcome defaults, • Control data usable to control the active single or redundant components and circuits. the
These statements, to monitor and to control the transients of the process, should be structured with respect to the different types of transients between the steady-states of the process. These statements should be represented as graphs of serial/parallel instructions. From the ergonomic point of view, there is a large variety of possible graphical representations. These transient monitoring and control documents should be validated by simulation . within the
20
controllers, and the sequences. transformation synopsis
=>--~~---~
-'->,>- " -"-
......../ ' :J-
schemes, to understand the activities control processing is carrying out. We should also have logbooks, technical sheets ...
y
CONCLUSION
)'-.=-::::-o.
pi'
We focussed on up-stream aspects of engineering. we should investigate the full process control life cycle and further on, for maintenance and management.
LI - ... _-- - '-' -~::.-. water 'ft conditionning ~ - . - - . - - - -""A-
<-..;;<_ n
_- \3 Pl ~.
- -:~ -:;:;t._-/"", P2 . ' , F3 ._- ~/ -1f--~~ ~ ->
However, it seems system analysis should be one way to rationalize process control and to stand it on strong definitions, concepts and theories, able to take into account: • the complexity of process and of process control with different and consistent levels of knowledge and point of views, without limitation for knowledge improvement, • the organization of a process as an ordered network of transformations, inter-layered graphs of monitoring and control activities (carried out by operators) and processing (supported by a control system), without limitation for future integration of maintenance and management, • The behavior of an autonomous system as networked process, control system and control operators activities.
P3 ~
--- =-> ---
I
transfonnation control requests. ~. ... --.-.-/~~
i
transfonnation monitoring reports
~~~
~,,>~,
~~>tt';~~ ~./ ~V ~PU~ . ~
#### ## MSB
ASB
MSB
ASB
MSB .MSB '
ASB . ·ASB i
Figure 19 : Transformation synopsis.
From the transformation synopsis, control operators should interact directly on the active components (individual actuators, switch on/off redundant actuators or circuits).
A difficulty should be to set up a multi-representation language and a structured data base able to support the different types of representations needed for the different points of view (process operation, specifications of monitoring and control processing and documents, and control operator activities). This should need cross-fertilization and co-operation between different worlds, which is not usually a natural attitude.
Control operators should also interact with the monitoring and control processing (adjust controller parameters, change limit parameters of measurements ... ). In these cases, control operators should access to dedicated menus. When disturbances occur, control operators should need alarm handling, listing activities control operators should carry out to recover the predefined disturbances.
REFERENCES
[1] Bateson G., Vers une ecologie de I'esprit, Paris, Le seuil, 1977. [2] Dellattre P., Systeme, structure, fonction, evolution, Paris, Maloine-Douin, 1971 . [3] Laborit H., La nouvelle grille, Paris, R Laffont, 1974. [4] Putnam H., Representation et realite . Paris, Gallimard , 1988. [5] Walliser B., Systemes et modeles. Introduction critique a I'analyse de systemes, Paris, Le seuil, 1977. [6] Watzlawick B., L'invention de la realite , Paris, Le seuil , 1988.
For abnormal operating, incident or accident, to move the process towards a safe low energetic state, control operators should use transient emergency monitoring and control documents. The presentation of these documents should be consistent with the normal monitoring and control documents. They should present specialized accidental instruction list to monitor and to control the process down to a safe lower energetic steadystate. Last but not the least, control operators should access to real time description of the programming
21
PROCESS CONTROL ENGINEERING TRENDS Dominique GALARA Electricite de France - Research and Development Department, Power Plant Control Branch 6, quai Watier, BP 49, 78401 Chatou, Cedex, FRANCE, eMail : [email protected] Jean Pierre HENNEBICQ Electricite de France - Generation and Transmission Division, Hydro and Fossil fuel plant Operation 34-40, rue Henri RegnauIt, PARIS LA DEFENSE, FRANCE.
Summary : The aim of this paper is to highlight some trends in the field of process control engineering to improve competitiveness of process control for new applications, maintenance of applications or retrofits. Rationale scientific foundations based on system approach should be a significant contribution to improve process control engineering of the future. We focus our interest in the definition of perennial process control specifications, whatever the implementation into offthe-shelves control system should be. Copyright © 1998 IFAC
INTRODUCTION
The aim of this paper is to point out some trends in the field of process control engineering, in particular to highlight the definitions we gave for "process" and "control system".
This paper is an attempt to pOint out productivity deposits to improve process control engineering . The aim of this paper is to focus on process control engineering.
Currently the notions of "process" and of "control system" are standing on the know how of engineering companies, the skill of engineers and the return of experience.
The first difficulty we have to cope with , is the lack of universal definition for "process", due to the variety of process from the hardware point of view. For example, it is rather easy to distinguish manufacturing and power plant hardware processes.
For new engineers starting in the field of process control, it is not trivial to make reference to an explicit life cycle for control system.
Now, if we adopt the point of view of engineering methodology, it becomes difficult to justify this variety. Currently company made engineering methods exit and are used but aren't they instances of a generic process engineering methodology?
Control system are defined, in general implicitly, as programming schemes and mimics from mechanical studies (Piping & instrumentation diagram and textual process operation description), as summarized on figure 1.
We should not define "process" with reference to the process hardware, but with respect to the set of transformations of material and energy into product. Each transformation could be supported by a large variety of process hardware. The second difficulty we have to cope with is the lack of definition for "control system". Each industrial area has its own definition, in general control system technology-driven . On the market, the number of control system vendors is decreasing, as a consequence the new control systems are usable in different types of process industries.
Control system driven Npf"Hentltions
Figure 1: From mechanics down to control .system.
We should not define "control system" with reference to the technology of control system , but with respect to the monitoring and control processing and the monitoring and control documents, control systems should have to support.
Defining a control system as programming schemes and mimics should not be a problem if they are perennial. Unfortunately, technology evolution is very fast, control systems disappear due to the
11
«Monopoly» between suppliers: Last but not the least, control system obsolescence is reached before 10 years. Problems occur for process control and control system maintenance.
PROCESS CONTROL - - - - - - - -------- - - -
~
studies
control system - > specification
process dependent control system in dependant
control system design
-
WORLD OF Material MA TERIAL~ ; & ENERGY Energy
process dependent control system dependant
Control operators are in the "world" of information. N. Wiener said "information is information not material nor energy". The distinction between data and information is not trivial. G. Bateson [1] said "a data is a track of an event, an information is a difference generating a difference". From figure 4, it appears that a control system is depending on the share of monitoring and control activities between control operators and the control system.
-
Process monitoring and control activites
~
proc:ess monitoring & 'controlllCtlvitles :
monitoring & :> control document .specifications
->
monitoring &
_> control proc:esslng __> specifications
,,- -
A
. -'-~~-'- '- ' -- ~ ' product
"
,--. >
Mechanical
-> _ _ _ _ _.-Waste
studies '
Process
I
----.Iwasl'e
A control system should be is in the "world" of data, processing data coming from and going to the process and operators. The interface with the process are data from/to transmitters and actuators. The interface with the operators is a set of monitoring and control documents.
Process control is a set of process monitoring and control activities, shared between control operators and a control system.
>
~>
Process
A process should be is in the "world" of material and energy, transforming material and energy into product and waste.
Before defining perennial speCifications of any control system. We point out that a control system is a part of a process control.
&.
- -.'I. - -product -
Figure 4: Control system within process control.
TOWARDS PERENNIAL CONTROL SYSTEM SPECIFICATION
Energy
_
- -- -- -- -- -- -- -- -- -- --=-- -- - -- - - - - -- -y
system Implementation
The main points of interest should be: • for new applications, to explicit and to be able to select the requested specifications of any control system, • to achieve perennial specifications whatever the control system, in which they will be implemented, is, • to integrate explicitly the evolutions of the specifications due to process modifications or to control system upgrades, • for retrofit, to reuse the specifications for implementation into upgraded or new control systems.
WORLD OF MATERIAL & ENERGY
monitonng & I documents supportBd by a control system CONTROL - monifoi'ing & COilfrOI -: SYSTEM processing supported by a control system : - -- -- -- - -- -- - .. -- -- ~ -- -~ - - - -- - -- - -- -- -~
WORLD OF DATA
Figure 2: Life cycle of a control system.
Material
A ____________ _ _
- - - . - - - - - - - - - - - - - - - - - - - -c:O.lri,- - - - - -
-CO~ ~
activities supportBd by control operators A
One way to improve control systems should be to achieve explicit perennial and reusable control system specifications implementable into any future control system, whatever the technology evolution should be, as summarized on figure 2. Mechanical
monitoring & contrOl
WORLD OF INFORMATION
- >
control ~ontrol system ._ > , ,. system specification / design
process dependant control sysbHn independant
Figure 3: Process control. A control system should support a subset of the process monitoring and control activities, figure 4.
_..>
control system Implementation
process dependant control sysbHn dependant
Figure 5: Control system life cycle.
12
activities is depending on the structure of the process. We should represent the structure of a process as an ordered network of active (pumps, valves .. . ) and passive (pipes, tanks ... ) components. We also have to identify the transmitters and actuators necessary for monitoring and to control activities. For normal process operation, the structure of a process is stable.
Process-engineers should establish the process monitoring and control activities taking into account process operation, experience feedback and requested performances (safety, availability). On the basis of the process monitoring and control activities, process-engineers should study and optimize the share of monitoring and control activities between control operators and a control system.
=----=-~~ -
=- - : : ( f - -
Once monitoring and control activities devoted to a control system should be identified; processengineers should describe perennial monitoring and control processing specifications, monitoring and control document specifications (as human machine interface), taking into account process operation constraints (safety , availability, performance).
-
----i<:~ ,., -= "'~--
We point out that the monitoring and control document speCifications should be completed and validated with control operators.
Figure 6: Scheme of the structure of a process.
The transformations are depending on the steadystates and transients of the process operation .
For engineering companies, anticipating the control system through engineering studies is important. The objective of reducing the cost and improving the quality of control engineering should be satisfied through explicit and standardized studies standing on reusable library of generic elements.
For steady-state process operation, we should define a set of activities to monitor and to control an ordered network of synchronic (Walliser [5]) transformations of material and energy within a structure.
PROCESS MONITORING AND CONTROL ACTIVITY REPRESENTATION from control point of view.
For transient process operation, we should define a set of activities to monitor and to control an ordered network of diachronic (Walliser [5]) transformations of material and energy within a structure.
In this section, the intention is to highlight what should be a representation of the process monitoring and control activities.
For process control, the point of interest is not in the structure of the process, but in the definition of the monitoring and control activities of the steady-state and transient transformations of a process.
We point out that there is a rupture between the world of mechanics and the world of process control. Any representation is directed by an intention, see H. Putmann [4]. The intention of control engineering is to represent process monitoring and control activities. To achieve this, process engineers need to know what to control, unfortunately this description is carried out by mechanical engineering, with another intention. We have to cope with two representations understandable in these two worlds. Here is a difficu Ity .
Process monitoring and control activities are depending on characteristics allowing to monitor and to control the steady-state and transient process operation . For the characteristics, P. Delattre [2] distinguishes qualitative and quantitative properties. Each characteristic is a unique qualitative alphanumerical identifier. Each characteristic is dedicated to a unique and unambiguous property of the structure or the transformations. Each characteristic can be completed with a quantitative value (a data and its measurement reference).
To define the process monitoring and control activities, we should used system approach, in particular from P. Delattre [2] suggesting the definition of a system as three interleaved representations : the structure, the transformations and the behavior.
Characteristics should be used to monitor and to control the structure and the related transformations of material and energy, such as : • The status and the state of the active
The structure of the process monitoring and control
13
•
• • •
components of the structure of the process, The status and the state of redundant components and circuits of the structure of the process, The status and the state of material and energy in the components and circuits, The status and the state of the transformations of material and energy, The status and the state of material and energy balances.
•
•
• •
We point out that these characteristics must be understandable in the worlds of plant operation (control operators ... ) and engineering (mechanics, process control).
•
These characteristics are chosen to be, on one hand , explicitly related to the physical characteristics of the structure and of the transformation of material and energy, and on the other hand , to be easily identified in the process monitoring and control activities.
operating such as chemical, thermal, hydraulic .. . Values of the characteristics of the structure and of the transformations, allowing the identification of the steady-state, Reconfiguration of the structure (components and circuits) to maintain the availability of the transformations, Upstream and down stream conditions of the transformations, Color of the graphic symbols of the structure supporting the transformations, Semiotic ; on a structure scheme components should be identified by names (tank, exchanger, boiler. .. ), instead on a steady-state schemes components should be identified by activities (tanking, cooling water, boiling water. .. ) representing the transformations in progress.
For the representation of the steady-state transformations of material and energy, we should propose a suggestive representation. For example, to value the characteristics on steady-sate transformation schemes. On figure 7 is represented a feed water conditioning steady-state transformation . .':>
~ ~-i=-~---
------"--'
-- -_._- ~ ------....-.
Figure 8: Steady-states of a process operation.
~~ -- ~ ~
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For normal operating , in a steady state, the activated transformations are synchronic together, it means we have steady-state relationships between the characteristics of the different transformations.
-"
$
__ n~
For complex processes, it may be difficult to manage the complexity of a process on a single scheme. To cope with this complexity, we should propose different layers of representation.
----
Figure 7: Feed water conditioning steady-state transformation scheme.
For complex processes we could have a synopsis of the process represented as a network of icons. Each icon should summarize a part of the process. Under each icon we should have the detailed part of the process.
The boarders between the transformations of material and energy should be defined with reference to principles of equilibrium within the different domains (electrical, mechanical, hydraulic, thermal, chemicaL .. ).
In this case we should have two layers of representation of the process ; the higher layer is the synopsis, the lower layer is a detailed partition of the process.
Figure 7 should be an attempt to represent a steadystate transformation scheme. The difference with a structure scheme should be the identification and the characterization of the transformations of material and energy , for example : • Identifiers of the transformations of material and energy , • Domain in which the transformations are
We point out that these two layers are not representing a hierarchical system , they only represent different aggregations of a single an unique process .
14
operation" steady state. In the fourth step, "in operation" steady-state, we should have monitoring and control activities to maintain the transformation in a steady state, as far as some incident can be recovered within the structure (redundant components, and circuits), and preventing the environment against accident. In the fifth step, we should have monitoring and control activities to shutdown the transformation on material and energy .
-
,
In the sixth step, we should have monitoring and control activities to remove residual material and energy to clean the structure.
.
.
Figure 9: From process steady-state operation down to transformation steady-state operation.
In the seventh step, we should have monitoring and control activities to restore the "transformation " in the "out of operation" steady state.
A process steady-state operation is "time independent". To complete the interpretation of P. Delattre [2] work, we have to introduce the behavior of the process ; the transient process operation. For normal operating, a transient process operation is the evolution from one steady-state process operation to another steady-state process operation . The transformations of material and energy are diachronic, some should move to "in operation" some others to "out of operation". It means we have relationships between the characteristics and the steady-state process operation. For normal operation , most of transformations should have two steady states: "out of operation" and "in operation". Some others should have more than two; the principle we describe should be the same.
Figure 10: Monitoring and control activities to monitor and control a transformation.
For normal operation , most of transformations should be monitored and controlled in four steps, to move from "out of operation" steady-state to "in operation" steady state and vice versa, as suggested on figure 10.
The monitoring and control activities should be encapsulated into blocks corresponding to the different steps. Each monitoring and control activity block should contain a graph of serial/parallel monitoring and control statements
In the first step, "out of operation" steady state, we should have monitoring activities to check the integrity and the availability of the structure and of up-stream material and energy .
For each step, the monitoring and control statements are described in a textual way. For each monitoring statement, we should have a verb describing the activity (verify ... ), a characteristic identifier (the temperature of ... ) and the aim of the monitoring (is lower than ... ) For each control statement, we should have a verb describing the control activity (open , maintain ... ), a characteristic identifier on which the control is applied (the valve, the level in the tank .. . ). These statements should take profit of the return of experience, from process operation .
In the second step, we should have monitoring and control activities to configure the structure (for example the circuit alignment to prepare the introduction of material and energy) and to check the upstream and downstream pre-conditions and interlocks allowing to start the transformation . In the third step, we should have monitoring and control activities to launch the transformation on material and energy and to achieve the nominal "in
The monitoring and control activities should also include, for example :
15
•
•
Requirements for the measurements and actuations (accuracy, performance, availability, and safety), Requests and reports for and from the upstream and down-stream pre-conditions and interlocks allowing to network the transformations.
energetic state and minimizing the impact on the environment. To improve the quality and to reduce the costs, the monitoring and control activities should be written with a monitoring and control statement block language. This language should allow to build blocks and to reuse standardized blocks available in a library. For the language we should distinguish different types of statements, for example related to : • Process operation (to monitor and to control in order to produce), • Structure test (to monitor and to control the ability of the structure to support the operations), • Transformation test (to monitor and to control the efficiency of the transformations), • Maintenance (to monitor and to control preventive maintenance), • Management (to monitor and to control process efficiency).
From a complete process operation, we should identify the different transformations within the different steady-states and the transients between the steady-states, in particular the pre-conditions and the interlocks between the transformation . On this basis, we should describe the ordered network of steady-state and transient monitoring and control activity blocks, with respect to the steadystate and transient process transformations. Process
Process
steady Proc... state tr'llnsient
Transformations"
1
t to 2
steady state
2
Process Process transient 2103
Process
steady Proc...
steady
sate
transient
stlde
3
3110.
4
This language should allow to describe and to validate by simulation the definition of the monitoring and control activities.
TURBlNNG STEAM
GENERATING
FEEDWATERlNG
~~~
It should be also interesting for process-engineers to optimize the ordering and the coordination of the different types of monitoring and control activities to optimize the start-up and shut-down of the process.
EYOIution
Last but not the least, plant management should be interested to prepare the start-up, the shut-down and the non scheduled activity in order to optimize the monitoring and control activities to save time. The interest of the definition of the monitoring and control activities is two folds: • Anticipate the behavior of the process monitoring and control before specifying the control system, in particular for critical situations, • Explicit the behavior of the process monitoring and control for diagnosis purposes.
Figure 11 : Monitoring and control activities to monitor and to control process steady-state and transient transformations,
The monitoring and control activities blocks should contain sets of monitoring and control statements to maintain the transformations in the predefined steady-states, to master the transients between the steady-states and to recover a predefined set of incidents and accidents of the structure and of the transformations.
SHARE OF MONITOmNG AND CONTROL ACTIVITIES BETWEEN OPERATORS AND A CONTROL SYSTEM.
Once the monitoring and control activities are defined and validated, these activities should be distributed between control operators and a control system.
An incident is a failure occurring on a component of the structure or on a transformation but the integrity of the process is maintained. In general an incident can be recovered by acting on the structure or moving towards a safe lower energetic state, without impact on the environment.
Here is another difficulty to share monitoring and control activities processing between operators and a control system.
An accident is a failure occurring on a component of the structure or on a transformation but the integrity of the process is not maintained. We can recover accidents, which have been studied by acting on the structure, to move the process towards a safe lower
Currently, in general, the share of monitoring and control activities between control operators and control systems is implicit standing on the company
16
know-how, the skill of engineers and the return of experience.
maintenance and technical management activities. In the allocation of monitoring and control activities to a control system , requirements should be included for Human machine interface, for example safety, availability and performances of monitoring measurements and control actuations, alarm presentation ...
There is not a rigorous method to share and to optimize the activity processing , between control operators and a control system , taking into account control operator and control system performances, such as activity duration, work load , serial and parallel activity management capacity, activity planning, availability, safety, cost.
Let us make the assumption that the monitoring and control activities should be shared between operators and a control system.
Nevertheless, monitoring and control activities could be devoted to control operators when the monitoring activities have to access to local indicators and the control activities to manual actuators.
The set of monitoring and control statements devoted to a control system should be translated into speCifications of the : • monitoring and control processing, • monitoring and control documents as human machine interface. These specifications should be standardized and written with ad hoc function block languages.
The allocation of monitoring and control activities to control operators should also take into account the desired level of automation and the related definition of the control team and the distribution of the activities between the operators (unit chief operator, process part operators, shift operators .. . ).
SPECIFICATIONS OF THE MONITORING AND CONTROL PROCESSING.
Monitoring and control activities should be devoted to a control system when the monitoring and control activity performances (accuracy, availability, safety) are too high or repetitive for control operators (activities related to protection, control loops and surveillance). - . - -- -
stioay' state 1
_.. -
-
-
The specifications should be control system supplier independent.
st.oay-· .- - - _. steiiIY- ---- -steaay-
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Figure 13 : SpeCifications of the monitoring and control processing and documents of a transformation.
Figure 12: share of monitoring and control activity between control operator and system.
The speCifications should structured with respect to the structure of the process; a network of transformations.
A monitoring and control statement block language. should be an interesting tool for process-engineers. They could analyze and optimize meaningful and rational share of activities between control operators and a control system , taking into account estimated activities duration (from return of experience of unit operation) , response time of the process and performances of control operators and control systems. Activity processing should not be restricted to control but should also include periodic tests,
The description of the speCifications to monitor and to control transformations should be organized into different layers. The first layer should dedicated measurements and actuations.
17
to
the
The specifications of the measurements should be described as : • requirements for performances (engineering unit, accuracy, update, safety, availability, failure reporting), • requirements for monitoring and control processing interfaces (data inputs and outputs networked with up-stream and down-stream processing) , • requirements for operator interfaces (types of interface, locations of interface, data access from the interfaces, safety and availability of interfaces) . These requirements should be a set of attributes encapsulated into measurement specification blocks (MSS). The specifications of the actuations should be described as : • requirements for performances (such as type of actuator, power supply, type of power interface, instrumentation, response time, safety, availability, failure reporting), • requirements for monitoring and control processing interfaces (data inputs and outputs networked with up-stream and down-stream processing), • requirements for operator interfaces (types of interface, locations of interface, data access attached the interface, safety and availability of interface ). These requirements should be a set of attributes encapsulated into actuation specification blocks (ASS).
• •
• • •
processing Identifier, Inputs (from measurements and actuations, from other monitoring and control processing , from control operators) , Outputs (to actuation, to other monitoring and control processing, to control operators), Performances (such as accuracy, safety, availability), Requests and reports from up-stream and downstream transformation monitoring and control processing . Reque.ts and Npotta from c::ontrol operatoB
__ .
~
7-
~
;...
Monitoring and controf docunwm apecif"te:ltions
Figure 14: monitoring and control processing of a transformation.
We point out that the layers of the specifications are not a hierarchy of power, it is a hierarchy of organization where the data must flow between each processing to achieve and autonomous sub system able to monitor and to control each transformation, see figure 14.
The second layer should be reflex processing to monitor and to control the structure and characteristics of the transformations, such as : • Protection controller : controlling physical characteristics of transformations via actuation and with respect to thresholds, • Measurement monitoring : measuring structure and transformation characteristics calculated from one or several measurement, • Open loop controller : controlling redundant components and circuits with respect to requests from monitoring and control processing or operators, • Closed loop controller : controlling physical characteristics of transformations via modulated actuations and with respect set-points, • Sequence control : controlling on/off actuations, open and closed loop controllers,
We can refer to Laborit [3] "each cell, each organ, each system control nothing, but each cell, each organ, each system, receive information to know what it has to do, to co-operate to the operation of the overall system. Each cell, each organ, each system send information to the rest of the overall system to request needs to be satisfied to continue to operate in good conditions . . . This double circulation of information is fundamental to understand .. .to achieve an autonomous overall system" . The speCifications of the monitoring and control processing should be detailed, with a function block language, as networks of Elementary Function Slocks (EFS) and Application SpeCification Slock (ASS) , such as Measurement and actuation specification blocks.
The third layer should be processing to monitor the status and to control the state of the transformations. For the speCifications of the second and third layers, process-engineers should identify for each monitoring and control processing :
Process engineers should also define constraints,
18
attached to the elementary and application specification blocks such as safety, availability and time constraints, for example blocks processing order, constraints of implementation of blocks (in the same I&C device, or not) ...
For the specifications of the overall process, taking into account the level of automation we should have process speCification blocks networked to the transformations speCification blocks.
Requests and reports from control operators
.. .- -- ~ .. ~-- .
Requests ."d reporl$ from control operators
~ ;:.. ~--
................................ :~ ...~..........................~ ...?':-................................ ,
Monitoring and control document speeirtcations
l : :
~
Figure 15: Functional Specification Diagram of the control processing "to control the pumps".
Rq
4Rq.RP1
~
Rq
monitoring and
control processing
l : :
~
Figure16: Process monitoring and control processing.
The monitoring and control requirement diagrams should be considered as perennial requirements implementable into any I&C devices.
We should paraphrase Laboritt [3] each specification block should receive data to know what it has to do in order to co-operate with the other speCification block to achieve an autonomous processing system, able to help control operators to monitor and to control the steady-states and the transients of the process operation.
We point out that the International Electrotechnical Commission is working on the standardization of elementary function blocks and should also propose standardized rules to build application speCification blocks.
A complete simulation of the overall monitoring and control processing should be interesting to validate: • The share of activities between control operators and the control system, • The behavior of the monitoring and the control processing, • The measurement and actuation of the monitoring and the control processing, • The data requests and reports of the control operators.
It should be possible to validate by simulation the speCifications of the monitoring and control processing for each transformations. Different types of simulation should be possible. The monitoring and control processing should be connected to a simulation of the measurements and actuations to take into account a predefined and realistic set of failures . The monitoring and control processing should be connected to a simulation of the measurements and actuations and of the process. The quality of the simulation is depending on the quality of the model. We point out that for new processes it should be difficult to anticipate the behavior of this process. But for well-known process it should be possible to set up a simulation of the process.
SPECIFICATIONS OF THE MONITORING AND CONTROL DOCUMENTS. These documents should support data exchanged between control operators and the monitoring and control system; it should be user friendly . The definition of speCifications for monitoring and control documents is not trivial. Ergonomic monitoring and control documentation should be a set of documents easy to understand and to use.
The speCifications of the monitoring and control processing of the transformations should be organized into reusable monitoring and control processing specification blocks.
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Monitoring
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engineering, and during the commissioning where they should be adapted to the real life process monitoring and control. ~
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For the process and transformations synopsis, we should have two complementary representations ; a global one representing the process and detailed representations of the transformations.
Monitoring .nd control processing SpKifiClltions
On the process synopsis we should have : • The process should be represented as a network of graphic symbols summarizing the main structure supporting the main transformations, • The graphic symbols should be dynamically colored with respect to the state and status of the structure supporting the transformations, • Monitoring data should summarize the main balances of material and energy, • Monitoring data should summarize the status of the structure supporting the transformations, • Control data should be usable to control the state of the process.
Figure 17 : Specifications of the process monitoring and control documents.
To monitor and to control a process, control operators need different types of documents: •
•
•
Transient monitoring and control documents, to start and to shut-down the process for normal operation, Process and transformation synopsis : • To monitor the normal transients of the process and of the transformations, • To monitor and to control the steady-states of the process and of the transformations, • To monitor emergency transients of the process and the transformations for abnormal operation . Transient monitoring and control documents, to move the process towards a safe energetic state in case of abnormal operation (incidents, accidents), requiring emergency monitoring and control activities.
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For the normal transient monitoring and control documents, control operators should use the ordered graphs of serial/parallel statements to monitor and to control the transients from cold shutdown steadystate up to nominal operation steady-state. These graphs of serial/parallel statements should be issued from the share of monitoring and control activities between control operators and the control system.
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Figure 18 : Process synopsis.
On the transformation synopsis we should have : • The transformations as a network of graphic symbols representing the components and the circuit of the structure supporting the transformations, • The graphic symbols dynamically colored, with respect to the state and status of the components, the circuit and the transformations. • Monitoring data representing characteristics of the components, circuits, transformations and balances of material and energy. • Alarm handling alerting control operators to overcome defaults, • Control data usable to control the active single or redundant components and circuits. the
These statements, to monitor and to control the transients of the process, should be structured with respect to the different types of transients between the steady-states of the process. These statements should be represented as graphs of serial/parallel instructions. From the ergonomic point of view, there is a large variety of possible graphical representations. These transient monitoring and control documents should be validated by simulation . within the
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controllers, and the sequences. transformation synopsis
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schemes, to understand the activities control processing is carrying out. We should also have logbooks, technical sheets ...
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CONCLUSION
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We focussed on up-stream aspects of engineering. we should investigate the full process control life cycle and further on, for maintenance and management.
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However, it seems system analysis should be one way to rationalize process control and to stand it on strong definitions, concepts and theories, able to take into account: • the complexity of process and of process control with different and consistent levels of knowledge and point of views, without limitation for knowledge improvement, • the organization of a process as an ordered network of transformations, inter-layered graphs of monitoring and control activities (carried out by operators) and processing (supported by a control system), without limitation for future integration of maintenance and management, • The behavior of an autonomous system as networked process, control system and control operators activities.
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Figure 19 : Transformation synopsis.
From the transformation synopsis, control operators should interact directly on the active components (individual actuators, switch on/off redundant actuators or circuits).
A difficulty should be to set up a multi-representation language and a structured data base able to support the different types of representations needed for the different points of view (process operation, specifications of monitoring and control processing and documents, and control operator activities). This should need cross-fertilization and co-operation between different worlds, which is not usually a natural attitude.
Control operators should also interact with the monitoring and control processing (adjust controller parameters, change limit parameters of measurements ... ). In these cases, control operators should access to dedicated menus. When disturbances occur, control operators should need alarm handling, listing activities control operators should carry out to recover the predefined disturbances.
REFERENCES
[1] Bateson G., Vers une ecologie de I'esprit, Paris, Le seuil, 1977. [2] Dellattre P., Systeme, structure, fonction, evolution, Paris, Maloine-Douin, 1971 . [3] Laborit H., La nouvelle grille, Paris, R Laffont, 1974. [4] Putnam H., Representation et realite . Paris, Gallimard , 1988. [5] Walliser B., Systemes et modeles. Introduction critique a I'analyse de systemes, Paris, Le seuil, 1977. [6] Watzlawick B., L'invention de la realite , Paris, Le seuil , 1988.
For abnormal operating, incident or accident, to move the process towards a safe low energetic state, control operators should use transient emergency monitoring and control documents. The presentation of these documents should be consistent with the normal monitoring and control documents. They should present specialized accidental instruction list to monitor and to control the process down to a safe lower energetic steadystate. Last but not the least, control operators should access to real time description of the programming
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