Knowledge based systems for pressure vessel design

Int. J. Pres. Ves. & Piping 53 (1993) 23-38

Knowledge Based Systems for Pressure Vessel Design

John Lawson

Engineering Design Research Centre, 1 Todd Campus, West of Scotland Science Park, Maryhill Road, UK, G20 OXA

&

John A n d e r s o n

University of Paisley, High Street, Paisley, UK, PAl 2BE

(Received 13 December 1991; accepted 28 December 1991)

A B S T R A C T

This paper describes a knowledge based system for the design o f pressure vessels. The system provides a unifying framework for the currently disparate pieces of software that support the design process. The system also provides a flexible platform for the incremental expansion of automated design capability.

1 INTRODUCTION

The application of computer aided engineering (CAE) software to the design of pressure vessels and other engineered products has had a significant impact on the design and manufacturing process. The application of computers to different activities within manufacturing has been evolutionary. Computer hardware and software has been applied to provide 'islands of automation' in design and manufacture whenever the continuing improvements in hardware and software technology has made it cost effective to do so.

CAE software and hardware is now so low cost that there are few areas of technical endeavour that are not supported by general information management, and specialist technical software.

This evolution has resulted in software and hardware systems which, because of their disparity, offer some obstacles to the further development of CAE support tools.

Expert system shells provide a vehicle for the construction of large scale design systems which can fully utilize existing software, thus

23 Int. J. Pres. Ves. & Piping 0308-0161/92/$05-00 (~) 1992 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland

24 John Lawson, John Anderson

providing a vehicle for the development of future design automation and support systems.

2 C U R R E N T A P P L I C A T I O N OF C O M P U T E R S TO P R E S S U R E VESSEL DESIGN AND M A N U F A C T U R E

2.1 Support for numerical analysis of analytical solutions

The solution of some systems of equations used to model the behaviour of pressure vessel components is only practical by the application of numerical methods, using a computer.~'2

Seng z justifies this type of application in stress analysis by the high cost of the alternative use of the finite element method (FEM). The high cost of FEM has been due to its requirements for high levels of computer processing power, but in the last decade the processing power of computer systems delivered per unit cost has doubled each year.

Regardless of the increases in processing power, this type of software will still have a place because it is simpler and more effective to use than the FEM for specific applications, and can provide a useful check on the FEM, which because of its generality will always require reliable verification procedures.

2.2 Finite element method

This can be used as a very powerful stress analysis tool and, although it requires very powerful computers for realistic application, the continuing drop in the real price of processing power is making it more accessible.

It will still be some time before the developing sophistication and intelligence of the software has an impact on the high level of skills required by practitioners to use these systems safely.

The direction of system development will be problematical, as increasing ease of use will be seen as encouraging users with decreasing levels of skills to use the systems. This gives cause for concern when at this time there is a lack of validation of the competence of users of these systems.

2.3 Design code calculation automation

The design and manufacture of almost all pressure vessels is governed by the relevant design codes that have been adopted as national standards, e.g. BS 55003 and ASME VIII division 1. 4

Knowledge based systems for pressure vessel design 25

These codes provide rules and formulae for the construction of pressure vessels based on research, experience and current best practice. They provide little or no support for conceptual design. Having selected a geometry, the codes can be used to ascertain the required thickness of the vessel components.

The design methods used in these codes are often iterative, time consuming and tedious. However, as they are constrained by rules and formulae, they have been amenable to computerisation 5"6 with very high gains in productivity.

Such has been the success of this particular application that several commercial packages such as PVE5 (Whessoe Technical and Comput- ing Systems, Darlington, UK) are available.

2.4 CAD (Computer aided design systems)

Systems such as AutoCad (Autodesk, Guildford, Surrey) are widely used; their cost of ownership is usually a direct function of the level of the three-dimensional (3D) capability of the system. This has meant that these systems have been draughting, rather than design systems. The productivity impact of these systems takes place at four levels, as follows.

(i) Improved accuracy, control and ease of modification through the flexibility and precision of these systems.

(ii) Creation of standard libraries of graphical components (e.g. nuts, bolts and flanges) that can be scaled and added to drawings.

(iii) Output of data to downstream manufacturing activities (e.g. profiles for numerically controlled machining).

(iv) Creation of programs within the CAD systems allowing design knowledge and procedures to be committed to the system so that elements of the design process can be fully automated.

The majority of users usually achieve the first two levels; indeed, failure to do so would make their investment worthless. It is only at level (iv) that these systems begin to impact on the design process, but this impact has been limited by the capabilities of the CAD system programming languages, the best of which have no better capabilities and facilities than a modern BASIC interpreter.

2.5 General computer tools

The design and manufacturing process is supported by a raft of general information management and transformation tools, which are, typi- cally, database systems, spreadsheets and documentation systems.


3 PRESSURE VESSEL DESIGN PROCESS

A pressure vessel provides a constraining envelope around a physical or chemical process. The geometric and the volumetric constraints depend on the parameters of the process, e.g. process fluid throughput, 'hold up time' and dimensions of proprietary internals. The materials selected will depend on the operating temperatures, pressures and the corrosive properties of the process fluid.

The aim of the reported design system, which will be described, is to provide a system that can be used to support the conceptual design of pressure vessels from the earliest stages within the design process. The system can make decisions on the basis of criteria such as the design for manufacture, explore different geometric and material conceptual design options and optimise the design on that basis.

4 ARCHITECTURE OF THE DESIGN SYSTEM: OVERVIEW

The system was built from major design support components as described in Section 2, arranged as shown in Fig. 1, and listed as follows:

(i) AutoSolid (solid modeller and CAD sysem) (Autodesk). (ii) Nexpert Object hybrid expert system shell (Neuron Data, Palo

Alto, California). (iii) PVE5 BS 5500 & ASME VIII division 1 design code software. (iv) INGRES relational database (Relational Technology Ltd,

London, UK).

The system was developed and runs under UNIX on an Apollo DN3500 workstation.

Fig. 1.

PVE5 I Ingres I Calculating Relational Engine Database

Arrangement of basic components of the design system.

Knowledge based systems for pressure vessel design

5 EXPERT SYSTEM SHELL

27

5.1 Introduction

The central core of the design system is the expert system shell. The shell that was chosen was Nexpert Object. It is a rule based and object oriented system, which is capable of representing knowledge in a natural and powerful way. Sim et al.7 highlighted the shortcomings of a rules only based system, and pointed out the need for frame or object based representation when tackling this type of application.

The expert system provides a medium for representing, and conserv- ing, design knowledge, and as a vehicle for the investigation of the science of design. 8 The expert system technology has emerged from research into artificial intelligence.

5.2 Rule based reasoning

As previously stated, the knowledge base is built of rules and objects. The rules are of the form shown in Fig. 2. The rules form a network of conditions that can be traversed in the backwards or forwards direction to satisfy the design constraints.

5.3 Object oriented representation

The system has object orientated data representation, which provides a very powerful tool for modelling product design data.

Meyer 9 covers this as one of the most important programming paradigm shifts in recent years, and Warman a° its importance in the development of new generations of CAD systems.

Figure 3 shows the typical object oriented representation of a hemispherical head. The object is an instance of the class Hemispherical

The format of the rules are ,

I 1F conditions .... I THEN conclusion,] AND I

actions... ]

Then a typical rule looks as follows

IF corrosion allowance is UNKNOWN, and material is carbon steel.

THEN corrosion allowance is set to default, AND

Let corrosion allowance = 3.0

Fig. 2. Example of the format of knowledge base rules.

28 J o h n L a w s o n , John A n d e r s o n

Fig. 3.

Shell

Component

Head

Hemi-head 1 - -

Head

Class : Shell Component

Property Slots: diameter; thickness; volume;

- - [ Class : Head

I Property Slots: , end;

Class : Hemispherical Head Classes:

Shell Component; Head

Property Slots: diameter; thickness; volume; end;

Methods: volume = 4/3*~r*(diameter/2) ^ 3

Object :Hemi-head 1 _

Classes: Hemispherical Head

Property Slots: diameter = 2000; thickness = 50; volume =

4/3*0*(2000/2) ^ 3 end= TOP,

[ I~ diameter = 200~_~ thickness = 50

Example of class h ie rarchy of a hemispher ica l head c o m p o n e n t objec t .

Head. This class has been derived from the base classes Head and Shell Component. The object Hemi-Head_l will inherit the property slots of its parent classes. Thus, the object has data slots which describe the object, and methods are attached to the object to manipulate that data, such as the method shown to evaluate the volume property slot.

5.4 System integration capability

The shell, written in the 'C' programming language, is provided as a development system and a set of libraries. This allows it to be used as

Knowledge based systems for pressure vessel design

an intelligent subsystem within an application, or as a means to provide the main system with the ability to communicate with other pieces of software. The reported system that has been constructed uses both of these capabilities, in that it runs as a subsystem from within AutoSolid, and PVE5 and the INGRES relational database run as communicat ing subsystems from within the expert system shell.

Nexpert has a 'callable interface' which allows functions that are written in the 'C' programming language to be installed within Nexpert, allowing program control to flow between Nexpert and installed functions.

6 DESIGN C O D E S O F T W A R E

6.1 Pressure vessel design code software

When the user, assisted by the expert system, has selected a geometric configuration for the pressure vessel, it is then necessary to apply the rules and formulae, from the relevant design codes, to calculate the required thicknesses of the vessel components .

Many man-years of effort have been expended on the development of code design software systems. To avoid needless duplication of effort and to address realistically some of the problems involved with system integration and software reuse, it was decided that the system would use commercially available software.

Whessoe generously provided a copy of their very comprehensive BS 5500 and ASME VIII division 1 design code software, PVE5. They were able to supply a version of the software that would run under the UNIX operating system.

6.2 Integration with expert system

The expert system communicates with PVE5 through UNIX PIPES, which are interprocess communicat ion channels. When the expert system is ready to execute code calculations, it executes PVE5 to carry out an interactive dialogue with it. The expert system interacts with PVE5 in the way that the designer would, and the dialogue appears in a transcript window so that it can be viewed by the designer and can be saved in a file to provide a permanent record of the design session.

PVE5 is run by the expert system shell as a separate process, and has had no modifications made to its code to allow it to be used with the system. It has been built into the system in exactly the same form as it is normally commercially supplied. Communicat ion with PVE5 through


UNIX PIPES has been achieved through a layer of interface code written in the 'C' and ' C + + ' programming languages. This interface layer of code is communicated with by using the Nexpert 'callable interface'.

7 I N T E G R A T I O N WITH R E L A T I O N A L D A T A B A S E SYSTEMS

7.1 Design system database requirements

In the course of the design process, the designer requires to make component selections, e.g. flanges, pressed head sizes, bolting or gaskets. Relevant component information is normally held in paper based catalogues, but increasingly can be supplied on magnetic media for use by computer systems.

Information in paper based catalogues is normally held in tables, a format easily represented in a relational database system, 1~ with little transformation required. Relational databases have the advantage that they have an ISO/ANSI standard structured query language (SQL) communication interface, allowing intercommunication with other relational databases that support the above standard. INGRES, which supports the SQL standard, was chosen because of its distributed data management capability.

7.2 Integrating the relational database

The INGRES relational database is based on a client/server model and has a distributed capability. In the client/server model a client

application requests a service, e.g. answer a query or process some transactions, from a server process, which may be running on the same computer locally or on some remote machine.

INGRES provides an embedded SQL for a C module, which allows a client application to be written in the C programming language with SQL statements embedded within the language. A special preprocessor turns the SQL statements into low level 'C'. These functions are then installed in the Nexpert 'callable interface', which allows the design system to become a client application.

The system passes out queries to the database server, and objects, which map to the records that have been selected, are created within the knowledge base. The queries are kept as general as possible, to


allow:

- - the greatest possible choice of parts; - - the knowledge based system to use its powerful pattern matching

capabilities; - - the knowledge based system to modify or change its pattern

matching criteria as it receives information.

7.3 Populating the relational database

When one decides to commit a large amount of paper based data to a database system, it soon becomes apparent that a massive manual effort has to be expended entering the information. To avoid this, the catalogue information was scanned in using optical character recogni- tion (OCR) software. Such software can be trained to recognise characters, and output them to a computer file. The file was then input to the relational database, using the SQL load table command.

The reliability of OCR software is dependent on the print quality of the original paper based data; to compensate for the variability in original print quality, the output from the OCR system was verified by a checking program.

7.4 Relational database suitability and alternatives

The relational database is a reliable, mature, standardised technology and is suitable for holding the 'flat' type of information normally found in component catalogues. Unfortunately, these systems are poor in supporting complex dynamic hierarchical structures such as the complex model of a pressure vessel, which is made up of many component parts. Object oriented database management systems (OODBMS) provide a vehicle which could give better support for complex engineered products. ~2 This is an area under active research, and the commercial products are still immature; other researchers have proposed a hybrid approach by providing an object oriented layer over a relational database. ~3

8 SOLID M O D E L L E R

8.1 Why a solid modeller?

A goal of the project was that the system would be able to provide graphical output for the production of fabrication drawings. It would be


a very difficult task to decide which set of two-dimensional (2D) projections to output as a set of manufacturing drawings for a pressure vessel.

A large amount of work would be required to write functions for the expert system which would output the graphics necessary for manufacturing drawings; attempting to write such software from first principles would be unrealistic and pointless.

Outputting the graphical information in IGES ~4 or DXF (AutoCad drawing exchange format) was another possibility. This had the advantage of using a standard method that would allow a commercial CAD system to be used. However, a large amount of production information would be lost, since these exchange formats were primarily developed for the exchange of geometric drawing information, and were thus based on the limited requirements of 2D drafting information. These data exchange formats have only a very limited useful 3D and nongraphical data representational capability.

The method adopted was to use the solid modeller AutoSolid (Autodesk). This is a constructive solid geometry (CSG) and boundary representation (B-Rep) modeller. The system is provided with a programming interface library, which gives the same type of integration capabilities as described for Nexpert.

8.2 Output from the modeHer

The modeller holds the complete geometric information that describes the vessel. The requirement is to be able to provide manufacturing drawings from the solid model as simply as possible.

The system has various rendering capabilities, and these can be used to generate profile line drawings as shown in Fig. 4, which is the profile drawing of a cylindrical vessel shell with ellipsoidal heads. These profile line outputs can be produced in DXF or IGES format. The profile line

. .

Profile line drawing of cylindrical shell with ellipsoidal heads produced by the design system.


drawing can be dumped at various chosen projections to provide the basic data for a fabrication drawing.

To bench mark the system, it was fired to design automatically a vessel to stated input design parameters. The design system output was offered as input to a randomly chosen alternative CAD system, viz, DAXCAD (Practical Technology International, Glasgow). D A X C A D successfully plotted out hard copy on A0.

This gave a reasonable level of confidence that the chosen route was a realistic method of transferring graphical information from the system to other systems using available data exchange standards.

8.3 Original system with modeller running standalone

Figure 5 shows the original configuration of the system. In the early development stages of the system, a library of standalone functions supplied with the modeller were used to generate a file containing a solid model of the design vessel. The modeller was run completely separately from the design system, and was used independently to read in and display the model.

8.4 Modeller as front end

It was decided that it would be much simpler in operation, and aid further development work to have the modeller and the other parts of the design system fully integrated. This was achieved, as shown Fig. 1, with AutoSolid running as the top level process in the design system, i.e. the design system is started by selecting an entry on a popup menu within AutoSolid. The system then goes through the design process and the vessel produced can then be viewed within AutoSolid.

Fig. 5.

~ e e e ~ e ~ e e e ~ e ~ e ~ . e e e e e e

~ Calculating Relational !1 Engine Database *

• Design System ' . . . . . . . . . . . . . . . . . . . . . ,

Original arrangement of basic components of the design system, with modeiler as the standalone system.


8.5 Modeller migration and system development

The development of AutoSolid by AutoDesk has terminated, and it is no longer available as a product. Fortunately AutoCad Release 11 Advanced Modelling Extension has inherited the same types of integration facilities as AutoSolid. Thus, for future development work, the solid modelling part of the process will migrate to AutoCad.

9 DISCUSSION

9.1 Construction of the system

The Construction of this type of design system is operation that follows this sequence:

a three-phase

(i) Identity major activities that are part of the design process and decide on the type of software elements that will provide the right functionality to support and integrate this activity within the expert system model of the design process. Explore the different methods that can be used to integrate the chosen elements to ensure that system development does not become bogged down in low level technical difficulties at a later stage, which may be very difficult to solve.

(ii) Build a framework which integrates the elements in (i) based on limited and general heuristics. (Some limited testing is carried out on this framework as it is constructed to ensure its correct operation.) Continue building this 'narrowly based system' until it can proceed from an identifiable start position to achieve, for the limited case studies used to build this stage of the system, the required design goals.

(iii) Expand the breadth of capability of the already constructed framework by filling the system with expert knowledge, and testing its performance against experts.

9.2 Current development position

The system at this time is well advanced in stage (ii) of the system construction process outlined in Section 9.1. As identified, the elements chosen to make up the design system were:

(i) AutoSolid (solid modeller and CAD system). (ii) Nexpert Object hybrid expert system shell.

(iii) PVE5 BS 5500 & ASME VII1 Division 1 design code software. (iv) INGRES relational database.


The design system interfaces between the above elements have been developed and tested to the point that integration and communication, between the various software elements, poses no constraint on system development. Thus, stage (i) of the system construction process has been successfully concluded.

Stage (ii) of the system construction process is well advanced. The development of the 'narrowly based system', to provide an integrating framework, has reached an identifiable milestone position.

The user provides the system with a minimal set of constraints defining the vessel geometry (e.g. upper and lower limits on length, diameter and volume) and loading. The design system will then select optimum vessel sizes, on the basis of the input data, and the expert knowledge contained in the system. If unconstrained on head type, it will select the optimum head.

The system will then interact with PVE5, carrying out a dialogue emulating a design engineer. On completion of this stage, the scantlings have been finalised, including the design of a vessel skirt, and the system generates a solid model of the pressure vessel.

Thus, from a minimal set of constraints the system can automatically generate a vessel which may be made up from hemispherical ellipsoidal, torsipherical, flanged, conical and cylindrical components. If the constraints are sufficiently minimal, the system will carry out multiple designs, exploring the impact of different geometric configurations, e.g. selection of different head types.

The ability to perform multiple designs from a set of base constraints

Fig. 6.

I

I

~ m ~

Some examples of output from the system.


allows the system to carry out design optimisation, and this will be broadened in the next phase of development as the system gains a higher degree of expert knowledge, and cost information is added.

Further examples of some of the output from the system are shown in Fig. 6.

The graphics output in Figs 4 and 6 were dumped from the system in DXF and IGES format, the output was then read into D A X C A D and transferred to the INTERLEAF desktop publishing system with which this document was prepared.

The next stage of development is the extension of the system framework to support design for nozzle compensation, and local loading of nozzles. In carrying out the design work that the system can execute at this time, it fully and successfully exercises the elements of the system described; thus, completion of this next stage of the system is expected without unforeseen problems.

10 CONCLUSIONS AND FUTURE WORK

The goal of building a knowledge based design system for the design of pressure vessels has progressed well; the basic system framework that it requires has been developed.

This project has shown that it is possible to build sophisticated large scale design systems without writing enormous amounts of low level code, as would be required if writing each of the elements used in the system for first principles.

The most attractive feature about using software to solve problems is that software has great potential for reusability. Unfortunately, for many complex reasons, software has failed to deliver the levels of reusability that has been hoped for. This project addresses the reuse of current design software components on the macroscale with promising results so far.

An issue which was examined in the selection of the modeller was the output of data to CAD systems for the production of manufacturing drawings. The use of the term data exchange is a complete misnomer to describe the current state of the art in this activity. In the transformation from the numeric and symbolic information in any expert system to a solid modeller, and from there to a CAD system, almost all of the information originally in the model is lost.

CAD system data exchange is based almost entirely on the exchange of geometric data, and the bulk of that activity, industrially, to date has been the transference of 2D draughting information. A large amount of


effort, expended in building design systems of this type, is on the design of internal data models used by the system, and the transformation of that data so that it can be passed to another part of the system or to other separate systems.

A very large part of development overhead associated with the design and implementation of internal product model representation in design and manufacturing systems could be avoided if product models were based on standard forms of representation and data exchange.

The ISO standard standard for exchange of product data (STEP) is currently undergoing balloting, and it should provide a solid layer of enabling technology and standardisation on which to develop design systems. Some interesting and well timed work 15 has been done on the basis of draft information available on the proposed standard. Future development work on the system will utilise the STEP standards as they become available.

The STEP standard has 'object oriented' features which will allow for near 'seamless' integration between 'object oriented' systems, such as the expert system used within the reported design system, and other systems using the STEP standard. This will allow product design data to be represented in a standard fashion and to be transferred from system to system with the minimum level of transformation and subsequent loss of information.

A C K N O W L E D GEMENTS

This work is funded by SERC grant SERC-GR/E/85-614, and the authors would like to acknowledge the support given by Whessoe Technical and Computing Systems Ltd and by Babcock Energy plc.

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5. Bilir, O. G. & Ozgen, M. K., Computer aided design of pressure vessels. Int. J. Pres. Ves. & Piping, 40 (1989) 161-72.


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Knowledge based systems for pressure vessel design