The engineering design functions which the computer can most effectively aid

The engineering design functions which the computer can most effectively aid

E. H. Masters The engineering design functions which the computer can most effectively aid This paper shows how design work can be divided into that ...

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E. H. Masters

The engineering design functions which the computer can most effectively aid This paper shows how design work can be divided into that which requires human intelligence and that which is best performed by a computer. The man/machine relationship, the intuition of man and the accurate logical drudgery performed by the machine. A computer application for the production of piping isometric drawings with material schedules is described and the planning and training that was necessary for its introduction into the design department is discussed.

When considering how a computer can aid design there seems to be a danger of thinking that the whole function of design can be done on a computer and implying that it will be a better design at less cost. Computer-aided design is unfortunately usually interpreted as computer graphics with cathode ray displays, light pens and plotters producing design drawings, without emphasising the importance of a man-machine relationship. The computer can only produce that which it has been fully instructed to do. This obvious fact is too often overlooked when considering design problems. Design is not a mathematically exact process; there are many variables and influencing factors to consider and usually a selection has to be made from many alternatives. This is why de.sign is sometimes described as part science, part art. It is only possible to instruct the computer to arrive at one answer; it cannot bring human judgement to bear on several alternative solutions. I will illustrate these points by considering the design of piping in a chemical or oil process plant. The equipment items have to be arranged on the available site and connected together by pipes according to the process flow diagrams. It would seem worthwhile, if the computer could evaluate the layout, to determine the optimum equipment arrangement; the optimum usually meaning the cheapest in either capital or operating cost. N o w imagine writing the instructions for the computer to perform this design function. The site has to be defined covering all the information relative to the positioning of items of equipment, such as ground slope, loading, access, hazardous areas and considerations to be taken into account from adjacent plants. Then each possible pipe route has to be considered by the computer to determine an acceptable route. How is acceptable to be measured ? The route may be acceptable AUTUMN

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1968

if it meets requirements such as: suitability for the specified duty, minimum capital and/or operating cost, not an obstacle to access or equipment maintenance. These requirements could be weighted to determine what is acceptable in the particular circumstances. Having determined one pipe route, how are the other pipe routes assessed in relationship to each other so that they can be grouped on pipe bridges or in trenches? I have given enough evidence to show that the number of logical decisions to perform this layout and piping task is immense and, on top of this, the answer must be aesthetically acceptable. A trained man with the necessary experience and knowledge easily performs the task of laying out a process plant and determining the piping, taking into account all the required logical decisions and applying his artistic skill intuitive. Man is far better at this work than a computer because he cannot logicalry define how he performs this work. Remember that, if the computer instructions can be specified, they must be in a general and not a particular sense so that they can be applied to all problems. This introduces the difficulty of specifying the particular problem to the computer. The volume of input data could also prove immense and may be a task as great as producing the design. I have tried to show that some design functions are not suitable for computer solution and I wish now to illustrate that there are other design functions which are suitable for compt~ter solution. Design work includes calculations, simple ari~thmetic, looking up information and listing. Calculations are defined mathematical procedures and, when justified, can be calculated on the computer. Simple This paper was first read to the Institute o f Chemical Engineers and is reproduced with their permission.

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arithmetic is used to determine overall dimensions by adding item dimensions together. Man finds simple arithmetic and the listing of material quantities boring and he is prone to errors. The computer, however, revels in this type of work as the instructions are extremely logical and easily defined and the boring and repetitive nature of the work does not cause the computer to make mistakes. Returning to the design of process plant piping, the listing of piping materials required and the detailing of each pipe run for fabrication and erection are excellent applications for the computer as they embrace simple arithmetic, looking up information and listing. Design work therefore, can be divided into that best performed by man and that by computer. A man-machine relationship in computer graphics illustrates this principle. A computer which is not capable of man's logical and intuitive thought hands over this part of the design process to man. The computer provides the look-up information, performs calculations, does the simple arithmetic and lists quantities. Computer Piping Isometric Drawing Program In Humphreys & Glasgow we have used our computer to assist design where it can be most effectively used. Our Piping Isometric Drawing Program and its associated Material Listing Programs are good examples of this principle. The stages in piping design covered by the computer programs are: Preliminary Material 'Take-Off'. Piping Isometric Drawings. Final Material 'Take-Off'. The first requirement is to place orders for the piping material before the piping design is complete, as most of the material is not purchased ex-stock and the delivery time would cause unacceptable delays if the material was ordered on completion of the design. This advance ordering is referred to as a 'Preliminary Material "Take-Off" '. As soon as the pipes have been routed, either on a Model or a General Arrangement Drawing, the detailed Piping Isometric Drawings for fabrication and erection of the material already ordered can be commenced. On completion of the isometric drawings the actual material requirements are known and a comparison is made with the Preliminary 'Take-Off' to determine surpluses and deficits for reallocation or further ordering of material. This is referred to as a 'Final Material "Take-Off" '.

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The piping materials suitable for handling a particular fluid are referred to as a pipe specification for that fluid.

03/05/68

PAGES*

It covers all materials required, e.g. valves, fittings, gaskets, bolts, etc. within defined pressure and temperature ranges. Therefore, before the material quantities can b e assessed, each pipe line on a plant has to be referenced to a pipe specification. A specification is coded in a flexible alphanumeric f o r m o n a computer input sheet, the material dimensions being entered on a separate input sheet. Specifications, including dimensions, are stored o n magnetic tape for access as and when required. E d i t programs allow for amendments or additions to a specification. A specification can be compiled for any combination of materials within the ASA and D I N codes of practice. Working from Engineering Line Diagrams and P l o t Plans, piping material requirements are dictated to a tape recorder for direct transcribing by a punch operator o n to paper tape for computer input. The material types are described by using a simple mnemonic code, e.g. J U N C = tee junction. The computer program produces bulk material requirements by assessing the appropriate piping specifications and translating the material types into unique material purchasing codes. See Fig. 1. ISOMETRIC

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The Isometric Drawing Program input data is taken from either the General Arrangement Drawing or a Model and is the minimum required to specify a pipe line. See Figs. 2 and 3. Following the pipe specification identifier, co-ordinates are inserted for the start and finish points of the pipe line, at each point of change in direction, and for the centre point of every valve and fitting excluding elbows. Only the changing co-ordinates need be given. The pipe line size is entered and the computer calculates all pipe lengths and specifies all items in accordance with the pipe specification. One sub-routine selects the valve and, if it is flanged, also calls up the mating flanges, gaskets and bolts. This is but one of many similar sub-routines, e.g. vents, drains and orifice plates. The dimensions of non-specification items, e.g. special materials (SMs) must, of course, also be included on the input data sheet. Facilities are also built in to enable such things as handwheel orientation to be shown and for fabrication and erection notes to be printed on the final drawing. The output is in three parts: (i) Paper Tape Punch The output for the actual Isometric Drawing is adjusted to fit into a standard drawing sheet, in this case 15 × 15in. The adjustment is not to scale, otherwise complex areas would become so cramped as to be unreadable. A simple symbol representation has been developed which results in an exceptionally clear and easily understood drawing. All line breaks to indicate 'passing behind' are properly shown and loops are closed, just as in a manual drawing. The punched paper tape is fed to an off-line Analogue

AUTUMN 1968 D

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Plotter on which the actual drawing is made. The drawing is neatly finished off with a boundary line, the North arrow, and the drawing number printed in the bottom right-hand corner. Drawing trimming marks are also added. Three different line thicknesses are used for clarity. Paper advance is automatic so that drawings are processed continuously. (ii) Line Printer The line printer output for each drawing is on tracing paper and gives the following information: (a) All co-ordinates. (b) All dimensions between significant points numbered by the program and included on the drawing. (c) All material required, sorted and summed, split into 'shop' and 'field'. (d) All reference data input, e.g. contract number, line number, specification number, etc. (e) Notes. ( f ) Drawing number. The paper is trimmed to remove the sprocket edges and joined to the plotter drawing by means of a special transpa[ent adhesive strip. Transparent 'Company Stamp' transfers are added if desired. See Fig. 4. The drawing is now complete and can be reproduced by ordinary dye-line printing. (iii) Magnetic Tape In addition to preparing the material list for the drawing, the material is transferred at the same time to a magnetic tape which is used to produce the Final Material 'Take-Off'. The computer processing is shown on the Data Flow Diagram. See Fig. 5. 37

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Final Material 'Take-Off' The material from the Isometric Program is stored on magnetic tape in random line and contract order. The material for a contract is sorted and compared with the Preliminary 'Take-Off'. The computer output is as follows: (a) When the Preliminary 'Take-Off' contains more of an item than is shown in the Final 'Take-Off' the quantity of 'surplus' material is output. This permits the surplus ordered to be either cancelled or transferred to another contract; alternatively, if the contract being considered is not complete it indicates the rate of usage. (b) When the Preliminary 'Take-Off' contains less of an item, a requisition is printed out for the deficit material. (c) Material is sorted by type and referenced to line numbers. See Fig. 6. In addition the program adds to the quantities required any predetermined allowances to cover such things as cutting and construction wastage, loss or damage. After each run the program produces a 'History' which is used as the comparison the next time the program is run. As already mentioned the system also caters for other than the standard material. For each contract a Sundry Material List may be prepared. When this is input the SMs are treated as standard, for that contract only, in the material correlation. In addition facilities are available for adding to or deleting from the contract material lists. Where it is necessary either to make a small late modification or adjust for material not processed through the whole system, this is done by inputting adjustment tapes which add to or subtract from the History File as specified.

Introductory Training One of the requirements of the Piping Isometric Drawing Program was to produce General Arrangement Drawings to a larger scale so that three dimensional co-ordinates could be added to the drawing to position equipment.

Whilst the program was being developed three dimensional co-ordinates were introduced into the drawing office and manual isometric piping drawings changed over to this form of dimensioning. The Preliminary Material 'TakeOff' Programs were the first programs to be used and these brought familiarisation with the mnemonic codes for the identification of material types. Thus when the isometric piping drawing program was ready for use in July 1967 the Piping Engineers and Draughtsmen were prepared to accept the requirements for completing computer input data sheets.

Cost Analysis On our first contract we found ~.hat-oUr"time on supervison and clerical work increased and it took longer to complete an input data sheet than was estimated, but with familiarisation these figures have considerably improved. We estimate the average time for completing an input data sheet, taking into account all checking and rewrites due to errors, to be 3.5 hours per data sheet and this figure results in a computer produced drawing being half the cost of a manual drawing. It is difficult to quote actual cost figures as they can be very misleading if they are not absolutely defined. Each Company has a different figure for their costs because their cost boundaries are different. In addition to reducing drawing office time and increasing throughput there are considerable indirect savings arising from the cost of errors which have been avoided by using the computer. Material is more accurately ordered and controlled and fabrication and erection delays avoided. On a manual drawing there is usually one material or dimensional error even after checking. The reduction of errors has been most noticeable on the contracts where the isometric piping drawings have been produced by the computer. These indirect savings are of far greater magnitude than the direct drawing office labour savings, thus derhonstrating how the computer can aid design by eliminating errors in activities in which man is particularly bad. Received August 1968

E. H. Masters M.A., C•Eng., M.l.Mech.E., was educated at Kings School, Rochester and Plymouth College, followed by St. John's College, Cambridge, where he obtained a second class Honours in Mechanical Sciences Tripos. Subsequent to this he served in the Royal Engineers from 1940-46 and prior to leaving he was a Captain working on Engineer Intelligence• He spent eight years in ICI on Chemical Plant Maintenance and Design and has been with Humphreys 8- Glasgow Ltd., Chemical Plant Contractors, for 11 years• At present he is an Associate Director and Manager of the Systems Department.

38

COMPUTER AIDED DESIGN