Design

• Design is the search for the optimum solution, not the verification of any old solution.
• Design is undertaken by stating the problem completely, creating a bank of solution candidates, setting up clear criteria and constraints, practicalising the candidates and finally evaluating them.

The design of a complete pressure vessel is complex, especially if the vessel is a heat exchanger where wide scope exists for the choice of pass and tube layouts. For a straightforward container however the scope is more restricted and the design procedure simpler. If it is not clear where to start a design (of pressure vessel or of anything else) then a start can be made anywhere, realising that the first pass through the design process will be something of a trial run. The skeleton procedure presented here for a container-type pressure vessel is recommended as a starting point.

There are two extreme approaches to the use of such a design procedure. The first approach is mechanistic, suitable for computer implementation, and involves the mindless selection of a number of alternatives for each parameter. Thus, following the procedure, one might select both extremes of design pressures, all four classes, all available heads etc. Each of the resulting large number of candidate solutions would incorporate a unique combination of all parameters. Only a few of these candidates would on later analysis be found practicable. Of these, the optimum solution would be the cheapest - within the usual constraints of safety over the vessel's life, consumer acceptance &c.

The apparent drawback to this approach - apart from stifling any individuality on the part of the designer, and the large bank of candidates - is the difficulty in obtaining realistic cost figures for all the alternatives; normal commercial confidentiality precludes it. Head costs are known to be approximately proportional to thickness and to ( diameter )^7/4, but this is of little use as heads are only a part of the total vessel. Graphs in Sinnott op cit suggest that in very general terms, cost is proportional to   diameter^1.05. length^0.85 - but one should not read too much into this as it does not refer to one fixed pressure.
 

		head	cylinder
candidate	bore	thickness	length	thickness
1	2134	25	3200	28
2	1372	16	9200	20

Such approximate generalisations notwithstanding, it is possible to formulate simple realistic quantifiable relative cost utilities - on the basis of known trends in relative welding costs for example. Thus it is possible to compare relatively but realistically the costs of the two tabled candidates, incorporating Rheem semi- ellipsoidal heads, for a 14.5 m³ capacity class 2B vessel whose plate widths are limited by the bending machine's 2 m capacity.
What criteria would you adopt for such a comparison?

A second design approach comprises a single pass through the procedure, at each step making a single choice of the corresponding parameter, and leading to a single solution. As the design unfolds however, the designer continually questions the effects of earlier decisions, and if a change appears likely to be beneficial then the designer must double back to the earlier stage, confirm whether or not the change is indeed beneficial, and if it is then effect the change. In the hands of an experienced designer such an iterative approach can lead to a close-to-optimum solution, however the novice designer often cannot weigh up the probable outcome of an altered earlier decision, so the novice does not iterate adequately and ends up with a far- from- optimum solution. With experience the need for iterative review lessens and the order of the procedural steps may profitably be altered.

An inadequate search is a common failing with novices' designs - not just of pressure vessels. For example the required thickness of a shell may work out to be 12.4 mm, so novices select the next highest available plate - 16 mm - rather than seeing whether an earlier decision can be altered so that the required thickness is 12.0 mm, allowing selection of standard 12 mm plate. The search and alteration to an earlier decision saves not 0.4 mm here, but ten times this.

The approach recommended is not unlike the first approach, however the choice of parameters at each step is both intelligent and limited. Thus at the procedural step for selecting the parameter class, the designer might know from the vessels' general specification that class 1 is too sophisticated and expensive, and that class 3 is impractical owing to the vessel's likely diameter and hence thickness over 12 mm. So the limited range of 2A and 2B might be selected as candidate attributes, instead of all four classes as in the first approach. Clearly this leads to a more manageable bank of candidates than that resulting from the first approach. Using the cheapest suitable material, simple weld preparation and the like will all promote the optimum solution. Selection of the optimum solution will consequently also define the design pressure, the class etc. because these parameters are intrinsic attributes of any candidate.

It should be realised that one's decisions are always subject to criticism by superiors or by those who hold the purse- strings, so any decisions must be capable of rational justification. Designers worth their salt should always question the input specification for the design process, since actual constraints may in fact not be so rigid as they first appear.

In the absence of a specialised computer program, spreadsheets can assist greatly in the design task because they foster consideration of a bank of solution candidates more comprehensive than could ever be attempted by hand. It is vital to accompany each spreadsheet with one complete set of clear manual calculations, both as a check for the designer on the spreadsheet's correctness and as a guide for the reader on the logic and equations used. A spreadsheet appearing in a report should be edited to remove the bulk of confusing irrelevant data especially unnecessary / downright misleading insignificant figures, and formatted so that like tabulated values appear with the same number of decimal places. It must be stressed that spreadsheets do not absolve the designer from the need to think.

Design is an exercise in trying to keep everybody happy - the welder, the user, the production manager, the salesman, the cleaner, the repairer and so on - in fact everybody who is or who might be affected by the design. This includes the reader of the design report. Designers must put themselves in these peoples' shoes and pay due attention to their wishlists.

With regard to the presentation of a design report, the prospective reader (who is probably busier and more senior than the designer) should be offered an easily assimilable document - a report must be short & sweet. The following layout is suggested for a pressure vessel report :

• the title page, including at least the addressee, the author(s) and the date;
• the Executive Summary appears on the next page: this outlines in a paragraph what the report is all about, and tabulates the major features of the chosen solution;
• the list of contents with correct pagination appears on the next page;
• then follow word processed pages which summarise the design process and attendant logic;
• the drawings if bound with the report come next, noting that :
  • all drawings must be to accepted standards, and sketches must be approximately to scale
  • the general arrangement (GA) is of particular interest to the addressee and must show all major dimensions so that the vessels' integration with other plant can be quicklyappreciated;
• the report is concluded by appendices, each containing longhand calculations on a major aspect of the design, with all significant decisions in the RH margin (see the attached sample calculation sheet) to assist both the designer(s) and the reader.

Results of similar calculations for a number of candidates should be tabulated, with a single specimen calculation only being provided. Code equations must be used wherever relevant. Sophisticated maths should not be over- used (unless absolutely necessary); it is better to demonstrate a practical grasp of the problem using simple approximations rather than to indulge in mathematical complexities.

All members of a vessel design group should contribute to the major task, that of selecting the vessel's overall parameters such as bore, class etc. Individual members of the group may consider separate features such as inspection openings, compensation and the like - remembering always the need to question earlier decisions.