Calculation Set Specifications

Defines minimum acceptable procedures and uniform practice for the preparation of manually recorded engineering calculations, to control the quality of the work, legibility and suitability for normal uses. Introduces specific organization techniques: indexing, design methods, physical properties, nomenclature and units, references, flow diagram, standard forms, and details of the calculation format. Numerous examples are included.

A. Index

The most important single element for organization of a calculation set is the index. Without an index it is impossible for any one else (and possibly even yourself) to find what specific information without paging through the entire calculation set.

1. Indexing Plan

An effective method of indexing calculations is technically known as the classified subject index.  This is an index of subjects which are arranged according to some logical classification as is illustrated in Figure 1.

This type of index is best known as a table of contents in books.

2. Preparing the Index

Start the index when the calculations are begun. Add to the index as additional calculations are prepared thus keeping the index current with the calculations. Then you can quickly find what you did yesterday, last week, or last month. If your work on the project is interrupted and have to temporarily lay it aside, then when you can resume work on it the index will prove to be a big time saver.

B. Standard Calculation Sheet Forms

Use standard calculation paper, unless pre-printed forms prepared for repetitive types of calculations peculiar to a particular calculation are used. In any event, essential calculations should not be made on "the back of an envelope", where they are lost for future reference.

Figure 2 illustrates the standard form, which has been designed for calculations. The form is ruled with light lines to assist in tabulating data and constructing drawings.

1. Title Blocks

The title blocks at the top of the form provide a place to fill in designer identity, the date, and the subject of the calculations on each page. Fill in the blocks completely, including subject, starting date, name of designer, date of completion, and page number.

2. Use Front Side Only

Begin each calculation set on a new page and use only the front side of the calculation paper. There is a loss of continuity when a part of the calculation set is on one side of a sheet and another part is on the opposite side.

3. Oversized Sheets

For calculations which will not fit the standard 8-1/2" x 11" calculation forms, use reproducible paper in a manner and of a size which can be neatly folded to fit standard binders without impairing the legibility of the contents (such as paper from quadrille-ruled 11" x 17" pads, folded to 8-1/2" x 11" size).

4. Page Numbering

Number the pages consecutively as calculations are completed. Thus, the pages are chronological with respect to their execution. As a result, the calculations "tell the story", in sequence, of how they were performed.

a. Added Sheets

Sheets which are inserted at a later time shall be identified by the first preceding numbered page and a letter. For example, if it necessary to insert a few extra pages following "page 5", number them 5a, 5b, 5b....

b. Do Not Renumber Pages

By not renumbering calculation pages, the index never needs correction of page numbers. Also, page numbers can freely be used as references in other parts of the calculations, e.g. "see page 57".

C. Illustrations

1. Flow Diagrams

The most valuable devices for coordinating process design calculations are the process and the material and energy balance flow diagrams. These instruments conveniently and explicitly tie together all the bits and pieces of process information. They show the interrelation of all of the working parts: equipment items, material balance, operating conditions, heat balance, utilities requirements, etc.

An effective method of constructing a flow diagram is to develop it as the process engineering calculations proceed. Start with a simple flow sketch. Add to or modify the sketch as your calculations proceed. When necessary, start with a fresh sheet and redo the flow diagram. When your calculations are completed, your process and material balance flow diagram is final and ready for the drafting room.

The above describes the design control device (flow diagram) most used by process engineers. Similar and analogous design control devices are used in other engineering disciplines.

2. Sketches

Sketches are highly desirable as parts of the calculations. For example, in Figure 3 is illustrated a sketch of a refrigeration cycle. The calculations that follow would be very difficult to follow without the sketch. The sketch ties all of the pieces together.

D. Calculation Body Details

1. Headings

Headings should be used to organize the calculations. For example, in Figure 3, lines 3, 5, 27 and 30 are shown with three generic levels of headings, together with the appropriate subject codes.

2. Subject Code

Note the convenient subject code pattern

A-1-a

used in the example figures. For a more complex calculation set requiring more hierarchal levels, one might use the following subject code pattern:

I-A-1-a-(1)-(a)

It really does not matter what code is used so long as it is consistent. These codes and headings are keyed to the calculation set index.

3. Indentation

An example of indentation is shown in Figure 4, lines 5-23.  Here are shown five subjects, the details of each indicated by indentation.

4. Identification of Results

Underline and clearly label all important intermediate results. Final answers should be boxed and clearly identified as for example:

E. Calculations

1. Equations

A good method of documenting the solution of equations is to list the equation first, and then substitute numerical values in it below. Examples of this are shown in Figure 5, lines 33-57.

2. Arithmetic

Although arithmetic is elementary, it can become involved, and fraught with opportunities for errors. An example of one way to handle this problem is shown in Figure 5, lines 7-26. Note the use of braces to indicate the various steps in the arithmetic operations. With this form of record, anyone can easily double-check the arithmetic. At the same time, the notation is compact and easy to record.

3. Trial-and-Error

These kinds of calculations present special problems in recording calculations. It is very tedious to repeat the calculations for each trial with complete detail. One way of handling this is illustrated in Figure 6.  First, the basic model equation is presented. This is followed by a tabulation of trials. The independent trial variable is identified on line 20, and the dependent variable to be satisfied, on line 40. The lines in between record the calculations. The final answer is recorded on line 44.

4. Tabulation

Tabulations make calculations compact and easy to follow. For example, Figure 4, lines 44-52 illustrate a tabulation of calculations. The headings of all columns and rows should be labeled and include units.

5. Graphics - Identification/Labeling

The use of graphics to present results of calculations is highly recommended. All graphics must be clearly and properly identified. The variables and units of all coordinates must be indicated on the appropriate axis. An identification box should include all conditions and parameters of the plot.

6. Notes

At the time of performing calculations, it may seem that everything is obvious. However, with the passage of time and changing of personnel, the original logic may become elusive. One of the most valuable aids to original calculations is the introduction of extensive notes. These notes may be in any of the following categories:

• Basis
• Assumptions
• Literature Citations
• Explanations
• Objectives
• Conclusions
• Recommendations
• Interpretations
• Comparisons to Known Criteria
• Voided Calculations

Examples of these types of notes are indicated in Figure 4, lines 6, 23, 54, 63; in Figure 5, line 45; in Figure 3, line 57.

7. Legibility

The use a No.2 or HB black pencil and keep it sharp. Pens and colored pencils are not to be used as they do not reproduce well on many copy machines. The calculations may be recorded either in printing or writing. If you print easily, this is preferred. However, if you are not used to printing, handwriting is acceptable.

Do not recopy your calculations at the end. Recopying may result in typographical errors and is considered a waste of time. With a little experience, you can learn to do your calculations neatly and legibly the first time.

8. Computer Output

When computers are used to complete portions of the calculations, insert the actual computer printout as part of the original calculations. Some computer programs confirm all input data in the output. In the event this feature is not part of the program, it is essential to list by machine all input data. Without this, it is impossible to interpret the significance of the output.

F. Analysis Methods

The analysis methods used are basic to the calculation. If the analysis methods are unreliable, so will be the calculations resulting from them. Frequently there are alternate analysis methods available, and it is critical to know which was employed.

1. Standard Procedures

If the design method is taken from a book, literature article, or report, then a citation will suffice. However, if you made any departures from the published design method, you should explicitly record these in your calculations.

2. Computerized Procedures

If the calculation method is implemented as a standard computer program, then the computer program should be explicitly identified.

If you develop a special computer program to complete your current project, you should include in the recorded calculations all information necessary for others to understand and use your program. This includes as a minimum the following:

• Listing of source code
• Definitions and units of all program variables
• Subprogram linkage diagram
• Basis of the model
• Example Input
• Example Output

G. Property Data/Estimation

The physical properties employed in the calculations are extremely important. Even though the calculation method is perfect, the use of unreliable physical property values/estimation procedures will result in an unreliable plant design.

1. Sources

For this reason, the physical properties employed should be clearly defined. This should include the sources of the physical property data or prediction methods, ranges of applicability, and any assumptions made where data were unavailable.

2. Tables/Correlations

When physical property tables and/or charts are extracted from the literature, it is convenient to include a photo-reproduction of the actual charts as part of the calculations.

Where certain physical properties are used repeatedly during the process engineering calculations, it is frequently advantageous to prepare charts or graphs of these properties, as for example vapor pressure vs. temperature, etc.

In any event, the physical properties should be documented in such a way that anyone reviewing the calculations can determine precisely where all physical/chemical/thermodynamic property values came from.

H. Citations

Literature citations can be used to precisely define a calculation method or property source. By citation of these references, the method need not be copied into the calculations. One practical method is to start a section on references. Then these references can be cited by author and date anywhere during the course of the calculations.

Besides the public literature, internal letters, memos, and reports can become reference citations.

I. Nomenclature

Symbols and unusual abbreviations should be clearly defined, preferably in a single list. Symbols which are a part of the nomenclature of recognized standard design procedures need not be defined.

1. Symbols

The use of mathematical symbols for variables can be of considerable assistance toward making calculations explicit, compact, and easy to follow. However, much of these advantages will be lost for future reference if the nomenclature is not explicitly defined. You should make certain that every symbol of nomenclature you employ is properly defined somewhere. All symbols should be carefully listed and defined in the calculations.

2. Units

Equally important as the mathematical symbols are the units employed. These may seem obvious to you at the time the calculations are made, but at some later date you or someone else will have no idea the units employed unless they are specified in the record.

Figure 6 illustrates the convenient technique of including the definitions at the point of use in the calculation set. Whenever a significant conglomeration of variables or terms are accumulated, they are assigned special symbols, as for example 1, 2, 3, etc. These symbols can be freely reused on succeeding lines. Note that all such special symbols are automatically defined without any additional notes.

J. Specification and Rating Forms

Certain types of process information are required so often that standard forms have been developed for this purpose. Employ these forms wherever applicable. These forms provide a valuable checklist to make sure that nothing important has been overlooked. In addition, those familiar with the forms can read them much more quickly and efficiently than unformatted information.

K. Error Control

No engineer is perfect. We all make errors in calculations.

1. Checking for Errors

The trick is to find the errors before the plant is built that won't operate. Methods for checking your calculations are discussed in a separate guide.

2. Correcting Errors

A disturbing problem arises when an error occurs early in the calculations, particularly when the erroneous value is used again and again in subsequent calculations. One practical method of handling this problem is to strike through the first erroneous value, and then all other key values computed on succeeding pages. In this way the corrected values are clearly apparent, and at the same time the original values are still readable, thereby permitting verification that the error was properly corrected.

L. Filing the Calculation Sets

Regardless of how many calculation sets are produced on the project, each and every one should be easily retrievable from the project files.

1. Working Calculations

The Project Notebook/File provides a place for each engineer to file his/her working calculations. File related calculations together preferably by equipment item number. Further discussion of the preparation of Project Notebooks is available.

2. Retention

Calculations are not disposable. After completion of the project, the Project Notebooks shall be sent to Records Storage for archiving with other documents on the specific project. The Project Notebooks also bring together in one place those calculations that are designated permanent files

3. Retrieval

The procedure given for Project Files provides a place for each engineer to file his calculations and then provides for bringing together in one place those calculations that are designated permanent files

Consider how this works when you have need for certain calculations on a project. Given a project name or number, you find the total project files. Referring to the outline, you determine the particular file covering the subject of your current needs. If all has been done properly, this file should contain the calculation set you are seeking, and the calculation set should provide all the information you need.

Adapted from:  Bolles, W.L., "Organization of Engineering Calculations," CED No. 106 Monsanto, St. Louis, MO (February, 1971)