How To Build A Good Design Calculation Document

Working for now more than 15 years in an engineering consultancy has given me some insight in how to build a design calculation document which is concise and complete and can be also submitted for certification to certification authorities.

Many good engineering consulting companies have standard templates for preparing a design calculation report which help in saving time and are helpful to fresh engineers from college.

Certain engineering calculation reports are prepared on the basis of the format prepared and provided by certification authorities and these are considered as legal documents which can be presented as evidence in a court of law in case of arbitration.

However, if you do not have any standard format for an engineering calculation report you don't need to worry. Just follow the following general guidelines and I can assure the readers, that you won't go wrong:

1. The first item in the calculation would be the objective of the calculation. In brief mention what are you trying to achieve, for example if you are trying to size an equipment (separator / vessel, pump, compressor etc.) mention the calculation objectives such as dimensions of a separator / vessel, total differential head and NPSHa of a pump, head and power of a compressor etc. Provide the exact description as per the PFD / P&ID and the tag / item number of the particular equipment for the sake of clarity.

2. Inputs: Provide the references with document numbers for the input data for the calculations. Do not forget to provide the revision number / status of the reference documents. These could be the PFD(s), P&ID(s), simulation software output such as physical properties, Design Basis Report, Project report etc. Multiple inputs should be entered as individual bullet points with document description, document number, revision status and date.

Inputs could also include inputs from standard books in chemical engineering, internal and external company standards and e-mail communication by client / vendor / 3^rd party consultant. All of these need to be mentioned.

3. Assumptions: Assumptions need to be mentioned separately. Since assumptions lead to a certain degree of uncertainty in the calculations, a reasonable amount of caution needs to be exercised when putting up assumptions. Assumptions should be based on sound logic and past experience for similar calculations leading to a successful design. The focus should be to minimize the assumptions.

4. Calculations: It is always good practice to write the equations used for the calculations in this part of the design calculation report instead of just providing a reference to them. I follow this and find that reviewers feel more comfortable about seeing the design equations in the actual document then trying to look for them at some other place. The calculations should be done step-by-step following a logical progression. The reviewer of your document will feel lost if the calculations do not follow a logical progression. Try to avoid long descriptive paragraphs in this section of the design document since it breaks the continuity of the calculation and makes for tedious reading and review.

5. Conclusion / Summary: This section of the design calculation report summarizes the result of the calculations in the earlier section and provides figures and numbers for the intended calculation such as "Total Differential Head" and "NPSHa" for a pump.

6. Annexure / Attachments: This section will include the actual copies of the references provided in the input section. These may include part or full copies of PFDs / P&IDs, simulation software print-outs, Plot Plans, Piping GA drawings / Isometrics, E-mail correspondence copies with client / vendor / 3^rd party consultant, relevant pages of engineering textbooks, relevant pages of company engineering standards etc.

The above mentioned methodology has been successfully followed by many top notch engineering consulting companies in generating engineering calculation reports and many such reports have been used as legal documents while arbitration in a court of law.

I would welcome comments from fellow forum members and peers to provide comments on my blog entry which I believe would be quite helpful to young engineers starting their careers in engineering design.

Regards,
Ankur