Ask three engineering companies for a design basis and you will get three different documents: one a two-page summary, one a 90-page compilation of standards, one a spreadsheet. None of them are wrong exactly, but only one shape of document is genuinely useful to a company deciding whether to spend capital.
Here is what we think belongs in it, and why.
1. Purpose and battery limits
One paragraph on what the plant is for, then a hard statement of where it starts and stops. Battery limit definition is where most scope disputes originate. Name the streams crossing the boundary, their conditions, and who is responsible on each side.
2. Capacity and turndown
Nameplate capacity, the basis for it (operating hours per year, availability assumption), and the required turndown. An availability assumption of 8,000 hours a year versus 7,000 changes equipment sizing by more than ten per cent and is frequently never stated.
3. Feed and product specifications
Not just the main components. Trace species decide materials of construction, catalyst life and effluent treatment, and they are almost always the ones missing from an early specification. If a trace level is unknown, say so and mark it in the assumptions register.
4. The assumptions register
The most important section, and the one most often absent. Every number in the design comes from one of three places: it is measured, it is taken from literature, or it is assumed. Tabulate them, mark which is which, and note what would happen if an assumed value is wrong by a factor of two.
This single table is what makes a design basis auditable. It is also what a technical diligence consultant will look for first.
5. Site and utility conditions
Ambient design temperatures — both extremes, not an annual average — cooling water supply and return temperature, steam levels available, electrical supply, and altitude. A condenser designed for a 32 °C ambient will struggle in a location that reaches 42 °C, and that is a design basis failure rather than a design failure.
6. Materials of construction philosophy
Not a full material selection, but the reasoning: which streams are corrosive, what the philosophy is for wetted parts, and where a corrosion allowance is being used instead of an upgrade.
7. Codes, standards and statutory framework
Which pressure vessel code, which electrical area classification standard, which effluent norms apply. In India this means naming the applicable state pollution control board consent conditions early, because they can change the design.
8. Safety and environmental basis
Relief philosophy, the hazardous materials inventory, area classification approach, and the intended effluent and emissions routes. Effluent handling is regularly left until after the process is fixed, and it is regularly the thing that makes the economics fail.
9. Scale-up risk register
For a first-of-a-kind process, this belongs in the design basis rather than in a separate report nobody reads. Each scale-dependent unknown, the test that resolves it, and what it costs to run that test.
10. What is explicitly excluded
A short list of what this document does not cover. It prevents a surprising number of later arguments.
How long should it be?
For a pilot plant, ten to twenty pages. For a commercial FOAK plant, thirty to fifty. If it is longer than that, standards have been pasted in that nobody will read. If it is shorter, the assumptions register is probably missing.
Working on something like this? Sherle designs pilot plants and basic engineering packages for companies scaling a chemical process. Book a technical call.