How to write a process design basis: a practical checklist

The process design basis is the document that says what the plant must do and under which conditions. It is written before flow diagrams and calculations, and every later step, from the material balance to equipment sizing, depends on it. A clear design basis prevents the most expensive kind of rework: designing the right equipment for the wrong requirements.

What a design basis is for

• It turns the client’s or business’s objectives into engineering requirements.
• It records assumptions so they can be checked, challenged and approved.
• It gives every discipline the same starting point.

The checklist

1. Scope and objectives

What the unit is, what it is not, and why it is being built. State what is inside the project and what is supplied by others, such as an existing utility system or storage.

2. Capacity and operating time

• Design capacity, in product rate or feed rate, and which one governs.
• Turndown: the minimum rate the plant must run at.
• Annual operating hours or on-stream factor, and whether the plant runs continuously or in batches.
• Design margins on top of normal capacity, and where they apply.

3. Feedstocks

Composition, impurities and their limits, physical state, and the range of feed qualities the plant must handle. If there are several feed cases, list each one; the worst case often sizes different equipment.

4. Products and by-products

Product specifications such as purity, maximum impurities, water content and colour, along with the test method behind each figure. Include by-products and how they leave the plant.

5. Battery limit conditions

For every stream that crosses the boundary of the unit, including feeds, products, effluents and utilities, state the temperature, pressure and physical state at the battery limit. These numbers decide pump heads, heat duties and line sizes later.

6. Utilities

• Cooling water: supply and maximum return temperature, available pressure.
• Steam: levels available, with pressure and temperature for each.
• Electrical supply, instrument air, nitrogen, fuel and refrigeration where relevant.

7. Site and climate data

Minimum and maximum ambient temperatures, design wet-bulb temperature for cooling, altitude, and any site factors such as wind or seismic requirements that affect process design.

8. Environmental and safety requirements

Emission and effluent limits, noise limits, and the safety philosophy: for example how relief and flare systems are handled, and requirements for hazardous area classification.

9. Codes, standards and conventions

Applicable codes and client standards, units of measure, equipment numbering and drawing conventions. Agree these early; changing them later touches every document.

10. Equipment philosophy

Sparing (for example, installed spare pumps), preferred equipment types, and any equipment that must be reused or bought from a particular source.

Keep it alive

1. Mark assumptions as holds. Anything not yet confirmed should be flagged and given an owner and a date.
2. Get it approved. A design basis without sign-off is a draft, and drafts get reinterpreted.
3. Revision-control it. When a basis value changes, every calculation that used it needs to know.

Where Black Owl fits

In Black Owl the design basis is the first step of the project, alongside components and utilities with their supply and return conditions. Later steps such as the block flow diagram, material balance and process flow diagram build on the same record, and calculation freshness tracking shows when results need to be rerun. Request early access for a walkthrough.