The design basis is a short document, usually 10 to 20 pages. It records the decisions that every other engineering document depends on. If one of them changes later, the change runs through the whole design.
Post 1 set out the five project stages. The design basis is drafted in pre-FEED and frozen at the start of FEED. This post lists the ten decisions it should contain, using the same $100M specialty chemical plant.
The ten decisions
Table 1. The ten decisions in a design basis
| Decision | What to fix | What it drives |
|---|---|---|
| 1. Capacity | Annual production, in tonnes of saleable product. Minimum rate the plant must run at (turndown). | Size of every item of equipment |
| 2. Operating schedule | Hours per year, shifts, planned shutdowns. | Hourly design rate, staffing |
| 3. Feedstocks | Specification of each raw material, its range of quality and how it is delivered. | Reactor design, purification steps, storage |
| 4. Products | Specification and grades of each product, by-products and how they leave the site. | Separation and finishing equipment, packaging |
| 5. Site conditions | Location, design temperatures, elevation, seismic and wind data, soil conditions. | Cooling design, structures, winterization |
| 6. Utilities | What the site supplies and at what conditions: power, steam, cooling water, nitrogen, compressed air. | New utility systems or tie-ins |
| 7. Battery limits | What is inside the project and what is outside. Every tie-in point. | Scope and cost estimate |
| 8. Codes and permits | Design codes, provincial and municipal rules, emission and effluent limits. | Materials, safety systems, treatment units |
| 9. Operating philosophy | Batch or continuous, level of automation, spare equipment, days of storage. | Control system, equipment count, tank sizes |
| 10. Future expansion and margins | Allowance for later capacity, design margins on equipment, plant life. | Plot space, line and utility sizing |
Most of these are business decisions, and only the owner can make them. The engineer's role is to ask for each one, explain what depends on it and record the answer.
Worked example: capacity and operating hours
Suppose the owner wants 50,000 tonnes of product a year. That figure does not set the size of the plant until the operating hours are fixed. Equipment is sized for an hourly rate:
Design rate = 50,000,000 kg ÷ 8,000 h = 6,250 kg/h
A continuous plant typically runs about 8,000 hours a year, which leaves time for maintenance shutdowns. Fewer hours mean a higher hourly rate and larger equipment for the same annual output.
Table 2. Effect of operating hours on design rate and equipment cost
| Schedule | Operating hours | Design rate | Rate against base | Equipment cost against base |
|---|---|---|---|---|
| Continuous, 24/7 | 8,000 h/y | 6,250 kg/h | Base | Base |
| Continuous, longer shutdowns | 7,000 h/y | 7,143 kg/h | +14% | +8% |
| Five days a week, 24 hours | 6,000 h/y | 8,333 kg/h | +33% | +19% |
| Two shifts, five days | 4,000 h/y | 12,500 kg/h | +100% | +52% |
The cost column uses the six-tenths rule, in which equipment cost rises with capacity to the power of 0.6. It is an approximation for screening. Even so, the choice between 8,000 and 6,000 hours changes the equipment cost by about 19%, and it is made in one line of the design basis.
Why a late change costs so much
The design basis sits at the start of a chain. It sets the heat and material balance, and the balance sets the documents that follow.
Table 3. What a 20% capacity increase affects, by when it is made
| When the change is made | What has to be redone | Relative cost |
|---|---|---|
| Pre-FEED | One line in the design basis and the option study | Low |
| During FEED | Balance, PFDs, equipment sizes, utility loads, estimate | Medium: weeks of rework |
| Detailed design | All of the above, plus vendor quotations, piping and structural design | High: months of delay |
| Construction | Purchased equipment replaced or modified, installed work removed | Very high |
What a completed design basis looks like
For the example plant, the summary page might read as follows.
Table 4. Design basis summary for the example plant
| Item | Basis |
|---|---|
| Capacity | 50,000 t/y of product, turndown to 50% |
| Operating schedule | 8,000 h/y, continuous, one planned shutdown a year |
| Design rate | 6,250 kg/h |
| Main feedstock | Delivered by rail car and tank truck, minimum purity 98%, 10 days of storage |
| Product | Two grades, minimum purity 99.5%, shipped in bulk and in 1,000 kg totes |
| Site | Existing industrial site, design temperatures −30°C to 35°C |
| Utilities from site | Power and natural gas. Cooling water, nitrogen and steam are in project scope. |
| Operating philosophy | Automated control from one control room, spare pumps on critical services |
| Expansion | Plot space and utility headers sized for a second train |
Three rules for managing it
- The owner signs it. A signed design basis is the agreed starting point. Later changes go through a formal change request with a cost and schedule estimate.
- Open items are listed. Any value still unknown is marked as a hold, with a named person and a date. A hold that is still open at the end of FEED becomes a risk in the estimate.
- Each value has a source. A specification from a customer contract carries more weight than a figure someone remembered. Record which one it is.
Takeaway. Fix the ten decisions before FEED starts and have the owner sign them. On the example plant, one of them alone, operating hours, moves equipment cost by 8% to 52%.
The plant, its capacity and its costs in this series are an illustrative example. The ranges and percentages are typical industry values and do not replace an estimate prepared for your project.
