Page 183 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

In Table 7.2, the accuracy range associated with each class of estimate and the costs associated with
carrying out the estimate are ranked relative to the most accurate class of estimate (Class 1). In order to
use the information in Table 7.2, it is necessary to know the accuracy of a Class 1 estimate. For the cost
estimation of a chemical plant, a Class 1 estimate (detailed estimate) is typically +6% to –4% accurate.
This means that by doing such an estimate, the true cost of building the plant would likely be in the range
of 6% higher than and 4% lower than the estimated price. Likewise, the effort to prepare a Class 5
estimate for a chemical process is typically in the range of 0.015% to 0.30% of the total installed cost of

the plant [1,2].

The use of the information in Table 7.2, to estimate the accuracy and costs of performing estimates, is

illustrated in Examples 7.1 and 7.2.

Example 7.1

The estimated capital cost for a chemical plant using the study estimate method (Class 4) was calculated
to be $2 million. If the plant were to be built, over what range would you expect the actual capital
estimate to vary?

For a Class 4 estimate, from Table 7.2, the expected accuracy range is between 3 and 12 times that of a
Class 1 estimate. As noted in the text, a Class 1 estimate can be expected to vary from +6% to –4%. We
can evaluate the narrowest and broadest expected capital cost ranges as follows.

Lowest Expected Cost Range
6
High value for actual plant cost ($2.0 × 10 )[1 + (0.06)(3)] = $2.36 × 10 6
6
Low value for actual plant cost ($2.0 × 10 )[1 – (0.04) (3)] = $1.76 × 10 6

Highest Expected Cost Range
6
High value for actual plant cost ($2.0 × 10 )[1 + (0.06)(12)] = $3.44 × 10 6
6
Low value for actual plant cost ($2.0 × 10 )[1 – (0.04)(12)] = $1.04 × 10 6

The actual expected range would depend on the level of project definition and effort. If the effort and