Page 62 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Planning, Scheduling and Flowsheet Design 47

ments will give total plant costs. The plant will include Pa = Cost of plant or section of plant of original capac-
usual control buildings, structure, foundations, overhead ity "a."
charges, construction fees, engineering costs, etc. A value Cb = Capacity of plant or section of new requirements.
of 4.0 is usually quite good. Ca = Capacity of plant or section of original requirements.
The process designer must be aware of costs as reflect-
ed in the ( 1) selection of a basic process route (2) the This is applicable for any given year of installation but
equipment to be used in the process and (3) the details does not correct for the differences in cost from year to
incorporated into the equipment. The designer must not year. This is conveniently done as described in the section
arbitrarily select equipment, specify details or set pressure for year indices. Experience has indicated that this six-
levels for design without recognizing the relative effect on tenths rule is reasonably accurate for capacity scale-up of
the specific cost of an item as well as associated equip- individual items of equipment. Thus, if the cost of one size
ment such as relieving devices, instruments, etc. of a piece of equipment is known, an estimating figure for
With more and better information regarding the one twice as large can be found by multiplying by (2) 0·6.
process and layout plans, estimating engineers can pre- The most difficult feature of this method is that for
pare detailed estimates which are often quite accurate, each type of plant or plant product as well as for each type
usually ±10 percent for the best. It is the duty of the of equipment there is a break-point where the 0.6 no
process designer to supply the best information in order
to contribute to better or improved estimates. longer correlates the change in capacity. For small equip-
ment or plants in reasonable pilot or semi-works size, the
Estimating equipment costs is a specialty field in itself. slope of the cost curve increases and the cost ratio is
Therefore, the estimator must have access to continuous- greater than 0.6, sometimes 0.75, 0.8 or 0.9. From several
ly updated basic reference costs and to graphical costs cost values for respective capacities a log-log plot of capac-
relations which are a function of capacity of this equip- ity versus cost will indicate the proper exponent by the
ment. Page's [10] Estimator's Manual of Equipment and slope of the resultant curve. Extrapolation beyond eight
Installation Costs is a helpful reference. Since the equip- or ten fold is usually not too accurate.
ment is only a portion of the total cost of a plant, or an
addition to a chemical project, installation costs which
reflect the labor portion of the total cost must also be Yearly Cost Indices
determined. Useful and comprehensive data for such
needs are presented for equipment [10], general con- The three most used cost indices for the chemical,
struction [ 11], heating, air-conditioning, ventilating, petrochemical, and refining industry for relating the cost
plumbing [12], piping [13], electrical [14] and all disci- level of a given year or month to a reference point are
plines [ 42] in the references indicated.
From such information even the inexperienced esti- 1. Chemical Engineering Plant Cost Index [42]. Probably
mator can establish an approximation of the costs, pro- the most commonly used cost adjusting index print-
vided he adequately visualizes the work functions and ed/updated monthly is in Chemical Engineering Mag-
steps involved. From the same type of work reference, the azine and has established continuity over many years.
experienced estimator can develop a realistic ccst, usual- Its breakdown component costs apply to plants and
ly expressed with certain contingencies to allow for plant equipment/systems.
unknown factors and changing conditions. The profes- 2. Marshall and Swift Equipment Cost Index [57]. Com-
sional estimator will normally develop cost charts and monly used for process industry equipment and
tables peculiar to the nature of his responsibilities and index numbers presented by industries in Chemical
requirements of his employer.
Engineering Magazine on a monthly basis.
Six-Tenths Factor 3. Nelson Index [58]. This is generally suited to petrole-
um refining plants and is referenced to them. It is
This factor as presented by Chilton [ 4] has been used updated and published regularly in The Oil and Gas
fur scale-up of total or segments of plant cost. Journal.

These indices are used to up-date costs when values at
( C • b )0.6 some date are known. The new costs are of estimating
Pb = pa c-:- accuracy and should be verified whenever possible.jusr as
the results of using the 0.6 power for correlating cost and
where Pb = Cost of plant or section of plant of new capacity "b." capacity.