Page 232 - Plant design and economics for chemical engineers

COST ESTIMATION 203
the service cost is apportioned among the different products at a rate based on
the amount of individual consumption.
Steam requirements include the amount consumed in the manufacturing
process plus that necessary for auxiliary needs. An allowance for radiation and
line losses must also be made.
Electrical power must be supplied for lighting, motors, and various pro-
cess-equipment demands. These direct-power requirements should be increased
by a factor of 1.1 to 1.25 to allow for line losses and contingencies. As a rough
approximation, utility costs for ordinary chemical processes amount to 10 to 20
percent of the total product cost.

Maintenance and Repairs
A considerable amount of expense is necessary for maintenance and repairs if a
plant is to be kept in efficient operating condition. These expenses include the
cost for labor, materials, and supervision.
Annual costs for equipment maintenance and repairs may range from as
low as 2 percent of the equipment cost if service demands are light to 20
percent for cases in which there are severe operating demands. Charges of this
type for buildings average 3 to 4 percent of the building cost. In the process
industries, the total plant cost per year for maintenance and repairs is roughly
equal to an average of 6 percent of the fixed-capital investment. Table 25
provides a guide for estimation of maintenance and repair costs as a function of
process conditions.
For operating rates less than plant capacity, the maintenance and repair
cost is generally estimated as 85 percent of that at 100 percent capacity for a 75
percent operating rate, and 75 percent of that at 100 percent capacity for a 50
percent operating rate.

TABLE 25
Estimation of costs for maintenance and repairs

Maintenance cost as percentage
of fixed-capital investment
(on annual basis)
Type of operation Wages Materials Total

Simple chemical processes 1-3 1-3 2-6
Average processes with normal 2-4 3-5 5-9
operating conditions
Complicated processes, severe 3-5 4-6 7-11
corrosion operating conditions,
or extensive instrumentation