Page 318 - Chemical engineering design

The advice of the technical service department of the company supplying the materials
should also be sought. MATERIALS OF CONSTRUCTION 293
7.6. MATERIAL COSTS
An indication of the cost of some commonly used metals is given in Table 7.5. The
actual cost of metals and alloys will fluctuate quite widely, depending on movements in
the world metal exchanges.

Table 7.5. Basic cost of metals (mid-2004)
Metal £/tonne US$/US ton
Carbon steel 300 500
Low alloy steel (Cr-Mo) 400 500 700 850
Austenitic stainless steel
304 1400 2400
316 1900 3200
Copper 1500 2500
Aluminium 900 1500
Aluminium alloy 850 1400
Nickel 6400 11,000
Monel 5000 8500
Titanium 20,000 34,000

The quantity of a material used will depend on the material density and strength (design
stress) and these must be taken into account when comparing material costs. Moore (1970)
compares costs by calculating a cost rating factor defined by the equation:
C ð
Cost rating D 7.2
d
where C D cost per unit mass, £/kg,
3
D density, kg/m ,
2
d D design stress, N/mm .
His calculated cost ratings, relative to the rating for mild steel (low carbon), are shown in
Table 7.6. Materials with a relatively high design stress, such as stainless and low alloy
steels, can be used more efficiently than carbon steel.
The relative cost of equipment made from different materials will depend on the cost of
fabrication, as well as the basic cost of the material. Unless a particular material requires
special fabrication techniques, the relative cost of the finished equipment will be lower
than the relative bare material cost. For example; the purchased cost of a stainless-steel
storage tank will be 2 to 3 times the cost of the same tank in carbon steel, whereas the
relative cost of the metals is between 5 to 8.
If the corrosion rate is uniform, then the optimum material can be selected by calculating
the annual costs for the possible candidate materials. The annual cost will depend on the
predicted life, calculated from the corrosion rate, and the purchased cost of the equipment.
In a given situation, it may prove more economic to install a cheaper material with
a high corrosion rate and replace it frequently; rather than select a more resistant but
more expensive material. This strategy would only be considered for relatively simple