Equipment costs (15): COLUMNS AND TOWERS
15 Jan 2000
Last month Costimator provided information on the budget costs of sieve trays, valve trays and bubble cap plates - the innards of many mass transfer operations such as distillation and absorption. We now follow this with cost data on the columns themselves, which are equipped with such internals. Here, we assume the term `columns' to be synonymous with `towers'.
The base case is for carbon steel equipment specified for operation at 3bar pressure. Costs are adjusted to January 1995. The available data excludes ladders and other external fittings, but includes a reasonable number of manways (say, three or four) and nozzles. Figure 15 here shows costs in £'000s per metre of column height against column diameter, also in metres.
Because of the wide variety of applications for this type of equipment, an equally wide range of materials of construction is used in their specification. Thus, Table 15, which cross-refers to Figure 15, lists factors for ten different materials. Thus, for example, a column fabricated in 410 stainless steel is twice as expensive as the carbon steel equivalent. Likewise, correction factors for design pressure are provided for between 3 and 21bar.
It will be noted that the graph in Figure 15 is curved despite the use of logarithmic scales. This point was covered by Costimator in PE in September and October 1987. With regard to the current issue, columns, there are significant end-effects (for example, dished ends) probably accounting for this curvature.
In brief, the correlation can, however, be linearised using an equation of the form:
Cost = a(b + Diameter)n
The articles published in 1987 (copies available on request) show how to find a, b and n. Applying this technique, we obtain:
Cost/m of height =
0.85(0.99 + Diameter)1.02
and this is shown inset on Figure 15 as the linear plot in the upper right hand quadrant. Thus, when we eliminate the end-effects using the above form of correlation, we obtain the expected result using logarithmic scales.
They come in all sizes and in many materials, but what do columns and towers cost? Use the correlations here for a budget estimate
{{Table 15: Factors to correct base case for design variations
Material factors Pressure factors
Carbon steel 1.00 Pressure (barg)410 stainless steel 2.00 3 1.00304 stainless steel 1.98 7 1.26316 stainless steel 2.28 14 1.56321 stainless steel 2.71 21 2.02304L stainless steel 2.09Nickel 5.96Inconel 4.78Hastalloy C 8.17Glass lined carbon steel 2.33}}
