Equipment costs (14): COLUMN TRAYS AND PLATES

After last month's festive diversion, we start the New Year with yet another contribution in Costimator's equipment cost series. This month we present data on the cost of trays or plates. These terms may be used synonymously, although it is more common to refer to bubble-cap trays, valve trays and sieve plates.

Now, as any chemical or process engineer knows, these trays and plates are typical internal fittings for mass transfer equipment such as distillation and absorption/desorption columns. Since distillation is certainly still the most common unit operation in the oil and gas, petrochemical and chemical industries, it is clear that the costs and types of trays and plates used are of immense economic and technical importance.

For example, a petrochemicals complex containing 50 distillation columns might be using, say, 1000 trays or plates. The initial capital cost of these items is high, as is the cost of replacements to improve separation efficiency or for maintenance requirements.

While we shall cover the costs of packing and columns (towers) in a future issue, here we deal just with the costs of:

{{* sieve plates* valve trays* bubble-cap trays}}

Table 14 gives a key to these and cross-refers to Figure 14 above. The data illustrated are based on carbon steel. However, as before (see Process Economics sections in PE September and November 1996), conversion to stainless steel equipment can be achieved using factors. Also, because the graphs are of near identical gradient (0.66 - 0.68, corresponding well with the two-thirds power law), the ratios of costs between the three types of fittings remain roughly constant.

Thus, as a rule of thumb, valve trays cost 50 per cent and bubble-cap trays 150 per cent more than sieve plates of the same diameter. These are useful, and easy, figures to remember.

{{Table 14: Costs of trays and plates (see Fig 14)

Key to Type Material of Cost factorFigure 14 construction

1 Sieve plates Carbon steel 1.0 2 Valve trays Carbon steel 1.5 3 Bubble-cap trays Carbon steel 2.5

Notes: Numbers of trays/plates range from 1 to 20Diameters of trays/plates range from 0.31 to 4.5m}}