Economic Sizing of Steam Piping and Insulation

Abstract

This paper presents a simple method for the optimal economic selection of pipe size and insulation thickness for steam piping systems. The primary operating costs inherent in any such system are consequences of fluid-flow friction and heat transfer losses. Striving to conserve energy, the engineer is motivated to select large pipe diameters and insulation thicknesses. But how large should the pipe diameter be and how much insulation is necessary? The answer is simply to make that investment in piping and insulation which minimizes the sum of the capital and operating costs (friction and heat transfer). Thus it is imperative that the operating expenses be precisely evaluated. The key is the recognition that it is available energy which is the commodity of value—that it is necessary to assign an economic value (cost) to the steam based on its available energy content. Because friction and heat transfer destroy available energy, their respective costs can then be accurately assessed. As an example a bleeder steam line that delivers 55,300 lbm/hr (7 kg/s) at 93.8 psia (0.65 MPa) and 603 F (317 C) to a feedwater heater is analyzed to show the optimal nominal pipe diameter and insulation thickness to be 12 in. (30.48 cm) and 3.5 in. (8.89 cm), respectively.