Pipes materials selection

Pipe-Wall Thickness. Once the design pressure and temperature have been established and the pipe material and size selected, the wall thickness is calculated using the appropriate section of the code. In rare cases, a thin pipe must be made thicker to withstand handling. Occasionally the thickness is affected by external loads or vibrations. All codes prescribe essentially the same design formula for metallic hoUow circular cylinders under internal pressure ... 

Provide permanent brazed or welded pipelines from the cylinders to near the points of gas use. Select pipe materials suitable for the gas and its application. Any flexible piping used should be protected against physical damage. Never use rubber or plastic connections from cylinders containing toxic gases. 

The process designer must also consider the corrosive nature of the fluids involved when selecting construction materials for the various process and utility service lines. Some designers attach these materials designations to the line designation on the flowsheets, while others identify them on the Line Summary Table (Figure 1-24D). Some typical pipe materials designations are ... 

Piping joints shall be selected to suit the piping material, with consideration of joint tightness and mechanical strength under expected service and test conditions of pressure, temperature, and external loading. Layout of piping should, insofar as possible, minimize stress on joints, giving special consideration to stresses due to thermal expansion and operation of valves (particularly a valve at a free end). 

Confirmation of corrosion performance of the materials selected for process piping, vessels, and other components the selected materials were polytetrafluoroethylene (PIPE) and perfluoro-alkoxy (PFA) linings. 

At this point, experiments must be performed. Experimental results for the erosive wear of the selected candidate ceramic materials in coal slurries are presented in Table 8.4. Notice that the wear rate has a very rough inverse correlation with which is consistent with some of the descriptions of erosive wear from the previous section. Any of these ceramic materials is suitable for the piping and pump components based solely on wear rate, with the lowest wear rate for SiC being the most attractive. Formability and economic criteria can be applied to assist in the final material selection. 

FIGURE 3B Roughness factors for selected types of pipe materials. [Adapted from Moody, L. F. (1944). Trans. ASME 66, 671-684.]... 

Appendix A contains a materials selection guide for aerated freshwater systems. As indicated in Note 27 of Appendix A, in freshwater systems, admiralty brass should be limited to a maximum pH value of 7.2 from ammonia and copper-nickel alloys and should not be used in waters containing more sulfides than 0.007 mg/L The materials selection guide is also satisfactory for seawater, although pump cases and impellers should be a suitable duplex stainless steel or nickel-aluminum-bronze (properly heat treated). Neoprene-lined water boxes should be considered. For piping, fiber-reinforced plastic (up to 150 psi [1,035 kPa] operating pressure) and neoprene-lined steel should also be considered. Titanium and high-molybdenum SS tubes should be considered where low maintenance is required or the cost can be justified by life expectancy. 

Four appendixes are included. Appendix A contains standard materials selection used by many refiners and contractors in petroleum processing equipment. Appendix B contains a rules of thumb overview of refinery materials of construction. Appendix C contains background information on hydrogen diffusion through vessel walls, and Appendix D contains a standard specification for steel line pipe. 

Selected blends of styrene-acrylonitrile copolymer (30 to 55%), a styrene-butadiene copolymer grafted with styrene and acrylonitrile (45 to 70%), and a coal-tar pitch (0 to 25%), were prepared. Physical properties of the experimental blends were determined and statistical techniques were used to develop empirical equations relating these properties to blend composition. Scheff canonical polynominal models and response surfaces provided a thorough understanding of the mixture system. These models were used to determine the amount of coal-tar pitch that could be incorporated into ABS compounds that would still meet ASTM requirements for various pipe-material designations. ... 

FIGURE 9.7 Material selection for gaseous oxygen (for pipe, fittings, components, and valves). A = Carbon steel, stainless steel, or copper or nickel alloys. B = stainless steel (1 in. NPS min. and Vs in. thick min.) or copper or nickel alloys. C = Copper or nickel alloys only. Note (1) The material choice for each site is based on the maximum velocity and pressure at each site. (2) Temperature less than 200°F. (From CGA G-4.4, Compressed Gas Association, 1997, 10. With permission.)... 

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