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Views: 0 Author: Site Editor Publish Time: 2022-01-04 Origin: Site
Maurice Stewart, in Surface Production Operations, 2016
Carbon steel pipe is the most commonly used material for process piping. It has the advantage of wide availability, high strength, and a large array of connection possibilities, for example, screwed, socket-welded, and butt-welded. Steel pipe should be selected for the required strength and durability required for the application and the ductility and machinability required to join it and form it into piping spools. The pipe must withstand the pressure, temperature, and corrosion conditions of the application. These requirements are met by selecting pipe made to an appropriate ASTM or API standard.
Carbon steel pipe is used for liquid, gas, and steam services both above- and belowground services. It is not recommended for use in corrosive services but may be used in caustic services.
There are many grades or strengths of carbon steel pipe and they are available in a number of wall thicknesses. We have seen that the allowable stress is used to determine what wall thickness is required. The allowable stress is a function of both the metallurgy of the material and the method of manufacturer.
The various piping specifications provided by ASTM and API provide guidelines for both the metallurgy and the method of manufacture. The most widely piping specifications for process lines are ASTM Specifications A-53 and A-106 and API Standard 5L. The principal wall thicknesses used are defined by schedules, for example, Schedule 40, Schedule 80, and weights, for example, STD, XS, and XXS. Both ASTM A53 and ASTM A106 pipe are fabricated SMLS or seamed, by electric resistance welding, in Grades A and B. Grades B have higher tensile strength. Three grades of ASTM A106 are available—Grades A, B, and C, in order of increasing tensile strength.
Table 3.2 provides the specifications for a given temperature range. For years, most pipes were made from Grade B steel, which has a minimum yield strength of 35,000 psi. Construction of high-pressure, large-diameter, cross-country transmission lines created a need for high-strength field-weldable steel that would allow a substantial savings in steel tonnage. API Grades X-42 through X-70 were developed with strengths of 42,000-70,000 psi.
Table 3.2. Metal design temperature for piping
| Design temperature | Specification |
|---|---|
| 650 to 60 °F 59 to − 20 °F − 21 to − 50 °F − 51 to − 150 °F − 151 to − 325 °F | API 5L A-53 A-106 API 5L A-106 A-333 Grade 1 or 6 A-333 Grade 304 A-312 Grade 304 A-358 Grade 304 A-312 Grade 304 A-358 Grade 308 |
Note: Metal design temperature shall be the design operating temperature plus 50 F or 10%, whichever is greater, for services 60 °F and above. For services 59 °F and below, subtract 5 °F or 10% whichever or greater from the design operating temperature where applicable; allowance should also be included for temperature effects due to process variation, especially at low temperature.
Figure 3.2 illustrates a typical stress-strain diagram for steel pipe. Table 3.3 illustrates the savings that can be realized by using Grade X pipe. Care must be taken to balance savings against corrosion allowances, special welding techniques required, minimum wall thickness criteria of the codes, and reduction in safety for hot-tapping operations. Some high-strength or alloyed pipe may not be suitable for certain corrosive environments. Table 3.4 compares the relative cost of steel pipe versus common alloys.
