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API 941-2004 pdf free download

API 941-2004 pdf free download.Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants .
This recommended practice discusses the resistance of steels to high temperature hydrogen attack (HTHA). At nor- mal atmospheric temperatures, gaseous molecular hydrogen does not readily permeate steel, even at high pressures. Car- bon steel is the standard material for cylinders that are used to transport hydrogen at pressures of 2000 psi (14 MPa). Many postweld heat treated carbon steel pressure vessels have been used successfully in continuous service at pressures up to 10,000 psi (69 MPa) and temperatures up to 430°F (221°C). However, under these same conditions, highly stressed car- bon steels and hardened steels have cracked due to hydrogen embittlement. The recommended maximum hydrogen partial pressure at atmospheric temperature for carbon steel fabricated in accor- dance with the ASME Boiler and Pressure Vessel Code is 13,000 psia (90 MPa). Bclow this prssurc, carbon stecl equipment has shown satisfactory performance. Above this pressure, very litle operating and experimental data are avail- able. If plants are to operate at hydrogen partial pressures that exceed 13.000 psia (90 MPa), the use of an austenitic stain- less steel liner with venting in the shell should be considered. At elevated temperatures, molecular hydrogen dissociates into the atomic form, which can readily enter and diffuse through the steel. Under these conditions, the diffusion of hydrogen in steel is more rapid. As discussed in Section 4, Forms of High Temperature Hydrogen Attack, hydrogen reacts with the carbon in the steel to cause cither surface decarburization or internal decarburization and fissuring, and eventually cracking. This form of hydrogen damage is called high temperature hydrogen attack.
Hydrogenation processes usually require standards and materials that may not be warranted in other operations of the petroleum industry. At certain combinations of elevated tem- perature and hydrogen partial pressure, both chemical and metallurgical changes occur in carbon steel, which in advanced stages can render it unsuitable for safe operation. Alloy steels containing chromium and molybdenum can be used under such conditions. The steels discussed in this recommended practice resist HTHA when operated within the guidelines given. However, they may not be resistant to other corrosives present in a pro- cess stream or to other metallurgical damage mechanisms operating in the high temperature hydrogen attack range. This recommended practice also does not address the issues sur- rounding possible damage from rapid cooling of the metal after it has been in high temperature, high pressure hydrogen service (e.g., possible need for outgassing hydroprocessing reactors). This recommended practice will discuss in detail only the resistance of steels to high temperature hydrogen atack. Presented in this document are curves which indicate the operating limits of temperature and hydrogen partial pressure for satisfactory performance of carbon steel and Cr-Mo steels in elevated temperature, hydrogen service. In addition, it includes a summary of inspection methods to evaluate equip- ment for the existence of HTHA. 2 References 2.1 STANDARDS Unless otherwise specified, the most recent editions or revisions of the following codes shall, to the extent specified herein, form a part of this publication.
Figure 1 is based upon experience gathered since the 1940s. Supporting data were obtained from a variety of com- mercial processes and laboratory experiments (sce the refer- ences to Figure 1). While temperature and hydrogen partial pressure data were not always known precisely, the accuracy is often sufficient for commercial use. Satisfactory perfor- mance has been plotted only for samplcs or cquipment exposed for at least one year. Unsatisfactory performance from laboratory or plant data has been plotted regardless of the length of exposure time. The chemical compositions of the steels in Figure 1 should conform to the limits specifed for the various grades by ASTM or ASME. Since the original version of Figure 1 was prepared for API in 1949,1 further experience has enabled curves for most commonly used steels to be more accurately located. A major exception has been for C-0.5Mo steel. This edition consoli- datcs all information rclevant to 0.5Mo stecls (C-0.5Mo and Mn-0.5Mo) in Appendix A. The ffth edition of this recommended practice also added three data points, which show high temperature hydrogen attack of 1.25Cr-0.5Mo stecl bclow the current 1.25Cr-0.5Mo curve. See Appendix B for more discussion of 1.25Cr-0.5Mo steel. Appendix C gives a similar discussion for 2.25Cr- 1.0Mo steel.

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