Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants

API RECOMMENDED PRACTICE 941 SEVENTH EDITION, AUGUST 2008

Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants

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API RECOMMENDED PRACTICE 941 SEVENTH EDITION, AUGUST 2008

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Copyright © 2008 American Petroleum Institute

Foreword

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Contents

Page

1. Scope 1

2. References 1

   1. Standards 1

3. Operating Limits 2

   1. Basis for Setting Operating Limits 2

   2. Selecting Materials for New Equipment 2

   3. High Temperature Hydrogen Attack (HTHA) in a Liquid Hydrocarbon Phase 2

   4. References and Comments for Figure 1 4

4. Forms of HTHA 6

   1. General 6

   2. Surface Decarburization 6

   3. Internal Decarburization and Fissuring 7

5. Factors Influencing HTHA 7

   1. Incubation Time 7

   2. Effect of Primary Stresses 9

   3. Effect of Secondary Stresses 9

   4. Effect of Heat Treatment 9

   5. Effect of Stainless Steel Cladding or Weld Overlay 10

6. Inspection for HTHA 11

Annex A High Temperature Hydrogen Attack (HTHA) of 0.5Mo Steels 15

Annex B HTHA of 1.25 Cr-0.5Mo Steel 25

Annex C HTHA of 2.25Cr-1Mo Steel 27

Annex D Effective Pressures of Hydrogen in Steel Covered by Clad/Overlay 29

Annex E Summary of Inspection Methods 31

Annex F Request for New Information 37

Bibliography 39

Figures

1. Operating Limits for Steels in Hydrogen Service to Avoid Decarburization and Fissuring 3

2. C-0.5Mo Steel (ASTM A204-A) Showing Internal Decarburization and Fissuring in High

   Temperature Hydrogen Service 8

3. Time for Incipient Attack of Carbon Steel in High Temperature Hydrogen Service 12

1. Experience with C–0.5Mo and Mn–0.5Mo Steel in High Temperature Hydrogen Service 17

2. Steels in High Temperature Hydrogen Service Showing Effect of Molybdenumand Trace

   Alloying Elements 22

3. Time for Incipient Attack of 0.5Mo Steels in High Temperature Hydrogen Service 24

B.1 Operating Conditions for 1.25Cr-0.5Mo Steels That Experienced HTHA Below the Figure A.1 Curve . . 26

C.1 Operating Conditions of 2.25Cr-1Mo Steels That Experienced HTHA Below the Figure A.1 Curve 28

Tables

A.1 Operating Conditions for C-0.5Mo Steels That Experienced High Temperature Hydrogen

Attack Below the 0.5Mo Steel Curve in Figure A.1 21

B.1 Experience with HTHA of 1.25Cr-0.5Mo Steel at Operating Conditions

Below the Figure A.1 Curve 25

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Page

C.1 Experience with HTHA of 2.25Cr-1Mo Steel at Operating

Conditions Below the Figure A.1 Curve 27

1. Summary of Inspection Methods for HTHA 32

2. Summary of Ultrasonic Inspection Methods for HTHA 35

Introduction

At normal atmospheric temperatures, gaseous molecular hydrogen does not readily permeate steel, even at high pressures. Carbon 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 carbon steels and hardened steels have cracked due to hydrogen embrittlement.

The recommended maximum hydrogen partial pressure at atmospheric temperature for carbon steel fabricated in accordance with the ASME Boiler and Pressure Vessel Code is 13,000 psia (90 MPa). Below this pressure, carbon steel equipment has shown satisfactory performance. Above this pressure, very little operating and experimental data are available. If plants are to operate at hydrogen partial pressures that exceed 13,000 psia (90 MPa), the use of an austenitic stainless 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, hydrogen reacts with the carbon in the steel to cause either surface decarburization or internal decarburization and fissuring, and eventually cracking. This form of hydrogen damage is called high temperature hydrogen attack (HTHA) and this recommended practice discusses the resistance of steels to HTHA.

Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants

1. Scope

   This recommended practice (RP) summarizes the results of experimental tests and actual data acquired from operating plants to establish practical operating limits for carbon and low alloy steels in hydrogen service at elevated temperatures and pressures. The effects on the resistance of steels to hydrogen at elevated temperature and pressure that result from high stress, heat treating, chemical composition, and cladding are discussed. This RP does not address the resistance of steels to hydrogen at lower temperatures [below about 400°F (204°C)], where atomic hydrogen enters the steel as a result of an electrochemical mechanism.

   
   

   This RP applies to equipment in refineries, petrochemical facilities, and chemical facilities in which hydrogen or hydrogen-containing fluids are processed at elevated temperature and pressure. The guidelines in this RP can also be applied to hydrogenation plants such as those that manufacture ammonia, methanol, edible oils, and higher alcohols.

   
   

   Hydrogenation processes usually require standards and materials that may not be warranted in other operations of the petroleum industry. At certain combinations of elevated temperature 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 RP resist high temperature hydrogen attack (HTHA) when operated within the guidelines given. However, they may not be resistant to other corrosives present in a process stream or to other metallurgical damage mechanisms operating in the HTHA range. This RP also does not address the issues surrounding 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 RP will discuss in detail only the resistance of steels to HTHA.

   
   

   Presented in this document are curves that 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 equipment 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.

ASME Boiler and Pressure Vessel Code, Section II 1—Materials—Part A: Ferrous Material Specifications, and Part D: Properties, Section III—Rules for Construction of Nuclear Power Plant Components, and Section VIII—Pressure Vessels, Divisions 1 and 2

ASME Code for Pressure Piping, ASME/ANSI B31.3—Chemical Plant and Petroleum Refinery Piping

1 ASME International, 3 Park Avenue, New York, New York 10016, www.asme.org.

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