Fabrication Considerations for Vanadium-Modified Cr-Mo Steel Heavy Wall Pressure Vessels

API TECHNICAL REPORT 934-B FIRST EDITION, APRIL 2011

Fabrication Considerations for Vanadium-Modified Cr-Mo Steel Heavy Wall Pressure Vessels

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API TECHNICAL REPORT 934-B FIRST EDITION, APRIL 2011

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Copyright © 2011 American Petroleum Institute

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Contents

Page

1. Scope 1

2. Normative References 1

3. Definitions and Acronyms 2

   1. Definitions 2

   2. Acronyms 3

4. Design and Fabrication 4

   1. Background 4

   2. Identified Fabrication Problems. 4

   3. Code References and Design Allowables 8

   4. Incorporation Into API 941 10

   5. Additional Characteristics 11

   6. Metallurgical Concerns During Fabrication 11

   7. Best Practices for Fabricating V-Modified 21/4Cr-1Mo Vessels 15

5. Assessment of Fabricators 18

   1. Current Worldwide Fabrication Capability 18

   2. Approving New Fabricators 18

Annex A (informative) SAMPLE Fabrication Plan 22

Annex B (informative) Example of a Detailed Fabrication Plan 23

Annex C (informative) Sample Fabrication Schedule 27

Bibliography 28

Figures

1. Charpy V-Notch Toughness of Conventional and Vanadium-modified 21/4Cr-1Mo

   Deposited Weld Metal after DHT and ISR Heat Treatment 6

2. Vanadium Content versus Creep Rupture of Cr-1Mo-V Steels 11

3. Typical Conventional and Vanadium-modified 21/44Cr-1Mo Weld Metal Hardness After

   Various Heat Treatments 13

4. Diffusible Hydrogen Test Results for 21/4Cr-1Mo Weld Metal 13

5. Charpy V-Notch Toughness of Vanadium-modified 21/4Cr-1Mo Deposited Weld Metal

after ISR Using a Second Alternative Wire Flux Combination 15

A.1 Sample Fabrication Plan 22

Tables

1. Typical Properties of Materials of Construction 9

2. Toughness Properties of Vanadium-modified Sub-Arc Weld Metal as Function

   of Heat Treatment Condition 15

3. Point System for Evaluating New Fabricators 19

B.1 Example Construction Schedule 23

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Introduction

This document is intended as a best practice guideline to be used by fabricators, in conjunction with API 934-A, when constructing new heavy wall pressure vessels with vanadium-modified Cr-Mo steels intended for service in petroleum refining, petrochemical, or chemical facilities. These materials are primarily used in high temperature, high pressure services which contain hydrogen.

The document provides typical practices to be followed during fabrication based upon experience and the knowledge gained from actual problems that have occurred during the fabrication of vanadium-modified Cr-Mo steels.

Background

The use of chrome-molybdenum steel vessels in hydrogen service can be traced back to the mid 1920s in Germany where they were used for reactors in high pressure hydrogenation plants. These vessels were fabricated from 2.25 % to 3.8 % chrome-molybdenum alloys operating in the pressure range of 28 to 70 MPa (4000 to 10,000 psi). [1] These steels are now referred to as the “First Generation” technology and were in use until the mid 1960s. The steels have evolved with significant improvements in each generation that can be summarized as follows.

Second Generation (Mid 1960s to 1970s) The birth of modern hydroprocessing reactors manufactured from heavy wall 21/4Cr-1Mo alloys with improved toughness (54 Joules at 10 ºC [40 ft-lbs at 50 ºF]), but with no particular temper embrittlement controls.

Third Generation (1970s to 1980s) Added J-factor limit of 180 to control temper embrittlement and developed improved toughness to 54 Joules at –18 ºC (40 ft-lbs at 0 ºF). Also, began step cooling tests with varying criteria, and precautions against weld overlay disbondment.

Fourth Generation (1980s to 1990s) Improved temper embrittlement control by lowering J-factor limit to 100 and achieving better results after step cooling. This generation also had toughness improvements to 54 Joules at –32 ºC (40 ft-lbs at –25 ºF)

Today “Fifth Generation” grades [2] of conventional 2¼Cr-1Mo steels have a 54-Joule (40 ft-lb) transition temperature typically lower than –40 ºC (–40 ºF), and even lower for conventional 3Cr-1Mo steels. Vanadium-modified Cr-Mo steels with 21/4 % and 3 % Cr were introduced for service with higher strength levels and increased hydrogen attack resistance. These grades achieved a 54 Joule (40 ft-lb) transition temperature typically around –29 ºC (–20 ºF). The vanadium-modified steels also offered enhanced creep rupture properties, lower temper embrittlement susceptibility, and a much lower susceptibility to hydrogen disbonding of weld overlay compared with conventional Cr-Mo steels.

As of 2009, over four hundred vanadium-modified reactors have been fabricated around the world with many more under construction.

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Fabrication Considerations for Vanadium-Modified Cr-Mo Steel Heavy Wall Pressure Vessels

1. Scope

   This best practice report complements API 934-A and specifies additional fabrication considerations that should be observed when constructing a new heavy wall pressure vessel using vanadium-modified Cr-Mo materials intended for hydrogen service at elevated temperature and pressure. It applies to vessels that are designed, fabricated, certified and documented in accordance with ASME Code Section VIII, Division 2, including Paragraph 3.4 of the ASME Code, Supplemental Requirements for Cr-Mo Steels and ASME Code Case 2151, as applicable (or equivalent international codes).

   
   

   Nominal material chemistries covered by this report are the vanadium-modified steels including 21/4Cr-1Mo-1/4V, 3Cr- 1Mo-1/4V-Ti-B, and 3Cr-1Mo-¼V-Cb-Ca steels. The interior surfaces of these vessels may have an austenitic stainless steel cladding or weld overlay to provide additional corrosion resistance.

   
   

2. Normative References

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

API Recommended Practice 582, Welding Guidelines for the Chemical, Oil, and Gas Industries

API Recommended Practice 934-A, Materials and Fabrication Requirements for 2 1/4Cr-1Mo & 3Cr-1Mo Steel Heavy Wall Pressure Vessels for High Temperature, High Pressure Hydrogen Service

API Recommended Practice 941, Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants

ASME Boiler and Pressure Vessel Code 1, Section II, Materials, Part A, Ferrous Material Specifications

ASME Boiler and Pressure Vessel Code, Section II, Materials, Part C, Specifications for Welding Rods, Electrodes and Filler Metals

ASME Boiler and Pressure Vessel Code Section II, Materials, Part D, Properties

ASME Boiler and Pressure Vessel Code Section VIII, Div. 2 Rules for Construction of Pressure Vessels – Alternate Rules

ASME SA 20, Specification for General Requirements for Steel Plates for Pressure Vessels

ASME SA 182, Specification for Forged or Rolled Alloy Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service

ASME SA 335, Specification for Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service

ASME SA 336, Specification for Alloy Steel Forgings for Pressure and High-Temperature Parts

ASME SA 369, Specification for Carbon and Ferritic Alloy Steel Forged and Bored Pipe for High Temperature Service

1 ASME International, 3 Park Avenue, New York, New York 10016-5990, www.asme.org.

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