STP/PT-004

IMPREGNATED GRAPHITE FOR PRESSURE VESSELS

STP/PT-004

IMPREGNATED GRAPHITE FOR

PRESSURE VESSELS

Prepared by:

The Special Working Group on Graphite Pressure Equipment

Date of Issuance: November 10, 2005

This report was prepared as an account of work sponsored by ASME Pressure Technology Codes and Standards (C&S) under the Special Working Group on Graphite Pressure Equipment and ASME Standards Technology, LLC (ASME ST-LLC).

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TABLE OF CONTENTS

List of Tables iv

List of Figures iv

List of Equations v

FOREWORD vi

ABSTRACT vii

1. INTRODUCTION 1

2. IMPREGNATED GRAPHITE 2

   1. History 2

   2. Manufacturing of Impregnated Graphite Products 2

   3. Material Specifications 3

3. PROPERTIES OF IMPREGNATED GRAPHITE 6

   1. General 6

   2. Union Carbide Tests 8

   3. Creep 9

   4. Fatigue Strength of Impregnated Graphite 9

      1. Data from SGL Carbon Group 9

      2. Fatigue Tests by Hoechst 9

   5. Modulus of Elasticity 10

   6. Notch Sensitivity of Impregnated Graphite 10

   7. Corrosion Resistance 12

   8. Coefficient of Thermal Expansion 12

4. DESIGN 13

   1. Existing Codes for Graphite Pressure Vessels 13

   2. Permissible Design Temperature 13

   3. Properties for Design of Impregnated Graphite Vessels and Components 13

   4. Design Margins 13

   5. Buckling of Cylindrical Shells 13

   6. Buckling of Tubes 15

   7. Thicknesses of Flat Heads, Covers, and Tubesheets 16

5. NOZZLES 18

6. LETHAL SERVICE 22

7. CERTIFIED MATERIAL SPECIFICATIONS 23

   1. Qualification of Certified Materials 23

8. CERTIFIED CEMENT SPECIFICATIONS 26

   1. Cementing Procedure Specification 26

9. TESTING OF GRAPHITE MATERIALS 27

   1. Block Material 27

   2. Tube Material 27

10. FABRICATION OF GRAPHITE VESSELS AND COMPONENTS 33

11. NONDESTRUCTIVE EXAMINATIONS 34

12. HYDROSTATIC PRESSURE TEST 35

13. SUMMARY 36

REFERENCES 37

ACKNOWLEDGMENTS 38

ABBREVIATIONS AND ACRONYMS 39

List of Tables

Table 1 - Required Properties of Certified Material 5

Table 2 - Properties of Impregnated Graphite and Other Materials 6

Table 3 - Hoechst Fatigue Tests 10

Table 4 - Value of [Term] for Various Tube and Cylinder Parameters 15

Table 5 - Hypothetical Test Data (1000 psi) 24

Table 6 - Testing Requirements for Certified Materials 28

List of Figures

Figure 1 - Typical Graphite Shell and Tube Heat Exchanger 3

Figure 2 - Typical Flow Diagram for Manufacture of Impregnated Graphite Products 4

Figure 3 - Schematic View of Resin-Filled Pore and Microcracks Under Stress 7

Figure 4 - Tensile Strength of Impregnated Graphite and Nonimpregnated Graphite as a Function

of Temperature 7

Figure 5 - Tensile Strength vs. Temperature Union Carbide Tests 8

Figure 6 - Flexural Stress vs. Temperature 8

Figure 7 - Typical Fatigue Strength of Process Equipment Impregnated Graphite Under

Alternating Bending Stresses 9

Figure 8 - The Assumed Profile of a Doubly Notched Plate 11

Figure 9 - Notch Sensitivity vs. Root Radius for Three Defect Sizes, c, and Three Notch Depths, t...11 Figure 10 - Tube Center Limit Perimeter 17

Figure 11 - Acceptable Nozzle Details in Graphite Vessels, Example A 18

Figure 12 - Acceptable Nozzle Details in Graphite Vessels, Example B 19

Figure 13 - Acceptable Nozzle Details in Graphite Vessels, Example C 19

Figure 14 - Acceptable Nozzle Details in Graphite Vessels, Example D 20

Figure 15 - Unacceptable Nozzle Details 21

Figure 16 - t-Distribution of Material Test Data 24

Figure 17 - Tensile Test Specimen 29

Figure 18 - Cement Material Tensile Test Specimen 30

Figure 19 - Tube Tensile Test Specimen 31

Figure 20 - Tube Cement Joint Tensile Test Specimen 32

List of Equations

Equation 1 - Notch Sensitivity 10

Equation 2 - Experimental Stress Concentration Factor 12

Equation 3 - Experimental Notch Sensitivity Index 12

Equation 4 - Allowable External Pressure for Plastic Deformation 14

Equation 5 - Allowable External Pressure for Plastic Deformation Term 14

Equation 6 - Simplified Allowable External Pressure for Plastic Deformation 14

Equation 7 - Allowable Load on Tubes, Buckling 15

Equation 8 - Allowable Load on Tubes, Compressive 16

Equation 9 - Minimum Thickness of Flat Heads and Covers 16

Equation 10 - Minimum Thickness of Graphite Flat Head or Cover 16

Equation 11 - Minimum Thickness of Fixed and Floating Tubesheets 16

Equation 12 - Design Pressure of Floating Tubesheets 17

Equation 13 - Confidence Interval 23

Equation 14 - Simplified Confidence Interval 23

Equation 15 - Allowable Stress 25

Equation 16 - Allowable Stress, Lethal Service 25

FOREWORD

This Standards Technology Publication is the result of a development project sponsored by ASME Pressure Technology Codes and Standards and performed under the oversight of the Special Working Group on Graphite Pressure Equipment, and the ASME Standards Technology, LLC.

Established in 1880, the American Society of Mechanical Engineers (ASME) is a 120,000 member professional not-for-profit organization focused on technical, educational and research issues of the engineering and technology community. ASME conducts one of the world's largest technical publishing operations, holds numerous technical conferences worldwide, and offers hundreds of professional development courses each year. ASME maintains and distributes 600 Codes and Standards used around the world for the design, manufacturing, and installation of mechanical devices. Visit www.asme.org for more information.

ASME Standards Technology, LLC (ASME ST-LLC) is a not-for-profit Limited Liability Company, with ASME as the sole member, formed in 2004 to carry out work related to newly commercialized technology, expanding upon the former role of ASME’s Codes and Standards Technology Institute (CSTI). ASME ST-LLC mission includes meeting the needs of industry and government by providing new standards-related products and services, which advance the application of emerging and newly commercialized science and technology and providing the research and technology development needed to establish and maintain the technical relevance of codes and standards. Visit www.stllc.asme.org for more information

ABSTRACT

Impregnated graphite (also called impervious graphite) is a material that has been in industrial use for the past 60 - 70 years. The primary industrial use has been in the construction of chemical processing equipment where the exceptional corrosion resistance and high thermal conductivity of graphite is particularly advantageous. Typical applications include the manufacture of pharmaceuticals and phosphate fertilizer, steel pickling, processing of chlorinated organics, flue gas treatment, HCl and H2SO4 production and recovery, plus the manufacture of chemical intermediates.

The impervious graphite used for the construction of graphite pressure vessels is a composite material, consisting of “raw” graphite that is impregnated with a resin using a tightly controlled pressure/heat cycle. The interaction between the raw material and the resin is the determining factor when considering the design characteristics of the material. The design characteristics include the strengths (flexural, compressive, tensile), porosity, coefficient of thermal expansion, thermal conductivity, and ultimately the safe operating life of the vessel.

Proposed new pressure vessel rules will apply to the impregnated material only. There are two main reasons for this. First, the raw material is porous in nature and cannot be used as a pressure-containing material. Second, the resin impregnation process is a major factor when considering the properties of impregnated graphite. To consistently meet the minimum design values, the resin impregnation process must be tightly controlled. The resin impregnation processes used today have been developed over a 70-year period. The essential variables of the process have been defined and apply universally to all manufactures of impervious graphite equipment. By verifying the essential variables, it is possible to assign a lot number to all certified materials. The manufacturer’s control of this process is assured through meaningful and consistent test data. The long and successful worldwide experience with impregnated graphite vessels demonstrates that impregnated graphite vessels are safe and reliable under various aggressive service conditions.

This report presents a view of current best practices and recommendations for development of new rules. This paper describes many of these rules and much of the logic that has gone into creating the proposed section, and it is intended to provide a basis for the development of consensus standards addressing the use of impregnated graphite for ASME Section VIII Division 1 pressure vessels. It is the hope of the committee that this document will help to provide a strong background of information supporting continued efforts directed at inclusion of the proposed part UIG in Section VIII.

Intentionally Left Blank

1. INTRODUCTION

   Graphite is a form of carbon possessing good corrosion resistance. Impregnated graphite (also called impervious graphite) is a material that has been in industrial use for the past 60 to 70 years. The primary industrial use has been in the construction of chemical processing equipment where the exceptional corrosion resistance and high thermal conductivity of graphite is particularly advantageous. It is usual for this equipment to remain in service for many years.

   Impregnated graphite is commonly used for chemical processes involving sulfuric acid (H2SO4), hydrochloric acid (HCl), hydrobromic acid (HBr), phosphoric acid (H3PO4), and hydrofluoric acid (HF). Typical applications include the manufacture of pharmaceuticals and phosphate fertilizer, steel pickling, processing of chlorinated organics, flue gas treatment, HCl and H2SO4 production and recovery, and the manufacture of chemical intermediates.

   Major components of chemical process equipment are manufactured from impervious graphite. These include tubes for heat exchangers and solid blocks. Components, such as tubesheets, headers, and nozzles are obtained by machining impervious graphite to size.

   Numerous companies have been engaged in the manufacture of impervious graphite equipment and have produced over 50,000 heat exchangers on a worldwide basis. Some companies that have been or are currently engaged in the manufacture of new impervious graphite equipment are: Union Carbide, Carborundum, SGL Carbon, Carbone Lorraine, Kearney Industries, Ralph Coidan, and Metaullics. Typical trade names in use today are: Diabon, Graphilor, Impervite, and Karbate.

   Currently there are no rules in the ASME Boiler and Pressure Vessel Code for design and construction of graphite pressure vessels. Many of the older graphite pressure vessels were designed and manufactured to the manufacturer’s in-house standards. However, requirements for pressure equipment made of graphite are included in some of the European and Asian pressure vessel codes. Most of the more recently produced graphite pressure vessels in the United States are based on the design and construction requirements of the German pressure vessel code AD Merkblatt N2.

   ASME has established a Special Working Group on Graphite Pressure Equipment (SWG-GPE) to develop rules for design and construction of impregnated graphite pressure equipment. The objective is to include these rules as Part UIG of Section VIII, Division 1 (Part UIG is the designation of the requirements for impregnated graphite intended for publication in Section VIII Division 1 of the Boiler and Pressure Vessel Code). These rules will incorporate generally accepted international practices that have resulted in reliable and safe impregnated graphite pressure equipment.

2. IMPREGNATED GRAPHITE

   1. History

      Graphite is a naturally occurring form of carbon. Edward G. Acheson accidentally synthesized graphite while he was performing high-temperature experiments on silicon-carbide. At about 4,150°C (7,500°F) he found that the silicon in the silicon-carbide vaporized, leaving the carbon behind in graphitic form. A patent for the manufacture of graphite was granted to Acheson in 1896, and commercial production started in 18971.

      Graphite has been used in the chemical and metallurgical industries since the time of Sir Humphrey Davy; circa 1800. However, porosity (on the order of 25%) prevented the use of graphite in many chemical process applications. In the 1930s, the then National Carbon Company developed a process (impregnation) to make the graphite impervious to fluids. The imperviousness is obtained by filling the pores of the graphite material with thermosetting resins, such as phenolics, furans and epoxies, during a vacuum/pressure impregnation process.

      Today, major elements of process equipment are manufactured from impregnated graphite. These elements include tubes, nozzles and cylinders, solid blocks, plates, tubesheets, and more. The impregnation process has allowed graphite with its excellent corrosion resistance and thermal conductivity properties to be used in chemical process equipment under pressure, such as impervious graphite shell and tube heat exchangers (see Figure 1). Typical applications include condensing and evaporating acids, such as hydrochloric and phosphoric. Over the past 60 years, improvements in the impregnation process allowed shell and tube heat exchangers to increase in size from 7-tube units 9 ft long to 2,125 tube units 27 ft long.

      
      

   2. Manufacturing of Impregnated Graphite Products

The materials used in manufacturing graphite are fillers (petroleum coke, natural and synthetic graphite) binders (coal tar pitch), and additives. Their conversion into machined impervious graphite material for process equipment involves the following steps:

• Prepare the raw materials by grinding, sorting and blending.

• Mix the raw materials to form a molding flour.

• Compress the molding flour into tubes and blocks.

• Bake the formed components at temperatures of about 1,800°F (980°C). Baking the shaped components (to carbonize the binder) produces a porous solid material whose pore structure changes only very slightly in the graphitizing process.

• Graphitizing is carried out in graphitizing furnaces at 4,700 to 5,400°F (2,595 to 2,980°C). This process rearranges the carbon-rich raw materials and carbonized binders at a molecular level into graphite. The resulting material exhibits high thermal shock resistance and conductivity, as well as strong resistance to chemical attack.

• Graphite is machined with standard machine tools into process equipment components, such as heat exchanger tubes, heat exchanger blocks, tube sheets, and headers.

1 Impervious Graphite for Process Equipment, Parts 1 and 2, Chemical Engineering, February 18 and March 18, 1974.