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Specification and Inspection of Thermoplastic Storage Tanks

HSL/2006/21

J Stonehill (SMS), H Bainbridge (HSE) P F Heyes (HSL)

Project Leader: P F Heyes

Engineering Control Group

Crown copyright 2002

SUMMARY Objectives This project was planned initially to review inspection methods for thermoplastic hydrofluoric acid storage tanks. It was later expanded to include a range of thermoplastic tank materials and contents, and to include aspects of specification, design and fabrication. The work was carried out by Mr J Stonehill (Stonehill Materials Services) under the direction and supervision of Mr H Bainbridge (HSE, TD5) and Mr P F Heyes (HSL). Main Findings Operators of metallic and reinforced plastic (GRP) storage facilities have well established requirements for inspecting their tanks. Thermoplastic tanks are an attractive alternative to metal and (GRP) for many products but are often installed with the view that they will not deteriorate and do not need to be inspected or assessed. There are no established procedures for inspecting these tanks and consequently they are often used without due consideration of their condition and fitness for service. Initially this project concentrated on helix wound high density polyethylene tanks used to store hydrofluoric acid but, as the work progressed, a need to consider the hazards of other products stored in large polyethylene tanks particularly of non-homogeneous forms of construction was identified. This report offers guidance on the specification, design, fabrication and in-service inspection of thermoplastic storage tanks in general. The emphasis is on tanks installed over the last 20+ years although it is acknowledged that a standard will soon be published which addresses many of the concerns and issues raised in this document. It has been recommended that consideration be given to publication of an HSE guidance note on thermoplastic storage tanks.

Contents

Page 1 INTRODUCTION 1 2 MATERIALS OF CONSTRUCTION 3 3 DESIGN & SPECIFICATION 4 3.1 Introduction 4

3.2 DVS 2205 5

3.3 European standard prEN 12573 11

3.4 NACE Publication 5A171 (1995 Revision) 12

3.5 Compatibility data 13

4 FABRICATION 13 4.1 Welding 13

4.1.1 Butt fusion welding 13

4.1.2 Extrusion Welding 13

4.1.3 Hot gas welding 14

4.2 Methods of fabricating thermoplastic tanks 14

4.2.1 Sheet forming 14

4.2.1.1 Background 14

4.2.1.2 Fabricators 15

4.2.2 Helix winding 16

4.2.2.1 Background 16

4.2.2.2 Fabricators 17

4.2.3 Rotational moulding 17

4.2.3.1 Background 17

4.2.3.2 Design Considerations 18

4.2.3.3 Fabricators/suppliers 18

5 TRANSPORT & INSTALATION 18 6 INSPECTION & QUALITY ASSURANCE 19 6.1 Pre-service inspection and quality assurance 19

6.2 In-service inspection 20

6.2.1 Techniques 20

6.2.2 Frequency 21

6.2.3 Acceptance standards 22

6.2.4 In-service testing 22

7 SERVICE EXPERIENCE: FALIURES & DEFECTS 23 7.1 Fabrication defects 23

7.1.1 Weld defects 23

7.1.2 Delamination of wall materials 23

7.1.3 Winding defects 24

7.2 Chemical degradation 25

7.2.1 Nitric acid 25

7.2.2 Sodium hypochlorite 26

7.2.3 Sulphuric acid 27

7.3 Other chemical effects 27

7.3.1 Hydrofluoric acid blistering 27

7.3.2 Hydrofluoric acids, leaching of additives of HPDE 28

7.4 Aspects of tank design 28

7.5 Operational defects and failures 29

7.5.1 Lifting of wall 29

7.5.2 scuffing of base 30

7.5.3 Embrittlement of outlet 30

7.5.4 Perforation of tank base by level sensor 31

7.5.5 Splitting of seal welds 31

7.5.6 Failures of sheet formed tanks 31

7.5.6.1 Splitting of seams 31

7.5.6.2 Internal bulging 32

7.5.6.3 Uneven discolouration of sheet tanks in PVDF 32

7.6 Effects of under and over-pressure 33

7.7Installation issues 33

7.8 Maintenance issues 34

7.9 Exposure to excessive temperature 35

7.10 Bund issues 36

8 CONCLUSIONS 37

9 RECCOMENDATIONS 37

APPENDIX 1 Acknowledgements 38

APPENDIX 2 Guidelines for the operation of HDPE tanks 39

APPENDIX 3 References 41

1 INTRODUCTION

A review has been requested of the specification, design, fabrication and in service inspection and assurance of fitness for service of thermoplastic storage tanks to provide HSE personnel with an overview and guidance notes. It must be emphasised that thermoplastics have a good record, established over more than 30 years, of safe storage of a wide range of chemicals and with few reported incidents of major failure. An estimated 5500 High Density Polyethylene & Polypropylene helix wound tanks of different sizes have been supplied over the last 20 years; of these more than 60 having typical capacity of ~23 m3 have been supplied for HF duty. Maximum tank capacities of around 70 m3 are supplied by fabricators of helix wound tanks. One manufacturer of sheet formed thermoplastic tanks, Niplast Ltd, indicates a capability of fabricating tanks up to 100m3.

Fabricators and users of these tanks consulted during preparation of this report are acknowledged in Appendix 1.

Chemicals regularly stored in these tanks are shown in Table 1.

Specific guidance was requested for helix wound high density polyethylene tanks used to store hydrofluoric acid but an additional need has been identified to consider the hazards of other products such as sodium hypochlorite particularly when stored in large tanks of non-homogeneous forms of construction.

Operators of all storage tanks have a well established duty to inspect them periodically to assess their fitness for service; standards and guidelines covering specification, installation and inspection of tanks fabricated from steel and GRP have been published. The design code for most thermoplastic tanks used in the United Kingdom, DVS22051, is outdated and will shortly be replaced by a European Standard; neither document gives guidance on in service inspection and there are no established procedures or guidelines for inspection of these

tanks.

Table 1 aluminium sulphate beers caustic soda cider detergents dyes & bleaches ferric chloride fertilisers hydrochloric acid hydrogen peroxide phosphoric acid sodium hypochlorite sulphuric acid wines

Many operators install thermoplastic storage tanks with the view that they will not deteriorate and therefore do not need inspection or maintenance. Other operators may take the view that thermoplastic tanks should simply be replaced at the end of their design life but may not implement any formal intermediate inspection. There is often economic pressure for tanks to be used beyond their design life sometimes without due consideration of the condition and fitness for service.

This report concentrates on tanks which are currently in service and which should have been designed and fabricated in accordance with the appropriate standard, DVS2205 and

1

best practice at the time. Many of the issues raised here have been addressed by the European Standard which will be issued shortly and which reputable fabricators will be applying now.

Two examples illustrate the importance of inspection and assessment of storage tanks and the potential hazards of incorrect design and/or installation and emphasise the need for operators to understand their duty to assure themselves of the correctness of a design and installation. Both incidents happened within the last 10 years and are from the authors own records .

The first is an example of how a small error during installation can negate the most careful and detailed pre-service controls and subsequent in-service inspection that included detailed thickness checks at 3 monthly intervals: The traditional material of construction

for storage of 70% hydrofluoric acid is carbon steel. A 22 mm thick horizontal cylindrical steel tank perforated within 12 months service due to locally high flow rates arising from an incorrectly installed dip pipe, Figure 1.

The second example illustrates what can happen when operators and fabricators are unaware of the existence of standards or guidance notes such as HSE PM752.

A very poorly designed and fabricated lined GRP horizontal cylindrical tank handling acidic (including weak hydrofluoric acid) effluent failed catastrophically by complete separation of one end of the tank, following a history of minor leaks during less than 12 months service, Figure 2. The fabricator had never made a tank before and had no knowledge of the requirements with regard to shell thickness, welding, supports or openings. The operator had simply requested a tank of the required size and configuration. Little consideration was given by the operator to the effects that acid leaks might have on the integrity of the GRP reinforcement. This is an aspect of operation of both GRP and steel stock tanks which