ADDITIONAL REQUIREMENTS FOR PRESSURE VESSELS FOR APPLICATIONS IN H2S CONTAINING ENVIRONMENTS

8/9/2019 ADDITIONAL REQUIREMENTS FOR PRESSURE VESSELS FOR APPLICATIONS IN H2S CONTAINING ENVIRONMENTS

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05489.MAT.COR.SDSRev. 2 March 1997Sheet 2 di 21

This document is property of Eni S.p.A. Divisione Agip. It shall neither be shown to Third Parties not used for purposes other than those for which it has been sent.

Eni S.p.A.Agip Division

FOREWORD

Rev. 2 General Revision No. Sheets 21March 1997


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CONTENT

1 GENERAL

1.1 Scope1.2 Reference Normative1.2.1 European normative references1.2.2 Normative references of ISO, IEC and national organizations1.2.3 Normative references of other organizations

1.2.4 Company Normative

2 FUNCTIONAL REQUIREMENTS

2.1 Definitions2.2 Symbols and Abbreviations2.3 Operating Environment2.4 Functional Requirements for Piping2.4.1 Fabrication process2.4.2 Chemical composition2.4.3 Heat and product analyses2.4.4 Hardness2.4.5 Defect repair 2.4.6 Non complying pipes and fittings2.4.7 In service non complying pipes and fittings

2.5 Functional Requirements for Sheets for Pressure Vessels2.5.1 Fabrication process2.5.2 Chemical composition of sheets2.5.3 Heat and product analyses

2.5.4 Non destructive testing2.5.5 Hardness2.5.6 Defect repair 2.5.7 Non complying sheets2.5.8 Existing non complying pressure vessels2.5.9 Stepwise Cracking tests

2.6 Ergonomics2.7 Safety2.8 Documentation

2.8.1 Piping2.8.2 Pressure Wells Sheets


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3 ADDITIONAL INFORMATION

3.1 Annexes and appendixes. Corrosion mechanisms in H 2S containingenvironments

3.1.1 Sulphide Stress Cracking3.1.2 Hydrogen Induced Cracking3.1.3 Stress Oriented Hydrogen Induced Cracking (SOHIC) e Soft Zone Cracking (SZC)

3.2 Annexes and appendixes. H 2S containing environments3.2.1 Sour Conditions According to NACE3.2.2 Sour Conditions According to EFC


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1 GENERAL

1.1 Scope

This specification applies to materials for construction of pressure vessels for applications in H 2S containing environments.

The specification includes materials both as sheets and pipes.

Pipes and fittings are intended as made in carbon and low alloy steels and seamless.

In this document the requirements are provided to prevent Sulphide Stress Crackingand Hydrogen Induced Cracking phenomena.

The specified requirements shall be intended as additional to those ones reported in thefollowing Company Specifications: 05490.EQP.MEC.SDS; 00190.PIP.MME.SDSand 00191.PIP.MME.SDS.

The following topics are outside the scope of this document: requirements for tubular to applied downhole; fluid treatments to be performed in order to reduce their corrosivity; corrosion monitoring methods; requirements for materials different from carbon and low alloy steels.


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1.2 Reference Normative

1.2.1 European normative references

EN 10028 Flat products made of steels for pressure purposes. 1 st,2nd and 3 rd part.

UNI 663 Unalloyed steel seamless tubes. Plain ends tubes for general purposes. Qualities, requirements and tests.

1.2.2 Normative references of ISO, IEC and national organisations

ISO 6507 1-3 Metallic Materials. Hardness Test. Vickers Test.

1.2.3 Normative references of other organisations

NACE MR0175 Standard Material Requirements - Sulphide StressCracking Metallic Material for Oil Field Equipment.

NACE TM0177 Standard Test Method - Testing of Metals for Resistance to Sulfide Stress Cracking at AmbientTemperatures.

NACE TM0284 Standard Test Method - Evaluation of Pipeline Steelsfor Resistance to Stepwise Cracking.

EFC 16 Guidelines on Material Requirements for Carbon andLow Alloy Steels for H 2S Containing Oil and Gas fieldService. The Institute of Materials.

ASTM A 578 Standard Specification for Straight-Beam UltrasonicExamination of Plain and Clad Steel Plates for SpecialApplications.


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1.2.4 Company Normative

02555.VAR.COR.PRG Design criteria. Internal corrosion. Corrosion parametersand classification of the fluids.

00190.PIP.MME.SDS Pipes for pressure vessels.

00191.PIP.MME.SDS Sheets for pressure vessels.

05490.EQP.MEC.SDS Pressure vessels.

20197.VAR.COR.SDS Internal corrosion monitoring. Test point systems.

03587.MAT.COR.PRG Metallic materials in contact with H 2S containingenvironments. Corrosion tests methods and evaluationcriteria.


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2 FUNCTIONAL REQUIREMENTS

2.1 Definitions

Carbon steel An alloy of carbon and iron containing up to 2% carbon and up to 1.65% manganeseand residual quantities of other elements, except those intentionally added in specificquantities for deoxidation (usually silicon and/or aluminium).

Low alloy steel Steel with a total alloying element content lower than about 5 %. This definition includes

micro-alloyed steels containing small quantities of elements able to form carbides andnitrides, as niobium, titanium and vanadium

Micro-alloyed steel Steel with a total alloying element content lower than about 5 %.

H 2S containing environment See Par. 3.2 in this document.

Vickers Hardness

Vickers hardness is the recommended test for measuring hardness at welds. It is performed by applying a load ranging from 50 g to 30 kg on a pyramidal diamond tipand measuring the size of the impression after removing the load.

2.2 Symbols and Abbreviations

CE Equivalent CarbonHIC Hydrogen Induced CrackingSCC Stress Corrosion CrackingSSC Sulphide Stress CrackingSWC Stepwise CrackingSOHIC Stress Oriented Hydrogen Induced CrackingSZC Soft Zone Cracking

2.3 Operating Environment

Piping and pressure vessels shall be suitable for operating at the environmentalconditions existing at the installation site as specified in the design datasheet.

2.4 Functional Requirements for Piping

Pipes shall be fabricated with steels covered by UNI 663 norm and in accordance withCompany Specification 00190.PIP.MME.SDS.


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2.4.1 Fabrication process

The steel shall be produced to be resistant to HIC (Hydrogen Induced Cracking) andto SSC (Sulphide Stress Cracking).

The steel shall be produced by the basic oxygen process or by electric oven and withlow sulphur content.

All pipes and fitting shall be of the seamless type. The seamless pipes shall be hot rolledand supplies in the as rolled conditions, normalised or normalised and tempered.

2.4.1.1 Cold working

For any pipe and fitting, cold working operations leading to a permanent outer fibredeformation greater than 5 % are not admitted, if not followed by suitable heattreatment.

When the permanent outer fibre deformation is greater than 5 %, resistance to SulphideStress Cracking of cold worked materials shall be restored by normalisation andtempering heat treatment. Also the stress relieving heat treatment at temperature of 620C, or 650 C for micro-alloyed steels, is acceptable.

2.4.2 Chemical composition

2.4.2.1 Pipes

It is requested a sulphur content lower than 0.010 %.

The equivalent carbon calculated in accordance with the requirements of the EN 10028norm:

15 NiCu

5VMoCr

6Mn

CCE +

+++

++=

shall be lower than 0.43.

If products are proposed with greater equivalent carbon contents, but in any case lower than those one specified in the Company Specification 00190.PIP.MME.SDS, anexception can be granted based on the results of weldability and hardness tests.

The weld shall be performed with automatic technique by gas metal arc welding(GMAW) and at low heat input, not greater than 1,0 KJ/mm. The maximum hardness of


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the weld metal, at the fusion boundary and of the heat affected zone shall be within thelimits specified at Par. 2.4.4.

2.4.2.2 Fittings

It is requested a sulphur content lower than 0,025 %.

The equivalent carbon shall comply with the above specified limit.

2.4.3 Heat and product analyses

The heat analysis shall be performed on each heat.

The product analysis shall be performed on each lot of pipes belonging to the same heat.The number of product analysis shall be defined in the project documentation.

The above specified limits for sulphur and equivalent carbon shall be respected in bothheat and product analyses.

2.4.4 Hardness

2.4.4.1 Base metal

An hardness is requested lower than 220 HV10.

A hardness test shall be performed for each heat. The hardness test shall be performedon a through thickness metallographically prepared sample, at the following locations: in correspondence to the internal diameter; in correspondence to the midline; in correspondence to the external diameter.

2.4.4.2 Girth welds

During the welding procedure qualification, hardness tests shall be performed in order toverify the above specified limits.

Girth welds shall be performed by qualified personnel in accordance with proceduressuitable to meet the following hardness limits: root side in contact with process fluid: max 250 HV10 cap side, in contact with atmosphere: max 275 HV10

The hardness tests shall be performed in correspondence to: weld metal, fusion boundary and heat affected zone.


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Materials used for welding shall have a nickel content lower than 2.2 %.

Hardness tests on girth welds performed by sampling on site, if requested, shall bespecified in the project documentation.

The post-weld heat treatment, at a temperature of 620 C or 650 C for micro-alloyedsteels, is requested when the above specified maximum hardness limits are notrespected.

The post-weld heat treatment shall be qualified by the stress corrosion cracking testsspecified in the Company Specification 03587.MAT.COR.PRG.

Number and type of samples and sampling procedure shall be defined case by case.

2.4.5 Defect repair

Defect repair by welding of the base metal of pipes and fittings is not allowed.

2.4.6 Non complying pipes and fittings

Use of pipes and fittings not complying with the requirements fixed in this document isnot recommended. Use of not complying sheets is permitted, within the limits fixed inthis document, only for the following parameters: equivalent carbon, as in paragraph 2.4.2.1; hardness, as in paragraph 2.4.4.2.

2.4.7 In service non complying pipes and fittings

In case of plants with in service piping non complying with the requirements specified inthis document, the following tests shall be carried out: hardness tests; stress corrosion cracking tests; Hydrogen Induced Cracking tests.

The Stress Corrosion Cracking and Hydrogen Induced Cracking tests shall be performed in accordance with the Company Specification 03587.MAT.COR.PRG.

Number and type of samples as well as sampling procedure shall be defined case bycase.


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2.5 Functional Requirements for Sheets for Pressure Vessels

Sheets for the construction of pressure vessels shall be fabricated with steels complyingwith the norm EN 10028, Part 3 and in accordance with the Company Specification00191.PIP.MME.SDS.

2.5.1 Fabrication process

The steel shall be: produced by the basic oxygen process or by electric oven; with low sulphur content;

calcium treated for inclusion control.

Steel produced by continuous casting and degassed under-vacuum are preferred.

2.5.1.1 Cold working

In case the cold working of the sheets during the bending phase was greater than 5 %,same requirements defined for piping will apply.

2.5.2 Chemical composition of sheets

It is requested a sulphur content lower than 0.003 %.

The Ca/S ratio shall be greater than 2.

The equivalent carbon calculated with the CE formula reported at paragraph 2.4.2.1shall be below the following limits:

Grade P275 P355 and P460

CEV 0.40 0.43

If products are proposed with greater equivalent carbon contents, but in any case lower than those one specified in the Company Specification 00191.PIP.MME.SDS, anexception can be granted based on the results of weldability and hardness tests. Theweld shall be performed with the same procedure adopted for the construction of thevessel.

The maximum hardness of the weld metal, at the fusion boundary and of the heataffected zone shall be within the limits specified at Par. 2.5.5.2.


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2.5.3 Heat and product analyses

The heat analysis shall be performed on each heat.

The product analysis shall be performed on each sheet or part of it used for pressurevessel construction.

The above specified limits for sulphur and equivalent carbon shall be respected in bothheat and product analyses.

2.5.4 Non destructive testing

All the sheets shall be ultrasonically inspected in accordance with ASTM A578.

The inspection procedure and the defect acceptability level shall be in accordance towhat specified as Level 2.

In case of sheets with thickness greater than 100 mm, the additional requirementsspecified in ASTM A578 Section S8 shall be applied.

2.5.5 Hardness

2.5.5.1 Base metal

An hardness is requested lower than 220 HV10.

A hardness test shall be performed for each heat. The hardness test shall be performedon a through thickness sample, metallographically prepared in the following locations: in correspondence to the external and internal sides; in correspondence to the midline.

2.5.5.2 Welds

During the welding procedure qualification, hardness tests shall be performed in order toverify the above specified limits.

Welds shall be performed by qualified personnel in accordance with procedures suitableto meet the following hardness limits: root side in contact with process fluid: max 250 HV10 cap side, in contact with atmosphere: max 275 HV10


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The hardness tests shall be performed in correspondence to: weld metal, fusion boundary and heat affected zone, root pass, filling pass and cap pass.

Materials used for welding shall have a nickel content lower than 2.2 %.

The post-weld heat treatment, at 620 C or at 650 C for micro-alloyed steels, isrequested when the above specified maximum hardness limits are not respected.

The post-weld heat treatment shall be qualified by tests to be defined among those onesspecified in the Company Specification 03587.MAT.COR.PRG.

2.5.6 Defect repair

Defect repair by welding is not allowed.

2.5.7 Non complying sheets

Use of sheets not complying with the requirements fixed in this document is notrecommended. Use of not complying sheets is permitted, within the limits fixed in thisdocument, only for the following parameters: equivalent carbon, as in paragraph 2.5.2; hardness, as in paragraph 2.5.5.2.

Sheets not complying with the requirements for the fabrication process and for sulphur content fixed in this document, shall be positively pass the Hydrogen Induced Crackingresistance test (Stepwise Cracking test) in accordance with requirements andacceptance criteria defined in the Company Specification 03587.MAT.COR.PRG.

The test shall be performed for each non complying sheet. In case of sheets comingfrom the same heat, a test shall be performed for each non complying heat.

2.5.8 Existing non complying pressure vessels

In case of pressure vessels already in service and non complying with the requirementsspecified in this document, the following tests shall be carried out: Stress Corrosion Cracking tests; Stepwise Cracking tests.

The tests shall be performed in accordance with the Company Specification03587.MAT.COR.PRG.

Number and type of samples as well as sampling procedure shall be defined case bycase.


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Stepwise Cracking tests

Hydrogen Induced Cracking tests could be in any case requested in the projectdocumentation in connection with specific aspects including: high H 2S content in the fluids treated in the pressure vessels; high sheet thickness; design pressure.

2.6 Ergonomics

Not applicable.

2.7 Safety

Not applicable.

2.8 Documentation

2.8.1 Piping

Documentation stating the fabrication process of pipes and fittings;

certificates for heat analysis for pipes and fittings; certificates for product analysis for pipes and fittings; certificates of welding procedure qualification; certificates for base metal hardness test; certificates for girth weld hardness test.

2.8.1.1 Non Complying Pipes and Fittings

The following additional documentation is requested: certificates for hardness tests after post-weld heat treatment;

certificates for Stress Corrosion Cracking laboratory tests;

2.8.1.2 Existing Non Complying Piping

The following additional documentation is requested: certificates for Stress Corrosion Cracking laboratory tests.

2.8.2 Pressure Wells Sheets

Documentation stating the fabrication process; heat analyses certificates; product analyses certificates; certificates for welding procedure qualification; certificates for base metal hardness test;


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certificates for weld hardness test; certificates for non destructive test examination.

2.8.2.1 Non Complying Sheets

The following additional documentation is requested: certificates for hardness tests after post-weld heat treatment; certificates for Stress Corrosion Cracking tests; certificates for Stepwise Cracking tests.

2.8.2.2 Existing Non Complying Pressure Vessels

The following additional documentation is requested: certificates for Stress Corrosion Cracking tests; certificates for Stepwise Cracking tests.


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3 ADDITIONAL INFORMATION

3.1 Annexes and appendixes. Corrosion mechanisms in H 2S containingenvironments

3.1.1 Sulphide Stress Cracking

Sulphide Stress Cracking (SSC) occurs on susceptible metallic materials when specificconditions are met, normally represented by presence of H 2S and mechanical stresssolicitations, applied or residual.

With reference to the conditions met in the oil and gas production, the environmentswhich lead to SSC of susceptible materials, in particular carbon and low alloy steels, aredefined acid by H 2S or sour.

3.1.2 Hydrogen Induced Cracking

The term Hydrogen Induced Cracking includes both superficial damage phenomena(blistering) and crack phenomena occurring without any applied or residual stress

contribution, as stepwise micro cracks.

Such cracks occur when atomic hydrogen diffuses in the metal and then recombines ashydrogen molecular at trap sites in the steel matrix. Favourable trap sites are typicallyfound in rolled products along elongated inclusions or segregated bands of microstructure. The molecular hydrogen is trapped within the metal at interfaces

between the inclusions and the matrix and in microscopic voids, with first a crack initiation phase and then propagation along the metallurgical structures sensitive to thistype of hydrogen embrittlement.

As more hydrogen enters the voids the pressure rises, deforming the surrounding steelso that blisters may become visible at the surface. The steel around the crack becomeshighly strained and this can cause linking of adjacent cracks to form SWC. The array of cracks have a characteristic stepped appearance.

Whilst individual small blisters or hydrogen induced cracks do not affect the load bearing capacity of equipment they are an indication of a cracking problem which maycontinue to develop unless corrosion is stopped At the stage when cracks link up toform SWC damage these may serious affect the integrity of equipment. Failure due tothese types of cracking have arising within months of start-up. Control of themicrostructure reduces the availability of crack initiation sites and is therefore critical tothe control of SWC.


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Cathodic protection, when applied, opposes to hydrogen escape from the metal matrix.

The most susceptible materials are rolled steels containing inclusions in the metal matrix,in particular C-Mn steels containing manganese sulphide Type II inclusions.

The likelihood of HIC occurrence in seamless and forged pipes is lower than in welded pipes.

3.1.3 Stress Oriented Hydrogen Induced Cracking (SOHIC) e Soft Zone Cracking (SZC)

SOHIC and SZC are related to both SSC and SWC.

SOHIC and SZC are related to both SSC and SWC. In SOHIC staggered smallcracks are formed approximately perpendicular to the principal stress (applied or residual) resulting in a "ladder-like" crack array. The mode of cracking can becategorises as SSC caused by a combination of external stress and the local strainingaround hydrogen induced cracks. SOHIC has been observed in patent material of longitudinal welded pipe.

Soft Zone Cracking is the name given to a similar phenomenon when it occursspecifically in softened heat affected zones of welds in rolled plate steels. The

susceptibility of such weld regions to this type of cracking is thought to arise because of a combination of microstructural effects caused by the temperature cycling duringwelding and local softening in the intercritical temperature heat affected zone. Thisresults in strains within a narrow zone which may approach or even exceed the yieldstrain.

SOHIC has caused service failures of pipelines in the past but there are no reportedfailures by SOHIC in modern micro alloyed line pipe steels produced for service in H 2Swith mandatory testing for SWC and SSC.

3.2 Annexes and appendixes. H 2S containing environments

The requirements specified in this document applies to piping and pressure vessels in oiland gas treatment plants in contact with fluids containing H 2S at conditions defined sour conditions.

Sour conditions are defined in accordance to NACE MR0175 or to EFC 16, assummarised here below.

Conservatively, any environment shall be defined sour when the H 2S partial pressure,

pH 2S, is greater than 0.0035 bar (0.00035 MPa).


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3.2.1 Sour Conditions According to NACE

In accordance to NACE MR0175 the following conditions are defined sour:

gas systems P > 5 bar (0,5 MPa) and pH 2S > 0,0035 bar (0,00035 MPa)

multi-phase systems P > 20 bar (2 MPa) and pH 2S > 0,0035 bar (0,00035 MPa); 5 < P < 20 bar (0,5 < P < 2 MPa) and pH 2S > 0,65 bar (0,065 MPa); P < 5 bar (0,5 MPa) and yH 2S > 15 % (molar).

Figure 1. and 2. show graphically the above defined domains.

1

10

100

1000

0.0001 0.001 0.01 0.1 1 10

Hydrogen Sulphide molar fraction in gas phase yH2S - %

T o

t a l P r e

s s u r e -

b a Sour Service

Fig. 1. - Sour environments according to NACE. Gas systems.


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1

10

100

1000

0.0001 0.001 0.01 0.1 1 10 100

Hydrogen Sulphide molar fraction in gas phase xH2S - %

S y s

t e m

P r e s s u r e -

b a

Sour Service

Fig. 2. - Sour environments according to NACE. Multi-phase systems.

3.2.2 Sour Conditions According to EFC

The definition of sour environment according to EFC accounts for, beyond H 2S content,the pH of the water phase at operating conditions (pH in situ ).

In accordance with the EFC standard, the following environments, or domains, aredefined (see also fig. 3.):

sour environments domain according to EFC pH in-situ 3,5 pH 2S = 0,01 1 bar (0,001 0,1 MPa) and pH in-situ 5,5 + log pH 2S (bar) pH

2S > 1 bar (0,1 MPa) and pH

in-situ 5.5

sour / non-sour transition domain according to EFC pH 2S = 0,001 0,01 bar (0,0001 0,001 MPa) and 3,5 < pH in-situ 5.5 + log

pH 2S pH 2S > 1 bar (0,1 MPa) and pH in-situ = 5,5 6,5

The limits proposed by EFC were derived for P110 Grade and shall be consideredapplicable for steels normally used for pipes and for pressure vessel sheets.

The pH in-situ shall be determined as defined in the EFC 16 document, Annex C, or it can be calculated using suitable software programs.


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All situations outside the above specified limits define the non-sour domain, where alsosusceptible materials can be used.

Conservatively, conditions within the sour / non-sour transition domain shall be regardedas sour conditions.

2.5

3.5

4.5

5.5

6.5

7.5

0.0001 0.001 0.01 0.1 1 10 100

Hydrogen Sulphide Partial Pressure (pH2S - bar)

S o

l u t i o n p

H

Sour Service

Transition Region

N o n S o u r S e r v i c e

Fig. 3. Sour environments according to EFC.

ADDITIONAL REQUIREMENTS FOR PRESSURE VESSELS FOR APPLICATIONS IN H2S CONTAINING ENVIRONMENTS

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