Api 571 Damage Mechanism Questions [PDF]

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API 571:CONTENTS

Now we will look at the last group of damage mechanisms covered by API 571 4.2.3 – Temper Embrittlement 4.2.7 – Brittle Fracture 4.2.9 – Thermal Fatigue 4.2.14 – Erosion/Erosion-Corrosion 4.2.16 – Mechanical Fatigue 4.3.2 – Atmospheric Corrosion 4.3.3 – Corrosion Under Insulation (CUI) 4.3.4 – Cooling Water Corrosion 4.3.5 – Boiler Water Condensate Corrosion 4.4.2 – Sulfidation 4.5.1 – Chloride Stress Corrosion Cracking (Cl-SCC) 4.5.2 – Corrosion Fatigue 4.5.3 – Caustic SCC (Caustic Embrittlement) 5.1.2.3 – Wet H2S Damage (Blistering/HIC/SOHIC/SCC) 5.1.3.1 – High Temperature Hydrogen Attack (HTHA

These are fairly complicated corrosion mechanisms,many of them related to higher temperature and oil/gas industry/ refinery applications Slide 1

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REMEMBER THE WAY THAT API 571 COVERS EACH OF THE MECHANISMS

Description/appearance of the damage mechanism

Critical factors

Affected equipment Prevention/ mitigation

Related mechanisms

Inspection/ monitoring Slide 2

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SULFIDATION

This is a high temperature corrosion mechanism WARNING: This is a common closed-book exam topic

Carbon and alloy steels

+

Sulphur compounds

+ High temperatures (260 degC +)

= Sulfidation corrosion Slide 3

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SULFIDATION

API EXAMINATIONS NEARLY ALWAYS HAVE QUESTIONS ABOUT SULFIDATION

What is it?

Slide 4 © Matthews Engineering Training Ltd

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SULFIDATION

HS

The main problem is caused by 2 (formed by the degradation of Sulphur compounds at high temperature) Occurs in crude plant,cokers,hydroprocessor units,fired heaters etc….anywhere where there are high temperature sulphur streams Sulfidation starts to degrade steels abve about 500degF (260 degC) Slide 5 © Matthews Engineering Training Ltd

SULFIDATION

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The susceptibility of steels to Sulfidation is shown in the McConomy curves shown in API 571

As the temperature rises above 500 degF,the sulfidation corrosion rate goes up

Watch out for exam questions on this © Matthews Engineering Training Ltd

Slide 6

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SULFIDATION

MITIGATION Higher Cr alloys(300-400 series stainless steels) may be more resistant to sulfidation corrosion

Slide 7 © Matthews Engineering Training Ltd

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STRESS CORROSION CRACKING API terminology also calls it ‘Environmental–assisted cracking’ •One of the most common corrosion mechanisms

316

304

•Prevalent in 300 series austenitic stainless steel and high chromium alloys

Where does the

come from?

Often from residual stresses caused by welding Slide 8 © Matthews Engineering Training Ltd

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STRESS CORROSION CRACKING The stress exposes the grain boundaries to corrosion

Temperature range above 60 degC (140 degF) and pH>2 © Matthews Engineering Training Ltd

Slide 9

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SCC Rule of thumb

In many cases ,SCC is due to

Salt

Chlorides attaching the stainless steel Slide 10 © Matthews Engineering Training Ltd

API 510 CERT SCC CAN BE VERY DIFFICULT TO DETECT….ALMOST IMPOSSIBLE IN ITS EARLY STAGES UT will not show very small SCC cracks

Neither will RT

Slide 11 © Matthews Engineering Training Ltd

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CLASSIC SCC BRANCHED CRACKS

Also known as bifurcated cracks Slide 12 © Matthews Engineering Training Ltd

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SCC DETECTED BY PT

Surface abrasion may be needed before PT in order to show fine SCC cracks

Cracks would remain hidden without surface abrasion Slide 13 © Matthews Engineering Training Ltd

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CAUSTIC EMBRITTLEMENT

A specialist type of SCC caused by alkaline conditions

The worst offenders are : •Sodium Hydroxide (NaOH) •Caustic Potash (KOH) Caustic attack in a heat exchanger tubesheet

Typically found in H2S removal units and acid neutralisation units Slide 14 © Matthews Engineering Training Ltd

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CAUSTIC EMBRITTLEMENT

Cracks normally start from the surface of the material

Look at Fig 4-85 in API 571 showing how temperature and NaOH concentration affects the susceptibility of Carbon steel to NaOH

Slide 15 © Matthews Engineering Training Ltd

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CORROSION FATIGUE Cracks caused by a combination of:

Corrosion

+

Cyclic loadings

These cracks often initiate at pits or under deposits Slide 16 © Matthews Engineering Training Ltd

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CORROSION FATIGUE

Secondary brittle fracture

Fatigue cracks in corroded area: initiated the failure Slide 17 © Matthews Engineering Training Ltd

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CORROSION FATIGUE AN IMPORTANT POINT

Unlike normal fatigue, there is no endurance limit for corrosion-assisted fatigue

Stress S

UTS

No endurance limit Cycles N © Matthews Engineering Training Ltd

Slide 18

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WET H2S DAMAGE

API 571 Sec 5.1.2.3 identifies

4 damage mechanisms

They affect carbon steels and low alloys steels in wet H2S environments Sulfide SCC

Hydrogen blistering Hydrogen induced cracking (HIC)

Stress Oriented Hydrogen induced cracking (SOHIC) Slide 19

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WET H2S DAMAGE

The actual damage mechanism for all 4 categories is the permeation of the H2 into the material’s grain boundaries This weakens the material and causes failure

The type of wet H2S damage that occurs is related to these factors(see 570 Sec 5.1.2.3..3.) •pH •The H2S level present •Temperature •Hardness •Type of steel •PWHT (an important one) Slide 20 © Matthews Engineering Training Ltd

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WET H2S DAMAGE

Hydrogen blistering The Hydrogen is liberated from corrosion (not the process fluid)

It weakens the material structure causing a blister (and eventual failure) © Matthews Engineering Training Ltd

Slide 21

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WET H2S DAMAGE

HIC Sometimes called ‘stepwise cracking’ as hydrogen causes cracks in the structure

Can be worse near a weld

The cracks weaken the structure and cause failure) Slide 22 © Matthews Engineering Training Ltd

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WET H2S DAMAGE

SOHIC Stress Oriented Hydrogen induced cracking (SOHIC)

Occurs in HAZ at weld toes

A type of HIC in which the cracks are made worse by stress concentrations © Matthews Engineering Training Ltd

Slide 23

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WET H2S DAMAGE

Sulfide SCC Essentially ……..SCC made worse by the presence of water and H2S Weld preheat and PWHT can help reduce the risk (depending on the alloy)

Can appear in areas of high hardness (e.g. in welds) Slide 24 © Matthews Engineering Training Ltd

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HIGH TEMPERATURE HYDROGEN ATTACK(HTHA)

This is a specialist and complex corrosion mechanism

In simple terms: At high temperatures,H2 reacts with the Carbon in the steel forming Ch4 (Methane) The resulting loss of Carbides weakens the steel

Fissures start to form,and propagate into cracks Slide 25 © Matthews Engineering Training Ltd

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HTHA TUBE FAILURE Thick-walled tube

failure

A neat rectangular section is ‘blown out’ without any bulging

HTHA has attacked the grain boundaries © Matthews Engineering Training Ltd

Slide 26

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API 571 SUMMARY

In these 3 presentations we have looked at all of the mechanisms in API 571 that are in the API 510 exam syllabus 4.2.3 – Temper Embrittlement 4.2.7 – Brittle Fracture 4.2.9 – Thermal Fatigue 4.2.14 – Erosion/Erosion-Corrosion 4.2.16 – Mechanical Fatigue 4.3.2 – Atmospheric Corrosion 4.3.3 – Corrosion Under Insulation (CUI) 4.3.4 – Cooling Water Corrosion 4.3.5 – Boiler Water Condensate Corrosion 4.4.2 – Sulfidation 4.5.1 – Chloride Stress Corrosion Cracking (Cl-SCC) 4.5.2 – Corrosion Fatigue 4.5.3 – Caustic SCC (Caustic Embrittlement) 5.1.2.3 – Wet H2S Damage (Blistering/HIC/SOHIC/SCC) 5.1.3.1 – High Temperature Hydrogen Attack (HTHA

NEXT STEP

Now finish off the module text and try the test questions Slide 27

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