API 510 Chapter 7

API 510 Chapter 7 – API 571 Damage Mechanisms

7.1 API 571 introduction

This chapter covers the contents of API 571: Damage Mechanisms Affecting Fixed Equipment in the Refining Industry: 2003. API 571 has only recently been added to the syllabus for the API 570 and 510 examinations and replaces what used to be included in an old group of documents dating from the 1960s entitled IRE (Inspection of Refinery Equipment).

The first point to note is that the API 571 sections covered in the API 510 ICP exam syllabus are only an extract from the full version of API 571.

  • Temper embrittlement
  • Brittle fracture
  • Thermal fatigue
  • Erosion/corrosion
  • Mechanical fatigue
  • Atmospheric corrosion
  • Corrosion under insulation (CUI)
  • Cooling water corrosion
  • Boiler condensate corrosion
  • Sulphidation
  • Chloride SCC
  • Corrosion fatigue
  • Caustic SCC/caustic embrittlement
  • Wet H2S damage (HIC/SOHIC)
  • High-temperature hydrogen attack (HTHA)

Figure 7.1 The 15 damage mechanisms from API 571 in the API 510 exam syllabus

Figure 7.2 API 571 DMs revised order
Figure 7.2 API 571 DMs revised order

7.1.1 The 15 damage mechanisms

Your API 510 exam copy of API 571 contains (among other things) descriptions of 15 damage mechanisms (we will refer to them as DMs). They are shown in Fig. 7.1. Remember that these are all DMs that are found in the petrochemical/refining industry (because that is what API 571 is about), so they may or may not be found in other industries. Some, such as brittle fracture and fatigue, are commonly found in non-refinery plant whereas others, such as sulphidation, are not.

7.1.2 Are these DMs in some kind of precise logical order?

No, or if they are, it is difficult to see what it is. The list contains a mixture of high- and low-temperature DMs, some of which affect plain carbon steels more than alloy or stainless steels and vice versa. There are also several various subdivisions and a bit of repetition thrown in for good measure. None of this is worth worrying about, as the order in which they appear is not important.

In order to make the DMs easier to remember you can think of them as being separated into three groups. There is no code-significance in this rearrangement at all; it is simply to make them easier to remember. Figure 7.2 shows the revised order.

One important feature of API 571 is that it describes each DM in some detail, with the text for each one subdivided into

REMEMBER THE WAY THAT API 571 COVERS EACH OF THE DAMAGE MECHANISMS

Figure 7.3 API 571 coverage of DMs
Figure 7.3 API 571 coverage of DMs

Caused by hydro- testing and/or operating below the Charpy impact transition temperature

Figure 7.4 Brittle fracture
Figure 7.4 Brittle fracture

On a macro scale, thermal fatigue cracks tend to be dagger shaped, wide and oxide-filled (caused by the oxidizing effect of the temperature cariations)

Figure 7.5 Thermal fatigue
Figure 7.5 Thermal fatigue

six subsections. Figure 7.3 shows the subsections and the order in which they appear.

These six subsections are important as they form the subject matter from which the API examination questions are taken. As there are no calculations in API 571 and only a few graphs etc. of detailed information, you can expect most of the API examination questions to be closed book, i.e. a test of your understanding and short-term memory of the DMs. The questions could come from any of the six subsections shown in Fig. 7.3.

7.2 The first group of DMs

Figures 7.4 to 7.7 relate to the first group of DMs in API 571. When looking through these figures, try to cross-reference them to the content of the relevant sections of API 571. Then read the full sections of API 571 covering the four DMs in this first group.

Figure 7.6 Mechanical fatigue
Figure 7.6 Mechanical fatigue
Figure 7.7 Corrosion fatigue
Figure 7.7 Corrosion fatigue

7.4 The second group of DMs Figures 7.8 and 7.9 relate to the second group of DMs. Note how these DMs tend to be process environment-related. Remember to identify the six separate subsections in the text for each DM.

High fluid Velocities cause scouring. Bends and welds are particularly suscptible

Figure 7.8 Erosion/corrosion
Figure 7.8 Erosion/corrosion

CUI hides under lagging , and is often widespread

Figure 7.9 Corrosion under insulation (CUI)
Figure 7.9 Corrosion under insulation (CUI)

Chloride contamination (from water or lagging) makes cui much worse

7.6 The third group of DMs

Now look through Figs 7.10 to 7.15 covering the final group of DMs. These DMs tend to be either more common at higher temperatures or a little more specific to refinery equipment than those in the previous two groups. Again, remember to identify the six separate subsections in the text for each DM, trying to anticipate the type of examination questions that could result from the content.

This is high-temperature corrosion mechanism

Figure 7.10 Sulphidation corrosion

The main problem is caused by H2S (formed by the degradation of sulphur compounds at high temperature) Occurs in crude plant, cokers, hydroprocessor units, fired heaters, etc. – anywhere wheree there are high-temperature sulphur stream

Figure 7.10 Sulphidation corrosion

API Terminology calls it Environmental-assisted cracking

Figure 7.11 Stress corrosion cracking (SCC)
Figure 7.11 Stress corrosion cracking (SCC)

A Specialist type of stress corrosion cracking caused by alkaline condition. The Worst Offenders are:

                                                          *Caustic Potash (KOH)

                                                          *Sodium hydroxide (NaOH)

Caustic attack in aheat exchanger tubesheet

Figure 7.12 Caustic embrittlement
Figure 7.12 Caustic embrittlement

Typically Found in H2S removal units and acid neutralization units

This is a specialist and complex corrosion mechanism

Figure 7.13 High-temperature hydrogen attack (HTHA)
Figure 7.13 High-temperature hydrogen attack (HTHA)

Figure 7.14 Wet H2S damage

All result in blistering or cracking of low carbon and low alloy steels

These wet H2S DMs have quite complex descriptions and critical factors. Please refer to API 571(5.1.2.3)

Figure 7.14 Wet H2S damage

This is a specialist type of brittle fracture, not related to low temperatures

Figure 7.15 Temper embrittlement
Figure 7.15 Temper embrittlement

Attempt this first set of self-test questions covering the first group of API 571 DMs.

7.3 API 571 familiarization questions (set 1)

7.5 API 571 familiarization questions (set 2)

1.

Q1. API 571 section 4.2.7.1: brittle fracture
Which of these is a description of brittle fracture?

 
 
 
 

2.

Q2. API 571 section 4.2.7.2: brittle fracture: affected materials
Which of these materials are particularly susceptible to brittle fracture?

 
 
 
 

3.

Q3. API 571 section 4.2.7.3: brittle fracture: critical factors
At what temperature is brittle fracture most likely to occur?

 
 
 
 

4.

Q4. API 571 section 4.2.7.4: brittle fracture
Which of these activities is unlikely to result in a high risk of brittle fracture?

 
 
 
 

5.

Q5. API 571 section 4.2.7.6: brittle fracture: prevention/mitigation
What type of material change will reduce the risk of brittle fracture?

 
 
 
 

6.

Q6. API 571 section 4.2.7.5: brittle fracture: appearance
Cracks resulting from brittle fracture will most likely be predominantly:

 
 
 
 

7.

Q7. API 571 section 4.2.9: thermal fatigue: description
What is thermal fatigue?

 
 
 
 

8.

Q8. API 571 section 4.2.9.3: thermal fatigue: critical factors
As a practical rule, thermal cracking may be caused by temperature swings of approximately:

 
 
 
 

9.

Q9. API 571 section 4.2.9.5: thermal fatigue: appearance
Cracks resulting from thermal fatigue will most likely be predominantly:

 
 
 
 

10.

Q10. API 571 section 4.2.9.6: prevention/mitigation
Thermal fatigue cracking is best avoided by:

 
 
 
 

11.

7.5 API 571 familiarization questions (set 2)

Q1. API 571 section 4.2.14
A damage mechanism that is strongly influenced by fluid velocity and the corrosivity of the process fluid is known as:

 
 
 
 

12.

Q2. API 571 section 4.3.2: atmospheric corrosion
As a practical rule, atmospheric corrosion:

 
 
 
 

13.

Q3. API 571 section 4.3.2.3: atmospheric corrosion: critical factors
A typical atmospheric corrosion rate in mils (1 mil = 0.001 inch) per year (mpy) of steel in an inland location with moderate precipitation and humidity is:

 
 
 
 

14.

Q4. API 571 section 4.3.3.3: CUI critical factors
Which of these metal temperature ranges will result in the most severe CUI?

 
 
 
 

15.

Q5. API 571 section 4.3.3.6: CUI appearance
Which other corrosion mechanism often accompanies CUI in 300 series stainless steels?

 
 
 
 

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