4.2.14 Erosion/Erosion – Corrosion

Question 1

4.2.14.1 Description of Damage

a) ___1___ is the accelerated mechanical removal of surface material as a result of relative movement
between, or impact from solids, liquids, vapor or any combination thereof.

b) ______2______ is a description for the damage that occurs when corrosion contributes to erosion
by removing protective films or scales, or by exposing the metal surface to further corrosion under the combined action of erosion and corrosion.

Answer 1

Answer

1. Erosion
2. erosion- corrision

Question 2

b) Metal loss rates depend on the velocity and _____1_____ of impacting medium (i.e., particles,
liquids, droplets, slurries, two-phase flow), the size and ____2____ of impacting particles, the hardness and corrosion resistance of material subject to erosion, and the __3__ of impact.

Answer 2

Answer

1. concentration
2. hardness
3. angle

Question 3

c) Softer alloys such as copper and aluminum alloys that are easily worn from mechanical damage may be subject to severe _____1____ under high velocity conditions.

d) Although increasing hardness of the metal substrate is a common approach to ___2___ damage, it is not always a good indicator of improved resistance to erosion, particularly where ____3____ plays a significant role.

Answer 3

Answer

1. metal loss
2. minimize
3. corrosion

Question 4

e) For each environment-material combination, there is often a ____1___ velocity above which impacting objects may produce metal loss. Increasing velocities above this threshold result in an increase in ____2___ rates as shown in Table 4-5. This table illustrates the relative susceptibility of a variety of metals and alloys to erosion/corrosion by seawater at different velocities.

f) The size, shape, density and hardness of the impacting ________ affect the metal loss rate.

Answer 4

Answer

1. threshold
2. metal loss
3. medium

Question 5

g) Increasing the corrosivity of the environment may reduce the stability of _________1_________ and increase the susceptibility to metal loss. Metal may be removed from the surface as dissolved __2__, or as solid corrosion ___3___ which are mechanically swept from the metal surface.

h) Factors which contribute to an increase in corrosivity of the environment, such as temperature, pH, etc., can increase ____4____ to metal loss.

Answer 5

Answer

1. protective surface films
2. ions
3. products
4. susceptibility

Question 6

4.2.14.4 Affected Units or Equipment

a) ___1____ of equipment exposed to moving fluids and/or catalyst are subject to erosion and erosion-corrosion. This includes piping systems, particularly the bends, ___2___, tees and ____3___; piping systems _____4____ of letdown valves and block valves; ___5___; blowers; propellers; impellers;
agitators; agitated vessels; heat exchanger tubing; measuring device orifices; turbine blades;
nozzles; ducts and vapor lines; scrapers; cutters; and wear plates.

Answer 6

Answer

1. All types
2. elbows
3. reducers
4. downstream
5. pumps

Question 7

b) Erosion can be caused by gas borne catalyst particles or by particles carried by a liquid such as a __1__. In refineries, this form of damage occurs as a result of catalyst ____2____ in FCC reactor/regenerator systems in catalyst handling equipment (valves, cyclones, piping, reactors) and slurry piping (Figure 4-43); coke handling equipment in both delayed and fluidized bed cokers (Figure 4-44); and as wear on pumps (Figure 4-45), compressors and other rotating equipment.

Answer 7

Answer

1. slurry
2. movement

Question 8

c) Hydroprocessing reactor effluent piping may be subject to erosion-corrosion by ammonium ___1___.
The metal loss is dependent on several factors including the ammonium bisulfide ______2______, velocity and alloy corrosion resistance.

d) Crude and vacuum unit piping and vessels exposed to naphthenic __3__ in some crude oils may suffer severe erosion-corrosion metal loss depending on the temperature, velocity, sulfur content and TAN level.

Answer 8

Answer

1. bisulfide
2. concentration
3. acids

Question 9

4.2.14.5 Appearance or Morphology of Damage

a) Erosion and erosion-corrosion are characterized by a localized loss in thickness in the form of _1_, grooves, gullies, waves, rounded __2__ and valleys. These losses often exhibit a ____3____ pattern.

b) Failures can occur in a relatively __4__ time.

Answer 9

Answer

1. pits
2. holes
3. directional
4. short

Question 10

4.2.14.6 Prevention / Mitigation

a) Improvements in design involve changes in shape, geometry and materials ____1____. Some examples are: increasing the pipe diameter to ___2___ velocity; streamlining bends to reduce impingement; increasing the wall thickness; and using replaceable _____3_____ baffles.

Answer 10

Answer

1. selection
2. decrease
3. impingement

Question 11

b) Improved resistance to erosion is usually achieved through increasing substrate ____1____ using harder alloys, hardfacing or surface-hardening treatments. Erosion resistant refractories in cyclones and slide valves have been very ____2____.

Answer 11

Answer

1. hardness
2. successful

Question 12

c) Erosion-corrosion is best mitigated by using more corrosion-resistant __1__ and/or altering the
process environment to reduce ____2____, for example, deaeration, condensate injection or the addition of inhibitors. Resistance is generally not improved through increasing substrate hardness alone.

Answer 12

Answer

1. alloys
2. corrosivity

Question 13

d) Heat exchangers utilize impingement __1__ and occasionally tube ferrules to minimize erosion
problems.

e) Higher _____2_____ containing alloys are used for improved resistance to naphthenic acid corrosion.

Answer 13

Answer

1. plates
2. molybdenum

Question 14

4.2.14.7 Inspection and Monitoring

a) Visual examination of suspected or troublesome areas, as well as __1__ checks or _2_ can be used to detect the extent of metal loss.

b) Specialized corrosion ___3___ and on-line corrosion monitoring electrical resistance __4__ have been used in some applications.

c) IR scans are used to detect ____5____ loss on stream.

Answer 14

Answer

1. UT
2. RT
3. coupons
4. probes
5. refractory

Question 15

4.2.14.8 Related Mechanisms

Specialized terminology has been developed for various forms of erosion and erosion-corrosion in specific environments and/or services. This terminology includes cavitation, liquid impingement ___1___, fretting and other similar terms.

Answer 15

Answer

1. erosion

Question 16

4.2.14.9 References

1. ASM Metals Handbook, Volume 13, “Corrosion,” ASM International, Materials Park, OH.
2. ASM Metals Handbook, Volume 11, “Failure Analysis and Prevention,” ASM International, Metals Park, OH.

Question 17

4.2.16.2 Affected Materials

All engineering alloys are subject to fatigue cracking although the stress levels and number of cycles necessary to cause failure vary by ____1____.

Answer 17

Answer

1. material

Question 18

4.2.16.3 Critical Factors

Geometry, stress __1__, number of cycles, and material ____2____ (strength, hardness, microstructure) are
the predominant factors in determining the fatigue resistance of a component.

Answer 18

Answer

1. level
2. properties

Question 19

4.2.16 Mechanical Fatigue

4.2.16.1 Description of Damage

a) Fatigue ___1___ is a mechanical form of degradation that occurs when a component is exposed to cyclical stresses for an extended period, often resulting in sudden, unexpected ___2___.

b) These stresses can arise from either mechanical ___2___ or thermal ___4__ and are typically well
below the yield strength of the material.

Answer 19

Answer

1. cracking
2. failure
3. loading
4. cycling

Question 20

a) Design: Fatigue cracks usually initiate on the surface at ___1___ or stress raisers under cyclic loading. For this reason, design of a component is the most important factor in determining a component’s resistance to fatigue ___2___. Several common surface features can lead to the initiation of fatigue cracks as they can act as stress concentrations. Some of these common features are:

Answer 20

Answer

1. notches
2. cracking

Question 21

• *1)** Mechanical ___1___ (sharp corners or groves);
• *2)** Key holes on drive shafts of ___2___ equipment;
• *3)** Weld joint, __3__ and/or mismatches;
• *4)** Quench nozzle __4__;
• *5)** _5_ol markings;
• *6)** Grinding __6__;
• *7)** Lips on drilled __7__;
• *8)** Thread root __8__hes;
• *9)** ___9___osion.

Answer 21

Answer

1. notches
2. rotating
3. flaws
4. areas
5. To
6. marks
7. holes
8. notc
9. Corr

Question 22

b) Metallurgical Issues and Microstructure

1) For some materials such as titanium, carbon steel and low alloy steel, the number of cycles to fatigue fracture ____1____ with stress amplitude until an endurance limit reached. Below this stress endurance limit, fatigue cracking will not occur, regardless of the number of cycles.

Answer 22

Answer

1. decreases

Question 23

2) For alloys with endurance limits, there is a correlation between Ultimate Tensile Strength (UTS) and the minimum stress amplitude necessary to initiate fatigue ____1____. The ratio of endurance limit over UTS is typically between _2_ and _3_. Materials like austenitic stainless steels and aluminum that do not have an endurance limit will have a fatigue limit defined by the number of __4__ at a given stress amplitude.

Answer 23

Answer

1. cracking
2. 0.4
3. 0.5.
4. cycles

Question 24

3) ____1____ found in metal can have an accelerating effect on fatigue cracking. This is of importance when dealing with older, “dirty” steels or weldments, as these often have ____1____ and discontinuities that can degrade fatigue resistance.

Answer 24

Answer

1. Inclusions

Question 25

4) __1__ treatment can have a significant effect on the toughness and hence fatigue resistance of a metal. In general, finer grained microstructures tend to perform better than coarse grained. __1__ treatments such as quenching and tempering, can improve fatigue resistance of carbon and low alloy steels.

Answer 25

Answer

1. Heat

Question 26

c) Carbon Steel and Titanium: These materials exhibit an endurance limit below which fatigue cracking will not occur, regardless of the number of ___1___.

Answer 26

Answer

1. cycles

Question 27

d) 300 Series SS, 400 Series SS, aluminum and most other non-ferrous alloys:

• *1)** These alloys have a fatigue characteristic that does not exhibit an ____1____ limit. This means that fatigue fracture can be achieved under cyclical loading eventually, regardless of stress amplitude.
• *2)** Maximum cyclical stress amplitude is determined by relating the stress necessary to cause ___2___ to the desired number of cycles necessary in a component’s lifetime. This is typically 106 to 107 cycles.

Answer 27

Answer

1. endurance
2. fracture

Question 28

4.2.16.4 Affected Units or Equipment

a) Thermal Cycling

1) Equipment that cycles daily in operation such as __1__ drums.
2) Equipment that may be auxiliary or on continuous standby but sees intermittent service such as ___2___ boiler.
3) Quench nozzle connections that see significant temperature deltas during operations such as
__3__ washing systems.

Answer 28

Answer

1. coke
2. auxiliary
3. water

Question 29

b) Mechanical Loading

1) Pressure Swing Absorbers on hydrogen purification __1__.

2) Rotating shafts on centrifugal __2__ and compressors that have stress concentrations due to changes in radii and key ways.

Answer 29

Answer

1. units
2. pumps

Question 30

3) Components such as __1__ diameter piping that may see vibration from adjacent equipment and/or wind. For small components, resonance can also produce a __2__ load and should be taken into consideration during design and reviewed for potential problems after installation.

4) High pressure drop control __3__ or steam reducing stations can cause serious vibration problems in connected piping.

Answer 30

Answer

1. small
2. cyclical
3. valves

Question 31

4.2.16.5 Appearance or Morphology of Damage

a) The signature mark of a fatigue failure is a “clam shell” type fingerprint that has concentric rings called “____1______” emanating from the crack initiation site (Figure 4-50 and Figure 4-51). This signature pattern results from the “waves” of crack propagation that occur during cycles above the ____2____ loading. These concentric cracks continue to propagate until the cross-sectional area is reduced to the point where failure due to overload occurs.

Answer 31

Answer

1. beach marks
2. threshold

Question 32

b) Cracks nucleating from a surface stress concentration or defect will typically result in a single “clam shell” fingerprint (Figure 4-52 to Figure 4-56).

c) Cracks resulting from cyclical overstress of a component without significant stress concentration will
typically result in a fatigue failure with multiple points of nucleation and hence multiple “clam shell” fingerprints. These multiple nucleation sites are the result of microscopic yielding that occurs when the component is momentarily cycled above its yield strength.