Erosion and Erosion-Corrosion in Process Equipment and Piping

Summary:

Erosion and erosion-corrosion are forms of damage that can occur in process equipment and piping exposed to moving fluids and/or catalysts. These damages can cause a localized loss in thickness in the form of pits, grooves, gullies, waves, rounded holes, and valleys. The metal loss rates depend on various factors, including the velocity and concentration of the impacting medium, the size and hardness of the impacting particles, the hardness and corrosion resistance of the material subject to erosion, and the angle of impact.

Preventing and mitigating erosion and erosion-corrosion involve improvements in design, changes in shape, geometry, and materials selection, and utilizing impingement plates and specialized corrosion coupons. Visual examination and specialized corrosion monitoring electrical resistance probes are among the inspection and monitoring techniques used to detect the extent of metal loss.

Electrical Resistance (ER) probes and instruments determine metal loss from corrosion or erosion by the electrical resistance method.

1. Description of Damage

a) Erosion 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) Erosion-corrosion is a description of 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.

2. Materials Affected by Erosion-Corrosion

 All metals, alloys, and refractories are susceptible to erosion-corrosion.

3. Key Considerations

a) In most cases, erosion-corrosion occurs as a result of the combined action of mechanical erosion and chemical corrosion, with pure erosion (abrasive wear) being rare. Thus, it is important to consider the role of corrosion in contributing to damage.

b) Metal loss rates are influenced by several factors, including the velocity and concentration of the impacting medium (such as particles, liquids, droplets, slurries, and two-phase flow), the size and hardness of the impacting particles, the hardness and corrosion resistance of the material subject to erosion, and the angle of impact.

c) Softer alloys, such as copper and aluminum, may be more vulnerable to erosion-corrosion under high-velocity conditions due to their susceptibility to mechanical damage.

d) While increasing the hardness of the metal substrate is often considered a means to minimize damage, it may not necessarily improve resistance to erosion, especially if corrosion plays a significant role.

e) For each environment-material combination, there is usually a threshold velocity above which impacting objects may cause metal loss. Increasing velocities beyond this threshold results in higher metal loss rates, as shown in Table 1, which demonstrates the relative susceptibility of different metals and alloys to erosion-corrosion by seawater at varying velocities.

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

g) Increasing the corrosivity of the environment can reduce the stability of protective surface films and increase the susceptibility to metal loss. Metal can be removed from the surface as dissolved ions or solid corrosion products that are mechanically swept from the metal surface.

h) Factors that increase the corrosivity of the environment, such as temperature and pH, can also increase the susceptibility to metal loss.

Table 1– Typical erosion-corrosion rates in seawater (API 571 Section 4.2.14)

4. Affected Equipment and Components

a) Erosion and erosion-corrosion can affect all types of equipment exposed to moving fluids and catalysts. This includes piping systems, such as bends, elbows, tees, and reducers, as well as downstream piping systems from letdown valves and block valves. Additionally, pumps, blowers, propellers, impellers, agitators, agitated vessels, heat exchanger tubing, measuring device orifices, turbine blades, nozzles, ducts, vapor lines, scrapers, cutters, and wear plates can be affected.

b) Erosion can be caused by gas-borne catalyst particles or particles carried by a liquid, such as a slurry. Refineries are particularly susceptible to this type of damage, as it can occur in catalyst handling equipment (valves, cyclones, piping, reactors) and slurry piping in FCC reactor/regenerator systems, coke handling equipment in both delayed and fluidized bed cokers (figure 1), and as wear on pumps (figure 2 and figure 3), compressors, and other rotating equipment.

Figure 1: Erosion of a 9Cr coker heater return bend (API 571 Section 4.2.14)

Figure 2: Cast iron impeller in untreated cooling water after four years of service (API 571 Section 4.2.14)

Figure 3: Close-up of Figure 2 showing both erosion-corrosion at the vane tips and pitting on the pressure side of the vanes (API 571 Section 4.2.14).

c) Hydroprocessing reactor effluent piping may be subject to erosion-corrosion by ammonium bisulfide, with the degree of metal loss dependent on several factors, including the concentration of ammonium bisulfide, velocity, and alloy corrosion resistance.

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

5. Appearance or Morphology of Damage

a) Erosion and erosion-corrosion result in a localized loss of thickness, typically in the form of pits, grooves, gullies, waves, rounded holes, and valleys. These losses often exhibit a directional pattern.

b) Failures can occur quickly, making it crucial to address and monitor these types of damage.

6. Prevention and Mitigation Techniques for Erosion and Erosion-Corrosion

Erosion and erosion-corrosion can cause serious damage to equipment and structures, leading to costly repairs and downtime. Fortunately, there are several prevention and mitigation techniques that can be employed to minimize the impact of these damaging processes.

a) Design improvements are a crucial aspect of preventing erosion and erosion-corrosion. Changes in shape, geometry, and materials selection can all play a role. Examples of design improvements include increasing pipe diameter to decrease velocity, streamlining bends to reduce impingement, increasing wall thickness, and using replaceable impingement baffles.

b) Improving resistance to erosion is often achieved by increasing substrate hardness using harder alloys, hardfacing, or surface-hardening treatments. Erosion-resistant refractories, such as those used in cyclones and slide valves, have also been effective.

c) Erosion-corrosion can be mitigated by using more corrosion-resistant alloys and/or altering the process environment to reduce corrosivity. Techniques such as deaeration, condensate injection, or the addition of inhibitors can all help to reduce corrosion. It is important to note that increasing substrate hardness alone generally does not improve resistance to erosion-corrosion.

d) Heat exchangers can utilize impingement plates and tube ferrules to minimize erosion problems.

e) In applications where naphthenic acid corrosion is a concern, higher molybdenum-containing alloys can be used to improve resistance to this specific form of corrosion.

By employing these prevention and mitigation techniques, the impact of erosion and erosion-corrosion can be minimized, ensuring the longevity and reliability of equipment and structures.

7. Inspection and Monitoring

a) Metal loss can be detected through visual examination of suspected or troublesome areas as well as through ultrasonic (UT) or radiographic testing (RT).

b) In some applications, specialized corrosion coupons and on-line corrosion monitoring electrical resistance probes are used for monitoring purposes.

c) Infrared (IR) scans are employed to detect refractory loss in service.

8. Related Mechanisms

Specific terminology has been developed for various forms of erosion and erosion-corrosion in particular environments and/or services. These terms include cavitation, liquid impingement erosion, fretting, and other similar terms.

9. Conclusion

Erosion and erosion-corrosion can cause severe damage to equipment and facilities in the refining and petrochemical industry, leading to safety hazards, production losses, and increased maintenance costs. Effective prevention and mitigation measures are crucial to minimize the impact of these damaging mechanisms. This requires a combination of proper material selection, design improvements, and regular inspection and monitoring.

10. Future Scope

Further research and development in materials science and corrosion engineering can lead to more effective solutions for preventing erosion and erosion-corrosion in the refining and petrochemical industry. This includes the development of new alloys and coatings with improved erosion and corrosion resistance, as well as the advancement of non-destructive testing techniques for early detection of damage. Additionally, continued education and training for industry professionals on the importance of erosion and erosion-corrosion prevention and mitigation can help reduce the frequency and severity of incidents caused by these mechanisms.

11. References

1. ASM Metals Handbook, Volume 13, “Corrosion,” ASM International, Materials Park

2. ASM Metals Handbook, Volume 11, “Failure Analysis and Prevention,” ASM International, Metals Park.

12. Case Studies and Examples of Erosion and Erosion-Corrosion in Refining and Petrochemical Industry

Here are some examples of erosion and erosion-corrosion in the refining and petrochemical industry:

1. Erosion-corrosion in a crude oil unit atmospheric distillation column: In this case, the metal loss occurred due to the presence of naphthenic acid corrosion in the column's top tray. The damage was detected during an inspection, and the tray had to be replaced.
2. Erosion in FCC regenerator cyclones: The cyclones in an FCC regenerator were suffering from erosion due to the high-velocity flow of catalysts. The damage was addressed by replacing the existing cyclones with erosion-resistant ceramic cyclones.
3. Erosion-corrosion in hydroprocessing units: Ammonium bisulfide was causing erosion-corrosion in the effluent piping of hydroprocessing units. The problem was mitigated by using more corrosion-resistant alloys and altering the process environment.
4. Erosion in piping downstream of letdown valves: In a petrochemical plant, the piping downstream of letdown valves was experiencing erosion due to high-velocity flow of fluids. The problem was solved by installing replaceable impingement baffles.
5. Erosion-corrosion in FCC reactor feed nozzles: The nozzles in an FCC reactor were experiencing erosion-corrosion due to the high-velocity flow of catalysts. The damage was addressed by changing the material of the nozzles to a more erosion-resistant alloy.

By NTS