Erosion-Wear | Materials And Corrosion Control
| Damage Mechanism | Erosion/Wear |
Damage Description |
Erosion/wear is the accelerated mechanical removal of surface material as a result of relative movement between, or impact from particles, liquids, droplets, slurries, and two-phase flow. Erosion is characterized by a localized loss in thickness in the form of pits, grooves, gullies, waves, rounded holes and valleys. These losses often exhibit a directional pattern. Failures can occur in a relatively short time.
· Metal loss rates depend on: o The velocity and concentration of impacting medium o The size, shape, density and hardness of impacting particles o The angle of impact of the particles o The hardness and corrosion resistance of material subject to erosion. · Softer alloys such as copper and aluminum alloys that are easily worn from mechanical damage may be subject to severe metal loss under high velocity conditions. · Increasing hardness of the metal substrate is not always a good indicator of improved resistance to erosion, particularly where corrosion plays a significant role. · For each environment-material combination, there is often a threshold velocity above which impacting objects may produce metal loss. |
Materials & Equipment |
Materials
All metals, alloys and refractories. Equipment Most prevalent at changes in direction, pressure drop locations or where there is extreme turbulence/velocity. FCC Catalyst – Cyclones, cyclone dip legs, flapper and slide valves, catalyst and flue gas piping, and the fractionators bottoms circuit (pumps, piping, heat exchanger tubes), Small cracks in refractories can propel catalyst at a very high velocity eroding the vessel wall. Cokers – Heater tube bends or where injection quills and thermowells jet out into the coke solids stream. |
Control Methodology |
· Improvements in design involve changes in shape, geometry and materials selection e.g., increasing the pipe diameter to decrease velocity; streamlining bends to reduce impingement; increasing the wall thickness; and using replaceable impingement baffles.
· Improved resistance to erosion is usually achieved through increasing substrate hardness using harder alloys, hardfacing or surface-hardening treatments. Erosion resistant refractories in cyclones and slide valves have been very successful. · Heat exchangers utilize impingement plates and occasionally tube ferrules to minimize erosion problems. |
Monitoring Techniques |
· Visual examination of suspected or troublesome areas, as well as UT checks or RT can be used to detect the extent of metal loss.
· IR scans are used to detect refractory loss on stream. |
Inspection Frequency |
· Periodically at T&Is
· On-Stream Inspection schedule |
KPIs |
None |
Reference |
NACE International Course – Corrosion Control in the Refinery Industry. |
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