Erosive wear
Introduction
Erosive wear results from the impact of particles against surfaces. Typical erosive wear situations in bulk solids handling plant are in the loading and off-loading of materials, and with free fall onto surfaces. The blowing of materials into cyclones, their loading into hoppers and onto chutes, off-loading from hoppers, conveyor belts and bucket ele- vators, are common examples. These are all cases where particles impact against sur- faces and cause erosive wear, rather than slide against a retaining surface and cause abrasive wear.
Erosion represents a major problem, not only in bulk solids handling plant, but in many other areas. In thermal power plant pulverized fuel causes erosive wear of supply lines and nozzles, and the resulting fly ash is a problem with respect to boiler tubes. Both pneumatic and hydraulic conveying of particulate materials in pipelines can result in severe erosion problems, and aircraft, rockets and missiles are eroded by rain drops and ice particles. The area that has probably received most attention, how- ever, is aircraft engines, and in particular helicopters, for dust ingestion can cause con- siderable damage, and has resulted in several catastrophic failures in service.
It is a major feature in the wear of pneumatic and hydraulic pipeline transport sys- tems. In pneumatic conveying, in particular, it can be a severe problem because of the high velocities required for conveying bulk particulate materials. The erosion of sur- faces by solid particles in a fluid stream is probably the main reason why industry is often reluctant to install pneumatic conveying systems, particularly when abrasive materials have to be handled. In several other areas, however, erosion has many prac- tical uses and advantages, such as in erosive cleaning of surfaces and erosive drilling and cutting.
Data sources
Information on erosive wear comes from a very wide range of sources, therefore. Until recent years little was known of the fundamental mechanisms of the erosion process or of the variables that influence the problem. There are, in fact, so many vari- ables that influence the problem that advances have only been made by the develop- ment and use of specially designed erosive wear testing rigs. In these a wide range of powdered and granular materials have been impacted against a wide range of surface materials over carefully controlled conditions of velocity, particle concentration, tem- perature, impact angle, etc.
Many studies have been of a general nature with a view to getting a better under- standing of the basic mechanisms of the process, and for this purpose numerous single particle impact investigations have been undertaken. Other studies have been conducted for specific purposes, and so the range of variables investigated can be extremely wide. For particle impact velocity, for example, tests have been carried out at about 1–3 m/s for hydraulic transport, from 15 to 35 m/s for pneumatic conveying, from 100 to 500 m/s for aircraft applications and up to 8000 m/s for rockets.
Issues considered
The information presented on erosive wear, therefore, is in two sections. In the first the influence of the major variables in the problem of erosive wear is presented, and the data for this has been obtained from a very wide range of sources. This provides general information on the basic mechanisms of the erosive wear process and influence of the variables involved, and will provide a useful background and basis for subsequent deci- sions in relation to the wear of pneumatic conveying system pipelines and components.
In the second part a review of industrial solutions to the problem of pipeline wear is presented, with particular reference to bend wear. Bends in pneumatic conveying system pipelines are probably the most vulnerable of all components to wear. If silica sand is conveyed through a pipeline with a conveying air velocity of about 25 m/s, for example, an ordinary mild steel bend could fail well within 2 h. This review, therefore, covers issues such as bend geometry and the use of wear resistant materials.