SLURRY EROSION OF HYDRO TURBINE STEELSBy: Harinderpal singhM.TECH Mech. Engg.Roll No:- 11393064

INTRODUCTIONSlurry erosion is defined as degradation of surfaces upon interaction with solid particles entrained in liquid medium. This erosion occur either material moves through slurry or slurry impinges on the material at certain velocity. Slurry erodes by the action of abrasive particles in liquid which result in the action of wear of material depend upon the conditions to which the system is exposed. Slurry erosion is major problem for the industries which deal with liquids having abrasive particles.

FACTORS AFFECTING SLURRY EROSION

Several types of factors are: Factors depending on the operating conditions are Velocity, concentration, impact angle, temperature etc. Factors that depend on erodent's are particle size, particle shape, hardness etc. Factors that depend on the properties of target surface are hardness, microstructure, mechanical properties etc.

Various erosion mechanisms responsible for the loss of material in slurry erosion are: EROSION MECHANISMS IN SLURRY EROSION1. CUTTING EROSIONIt takes place when the particles impacting are very sharp. When particles impact on the surface micromachining action takes place on the material and the material is removed.

2. PLATELET MECHANISMIn this process when solid particle impacts on the target material, it spreads the target material in the direction of impact. This target material further flattens and extends to form a platelet.

3. PLOUGHING EROSIONThis process is a two stage process in which plastic deformation of the surface takes place by particle impacts. First of all due to particle impact on the surface, it deforms plastically and a shear lip is formed. In the second stage due to repeated particle impacts this shear lip fails and breaks down due to fatigue.4. SUBSURFACE DEFORMATION AND CRACKINGWhen a blunt particle strikes the target surface leading to its localized plastic deformation, which develops cracks on the surface which leads to brittle fracture of the material. This type of wear mechanism is known as subsurface deformation or cracking.

SLURRY EROSION PREVENTIONThere are different methods of combating sediment erosion in hydropower plants. Damage due to sand erosion portrays serious issues in hydropower plants due to increased shut down time during maintenance and subsequent revenue losses as a result of the damage . However, the impact of sediment on turbine blades and its components can be minimised to an acceptable limit by: Constructing civil structure such as dam and de-silting chambers. Monitoring the concentration of sediment flow to power house. Coating to improve resistance against erosion.

LITERATURE REVIEWBhandari et al. (2012) studied slurry erosion performance study of detonation gun sprayed WC - lOCo-4Cr coatings on CF8M steel under hydro accelerated conditions. They investigated the erosion behavior of coated and bare steel on high speed erosion test rig. Hollow cylindrical specimens of external diameter 12 mm, internal diameter 6 mm, and length 10 mm were prepared from the steel. Silt used as abrasive media collected from actual working conditions of power plant and then sieved and grains of 100 m and 300 m were obtained. Parameters like rotational speed, particle size, and concentration were studiedThey observed that under all the investigated experimental conditions, specific mass losses were higher for CF8M steel than WC-10Co- 4Cr coating.

They revealed that with the change in slurry concentration from 10,000 and 30,000 ppm, there is an increase in specific mass loss of the coating as well as of CF8M steel.They observed that with the change of average particle size from 100 to 300 lm, there is an increase in specificmass loss of the coating as well as of CF8M steel. Thiseffect is more pronounced in the coating.They concluded that most of erosion in CF8M steel was due to cutting/ploughing whereas in coated steel erosion was due to brittle fracture.

Goyal et a1. (2012) Compared slurry erosive wear of HVOF spray Cr203 coatings on CF8M and CA6NM turbine steel High Speed Erosion Test Rig was used for slurry erosion tests, and the effects of three parameters, namely, average particle size, speed (rpm), and slurry concentration on slurry erosion of these materials were investigated. Test rig consist of erosion chamber, rotor to rotate work piece up to 5000 rpm was usedSilt used in slurry was of irregular shape and mainly Si02 particles of 1161 HV. The average particle size used was 100 m and 300 m. They observed that uncoated CA6NM steel comparatively showed higher erosion resistance in comparison with uncoated CF8M steel. HVOF-sprayed Cr2O3 coating showed comparatively higher erosion resistance in comparison with uncoated CF8M and CA6NM steels.

They observed that the erosion rates increase with increase in average particle size. This increase in the erosion rate with increase in average particle size was mainly due to the increase in kinetic energy of impacting particles. The effect of particle size was more pronounced in case of uncoated CA6NM steel as compared to CF8M steel.The main mechanism for material removal from steels was crater and lip formations, plastic deformation, cutting and ploughing, whereas in case of the HVOF-sprayed Cr2O3 coating, the main mechanism responsible for material removal was the micro chipping at splat boundaries and removal of protuberances from the surface.Uncoated CF8M and CA6NM steels showed the ductile behaviour during slurry erosion, the HVOF sprayed Cr2O3 coating exhibited a mixed behaviour (mainly brittle) during the slurry erosion testing.