excavator dredge pump track

A Review On Erosion Wear of Different Types of Slurry Pump Impeller

Introduction

Slurry transportation system has two basic types; packed flow and slack flow. While the slack flow also known as open channel flow is characterized by a pipe running less than full, packed flow is characterized by a pipe running full. Depending upon the sys-tem‘s slope, slack flow limits on particle size and lesser than 65%solid flow is observed. A relia ble design require ment in today‘s era has become a necessary requirement due to the flow structures being expensive and large. A typical slurry transportation system considering conditions like maximum design and minimum flow,may work well for a maximum flow, but may function incorrectly in case of minimum volume. With over 10 m/sec of velocity in cascades, slurry is conveyed down a stee p terra in which would otherwi se be impra ctica l to maintain at lesser slopes. This can cause high rates of wear. A cas-cade has a receiving sump attached to it just below, which acts as a sti lli ng bas in and dis sip ate s the ki net ic ene rgy of the inc omi ng hig h velocity flow. Impact of the incoming flow impacts the slurry sur-fac e pre sen t in the sum p. For ve rtic al ele va tio n of the slu rry,devices called drop boxes are often installed and a typical drop box may include inlet, height, outlet and diameter for a given type of flow.

Ash handling system

The method of collecting, conveying, storage and load-out of various types of ash residues remaining as leftovers while carry-ing dif fer ent sol id fue l com bus tio n pro ces ses is cal led as ash handling. There are four common types of ash, such as: Bed Ash, Bottom Ash, Fly Ash, Ash Clinkers. Different pneumatic or mechanical ash conveyors are employed based on the ash handling systems. A vacuum pneumatic ash con-veyor may be used in a typical ash handling system delivering ash from several pick-up stations to an ash storage silo for holding to load out for reus e or dispo sal. Howeve r, press urize d pneu matic ash conveyor may also be used. In ash handling, the major portion of the material conveyed consists of finely sized fly ash. Fly ash accounts for almost 80% of total ash generated in a thermal power plant in the form of very fine particles. While as 20% of the remaining ash consists of bed ash, bottom ash and ash clinkers thus comprising a minor portion. The ash handling system cools down the ash to manageable tem-perature and transfers it to a disposal or storage silo.

Problem identification

Due to different mechanisms like ploughing etc., there is a loss in materials. Various mechanisms resulting in erosion wear prob-lems include: Cutting mechanism, Ploughing mechanism, Surface deformation and cracking.

Materials required

For the experimentation purpose and analyzation of the erosion wear in pump impellers, it is necessary to choose suitable materi-als in order to get accurate and precise results. For the study, var-ious materials like pump impeller material, erodent and coating powder were considered. Pump impeller material: Pump impeller can be made up of different materials like cast iron and stainless steel. Stain less stee l havi ng compa rativ ely lowe r brittl e natu re has an advantage over cast iron and is one reason to choose stain-less steel as pump impeller material. These materials are chosen bas ed on the Vic ker s Har dne ss Num ber,VHN ac qui red dur ing Vic k-ers Hardness Test which consists of material indenting using a dia-mond indenter in order to find the hardness of the stainless steel. The materials used in this work are stainless steel SS-404. How-ever, cast iron may also be considered in addition.

Coatings

In order to increase the reliability factor and life span of the pump impeller while reducing or eliminating the risk of corrosion and fatigue wear, materials are coated with special and suitable coating powders. Thermal spraying is conducted in case of coating the material where melted or heated coating powders are sprayed ont o the sur fac e of the mat eri al. The pro ces s of coa tin g the mat eri al inclu des vario us coati ng techn iques like spray paintin g, HVOF(High Velocity Oxy Fuel) spraying, plasma spraying, plasma trans-ferred wire arc thermal spraying etc. Spraying techniques can pro-vide coatings ranging from 20mm to several mm, depending on the process. For the work, HVOF coating was considered and imple-mented to get the coating done selected on the basis of advantages over other coating techniques and applications. Benefits over other coating techniques: Improved and enhanced efficiency, Improved electrical properties, Cost reduction, Enhanced performance, Increased life span etc. HVOF is an attractive technique and has gained popularity as it can be applied to metals, alloys, composites, plastics and ceramics. The common powders used for coating in HVOF spraying include WC, alumina and chromium carbide. There are various other pow-ders which include Zn, Yt, MCrAIY, TiO2, AL2O3. For the research work, two combinations of coating powders were prepared. Furthermore, Liu et al. (2019) 35Cr-Mo steel substrate material was coate d with WC-10 Co-4 Cr compo site coatings and coati ng with bimodal struc tures was depos ited using HVOF techniq ue. SEM and XRD meth ods were impleme nted and micro-har dnes s,por osi ty and rou ghn ess wer e als o cal cul ate d. Den ser mic ro-struct ure, enha nced mech anic al prope rties, lower poros ity and better resistance to slurry erosion were observed in bimodal coat-ing in comparison to conventional coating. The erosion was mainly due to two mechani sms such as spa lling and propagation of fatigue wear. Ache bo (2009) appli ed the pract ical mass load ing equa tion,sto ke nu mbe r an d det erm ine d the ma ss tra nsf er coe ffic ien t in wea r severity in pipeline and evaluated the wear occurrence in pipeline. Significant particle–wall interaction was found on applying mass loa din g sto kes num ber equ ati ons and the hig h flow vel oci ty of flui d coupled with the size of the pipeline aided in high rating of wear resulting in oil leakages. Clar k (2002) studied and eval uated the partic le velo city and effects of particle grain size in laboratory slurry erosion. The influ-ence of the impact parameters such as impact angle, impact fre-qu en cy et c. on er os io n ra te s we re de pi ct ed in te rm s of a quantitative model for flow of slurry and impact of soli particles. It was found that aerodynamic and liquid drag influence or impact dominates the particle impact in slurry erosion. Xie et al. (1999) It was found that the slurry concentration plays a vita l role. Most erode nt parti cles showed repeate d bounc ing along the surface in case of dilute slurry. But when the slurry i s less dilute or in other terms concentrated, the rebounding was found to be depressed by particle interactions resembling in sliding of the particles along the surface. Erosion damage was found to be dom-inated by low-angle impact due to the contact pressure being too small.

Experimental setup

To study the erosion wear in materials and analyze the influ-enc e of va rio us imp act par ame ter s an d coa tin gs,sev era l ele ctr ome-chanical devices or setups are used depending upon the nature and/or scope of the research work. In this work, devices or setups used throughout, include: Slurry pot tester, HVOF thermal spraying system, Double disc polishing machine, Weight balance, Drill machine. Thes e devi ces al ong with their speci ficati ons and/o r featu res are discussed, as:

Slurry pot tester

A slurry pot test rig is mainly suitable for steel and rubber used to provide a rapid erosion wear response of the materials or a slurry pot tester is an electro-mechanical machine used to study the erosi on wea r rat e or ma ter ial loss by mas s of the mate ria l placed inside. It is an easy to operate device due to its simple con-figuration and often provides rapid results to slurry erosion. Slurry pot tester Components of Slurry pot tester: Cylindrical pot: used to pour the slurry mixture, Rotating shaft: to which the testing material is attached, Electric motor: it rotates the shaft and is placed at the top of the pot tester, having the maximum speed of 1500RPM, Height adjusting jack: it adjusts the height of the cylindrical pot as per the requirement.

Features of a slurry pot tester

Preset revolution count, Duration count, Fluid circulation jacket. A rectangular sample of pre-established dimensions is placed inside the cylindrical pot and is made to rotate using the adjust-me nt kn ob s av ai la bl e fo r re vo lu ti on co un ts. Th e sa mp le is weigh ted before and afte r the test. The differe nce between the two, determines the mass loss caused due to slurry erosion.

Working methodology of slurry pot tester

The basic methodology opted throughout the study can be put as: Clamping the specimen in the pot tester fixture, Plaining the fixture at required angle/s, We ig ht in g th e sa nd as pe r th e re qu ir em en ts fo r sl ur ry preparation, Combing sand and water as required, Starting the pump, Calculation of the initial weight, Calculation of the final weight, Repeating the steps as per requirement, Delibera tion of the specime n to meas ure mass loss durin g and after erosion, Clean drying the specimen, Taking the specimen out of the pot tester.

Weigh balance

In this work, two digital precision balance machines were used. For measuring the solid–liquid concentration, FX-5000i precision balance was used having readability of 0.01 g and load capacity of 5200 g. However, a more precise weigh balance was used to calculate the initial and final weight of the substrate specimens. A&D weighin g bala nce with model number HR100AZ was used with readability of 0.0001 and load capacity of 102 g.

Conclusion

The complete research work has been conducted keeping into consi dera tion vario us concl usion that have been achi eved throug h-out by conducti on serie s of expe rime nts and tests. Thes e objective s include: Investigation of slurry erosion in slurry pot tester under the effe ct of vari ous impac t param eter s of spee d, conce ntrati on and time, Investigation of physical, chemical, mineralogical and rheologi-cal properties of fly ash by conducting tests like specific gravity test and pH value of fly ash, Inv estig ation of erosi on wear characte risti cs of HVOF coated pump impeller material SS-404, Better understanding of influence on resistan ce towards erosion wear in coated materials.