Introduction to Wear in Solids Handling...

Introduction to Wear in

Solids Handling SystemsFundamentals: Controlling: Predicting

Dr Pablo García-Triñanes

Wolfson Centre for Bulk Solids Handling Technology

Faculty of Engineering and Science

University of Greenwich, UK

[email protected]

mailto:[email protected]

Wear in Solids Handling Plant

Principal effects:

• Unplanned maintenance

• Process downtime

• Iron in product

• Mess from spillage

The content of this presentation is proprietary and confidential information of the Wolfson Centre for Bulk Solids Handling Technology (University of

Greenwich, UK). It is not intended to be distributed to any third party without the written consent of Wolfson Centre for Bulk Solids Handling Technology

(University of Greenwich, UK)

Types of Wear

Two distinct types in solids handling plants:

• Erosion (erosive wear)

– From particles hitting a surface

• Abrasion (abrasive wear)

– From a bed of particles sliding along a surface

• May occur together





Erosion:

• Impact of particles on surface from a stream of bulk

solid, for example in:-

– Pneumatic conveyors

– Transfer points between belt conveyors





Abrasion:

• Sliding of a body of particles against a surface, for

example

• Hoppers (Silos/bins/bunkers)

• Chutes

• Screw and drag chain conveyors.




Erosion Example - Pipeline Wear

• Usually most severe

at bends

Puncture point




Bend wear

in pneumatic

conveying

With erosive products:

• Hard

• Angular




Impact erosion of solids handling equipment




Erosion - Effect of Velocity

Erosion = k . (particle velocity)n

where

k = a constant

n = a power; 2.2 to 2.8 for most materials

Example: +25% velocity

leads to + 60% to +90% wear!




Erosion - Other Variables

Solids concentration - e.g. dilute versus dense phase

Number of changes of direction - bends, transfer points

Geometry of impact - e.g. radius, shape of bends or

impact zone

Material properties - impact zone and particles

Quality of pipe fitting




Dilute phase conveying

Particles suspended in air flow

(typical air velocity 15-20 m/s)The content of this presentation is proprietary and confidential information of the Wolfson Centre for Bulk Solids Handling Technology (University of



• Dense phase conveying

• Air velocity about 6 m/s at inlet

Solutions

Erosion - Particle Properties

Increased erosion results from:

• Harder particles

• More angular particles

• Larger particles - up to about 200 microns

• Abrasive inclusions in soft particles




Erosion - Material Properties

• Hardness of surface not a good guide

• Different materials respond differently to angle of

impact




Erosion - Effect of Angle

• Brittle response

– ceramics

– glass

– rock

Erosion

rate

Angle of attack, alpha 90

(Normal

impact)alpha

0




Erosion - Effect of Angle

• Ductile response

– steel

– most metals

Erosion

rate


(Normal

impact)alpha

0 30




Erosion - Effect of angle

• Elastomers

– rubbers

– polyethylene

Erosion

rate


(Normal

impact)alpha

0




Erosion - Prediction of wear rate

• Laboratory scale erosion test

– Samples of bulk solid and surface material

– Velocity, angle, concentration comparable to

real system

• E.g. rotating disc tester

• Scale material loss to real system




EROSION - Centrifugal erosion tester

• Kleis (Tallinn)

• Soderberg (Trondheim)

• Burnett, Deng (Greenwich)




Impact wear

tester

Centrifugal feed of material to impact

targets




Erosion - Protection techniques I

Minimise impact velocity

– For example, with velocity exponent of 2.6

– Reduce velocity

from 18 to

12 m/s (33%)

– Erosion reduced

by 65%! 0

0 10 20 30

Ero

sio

n r

ate

Impact velocity m/s

E=k.v2.6




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Erosion - Protection techniques II

Select “wear resistant” material carefully

• Elastomers for high angle impact

– Rubber

– Polyethylene

• As an impact zone liner

• Or as a hanging curtain

– Weight important




Erosion - Protection techniques III

• Ceramics for low angle impact

– Fused alumina

– Cast basalt

• Erosion tests!

– To identify most cost-effective material




Erosion - Protection techniques III

Retain particles in impact zones

– “Dirt box” in transfer point

– Can compromise flow

– “Blind tee” bend in

pneumatic conveyor

– Gives high pressure drop




Erosion - Protection techniques IV

Sacrificial protection

– Expendable liner

• Simple shape

• Cheap to make

• Easily changed

• Must check it regularly!

– Choose steel carefully

• Hardness not a guide

• Erosion test needed

– Extra thickness “wear back” locally

• Erosion test to identify thickness




Abrasion

• Sliding of bulk solid against surface

• Abrasion rate

A = k.p.v

• Found in

– Silos

– Chutes

– Mechanical conveyors

– “Hood and spoon” transfers

Velocity v




Abrasion - Prediction of Wear Rate

• Laboratory abrasion testers

– Press bulk solid against surface at controlled

pressure

– Sliding at known velocity

• Scaling for velocity and pressure is reliable

– Can test at accelerated abrasion rate




Abrasion - Laboratory testers

• Linear Abrasive Wear Tester

– Roberts (Newcastle), Mengistu (Greenwich)

• Test at increased velocity and scale down

• Many other types

Force applied

by dead load

Test material

Velocity v

Bulk solid in

feed hopper

Conveyor belt

Bed of bulk solid




Sliding Abrasion Wear Test

Rotating annulus of test material and wear sample




Abrasion - Protection I

• Difference in hardness a key factor

– if Hsurface > 1.2 x Hbulk solid, no wear

– if Hbulk solid > 1.2 x Hsurface, significant wear

– Greater difference, greater wear

– More angular particles - greater wear

• Select surface harder than bulk solid

– Hard steel (for some bulk solids)

– Hard-faced steel (but care with surface finish and high friction)

– Ceramics e.g. alumina, basalt

– Hard materials difficult to work!




Material options for abrasive wear resistance

• Austenitic Stainless– 304 or 316

– Significantly better than carbon steel against corrosion-erosion (eg damp conditions)

– Limited abrasion resistance

– Can bend and fabricate easily

• Maternsitic stainless– Significantly harder, so better abrasion resistance

– Hard to bend and fabricate

– Air-hardening (e.g. 420) cracks when welded

– Quench-hardening softens when welded

• Low alloy/high carbon– Harder, but susceptible to corrosion

– Poor under wet abrasion conditions

– Softened by welding

Hard steel - bolted

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Material options for abrasive wear resistance II

• Clad plate– Mild steel with surface of chromium-carbide hard facing

welded on

– Mild steel backing can be welded

– Hard facing not much softened by welding

– Cannot be bent much

– Surface not smooth – high wall friction

• Alumina ceramics– Very hard

– Brittle

– Not impact resistant

– Stick-on tiles




Abrasion - Protection II

• Lubrication reduces wear

– Wet abrasion usually less

– Not if corrosion can result (corrosion and abrasion

have synergistic effect!)

• Sacrificial protection

– Cheap replaceable liner (Check regularly!)

– Additional wear allowance determined from test

• “Dirt box” technique (retained particles)

– Cheap

– Can compromise flow





SUMMARY

• Two main phenomena

– Impact EROSION

– Sliding ABRASION

• Rates of wear depend on

– Bulk solid properties (hardness, angularity)

– Surface properties (toughness, hardness, elasticity)

– Conditions of interaction (impact angle, velocity,

concentration, contact pressure)




Wear In Solids Handling Plant

SUMMARY (cont’d)

• Protection

– Choice of material at impact site

• Many factors to consider

• Wear testing an essential guide

– Sacrificial protection

• Wear allowance, replaceable liner

– Geometry

• Optimise angle for material in use

• Retain particles

www.bulksolids.com

For further questions contact:

Dr Pablo García-Triñanes

Wolfson Centre for Bulk Solids Handling Technology

Faculty of Engineering and Science

University of Greenwich, UK

[email protected]

http://www.bulksolids.com/

mailto:[email protected]

Introduction to Wear in Solids Handling...

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