THE USE OF HIGH PERFORMANCE POLYMERS IN BULK HANDLING

Author: John G. Riordan

Project Manager / DOTMAR Engineering Plastic Products –

Sydney, NSW, Australia

SUMMARY

This paper will address the virtues of ultra high molecular weight polyethylene (UHMWPE) liners and the importance of correct flow promotion liner installation. Various installation conditions and attachment methods will be considered. Flow promotion liner selection and purpose is reviewed for various devices and applications. Empirical observations will be translated from extensive exposure to and across many industries.

1. INTRODUCTION

Many types of polymers exist in today’s engineering world, all designed for specific applications most suited to their particular physical properties. Amongst the seemingly endless variety of polymers exists many grades of Polyolefins. One such polyolefin is polyethylene and was discovered in England in 1933 by the Research Department of Imperial Chemical Industries. Polyethylene basically consists of long chain molecules of carbon and hydrogen. Commercial production commenced in England in 1939 and in the USA in 1943. High Density Polyethylene was the next major development by Ziegler in Germany and Phillips Petroleum Co in USA in 1953. Polymerization of UHMWPE was commercialised in the 1950s by Ruhrchemie AG.

2. POLYETHYLENE PROPERTIES AND FORMULE

The two most important properties of any polyethylene are molecular weight and density. The molecular weight is a measure of the average size of a polyethylene molecule or chain. Density is defined as the mass per unit volume of the material. The specific gravity of most polyethylene grades are in the range of 0.93 – 0.96.

Ensuing benefits are the ability to display high sliding abrasion resistance, good impact strength and a low friction co-efficient against a wide variety of suitable bulk solids.

Formulations of UHMWPE vary from one manufacturer to the next. and will typically commence with a base resin that is most suited to the properties considered important for the target duty. Additives are introduced to enhance desired properties of the end product such as UV stabilizers, carbon, silicone, silica and titanium dioxide. Formulations can encompass a combination of these additives or from a range of many others.

For example, some of these thermoplastic formulations manufactured by engineering plastics producers in sheet form are specifically designed for bulk materials handling by reducing boundary friction in storage and transfer equipment.

These UHMWPE grades are designed as flow promotion liners that exhibit superior surface release properties in most application and high sliding abrasion resistance, by starting with an ultra high molecular weight base resin of 7 million. UV stabilizers and silicone are added to enable outdoor applications and to increase liner release properties.

UHMWPE is typically manufactured by two molding methods. These are compression molding for sheet and ram extrusion molding for rod. Both methods are reliant upon correct temperature, pressure and time settings.

Clarification of resultant interaction between selected bulk solids and a proposed liner material by an authoritative test facility is generally advised.

3. UNDERSTANDING FLOW PROBLEMS

Understanding flow patterns is a specialised area of study. The expertise of those that reside in this field should be consulted where accurate and reliable guidance is sort. Complete lining system trials and testing over a range of operating conditions will benefit the end user. Results of liner interaction with the bulk solid over the life of the liner should become evident.

Typically flow patterns of bulk materials can be influenced by direct or indirect intervention.

These influences can be:

•Climatic change

•Geological variations

•Human intervention

•Mechanical inadequacies

•Production demands

These causes, and many other contributing factors, often result in negative operating conditions needing a suitable solution to be identified and implemented. A suitable option to address a pre-existing or foreseen issue could be through appropriate lining methods.

Lining production equipment requires a great deal of consideration. A full understanding of the behavior of the bulk solid, critical to quality measurements of the bulk solid, equipment geometry, functions of areas within the equipment, charging and extraction methodology all need to be considered.

Transfer equipment input could envelop:

•Specific working zones within a single transfer item should be understood. Primary impact zone, secondary impact zone, primary sliding surfaces, secondary sliding surfaces.

•Liner combinations for specific duties to achieve maximum performance.

•Motivation for liner selection chosen through maintenance convenience, cost restrictions or information based on sound engineering foundations.

•Comfort gained by previous exposure to common place lining materials, opposed to selecting new options such as a polymer lining.

Selecting the correct liner material or combination of materials will be a reaction to, or proactive research based upon, influences contributing to understanding individual flow problems.

4. Application images

(please see Figures 1-8 below)

5. Installation methodology

Installation methodologies vary to suit a wide range of applications and operating environments.

Main contributing factors for the most suitable installation methodology are:

1.How well informed the client or end user is in regard to installation options.

2.How well the client accepts correct installation recommendations.

3.Should the liner need to be changed out during regular scheduled shut down cycles.

4.Technical skills and the availability of specific specialized tools and equipment.

5.Capital made available to the project.

Correct installation methodology is recommended to be employed at all times to enhance the success, longevity and effectiveness of the new flow promotion liner.

Important installation functions include:

1.Use of specially designed fasteners – stud, nut & plug or poly capped bolt assemblies.

2.Correct application of the specially designed fasteners.

3.Correct fastener patterns.

4.Correct clearance between hole and fastener.

5.Extrusion welding of joints.

6.Exit bar welded to the discharge perimeter.

7.Z-section cover strip installed to cover the top edge of the liner.

(please see Figures 9-13 below)

It is advised that an exit bar is stitch welded to the discharge perimeter of the device prior to commencement of liner installation. Exit bars are typically of equal thickness of that of the liner. The function of the exit bar is to exclude ingress of fines beneath the liner, minimize wear life at the liner edge and provide a ledge on which to commence installation.

Extrusion welding is an effective method of preventing fines ingress behind the liner panels. And at the same time address the transition issue of joining liner panels. Independent weld strength tests have returned results of not less than 85% of parent material tensile strength. Liner joint welding is not considered or intended to be a structural weld. Welding HDPE in tank and vessel fabrications are considered and intended to be structural welds whist conforming to the German DVS 2212 guidelines for plastic welding.

The condition of internal wall surfaces of various bulk handling devices will alter over time due to wear, impact, corrosion, chemical reaction or a combination of these influences, plus, many others. There is a multitude of operating conditions influencing change in flow rate during the life of these devices.

•Variable physical properties of bulk solids handled

•Alternative charging methods

•Variations to discharge methods

•Corrosion

•Mechanical or programming malfunctions causing damage

•Poor liner installation techniques

•Questionable liner system integrity

•Operator intervention

•Trial and error flow promotion aids

Successful performance and longevity of a polymer liner is largely reliant upon correct installation methodologies chosen and implemented. Installation environment is also important.

6. COMMON MISCONCEPTIONS WITH PRODUCT FAILURE

Equipment failure of many kinds, are born from misinformation, being ill-prepared or through poor fitment. Polymer liners are subject to similar issues.

It is imperative that application information supplied by the end user or engineering delegate is accurate. An appropriate testing facility can be employed to collate information and subsequently submitted for suitable liner material selection.

Incorrect material specification has been attributed to past failures. Correct material designation must be allocated to a specification when a polymer liner is identified. Polyethylene grades differ greatly in formulation and purpose. Low density, high density, high molecular weight and ultra high molecular weight polyethylene are the major family members. Variations within these family members exist with attributes suitable to a wide range of applications.

Ultra high molecular weight polyethylene possesses impressive sliding abrasion and impact resistance in suitable applications. Applications where extremely high tonnage throughput, hard and sharp particles, high velocities and heavy impact occur should be considered with extreme caution.

Preparation for major installations must be thorough and involve persons with extensive subject knowledge. A capability statement is a good guide to assessing suitable candidates. Short cuts in preparation through poor access, inadequate or unreliable power supply, rough or corroded substrate, unsuitable installation environment and inappropriate personnel are to be avoided.

Choosing a highly developed lining material and then choosing inferior or unsuitable fasteners and methods of attachment also have the potential to cause failure. Termination of the liner extremities and joins are details that need consideration. Inclusion of cover strips, discharge bars and join sealing, such as extrusion welding work well when properly conducted.

Misconceptions of all polyethylene being the same, that they may be used in contact with vigorously aggressive bulk materials, use of unsuitable fastening methods and materials are all typical modes of potential failure.

7. Economic, OH&S and environmental advantages of high performance polymers

Advantages of working with highly developed polymers become evident rapidly. Positive impacts on persons working on sites where polymer liners have been implemented are enjoyed from those persons coming in direct or indirect contact with the material.

OH&S and environmental issues are prominent and serious management issues on all Australian bulk solids materials handling sites. Efforts to reduce or eliminate lost time injury rates are a major benefit attributed to the use of polymers.

Acoustic levels reduce considerably in polymer lined equipment to make a more pleasant work environment. A leading mobile plant manufacturer installed polymer pads to the back of large dozer tracks to improve operator comfort with excellent results.

Reducing or eliminating the need to manually free blockages with bars, air lances and hammers is a corrective action to reduce repetitive strain injuries. Reduced material mass of UHMWPE being 1/8th the weight of steel avoids lifting related injuries (see Figs. 14-16).

Many tradespersons originating from metal or building trade backgrounds find handling polymers far more comfortable than metals and alloys without risk of cuts or abrasions. Safety issues raised through thorough job safety analysis prior to conducting onsite installations have identified the passive nature of the undertakings.

High visibility grades are used in applications such as dock fenders to avoid potentially serious craft to port collisions and to assist tug drivers with visible dock location. Polymer dock fender rubbing strips are an environmental step forward as the cutting of aged hardwoods has been substituted (see Figs. 17-19).

Economical benefits of utilising polymers in bulk materials handling industries are many and varied from reducing the build height of bins and hoppers by reducing hopper half angles, increasing volumetric capacities, reducing carry back in mobile plant to providing increased site safety.

8. CONCLUSION

Virtues of using UHMWPE liners have been addressed in this paper. Understanding these virtues should have become more apparent to design engineers, maintenance supervisors, operations supervisors and others involved in the bulk solids handling industries. UHMWPE can be considered as an alternative to traditional liner materials or utilized in conjunction with other materials with new and improved awareness.

High performance polymers employed to execute specific tasks will do so with far reaching benefits and challenge the minds of those that consider their inclusion at either the design or maintenance phase.

For more information, please visit:

https://edir.bulk-online.com/profile...s-products.htm11014 domtar engineering plastics products.htm

Additional information:

http://www.google.com/search?hl=de&c...btnG=Suche&lr=search?hl=de&client=safari&rls=de de&q=UHMWPE+site%3Abulk online.com&btnG=Suche&lr=

Fig. 1 + 2: Coal bunker

Fig. 3 + 4: Mineral sands bunker

Fig. 5: Ore fines bin

Fig. 6: Dolomite hopper

Fig. 7: Off-road dump truck

Fig. 8: Dozer bucket

Fig. 9: Material, fastener and fixture options

Fig. 10: Drawn arc stud welding application

Fig. 11: Extrusion welding

Fig. 12: Weld tensile test

Fig. 13: Weld flex test

Fig. 14 + 15: Potential for injury

Fig. 16: Dangerous blockage clearing method

Fig. 17: Timber dock fender

Fig. 18 + 19: UHMWPE fender

Attachments

fig.-1_-coal-bunker (JPG)

fig.-2_coal-bunker (JPG)

fig.-3_mineral-sands-bunker (JPG)

fig.-4_mineral-sands-bunker (JPG)

fig.-5_ore-fines-bin (JPG)

fig.-6_dolomite-hopper (JPG)

fig.-7_off-road-dump-truck (JPG)

fig.-8_dozer-bucket (JPG)

fig.-9_material,-fastener-a (JPG)

fig.-10_drawn-arc-stud-weld (JPG)

fig.-11_extrusion-welding (JPG)

fig.-12_weld-tensile-test (JPG)

fig.-14_potential-for-injur (JPG)

fig.-15_potential-for-injur (JPG)

fig.-16_dangerous-blockage- (JPG)

fig.-17_timber-dock-fender (JPG)

fig.-18_uhmwpe-fender (JPG)

fig.-19_uhmwpe-fender (JPG)

fig.-13_weld-flex-test (JPG)

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