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Mar 10, 2013

Liners: Long-Term Protection for Your Elevator's Critical Infrastructure

In today's current processing environment the amount of grain being handled through the same system has greatly increased compared to a few years ago. Over a course of a year, thousands or millions of bushels are being handled through these systems. As seed technology and farming practices keep improving the volume of grain being processed will also increase. In addition, the cost of building new facilities is continually increasing along with the costs to keep these facilities in top running condition. In order to minimize system downtime and incurred costs, all facilities are looking at what can be done to maximize the system for the best up-time. Facility owners' expectations of up time greatly impact the purchasing decisions and maintenance costs.

This paper is going to look at some of the different wear locations in a grain facility, the type of impact zones created by the grain handling equipment and what available material is there to improve the life cycle of these wear areas, and how the choice of different lining material affects the overall cost of the facility. While this paper is designed to provide insight into why a lining material is performing better than a previous lining material it is not intended to recommend any lining material or specify the best use of that lining material.

What is wear and what factors are at work to cause wear? Starting off a brief description of wear is a process of gradual removal of a material from a surface subject to contact and sliding. Damages of contact surfaces are results of wear. They can have various patterns (abrasion, fatigue, ploughing, corrugation, erosion and cavitation). The results of wear, in most cases are identified as the irreversible changes in body contours. The forms of wear discussed in this paper are focusing on the sliding abrasion and impact abrasion [1].

Sliding abrasion is defined by the surface being worn away by friction — due to sliding of one material verses another. In the diagram below the plate and block are two opposing surfaces. All surfaces have some variation of flatness as shown in the diagram below. These irregularities in the surfaces are called asperities. The resistance of movement between the two items results in the frictional force.

 
Figure 1

As the blocks slide over each other the blocks cause friction between the two surfaces but it also causes both surfaces to lose some material, even when one surface is much harder than the other. Many materials especially in the plastic industry define material performance by a wear rate which is defined by the volume of material removed divided by the slide distance. In addition a wear factor, used for comparison in plastic material can be calculated as Wear Factor K = Wear rate / PV. (P is Pressure applied and V is velocity) [2].

Impact abrasion is the result of a surface being worn away by gouging, spalling, or cutting caused by impact from other material. The bombardment of particles against another surface depending on speed and angle impact and slide along the surface producing scratches, grooves and wear debris. The bombarding particle then finally bounces off. The repeated impacts of the particulate eventually wear away the other surface. There have be multitude of tests performed in controlled conditions which have confirmed the higher the eroding velocity the higher the erosion loss. In addition the type of material being eroded is effected by the angle of bombardment. For more ductile material, the impact angle around 30 degrees imparted more material loss while on more brittle surfaces the maximum wear occurred at a 90 degree impact angle.

 
Figure 2

Many studies done on the wear effects of ductile and brittle material have tried to understand the factors involved in regards to why each material performs differently in regards to the impact angle. The studies point to the ductile material is constantly deforming in the plastic range until the material finally fatigues and fails. The brittle material sees more impact force at the 90 degree angle which then fractures the material and the material begins to wear. The above phenomena are understandable. For a ductile material, it has a relatively high resistance to impact due to its good capability to accommodate plastic deformation. It is known that the fracture is generally caused by tensile or shear stress. When impinged by solid particles at 90 degrees, the lateral tensile stress may not effectively result in fracture due to the large fracture strain of the ductile material. As a result, a ductile material should have less damage when impacted at 90 degrees. However, for a brittle material the situation changes. The low ductility of the brittle material makes it vulnerable to deformation. Even under compressive stress, the lateral expansion could be sufficient to cause fracture. Therefore, a brittle material is more sensitive to the impact energy. The impact energy increase when the impact angle changes from 30 to 90 degrees. Consequently, the erosion damage increases as the impact angle is increased and reaches a maximum of 90 degrees [3]. During particle impact, when yield strength of the material is locally exceeded, plastic deformation takes place in the vicinity of the impact. After multiple impacts, a plastically deformed surface layer may form near the eroded surface and , therefore, the yield strength of the material increases due to strain hardening. Upon further deformation, the yield strength at the surface will eventually become equal to its fracture strength, and no further plastic deformation will occur. At this point, the surface becomes brittle and it fragments may be removed by the subsequent impacts [5].

 
Figure 3

The two illustrations above show on a microstructure level how the impact angle and shape of a particle erodes a surface. The first illustration has a high impact angle and a brittle surface. As mention previously this creates lateral and radial cracks in the ceramic surface. As the surface is continually bombarded it fractures the surface which eventually wears it down. In comparison the illustration on the right depicts a lower incident angle with a particle impacting a metal surface. In this example the base metal is continuously deforming in the plastic range until it finally fatigues and fails. In the two previous illustrations, each material would not be classified as having high percentage of "rebound". Later in this paper we will discuss the properties of the lining material and how they are affected by the impact angle.

In addition to material speed and angle, the understanding of particle shape also influences how different material is being eroded during impact. Many studies done on particle orientation show that as the base material is bombarded with irregular shaped particles the different orientation causes plastic deformation on the base material eventually wearing it away.

The next two charts summarize the effects of particle shape and orientation on the base material. These charts are a result of a study by Q. Chen using a computer simulation of the effects of solid particles [3]. The chart on the left shows the different erosion loss caused by impacting different shapes into the base material. What this chart is illustrating is continual impacts from the same shape particle in the same orientation results in higher wear for the triangular particle. The chart on the right summarizes the increased wear caused by the different angle of rotation of the particle. So even when the square particle is being used which in the first chart had the mid-range amount of erosion loss if the particle is rotated the amount of material lose increases. The grain commodities being handled are of a consistent size and shape. While most of the time the grain is softer than the impacting surface, the impacting surface along with dirt, sand, etc which is in the system continually wears away at the lining material which then adds more abrasive particles to the mix, potentially causing more wear. While is not possible to control particle orientation the intent of this discussion is to highlight that this along with a multitude of other factors are at work in the erosion process. The study of wear and friction are very complex, and little information has been documented and published on wear effects of grain handling. What is well documented is the various factors that influence erosion: particle characteristics, substrate properties, impingement angle and temperature.

 
Figure 4

Today there are many types of abrasion / wear resistant products available. Some of these products have been around for many years, others are new and yet to be tested in a grain facility. The more common lining material today in grain handling facilities would be ultra high molecular weight (UHMW), urethanes, abrasion resistant steel (AR), and ceramic. In addition to lining material other industries like coal handling use material on material surfaces to reduce the erosion in some equipment. For this article we will focus on the materials used today in a facility. The current materials are cost effective, easier to apply and have demonstrated their performance over time.

A brief comment on some newer materials like thermal spraying, this process is the application of an alloy surface onto a base metal substrate. Previous experience with some of these application methods result in a rougher finished surface, more heat imparted into the substrate, along with more of a surface hardening than a through hardness like AR or ceramic. In addition the specialty equipment to perform the alloy application has been expensive and a slower application process. In addition there are other materials which are available with harder surfaces this paper is only reviewing the current materials.

Like the discussion on what causes wear, in selecting the appropriate lining material there are several factors which need to be considered. Some of these factors are the properties associated with the lining material, the environment the lining material will be subjected to, life expectancy of the lining, initial cost and replacement costs and product damage. Past experience is also valuable in the assessment of lining material but a solution in one application may not be a good solution in another.

A basic starting point to make comparisons between the different material are listed in the table below. The first material property is hardness, this is defines the materials ability to resist plastic deformation, penetration, indention or scratching. The UHMW and polyurethane are classified as plastics in this case the hardness rating between the plastics and steel is vastly different. The ranking of material hardness from softest to hardest is Polyurethane, UHMW, AR, and then ceramic. Tensile strength is the maximum load a material can support without fracture when being stretched, divided by the original cross-sectional area of the material. Again the material ranking is polyurethane, UHMW, Ceramic and then AR. And finally the density of each is rank as follows: UHMW, polyurethane, ceramic and AR.

 

Material

UHMW

Polyurethane

AR-400

Ceramic

Hardness

Durometer D60

Durometer A80

43

69

Tensile strength (MPa)

35

24

1241

155

Density (gm/cc)

0.95

1.25

8.30

3.42

 

With the information on the effects of impact angle, velocity, plastic deformation and the data in the preceding table. Assumptions could be made that the softer material UHMW and polyurethane would not hold up well in the grain handling environment. While there are no recent published studies which compare UHMW, urethane, AR or ceramic on the same test basis, there is test data from Argonics which has made a comparison between 6 similar products. The abrasion test that was performed used a sandblaster with crushed walnut shells, in the test crushed walnut shells were impacting the test specimens at a maximum angle of incidence of 30 degrees. From this test there are differences in material performance [4].

The test results shows that under the same condition there are differences in how a material performs. Without comparing all of the properties of the materials the Kryptane Red had the lowest hardness rating. In addition the Kryptane Red also advertises the largest percentage of rebound. This may help explain the better performance of the material.

Other issues to consider in addition to the mechanical properties of the lining material would be installation and replacement costs. The ability to cut, form, and install UHMW and urethane linings are similar for each product. Urethane lining with a metal backing allows the material to formed and hold its shape better than if there was no metal backing. Material may be cut using a ¼" metal shear, power jig saw or hand operated nibbling tools. Installation of the lining can be with mechanical fasteners, either with the use of self-drilling metal screws from the outside or the use elevator bolts or stove bolts. The standard rule of thumb for fastening is 1-1/2 to 2" from any edge and a spacing of 6-18" center to center spacing throughout the sheet, depending on backing type and material thickness. In the example below it is recommended that a piece of chute work be lined 75-80% of the side wall. It is also recommended that the sides are lined first and then the bottom is lined per the example.

 
Figure 5

Installation of AR material is different than polyurethanes. Cutting of the hardened material can be accomplished with a power shear, torch or similar equipment. Drilling holes and countersinks into the material is also more difficult. Fastening AR material for lining could be bolted in with mechanical fasteners or welded in. Again a good rule of thumb is 1-1/2 to 2" from the edge and center to center bolt spacing can be increased to 24".

Ceramic tile installation can be more complex if the tile needs to be cut. In addition ceramic tile can be welded in or installed with adhesive. If installing with adhesive the substrate which the tile is being bonded to needs to be free of paint, rust and grease. Proper installation of ceramic tiles should have no gap between tiles. As shown in the picture below large gaps between the tiles will cause increased wear. The best method to install is with the seam of the next tile not in line with the tiles above it.

 
Figure 6

The wearability of the lining material is not the only factors to be considered when selection a lining system. As an example the cost of initial purchase, installation and replacement costs will be compared. In this example it is assumed the previous spout drawing is 24" square. The table below summarizes the weights and costs associated with the different lining options. In the example the assumption the spout will have two thirds material depth and no other loads will be applied to the spout. In addition the lining material being installed is based on same thickness not equal life span to generate a baseline for cost comparison.

 

Lining Material UHMW Polyurethane AR400 Ceramic
Lining thickness 1/4 1/4 1/4 1/4
Spout weight/ft 50.5 50.5 50.5 50.5
Material weight/ft 149 149 149 149
Lining weight /ft 5.7 11.1 47.6 22.3
Lining cost / # $1.23 $4.08 $1.40 $3.51
Installation cost $25 / hr $1.25 $1.25 $1.67 $8.33
Spout & liner weight / ft 56.2 61.6 98.1 72.8
Total cost/ foot $12.84 $51.11 $74.42 $117.09

 

The information in the table is slightly misleading. The life expectancy from the 1/4" AR and ceramic in most instances would be greater than the UHMW and polyurethane. In addition the ability to use thinner AR material would reduce the cost and weight of the spout while still providing very good performance. Changing the AR to 10 gauge would reduce the spout weight by 21 pounds and save $30 in cost. The ceramic on the other hand is the thinnest piece. Other factors which would impact the decision on what lining option to use would be the upfront cost to install and support the spout along with the costs to access the spout to replace the lining in the future.

The next thing to discuss is the effects of equipment on how grain is being handled and how that would affect lining wear. Today's facilities are able to handle larger volumes of grain in short time periods. The following chart lists some of the typical speeds and capacities.

 

Equipment En-masse Enclosed Belt Bucket Elevator Bulkweigher
Speed range (FPM) 90-160 400-700 400-700 Draft Size (lbs)

 

240-60,000

BPH 2,000-50,000 3,000-80,000 2,000-70,000 723-90,000

 

The selection of equipment based on capacity and speed does affect the type of lining need. In the following example assume a belt conveyor running horizontal, the material trajectory is plotted at 400 FPM and 700 FPM. The extra distance which the material is being discharged is 22". If you assumed the discharge hood of the belt conveyor end where the 400 FPM trajectory stopped the impact area on the hood would be negligible. If in the case of the 700 FPM trajectory, the hood still stops at the 400FPM trajectory line. There would be more impact on the hood in this case compared to the 400 FPM speed.

 
Figure 7

As the previous example shows equipment selection and facility lay does affect the need for lining and may dictate a more expensive lining option to handle the increased wear. In addition to the speeds created by the handling equipment velocities in the spouts need to be considered not only for the wear of the spouts themselves but also in the impact zones created by the slowing, sudden stop or changing of direction of the material. In some instances wear can be reduced if the system is able to have grain on grain impact. A classic example of this is in the upper garner of bulk scale. In the upper garner, once the conveying system is started there should always be a small amount of grain in the bottom of the hopper. The new grain entering the scale is impact on this pile and acting like the plastic deformation zone of the lining. This minimizes the amount of wear in the upper garner.

In summary, there are many factors which can dictate what lining material may be the better choice. As discussed the effects of sliding or impact friction, particle shape, impingement angle and velocity all play a critical part. The use of one lining material compared to the others needs to be decided based on actual performance and cost. While the two different approaches to wear — energy absorbing liners to cushion the impact or harder surface liners to minimize the total wear- both have had many successes in real world applications.

In existing facilities the ability to change some of the underlining principles (like product speed and impact angle) are not possible. Understanding the dynamics and princeple involved in the wear process should help in selecting the best possible wear protection. If on the other hand a new facility is being built the design should be evaluated to minimize the areas of high wear and potential product degradation. With the same understanding of what causes wear and how the ystem interacts would help identify the better lining options.

The point to be made is the understanding of the how and why of what material performs best is a very difficult process. The multitude of variations between materials and work conditions along with the collection data over long time periods to confirm some of the concepts makes it difficult to state what material is best in each application. The best indicator of lining performance is personal experience and performance of lining material in like conditions.

References

[1] Alfred Zmitrowicz, Wear patterns and laws of wear — a review 2006

[2] Zeus, Friction and Wear of Polymers 2005

[3] Q.Chen, Computer Simulation of solid Particle Erosion 2002

[4] Argonics Engineered Polyurethane Abrasion Test Results

[5] BF Levin, KS Vecchio, JN DuPont and AR Marder, Modeling Solid-Particle Erosion of Ductile Alloys 1999


* As presented by Ted Sondgeroth at GEAPS Exchange 2013
 

Speaker Contact Info

Contact info for Ted Sondgeroth:
Phone: (402) 330-1500 ext 6456 
Email: tsondgeroth@intersystems.net

 
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