Problem solving at a UK cement plant

When a cement plant in the UK experienced operational problems with the drag chain system supplying biomass-derived fuel to its kiln, UK based John King Chains Ltd (John King) was able to help.

At the UK cement plant, the mechanical handling equipment that was experiencing operational problems comprised of a drag chain system used for the transfer of a biomass-derived fuel into the kiln, in which two strands of a forged fork link chain were made common with a steel flight. The drag chain system pulled the material a total of 80m along a horizontal section and then up an inclined section.
Since the commissioning of the plant in the recent past (within four years), the plant operator had experienced
operational issues with the drag chain system.


Plant visit

The rising section of the old drag chain system installed at the UK cement plant
The rising section of the old drag chain system installed at the UK cement plant
A meeting with representatives of the cement plant operator was arranged to discuss the experiences of the existing poor chain system, establish the symptoms and generate initial theories and hypotheses. A site visit was then arranged to investigate the current system, physically examine drag chain components, gather information, identify the cause(s) of the operational problems and formulate solutions.
 
 

The problem

The chain system consisted of a doublestrand conveyor belt system with a drag bar attached between the two. The chain iself comprised of two strands of a forged link, a headed pin with collar and role pin retainer. Two strands had a rectangular welded attachment to which a flight bar was bolted rigidly against the attachment. The flight bars were spaced every third link, with two plain links in between.
Analysis by John King established that the forged links were from a low alloy case-hardening steel and that the pin was from a case-hardened mild steel. The attachments and scrapers were made from standard steel. The fixing bolts were high tensile with all-metal locking nuts.

The main problems that the cement plant operator described were:

  • Poor service life, which caused two chains to wear out within 18 months,
  • Intermittent component failure causing damage, downtime and loss of equipment availability,
  • Chains extending up to 1.4m in length,
  • Slat deformation and resultant damage,
  • Bolt failure and resultant damage.


The root of the problem

Based on the analysis carried out by John King and its past applicational experience, two principal problems were identified:

  • A combination of corrosion and abrasion was causing degradation of the case-hardened surface of the pin and resultant accelerated wear of the relatively soft core material.
  • The extension on chain pitch resulting from accelerated pin wear was found to be unequal between the two strands, causing a mismatch.
  • Considering that the two strands were made common with a fixed attachment, the resulting mismatch between the strands created a sheer condition causing slat twisting and bolt failure

Cutlery - A strange but not uncommon component of the biomass-derived fuel that the cement plant operator has to handle
Cutlery - A strange but not uncommon component of the biomass-derived fuel that the cement plant operator has to handle
Examination of the biomass derived fuel and information provided by the cement plant operator established that it was a mixture of shredded domestic waste primarily plastics and paper products. It was clear that the material was in the process of decomposition and was therefore corrosive. This was further aggrevated by the deteriorated condition of the inside of the casing of the conveyor. The material also contained contaminants such as glass particles, silica and metals, which caused abrasion of the chain components. It is known from past experience that where a combination of corrosion and abrasion exists, accelerated component wear is often experienced.

Upon further investigation, a representative of the cement plant operator said that the plant was having an issue with the quality, consistency and preparation
of its biomass-derived fuel. It was noted that similar systems in Germany had better success and that lessons may be learnt from these examples.


Data collection methods

The initial data collection methods involved using verniers and micrometers along with photographic proof on- and off-site.

A total of 10 pins were removed from the chain (five from each strand) and were subjected to dimensional analysis. Checks were also carried out on the pitch holes of the forged links. It was established that there was a limited degree of wear in this element of the chain assembly so attention was directed to the
chain pins.

Three images showing use of a micrometer in assessing pins from the old drag chain system
Three images showing use of a micrometer in assessing pins from the old drag chain system

The 10 pins were taken for dimensional analysis to establish the extent of the wear. Dimensional checks were taken and at various points using verniers and micrometers. The results are shown in Table 1 based on the two individual strands of chain. There was a total mismatch of 139.32mm between the
two sides.

Drive side (mm)Non-drive side (mm)
Pin 1     28.76Pin 6     29.02
Pin 2     28.79Pin 7     29.04
Pin 3     28.82Pin 8     28.86
Pin 4     28.57Pin 9     28.67
Pin 5     28.84Pin 10   29.02
Mean 28.76Mean 28.92
Extension per link (mm)1.241.08
No. of links in conveyor860860
Total extension (mm)1066.4927.1
Mismatch between sides (mm)139.32
Table 1: The degree of pin wear (in terms of pin diamter) on the drive side compared to the non-drive side (Pins had an initial diamter of 30mm)


Worn out chain pins from the old design
Worn out chain pins from the old design
The same ten pins were also checked for hardness on the unworn portion of the body and the worn portion to obtain hardness values. This particular testing was carried out using a Rockwell C hardness tester. Table 2 shows the hardness testing values of pins on drive and none drive side on worn and unworn areas of the pin. It clearly demonstrates that the primary wear condition was taking place on the pin body where the link articulates. The articulation will only take place when the chain moves around the sprockets on the head and tail shaft.

Drive sideDrive sideNon-drive sideNon-drive side
UnwornWornUnwornWorn
58176116
58186218
60165918
59176016
60185816
Mean5917.26016.8
Table 2: Hardness testing. Pins from the drive and non-drive side.


It was evident that the surface hardening, which was at a very high value, was completely undermined in the worn area where the values were very low and reflected the nature of the poor pin material.

It was noted that there was an increased degree of wear on the drive side of the two strands. The unequal extension was undoubtedly a negative influence on the operation of the chain and would be the subject of further consideration.

It was already understood that the link was made from nickel chrome case-hardened steel. It was concluded that the alloy content had provided a necessary degree of corrosion resistance. The pin however was made from a mild steel EN3A, a finding that was the result of an independant investigation by the cement plant operator. EN3A is a basic mild steel and although its is carbonised to create a high hardness case, it has no alloy content and therefore has a low core hardness.

It was concluded the failures of the experience in the slat arrangement was the result of the accelerated wear of the pins and uneven pin wear between the two strands taking into account that the slats were bolted solidly to the attachments without any elasticity. The data shown in Table 1 established that there was 139.32mm of unequal extension between drive side and non-driveside. When the chain met the drive sprockets this mismatch created a sheer condition, which caused the slats to twist.

The results of the analysis highlighted two areas for consideration; 1) Examination of the pin material and heat-treatment methods to establish if any better alternatives can be selected. 2) The introduction of some elasticity in the slat fixing arrangement to compensate for any mismatch between the two strands of chain. 3) An improvement in the material of the slat to reduce weight and corrosion.


Engineering a solution

It was clear that the pin material was inadequate from the perspectives of both corrosion and abrasion resistance, although corrosion seemed to be the main issue.

John King had past experience of many similar applications and was able to confirm that it was essential that to have built-in clearance between the slat and the attachment on double strand drags such as the system at the UK cement plant. Options 1 and 3 demonstrate designs that ensure that mismatch can be accommodated without mechanical damage.

Option 1 had a spacer bush, the length being in excess of the combined thickness of attachment and the slat so when it is bolted it does not clamp solid.

Sample of chain parts from option 1 as shown to the cement plant operator
Sample of chain parts from option 1 as shown to the cement plant operator
CAD drawing of option 1, which was selected by the cement plant operator
CAD drawing of option 1, which was selected by the cement plant operator

Option 3 incorporated a cast clevis, which is welded to the link with a swivel bolt to fix the slat. Plastic flights produced from polyethylene material were considered to reduce weight and offer corrosion resistance.

Options 2 and 4 were the same as options 1 and 3, but with plastic slats instead of metal slats.


Meeting with the cement plant operator

A meeting was arranged with engineers from the cement plant operator in order to discuss the proposals.

All four options were presented and discussed. It was recognised that the plant operator has first-hand and long experience in operation of chain systems, therefore it was understood that their opinion would be valued.

The plant operator discounted options 2 and 4, because it considered plastic slats to be too vulnerable to foreign objects entering the conveyor and causing impact damage. It was concluded by those present that option 1 with the cast clevis was more robust and offered better self-aligning characteristics. The meeting was concluded with a request for a commercial offer for a replacement chain based on option 1. A sample of chain parts was requested and subsequently an order was placed.


Conclusion

The project was completed by John King in January 2010 and the new system has since proven itself in terms of specification and construction.


Oliver Wadsworth, John King (USA) Inc.

 

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