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.

A UK cement plant operated a drag chain system for the transfer of a biomass-derived fuel into the kiln. 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, the plant operator had experienced operational issues with the drag chain system.

Site visit

A meeting with representatives of the cement plant operator was arranged to discuss the experiences of the existing and inadequate chain system, establish the symptoms and generate initial theories and hypotheses. This included a site visit 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 double-strand chain arrangement with a drag bar attached between the two strands. The chain itself comprised of two strands of a forged link, a headed pin with collar and role pin retainer. Each strand 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:

1) Poor service life, which caused two chains to wear out within six months,

2) Intermittent component failure causing damage, downtime and loss of equipment availability,

3) Chains extending up to 1.4m in length,

4) Slat deformation and resultant damage,

5) Bolt failure and resultant damage.

The root of the problem

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

a) 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.

b) The extension on chain pitch resulting from accelerated pin wear was found to be unequal between the two strands, causing a mismatch.

c) 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.

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 evidenced 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 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.

The 10 pins were taken for dimensional analysis in order to establish the extent of the wear. Dimensional checks were taken 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.

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 non-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.

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 (Din C20) (SAE1020), a finding that was the result of an independant investigation by the cement plant operator. EN3A is a basic mild steel and although it is carburised to create a high hardness case, it has no alloy content and therefore has a low core hardness.

It was concluded that the failures of the 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 flexibility. The data shown in Table 1 established that there was 139.32mm of unequal extension between drive side and non-drive side. 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 could be selected. 2) The introduction of some flexibility 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.

Table 1: The degree of pin wear (in terms of pin diameter) on the drive side compared to the non-drive side. (Pins had an initial diameter of 30mm)

Table 2: Hardness testing. Pins from the drive and non-drive side.

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.

A martensitic stainless steel Grade 420 was proposed. This material, with its unique hardening qualities, offers a combination of corrosion and abrasion resistance. A through hardening value of 45Hrc was selected allowing high hardness and core strength with adequate ductility through the component section. John King had past experience of many similar applications and was able to confirm that it was essential 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 that when it is bolted it does not clamp solid. 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.

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.