bulk-online, the Powder/Bulk Portal

The OLC-2 long-distance conveyor at KPC in Indonesia transports 4000 metric tons/hour (nominal) and 4500 metric tons/hour (peak) of washed coal (bulk density of 0.9 metric t/m³) with a belt speed of 8.45 m/s. The conveying length is 12 589 m and the conveying height is 29.1 m. For this task, KPC as the user of the facility and PT RSSI as the EPC contractor selected the Contitech Stahlcord belt 1100 St2250 5.5:5.5 X/XLL with the extra low loss bottom cover. This selection is based on the successful application of the Contitech extra low loss belt with XLL-bottom cover in the existing OLC-1 back to 2002 and the expected improvements can be summarized as follows:

1. At the expected 15 year service life (assuming round the clock, i.e. 24 h/day and 365 days/year) for such a Contitech belt and the power difference of ΔP_erf = 1751 kW, as well as the assumed electricity generation costs of k_e = 0.05 €/kWh, the following capital savings K are achieved: K = 15 y · 365 d · 24 h · 0.05 EUR/kWh · 1751 kW K ≈  EUR 11.5 millio
2. The belt amount ordered by KPC at 26032 m with a belt weight difference of Δm‘_G = 5.1 kg/m, yields weight savings G of: G = 26 032 m · 5.1 kg/m G ≈ 133 metric tons
3. Thanks to the maximum belt tension being reduced by ΔT_ANmax = 114 kN (approx. 12 metric tons), the steel structure of the system, pulley shafts, bearings, etc. may be selected lighter and more economical.

Fig. 5: Old OLC-1 and new OLC-2 long-distance conveyors positioned parallel to each other at KPC in Indonesia with XLL conveyor belts from Contitech. (Picture: © Contitech)

Thanks to the reduction in the number of motors (1×2200 kW), smaller pulleys, a lighter steel structure, as well as a conveyor belt with reduced nominal breaking strength and mass per meter, the initial procurement costs (incl. logistics costs) for the OLC-2 long-distance conveyor with an XLL conveyor belt are lower than the standard commercially available conveyor belt. Subsequent maintenance costs are lower as well, because “light” spare parts are less expensive.

Steel Cord Conveyor Belt Design acc. to Version 1982 and 2011 of DIN 22101

In 2011, the old standard DIN 22101 version 1982 “Belt Conveyors for Bulk Material – Principles for Calculation and Design” was replaced by a new version. The calculation example shown below illustrates the advantages for belt design that the new standard DIN 22101 version 2011 brings to the table.

DIN 22101 version 1982

According to DIN 22101 version 1982 (DIN 22101-1982), the minimum nominal breaking strength k_N,min of a belt is calculated according to Eq. (4):

where the safety factor for normal operation and favorable operating conditions is S_sta = 6.7 (S_sta = 9.5 for unfavorable operating conditions).

Breaking strength loss value rverb for steel cord conveyor belts is a function of the number of steps of a belt splice:

• r_verb = 0 for number of steps n ≤ 2 or
• r_verb = 0.05 for number of steps n ≥ 3

For example, for a steel cord conveyor belt of width B = 2100 mm and maximum belt tension T_max = 1000 kN when in normal operation, the maximum belt tension k_sta is resulting in:

According to Eq. (4), this yields the mini­mum nominal breaking strength of a belt:

For this numerical example, a standard steel cord conveyor belt St3500 is selected.

The breaking strength loss value is r_verb = 0.05, because a steel cord conveyor belt St3500 has a three-step splice.

DIN 22101 version 2011

According to DIN 22101 version 2011 (DIN 22101: 2011-12), the minimum nominal breaking strength k_N,min of a belt is calculated according to Eq. (5):

c_K is a coefficient for determining the minimum dynamic splice efficiency of a conveyor belt corresponding to the belt tension in the belt edge relative to the belt width. For steel cord conveyor belts, c_K = 1.25 is for trough transition zones and c_K = 1 for transition curves.

k_K,max is the maximum belt tension in the belt edge relative to the width of the belt, which is generally 1 to 1.2 times the mean belt tension.

The safety factor S₀ is a function of the features of splice manufacturing (competence of the splicers, quality of the splicing material, ambient temperature and conditions, etc.) whereas the safety factor S₁ is a function of the features of operating conditions (chemical/physical stress, rotati­o­nal frequency, expected service life, starting/stopping cycles, etc.)

The safety factors are selected within the range of S₀ = 1 ... 1.2 and S₁ = 1.5 ... 1.9.

k_t,rel designates dynamic splice efficiency of the belt splice, which according to DIN for steel cord conveyor belts should be at least 45%. For all Contitech “Stahlcord” steel cord conveyor belts, the relative reference fatigue strength of the belt splice is at least 50%!

Therefore, according to Eq. (5) for our numerical example (steel cord conveyor belt with B = 2100 mm belt width and maximum belt tension in normal operation T_max = 1000 kN resulting in the belt tension in the edge k_K,max ≈ 1.1∙476 N/mm = 524 N/mm), the following minimum nominal breaking strength k_N,min of a steel cord conveyor belt results

For this numerical example, a standard steel cord conveyor belt St2500 is selected.

In this example, according to a new standard a significantly lower required belt nominal breaking strength results. This results in enormous cost savings for a belt which, depending on the cover ratio, can be around a third. Moreover, an St3500 requires a three-step splice, which is more complicated and takes more time to produce than a two-step splice of an St2500.

Transverse Reinforcement on the Impact Strength of a “Stahlcord Barrier” type Belt

Fig. 6: A typical belt in surface mining areas close to heavy mining machinery must often transport coarse and sharp-edged bulk material (a). A typical impact break with two affected steel cords (b) and the causes for this (c). (Picture: © Contitech)

A transverse reinforcement (of whatever kind) is generally incorporated in the top (carry side) cover of a steel cord conveyor belt in order to prevent impact breaks and the associated belt rips in the longitudinal direction and to protect the belt carcass (steel cord). Often these are belts used in heavy mining machinery or in short- and middle-distance belt conveyors in surface mining areas close to heavy mining machinery that are very heavily stressed by coarse and sharp-edged bulk material (Fig. 6).

The installation of a transverse reinforcement in long-distance conveyors (A-A > 1000 m) is also possible, but often is not worth it because of the higher manufacturing costs of a belt. Therefore, for long-distance conveyors, sensor loops with a spacing of 50 to 250 m are installed in the belt and dangerous points of the system (generally at the head end and tail end of the system) are equipped with rip detection systems. In this case, the feeding and transfer points should be designed so that insofar as possible no impact breaks can occur because a cord in the penetration point can come loose from the belt and completely jam an idler on the conveying route and can rip the entire belt. Even modern rip detection systems are useless when it comes to preventing such damage because they are often installed for cost reasons at the head end and the tail end of the system.

Fig. 7 shows possible steel cord conveyor belt designs with sensor loops, fabric and steel cord transverse reinforcement for protection of the belt. A fabric transverse reinforcement is generally used in practice for steel cord conveyor belts, although steel cord transverse reinforcements are also being used more and more, especially in the mining industry.

Within the internal research project, the impact strength, troughing properties according to ISO 703 and according to Conti procedures, ply adhesion of cover/transverse reinforcement and transverse reinforcement/bead core according to DIN EN 28094 were studied in Conti­tech’s test laboratories on a so called 3D-test rig for a steel cord conveyor belt that is typical in the German lignite industry, 2200 St2500 20:8 DIN-X, for two types of steel cord transverse reinforcement, for three types of fabric transverse reinforcement, and for 2 mm auxiliary rubber covers. The width of the transverse reinforcement here was approx. half the belt width, i.e. B_QA ≈ B/2.

Fig. 7: Possible steel cord conveyor belt designs with sensor loops only (a), and with fabric (b) and steel cord (c) transverse reinforcement. (Picture: © Contitech)

With studies on the special Contitech 3D-test rig a belt sample undergoes multi-dimensional deformations to determine whether or not a steel cord transverse reinforcement creeps out from the top cover.

Fig. 8 shows a schematic diagram of an impact test rig, and Table 3 presents the results of the study where borderline impact energies and belt weights for various design variants of 2200 St2500 20:8 DIN-X are compared to each other.

Fig. 8: Schematic diagram of Contitech’s impact test rig. (Picture: © Contitech)

It is evident from Table 3 that a centrally arranged steel cord transverse reinforcement can increase the impact strength of a steel cord conveyor belt by a factor of two to three. The belt weight relative to width increases only slightly here.

For the new product Stahlcord Barrier the Type 2 steel cord transverse reinforcement was selected. Before approval of the product, it was also necessary to precisely test, in addition to the impact strength, the troughing according to ISO 703 and to Conti procedures, and ply adhesion of cover/transverse reinforcement and transverse reinforcement/bead core according to DIN EN 28094 on the 3D-test rig.

The advantages of the new product can be briefly summarized as follows:

• Two to three times more resistance to penetration damage. No belt rips possible in the steel cord transverse reinforcement area.
• Excellent troughability according to DIN ISO 703 and CONTI procedures (idler set test rig)
• High ply adhesion in accordance with ISO 15236 and CONTI procedures („3D“-test rig)
• Slight increase in the belt weight per meter of 5% compared to a steel cord conveyor belt without transverse reinforcement
• Long service live time à safer and more reliable operation of heavy-duty conveyor belts

Therefore, Stahlcord Barrier is an ideal solution for belts used in heavy mining machinery or in short- and middle-distance belt conveyors in surface mining areas close to heavy mining machinery and for transporting sharp-edged and coarse-grained bulk materials at several feeding and transfer points.

Conclusions

With increasing globalization, mining companies are also finding themselves under strong cost pressure. The investment plans in the mining industry are being adapted to the raw material market situation and were significantly reduced in many mining companies in 2014, for example, due to a lower world coal price. Therefore, it is essential for the survival of suppliers to the mining industry in particular to design their products in a cost-efficient and application-oriented manner without losing long service lives and proven quality characteristics of the product in the process.

On the other hand, a mining company can complete new projects through savings in terms of procurement and operating costs. Of course, environmental aspects are a point of focus both for suppliers in the mining industry and also for mining companies, not just because of the increased environmental requirements of the states but also by virtue of environment-oriented thinking, which has become corporate policy in many companies for some time now.