Two meter shields become reality

THE evolution of the two meter shield has been a significant step in the longwall industry’s continuous push for increased productivity.
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DBT shields at Cumberland longwall.

Angie Tomlinson

At Cumberland Resources, RAG and DBT bought the two meter shield from a vision to reality, with the successful design and operation of a two meter longwall system.


This was the topic of a paper written by RAG’s James Bryja, Douglas Conklin, Robert Robinson and DBT’s Keith O’Neil who explained the history, design, application and operation of the two meter shield.


The RAG and DBT team explained, to further improve longwall productivity, a longer and wider panel is a must.


“The application of a 2m wide system will shorten the longwall face move time by reducing the number of shields used for a fixed face length and minimize the overall maintenance time on face roof supports and this increase the availability for face production during longwall retreat,” the paper said.


“A longer panel will increase the normal mining period along the panel length and thus minimize the downtime at the start and end of each panel. A wider panel can reduce the ratio of the gate end cycle time to normal face cutting time per mining cycle, and thus increase face productivity.


“The most significant advantage of wider longwall faces is the reduction of continuous miner development required. As longwall panels continue to get wider in the future, the benefits of using 2m shields will be significant on the overall longwall performance.”


Preliminary investigations to develop and install the 2m shield were initiated in 2001 by RAG and DBT for RAG Cumberland Resources. Several design considerations had to be taken into account in the development of the shields for the 71 inch to 90 inch Pittsburgh seam with a depth cover of 590 to 1050 feet.


The roof loading requirements for a 2m shield at Cumberland indicated a web width of 42 inches, a support density of 5.3 to 5.4 tons per square foot at set and 6.8 to 7 tons per square foot at yield after cut, a top-to-face distance of 18 inches at a normal mining height of 84 inches, and a floor pressure of less than 800psi.


After DBT designed the shields based on the technical requirements and a force analysis was carried out, RAG requested DBT manufacture a prototype. In addition to the engineering specifications, the mine decided to incorporate an automated dust suppression system.


An evaluation of the transport system was carried out, analysing the slope hoist, the track haulage system, the rail mounted shield carriers, underground locomotives and the battery powered shield haulers.


An area of concern for RAG was the battery powered shield haulers. Cumberland decided to keep their existing fleet of four 590 class Eimco shield haulers and upgrade them to a single 595 class hauler, with modifications to the hydraulic system and the installation of heavier duty axles.


To cope with the higher ground pressures the tyre size was increased to the maximum width.


Not only did the new shields necessitate a change in the transport system, but they also necessitated the installation of a new high pressure pumping system with the increase in the size of the hydraulic cylinders.


“In order to account for pressure losses in the hydraulic hoses leading form the pump station to the headgate, and for the losses in the hoses within the face itself, the new pump system was specified to deliver a total capacity of 400gpm at an output pressure of 5000psi,” the paper said.


This meant day-to-day operation required the use of only three of the four pumps operated through a programmable logic controller.


Ladder crossovers to connect the main 1.5 inch pressure and 2 inch return lines running from shield to shield allowed fluid to flow to the “consumer” from two sources splitting. This allowed lower fluid velocities in the hoses, accompanied by an improvement in the shield setting pressures. The result was good roof control towards the tailgate end of the face.


Once all the specifications were finalised an on-site evaluation of the proto-type shield was conducted at the end of 2001. Once no structural defects were found, designers turned their attentions to fabrication and quality control.


Manufacturing began in January 2002 and final assembly took place in Germany during the spring of 2002, with 30 shields delivered to DBT’s Houston facility mid-year.


Underground installation began in June 2002, whilst the mine was in full production, transporting the entire longwall system 7.2 miles from the surface to the longwall face.


Production from the new system began in July 2002. Only one problem was encountered relating to the 2m shield widths. The pitch of the rack haulage system for the shearer was slightly out of tolerance causing several racks to break during snaking of the conveyor. The problem was fixed in the short term, with the long-term solution being to reduce the gap between the pan joints.

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