Hydraulic powered roof supports are self advancing structures which are interconnected along the length of the longwall face. Each unit along the line is also connected to each pan of the AFC.
The function of the powered supports are to:
- To control strata deformation, fracture and movement around the coal face,
- To maintain a safe and coal producing working environment,
- To limit the amount of roof to floor convergence,
- To prevent broken rock from entering the work area, and
- To secure and advance all plant on the coal face including the roof supports.
Each support structure consist of a roof canopy connected to a base via a shield and leminscate linkages. Side shields mounted to the canopy prevents excessive debris falling into the work space during support advancement. The hydraulic legs of a each type of support are inclined or splayed at an angle. This is to prevent the legs from buckling under the immense loads that they support. In Australia, the most comon types of supports used are :
- Chock shield supports
- Shield Supports
The forces required at the support line to control strata deformation are supplied by a set of hydraulic legs acting between the base and roof canopy. The hydraulic pressure to the legs are supplied from a power pack system which could be located remote from the longwall face or mounted on a Pantechnicon at the main gate end of the longwall face.
The initial force applied to the strata is known as “setting pressure”. Once this pressure level is achieved the hydraulic supply is removed but the pressure is retained in the support legs by means of a non return valve. A guaranteed setting pressure, also known as Positive Setting Pressure, insures the preset full pump pressure is being maintained on each support unit along the face. This is achieved by the incorporation of Positive Set Circuit in the hydraulic circuit of the support system.
The term “yield load” refers to the maximum resisting force of the support and is determined by a preset yield valve in the leg of the powered support. The yield valve generally releases the fluid from the support leg circuit at a constant pressure and in such a manner to insure that, even during yield, the fine control of convergence is still achieved. In Australia the setting load of the supports is 80% of the yield load.
Support capacities in Australia vary and are dependent upon geological and geoenvironmental factors. In general the support capacities can be between 650 tonnes to over 1100 tonnes. For example, Baal Bone mine, located near Lithgow NSW, uses a 630 tonne yield load 4 leg chock shiled support for a depth of cover up to 240 metres. Another example is Crinium mine, located near Lilyvale QLD, uses a 950 tonne yield 2 leg support for a depth of cover of 130 metres.
As the shearer traverses along the coal face the supports are advanced to enable the shearer to cut a fresh web of coal when it returns. There are two methods of advancing powered supports, these are:
Conventional Method of Advancement
In the conventional method the supports are stood up to the conveyor before the shearer cuts a web of coal. After the shearer passes the support an extension bar from within the canopy of the support is extended. This gives support to the newly exposed roof until the conveyor and the support are advanced to their new position.
Immediate Forward Support (IFS) Method of Advancement
In the IFS method the supports are stood back from the conveyor before the shearer passes. This is to allow the support to be advanced once the shearer has passed it to offer immediate forward support. The conveyor is then advanced afterwards.
SUPPORT CONTROL SYSTEM
The various functions of the powered supports are controlled by an electro-hydraulic control system. The simplest method utilises a control valve mounted on each support which is used to operate the functions of that support (lowering and raising legs, support advancement and conveyor push). This method is termed “unit control” and has a major disadvantage in that the operator is located in the powered support whilst it is moving. For this reason this method has been superseeded by more sophisticated systems.
The “adjacent control” method, as the name suggests, allows the operator to control the powered supports from the adjacent unit by using a similar type of control valve. This allows the operator to remain within a support which is set to the roof. This system can be extended so that not only does the adjacent support lower, advance and set to the roof, but once this is completed a signal is transmitted to the next support so that it too can be operated with the operator at the one location. This can be continued for any number of supports but is restricted to a comfortable seeing distance of around 8 to 10 supports. On completion of the advance cycle of this group of supports the operator will walk through to the start of the next group and continue advancement. The type of system is termed “batch control” or “bank control”.
It is also possible to remove the operator from the face completely and allow them to control the supports from a console at the face end. However, in most cases the operation is still carried out on the face because of mining considerations and the requirement to operate supports in conjunction with other face equipment.