The new valves will act as safety protection of the shield legs during the yielding process and will also act as passive protection against the consequences of a sudden impact from fracturing of upper strata.
West Wallsend in New South Wales' Hunter Valley lies under massive sandstone strata units, resulting in periodic weighting and windblast problems at the mine.
Periodic weighting occurs when the sandstone units are much stronger than normal and do not fail in the usual manner. This causes a cantilever effect on the longwall roof supports – causing poor face conditions and undue damage to equipment.
West Wallsend’s current yield valves are rated at 600l/second – simply not enough to cope with the impact of the roof hitting the supports during a weighting event.
During mining under a massive sandstone channel section during extraction of longwall panel 28 a very large section of strata fell from great height within the goaf, striking the rear of the roof supports causing them to be thrown violently into the face. Extensive damage to equipment resulted, along with injury to personnel involved.
Another incident at Longwall 27 caused pressure cylinder and structural damage costing West Wallsend A$1million at the time.
The new Ostroj Opava valves will be installed to put a stop to damage to the legs and associated seals during such events, in addition to offering a safer work environment for employees.
The new VR7100 two-stage spring valves have a maximum flow rate of 7100l/second. During normal operation of the roof support the pilot valve maintain nominal load bearing capacity at an adjustment rate of 22-50MPA and opening pressure of 30-50MPA.
Under impact loading, the main valve opens up to 7100 l/min within 0.01sec – allowing a large amount of fluid to discharge from the support leg.
After the pressure peak fades, the valve lets through the fluid from the leg at the pressure maintaining the nominal load bearing capacity of the leg.
The valve is fully closed when the pressure in the leg drops to below 95% of the valve required for the nominal bearing capacity of the support.
Xstrata Coal, NSW eastern underground general manager Glen Lewis, said the valve was a retrofit unit able to be fitted straight to a 360mm diameter leg with stacker pipe. No modifications would be required to current leg design.
He said the current valve was not designed for protection of the leg against dynamic impacts, where an amount of fluid is displaced over a given time.
The valve has been tested, and according to Lewis, exceeded expectations of the VR7100 unit on a 360 diameter leg. There was substantial reduction of pressure increase in the leg until the moment of the impact valve opening.
West Wallsend has ordered 200 valves which are currently being manufactured by Ostroj Opava and will be installed at the mine in late January 2005.
Ostroj Opava have targeted the Australian market with its valves and attended the Queensland Mining and Engineering Exhibition earlier this year. The company already produce hydraulic parts for OEM’s and have their products in 15 Czech Republic longwalls, four Spanish mines and longwalls in Russia and the Ukraine.
Ostroj produces a wide range of mining equipment – including roof supports, scraper and belt conveyers, plough sets, hydraulic posts and cylinders. According to Ostroj mining and machinery director Richard Beres the company will initially supply hydraulic parts into the Australian market before eventually looking at entire roof supports.
