When a longwall stops:
1 - the strata forces such as abutment loads, racking forces, horizontal and vertical stresses are all still active. Anyone who has worked on a longwall knows the sounds of those strata forces at work when the power goes off. The snap, crackle and thunder can be very disconcerting!
2 - the coal face can deteriorate, with coal spall filling the conveyor, the canopy tip to face distance gets bigger and the support system has more work to do as the roof load increases
3 - the roof supports can get loaded up to the point where they start to yield, with resulting convergence which in turn causes further roof deterioration, further coal face spall, and more work for the support system
4 - if the electricity is switched off, the pumps go down and guaranteed set systems are no longer active, keeping the support pressures in check. Any leaks or bypasses will lead to supports becoming less effective or even ineffective.
To summarise, stopping a longwall is easy but stopping the strata forces is impossible, and deterioration will occur to some degree. Starting a longwall up again requires special care and attention.
However, as always, prevention is better than cure, and if you want to minimise the effect of downtime, then planned stoppages need to be properly planned with flexibility, and unplanned stoppages need to be eliminated.
Longwall operations should be worked 24/7 for maximum roof control, however this is not possible because maintenance is also an essential ingredient for effective roof support. Shorter, more frequent stops are better than long shutdowns. For example, five 4 hour maintenance shifts are better than two 10 hour shifts.
There are many reasons for unplanned stoppages, but perhaps the biggest single cause of unplanned downtime relates to coal clearance issues such as belt conveyors and bunkers. Take a look at the top producing longwall operations in the world. Beltana, Twentymile, Bailey, Oaky North, Newlands, they all have very high availability of their coal clearance systems.
In the 90s I lived and worked in the USA for 2 years and visited some 60 longwall mines. I only recall seeing one conveyor stop during production ... and then all hell broke loose!
I can’t say the same for all Australian longwall mines. I recall one punch longwall operation in QLD that had only two belt conveyors. The outbye belt was a “surface” belt – and used to stop because the sun was too hot?
Finally there is another very good reason for keeping downtime to a minimum. Spontaneous combustion! If a longwall is stationary or very slow moving, there is a greater chance of spontaneous combustion of coal in the goaf. This is more likely to occur through major stoppages such as roof falls and longwall relocations.
Do’s and Don’ts
- DON’T ever stop a longwall for planned maintenance when the roof supports are yielding.
- This is indicative of a cyclic loading event and to stop the operation WILL result in major roof problems.
- DON’T shut the hydraulic pumps down unless essential for maintenance. Guaranteed set systems should remain active at all times
- DO keep the longwall moving consistently
- DO keep downtime to a practical minimum
- DO use face sprags to prevent coal spall in thick seams, and double chock the coalface if the tip to face becomes too large during shutdowns. If your face is prone to heavy face spall, double chock at the start of the shutdown.
- DO make sure you have good coal clearance availability at all times particularly during roof fall recovery and especially during a yielding event.