Coal Age

APR 2018

Coal Age Magazine - For more than 100 years, Coal Age has been the magazine that readers can trust for guidance and insight on this important industry.

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18 April 2018 longwall mining continued Delta P decreases as hose diameter increases. adjust these switches they are preset, if someone is getting pressure spikes that shut things down, they need to look at their system to see why this is happening. Accumulators Is a storage device and shock absorber pro- tecting the system from pressure spikes? They also protect the pressure transducer in the same way. Pressure spikes are created by the action of the unloading valves and the pressure band width settings of each pump if they are not set right, they will create this problem. The nitrogen charge on the accumu- lator should be part of the maintenance schedule and they should only be checked when there is zero pressure in the hydrau- lic system. Look at the system to determine the best method using the correct testing de- vice to check the nitrogen charge pressure on each accumulator, which is indicated on the name tag. Accumulators only use nitrogen. Return Relief Valve Usually the location of the return relief valve is determined by the shield manu- facturer and installed at the tank car, un- less otherwise specified. Today's modern shields are mostly two-leg configurations engineered for the seam height and geological condi- tions. The shields are designed to suit the geological mining conditions as dictated by the mine engineers and the shield makers. The shield valve banks are comput- er-controlled, electrohydraulic functions with each function incorporating a man- ual push button operation. Modern systems allow for individual or batching method to advance the ar- mored face conveyor (AFC) and shields as the shearer dictates by its cutting speed across the face. The shields can be shearer initiated after the shearer has passed. This is done by having the se- quence of operation controlled by the PLC and today's technology. Shield cy- cle times are relative to the number of shields in the cycle of operation and the total volume of the high-pressure pumps to operate the longwall. Fluid Distribution No matter how many pumps are used, if the distribution is not right, the system will be inefficient and production will suf- fer. The hose layout from the high-pres- sure manifold to the last shield is critical to the speed and pressure required for safe operation of the longwall. Today's hoses are rated for 5,000 psi with a 4:1 safety factor for burst pressure. When considering the hose layout, the three main factors are the total flow (gpm), velocity and the pressure drop to the last shield on the longwall. Fluid distribution is controlled by the PLC and the electrohydraulically controlled unloading valve on each pump. The PLC achieves this by con- verting a signal from the transducer and switching the unloading valves from unload to load and load to unload based on the pressure bandwidth set for each pump. This bandwidth setting on each pump is an important part of how the system will perform and what pressure transients (spikes) occur when the unloading valves switch from load to unload and vice versa. Accumulators do protect the system from some of these transients and the transducer, but they do not stop hoses from jumping, which is caused by an in- rush of high-pressure, high-volume into the system. As an example, the old me- chanical pre-control valve for instance had a 20% bandwidth from unload to load, which means that after unloading the pressure had to drop 20% before switching back to load. Electrohydraulic control has reduced this percentage for higher efficiency. If in- correct, it will cause problems in the fluid distribution system. The basic-ring main line drawing does not go into detail what is required to have a good fluid distribution system relative to hose sizes length and quantity of hos- es required based on total pump volume. Fluid velocity is critical to how the system performs in many respects. Basic circuit above does not detail the hose requirements for pressure and return of any working system. Filtration Safety first and filtration is second. Having good filtration will reduce downtime on all of the hydraulic components. Every com- FLOW / VELOCITY / PRESSURE LOSS Hose size GPM Velocity Delta P/lft (psig) 1/2" 00, 18.35 30 ft/sec 5.9420 3/4" 00, 41.30 30 ft/sec 3.5796 1" 00, 73.45 30 ft/sec 2.4980 1 1/4" 0, 114.70 30 ft/sec 1.8900 1 1/2" 0, 165.20 30 ft/sec 1.5050 2" 0, 293.60 30 ft/sec 1.0504 2 1/2" 0, 459.00 30 ft/sec 0.7950 3" 0, 660.00 30 ft/sec 0.6330 3 1/2" 0, 899.60 30 ft/sec 0.5218 4" 1,174.00 30 ft/sec 0.4420

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