Hydraulic System Considerations
The additional system pressure losses due to the additional face width, the adequacy of the monorail system due to the additional weight and lines, and the pump system setup and move requirements were investigated. A minimum of two shields are required to move at the same time to keep up with the shearer cutting rate. The movement of two shields with a flow rate of about 95 gpm per shield will allow the face to keep up with shearer maximum cutting rates of 60 fpm and approximate an 8-second shield cycle time. As a result, a total flow rate of 190 gpm is required for the shield advancement, which has an average flow of 105 gpm. The minimum setting pressure for the shields based on shield design and monitoring was established at 4,060 psi.
Several calculations were completed for the three face widths. The calculations were completed with the use of the shield manufacturer’s software program. This program requires input of all line sizes, fittings, valve banks, etc. to obtain accurate results. Based on lab testing results of fittings, valve banks and hoses, the friction/loss factors are input into the program to calculate the losses. The calculations were performed on two cases, one case with two shields being cycled at the tailgate and a second case with two shields being cycled past mid-face. The current hydraulic system, Figure 3, consists of two high pressure 2-in. hoses along the AFC for high pressure feeds, and two 2.5-in. hoses along the AFC for the return feeds.
The inter-shield hoses for the high pressure is 1.5 in., and the return inter-shield hoses are 2 in. The current hydraulic system contains no crossovers other than at the headgate and tailgate to connect the AFC and inter-shield hoses. The mine had a history of bad experience with maintenance issues and safety concerns dealing with crossovers along the face, and as a result did not use crossovers in the face area of the hydraulic system. Hosing from the longwall stageloader to the pump station consisting of two high pressure 2-in. hoses and three 2-in. return hoses. The worst case scenario for pressure loss occurs with the two shields being cycled past mid-face due to the lack of crossovers. Table 3 provides a summary of the details of the hydraulic system hosing attributed to the increase in face width.
Calculations based on this system resulted in an increase of approximately 72.5 psi pressure loss from 1,050 ft to 1,250 ft wide face. To overcome this pressure loss, one 2-in. high pressure hose and one 2-in. return hose was added to the monorail and longwall stageloader system. The addition of these hoses reduced the pressure loss by almost 130 psi, an improvement of 58 psi over the original face design of 1,050 ft.
Further calculations were completed to obtain the additional pressure drop to increase the face width to 1,600 ft. Increasing the face width by another 350 ft to a total longwall face width of 1,600 ft resulted in a pressure drop of approximately 145 psi. As previously mentioned, the face hydraulic design did not include crossovers. Additional crossovers will be necessary along the face to overcome the pressure loss from the additional face width, but the existing type 1.5-in. pressure lines and 2-in. return lines will be adequate for the crossover designs. The crossovers will be placed at one quarter of the way down the face, halfway down the face, and three quarters of the way down the face, with the original connection at the headgate and tailgate being maintained. The addition of the crossovers in the system design improved the hydraulic circuit by almost 290 psi when comparing the 1,250-ft face system. The combination of both changes from the original 1,050-ft face system yielded a 203 psi improvement.
As mentioned earlier, the hydraulic flow for this system was powered by four 100-gpm pumps operating at 5,000 psi. The pumps operate based on demand programming and were recently converted to VFD technology. To meet the minimum design pressure of the 4,060 psi or 280 bar, the minimum operating pressure of the pumps must be maintained to at least 4,500 psi or 310 bar. Table 4 provides a summary of the pressure losses attributed to the increase in face width.
Electric System Considerations
The increase in face length required the electrical system to be reviewed. The transformer was designed to handle the increased horsepower loads with considerations for ventilation and power moves. The initial increase in length from 1,050 ft to 1,250 ft required no changes to the current electrical system. The No. 2 cable to the AFC motors was adequate to handle the additional starting current, and the current 3,400 KVA and 7,000 KVA transformers were sufficient for motor loads during starting and operation.
The increase in length from 1,250 ft to 1,600 ft will require the increase in size of the AFC motor cables from the current No. 2 cable to a 2/0 cable due to the additional required horsepower, motor starting current and distance. Since the system operates on 4,160 volts, smaller cable sizes were possible than at lower voltages. Require-ments for the ground monitoring systems and potential problems associated with the shearer cable length and the pick up and drop out of the ground monitors were reviewed. Working with the electrical equipment manufacturer, new designed ground monitors were installed to provide required protection.
As determined by the face lighting and communication system manufacturer, additional power supplies will be required for the face lighting, communication system, and electrical hydraulic controls for the face at reduced shield spacing intervals to account for the voltage drops over the longer distance. The current 3,400-KVA power center will remain the same, while the 7,000-KVA power center will be upgraded and enlarged in length to 9,000 KVA to handle the additional motor load of the face conveyor. This increase will handle the additional 1,200 hp requirement added to the face to power the AFC.
As part of this review of electrical system, the entire electrical system for the mine was modeled for the additional loads required during starting up. The modeling indicated the system would be adequate, but alternative time delay motor starting was discussed as options should the in-rush current loads cause excessive voltage drops in the system. This process would involve developing a pattern for the start up of the high horsepower motors on the longwall face in sequence to avoid excessive loads. With the CSTs, one conveyor motor would start, then sequence to the next conveyor motor start, then to the final conveyor motor start, at which time the shearer would then start. This sequencing was not required because of the mine’s high quality electric system. The sequencing may be necessary at other mines, but will result in potential production delays due to the required slower start up.
The longwall monorail system was also reviewed. The additional weight of power cables and hydraulic hoses, as a result of the upgrades, were looked at for possible modifications to trolley carriers. The additional weight of hoses and cables would not affect the operation of the monorail with respect to current bolts used to suspend it, beams to carry the trolleys, or the trolley wheels and support pins. However, the trolley cable brackets will need to be widened to accommodate the additional cable widths and extra hoses.
Ground Control
Several factors were reviewed when looking at ground control issues. Additional pillar and shield loading was reviewed. As the gob caves behind the retreating longwall face, caved rock material will pile up behind the shields and take load from the upper strata. The gob pressure around the panel edges is mainly a result of the weight of the caved material. The gob pressure increases toward the center of the panel. This is a result of overburden weight compacting the caved material.
The width of the longwall panel determines whether the gob pressure reaches the full load of the overburden weight. Longwall panels that are subcritical do not reach full load of the overburden in the gob. This is because the panel is too narrow and the upper strata remains unbroken and will be bridged by the side abutments, resulting in gob pressure approximately equal to the weight of the rock fragments within the caving height. When the panel reaches critical width and length, then the maximum gob pressure reaches the overburden weight. When the panel exceeds the critical width and length, the panel is termed “supercritical.”
Since the existing face of 1,050 ft is already at supercritical width at a 750- to 800-ft overburden depth with an angle of draw of 21˚, the increased face width would have minimum or no additional loading impact on the shield supports or on the gate entry pillars.
The additional “stand-time” for the entries were reviewed. Since the wider face would have a slower retreat rate, as shown in Table 1, the entries would need to stay open longer. From calculations shown in Table 5 and in mine conditions observed, it was determined that the worst case of 8.4% reduction in longwall face advance (approximately 25 days) would have minimal impact to the gate road roof, floors and pillars due to the additional stand time from the roof or floor pressures created by the opening.
The extension to the 1,600 ft resulted in the need for larger drive units. The current tip to face distance provided by the existing shield design is 18 in. The larger drive units will cause additional shield tip to face distance of 4 in. along the headgate shields and tailgate shields. Gate-end shield canopies of 4 in. will be added to keep the same tip to face distances. This modification to the shields did not affect the integrity or geometry of the shield, and resulted in negligible change to the shields tip pressure. One additional gate-end shield was needed at the headgate due to the added length of the larger face conveyor drive gearbox. The larger drives did not require any additional headgate width or height for the entry to be mined over the current 16-ft wide and 8-ft high entries.
Considerations had to be made for the effect of the additional weight of the larger AFC drives when soft floor conditions would be encountered. The base plates for the larger drives were extended to accommodate the larger power units to help reduce the additional ground pressure.
The larger power units added about 3.9 tons of weight to the tailgate drive, and about 8.7 tons to the headgate drive unit.
The existing gate shield bases were modified to increase the ram size for the additional force to push the heavier drives.
Clevis connections and relay bars were upgraded to provide adequate strength for cylinder to panline connections.
Other roof control factors were considered during set-up and recovery. In the set-up entry, additional width was required. This additional width was because the new drives required the shields to sit back an additional 4 in. The mining width in the set-up notch was enlarged by 6 in. at both the headgate and tailgate areas. Although longwall moves were reduced by the wider face, each move would require additional time to roof bolt the wider face during recovery, and extract the increase in the number of panline and shield supports. Additional pullout chutes would be needed to improve the ventilation, roof control, and equipment extraction process by allowing quicker access and the use of extra equipment.
Ventilation & Degasification
Ventilation and degasification was another important factor that needed to be assessed. The liberation of methane and the resistance to the ventilating circuit is directly proportional to the width of the face. For an adequate evaluation, a study was conducted to evaluate face liberation. The initial goal was to determine the methane emissions from individual sections of the longwall face and to extrapolate that data to estimate emissions from a longer longwall face. The current ventilation schematic is shown in Figure 4. Using this approach, the face is divided into segments to characterize how methane emissions rates vary across the face. Face emissions are then predicted from a graphical solution using regression analysis at longer face lengths based on emissions data from shorter faces.
To further enhance the study, an analysis of the methane sources and their individual contributions to the total longwall methane emissions were determined from methane concentration data collected at the beginning and end of the longwall face, along with the shearer location and other relevant ventilation and mining data. The methane emissions contributors from mining on the longwall face that were evaluated were: gas released from the coal broken by the shearer; gas emitted from the broken coal on the face conveyor; gas emitted from the coal transported on the belt; and background gas emitted from the coal face and from the adjoining ribs in the intake gateroads. Once the methane contributions from the various sources were defined for an actual longwall cutting sequence, a delay free (no downtime) cut sequence can be predicted. The calculated methane emissions contributions was then extrapolated to longer longwall faces, taking into account the variations in coal production and transport factors, to accurately predict future methane emission rates from longer longwall faces.
Methane liberation was expected to increase by about 7% above its previous level for every 100-ft increase in face width, and the pressure drops about -0.04 in. for every 100 ft of panel width. Table 6 illustrates the required quantities and resistances for the extended face widths. The 1,050-ft face resistance was established from previous ventilation surveys, and the projected face widths are calculated from two basic ventilation formulas using substitution of values to solve for the unknowns:H = RQ2, and Hf = (KPLQ2)/ 5.2A3 (Atkinson Formula) where H and Hf are the head loss, R is the resistance, Q is the quantity of the airway, K is the friction factor of the airway, P is the perimeter of the airway, L is the length of the airway, and A is the area of the airway.
From the study, it was also determined that the expected peak methane emissions increases for wider longwall panels are primarily from the coal transported on the AFC, the background emissions (coal on section belt, rib liberation along belt, etc.), and from the exposed coal on the face. The intake belt air to the longwall face is used to carry all of the methane liberation associated with rib liberation of the gateroad section belt entry and the recently mined coal on the belt across the longwall face as intake air ventilation. The methane emissions increases related to the transport of coal on the face conveyor is more significant for longer longwall faces because a wider face has longer mining passes. The elimination of the use of section belt air to ventilate the face will be beneficial to reduce methane delays. The mine also currently uses vertical gob ventilation boreholes to remove methane from the gob prior to the methane reaching the bleeder system. To determine possible additional vertical degasification needs, a 3-D dynamic reservoir model was developed to simulate methane produced in the gob from the longwall operation. It was determined from current designs that the system had adequate capacity to support panel width up to 1,450 ft, and that additional boreholes be placed on the headgate side of the panel for the 1,600-ft panel width.
Mine & System Infrastructure
Changes created by widening the longwall face are likely to affect other factors related to the mine and system infrastructure that will need to be further reviewed. The handling of the larger and heavier AFC drives during installation and longwall moves were reviewed. An alternative that is commonly used at other mines was the use of a two-piece base for the headgate drive. This two-piece base design will eliminate handling of anything heavier or larger dimensions than is currently handled by the mine previously. Making the headgate drive a two-piece base eliminates concerns with entry dimensions of the mine slope, outby haulage entries dimensions, and track requirements to handle heavier loads.
The longwall tailgate rock dust system was modified to account for the wider face and subsequent longer transport distance of the dust through the system. The water system for dust suppression was reviewed for the increase in distance. The current water pumps were adequate with the minor additional line losses calculated.
Longwall moves were also reviewed for possible changes or affects. Approximate-ly, each additional 100 ft of face width requires the addition of 15 2-m shields. The normal move time for face width of 1,050 ft with a complete extra face conveyor and electrical system takes approximately 12 days. The increase in face width from a 1,050-ft face width to 1,250-ft face width resulted in an additional two days of move time for additional equipment and hosing required, resulting in a 14-day move time. The increase in face width to 1,600 ft resulted in an anticipated longwall move time of 17 days and two shifts. This additional move time was a result of the extra equipment, larger drive motor cables, extra hydraulic hosing, and two shifts were added to account for the removal of larger drive power units and the two-piece headgate drive base on recovery.
Power moves were also reviewed because of the larger power centers, additional hydraulic hose, and larger drive power cables. After review, no significant change in time or work resulted from the change.
Additional work was completed to review the effects on component life due to the extended panel life and additional tons, as shown in Table 7. Since there are considerably more tons in each panel, the equipment had to last longer between rebuilds, or in some cases, mid-panel repair change outs had to be considered. The higher wear items, such as the face conveyor chain and sprockets, shearer components, stageloader chain and sprockets were reviewed. The current wear on the components required no additional changes or maintenance time that was currently planned from the 1,050-ft face when extended to 1,250 ft width.
When the face was widened to 1,600 ft, additional maintenance would be required. The face conveyor chain would only be used for one panel instead of the reuse on the second panel as previously done. The headgate sprocket would be changed out during a mid-panel planned maintenance period. The tailgate sprocket would be strategically spotted in the tailgate entry or a box made up near the tailgate on the face to house a spare sprocket. The shearer would undergo a component change out including cutting drums during the mid-panel planned repair period.
While in the 1970s and 1980s, up to 25% of the coal produced from a longwall mine would come from development, this has dropped considerably and now tends to be around 10% in a modern operation. However, gateroad development is becoming the major cost driver in longwall mines. Improved longwall technology and resulting retreat rates continue to make advance rates of continuous miners critical factors in the cost-effectiveness of longwall mines.
The changes in longwall methods and mining operations are significant and are forcing a catch-up process for development with continuous miners. The aim of using a wider face is to reduce the longwall retreat rate without sacrificing productivity, and thereby reducing the pressure on increasing panel development rates. However, a systematic approach is being used to analyze potential increases in performance and production, as well as to assess the risk potential in the decision-making process to move forward with extended longwall face widths. Each part of the overall longwall system contains its own inherent potential risk characteristics. Moving forward to field test each part is the next step in this evaluation.
This process does not end with the analysis of one process, continued observations and data collection in variable conditions are required to evaluate the performance as a complete system. Also, the evaluation of the difference in required maintenance relative to the larger face conveyor equipment, multitude of mechanical and hydraulic component requirements, and manpower are required. The increase in face width inherently translates into increases in the level of, and an increase in the time required for tear-down and installation time during the moves between panels. These processes were also reviewed and evaluated through underground observations. Since technology is always advancing, projects need to be reviewed again and again to further reduce the risk.
Through engineering design and field observations, extended longwall faces have proven to be successful with the current available technology. It is recommended that each mine should conduct an in-depth review of the site specific conditions to identify differences and conduct a risk assessment to enable the advantages of wider longwall faces to become a successful part of their business model.
The junior author would like to acknowledge the support of Chinese NSF under project No. U1261207 during preparation of this paper.
References
1) “Longwall Mining”, Energy Information Administration, Office of Coal, Nuclear, Electric and Alternate Fuels, U.S. Department of Energy, March 1995.
2) “Longwall Mining” second edition, Syd S. Peng, 2006, 321 pp.
3) “Analysis and prediction of longwall methane emissions: a case study in the Pocahontas No. 3 Coalbed, VA.” NIOSH Report of Investigations (RI 9649), Diamond, W.P., Garcia, F. 1999.
4) “Data Prediction of longwall methane emissions and the associated consequences of increasing longwall face lengths: a case study in the Pittsburgh Coalbed”, S.J. Schatzel, R.B. Krog, F. Garcia, and J.K. Marshall, NIOSH, Pittsburgh Research Laboratory, Pittsburgh, PA, USA, J. Trackemas, 2006.
