PURPOSE:  In response to the 2006 MINER Act, this project will provide a scientific and engineering justification to increase the 140 kPa (20 psi) design standard for seals.

RESEARCH SUMMARY:  Seals are dam-like structures constructed in underground coal mines throughout the U.S. to isolate abandoned mining panels or groups of panels from active workings. Historically, mining regulations required seals to withstand a 140 kPa (20 psi) explosion pressure; however, the 2006 MINER Act requires MSHA to increase this design standard by the end of 2007. This report provides a scientific and engineering justification to recommend a three-tiered explosion pressure design criteria for new coal mine seals. Much of this information also applies to existing seals.

NIOSH engineers examined seal design criteria and practices used in the U.S., Europe and Australia and then classified seals into their various applications. They next considered various kinds of explosive atmospheres that can accumulate within sealed areas and used simple gas explosion models to estimate worst-case explosion pressures. Three design pressure pulses were developed for the dynamic structural analysis of new seals under the conditions in which those seals may be used: unmonitored seals where there is a possibility of methane-air detonation behind the seal; unmonitored seals with little likelihood of detonation; and monitored seals where the amount of potentially explosive methane-air is strictly limited and controlled. These design pulses apply to new seal design and construction.

For an unmonitored seal with the possibility of detonation, the recommended design pulse rises to 4.4 MPa (640 psi) and then falls to the 800 kPa (120 psi) constant volume explosion overpressure. For unmonitored seals without the possibility of detonation, a less severe design pulse that simply rises to the 800 kPa (120 psi) constant volume explosion overpressure, but without the initial spike, may be employed. For monitored seals, engineers can use a 345 kPa (50 psi) design pulse if monitoring can assure that the maximum length of explosive mix behind a seal does not exceed 5 m (15 ft) and that the volume of explosive mix does not exceed 40% of the total sealed volume. Use of this 345 kPa (50 psi) design pulse requires monitoring and active management of the sealed area atmosphere.

NIOSH engineers used these design pressure pulses along with the Wall Analysis Code from the U.S. Army Corps of Engineers and a simple plug analysis to develop design charts for the minimum required seal thickness to withstand each of these explosion pressure pulses. These design charts consider a range of practical construction materials used in the mining industry and specify a minimum seal thickness given a certain seal height. These analyses show that resistance to even the 4.4 MPa (640 psi) design pulse can be achieved using common seal construction materials at reasonable thickness, demonstrating the feasibility and practical applications of this report. Engineers can also use other structural analysis programs to analyze and design seals by using the appropriate design pulse for the structural load and a design safety factor of 2 or more. Finally, this report also provides criteria for monitoring the atmosphere behind seals.

NIOSH will continue research to improve underground coal mine sealing strategies and prevent explosions in sealed areas of coal mines. In collaboration with the U.S. National Laboratories, NIOSH´s new project will further examine the dynamics of methane and coal dust explosions in mines and the dynamic response of seals to these explosion loads. This work seeks better understanding of the detonation phenomena and simple techniques to protect seals from transient pressures. Additional work will conduct field measurements of the atmosphere within sealed areas. Successful implementation of the seal design criteria and the associated recommendations in this report for new seal design and construction should significantly reduce the risk of seal failure due to explosions in abandoned areas of underground coal mines.