EXPLOSION PRESSURE DESIGN CRITERIA FOR SEALS IN 

U.S. COAL MINES - AN UPDATE ON WORK AT NIOSH 

R. Karl Zipf Jr.t

INIOSH - Pittsburgh Research Laboratory, P.O. Box 18070,626 Cochrans Mill Rd. 


Pittsburgh, PA 15236, Email: rzipf@cdc.gov 


, Jiirgen F. Brune2

2NIOSH - Spokane Research Laboratory, 315 East Montgomery, Spokane 


WA 99207, Email: jbrune@cdc.gov 


and Edward D. Thimons1 

INIOSH - Pittsburgh Research Laboratory, P.O. Box 18070,626 Cochrans Mill Rd. 


Pittsburgh, PA 15236, Email: ebt7@cdc.gov 


ABSTRACT 

A fatal explosion in a sealed area at the Sago Mine in 2006 prompted researchers and 

regulators in the United States to re-examine the requirements for explosiol1-resistant 

mine seals. Seals are used in underground coal mines to isolate abandoned mining 

areas from active workings. Prior to the Sago disaster, mining regulations required 

seals to withstand a 140 kPa explosion pressure. Recent research at the National 

Institute for Occupational Safety and Health (NIOSH) Pittsburgh Research 

Laboratory showed that pressures from explosions can far exceed this pressure. U.S. 

mining regulations now require much stronger seals. Current research focuses on 

understanding the science, engineering and management of sealed areas of coal 

mines. Scientific efforts seek understanding of how explosive methane-air mixtures 

accumulate using measurements and models of the sealed area atmosphere. 

Additional scientific efforts aim to understand the explosion pressures that can 

develop using a combination of numerical modeling and experiments. Engineering 

efforts aim to produce design guidelines for seals that can resist the new higher 

pressure design criteria. NIOSH researchers are also developing guidance for 

monitoring of sealed area atmospheres and inertization of potentially explosive gas 

mixtures within sealed areas. This paper seeks to encourage scientific discussion 

among international peers since preventing explosions within sealed areas is of 

interest to underground coal operators and regulators. 

1. INTRODUCTION 

The Sago Mine disaster January 2, 2006 caused by an explosion within a recently 

sealed area precipitated many changes to mining regulations pertaining to seals. 

Mandates from the Mine Improvement and New Emergency Response Act (the 

MINER Act) of June 2006 required the Mine Safety and Health Administration 

(MSHA) to increase seal design pressures by the end of 2007. Scientific studies of 

gas explosions within sealed areas (Zipf et al., 2007) provided a basis for the new 

MSHA regulations on sealing of abandoned areas (Federal Register, 2008). 
NIOSH researchers conducted scientific studies of methane-air explosions within 

sealed areas of coal mines by first considering the formation of potentially explosive 

gas mixtures that can develop in sealed areas upon sealing. Starting with an 

atmosphere that is pure air, the methane concentration may increase; the oxygen 

concentration may decrease; or some combination of these changes may occur. 

Based on thermodynamics, chemistry, and physics, NIOSH researchers conducted a 

worst-case analysis of methane-air explosions within the sealed areas of coal mines 

and presented several important facts about possible explosions within sealed areas: 

1. 	 Combustion of stoichiometric (about 10%) methane-air mix in a closed volume 

increases the pressure about 807 kPa. This pressure is called the constant volume 

(CV) explosion overpressure. The CV explosion overpressure is greatest for a 

stoichiometric mix and decreases for fuel-rich, fuel-lean, oxygen-deficient, or 

carbon dioxide-rich mixtures. 

2. 	 Combustion of coal dust in air in a closed volume produces a somewhat lower 

CV explosion overpressure of about 690 to 790 kPa. 

3. 	 Due to dynamic effects, explosions in tunnels produce transient pressure waves 

that are greater than the CV overpressure. 

4. 	 When a blast-created shock wave with a quasi-static overpressure impacts a 

structure, it reflects with a transient reflected wave overpressure that is 2 to 8 

times greater than the incident quasi-static overpressure. 

5. 	 If detonation of a stoichiometric methane-air mix develops, the maximum 

detonation wave overpressure is 1.66 MPa. When a detonation wave impacts a 

structure, it reflects at a pressure of about 4.40 MPa which is about 2.54 times 

greater than the incident detonation wave pressure. 

The NIOSH report also presented simple numerical model calculations of explosion 

pressures within mine tunnels. Using the gas explosion models AutoReaGas (ANSYS 

Inc., 2009) and FLACS (Gexcon, 2009), which are commonly applied throughout the 

oil, gas, and chemical industries, researchers calculated explosion pressure at a seal. 

Figure I shows the simple mine layout and the calculated explosion pressure. The 160 

m long gas cloud that filled 3 entries and the cross-cuts developed explosion pressure 

ranging from about 2.4 to 3.3 MPa. 

To demonstrate the possibility of the high explosion pressures, recent experiments by 

NIOSH researchers at the Lake Lynn Experimental Mine (LLEM) produced such 

pressures from very small explosive gas clouds (Sapko et at., 2009). In one 

experiment, a methane and coal dust cloud with an effective length of 38 m long 

developed an incident quasi-static blast wave pressure of 324 kPa which then 

produced a reflected explosion overpressure of about 1.124 MPa on an experimental 

structure. 
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Figure 1: Calculated gas explosion pressure from numerical models at a seal fron 

160 m long gas cloud. Model gas cloud geometry is above (Zipf et aI., 2007) 
NIOSH researchers then presented a three-tiered recommendation for explosion 

pressure design criteria for seals as shown in Figure 2. 

FLOWCHART FOR SELECTING DESIGN OF NEW SEALS 


Monitored sealed area 

-Use 345-kPa (50-psi) 

pressure-time curve for 

seal design 

- Limit explosive mix 


behind seals to. 


less than 5m 


- Limit volume of 


explosive mil< to 


less than 40% 


ofsealed volume 


Use 4.4·MPa (640-psi) Use8l)O kPa (120-psi) 

pressure-time.curve pressure-time curve for 

for seal design seal desi n 

Figure 2: Design criteria for various sealing conditions (Zipf et aI., 2007) 

Application of these criteria 

depends on the monitoring regimen applied. If the sealed area is monitored 

continuously during and after sealing and if the potential size of explosive mixture is 

limited to a less than 5 m long space right behind a seal, then a 345 kPa explosion 

pressure-time curve applies. However, if the sealed area atmosphere is not monitored, 

then much larger explosive gas volumes and much higher explosion pressures can 

develop. If the open entry behind the seal is small with a length less than 50 m, then 

the 800 kPa pressure-time curve applies. If the open entry behind the seal is large 

with a length more than 50 m, then the 4.4 MPa explosion pressure-time curve 

applies. Note that it is not necessary for an explosive methane-air mixture to detonate 

to achieve high explosion pressure. Non-reactive blast waves from ordinary 

defiagrations can easily develop reflected pressures greater than 1 MPa as the 

experiments at LLEM have demonstrated for gas clouds as small as 26 m long. 

2. KNOWLEDGE GAPS WITH SEALS AND SEALED AREAS 

The recent NIOSH study (Zipf et aI., 2007) coupled with the U.S. Army Corps of 

Engineers (USACE) report (McMahon et aI., 2007) provided complementary, 

independent analyses of worst-case explosion pressures that could develop in sealed 

areas of coal mines. The discussions in the mining community ensuing from these 

reports and comments on the new seals regulation identified numerous unknowns and 

knowledge gaps with seals and the sealed areas of coal mines. NIOSH researchers 

categorize these knowledge gaps into three main themes: 
1. 	 Science of sealed areas including the sealed area atmosphere composition, 

explosion processes, and the explosion pressures that could develop. 

2. 	 Seal engineering including failure mechanisms of seals, analysis and design of 

seals, and engineering methods to account for the seal foundation and convergence 

in seal design. 

3. 	 Management of sealed areas and their atmospheres including ways to plan mines 

for future sealing along with monitoring and inertization techniques for sealed 

areas. 

3. NIOSH RESEARCH PROGRESS TO ADDRESS SEALING ISSUES 

3.1 Composition of Sealed Area Atmosphere 

The composition of the sealed area atmosphere is not well understood. Prior to the 

Sago Mine disaster, few researchers had ·measured the composition of the atmosphere 

within sealed areas. The U.S. mining community erroneously assumed that the sealed 

area atmosphere would become inert rapidly upon sealing and then remained inert. 

Gadde et al., (2009) presented the first composition data from thousands of sealed 

area atmosphere samples from a few mines. More than 99% of the samples were inert 

and less than 1% were potentially explosive with oxygen concentration above 10% 

and methane concentration between 8 and 12%. Thus, it appears that the probability 

of encountering a potentially explosive atmosphere within a sealed area is low. 

However, the 12 documented explosions within sealed areas that occurred between 

1986 and 2006 demonstrate the inadequacy of assuming an inert atmosphere. 

Understanding the composition of the sealed area atmosphere and how it changes 

across the sealed area and over time is important for understanding the blast pressure 

that could develop from an explosion and assessing the risk associated with this 

danger. NIOSH researchers have acquired from SIMTARS in Australia a monitoring 

system that is capable of measuring the composition of the sealed area atmosphere 

and tracking its evolution continuously over time. NIOSH researchers seek an 

appropriate coal mine with sealed areas in which to deploy this system for research 

and technology demonstration purposes. NIOSH researchers would place sampling 

tubes throughout an abandoned area prior to seal construction and final sealing. 

3.2 Gas Explosions - Numerical Simulations and Experiment 

Explosion processes within sealed areas are not well understood. Some question 

whether the high explosion pressures presented in the recent NIOSH and USACE 

studies could ever occur in a mine. Others question whether detonation of methane-air 

mixtures is a physical possibility. The process by which a weak spark ignition grows 

from a laminar flame to a deflagration and then possibly a detonation is not well 

understood for methane-air. Whether deflagration-to-detonation transition (DDT) can 

occur in methane-air mixtures and whether the process can occur in a mine requires 

further study. 

To simulate high pressure methane-air explosions in coal mine entries, NIOSH 

researchers have constructed a gas explosion tube with a diameter of 1048 mm and a 

length of 73 m as shown in Figure 3. This tube is almost twice the diameter of a tube 
used for similar experiments by Kuznetsov et aI. (2002), and it should have sufficient 

diameter to support a true detonation of methane-air. The objectives for experiments 

with this tube are (1) to measure maximum explosion pressures for various mixtures 

of methane, air and inert gases, (2) to measure the distance required to accelerate a 

flame, achieve high pressure and develop possible detonations, and (3) to validate 

numerical gas explosion models of these processes. In addition to varying the 

explosive gas mixture composition, the experimental program will vary the number of 

turbulence-generating obstacles in the tube along with the blockage ratio. 

Figure 3: 1048 mm diameter (42 in) gas explosion tube at NIOSH Lake Lynn 


Laboratory 


Naval Research Laboratory (NRL) researchers have recently simulated deflagrationto-detonation transition (DDT) of methane-air mixtures using state-of-the-art reactive 

flow programs (Kessler et aI., 2008). The NRL model solves numerically the NavierStokes equations for fluid dynamics and considering the viscosity, diffusion, and 

thermal conductivity of methane-air and the reaction products. Figure 4 shows 

calculations of a turbulent flame, shock wave development, and the initiation of a 

detonation in methane-air. The first two frames show the turbulent flame front and a 

shock wave traveling at the local sound speed ahead of the flame. When this leading 

shock wave impacts an obstacle (the second baffle), it ignites a detonation as shown 

beginning in the third frame. In Figures 4, 5, and 6, the detonation front travels 

supersonically and rapidly overtakes the leading shock wave. Beginning in frame 7, 

the reaction continues as a detonation. 

3.3 Seal Engineering 

Developing engineering procedures for the mining community to use for new seal 

design is another key component of on-going research. NIOSH researchers are 

collaborating with researchers at USACE and West Virginia University (WVU) to 

produce these guidelines. Major efforts to date include: (1) cataloging existing 20 psi 

seal test data, (2) developing basic seal analysis methods, (3) developing seal 

foundation analysis methods, and (4) developing simple, cost-effective seal designs to 

resist high explosion pressures. 
USACE researchers recognize three distinct analysis methods for seals: (1) bending 

beams and plates, (2) shear plugs, and (3) arching. A bending-beam analysis applies 

when the thickness-to-span ratio for the structure is less than 4. The failure mode for 

a bending structure varies, but may involve tensile failure of the outer fibers opposite 

the applied load, compressive failure on the same side as the applied load or shear 

failure near the supports. 

Figure 4: Calculations based on fundamental physics calculate distances to 

deflagration-to-detonation transition for stoichiometric methane-air 

mixtures in channels with obstacles (Kessler et al., 2008) 

Note: Blue represents unburned mixture; the line between green and blue is a shock 

wave, and the line between yellow and blue is a detonation wave 

All three failure possibilities require analysis and design consideration. Shear plug 

analysis applies when the thickness-to-height ratio for the seal is greater than 1. The 

failure mode is either via shear failure through the seal material or through the 

surrounding foundation rock. Arching analysis applies to articulated structures where 

the thickness-to-span ratio ranges from about 2 to 5. The arching failure mechanism 

is via compressive failure of the seal material at the supports in the outer fibers 

opposite the applied load and also at the mid-span on the same side as the applied 

load. Development of the ideal arching mechanism requires an infinitely stiff or rigid 

foundation. 

As documented by USACE researchers (O'Daniel et ai., 2009; Walker et al., 2009), 

the seal foundation influences the behavior of seal structures. For example, as shown 

in Figure 5, weak foundations prevent the development of arching, and average 

foundation conditions in a coal mine may only develop limited arching. Seal 

designers should not use an arching analysis to determine the load bearing capacity of 

a seal since; in general, the method tends to overestimate the strength of a seal and is 

therefore not conservative. However, if the soft foundation rock is excavated to 

competent material, then an arching analysis could apply. 
Loading -

Direction --'-1 

Floor 

No arching in Limited arching in 

weak foundations Strong foundations 

Figure 5: Arching analysis of articulated seal structures. Weaker foundations prevent 


development of arching. Arching cannot be assumed to determine load capacity 


of a seal (O'Daniel et aI. , 2009). 


3.4 Innovative Seal Designs 

NIOSH researchers in collaboration with USACE and the West Virginia Office of 

Miner's Health Safety and Training (WVOMHST) have developed several concepts 

that should provide simple, cost-effective seal designs to resist the explosion 

pressures specified in the new MSHA final rule on seals or even higher worst-case 

explosion pressures (Sapko et aI., 2009) . 

Figure 6 shows the Gob Seal with Load Collectors concept developed by USACE. 

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Figure 6: Side view of gob seal with block load collectors (Walker et aI., 2009) 

This seal uses dry-stacked, concrete block "load collectors" to compress a gob pile. 

Upon compression from an explosion loading, the gob pile expands laterally and 

locks into the surrounding rock. Preliminary analyses by USACE researchers showed 

tl).at a 2.4 m high, 5.6 m thick gob pile with the load collectors could resist the 800 

kPa design pressure. 
3.5 Management of Sealed Areas 

NIOSH researchers have advanced several concepts and technologies relevant to the 

management of sealed areas: (1) mine planning practices, (2) monitoring practices 

and (3) inertization methods. The increases in explosion pressure design criteria for 

new seals have increased the cost of seals which now require more material and labor 

to construct. The mining community has expressed concern that the cost of seals may 

lead to abandonment of sealing and continued ventilation of mined-out areas. 

Alternatively, NIOSH researchers suggest that better planning for future sealing can 

decrease the number of seals required for lesser net increase in the cost of sealing an 

area. Better pre-planning for sealed areas can decrease the number of seals required 

and decrease the ventilation pressure differential across sealed areas. NIOSH 

researchers plan to document improved mine layouts that facilitate mine sealing. 

Continuous monitoring behind seals to ensure that the sealed area atmosphere remains 

inert enables the use of 345 kPa seals. In Australian practice, continuous monitoring, 

coupled with inertization of the sealed area atmosphere as needed, eliminates the 

explosion hazard by preventing the accumulation of an undetected, potentially 

explosive mixture within the sealed area. The alternative approach is containment of a 

potential explosion with stronger seals. Tube-bundle gas monitoring systems are 

utilized at every underground coal mine in Australia, and NIOSH researchers hope to 

hasten their adoption by U.S. coal mining companies. 

To facilitate the development and adoption of inertization technology for sealed areas, 

NIOSH contracted with On Site Gas Systems (OSGS) to build an in-mine mobile gas 

generation plant to extract nitrogen gas from the mine atmosphere. The 

demonstration unit uses pressure-swing-adsorption separation technology, produces 

about 8.5 m3 per minute, is less than 1.3 m high, and fits on a standard shield carrier 

used in underground coal mines (Trevits et aI., 2009). 

4. SUMMARY AND CONCLUDING REMARKS 

NIOSH is undertaking a major research effort on seals and sealed areas of coal mines. 

Recent scientific studies of explosions within sealed areas and the new regulations 

have made the mining community aware of the potential danger posed by seals and 

sealed areas of coal mines. NIOSH researchers aim to provide engineering guidelines 

for meeting all aspects of the new MSHA regulations. The new guidelines should 

provide multiple solutions for sealing that go beyond a "one-size-fits-all" approach. 

In developing proper engineering design codes for seals and sealed areas, the authors 

advocate seal designs to resist pressure-time curves that are based on an 

understanding of the risk involved and not just a simple worst-caSe analysis. 

DISCLAIMER 

The findings and conclusions in this report have not been formally disseminated by 

the National Institute for Occupational Safety and Health and should not be construed 

to represent any agency determination or policy. Mention of any company or product 

does not constitute endorsement by the National Institute for Occupational Safety and 
Health (NIOSH). In addition, citations to Web sites external to NIOSH do not 

constitute NIOSH endorsement of the sponsoring organizations or their programs or 

products. Furthermore, NIOSH is not responsible for the content of these Web sites. 

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