Published in the May 2009 Coal USA Magazine
As part of the National Institute for Occupational Safety and Health’s ongoing mine fire research program, the institute partnered with Strata Products to test the sealing capability of temporary ventilation control device Ventstop.
The team, led by Minvent’s HJ (Bill) Reolofs, Strata’s Cliff McCartney and NIOSH’s Michael Trevits, wanted to see if Ventstop could be successfully used by the US coal mining industry to temporarily close an underground mine area in response to a fire or spontaneous combustion heating event, and to temporarily redirect mine ventilation during longwall equipment moves or construction of stoppings.
Ventstop, manufactured and marketed by suppliers Strata Products and Minvent Solutions, is a multi-purpose, inflatable device originally intended for use as an emergency, temporary ventilation control device. Its function was to control ventilation in breakthrough situations and to combat radiation dust, thermal heating return ventilation, blast fumes and fires.
Ventstop was initially introduced into the Australian coal industry in 2005.
Quickly deployed, the unit is reusable and can be placed in both horizontal and vertical mine voids.
The standard unit is constructed from a polyester scrim-based fabric with a double-sided polyvinyl chloride coating that meets US MSHA requirements.
In general, a Ventstop unit will accommodate entry-width variations of 2 feet and height variations of 3ft.
To ensure best fit, the unit is typically sized 1ft larger in diameter than the mine height, primarily due to rough and irregular mine opening dimensions. The unit should be no more than 1-2ft longer than the entry width to minimize the wrinkling possibility as well as enhance the ability to ventilate the space even when inflated.
Currently, the Ventstop unit is available in three standard sizes: 7, 10 and 13ft diameters by 18, 20 and 22ft lengths – designed to fit maximum mine openings of 6, 9 and 12ft, respectively, though a unit can be custom-built if needed.
The portable unit contains a single chamber that can be inflated from one fill port, using either mine compressed air or bottled compressed air, with a venturi or powered blower. The inflation pressure is very low, in the order of 0.25 to 0.35 pounds per square inch, and the unit can be kept inflated for days using a filtered pressure demand control system.
This system provides a small gas flow to the unit to maintain appropriate pressure, and can keep a unit inflated even when it contains small holes or cuts.
Holes or cuts can be temporarily repaired using a self-adhesive patch and permanently repaired by welding on a fabric patch in fresh mine air.
Taking a licking in testing
Deployment, multi-day inflation and air leakage tests were conducted at the NIOSH Lake Lynn experimental mine in Pennsylvania to determine Ventstop’s capability and limitations in a simulated mine setting.
In the first evaluation, a prototype constructed of fire-retardant and anti-static fabric was used. The cylindrically shaped unit measured 10ft in diameter, 20ft long and weighed 150lbs.
Installation tests showed Ventstop could be quickly deployed by two people from the carry bag to full inflation underground in approximately 7.5 minutes (5.5 minutes to inflate only). Deflation, folding and replacement into the carry bag took about 15 minutes (11 minutes to deflate only).
The tests noted difficulties installing in high-flow mine ventilation air, a problem that is solved with more robust hold-down loops or propping the unit during inflation against a series of posts, cribbing blocks or other supplemental roof support structure.
Once installed in the mine opening, 91.5ft2 of contact area (5ft by 18.3ft) was made at both the mine roof and floor areas, with a total contact area of 340ft2, including the roof, floor and side areas.
A series of air leakage tests determined the sealing capability and limitations of the prototype unit.
During one test, air leakage past the unit was measured at two different inflation pressures that would be used in an actual underground mine setting.
It was found that air leakage decreased by about 25% at the higher inflation pressure. In all likelihood, the increased pressure forced the unit to fit more tightly across the irregular surface of the mine opening.
Air leakage testing also showed that the seal provided by the unit could be further enhanced through the application of polyurethane (PUR) sealant. During a test, PUR sealant was sprayed along the entire perimeter of the upwind side of the Ventstop unit where it interfaced with the mine opening.
The expanded thickness of the applied sealant was about 5in and tapered in thickness to 1/8in at a depth of about 18in using the standard spray head provided with the PUR material.
Looking at the air leakage test results before and after PUR application, there was a dramatic decrease in air leakage (by 85%) after application.
Smoke tube testing before PUR application showed most of the air leakage occurred near the mine roof and rib interfaces, and these air leakage paths were mostly eliminated with the application of the sealant.
The unit was also subjected to oscillating pressure pulses from a methane gas ignition. Though the ignition zone was approximately 1250ft away and the resulting pressure pulse degraded as it traveled through a series of drift openings to the unit, the prototype successfully withstood a maximum peak positive pressure pulse of 0.5psi, maximum peak negative pressure pulse of 0.65psi and a peak sweeping pressure value in the 0.5-0.6psi range. Pre and post-explosion tests showed that air leakage past Ventstop was not affected by pressure pulse forces.
Ventstop vs. conventional stoppings
In a previous study by the former US Bureau of Mines, five different stopping construction techniques were evaluated and air leakage tests were performed. The stoppings studied included 1) conventional stoppings built by dry-stacking hollow-core concrete blocks and troweling mortar on one stopping face; 2) quick-build stoppings built in the same manner as conventional stoppings except brushed-on glass-fiber enhanced mortar (modified mortar) was substituted for troweling; 3) universal stoppings built of dry-stacked hollow-core blocks, which had a footer, keyed ribs, sealed roof and one face sealed with one coat of modified mortar; 4) high-pressure stoppings built of hollow-core wet-laid concrete blocks, which had a footer, keyed ribs, a sealed roof, two brushed-on coats of modified mortar on one face and one coat of brushed-on modified-mortar on the other face; and 5) hybrid stoppings built the same way as high-pressure stoppings, except that solid-core blocks were used instead of hollow-core blocks.
The leakage rates for Ventstop at 0.22, 0.36 and 0.36psi inflation pressure with PUR applied from the NIOSH study were compared to the other types of newly constructed stoppings from the USBM study (for comparative purposes, the air leakage rates were converted to cubic feet per minute per hundred square feet of stopping area per inch water gauge pressure differential).
The air leakage rates for Ventstop were among the lowest observed and the rate for Ventstop with PUR applied was the lowest rate measured.
DISCLAIMER: Any mention of a specific product or trade name does not imply endorsement by the National Institute for Occupational Safety and Health.
