1 AUSTRALIA Patents Act 1990 VALLEY LONGWALL INTERNATIONAL PTY LTD COMPLETE SPECIFICATION INNOVATION PATENT Invention Title: Vacuum relief valve The following statement is a full description of this invention including the best method of performing it known to us:- 2 Cross-Reference to Related Applications The present application claims priority from Australian provisional patent application No 2011903613 filed 6th September 2011 entitled "Vacuum relief valve" the content of which is incorporated herein by reference. 5 Field of the Invention This invention relates to a vacuum relief valve, particularly for use in underground mining. 10 Background of the Invention Underground mines are typically hot, dirty and dusty and require ventilation particularly for the removal of gas, dust and small particles of debris from mining operations. Ventilation ducts are used in mines to vent gas, dust and debris laden air from mines and are most commonly used in longwall coal mining to remove the gas, 15 dust and debris caused by the cutting of the coal at the coal cutting face. To meet the Australian standards, ventilation tubes have to be fire resistant and also anti-static, as the built up of static electricity underground can cause a spark and ignite a fire due to the presence of flammable gases such as methane, as well as combustible coal dust. One material which is commonly used is fibre glass, which is 20 fire resistant and can be formulated with anti-static properties. Fans or the like are provided to suck dust laden air through the tube system. In a typical system, the vent tubes are around 618mm diameter tubes and run to the coal face. The fans are positioned about 150m from the coal face, at a distal end of the tube and have a capacity of 18-23 cubic metres/second so are quite powerful. 25 One problem with ventilation ducts is that they can from time to time become blocked with debris and being fibre glass, may collapse or become damaged. Further, they may become blocked by a person being drawn into the end of the ventilation system, which can lead to serious injury to that person. The present invention aims to address and alleviate such problems. 30 Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. 35 3 Summary of the Invention In a first broad aspect, according to the present invention there is provided a ventilation tube for underground mining incorporating a relief valve and a means for detecting a decrease in pressure in the tube in use, and arranged to open the relief valve 5 on detection of the decrease in pressure. Typically, the valve is biased to the closed position against a differential pressure causing the valve to open, due to the pressure inside the tube being lower than that outside the tube, in normal use. The valve may be of the butterfly type including a disc pivoting about a central 10 pivot. The biasing means may include a lever having a length, and fixed to the pivot of the disc and arranged to turn as the valve opens and vice versa. The adjustment of the valve opening pressures may be via a magnet movable along the length of the lever. The magnet will apply a predetermined force on the lever 15 inhibiting it from moving. By moving the point of action along the lever the moment and torque applied by the magnet changes thus adjusting the sensitivity to opening of the valve. The use of a magnet is particularly effective for use underground as it is inherently safe, predictable and reliable under the extreme conditions present 20 underground, and requires a minimum of maintenance. Typically the settings on the valve will be adjustable to cause it to open at different pre-set negative pressure within the tube, in a range from -3kPa to -I6kPa. Typically, the relief inlet will be covered with a mesh or the like and most preferably will comprise a mesh or perforated skirt and a mesh or perforated end face. 25 Advantageously the present invention can automatically relieve suction in a vent line when a blocked line occurs in the vent line, increasing safety underground reducing the risk to underground workers and also reducing the likelihood of damage to the ventilation tubes which can fracture or implode under severe negative pressure. 30 Brief Description of the Drawings A specific embodiment of the present invention will now be described by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows an isometric view of a ventilation tube assembly incorporating a relief valve embodying the present invention; 35 Figure 2 shows a close up view of one end of the assembly of Figure 1 showing the relief valve closed; 4 Figure 3 is a similar view to Figure 2, but showing the relief valve open; Figure 4 is a perspective view of a latch for manual opening or closure of the relief valve and an associated cover; and Figure 5 is a view of the latch only of Figure 4. 5 Detailed Description of a Preferred Embodiment Referring to the drawings, Figures 1 to 3 show an underground ventilation tube assembly 10 incorporating a relief valve. The assembly 10 includes a first elongate tube 12 made of an anti-static rated fibre glass material. A second tube 14 made of the 10 same material is connected to the first tube 12 at an angle. The tubes are in fluid communication so that air can flow from one into the other. That second tube 14 has an open end 15 which in use is connected to a ventilation ducting line (not shown). The dusting system will typically comprise at least one ventilation line which extends to a coal cutting face. 15 At one end, the first tube 12 has a open end 16 for direct connection to a fan. At the opposite end 17, a relief inlet 18 is defined. The relief inlet is covered by a protective mesh safety grill/guard 20 which includes an end face 22 and a tubular mesh skirt 24. The mesh inhibits unauthorised access to the relief valve and also inhibits persons and foreign objects (of a size greater than the mesh openings) from being 20 drawn into the ventilation system when the relief valve is open. The mesh skirts help to reduce the intensity of the suction if a person or foreign object is sucked onto the end face 22. A relief valve of the butterfly type including a disc 30 pivotable about a central axis 31 and a valve seat 32 for sealing the tube is located adjacent the end 17. In the 25 open position shown in Figure 3 the disc is pivoted about the axis so that it is parallel to the tube and air can flow through the tube. In the closed position shown in Figure 2, the disc seals against the valve seat, preventing air flow through the inlet 18. In use, due to the fan, the pressure inside the tube 12 is less than that outside the tube so there is a differential pressure acting on the disc which would, in the absence of 30 any other forces, cause the disc to pivot and the valve to open. Hence, in use, the valve needs to be biased to the closed position. A latch 40 best seen in Figures 4 and 5, is provided for manual opening of the valve. The latch includes a lever 42 one end 43 of which is fixed to the pivot axis 31 of the disc 30 for operating the valve. When the valve opens, the lever moves and vice 35 versa. A locking pin 44 can be used to prevent movement of the lever and hence prevent opening of the relief valve in the event that it is not required. The latch 40 is 5 surrounded by a circular wall 46 which has a hinged mesh cover 50 to inhibit unauthorised access to the latch. A slot 47 extends along the lever and a magnet 48 can be moved back and forth along the slot to adjust the sensitivity of the relief valve and the negative pressure at which the valve opens, in use. Moving the magnet in the 5 direction A away from the pivot increases the leverage and the resistance to opening of the valve. Moving the magnet in the opposite direction reduces the resistance to opening. Once the correct sensitivity is achieved the magnet can be fixed in position. The valve can be adjusted to open when the negative pressure is from -3 to -13kPa (i.e the pressure on the interior face of the disc is from -3kPa to -13kPa less than that at its 10 outer face. The particular setting used will depend on the length and conditions in the system, and will typically be adjusted to suit once installed. The setting can be tested by installing a regulator in-bye of the relief valve and closing manually. The valve can be opened and closed manually at any time using the lever. A typical ventilation system operates at a pressure of from -5 to -10 KPa 15 depending on the fan being used and the distance of the fan to the coal cutting face. The biasing provided by the magnet needs to be sufficient to overcome the force tending to open the valve due to the differential pressure in the tube. When a blockage occurs in the vent line, the negative pressure increases to from -6 to -13 kPa. In use when there is a blockage in the line, the negative pressure in the tube increases and if it 20 exceeds the negative pressure at which the valve is set to open, the valve opens relieving straight through to the fan, bypassing the vent line to the face and allowing the blockage to be removed. When the valve is opened extreme suction will occur at the inlet end 18 of the tube which can be stopped either by closing the valve manually or switching off the fan 25 (not shown). It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not 30 restrictive.