BOREHOLE SHUT OFF VALVE FOR GAS DRAINAGE SYSTEMS

ABSTRACT

This report describes the development and form of a shut off valve designed for gas drainage systems in underground coal mines. The valve is designed to close off flow from boreholes in the event of damage to the gas drainage pipeline.

The shut off valve developed is designed to trigger on pressure changes within the gas drainage pipeline. The principal mode of operation is through a step function pressure change associated with pipeline breakage. The pressure change may be either positive or negative depending on the pressure within the pipeline prior to breakage.

Gradual pipeline pressure fluctuations will not trigger the valve to close however pressure rises beyond a threshold will do so. Thus pipeline crushing or inadvertent valve closure will lead to borehole shut off.

The valve actuation mechanism may be used to control larger valves in the main pipeline. It also has potential use in gas supply situations.

Ian Gray

June 1999

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BOREHOLE SHUT OFF VALVE FOR GAS DRAINAGE SYSTEMS

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1. BACKGROUND

The development of the borehole shut off valve was a direct result of comments made in the Report on an Accident at Moura No. 2 Underground Mine on Sunday 7 August 1994, Published by the Queensland Government in January 1996. On page 67 of this report the following two paragraphs can be found.

Methane Drainage Installation

There was evidence that the first explosion damaged the mine's methane drainage pipework underground. This may have provided the source of fuel for the second explosion.

There is, therefore, a need to consider the engineering of methane drainage installations so as to minimise damage in the event of an explosion and to prevent contamination of mine airways should damage occur.

The Australian Coal Association decided to put forward the need for a shut off valve to close gas drainage lines as part of its 1996 ACARP research priorities. Sigra put forward a proposal to build such a valve and was in early 1997 awarded a grant to do so.

The design was influenced by comments made by the ACARP adviser, Jon Sleeman who was keen to see a valve which would shut methane off at its source, namely the borehole, and would in addition work on methane drainage lines operating under positive pressure or vacuum.

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2. THE PURPOSE OF GAS DRAINAGE SHUT OFF VALVES

Gas drainage shut off valves should be incorporated into gas drainage systems to cut off gas flows from boreholes in the event of pipeline breakage. Such breakage may lead to the escape of gas into the roadway or of air being introduced to the pipeline. Either situation can lead to the creation of an explosive mixture with the risk of an explosion.

As gas drainage pipelines are inherently weak structures that can be broken by vehicles, rock falls or explosions the only safe place to shut of the gas is at the standpipe as close to the the ribside as is possible. Additional valves within the main pipeline are desirable so that downstream sections may be isolated for repair.

For maximum safety the actuation system controlling the shut off valves should be distributed over multiple sites and not generally reliant on the provision of electricity or compressed air etc.

In addition to valves which shut off flow from boreholes there is a need to shut down sections of methane drainage pipework to stop air being ingested into the pipeline and passing to surface.

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3. PREVIOUSLY EXISTING SYSTEMS

Gas drainage pipeline shut off valves have in the past been available in two forms. One relies on the maintenance of vacuum in the pipeline to keep the valve open. The second relies on the maintenance of compressed air in a secondary pipe to hold the valves open.

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3.1 Vacuum Operated Valves

Automatic shut off valves have in the past been designed to operate where drainage ranges operating under vacuum are in use. In this case the valve has been held open whilst an adequate vacuum has been maintained on the downstream side of the valve. An example is described in REF. 1 which was used as a large valve in a main drainage range. Here the valve is a gate valve with a piston above the gate. The vacuum operates on the top of the piston and atmospheric pressure on the bottom. Thus whilst adequate vacuum is maintained the gate is held open permitting gas to flow. If the vacuum drops in the range then the piston descends under vacuum shutting off the gas flow from the upstream end of the range.

The principal shortcoming of the vacuum actuated valves is the fact that they require vacuum in the gas drainage pipeline to remain open. In the event of a vacuum pump failure it is most undesirable to close the pipeline or boreholes as this is an unnecessary hindrance to gas drainage and will cause problems of pressure surges when these are reopened. These pressure surges may well cause shut off valve oscillations as pressures in the pipeline go from positive to negative and back again as the valve opens.

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3.2 Compressed Air Operated Valves

Shut off valve systems that are maintained by compressed air have advantages over vacuum operated systems. The most important of these is that they can be triggered independently of vacuum and by sensors other than those operated by pressure. Oxygen or gas concentration sensors are such an example. However the valves have significant disadvantages. Firstly they require a separate line to operate. To be effective this line needs to be tied to and break simultaneously with the gas drainage pipeline so as to trigger immediate valve closure. In the event of compressed air failure all valves close unnecessarily and pressures build up. The restoration of compressed air may mean that all valves open simultaneously with resulting huge flows unless manual valve by valve opening is incorporated.

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4. THE SIGRA SHUT OFF VALVE SYSTEM

The Sigra borehole shut off valve is designed to overcome the shortcomings of the vacuum operated valve or valves maintained by compressed air. They are designed to close drainage boreholes at the standpipe in the event of methane drainage pipeline breakage. They may be triggered by a variety of sensors which includes that of a pressure pulse in the gas drainage pipeline. They will operate with a gas drainage pipeline running under vacuum or positive pressure. Each valve has its own sensing mechanism and as such is inherently safer than systems controlled by a single sensor or group of sensors. The mechanism may also be used to shut butterfly or other large valves within the methane drainage pipeline in the event of an emergency.

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4.1 The Sigra Shut Off Valve Mechanism

The shut off valve is actuated by a differential pressure causing a force to be generated across a diaphragm which in turn releases a catch allowing a spring loaded valve to close.

The differential pressure across the diaphragm that leads to triggering may be generated in a number of ways.

The first of these is by the detection of a pressure pulse caused by a break in the gas drainage pipeline that is operating either in vacuum or in positive pressure. Provided that the break causes a significant change in pressure in the pipe, the pressure pulse so generated will trigger the valves upstream and downstream of the break to close. This pressure pulse is transmitted quickly to all valves in the line by the pipeline itself. The sensitivity of the shut off valve can be adjusted so that triggering does not occur as a result of minor pressure fluctuations caused by water surges from boreholes or from holes being added to the line. In the case where pipeline breakage is minor the break may be detected by other means discussed later.

The differential pressure control circuit comprises a simple gas operated analogue computer with a digital outcome - either the valve stays open or the valve is trigged to close. The computer comprises a small pipe connection from the gas drainage pipeline which bifurcates to each side of the diaphragm via control orifices and control volumes. One side of the diaphragm is designed to maintain average pipeline pressure and therefore the control volume is comparatively large and the orifice small, thus maintaining an average pipeline pressure which can be made to change with a time constant of several minutes. On the other side of the diaphragm the control volume is comparatively small with a larger orifice. This means that the time constant may typically be a second, thus damping out short term transients but enabling the diaphragm to be subject to, and deflected by, a differential pressure in the event of pipe line breakage.

The differential pressure threshold required to cause the shut off valve to trigger can be adjusted by changing a spring pressure on the catch.

To permit the valve to be reset the design incorporates a pressure equalization valve. This opens a passage from one side of the diaphragm to the other. Thus pressure transients caused by opening the valve do not cause the diaphragm to deflect and the valve to close.

Figure 1 shows the basic mechanism of the valve.

Figure 2 shows the valve in an installed form. In this form a tube is tapped into the gas drainage range (normally at the junction of pipes) and this is connected to a waste collection cylinder. The purpose of this is to collect any moisture or detritus and it is therefore furnished with drain and vent valves. One port of this cylinder is connected via a tube to the fast response side of the diaphragm. The tube diameter and volume adjacent to the diaphragm serve to damp unwanted pressure fluctuations. A second connection is made from the waste collection cylinder via a tube and orifice to a second cylinder. This acts as a control volume and the orifice as a control jet. The pressure in this cylinder is therefore an average of that in the pipeline. This control cylinder is connected to the slow response side of the diaphragm.

Plate 1

Plate 2

Plate 1 and Plate 2 show the valve prior to installation at Southern Colliery.

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4.2 Operation Following a Step Change in Pressure Caused by Pipeline Breakage

In this mode the shut off valve requires no external controls, power supply, compressed air or sensors. It is thus fundamentally safe, each valve being totally self contained. Thus no matter what damage occurs, such as roof falls, rib spall or vehicle accidents which may occur along the line, the system will remain operational and able to detect pressure pulses caused by pipeline breakage. These pressure pulses have been numerically modelled. Whether the drainage pipeline is operating in suction or in positive pressure a pipeline break in a properly designed gas collection system will lead to a pressure pulse that will trigger the shut off valves.

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4.3 Operation in the Event of a Slow Leak

In the event of a comparatively small leak or gradually enlarging leak in the gas drainage pipeline the pressure change will not be detected and the shut off valves will not be triggered unless some other leak detection system is used.

Where vacuum is used in the drainage ranges the triggering mechanism needs to be based on some system for detecting air (oxygen) in the pipeline. A normal electronic sensor can be used for this purpose. Slowly increasing leakages from pipelines are less easy to detect. Roadway gas sensors may, however, be useful.

The output from any electronic sensor may be used to turn on compressed air to a small diameter pipeline daisy chained to each shut off valve. Here the air will pass through a pressure relief valve into the slow acting side of the diaphragm thus causing it to deflect, the catch to release and the valve to close.

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4.4 Operation in the Event of Slow Pipeline Closure

In the event of a slow pinching of the pipeline or slow closure of a valve in the pipeline the pressure in the line could, if the borehole shut off valves were not actuated, lead to dangerous pressures developing within the pipeline. Despite the slow rise in pressure the valve will trigger and close. This is achieved by placing a pressure relief valve on the slow acting side of the diaphragm. As the pressure in the line rises, so does the pressure on both sides of the diaphragm until the pressure relief valve on the slow acting side comes into operation thus maintaining a constant pressure on that side of the diaphragm whilst the pressure continues to rise on the other side. This situation will only continue until the pipeline pressure exceeds that of the pressure relief valve plus the set threshold (approximately 4 kPa). By this means the shut off valve closes.

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4.5 Operation by External Control

Whilst it is not the preferred means of operation the valve may be triggered by an external compressed air source. In this form an external sensor is used to raise the pressure of compressed air supplied to a tube. Each shut off valve is fitted with a pressure relief valve arranged to allow air into the slow acting side of the diaphragm when the pressure rises above a preset threshold. The valve(s) may be thus triggered by an external compressed air source.

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4.6 Resetting the Valve

The Sigra shut off valve is not normally fitted with an automatic reset as the effect of resetting all valves simultaneously may be most detrimental to the pipeline and dangerous if a reset is actuated without rectification of the cause of valve closure. Rather it is intended that each valve be manually reset as part of a pipeline inspection following triggering.

In the event that a number of valves have triggered and closed it is assumed that a mine official will walk along the pipeline into which boreholes have been discharging and will examine the system for the cause of the valve closure. Whilst walking up the pipeline he will be able to open the pressure equalization valve between the two sides of the diaphragm in the shut off valve.

Having ascertained and repaired the problem that led to the valves closing, the official will return along the pipeline and lift the valve's safety cover. He will then insert a lever into a socket and open the valve slowly to avoid violent pressure fluctuations in the line. He will set the cocking mechanism by pulling the diaphragm and catch back to a central position whilst holding the valve open against the spring.

Having cocked the valve the pressure equalization valve can then be closed on each valve and the safety cover replaced. In the event that the valve is withholding a pipeline or borehole containing a large volume of high pressure gas it may be necessary to leave all pressure equalization valves open until the resetting process is complete and then to close them.

The process of resetting a valve need not take more than 30 seconds once some experience of the procedure is gained.

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5. COMPUTER SIMULATION OF PRESSURE TRANSIENTS IN GAS DRAINAGE PIPELINES

To be able to design the control mechanism for the valve a number of analyses of pipeline flow were undertaken. These involved the numerical simulation of transient compressible flow within the pipeline and the adjacent boreholes.

The results of simulation presented here are based on a gas drainage pipeline connected to a surface vacuum pump 250 m above on surface. The pipeline is of 2250 m length. The pipeline is of 10" (254 mm) diameter. It has eight inlets corresponding to gas drainage bays, each 300 m apart. The first of these is located 150 m along from the base of the vertical pipe section. The pipe is assumed to be flowing methane. Each inlet comprises four boreholes. The total flow, divided equally between the eight inlets is 2000 cu.m/hr at standard temperature and pressure.

Under steady state conditions the pressure distribution in the pipeline begins with a 10.6 kPa vacuum leading to a suction of 6 kPa (95 kPa absolute pressure) at the end of the pipeline.

Figure 3 shows the pressure traces in the event of a pipeline rupture 270 m length from the surface. Even at the end of the pipeline, 2180 m away, a sharp pressure pulse of 4.6 kPa will be obvious. This is more than is required to trigger the shut off mechanism. Pipeline rupture at the upstream end produces a similar effect.

Simulation shows that the effect of closing a valve to an individual borehole is inadequate to trigger other valves in the line. However if a borehole were shut in and developed a pressure of 1 MPa and it were then suddenly opened the pressure pulse would certainly trigger all valves in the line. For this reason any opening of a shut in borehole would have to be done gradually so as not to trip shut off valves.

If a valve in the line on surface were closed suddenly as might occur if the vacuum pump stopped instantly and a check valve closed, then the pressure transient generated would cause the shut off valves to close. Therefore vacuum pump shut downs should be effected gradually.

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6. LABORATORY TESTING OF THE VALVE

As part of the development process the valve was attached to a large gas cylinder (of approximately 0.2 cu.m volume) which simulated a methane drainage range. This cylinder could be either charged or evacuated via pumps. The pressure in this cylinder could be changed by opening a valve to atmosphere.

To monitor pressures during the test process pressure transducers were used. One was connected to the waste collection cylinder, one to the slow side of the diaphragm and one differential pressure transducer was connected between the two sides of the diaphragm. The output of the pressure transducers was logged via an analogue to digital conversion card installed in a computer.

In total nearly three hundred logged tests were performed on the shut-off valve with simulated line pressures between -50 and +50 kPa compared to atmospheric pressure. In its final form the valve was set to infallibly close when the pressure step in the gas cylinder (range) exceeded 3.4 kPa no matter what pressure existed in the cylinder prior to the fluctuation.

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7. FIELD TESTING OF THE VALVE

The valve has been installed for test purposes at Southern Colliery, German Creek Mine. The test involved placing the valve between a standpipe and a large manifold acting to gather gas and as a water trap. The connection between the standpipe and the manifold/water trap was 100 mm diameter pipe and the manifold/trap of 300 mm pipe. Four other boreholes drained into the manifold/trap. The outlet from the manifold/trap was via 150 mm pipe into a 300 mm drainage range which then passed up a cased vertical hole to surface. Within the 150 mm pipe adjacent to the 300 mm range was a tapping and valve. The tapping was used as the pressure sensing point for the shut off valve.

The gas make from the boreholes in the area was not very great and less than 1 kPa positive pressure existed in the drainage range. Under these pressures the shut off valve would not actuate. To simulate a more normal line situation the range pressure was raised slightly by partially closing a valve so that 4 kPa was generated upstream of it. Pipeline breakage was then simulated by rapidly opening this valve to drop the pressure in the range back to less than 1 kPa. This was repeated several times and the valve reliably shut.

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8. CONCLUSIONS

The Sigra borehole shut off valve system has significant advantages over other options. Each shut off valve can independently detect and close off flows from individual boreholes into a gas drainage pipeline in the event of pipeline breakages which lead to sharp steps in pressure of greater than 4 kPa. In addition it does not rely on any special connection to the valve other than the gas drainage pipeline.

This pipeline breakage detection operates whether the pipeline is operating in positive pressure or vacuum.

Slow removal of vacuum does not lead to line valve closure. Hence vacuum pumps can be taken off line in a controlled manner and the gas drainage system will continue to operate.

The shut off valves will trigger in the event that pressure in the pipeline exceeds a certain threshold.

It is possible to trigger the valves externally by the use of a compressed air line which could for example be triggered by an oxygen sensor in the pipeline.

A particular use of the valve is in protecting gas drainage systems using vacuum from ingesting air from a borehole when that borehole is intersected by mining operations. In this case the pressure sensing tapping may need to be connected more closely to the standpipe where a pressure fluctuation will be observed when the hole is mined through.

Whilst the unit produced operates a 50 mm bore ball valve rated to 7.0 MPa the mechanism can be used with a variety of ball and butterfly valves of differing sizes.

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9. REFERENCES

REF 1 Gray, Ian (1980). Overseas Study of Japanese Methane Gas Drainage Practice and Visits to Coal Research Centres June - August 1980. Australian Coal Industry Research Laboratories Ltd Published Report 80-15, September 1980. ISBN 0 86772 056

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10. ACKNOWLEDGEMENTS

Sigra wish to thank the Australian Coal Association Research Program for funding this project. ACARP project coordinator Jon Sleeman highlighted the need for this project and provided significant input to discussions on operational requirements of the valve.

John Wedmaier produced the first prototype from conceptual patent drawings whilst Fred Howie machined the final design. Both contributed significantly as did Gary Paradise who set up the test rig and made many little but important adjustments.

Ron Cassidy and Tim Harvey are thanked for their assistance in arranging the test at Southern Colliery.

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11. LIST OF FIGURES AND PLATES

Figure 1. Schematic Drawing of Shut Off Valve

Figure 2. Shut of Valve in Installed Form

Figure 3. Simulated Pressure Traces at Varying Distances Along A Gas Drainage Pipeline Ruptured 270 m from the Vacuum Pump.

Plate 1. Mechanism Side of Gas Drainage Shut Off Valve

Plate 2. Reverse Side of Gas Drainage Shut Off Valve