DE2719889A1 - Testing and regulating gas producing rubbish dumps - using sonde gauges to prevent extraction rate drawing in air - Google Patents

Abstract

A method of drawing gas from a rubbish dump on which there is a anaerobic fermentation, without introducing air in a quantity which would destroy its capacity to produce methane is carried out in a series of steps. Test shafts are bored, and gas is drawn off whilst maintaining a pressure which is low enough for the gas in the dump to flow into the boreholes being tested. At the time over a region near the test holes, other holes varying in depth and penetrating through the top covering layer into zones where fermentation is going on, are sunk for tubes fitted with pressure gauges. The magnitude of the first pressure at which the gasis extracted is then controlled continuously relatively to the pressure in the zones monitored by the sondes, so as to ensure that pressure does not fall below that of the atmosphere and air be drawn into the dump. On a basis of the tests carried out, a maximum extraction rate is decided. The tests are carried out on destroying the active bacteria by admitting oxygen.

Description

Method of withdrawing gas from a waste pit

Breaking waste in a pit creates gas that contains methane. In some mines the concentration of methane is sufficient for recovery of the gas so that the methane can be used. In some cases the iSlethane is separated from other components of the gas by absorption and in others the ethane is used without removing the impurities.

To recover the gas, a source is lowered into the pit chemical reaction in the pit creates a pressure that is greater than atmospheric pressure so that the gas migrates into the source. To reinforce this natural flow can use a pump to reduce the pressure in the source below the outside world be used.

From the standpoint of the greatest gas generation, it is desirable that the gas Withdraw from the source at the maximum speed at which the pump operated can be. Here, however, air can be drawn into it through the surface will. Air entry into the pit is allowed if the methane is to be avoided the hazard can easily be removed by its combustion properties. ienn However, the gas to be extracted from the source is the entry of air into it the pit completely inadmissible because the oxygen poisoned the microorganisms that are important for the production of oil through the pit. Accordingly, the entry must be avoided by air.

One attempt to avoid air ingress involves pulling the Gas and periodically analyzing its composition. When the gas has a certain Has oxygen content, the withdrawal rate is reduced to in the following remove the oxygen from the gas samples taken. The main problem here is that air entry is only indicated after air has entered and through parts the pit has flowed to the source. So there is no display until the oxygen poisoning takes place. Between the corrective action after the detection of oxygen and there is also a delay in stopping the air supply to the pit. That is why the analysis according to the composition is carried out to avoid poisoning Oxygen inadequate.

A primary problem in extracting ethane from manure pits is hence the choice of the withdrawal speed that is economically feasible and which avoids oxygen poisoning. Such an @roblem is determining of Space between neighboring sources in the pit. A correct distribution the well is important to ensure that all zones of the mine are exposed to the influence are subjected to the source without air ingress.

The invention shows testing a pit to determine the highest > Drawing speed from the source without air entering the pit. These Determination can take place in an initial phase before the source is used Production lies. In addition, the distance can be determined in advance. Finally the invention provides an early goods system for determining as a safety measurement air inlet into the pit during production.

Thus, if air enters during production, it will be early enough indicated before oxygen gets through the pit to the source.

Located without influencing the pit by facilities such as pumps the gas is at static pressure, which is usually slightly above atmospheric pressure lies. Under static conditions the pressure differential tries between the Pit and the atmosphere to allow the gas to escape to the atmosphere. The invention specifies a test program in which the highest pulling speed has a function is that peel speed at which the pressure is in a selected area in the pit is approaching atmospheric pressure. As long as the one in the pit in that selected area does not drop below atmospheric pressure, there is no pressure differential, that would allow air to enter the pit.

The test program according to the invention contains a method for determining before the production of the highest stripping speed, the the Desired pressure ratio between a selected area of the mine and the atmosphere maintains. This happens, for example, through at least one source in the pit and then withdrawing the gas from the source at two withdrawal speeds during two periods. The pressure in the selected area in the pit is during found in both periods. With knowledge of the pressure and the take-off speed can be a general relationship between the pressure in the selected area and the drawing speed getting produced. This relationship is used to determine the draft speed, which would generate a pressure close to zero in this area. This withdrawal speed is the maximum permissible withdrawal speed for the source, which is the one which generates a pressure in the selected area that is approximately equal to atmospheric pressure is.

The same speed can also be used for other wells in the pit used if the pit is assumed to be relatively homogeneous. At the If the maximum speed is exceeded, the pressure in the selected area would fall Atmospheric pressure would drop and the differential pressure at the surface of the pit would Try to draw air into them.

In order to maintain maximum methane production, the draft speed should be equal to the maximum allowable.

Using a slower speed will result in one o safety factor, but at which the gas generation is reduced.

The area in which the pressure is detected is preferably located near the surface of the pit and is for areas allowed that practically removed from the pit surface that long when pressed below @ atmospheric pressure, such as: n the top of the pit is a layer, which is at least at atmospheric pressure. thus the lowest possible pressures can be used in the pit and thus the highest drawing speed is obtained, the selected area is preferably directly under the decking material, that is characteristically placed over the surface of the waste in the pit will. This disorder results in the upper hand area of the waste in the pit a pressure barrier that isolates the interior of the pit from the atmosphere.

The chosen area should be horizontally next to or close to the source because the parts of the pit surface immediately next to the source are mostly i lowest pressure.

If the chosen area is at a considerable distance from the source horizontally is removed, and is held at zero pressure, the pressure is exactly below that Masking material close to the source under atmospheric pressure when the pressure barrier does not cover the whole area of the pit.

The area chosen may be the same as that for the different draft speeds or be different. It can also contain several different zones where Pressures can be read, for example, the uruck readings can be at different Depths of the pit, the accuracy of the pressure information being checked can.

To obtain more analytical data on the pit, is it is often desirable to have a second source in the pit remote from the first preferably to be provided. The probes are then made in a desired pattern and in the desired Lows placed between the two sources. The measures described for determination the maximum permissible draw speed can be used for the second source will.

A second phase of the testing process determines the existence of the sources for a given production speed.

this is the pressure in the pit in horizontally distributed areas determined during the two bzusperiods, which are preferably immediately below the <.covering material of the pit and where to determine the distance the Pressures can be read. The pressures noted during the first period are used then to build up the point closest to the source where the static There is pressure during this period. Those noted during the second periods Pressures will travel in the same direction to determine which is closest to the source Used where the static pressures occur during this period. These locations and the pulling speeds are then used to build a relationship between the shortest distance from the source at which there is static pressure, and the @drawing speed.

For greater accuracy one preferably has three such points. the The resulting curve shows the zone of influence of the source for a given production and speed with which, together with the distance, the greatest possible withdrawal speed can be determined.

because the two sources are mutually exclusive zones of influence own or not be operated at the same time, each can up to the maximum permissible @drawal speed can be operated. hen the sources operated at the same time and overlapping lin flow zones can have corresponding Settings are made.

For example, an empirical setting can be made, which is the maximum allowable withdrawal speed for a given source distance belittles.

After testing, the pit is used for production.

As a safety measure, the printing can be done in one selected area in the pit can be monitored, continuously or intermittently can take place when this pressure drops below atmospheric pressure, the drawing speed can be can be decreased, i.e. the pressure of the source can increase by in the selected Area to prevent pressure build-up to re-establish the pressure limit. When using this system, the print size can be set according to the Pressure can be regulated in the selected area. The area for the pressure measurement is preferably the same or similar to that used to determine the maximum allowable @pulling speed.

The invention is described on the basis of the drawings.

In these: Figure 1 is a plan view of a manure pit from which Mine gas containing ethane can be withdrawn; Figure 2 shows a partial section the line 2-2 of Figure 1; Figure 3 is a section of a preferred source form; figure 4 is a plan view of a preferred special construction; Figure 5 is a section the line 5-5 of Figure 4, with a preferred special arrangement; figure Fig. 6 is a graph of the average special pressure versus peeling speed; figure 7 a graph of the average cover custom print versus distance from source for various withdrawal speeds, and FIG. 8 is a diagram of the source spacing for production speed for both single source and double source production.

Figures 1 and 2 show a manure pit 11, in the anaerobic Decay creates a gas that contains ethane.

The pit 11 is usually filled with waste 13 in one by deposition Cavity 15 is formed in the earth 1 @ and also has a layer of covering material 19, e.g. soil brought over surface 21 of waste 13. Even though the waste 13 in Figure 2 is shown homogeneously - it is not used to this and may have several intermediate layers of earth between the surface 21 and the ground 23 of pit 11 included. The test program according to the invention can be used both in the Cavity pit 11 as well as other pits such as the iintiklinalgrube will.

If the pit 11 contains sufficient wienge ethane, the a To make pulling off economically feasible, the test program at the pit is used to Determine various factors such as the maximum permissible peeling speed and the space between production sources. The first step in the test program is the definition of the test area 25, which is preferably the shape has a circle and is offset from the edge 27 of the pit 11 inward. For example the test area 25 can one 150 meters in diameter and be at least 75 meters away from Kand 27. If the pit 11 for these dimensions is too small, the diameter of the test area 25 and the free space can be between the test area of the edge 27 can be reduced accordingly. For example, could the diameter of the test area> 5 by 120 meters and the free space accordingly can be reduced to 60 meters. Other dimensions can also be used.

The specified @ n are purely those that have been determined to be sufficient are.

It will be in the pit 11 in opposite directions on the perimeter of the Test area 25 production wells 29 and 31 drilled, which in the example described, 150 meters apart, they are constructed identically.

FIG. 3 shows the structure for each source 29 and 31. Because these are identical 3 shows the source 29 only.

The source 29 includes a bore 93, 58 in diameter up to 87 cm, which preferably leads to the bottom 23 of the pit. Located in the bore 33 a corresponding number of tube sections 35, which are connected to one another by couplings are connected. The rearmost Hohrabschnitt 35 is through a flexible connection 41 coupled to a perforated pipe section 39, which allows axial movement between the perforated pipe section 39 and the lowermost pipe section 35 allows. the Connection 41 can be of conventional construction. The perforated pipe section has a series of Lociiungen 43 and is in the bore 33 by @entriereinrichtungen 45 centered on the perforated ear portion 39 in a corresponding manner, e.g. by Clamps, are attached; The bottom of the perforated pipe section 39 is closed by a cap 49, which is at the bottom of the bore 33 on a layer Gravel lies. The pipe sections 35 and 39 can, for example, made of plastic such as Consist of polyvinyl chloride.

The bore 33 is most of the length of the perforated section packed with gravel. just at the end of this section 39 is a weighting 55 for the holes against the ooeren areas of the bore 33, which is shown in Example includes a layer of earth 57 and a layer of concrete 59. outside the seal 55 in the bore 33 there is a filling of unused soil 61. The top one Drilling section 35 is connected to a pump 63 via a throttle valve 65 and a pipe 67 coupled, between which there is a flow indicator 68 with a suction pipe can be located.

Between the sources 29 and 31 there are four rows 69, 71, 73 and 75 of probes 69a, 71a, 73a, and 75a, their number and depth and their arrangement can be varied within wide limits according to the desired results and in the example shown evenly, for example 3 meters apart are. The probes of the corresponding rows are at the source 29 and result in a transverse row. The other probes in the four rows are on a similar track arranged in transverse rows. In the exemplary embodiment, the probes at a distance of 3 peters from a reference line 77 passing through the source 29 goes through, and the periphery of the test area 25 is tangent. For example, can the next four special rows from the reference line 77 at a distance of 7.5 m, 15 m, 30 m or 75 m away. These distances are not critical and are only intended represent a suitable special arrangement.

In a similar way, probes 69b, 71b, 73b and 75b are attached to the Source 31. The distances between the reference line 79, which is the extent of the test area 25 at the source 31 is tangent, and the five rows of probes at the source 31 can be identical, so that only the one at source 29 has to be described.

The probes are preferably located at different depths in the pit. In line with FIG. 5, the probes 69a to 75a are arranged in progressively lower layers.

Although different arrangements are possible, the cut is made Assumed according to Figure 5 as typical for the embodiment example, i.e. all probes row 69 are at the same depth, and all irobes in row 71 are at the same depth Depth etc .. iiiier the length all probes can be constructed identically and therefore only the probe 69a and the respective reference numerals are described in detail apply to corresponding parts.

The probe 6 ° a is accommodated in a bore 81, which aureh the Cover material 19 passes through it and has a bottom 83 which, for example, 60 cm below this material can be 19. The probe 69a includes a tube 85 with a Cap 87 at the lower end, which leads over the ibdeck material 19 around the bottom 83. That Area of tube 85 under cover material 19 contains multiple perforations 89. Preferably the part of the tube 85 which has the perforations 89 is relatively short, e.g. 60 cm long to isolate the vertical point where the pressure is read can be. The area of the bore 81 around the pipe 85 becomes under the cover material 19 filled with a substance like gravel. The top of the bore 81 is of a Plate 93 sealed, which can be a concrete cast. The pipe 85 can have a small diameter and are made of plastic such as polyvinylchlor @ d broom.

Probe 69a is a @bdeck probe because it works with the area of the pit 11 is directly under the cover material 19 in connection. With the @nneren of the pipe 85 can be a pressure indicator 94, for example a manometer, which @en pressure of @rubgas immediately under @covering material 19 next to the probe 69a anzei @ t. The pressure corresponds to the barometric pressure and not the atmospheric one Standard pressure.

Probe 71a is no longer than probe 89a. For example, can the probe? 1a go to a depth which is equal to 20% of the depth of the bore 29. The probes 73a and 75a can lead 50% and 70% down into the source 29, respectively.

Each probe is installed in its own borehole 81.

The length of the tube 85 with the perforations 89 can be identical for each probe be.

The sources 29 and 31 and the probes can be installed in any order with the static pressures indicated by indicators 94 in all probes and in sources 29 and 31 continuously or intermittently for a sufficient duration can be found to be the static pressure for sources 29 and 31 and for each To determine the probe with sufficient accuracy. For example, the pressures Read on the indicators 94 every 24 hours at intervals of four hours will. In addition to determining the pressure, you can also check the temperature in the probes and read off in the sources as well as the atmospheric pressure and the outside temperature will.

Then @ie @ bzuggeschwindimkei @@ n are chosen empirically. For example The draft speeds are 2.83 m3 / min,, 5 m3 / min and 14.3 m3 / min. The pump 63 for the source @ 9 is started at a speed of 2.83 m3 / min elas @ en and operated to draw the gas from the source 29. The @pump of the source 3 @ is switched off during the @@ nzen test of source @ 9. Pumping @er source 29 verrin @@ rt, rogressiv the pressure in the eyes. Finally @ t @@ i @ isize lie press in the @orden. i.e. @er jerk does not fluctuate significantly when @n @@@ ern de @ @ s @ bziehens. @@ S @@ s works with a speed of 83 m3 / min in one Subtracted interval after pressure stabilization in the probes. @ie Press in @llen @ondes, as shown @ by the display devices 94 @@ are shown, either continuously or intermittently as well as monitored before @@@ @@ ch after @press stabilization. The @speed of the @pump is set manually when the required flow rate an @ should be kept.

Of particular importance is the pressure in the @bdecksonde 69a after pressure stabilization, after which a stable average pressure for the cover probe 69a is determined. This is done, for example, by means of the pressure displays after pressure stabilization.

The pressure on the @bdecksonde 69a is especially important because this is the point at which air usually enters first, because this is the next @bdecksonde at the holes 43 of the source 29 is. @for this reason there is during operation of the source 29 and the pump 63, the cover probe 69a possesses a lower pressure than one of the other @bdeckprobe 69, because it covers a larger @ t @ nd by @e @@@@@@ comments 43, however, if there is any country, the others @bdecksonden 69 are at a lower pressure than @bdecksonde 69a, these @other probe than critical @bdecksonde ground and i @ r mean pressure determined @nd used as described.

However, if a @bdecksonde has a lower pressure @ls the @bdecksonde 69a, the tube has a relatively unrestricted channel to it own a probe. In some cases this channel can be destroyed and the @bdecksonde 69a give the lowest pressure of the probe 69.

After a stable mean pressure for the critical @buecksonde at a drawing speed of 2.8 m3 / min it was determined that this should be the case the @onde 69a), @ie pump 63 is switched off and the pit 11 can go to your Static @ back, which is correct before using the @pump @ for the first time have been. This lasts 48 hours.

The roof of the return of the pit 11 to static pressure is jie lump operated at a drawing speed of 8.4 m3 / min and the process described will be repeated.

Then, like pit 11, it can return to static pressure again and the pump is operated at a speed of 14 m m3 / min and the process repeated.

Using the data obtained, the diagram can also be shown in Figure 6 be aufgemtellt. There the points A, B and r represent the stable average pressure on the deck probe 69a at speeds of 2.8 m3 / min, 8.4 m3 / min or

14 m3 / min and are used to build a relationship between the pressure @n the @bdecksonde 69a and @drawing speed. This relationship is then used to build up the take-off speed at which the pressure on the Probe 69a becomes zero, i.e. equal to atmospheric pressure, at the point shown in FIG For example, the pressure on probe 69a would be at a retraction speed of about 13 m3 / min become zero. This speed is then the in this example maximum permissible speed, because higher speeds result in negative pressure on the cover probe 69a, which is lower than atmospheric pressure and so a pressure differential arises which tries to draw air into the pit 11, while there is no pressure differential at zero pressure.

Whether or not there is a pressure differential can be barometric Changes in pressure arise. However, because the pit is relatively slow on the responds to low pressure differentials and the maximum permissible withdrawal speed Based on the mean pressure, the choice is the highest allowable draw speed in this way quite sufficient.

In the described embodiment, the relationship is between the withdrawal speed and the pressure on the probe 69a in a diagram as one Line 95 shown.

This relationship can also be determined without plotting.

The speed 1 at which the pressure on the probe 69a is approximately zero can be through trial and error procedures or through a combination of trial and errors and interpolation the results of which are obtained.

The stable average pressure at other probes can also be in the iagra according to Figure 6 can be entered.

For example, it can be useful to keep the stable average pressures in the probes 719 in Figure 6, even if one of the points A, B and C in Figure 6 is wrong. The deeper probes 73 and 75 are used to determine the zone of influence the source 29 at different take-off speeds.

m to indicate a zone of influence and to trigger a determination of how At the distance from the source, the entry in FIG. 7 can be made. This figure 7 is inclined the average cover probe pressure, i.e. that at the probes 69 against the distance from the source for the three selected @drawing speeds. Also shows figure 7 the static pressure at the probe 69 at various distances from the source 29, i.e. from the reference line 77. The three speed curves intersect the line there are static @ jerks at the D, E or F.

Figure 8 is a graph of source spacing versus haul-off speeds. The points D, E and F of FIG. 7 are taken over into the figure 8 in order to establish a relationship in the form of a line 97 between the source distances and the withdrawal speed to manufacture.

If the sources 29 and 31 are operated simultaneously and their Zones of influence overlap, the maximum permissible withdrawal speed for a given source distance can be reduced to the interactions between to compensate for neighboring sources. This is done by using the relationship according to line 99 in FIG. 8.

In particular, the train speed became a simultaneous double operation of sources is reduced by 25%, which is shown by line 99 in Eigur 8. This reduction factor is considered sufficient. When using the line 99 for all maximum permissible withdrawal speed of 13 m3 / min the sources should be at least 95 meters apart. The described Test procedures are repeated for the source 31, while the pump 63 for the source 29 is switched off. That would take the entries of Figures 6 to 8 for the source 31 result.

In this case, the figures for these two can swell the source distance and the maximum permissible withdrawal speed for sources 29 and 31 and also used for other wells drilled in the pit.

After the end of the test program, the uals 29 and 31 are used for Production of gas. Other wells can also be drilled into the pit. It is believed that the test results, i.e. the maximum take-up speeds and the spacing of the jielle 29 and 31 is also used for other sources in the pit can be.

Sources operated at production rates according to the invention have no Problem with the entry of air through the cover material 19 into the pit 11. However, the invention results in an early warning system for the air ingress. This happens by monitoring the pressure in the cover probes 69 in addition to one or more operated Sources. For example, the pressure in the cover probes 69 and particularly in the ltbdecksonde 69a can be monitored at source 29 during production. At the Falling the pressure on one of the cover probes 69 below zero for a given time or the take-off speed can be reduced accordingly by a given value in order to eliminate the pressure differential across the cover material 19, the air in tried to push the pit.

this would result in somewhat negative or completely negative pressures in the cover probes 69 that last only a short time because of the slow response of the air and the pit 11 on Pressure differentials of this would mean no air entry cause.

The speed of the pump 63 can be manually or automatically accordingly the pressure conditions in the cover probe 69 are regulated, which changes the withdrawal speed would ensure.

For example, the pump speed should correspond to the pressure in the covering probe 69a when falling by a given shipyard below the atmospheric pressure, like 0.25 in all20 or dropping below atmospheric pressure for 24 hours will.

Summary: In a process for withdrawing gas from a Pit without air entering, a source is provided in the pit and withdrawing the gas from the source at several different buttock velocities. The pressure in a given area in the pit is determined while the Gas is withdrawn. Then there will be a relationship between the speed and the Pressure built up in the chosen areas. This relationship is used to build the maximum permissible withdrawal speed, which is approximately at zero pressure. The gas is then taken from the source at no more than the maximum allowable speed deducted.

- patent claims -

Claims (15)

Patent claims: 1. Getting cast out of a garbage pit, without the air in these - enters in such quantities that the ethane is destroyed, which emerges in the pit, characterized in that in the pit (11) at least a source (29, 31) is provided that the gas through the source (29, 31) through Maintaining the pressure within the source at a first pressure released, the is so narrow that the gas flows from the pit into the source that it is pressure in at least one selected area in the pit is monitored, which is outside uer source is located, and that the size of the first print corresponds to the print is regulated in this area so that air ingress into the pit is practically prevented will. 2. The method according to claim 1, characterized in that the pit (11) contains a layer of cover material (19) and at least one cover probe (69a, 71a, 73a, 75a) is inserted into the pit (11) next to the source (29, 31) and that the area is that of the pit next to the probe and next to the .bdeckmaterial includes. 3. The method according to claim 2, characterized in that the first Pressure is regulated so that it does not fall below the atmospheric pressure at the probe. 4. The method according to 1en of the preceding claims, characterized in that that the pressure in at least one area of the pit (11) and outside of the one Source (29,31) is established in a first time, before and after the gas is discharged the source is subtracted that subsequently the pressure in at least one area outside the source in a second time it is found that between the withdrawal speed and the two detected pressures a relationship is established, and that the maximum production rate the take-off speed built according to the relationship that would exist at around zero pressure. 5. The method according to claim 4, characterized in that the pit (11) contains a covering material and a covering probe (69a) in the pit next to the Source is used and that the pressure at this probe is determined. 6. The method according to claim 4, characterized in that the pressure during the initial period it is found to be an average pressure for one Part of this period of time, and that this average pressure is used for this will. 7. The method according to claim 6, characterized in that the peeling of the gas to a relative pressure during the first period of time stable value progressively decreases, and that this Part of the first Time after reaching this relatively stable value occurs. 8. experience according to claim 4, characterized in that the pressure, which was detected during the second time period, during the first time period is reduced, and that there is such a long interval between the two times that the pressure in the area in which it was detected during the second period of time is practically reduced to static pressure. 9. The method according to claim 4, characterized in that the gas from one point in the pit during the production period at no more than about subtracted from the maximum production rate, preventing air from entering the pit becomes unlikely. 10. The method according to claim 9, characterized in that the pressure in an area adjacent to this location for at least part of the production period is monitored and the withdrawal speed according to the pressure in the area next to this point during the production period is regulated so that the entry of air into the pit is practically prevented during the production period. 11. The method according to claim 10, characterized in that the pit (11) contains a layer of masking material (19) that the area next to it is next to the cover material, and that the withdrawal speed of the source (29, 31) is regulated in such a way that the pressure in the area next to this point does not fall Atmospheric pressure falls. 12. The method according to the preceding claims, characterized in that that several probes (69a, 71a, 73a, 75a) arranged in a pattern in the pit are that in a first period of time gas is withdrawn and at least a first group It is determined by probes that a maximum permissible draw speed for the first source using these pressures is established that this velocity is practically the highest speed at which ds gas without the entry of air can be withdrawn into the pit, and that withdrawing the gas from the first Source is carried out at a speed no greater than the highest pound speed is. 1 method according to claim 12, characterized in that in the north of multiple wells, the first group of these wells at a first depth in the Pit (29, 31) is provided that a second group of probes are in the pit located deeper than the first group and a third lower than the second, and that a characteristic of the gas in the second and in the third special group is measured. 14. The method according to claim 12, characterized in that the gas is taken from the first source (29) during a second time and the pressure is found in at least some probes during the second time and that using the determined pressures, the maximum permissible withdrawal speed than that at which the pressure on at least one probe approximates is equal to atmospheric pressure. 15. The method according to the preceding claims, characterized in that that the static pressure in the pit (11) in several, stood up from the source (29,31) it is found that the gas during the two times with two different Speeds is deducted that the pressure in several areas of the pit (11) during both times is found in at least two areas that are different are far from the source that during part of the first time in the pressures determined in these areas to build up the first draft speed is used at about the shortest distance from the source at which during this Part of the first time there is static pressure that of during part of the second Time determined in these areas to build up the second draw speed near the shortest distance from the source is used during this Part of the second time there is a static pressure that between the shortest Distances and the haul-off speeds a relationship is established through which the source distance can be determined for different flow velocities, that at least one other source (41) is provided in the pit (11) that the maximum permissible withdrawal speed of the sources, which without air entry into the Can be used to build up and that from the source gas at a pit Speed is withdrawn, which is not more than about the lower of the withdrawal speed that is established by the relationship and is the maximum allowable í, pulling speed is.