METHOD AND AN APPARATUS FOR PREVENTING THE EMISSION OF HARMFUL GASES INTO ATMOSPHERE

Abstract

An apparatus (100) for conveying leaked gas streams (1.sub.1,1.sub.2,1.sub.3 . . . 1.sub.n) leaking at a substantially atmospheric pressure from an industrial plant to a reception body (9) that is pressurised at a predetermined reception pressure (P.sub.r), comprises a collector container (10) of said streams, pneumatically connected with a leaked gas-capturing means, a compressor (20) having a suction mouth (25) pneumatically connected to the collector container (10) and a delivery mouth (26) pneumatically connected to the reception body (9) and to the collector container (10) through a discharge pipe (6) and a recirculating duct (4), respectively. In order to adjust the compressor suction pressure (Ps) at a value below atmospheric pressure (P.sub.atm) and above a minimum value of 200100 Pa, the apparatus (100) comprises a control unit (40) in which a control valve (41) is arranged along the recirculation duct (4) and a pressure sensor/transducer (47) is arranged at suction side of compressor (20), the sensor/transducer (47) configured to generate a pressure signal (46) corresponding to the detected suction pressure (P.sub.s), preferably a differential pressure signal (46) between the atmospheric pressure and the compressor pressure suction. Moreover, a pressure controller (45) is configured to receive the suction pressure signal (46), to generate an open/close signal (43) responsive to pressure signal 46, and to transfer the open/close signal (43) to the control valve (41) so as to change its opening according to the detected suction pressure.

Claims

1. An apparatus for conveying leaked gas streams (1,1.sub.2,1.sub.3 . . . 1.sub.n) leaking at a substantially atmospheric pressure from an industrial plant to a reception body (9) that is pressurised at a predetermined reception pressure (P.sub.r), said apparatus comprising: a gas-capturing means for capturing said leaked gas streams (1.sub.1,1.sub.2,1.sub.3 . . . 1.sub.n); a collector container (10) for collecting said leaked gas streams (1,1.sub.2,1.sub.3 . . . 1.sub.n), said collector container pneumatically connected to said gas-capturing means; a compressor (20,30) having a suction mouth (25) pneumatically connected to said collector container (10); a recirculation duct (4); wherein said compressor (20,30) has a delivery mouth (26) pneumatically connected with: said pressurised reception body (9), through a delivery pipe (6) of said apparatus (100); said collector container (10), via said recirculation duct (4), so as to convey a recirculation portion of said leaked gas compressed by said compressor (20,30) back into said collector container (10), a control unit (40) for controlling a suction pressure (P.sub.s) at said compressor (20,30), comprising: a pressure sensor/transducer (47) arranged to measure said compressor suction pressure (P.sub.s) and to generate a suction pressure signal (46); a control valve (41) mounted along said recirculation duct (4); a pressure controller (45) configured to receive said pressure signal (46) and to generate an open/close signal (43) for modifying an opening degree of said control valve (41) responsive to said pressure signal (46), in order to maintain said suction pressure (P.sub.s) at a target pressure value between the atmospheric pressure (P.sub.atm) and a predetermined minimum pressure (P.sub.m).

2. The apparatus according to claim 1, wherein said pressure sensor/transducer (47) is further arranged to detect a current value of said atmospheric pressure (P.sub.atm) and to generate said suction pressure signal (46) as a differential pressure signal related to a difference between said atmospheric pressure (P.sub.atm) and said compressor suction pressure (Ps).

3. The apparatus according to claim 1, wherein said compressor (20,30) is a fully sealed compressor.

4. The apparatus according to claim 1, wherein said compressor (20,30) is a diaphragm compressor.

5. The apparatus according to claim 1, wherein said collector container (10) is provided with a safety vent device (5,50) configured to release said collected leaked gas (1), which is present in said collector container (10), into the atmosphere if the pressure of said collected leaked gas (1) exceeds a predetermined safety maximum threshold value (P.sub.Ms) higher than the atmospheric pressure (P.sub.atm), said safety maximum threshold pressure set between 1 mbar and 10 mbar.

6. The apparatus according to claim 1, wherein said safety vent device (5,50) comprises a pressure relief valve (50) arranged along a vent pipe (5) with an outlet in a safe area.

7. The apparatus according to claim 1, wherein a heat exchange device (42) is provided along said recirculation duct (4), said heat exchange device configured to cool said recirculation portion of said leaked gas conveyed to said collector container (10).

8. The apparatus according to claim 1, wherein a check valve (22,32,52) is arranged along said delivery pipe (6,6,6) of said compressor (20,30) to prevent a pressurised-gas backflow from said pressurised reception body (9) towards said compressor (20,30) and towards said collector container (10) via said recirculation duct (4), if said compressor (20,30) stops, wherein said delivery pipe (6,6,6) comprises a restricted-section pipe portion (29) serially arranged to said check valve (23,33,52) in order to limit said pressurised-gas backflow, if said compressor (20,30) stops and said check valve (22,32,52) does not close.

9. The apparatus according to claim 1, wherein said compressor is a first compressor (20), and said apparatus comprises a second compressor (30) arranged in parallel to said first compressor (20), and said pressure controller (45) is configured to start said second compressor (30) or said first compressor (20) in addition to said first compressor (20) or to said second compressor (30) respectively, if said suction pressure (P.sub.s) becomes higher than a predetermined maximum operating threshold value (P.sub.Me).

10. The apparatus according to claim 9, wherein said pressure controller (45) is configured to stop one of said first and said second compressors (20,30) if said suction pressure (P.sub.s) becomes lower than a predetermined switch-off threshold value (P.sub.R) that is lower than or at most equal to said maximum operating threshold value (P.sub.Me).

11. A method for conveying leaked gas streams (1,1.sub.2,1.sub.3 . . . 1.sub.n), leaking at a substantially atmospheric pressure from an industrial plant to a reception body (9) that is pressurised at a predetermined reception pressure (P.sub.r), said method comprising steps of: capturing said leaked gas streams (1.sub.1,1.sub.2,1.sub.3 . . . 1.sub.n); collecting said leaked gas streams (1.sub.1,1.sub.2,1.sub.3 . . . 1.sub.n) into a collector container (10), thus forming a collected leaked gas (1); in a compressor (20,30), compressing said collected leaked gas (1) up to said reception pressure (P.sub.r), thus forming a compressed leaked gas; measuring a suction pressure (P.sub.s) of said compressor (20,30); conveying a main portion of said compressed leaked gas into said pressurised reception body (9); recirculating a portion of said compressed leaked gas back into said collector container (10), wherein said main portion and said recirculation portion of said compressed leaked gas are established according to said measured suction pressure (P.sub.s), in such a way to maintain said suction pressure (P.sub.s) at a target pressure value between the atmospheric pressure (P.sub.atm) and a predetermined minimum pressure (P.sub.m).

12. The method according to claim 11, wherein said minimum pressure (P.sub.m) is set between 200 and 100 Pa relative.

13. The method according to claim 11, wherein said minimum pressure (P.sub.m) is about 150 Pa relative.

14. The method according to claim 11, wherein said target pressure value is between 50 Pa relative and said minimum pressure (Pm).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The invention will be illustrated below with a description of some embodiments and modifications thereof, by way of example and not of limitation, with reference to the accompanying drawings, in which

[0043] FIG. 1 is a flow diagram of an apparatus according to the invention;

[0044] FIG. 2 is a flow diagram showing features of apparatuses according to some embodiments of the invention;

[0045] FIG. 3 is a flow diagram of an apparatus according to the invention, in which two parallel-arranged compressors are provided;

[0046] FIG. 4 is a flow diagram showing features of apparatuses according to some embodiments of the invention, in which two parallel-arranged compressors are provided;

[0047] FIG. 5 is a flow diagram of a device for treating gas streams of gaseous matter leaking at substantially atmospheric pressure, according to the prior art.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0048] With reference to FIG. 1, an apparatus 100 is described for conveying streams 1,1.sub.2,1.sub.3 . . . 1.sub.n of a gas leaked at substantially atmospheric pressure in an industrial plant, into a pressurised reception body 9 at a predetermined reception pressure P.sub.r. For example the plant can be a natural gas compressor station of a pipeline, and the reception body can be the pipeline itself at its operating pressure P.sub.r.

[0049] Apparatus 100 comprises a gas-capturing means, not shown, for capturing gas streams 1.sub.1,1.sub.2,1.sub.3 . . . 1.sub.n, or is associated with a gas-capturing means existing in the industrial plant, as it is provided in the conventional technique. A gas-capturing means may comprise a chamber encircling a possible leakage points of the plant, such as a vent of a gas sealing system, a stuffing box of a rotating or otherwise movable organs, a sampling sockets, a flange connection, and the like.

[0050] Apparatus 100 further comprises a preferably elongated vertical collector container 10 of leaked gas streams 1.sub.j, j=1 . . . n, pneumatically connected with the gas-capturing means through collection pipes 2.sub.j, j=2 . . . n. This way, streams 1.sub.j form a collected leaked gas 1 that is conveyed into collector container 10. The collector container can be a reservoir 10, as shown, and/or a simple manifold. Collector container 10 advantageously comprises a drain nozzle 11 with a drain valve 12 for periodically removing any liquid present in collected leaked gas 1 and accumulated in the bottom of collector container 10.

[0051] Apparatus 100 further comprises a compressor 20, preferably a diaphragm compressor, or a compressor of any type from which substantially no gas is likely to leak.

[0052] Compressor 20 has a suction mouth 25 pneumatically connected with collector container 10 via a suction pipe 3, and a delivery mouth 26 pneumatically connected with reception body 9 via a delivery pipe 6. Shut-off valves 21,23 can be arranged along suction pipe 3 and delivery pipe 6. These valve are preferably manually operated and are maintained open during normal operation of apparatus 100. Shut-off valves 21,23 can be closed to isolate compressor 20 or apparatus 100 from the rest of the system in case of maintenance or replacement of compressor 20.

[0053] Compressor 20 is arranged to compress collected leaked gas 1 up to reception pressure P.sub.r of reception body 9, generally set between a few bars and a few tens of bars. Compressor 20 is also arranged to maintain a suction pressure P.sub.s, in particular the pressure in collector container 10, at a value of about few millibars below atmospheric pressure P.sub.atm.

[0054] To this purpose, according to the invention, delivery mouth 26 of compressor 20 is also connected with collector container 10 through a recirculation duct 4, and a pressure control unit 40 is provided to control suction pressure P.sub.s of compressor 20, comprising a control valve 41 arranged along recirculation duct 4.

[0055] Control unit 40 further comprises a pressure sensor/transducer 47 arranged to detect pressure P.sub.s at the suction to compressor 20 and to generate a suction pressure signal 46. As shown FIG. 1, pressure sensor/transducer 47 can be mounted, for instance, to collector container 10, or to suction pipe 3.

[0056] Control group 40 further comprises a pressure controller 45, preferably an electronic controller, configured to receive pressure signal 46 from pressure sensor/transducer 47, and to generate an open/close signal 43 for an actuator 49 of control valve 41. Actuator 49 is arranged to modify the opening of control valve 41 responsive to pressure signal 46, in order to maintain suction pressure P.sub.s at a target pressure value between atmospheric pressure P.sub.atm and a predetermined minimum pressure P.sub.m. In practice, by recirculating a part of the flow of leaked gas 1 back into collection container 10, a balance is established between the flow rate of leaked gas 1 from the leakage points and the flow rate delivered by compressor 20 at reception pressure P.sub.r.

[0057] Preferably, the minimum relative pressure P.sub.m is set between 200 Pa and 100 Pa, in particular P.sub.m is about to 150 Pa. The target pressure value, as a gauge pressure, is preferably set between a maximum gauge pressure of e.g., 50 Pa and the minimum gauge pressure P.sub.m.

[0058] Advantageously, pressure sensor/transducer 47 is a differential pressure transducer arranged between a location upstream of compressor 20 and the atmosphere, for example between collection container 10 and the atmosphere, and is configured to measure both positive relative pressures and negative relative pressures, generally in a measuring range of 20 mbar+20 mbar (2 kPa+2 kPa). In other words, pressure sensor/transducer 47 is arranged to detect a current value of atmospheric pressure P.sub.atm in addition to suction pressure P.sub.s of compressor 20, and is configured to generate suction pressure signal 46 as a differential pressure signal related to a difference between atmospheric pressure P.sub.atm and suction pressure P.sub.s of compressor 20.

[0059] Besides controlling suction pressure P.sub.s of compressor 20, recirculation duct 4 serves to make the compressor start-up easier.

[0060] With reference to FIG. 2, some embodiments of the apparatus further include respective features that are described hereinafter. Although all these features are represented in the same FIG. 2, they may be present independently from one another, alone or in combinations of some of these features.

[0061] In an apparatus 101 according to one embodiment of the invention, a check valve 22 is preferably arranged along delivery pipe 6 of compressor 20 in order to prevent pressurised-gas backflow from pressurised reception body 9 towards compressor 20 and collector container 10 via recirculation duct 4 if a shutdown of compressor 20 occurs, typically a shutdown due to a sudden compressor failure or to a sudden power supply interruption. Moreover, a restricted-section pipe portion 29, such as a calibrated orifice, can advantageously be serially arranged to check valve 22 along delivery pipe 6, preferably downstream of check valve 22, in order to limit the pressurised gas flow from pressurised reception body 9 towards compressor 20 and towards collector container 10 if check valve 22 does not close or does not close completely in case of a sudden stop of compressor 20.

[0062] In an apparatus 102 according to another embodiment, collector container 10 is provided with a safety vent device 5,50 for releasing collected leaked gas 1 from collector container 10 into the atmosphere in a safe zone 7, only if pressure P.sub.s of collected leaked gas 1 exceeds a predetermined safety maximum threshold value P.sub.Ms higher than atmospheric pressure P.sub.atm, due to any breakdown or failure of apparatus 102. In particular, as shown in FIG. 2, the safety vent device comprises a vent pipe 5 leading to atmosphere in safe area 7, and an overpressure i.e. pressure relief valve 50 such as a counterbalance valve 50 arranged along vent pipe 5. In particular, the safety vent device, i.e., the pressure relief valve 50, is configured to release collected leaked gas 1 from collector container 10 when pressure P.sub.s of leaked gas 1 exceeds a predetermined maximum pressure P.sub.Ms set between 1 mbar and 10 mbar (100-1000 Pa), for example 2 mbar (200 Pa).

[0063] Moreover, in an improved modification of apparatus 102, the safety vent device comprises a bypass pipe 5 connected between upstream and downstream of pressure relief valve 50, and first and second shut-off valves 51,52 arranged along vent pipe 5 and bypass pipe 5, respectively. First and second shut-off valves 51,52 are operated as a normally closed valve and a normally open valve, respectively, during normal operation of apparatus 102. This valve arrangement is useful to allow maintenance of pressure relief valve 50 while continuing to remove gas 1j from the plant and releasing it into safe area 7 also during the short time required by the maintenance.

[0064] In a further embodiment, an apparatus 103 comprises a safety valve 24 having an inlet port pneumatically connected with delivery mouth 26 of compressor 20.

[0065] In a modification of this apparatus 103, further including the above-described safety vent device 5,50 of apparatus 102, a discharge port of safety valve 24 is pneumatically connected with vent pipe 5, downstream of overpressure valve 50.

[0066] In an apparatus 104 according to a further embodiment, a heat exchange device 42 is provided along recirculation duct 4, preferably downstream of control valve 41. Heat exchange device 42 is configured to cool the gas recirculating back to collection container 10, so that leaked gas 1 in collection container 10 does not attain a temperature too high for admission to compressor 20.

[0067] In an apparatus 105 according to a further embodiment, control valve 41 is associated with a positioning device 48 including a position sensor configured to detect the position of a plug member of control valve 41 and to generate a position signal 49 for modifying the position of the plug member and the opening degree of control valve 41. Pressure controller 45 is configured to receive position signal 49 in addition to suction pressure signal 46, and to modify open/close signal 43 based on position signal 49.

[0068] With reference to FIG. 3, an apparatus 200, according to a further embodiment of the invention, comprises a plurality of parallel arranged compressors, in particular a first compressor 20 and a second compressor 30. In this case, control unit 40 is configured to automatically start second compressor 30 or first compressor 20 in addition to first compressor 20 or second compressor 30, respectively, if suction pressure P.sub.s upstream of compressor 20,30 exceeds a predetermined maximum operating threshold value P.sub.Me. This can occur if a breakdown of the other compressor 20 or 30 occurs. Control unit 40 can also be configured to automatically stop only one between first and the second compressors 20,30 if suction pressure P.sub.s becomes lower than a predetermined shutdown threshold value P.sub.R that is in turn lower than or at most equal to maximum operating threshold value P.sub.Me. As an alternative, once the breakdown has been corrected, one of the two compressors can be stopped manually by an operator.

[0069] Similarly to apparatus 100 shown in FIG. 1, each compressor 20,30 can be equipped with a respective shut-off valve 21,31 on its suction branch 3,3 and with a respective shut-off valve 23,33 on its discharge branch 6,6, to allow maintenance of one of the compressors 20,30 while the other compressor 30,20 is running, and apparatus 200 is operative. Delivery branches 6,6 come together into the common delivery pipe 6, along which an additional shut-off valve 53 can be provided.

[0070] With reference to FIG. 4, some embodiments of the apparatus further include respective features that are described hereinafter. Although all these features are represented in the same FIG. 4, they may be present independently from each other, alone or in combinations of some of these features.

[0071] In an apparatus 201 according to one embodiment of the invention, a check valve 52 is preferably arranged along common delivery pipe 6 of compressors 20 and 30 in order to prevent pressurised-gas backflow from pressurised reception body 9 towards compressor 20 and collector container 10 via recirculation duct 4 if a shutdown of compressor 20 or 30 occurs, typically a shutdown due to a sudden compressor failure or to a sudden power supply interruption. A similar flow-limiting function can have check valves 22,32 arranged on respective Moreover, a restricted-section pipe portion 29, such as a calibrated orifice, can advantageously be serially arranged to check valve 52 along common delivery pipe 6, preferably downstream of check valve 52, in order to limit the pressurised gas flow from pressurised reception body 9 towards compressor 20 and collector container 10 if check valve 52 does not close or does not close completely in case of a sudden stop of compressor 20 or 30.

[0072] An apparatus 202 according to another embodiment comprises a safety vent device 5,50 for releasing collected leaked gas 1 from collector container 10 into the atmosphere, if the pressure of collected leaked gas 1 exceeds a predetermined safety maximum threshold value P.sub.Ms higher than atmospheric pressure P.sub.atm, as in apparatus 102 of FIG. 2.

[0073] An apparatus 203 according to a further embodiment comprises two safety valves 24,34 to prevent exceeding the maximum operating pressure of compressors 20,30, each safety valve 24,34 having its inlet port pneumatically connected with delivery mouths 26,36 of compressors 20 and 30 respectively, and with its outlet port pneumatically connected with emergency vent pipe 5, if present.

[0074] An apparatus 204 according to a still further embodiment comprises a heat exchange device configured to cool the gas recirculating to collector container 10, such as the apparatus 104 of FIG. 2.

[0075] In an apparatus 205 according to a still further embodiment, a control valve 41 comprises a positioning device 48, as in apparatus 105 in FIG. 2.

[0076] The above description of embodiments and modifications of the invention is capable of showing the invention from a conceptual point of view in such a way that others, using the known technique, will be able to modify and/or adapt in various applications such specific embodiments without further research and without departing from the inventive concept, and, therefore, it is understood that such adaptations and modifications will be considered as equivalents of the modifications and specific embodiments. The means and materials for realising the various functions described may be of various kinds without departing from the scope of the invention. It is understood that the expressions or terminology used are purely descriptive and, therefore, not limitative.