APPARATUS AND METHOD FOR COMPRESSING A GAS WITH RECOVERY OF GAS LEAKAGED FROM A COMPRESSOR SEAL

Description

Description of the industrial invention entitled “APPARATUS AND METHOD OF COMPRESSING A GAS WITH GAS RECOVERY “LEAK FROM A COMPRESSOR SEAL” on behalf of SIMEREE Srl, an Italian company based in Livorno 5 DESCRIPTION Scope of the invention

[0001] The present invention relates to an apparatus and a method of compressing a gas without any problems emission into the air of small gas leaks that 10 inevitably occur through the sealing organs of the compressor(s) involved, typically compressors volumetric operators operating in the oil and gas sector. Notes on the prior art – Technical problem

[0002] Various solutions have been proposed to address the 15 problem of atmospheric emission of the above leaks indicated, among which those of gas are particularly critical harmful to health and / or flammable, and / or capable of forming explosive mixtures with air. In these cases, safety is normally restored by aspirating and conveying the 20 leaks identified in an area identified as “safe”, that is, not frequented by people and animals and without sources of triggers such as overheated surfaces or electrical devices not adequately protected.

[0003] In this way, however, the leaking gas is 25 emitted into the atmosphere. Being released under pressure substantially atmospheric, it cannot in fact be easily admitted into a combustion device. If then This gas contains hydrocarbons such as methane, it is obtained a modest but continuous emission of greenhouse gases, 30 a non-negligible problem given the widespread diffusion of installations that handle hydrocarbons such as gas pipelines, power plants of power and chemical plants in general. Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544

[0004] WO2023095079A1 relates to an apparatus and method for treat near-atmospheric pressure gas leaks such as hydrocarbons, avoiding their release into the air. The leaks they are collected, compressed at a modest pressure and 5 conveyed into a pressurized receiving body. Such solution is however complicated and expensive, in particular, for the presence of at least one auxiliary compressor. Summary of the invention

[0005] It is therefore an object of the present invention to provide a 10 apparatus and a method of compressing a gas which, in addition to allow for the collection of leaking gases substantially atmospheric pressure through the seals of the compressors, avoid their emission into the air, and at the same time same implementation simpler and less expensive than the 15 prior art.

[0006] It is also an object of the invention to provide such apparatus and method that allow for the useful reuse of the gas leaks in the main process line.

[0007] These and other purposes are achieved by an apparatus 20 and by a method as defined, respectively, in claims 1 and 10. Advantageous embodiments of the apparatus and method are defined in the claims respectively dependent.

[0008] According to one aspect of the invention, the apparatus 25 includes: – a compressor configured to compress the gas to obtain a compressed gas; – a gas supply passage to the compressor, – the compressor being equipped with a device 30 seal and a leak collection device gas leaking through the sealing device; – a discharge duct for the compressed gas from the compressor; wherein the collection device comprises: Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544 – a first duct arranged to carry the leaking gas from the sealing device to a first collection point; – a second conduit arranged to connect pneumatics the first collection point with a second point 5 compressor collection, where the second collection point is located on the passage of power supply, and where, in a section of the upstream feed passage at the second collection point there is a device 10 pressure control configured to impose in the second collection point a first pressure value lower than the atmospheric pressure.

[0009] According to another aspect of the invention, the method includes the phases of: 15 – gas supply to a compressor, the compressor being provided with a sealing device; – compression of the gas in the compressor in order to obtain a compressed gas; – collection of a gas leak seeping through the 20 sealing device; where the collection phase includes the following phases: – first conveyance of the leaking gas from the device held at a first collection point; – second conduction of the leaking gas from the first point 25 collection at a second collection point of the compressor, where the second collection point is located upstream of the compressor, and – control of the gas pressure upstream of the second 30 collection point where in the second collection point a first pressure value is imposed that is lower than the atmospheric pressure; – extraction of a flow of compressed gas from the compressor. Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544

[0010] In this way, it is possible to collect the leaking gas from the compressor sealing device and avoid it the emission into the air, without introducing particular system complications. Furthermore, the leaking gas is 5 usefully reintroduced into the treatment process of gas mainstream.

[0011] Preferably, the first pressure value is between -0.02 relative bar and -0.10 relative bar, plus preferably between -0.04 relative bar and -0.08 relative bar. 10 For example, the first pressure value is close to -0.06 relative bar, that is, it is close to 0.95 absolute bar.

[0012] In particular, the compressor is a compressor volumetric, in particular a reciprocating compressor. The invention is particularly suitable for these types of 15 compressors in which, for well-known technological reasons, it is avoided the use of gas or liquid flushed mechanical seals and are Instead, stuffing box type seals are used, practically subject to leaks.

[0013] In an advantageous variant of the method, the phase of 20 pressure control includes the following phases: – extraction of a fraction of the compressed gas delivered of the compressor; – reduction of the pressure of the gas fraction compressed; 25 – conveying the compressed gas fraction into a section upstream of the second collection point, in which the pressure reduction phase and the pressure reduction phase extraction of the compressed gas fraction is carried out in a the first value to be maintained in the second collection point 30 pressure.

[0014] To this end, in an advantageous embodiment of the apparatus, the pressure control device comprehends: Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544 – an auxiliary pipe arranged to place in pneumatic communication the exhaust pipe with the power supply passage in the upstream section of the second collection point; 5 – a pressure reducing organ arranged along the auxiliary pipe and configured to let through through the auxiliary pipe and convey from the exhaust pipe to the upstream section of the second collection point a fraction of the compressed gas from 10 keep the first value in the second collection point of pressure.

[0015] Advantageously, the control device of the pressure also includes – a pressure sensor arranged on the passage of 15 power supply near the second collection point, the pressure sensor being configured to generate a pressure signal at the second collection point; – a regulator configured for – receive the pressure signal and perform a 20 pressure signal comparison with first value of pressure of pressure – process a control signal based on the comparison between the pressure signal and the first value of pressure; 25 – an actuator configured to receive the signal from control and to locally modify the passage according to to the control signal, so as to maintain in the second collection point the first pressure value.

[0016] In particular, the actuator is associated with the control organ 30 pressure reduction and is configured to control an opening of the pressure reducing organ based on at the control signal. Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544

[0017] This variant / embodiment of the method also does not involves particular system complications, since essentially include more than an installation conventional without treatment of leaking gases, a 5 auxiliary pipeline for extracting a portion of the gas compressed upstream from the compressor, and a pressure reduction device from the delivery value of the compressor at the first pressure value, provided that choose / modify the compressor so that it can operate 10 starting from a slightly lower upstream pressure to the atmospheric and to size the reduction device in a suitable manner to obtain said pressure upstream of the compressor, or first pressure value.

[0018] Preferably, the power pass 15 comprises a pressure stabilizing tank having a predetermined volume, and the pressure sensor is arranged in order to detect a pressure value related to the first pressure value inside the stabilizer tank of the pressure. In this way, the pressure value that is 20 detected by the pressure sensor is a stable value, i.e. not significantly dependent on any disturbances due to variations in flow rate or other. Furthermore, the pressure that is detected in that tank, at difference in pressure detected in line along the 25 power supply path duct, is feels from kinetic pressure levels. This ensures a more stable regulation of the pressure immediately upstream of the compressor and, therefore, a regular course of operation recovery of leaking gases 30

[0019] Advantageously, the pressure sensor is a sensor of differential pressure between the pressure at the second point of collection and a current atmospheric pressure. In this way, the operation of the device is independent of variations Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544 atmospheric pressure climates, which can be significant given the small depression value set upstream of the compressor, i.e. given the first value of pressure to atmospheric pressure, having no particular 5 advantages of imposing a higher vacuum value.

[0020] In one embodiment, the apparatus comprises also a preliminary pressure regulation organ upstream of the power supply passage, configured for maintain a pressure upstream of the feed passage 10 meters above atmospheric pressure. In this way, it is possible to isolate the leak recovery system from other sections of the system upstream of the compressor, limiting the areas where negative pressure reigns, even if small in absolute value. This allows to limit / avoid 15 the entry of ambient air into such other sections, with well-known advantages in terms of maintaining purity of the treated gas and / or protection against the formation of potentially explosive mixtures, for example, of hydrocarbons with air. 20

[0021] Advantageously, the apparatus further comprises a pressure relief valve arranged to convey at least one amount of gas leaking into a predefined safe area if in leakage gas collection device is exceeded predetermined safety pressure value higher than 25 first pressure value. This measure allows you to make the recovery system more operable and safe vents, in case of any malfunction of the upstream pressure control device compressor, which does not allow to establish upstream of the 30 compressor a depression sufficient to dispose of the gas leaking through the sealing device. Brief description of the drawings Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544

[0022] The invention will be illustrated below with the description of some embodiments, made for the purpose of exemplary and non-limiting, with reference to the drawings annexes, in which: 5 – Figure 1 is a flowchart of a method for compress a gas according to an aspect of the invention; – Figure 2 is a block diagram of an apparatus for compress a gas in accordance with another aspect 10 of the invention; – Figure 3 is a flowchart of a method according to a variant of the invention, comprising steps of extraction of a fraction of compressed gas and its conveyed upstream of the compressor, after reduction 15 of the pressure; – Figure 4 is a block diagram of an apparatus according to an embodiment of the invention, wherein the pressure control device includes a auxiliary pipe to convey a fraction back 20 of compressed gas upstream of the compressor, and a pressure reducing valve mounted along said auxiliary pipe; – Figure 5 is a block diagram of an apparatus according to a variant of the embodiment of figure 4, in which 25 the pressure control device comprises a regulation loop associated with the reduction organ of the pressure; – Figure 6 is a block diagram of an apparatus according to another embodiment of the invention, provided with 30 a pressure stabilizer tank; – Figure 7 is a block diagram of an apparatus according to yet another embodiment of the invention, comprising a pressure regulating organ Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544 preliminary upstream of the gas supply passage to the apparatus; – Figure 8 is a block diagram of an apparatus according to a further embodiment of the invention, wherein 5 the leakage gas collection device comprises an emergency vent line equipped with a valve of overpressure. Description of preferred embodiments

[0023] With reference to figures 1,2 and 4,5 the following are described: 10 a method and apparatus 1a-c, for compressing a gas 3 available at an initial pressure P obtaining a gas compressed 4 at a pressure P .

[0024] The method of the invention first comprises a S1 phase of preparation of a configured 20 compressor 15 to compress a predetermined flow rate of gas 3 from a first pressure P lower than atmospheric pressure P 1 atm up to a second pressure or compressed gas pressure P .

[0025] The second pressure P that can be reached with the compressor 20 of the apparatus 1a of the invention may be of the order 20 of the tens or even hundreds of bars. In that case, the compressor 20 is typically a volumetric compressor, preferably a reciprocating compressor comprising one or more cylinder-pressure groups 21 each of which can express a single or double compression effect, such as 25 well known in the art, according to which each piston 23 is overlooking a 24' compression chamber or two chambers 24', 24” opposing compression as illustrated in the figures 4 and 5, which can be connected in series, as shown in the figures or, in a variant not shown, 30 in parallel to each other.

[0026] Still referring to figures 4 and 5, the compressor 20 is provided with a sealing device 22 on the outside. In the case of the reciprocating compressor in figure 2, but not only, Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544 the sealing device 22 is typically a sealing device stuffing box or packing gland.

[0027] Sealing devices such as the sealing device 22 are often the site of gas leaks 5 from the 5 or from the compression chambers 24',24” outside the compressor 20. Gas 5 is expelled under pressure substantially atmospheric or slightly higher than atmospheric pressure P , which does not allow the recovery of the ATM gas leaking 5 into a receiver at a higher pressure or 10 the conveyance to a torch for its combustion.

[0028] Referring again to figure 1, the method then includes a gas supply phase S10 3 to compressor 20 and a gas compression stage S20 3 in compressor 20 obtaining a compressed gas 4 at pressure P. 15

[0029] The method also includes a collection phase S40 of the gas stream 5, comprising in turn, according to the invention, a first gas conveying stage S41 drawing 5 from the sealing device 22 to a first point of collection 31, and a second gas conveyance phase S32 20 drawing 5 from the first collection point 31 to a second point collection tank 32 located upstream of compressor 20, in particular on a supply path or duct 10 of the gas 3 to be treated at compressor 20. Always according to the invention, the S40 collection phase comprises a phase of 25 S50 gas pressure control 3 upstream of the second collection point 32, where the first value is set of pressure P as a pressure value lower than the atmospheric pressure P . The method finally comprises a step ATM of S60 sampling of a flow of compressed gas 4 from the 30 compressor 20.

[0030] Preferably, the first pressure value P is between -0.02 relative bar and -0.10 relative bar. More preferably, the first pressure value P is included Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544 between -0.04 relative bar and -0.08 relative bar. For example, the first pressure value P is close to -0.06 bar relative, that is, it is close to 0.95 absolute bar.

[0031] With reference to figures 2, 4 and 5, the apparatus 1a-c 5 includes, in addition to at least one compressor 20 as above described, a gas supply passage 10 of the gas 3 to the compressor 20, typically comprising a duct power supply 12.

[0032] To perform the S40 step of collecting the leaking gas 10 5, the compressor 20 is equipped with a collection device 40 of gas leaks or leaking gas 5 comprising a first collection duct 41 to carry the gas leakage 5 from sealing device 22 to the first point of collection 31 and of a second collection duct 42 for 15 put the first collection point in pneumatic communication 31 with the second collection point 32 which is located along the feed passage 10, in particular along the supply pipe 12.

[0033] In a section 15 of the feed passage 10 a 20 upstream of the second collection point 32 there is a pressure control device 50 configured for impose a first value of in the second collection point 32 pressure P lower than atmospheric pressure P . 1 atm

[0034] The apparatus 1a-c further comprises an exhaust duct 25 60 of the compressed gas 4 from the compressor 20.

[0035] With reference to figure 3, in a variant advantage of the method, the S50 control phase of the pressure includes an extraction step S51 of a fraction 4' of compressed gas 4 in delivery of compressor 20, one phase 30 S57 of pressure reduction of this fraction 4', and a S58 phase of conveying the compressed gas fraction 4 in a section 15 upstream of the second collection point 32. The S57 pressure reduction phase and the Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544 S51 extraction of the 4' fraction of the compressed gas 4 are conducted in such a way as to maintain the second collection point 32 the first pressure value P .

[0036] With reference to figure 4, in one embodiment 5 advantageous, the 50 pressure control device of a compression apparatus 1b comprises a pipe auxiliary 51 arranged to put in pneumatic communication the exhaust duct 60 with the supply passage 10, typically with a 15 section of pipe 10 power supply 12, upstream of the second collection point 32. Along the auxiliary pipe 51 there is also a device pressure reducing valve 57, in the form of a valve self-regulating or pressure regulator 57 In this variant, to the pressure regulation organ 57 which can 15 be associated with a modulating actuator 56 of various types, for example of automatic or electric type. The reduction organ or pressure valve 57 is sized so as to let it pass through the auxiliary pipe 51 and convey from the exhaust duct 60 to the path of 20 power supply 15, upstream of the second collection point 32, a fraction of the compressed gas 4 such as to maintain in the second collection point 32 the first pressure value P above indicated.

[0037] With reference to figure 5, in a variant of the 25 embodiment now described, the control device 50 of the pressure of the apparatus 1c also includes a pressure sensor 52 arranged along the passage of power supply 10 near the second collection point 32. The pressure sensor 52 is configured to generate a 30 pressure signal 53 representative of the pressure in the second collection point 32. The control device 50 It also includes a pressure regulator 54 configured to receive the pressure signal 53 and to compare Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544 the latter with the first desired pressure value P, i.e. with a pressure set-point value introduced in the pressure regulator 54. Based on this comparison, i.e. in based on the difference between the desired value and the value 5 pressure current at the second collection point 32, the regulator 54 is configured to output a signal of control 55 receiveable by actuator 56 which is configured to locally change the power step 10, in this case, the opening of the reduction organ or valve 10 57 based on control signal 55, so that maintain the first value of in the second collection point 32 desired pressure P.

[0038] In a further not shown variant of this form embodiment, the pressure sensor 52 is a pressure sensor 15 differential pressure, i.e. a sensor capable of detecting, in addition to the pressure at the second collection point 32, the atmospheric pressure P current to calculate the difference ATM between these pressure values. In this case, both the signal pressure 53 that the set-point value of the regulator 54 is 20 refer to such a pressure difference or pressure differential, as well known in the art.

[0039] In another advantageous embodiment of the apparatus 1d, shown in figure 6, the passage of power supply 10 of the compressor 20, typically to one of its 25 ends, a tank 13 is provided with a capacity predetermined. The tank1 13, in addition to having the function gas collection in case of malfunction of the 1d apparatus, is sized to act as pressure stabilizing tank. In this case, the 30 pressure sensor 52 is advantageously arranged so to detect the pressure in the stabilizer tank 13, similar or in any case correlated to the pressure value P at collection point 32. Therefore, this form Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544 implementation is compatible and constitutes a refinement of the embodiment of figures 4 and 5.

[0040] In yet another advantageous embodiment of the apparatus 1e, shown in figure 7, an organ is provided 5 preliminary pressure regulation 9 upstream of the feed passage 10, sized to maintain a upstream of the feed passage 10 a relative pressure positive P ', i.e. higher than atmospheric pressure P , 0 atm so as to avoid intrusion of atmospheric air through 10 any defective seals and / or joints. This organ of regulation 9 can have the form of a self-regulating valve of pressure or a pressure regulating valve controlled by a regulation loop, not shown as obvious for those who are skilled in the field. 15

[0041] In a further advantageous embodiment still of the apparatus 1f, shown in figure 8, a valve is provided of overpressure 70 arranged to convey at least a portion 5' of the leaking gas 5 in a predefined safe area 7 if in the collection device 40 of the leaking gas 5 is 20 exceeded a predetermined safety pressure value higher than the first pressure value, for example, in case of failure of the collection device 40 / of regulation of the pressure 50 of the apparatus 1f. This valve overpressure is advantageously arranged along a pipe 25 emergency discharge 71 which preferably refers to a vent collector 80 which collects any other emergency atmospheric vents of the plant, typically the discharges from safety valves and other devices against overpressure, not shown, present in the apparatus 30 1f.

[0042] The above description of embodiments and variants of the apparatus and method is able to show the invention from the conceptual point of view so that Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544 others, using the known technique, will be able to modify and / or adapt these embodiments to various applications specifications without further research and without departing from the inventive concept, and, therefore, it is understood that such 5 adaptations and modifications will be considered as equivalents of the specific variants and embodiments. The means and materials to carry out the various functions described may be of various natures without this go beyond the scope of the invention. It is understood that the 10 expressions or terminology used have purpose purely descriptive and, therefore, not limiting. Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544

Claims

1. An apparatus (1a-f) for compressing a gas (3) comprising: - a compressor (20) configured to compress said gas (3) to obtain a compressed gas (4); - a passage (10) for supplying said gas (3) to said compressor (20), said compressor (20) being provided with a sealing device (22) and a device (40) for collecting a leakage of gas (5) through said sealing device (22); - a discharge duct (60) for discharging said compressed gas (4) from said compressor (20); wherein said collecting device (40) comprises: - a first duct (41) arranged to carry said leakage gas (5) from said sealing device (22) to a first collection point (31);- a second duct (42) arranged to put said first collection point (31) into pneumatic communication with a second collection point (32) of said compressor (20), wherein said second collection point (32) is arranged on said supply passage (10), and wherein, in a section (15) of said supply passage (10) upstream of said second collection point (32) there is a pressure control device (50) configured to impose in said second collection point (32) a first pressure value (P1) lower than atmospheric pressure (Patm).; 2. An apparatus (1a-f) as per claim 1, wherein said compressor (20) is a volumetric compressor, in particular a reciprocating compressor. Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register No. 544 3. An apparatus (1b-f) as per claim 1, wherein said pressure control device (50) comprises: - an auxiliary pipe (51) arranged to put said exhaust pipe (60) in pneumatic communication with said supply passage (10) in said section (15) upstream of said second collection point (32); - a pressure reduction member (57) arranged along said auxiliary pipe (51) and configured to allow a fraction (4') of said compressed gas (4) to pass through said auxiliary pipe (51) and convey from said exhaust pipe (60) to said section (15) upstream of said second collection point (32) such as to maintain said first pressure value (P1) in said second collection point (32).

4. An apparatus (1c,1d) as per claim 1, wherein said pressure control device (50) further comprises - a pressure sensor (52) disposed on said supply passage (10) in proximity to said second collection point (32), said pressure sensor (52) being configured to generate a pressure signal (53) at said second collection point (32); - a regulator (54) configured to - receive said pressure signal (53) and perform a comparison of said pressure signal (53) with a first pressure value (P1) - process a control signal (55) based on said comparison between said pressure signal (53) and said first pressure value (P1); Eng. Marco Celestino ABM Patent & Trademark Agency Registered in the register N.544 - an actuator (56) configured to receive said control signal (55) and to locally modify said passage (10) based on said control signal (55), so as to maintain said first pressure value (Pi) at said second collection point (32).

5. An apparatus (1c,1d) as per claims 3 and 4, wherein said actuator (56) is associated with said pressure reducing member (57) and is configured to command an opening of said pressure reducing member (57) based on said control signal (55).

6. An apparatus (id) as per claim 4, wherein - said supply passage (10) comprises a pressure stabilizing tank (13) having a predetermined volume, and - said pressure sensor (52) is arranged so as to detect a pressure value (Pi*) correlated to said first pressure value (Pi) inside said pressure stabilizing tank (13).

7. An apparatus (1c,1d) as per claim 4, wherein said pressure sensor (52) is a differential pressure sensor between the pressure (P5) at said second collection point (32) and a current atmospheric pressure (Patm).

8. An apparatus (1e) as per claim 1, comprising a preliminary pressure regulating member (11) upstream of said supply passage (10), configured to maintain upstream of said supply passage (10) an upstream pressure (P0') higher than atmospheric pressure.

9. An apparatus (1f) as per claim 1, comprising an overpressure valve arranged to convey at least a portion (5') of said leaking gas (5) into a predefined safe area (9) if a predetermined safety pressure value higher than said first pressure value (Pi) is exceeded in said collection device (40) of said leaking gas (5).

10. A method of compressing a gas (3) comprising the steps of: - feeding (S10) said gas (3) to a compressor (20), said compressor (20) being provided with a sealing device (22); - compressing (S20) said gas (3) in said compressor (20) so as to obtain a compressed gas (4); - collecting (S40) a leaking gas (5) through said sealing device (22); wherein said collecting step (S40) comprises the steps of: - first conveying (S41) said leaking gas (5) from said sealing device (22) to a first collection point (31);- second conveying (S42) of said leaking gas (5) from said first collection point (31) to a second collection point (32) of said compressor (20), wherein said second collection point (32) is located upstream of said compressor (20), and - control (S50) of the pressure of said gas (3) upstream of said second collection point (32) wherein in said second collection point (32) a first pressure value (Pi) lower than atmospheric pressure (Patm) is imposed; - extraction (S60) of a flow of said compressed gas from said compressor (20).; 11. A method (2) as per claim 10, wherein said pressure control step (S50) comprises the steps of: Ing. Marco Celestino ABM Agenzia Brevetti & Marchi Iscrizione all'albo N. 544 - extraction (S51) of a fraction (4' ) of said compressed gas (4) in the delivery of said compressor (20); - pressure reduction (S57) of said fraction (4') of said compressed gas (4); - conveying (S58) of said fraction (4') of said compressed gas (4) in a section upstream of said second collection point (32), wherein said pressure reduction step (S61) and said extraction step (S51) of said fraction (4' ) of said compressed gas (4) are conducted in such a way as to maintain said first pressure value (P1) in said second collection point (32).

12. A method as per claim 10, wherein said first pressure value (P1) is between -0.02 relative bar and -0.10 relative bar, in particular between -0.04 relative bar and -0.08 relative bar, more particularly said first pressure value (P1) is close to -0.06 relative bar.