Vacuum pump

Abstract

A dry vacuum pump (1) comprising: - a discharge line (14) fluidly connecting the lubricant housing (2) to the inter-seal volume (11) and - a pressure gradient control device (15) comprising a check valve (18) arranged on the discharge line (13) to allow the passage of purge gas from the lubricant housing (2) to the inter-seal volume (11) when the pressure difference upstream-downstream of the check valve (18) exceeds a predetermined opening threshold, such that the pressure in the lubricant housing (2) is greater than the pressure in the inter-seal volume (11). Figure 1

Description

Title of the invention: Vacuum pump Technical field of the invention

[0001] The present invention relates to a vacuum pump. The invention relates more particularly to the seal between the pumping chamber and the lubricant casing(s) of the vacuum pump. Technical background

[0002] Volumetric vacuum pumps comprise one or more pumping stages in series in which a gas to be pumped circulates between a suction and a discharge. A distinction is made between primary vacuum pumps with rotary lobes, also known as "Roots" with two or more lobes, or those with a claw, also known as "Claw" or even those with screws. Also known are Roots compressor type vacuum pumps (or "Roots Blower" in English) which are used upstream of primary vacuum pumps, to increase the pumping capacity in high flow situations. These vacuum pumps are called "dry" because in operation the rotors turn inside the stator without any mechanical contact between them or with the stator, which makes it possible not to use oil in the pumping stages.

[0003] The rotation of the rotors is synchronized by gears. The rotors are guided in rotation by bearings generally located on either side of the pumping chamber. These gears and bearings are lubricated by oil or grease contained in lubricant sumps which are isolated from the pumping chamber by a sealing means through which the shafts are always capable of rotating. The sealing devices mainly comprise physical barriers to lubricants such as rubbing lip seals, ejector discs, gas purges and / or obstacles such as labyrinths and baffles.

[0004] However, in operation, the pressures used in vacuum pumps fluctuate significantly, particularly in applications where a volume of gas is cyclically evacuated. Although the atmosphere of the lubricant casings is not evacuated but left at atmospheric pressure, it is also subject to a lesser extent to the pressure variations occurring in the pumping part. Indeed, the sealing means are not perfectly gas-tight since they must allow the rotation of the shafts. In addition, the contact of the lip seals rubbing on the rotating parts can be degraded, or even lost over time, making them ineffective, under the action of their own heating or wear of the lips linked to their movements, themselves generated by the variation in pressure differentials. It follows that gases pumped into the pumping chamber can penetrate into the lubricant sumps.

[0005] These pumped gases may contain corrosive species, precursors or even resin vapors, in the semiconductor, flat panel ("Fiat Panel Display" in English), solar and deposition industries. Some applications, such as "HARP" (for High Aspect Ratio Process in English), generate solid by-products, in large quantities (powders, pastes, pieces). These by-products can succeed in penetrating in large quantities into the lubricant sumps and into dead volumes of the sealing devices, in particular in the baffles or labyrinths, to accumulate and / or react there. This contamination can lead to premature degradation of the properties of the lubricants or poor lubrication of the bearings, which can lead in the more or less long term to their premature wear, or even their breakage, reducing the service life of the vacuum pumps.This phenomenon is accelerated for cyclic pumping applications where successive emptying and filling of the lubricant sumps promotes these gas transfers and therefore contamination of the lubricants. Summary of the invention

[0006] An aim of the present invention is therefore to propose a dry type vacuum pump which at least partially overcomes the drawbacks of the state of the art.

[0007] To this end, the invention relates to a dry vacuum pump comprising: - a lubricant sump, - a pumping chamber comprising at least one pumping stage, - two rotating shafts configured to drive rotors in rotation in the pumping chamber, the shafts being guided in rotation in bearings lubricated by a lubricant contained in the lubricant housing, - at least one first and one second annular lip seals arranged in series along each shaft, interposed between the lubricant casing and the pumping chamber, the lips of the sealing annular seals being oriented in the same direction and in such a way that a pressure in the pumping stage of the pumping chamber adjoining the second annular lip seals, greater than the pressure in an inter-seal volume located between the first and second sealing annular lip seals, presses the lips against the shaft, - a gas injection line fluidically connected to the lubricant housing, - a pressure relief device arranged in the gas injection line, characterized in that the vacuum pump further comprises: - a discharge pipe fluidly connecting the lubricant sump to the inter-joint volume and - a pressure gradient control device comprising a non-return valve arranged on the discharge line to allow the passage of purge gas from the lubricant sump to the inter-seal volume when the upstream-downstream pressure variation of the check valve is greater than a predetermined threshold so that the pressure in the lubricant sump is greater than the pressure in the inter-seal volume.

[0008] The purge gas supply flow rate and the overpressure device in the lubricant sump on the one hand and the gas discharge flow rate through the leaks of the first lip seals on the sump side and the discharge line via the pressure gradient control device on the other hand, ensure that the lubricant sump is continuously overpressured relative to the inter-seal volume, i.e. that the pressure in the lubricant sump is greater than that of the inter-seal volume, and that the gas continuously escapes from the inter-seal volume under the lips of the second lip seals towards the pumping chamber.

[0009] The pumping chamber thus makes it possible to lower the pressure in the inter-seal volume by suction through the leaks of the second annular lip seals. An advantage of this configuration is that the pumping of the inter-seal volume is carried out by a pumping stage of the pumping chamber, i.e. directly by the vacuum pump itself.

[0010] When the first lip seals wear (lubricant sump side first), the gas begins to pass under the lips of these first lip seals to reach the inter-seal volume, in addition to continuing to escape through the non-return valve. The proportion of gas flow escaping through the non-return valve decreases but the direction of flow from the lubricant sump to the pumping chamber continues to be ensured due to the pressurization maintained in the lubricant sump and it is thus ensured that there is no entry of particles, powders or other pumped contaminants into the lubricant sump.

[0011] The more the lips of the first annular seals wear, the more the purge gas flow can escape under the lips. The more the gas flow rate passing under the lips increases, the more the gas flow rate escaping through the non-return valve decreases. The flow rate evacuated from the inter-seal volume through the discharge pipe thus automatically adapts to the wear of the first annular lip seals.

[0012] Another effect obtained as a result of the increase in the gas flow under the lips of the first annular lip seals is that this also makes it possible to limit damage to these first annular lip seals on the casing side.

[0013] The vacuum pump may further comprise one or more of the features described below, taken alone or in combination.

[0014] The first and second annular lip seals are for example single lip.

[0015] The opening threshold of the non-return valve is for example greater than 40 mbars (4000 Pa), such as between 40 mbars (4000 Pa) and 100 mbars (10000 Pa), such as 75mbars (or 7500Pa). This threshold allows the check valve to open before gases leak through the first lip seals when they are new. Thus, when the first lip seals on the crankcase side are new, most of the purge gas flow escapes via the check valve.

[0016] The non-return valve comprises, for example, a spring urging the shutter of the non-return valve into the closed position on its seat.

[0017] The discharge line may fluidly connect the gas injection line, between the overpressure device and the lubricant sump, to the inter-joint volume.

[0018] The pressure gradient control device may comprise a lubricant filter interposed between the non-return valve and the lubricant sump.

[0019] The pressure relief device may comprise a restriction and a pressurization check valve mounted in series in the gas injection line.

[0020] The opening threshold of the pressurization non-return valve is for example less than 20 mbars (or 2000 Pa), such as less than or equal to 10 mbars (or 1000 Pa). The pressurization non-return valve is for example gravity-operated.

[0021] The vacuum pump may comprise a housing in which the gas injection pipe and the discharge pipe are at least partially arranged, the housing receiving the pressure relief device, the non-return valve and the lubricant filter. Brief description of the figures

[0022] Other advantages and characteristics will appear on reading the description of the invention, as well as the attached drawings in which:

[0023] [Fig-1] [Fig.l] shows a schematic view of an example of a vacuum pump.

[0024] [Fig.2] [Fig.2] shows a side sectional view of elements of the vacuum pump of [Fig.l] at the level of an inter-joint volume.

[0025] [Fig.3] [Fig.3] shows a top sectional view of the elements of [Fig.2].

[0026] [Fig.4A] [Fig.4A] shows a partial sectional view of the vacuum pump of the [Fig.l] at the level of a box.

[0027] [Fig.4B] [Fig.4B] shows a diagram of the housing of [Fig.4A] and the elements it contains.

[0028] [Fig.5] [Fig.5] shows a graph of pressures (in mbar) as a function of time (in seconds) for the pressure in the lubricant casing of the vacuum pump in [Fig.l] (curve PI in dotted lines), in the inter-seal volume (curve P2 in solid, thin lines) and in the pumping chamber (curve P3 in solid, thick lines).

[0029] In these figures, identical elements have the same reference numbers. Detailed description

[0030] The following embodiments are examples. Although the description refers to a or more embodiments, this does not necessarily mean that each reference relates to the same embodiment, or that the features apply only to a single embodiment. Single features of different embodiments may also be combined or interchanged to provide other embodiments, without departing from the scope of the invention, as defined by the claims.

[0031] “Upstream” means an element that is placed before another in relation to the direction of circulation of the gas to be pumped. Conversely, “downstream” means an element placed after another in relation to the direction of circulation of the gas to be pumped.

[0032] The invention applies to any type of dry vacuum pump, i.e. comprising one or at least two pumping stages, such as comprising one to ten pumping stages. This vacuum pump may be a primary vacuum pump comprising a plurality of pumping stages and configured to discharge the pumped gases at atmospheric pressure or a dry vacuum pump, called Roots or Roots compressor, with one to three pumping stages which, in use, is fluidically connected in series and upstream of a primary vacuum pump and whose discharge pressure is that obtained by the primary vacuum pump.

[0033] [Fig.l] shows an example of a vacuum pump.

[0034] The vacuum pump 1 comprises at least one lubricant casing 2, a pumping chamber 3 comprising at least one pumping stage T1-T6, two rotary shafts 4 (see [Fig. 3]) configured to drive rotors 5 in rotation in the pumping chamber 3, and at least one sealing device 6 interposed between the lubricant casing 2 and the pumping chamber 3.

[0035] The vacuum pump 1 may comprise only one or more pumping stages T1-T6, such as six, connected in series between the inlet and the outlet of the vacuum pump 1.

[0036] Each pumping stage T1-T6 comprises a respective inlet and outlet. When the vacuum pump 1 comprises several pumping stages, the successive pumping stages are fluidically connected in series one after the other by respective inter-stage channels fluidically connecting the outlet of the preceding pumping stage to the inlet of the following stage. The pumping flow rates of the pumping stages T1-T6 are decreasing or equal with their position between the inlet and the outlet of the vacuum pump, the flow rate generated by the first pumping stage T1 at the lowest pressure corresponding to the greatest pumping flow rate.

[0037] The shafts 4 drive rotors 5 in rotation in the pumping stages T1-T6 of the pumping chamber 3 to drive a gas to be pumped from the inlet to the outlet of the vacuum pump 1. The shafts 4 are driven in rotation by at least one motor M of the vacuum pump 1.

[0038] During rotation, the gas sucked from the inlet is trapped in the volume generated by the rotors 5 and the stator, then is driven by the rotors 5 to the next stage. The rotors 5 of the vacuum pump 1 have, for example, lobes of identical profiles, for example of the “Roots” type (section in the shape of an “eight” or a “bean”) or of the trilobed type or of the “Claw” type or are of the screw type or of another similar principle of volumetric vacuum pump.

[0039] The shafts 4 are guided in rotation in bearings lubricated by a lubricant contained in the lubricant casing 2. The lubricant, such as oil or grease, makes it possible to lubricate in particular the bearings 7 of the bearings and / or the synchronization gears 8 of the shafts 4.

[0040] The vacuum pump 1 is called “dry” because in operation the rotors 5 rotate inside the stator without any mechanical contact between them or with the stator, which makes it possible not to use oil in the pumping chamber 3.

[0041] The vacuum pump 1 comprises, for example, two lubricant casings 2 arranged on either side of the pumping chamber 3, a lubricant sealing device 6 being interposed between the lubricant casing 2 and a pumping stage T1, T6 at the level of each shaft passage on either side of the pumping stages T1, T6.

[0042] The sealing device 6 comprises at least one first lip seal 9 and one second lip seal 10 arranged in series along each shaft 4. There are therefore at least two first lip seals 9 and two second lip seals 10 ([Fig. 3]). The first lip seals 9 are interposed between the lubricant casing 2 and the second lip seals 10. The second lip seals 10 are interposed between the first lip seals 9 and the pumping chamber 3.

[0043] The first and second annular seals 9, 10 are for example single-lip (1 single lip). The lips of the annular sealing seals 9, 10 can rub against the friction rings mounted on the shafts 4.

[0044] Better visible in [Fig. 2], the lips of the annular seals 9, 10 are oriented in the same direction and in such a way that a pressure in the pumping stage T1, T6 of the pumping chamber 3 adjoining the second annular lip seals 10, greater than the pressure in the inter-seal volume 11 located between the first and second annular lip seals 9, 10, presses the lips against the shaft 4. Conversely, the lips of the annular seals 9, 10 tend to lift and thus to “leak” when the pressure in the inter-seal volume 11 is greater than that of the pumping chamber 3.

[0045] The pumping stages T1, T6 adjoining the first and second lip seals 9, 10 are here the first and last pumping stages.

[0046] The sealing device 6 may also comprise a “dynamic” seal, i.e. a non-friction seal, such as a segment seal, a labyrinth seal or a baffle. or a “wall” of gas. The sealing device 6 may also comprise a deflector disc having a general disc shape mounted integrally in rotation on the shaft 4. The sealing device 6 creates a very low conductance around the rotating shafts 4, which makes it possible to greatly limit the passage of lubricating fluids from the casing 2 to the dry pumping stages and vice versa while allowing the shafts 4 to rotate.

[0047] The vacuum pump 1 further comprises a gas injection pipe 12 fluidly connected to the lubricant casing 2, a pressure relief device 13 arranged in the gas injection pipe 12, a discharge pipe 14 fluidly connecting the lubricant casing 2 to the inter-seal volume 11 and a pressure gradient control device 15.

[0048] The injected purge gas is, for example, nitrogen.

[0049] The gas injection pipe 12 opens into the internal volume of the housing of the 2 in an injection zone topologically located so as to limit as much as possible the mixture of oil mist and purge gas in order to limit its carryover. The inlet of the gas injection line 12 is intended to be fluidically connected to a purge gas supply.

[0050] The overpressure device 13 allows a small flow of purge gas to enter easily and quickly into the lubricant sump 2. The maximum injection flow rate of the purge gas is for example 3.2 sim (or 5.4 Pa.mVs) for a lubricant sump 2 located next to the last pumping stage T6 of a multi-stage vacuum pump or 0.5 sim (or 0.8 Pa.mVs) for the lubricant sump 2 located on the side of the first pumping stage of a multi-stage vacuum pump or for a Roots compressor type vacuum pump. These small flows also allow the entry of the purge gas into the lubricant sump 2 not to stir the lubricants and not to carry them towards the pumping chamber 3.

[0051] According to an exemplary embodiment, the overpressure device 13 comprises a restriction 16, that is to say a low conductance orifice (also called a calibrated orifice or nozzle) and a pressurization non-return valve 17 mounted in series in the gas injection pipe 12. The pressurization non-return valve 17 is interposed between the restriction 16 and the lubricant casing 2.

[0052] Restriction 16 makes it possible to limit a maximum injection flow rate of purge gas.

[0053] The pressurization non-return valve 17 makes it possible to prevent the gas coming from the lubricant sump 2 from entering the gas injection pipe 12. Conversely, it is possible to supply the lubricant sump 2 with purge gas by opening the valve 17 when the pressure difference between the gas injection pipe 12 and the lubricant sump 2 is greater than a calibration threshold of the pressurization non-return valve 17, that is to say when the pressure is too low in the lubricant sump 2.

[0054] The pressurization non-return valve 17 is calibrated to allow the passage of gases going towards the lubricant casing 2 when the upstream-downstream pressure variation of the valve 17 is greater than a predetermined opening threshold. This opening threshold is low, it is for example of the order of a few mbars, such as less than 20 mbars (or 2000 Pa), such as less than or equal to 10 mbars (or 1000 Pa).

[0055] The pressurization non-return valve 17 is for example gravity-operated, the shutter, for example a ball, a warhead or other, being held in the closed position on its seat by gravity.

[0056] The restriction 16 and the pressurization non-return valve 17 thus allow mechanical control of the flow of injected gas, simple to implement and inexpensive.

[0057] Alternatively, the overpressure device 13 comprises a flow controller or flow meter (or “mass flow controller” in English).

[0058] The purge gas supply flow rate in the gas injection line 12 and the overpressure device 13 on the one hand and the purge gas discharge flow rate through the leaks of the first annular lip seals 9 and the discharge line 14 via the pressure gradient control device 15 on the other hand, make it possible to ensure that the lubricant sump 2 is continuously overpressurized relative to the inter-seal volume 11, i.e. that the pressure in the lubricant sump 2 is greater than that of the inter-seal volume 11. It is thus possible to pressurize the lubricant sump 2 with a clean gas.

[0059] The discharge pipe 14 fluidly connects, for example, the gas injection pipe 12, between the overpressure device 13 and the lubricant casing 2, to the inter-seal volume 11.

[0060] The pressure gradient control device 15 comprises a non-return valve 18 arranged in the discharge pipe 14. The non-return valve 18 is calibrated to allow the passage of the purge gas from the lubricant sump 2 to the inter-seal volume 11 when the upstream-downstream pressure variation of the non-return valve 18 is greater than a predetermined opening threshold so that the pressure in the lubricant sump 2 is greater than the pressure in the inter-seal volume 11. The non-return valve 18 also makes it possible to prevent gases from the pumping chamber 3 from entering the lubricant sump 2.

[0061] The opening threshold of the non-return valve 18 is for example greater than 40 mbars (4000 Pa), such as between 40 mbars (4000 Pa) and 100 mbars (10000 Pa), such as 75 mbars (or 7500 Pa). This threshold allows the non-return valve 18 to open before the gases leak through the first new annular lip seals 9. Thus, when the first casing-side annular lip seals 9 are new, the majority of the purge gas flow is evacuated via the non-return valve 18.

[0062] To facilitate the implementation of the predetermined threshold, the non-return valve 18 comprises for example a spring 19, for example compression, urging the shutter of the non-return valve 18, for example a ball, a warhead or other, into the closed position on its seat.

[0063] The pressure gradient control device 15 may comprise a lubricant filter 20 interposed between the non-return valve 18 and the lubricant casing 2, to trap any microdroplets of oil mist going from the lubricant casing 2 into the inter-seal volume 11 via the non-return valve 18.

[0064] The lubricant filter 20, such as a sintered stainless steel filter, makes it possible to separate the oil or other lubricant from the gas so as not to empty the casing 2 of its lubricant.

[0065] When the discharge line 14 is fluidically connected to the gas injection line 12 between the pressure relief device 13 and the lubricant housing 2, the lubricant filter 20 can be arranged on the gas injection line 12, between the intersection with the discharge line 14 and the lubricant housing 2.

[0066] According to an exemplary embodiment visible in Figures 4A and 4B, the vacuum pump 1 comprises a housing 21 fixed to the stator of the vacuum pump 1 and in which the gas injection pipe 12 and the discharge pipe 14 are at least partially arranged, the housing 21 receiving the overpressure device 13 (in particular the pressurization non-return valve 17 and the restriction 16), the non-return valve 18, and where appropriate, the spring 19 urging the non-return valve 18 into the closed position and / or the lubricant filter 20. The inlet of the gas injection pipe 12 may have a connection 22 external to the housing 21 for the fluid connection of the pipe 12 to a purge gas supply source.

[0067] According to another example, the elements of the housing 21 are arranged in the body of the stator 2 of the vacuum pump 1.

[0068] In operation, when the annular lip seals 9, 10 are new, the lips press against the shafts 4. In an equilibrium situation, the pressure being higher in the lubricant casing 2, the gas continuously escapes from the lubricant casing 2 through the non-return valve 18 into the inter-seal volume 11 and from the inter-seal volume 11 under the lips of the second annular lip seals 10 towards the pumping chamber 3.

[0069] The pumping chamber 3 thus makes it possible to lower the pressure in the inter-seal volume 11 by suction through the leaks of the second annular lip seals 10. An advantage is that the pumping of the inter-seal volume 11 is carried out by a pumping stage T1, T6 of the pumping chamber 3, i.e. directly by the vacuum pump 1 itself.

[0070] When the first annular lip seals 9 wear (lubricant casing 2 side first), the gas begins to pass under the lips of these first annular seals 9 to reach the inter-seal volume 11, in addition to continuing to escape through the non-return valve 18. The proportion of gas flow escaping through the non-return valve 18 decreases but the direction of flow from the lubricant sump 2 to the pumping chamber 3 continues to be ensured due to the pressurization always maintained in the lubricant sump 2 and it is thus ensured that there is no entry of particles, powders or other pumped contaminants into the lubricant sump 2.

[0071] The more the lips of the first lip seals 9 wear, the more the purge gas flow can escape under the lips. The more the gas flow rate passing under the lips increases, the more the gas flow rate escaping through the non-return valve 18 decreases. The pressure gradient control device 15 and the fact that the pressure in the lubricant sump 2 is greater than the pressure in the inter-seal volume 11 therefore makes it possible to adapt the flow rate evacuated from the inter-seal volume 11 via the evacuation pipe 14 as a function of the wear of the first lip seals 9.

[0072] Another effect obtained from the fact that the flow of gas escaping through the non-return valve 18 decreases following the increase in the gas flow under the lips of the first annular lip seals 9 is that damage to these first annular lip seals 9 is limited.

[0073] The advantages of the present invention can be better understood with reference to the graph of [Fig.5] showing pressure curves in the lubricant sump (curve PI) next to the last pumping stage T6 of a multi-stage vacuum pump 1, in the inter-seal volume (curve P2) and in the last pumping stage T6 of the pumping chamber (curve P3) at the second lip seals 10.

[0074] In an equilibrium situation and here while the annular lip seals 9 are new (very tight), the pressure in the lubricant casing PI is of the order of 1180 mbars (or 118000Pa) and the pressure in the inter-seal volume P2 is of the order of 1080 mbars (or 108000Pa). The difference of 100mbars (or 10000Pa) between the two values ​​results from the calibration value of the pressurization non-return valve 17 of the order of 75mbars (7500 Pa) and the different conductances.

[0075] The pressure in the pumping chamber P3 is an average value between the suction and discharge pressures in the pumping stage T6. This average value is here in an equilibrium situation, of the order of 760 mbar (or 76000 Pa).

[0076] The pressure in the lubricant casing PI is thus higher than the pressure in the inter-seal volume P2, itself higher than the pressure in the last pumping stage P3. The gas continuously escapes from the lubricant casing 2 through the non-return valve 18 to the inter-seal volume 11 and from the inter-seal volume 11 under the lips of the second annular lip seals 10 to the pumping chamber 3.

[0077] When an isolation valve arranged upstream of the vacuum pump 1 is opened tl in the direction of flow of the pumped gases for creating a vacuum in an enclosure (or equivalent) fluidically connected to the inlet of the vacuum pump 1, the sudden entry of the gases pumped into the vacuum pump 1 causes a pressure rise in the last pumping stage (curve P3). The pressure in the pumping stage T6 then becomes almost immediately higher than the pressure in the inter-seal volume P2, which tends to close the second lip seals 10. This isolation causes the pressure to rise in the lubricant casing PI and in the inter-seal volume P2 because the overpressure device 13 continues the injection of purge gas into the lubricant casing 2 and consequently into the inter-seal volume 11.

[0078] Then, once the volume of the enclosure has been emptied, the pressure drops again in the last pumping stage P3 and falls below the pressure of the inter-seal volume P2 (at time t2). We then fall back into the situation where the pressure in the lubricant casing PI is higher than the pressure in the inter-seal volume P2, itself higher than the pressure in the last pumping stage P3. The gas then escapes from the lubricant casing 2 through the non-return valve 18 towards the inter-seal volume 11 and from the inter-seal volume 11 under the lips of the second annular lip seals 10 towards the pumping chamber 3, so that the pressures in the lubricant casing PI and in the inter-seal volume P2 drop again.

[0079] The increase in pressure in the lubricant casing 2 and in the inter-seal volume 11 caused by the closing of the second annular sealing joints 10 and by the continuous injection by the overpressure device 13 made it possible to reduce the duration (here to 1.5 seconds) for which the pressure in the pumping chamber 3 was greater than in the inter-seal volume 11, where particles or contaminants can be entrained towards the lubricant casing 2 and where the second annular lip seals 10 on the pumping side wear out.

[0080] Although [Fig.l] illustrates a pressure boosting device 13 and a pressure gradient control device 15 for only one of the two lubricant casings 2, the invention can be applied to one or the other of the two lubricant casings 2 or to both lubricant casings 2 of the vacuum pump 1.

Claims

Claims

1. Dry vacuum pump (1) comprising: - a lubricant casing (2), - a pumping chamber (3) comprising at least one pumping stage (T1-T6), - two rotary shafts (4) configured to drive rotors (5) in rotation in the pumping chamber (3), the shafts (4) being guided in rotation in bearings lubricated by a lubricant contained in the lubricant casing (2), - at least one first and one second annular lip seal (9, 10) arranged in series along each shaft (4), interposed between the lubricant casing (2) and the pumping chamber (3), the lips of the annular sealing seals (9, 10) being oriented in the same direction and in such a way that a pressure in the pumping stage (T1, T6) of the pumping chamber (3) adjoining the second annular lip seals (10), greater than the pressure in a volume inter-seals (11) located between the first and second annular lip seals (9, 10),presses the lips onto the shaft (4), - a gas injection line (12) fluidly connected to the lubricant sump (2), - a pressure relief device (13) arranged in the gas injection line (12), characterized in that the vacuum pump (1) further comprises: - a discharge line (14) fluidly connecting the lubricant sump (2) to the inter-seal volume (11) and - a pressure gradient control device (15) comprising a non-return valve (18) arranged on the discharge line (13) to allow the passage of the purge gas from the lubricant sump (2) to the inter-seal volume (11) when the upstream-downstream pressure variation of the non-return valve (18) is greater than a predetermined opening threshold so that the pressure in the lubricant sump (2) is greater than the pressure in the inter-seal volume (11).,

2. Vacuum pump (1) according to the preceding claim, characterized in that the first and second annular lip seals (9, 10) are single-lip.

3. Vacuum pump (1) according to one of the preceding claims, characterized in that the non-return valve (18) comprises a spring (19) urging the non-return valve shutter (18) into the closed position on its seat.

4. Vacuum pump (1) according to one of the preceding claims, characterized in that the opening threshold of the non-return valve (18) is greater than 4000Pa, such as between 4000Pa and 10000 Pa.

5. Vacuum pump (1) according to one of the preceding claims, characterized in that the discharge pipe (14) fluidly connects the gas injection pipe (12), between the overpressure device (13) and the lubricant casing (2), to the inter-seal volume (H).

6. Vacuum pump (1) according to one of the preceding claims, characterized in that the pressure gradient control device (15) comprises a lubricant filter (20) interposed between the non-return valve (18) and the lubricant casing (2).

7. Vacuum pump (1) according to one of the preceding claims, characterized in that the overpressure device (13) comprises a restriction (16) and a pressurization non-return valve (17) mounted in series in the gas injection line (12).

8. Vacuum pump (1) according to the preceding claim, characterized in that the pressurization non-return valve (17) is gravity-operated.

9. Vacuum pump (1) according to one of claims 7 or 8, characterized in that the opening threshold of the pressurization non-return valve (17) is less than 2000Pa, such as less than or equal to 1000Pa.

10. Vacuum pump (1) according to claim 6, characterized in that it comprises a housing (21) in which the gas injection pipe (12) and the discharge pipe (14) are at least partially arranged, the housing (21) receiving the overpressure device (13), the non-return valve (18) and the lubricant filter (20).

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