CN111801498B

CN111801498B - Vacuum pump with pressure relief valve

Info

Publication number: CN111801498B
Application number: CN201980018247.7A
Authority: CN
Inventors: M·R·维克斯; N·特纳; A·E·K·霍尔布鲁克; S·凯拉萨姆
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2018-03-09
Filing date: 2019-03-08
Publication date: 2023-03-10
Anticipated expiration: 2039-03-08
Also published as: WO2019171075A1; CN111801498A; GB2571792B; TW201938916A; EP3762611B1; TW201938915A; TW201940815A; GB2571792A; EP3762611B8; WO2019171076A1; WO2019171074A1; EP3762611A1; GB201803859D0

Abstract

The vacuum pump includes: at least two rotors, each comprising at least one blade, the at least two rotors being mounted within the stator; at least one of the blades includes an inertial pressure relief valve comprising: a body mounted for movement between a closed position in which the body blocks a fluid flow passage between a front side and a rear side of the rotor and an open position in which the fluid flow passage is unobstructed; wherein the body is configured to move between the open position and the closed position in response to a change in a force acting on the movable body triggered by a change in a rotational speed of the rotor.

Description

Vacuum pump with pressure relief valve

Technical Field

The field of the invention relates to the field of vacuum pumps, and in particular to vacuum pumps having a pressure relief valve.

Background

The present application relates to vacuum pumps, and in preferred embodiments to booster pumps, and to Pressure Relief Valves (PRVs) for use in such pumps to alleviate problems that can arise when the input pressure to such pumps rises suddenly. Booster pumps are used to boost the capacity of vacuum pump assemblies. It operates at a high rate and during most of its operation there is only a small pressure difference between the inlet and the outlet of the pump. If the pressure at the inlet suddenly rises, the gas is rapidly transported by the rotor to the pump outlet, where the pressure may rise to 5 bar or even 8 bar. This pressure exerts a large force on the rotor, which can cause it to crack, or cause the shaft gear to slip. To address this issue, PRVs are typically incorporated into such booster pumps. These PRVs are typically mounted in a stator and are pressure actuated and opened when the pressure at the supercharger outlet exceeds the inlet pressure by more than a certain amount. This helps reduce the pressure differential between the inlet and outlet and protects the pump.

Such conventional PRVs are generally located in a path extending the stator that recirculates the gas at the outlet to the inlet, thereby increasing the pressure at the inlet. Such an arrangement increases pump footprint by making the stator wider and/or longer (than it would otherwise be). Fig. 1 shows an example of such a prior art arrangement.

It would be desirable to provide a vacuum pump with a pressure relief valve without unduly increasing the size of the pump.

Disclosure of Invention

A first aspect of the present invention provides a vacuum pump comprising:

at least two rotors, each comprising at least one blade, the at least two rotors being mounted within a stator; at least one of the vanes includes an inertial pressure relief valve comprising: a body mounted for movement between a closed position in which the body blocks a fluid flow passage between a front side and a rear side of the rotor and an open position in which the fluid flow passage is unobstructed; wherein the body is configured to move between the open and closed positions in response to a change in a force acting on the movable body triggered by a change in a rotational speed of the rotor.

The inventors of the present invention have addressed the problem of PRV increasing the footprint and cost of conventional pumps by incorporating the PRV within the rotor such that the pressure relief fluid flow path between the inlet and outlet is formed from one side of the rotor to the other, rather than via a path external to the stator.

While it may be desirable to incorporate a valve into a pump in this manner, conventional PRVs respond to pressure differentials, typically opening when the pressure acting on the outlet side of the valve is sufficient to move the movable valve element (either against its own weight, or against the force provided by a spring). This makes it unsuitable for use in rotors having blades, where the pressure differential across the rotor reverses depending on the position of the rotor in its rotation cycle. Thus, if such a conventional PRV were to be incorporated into a vacuum pump rotor, it would attempt to open and close as the pressure differential reverses during each rotation of the rotor. This would not only mean that the gas discharge channel would not be available for some time, but also that the rotation rate is such that the valve would not be able to open and close successfully within the time provided, and likely remain in some intermediate indeterminate state.

The inventors of the present invention have recognized that while conventional PRVs may not provide an acceptable valve when located within the rotor, if an inertial pressure relief valve is instead used, the desired operating characteristics may be achieved.

In this regard, it is recognized that the pressure increase experienced by such pumps will slow the rotor, and thus configuring the valve to react to this (rather than the pressure differential itself) will allow the valve to be provided on the rotor, which will not reverse position with varying pressures on different faces as the blades rotate. An inertia valve configured to move between an open position and a closed position in response to changes in the rotational speed of the rotor would provide the desired operational characteristics. Such valves incorporated into the rotor occupy no additional space and still provide relief against pressure fluctuations.

In some embodiments, the fluid flow channel comprises a passage between a rotor tip and the stator, and the body comprises at least a portion of the rotor tip.

In some embodiments, the body may form part of the rotor tip when in its closed position, providing at least part of the pump sealing face for the stator. When in the open position, the body does not extend out to the stator and a channel is formed between the stator and the rotor blade, allowing some pressure relief and reduction in the force experienced by the rotor.

In other embodiments, the fluid flow channel comprises a passageway through the vane.

Alternatively, the fluid flow passage may be a passage through the rotor which is sealed by the body when in its closed position. When in the open position, the body moves to open the passage and experiences some relief and the force on the rotor is reduced. One way of observing this is that when the body is in the open position, the rotor becomes more porous and, therefore, the force exerted by the fluid pumped over the rotating blades is reduced.

The body may have several different forms in the case where it is configured to move between an open position and a closed position in which it seals the fluid passage. In some embodiments, the body comprises a ball mounted within the fluid flow passage, the fluid flow passage comprising a constriction, the constriction comprising a seat into which the ball fits in the closed position, the ball being on the leading edge of the seat.

Providing a ball on the front side of the valve seat is not a conventional way of configuring a pressure relief valve. However, when installed in this position, deceleration of the rotor will cause the ball to move away from the valve seat and open the valve. Deceleration occurs in response to an increase in pressure at the fluid inlet and is an indication of a demand for pressure relief. Thus, having a valve that reacts to deceleration to open provides a suitable pressure relief function. Furthermore, once the conditions requiring pressure relief are reduced and the pressure differential between the inlet and outlet begins to decrease, the rotor will begin to accelerate and the acceleration forces will cause the body to move to the closed position and normal pumping operation will resume.

In some embodiments, the body is mounted such that centrifugal forces due to rotation of the rotor act to bias the body towards the closed position.

In some embodiments, the inertial pressure relief valve is designed such that centrifugal forces due to rotor rotation act to bias the movable body towards the closed position, and thus during steady state operation, the body is biased to the closed position, and the rotor has the functional characteristics of a solid rotor.

In some embodiments, the body is mounted within a guide that constrains movement of the body between the open and closed positions.

To control the movement of the body between the open and closed positions, it may be desirable to mount it within some type of guide such that its motion is constrained along a particular path, allowing for more predictable movement of the body and more controllable pressure relief operations.

In some embodiments, the fluid flow channel comprises the guide.

In some cases, the fluid flow path may act as a guide, and in particular where the fluid flow path includes a valve seat, then the body may move within the fluid flow path towards and away from the valve seat in accordance with the speed change of the rotor.

In some embodiments, the closed position for the body is located at a point within the guide that is radially furthest from the center of the rotor.

As noted previously, it may be desirable that centrifugal forces due to rotation of the rotor act to bias the body towards the closed position, and one simple way of providing such a function is to provide the closed position in the guide at a position radially furthest from the centre of the rotor. Thus, in case the body moves along the guide, when the rotor rotates, it will be thrown to this closed position due to centrifugal forces.

In some embodiments, the fluid flow path comprises two laterally offset channels, one extending from the front side of the rotor to a connection point and the other extending from the rear side of the rotor to a connection point, a connecting path extending between the channels, the body being mounted so as to block the connecting path when in the closed position.

In some cases, the movable body may be within a connection path between two offset fluid flow channels as opposed to having it within a fluid flow channel. One advantage of this is that the pressure difference on either side of the rotor blade acts perpendicular to the body and does not urge it towards or away from the open or closed position. Conversely, the guide in which the body moves may be designed so that the only significant force on the body is due to the movement of the rotor, and the appropriate angle and weight of the body may be utilized to achieve the desired features for opening and closing the valve.

In some embodiments, at least a portion of the body and at least a portion of the rotor adjacent the closed position are formed of magnetic material such that the body is biased toward the closed position by magnetic attraction between the body and the rotor.

In some cases, the only biasing force toward the closed position is due to centrifugal force. In other embodiments, there may be a magnetic biasing force, and in some embodiments, there may be both. Where magnetic force is used to bias the body towards the closed position then centrifugal force may not be required in steady state operation to keep the body in the closed position and in this case the guide need not be angled to provide the closed position at the radially furthest point. In this case, acceleration and deceleration of the rotor may be used to provide movement of the body, which is held in the closed position by magnetic force. In other embodiments, holding the body in the closed position may be a combination of centrifugal and magnetic forces.

In some embodiments, the body comprises a disc operable to roll within the guide.

As noted previously, the body may have several forms, and it may comprise a disc mounted to roll within a guide. In other embodiments, it may be a ball that rolls within a guide or a rod that rolls within a guide.

In some embodiments, the guide comprises a ramp such that an edge of the guide closer to the rear side comprises the closed position for the body is radially further from a center of the rotor than an edge of the guide closer to the front side.

The guide may be arranged such that the closed position is radially furthest along the guide, such that centrifugal force acts to close it. So that it is directed towards the rear side means that deceleration will provide a force on the body acting to open the valve, while acceleration will provide a force acting to close the valve.

In some embodiments, the vanes comprise hollow vanes, the at least one of the vanes comprises a passage through an outer surface of the rotor, one of the passages is located on the front side of the rotor and one of the passages is located on the rear side of the rotor, and the body comprises a rod mounted within the at least one of the vanes, the body being operable to block the passage in the closed position and to hold the passage open in the open position.

In some cases, the blades of the rotor may be hollow, and this has the advantage of reducing the weight of the rotor and therefore the power required to rotate it. Where the fluid flow path is a passage through the vane, then the hollow vane may form part of this passage, with the outer surface of the vane having passages on the front and rear sides on either side of the sealing surface, such that there is a fluid flow passage across the vane. In some cases, the body may include a rod mounted within at least one of the hollow vanes, the body moving in response to inertial forces to block the passage in the closed position and keep it open in the open position. In some cases, the rod may loosen within the hollow vane, and in other cases there may be some type of guide mechanism that rolls within between the open and closed positions.

In some embodiments, the vacuum pump comprises a plurality of fluid flow paths in each vane and a corresponding plurality of inertial pressure relief valves.

There may be only a single fluid flow path in one blade or there may be only a single fluid flow path in each blade of each rotor. Alternatively, the blade may have a plurality of fluid flow paths arranged in parallel. This has the advantage that the effect due to the opening of these paths increases with the number of paths and, consequently, the pressure relief provided increases.

In some embodiments, the body is mounted such that deceleration of the rotor provides a force biasing the body away from the closed position and acceleration of the rotor provides a force biasing the body toward the closed position.

In some cases, the body may be mounted such that deceleration of the rotor provides a force for biasing the body away from the closed position and acceleration provides a force biasing it toward the closed position. When pressure relief is required due to the sudden increase in pressure at the inlet providing a large force on the rotor, significant deceleration of the rotor occurs and this action can be used as a mechanism to trigger the opening of the pressure relief valve and provide effective pressure relief. The use of an inertial pressure relief valve with a moving body that reacts in this manner provides a rapid response to changes in inlet pressure.

In some embodiments, the body is mounted connected to a spring such that the spring force biases the body toward the open position and the centrifugal force biases the body toward the closed position; such that when the rate of rotation exceeds the predetermined speed, the body is in the closed position, and when the rate of rotation is below the predetermined speed, the body is in the open position.

Alternatively, the body may be connected to a spring, and the spring may bias the body toward the open position, while centrifugal force biases the primary force toward the closed position. In such cases, the rotation rate determines whether it is in the closed position or the open position. This is an alternative approach and provides predictable operation, although the pressure relief valve may react more slowly to pressure changes than when it reacts to deceleration. This predictable nature of the operation of the spring may enable the pump to be configured to operate in a green mode. When operating in the green mode, the pump is typically run slowly to reduce power consumption. The valve spring may be configured to open the PRV at the green mode rate and avoid any compression in the supercharger at this rate. This will reduce the power even further. During normal higher power operating modes, the valves will remain closed until the rate drops to these slower rates due to the deceleration caused by the pressure increase of the pumped gas.

In some embodiments, the at least one blade comprises a hollow blade and the body comprises two flaps, each mounted on a pivotable arm and configured to cover a passage through an outer surface of the rotor, one passage on the front side of the rotor and one passage on the rear side of the rotor, the two arms being connected via a spring biased to hold the two flaps in position to close the passages when the rotor is rotating at a speed above the predetermined rotational speed and not to block the passages when the rotor is rotating at a speed below the predetermined rotational speed.

In some embodiments, the at least one vane comprises a hollow vane and the body comprises a curved flap mounted on a spring biasing the flap away from the stator, the flap being operable to form an outer sealing surface of the rotor when the rotor is rotating at a speed above the predetermined speed.

Some embodiments may provide a movable body at the outer edge of the blade, allowing pressure relief in a simple manner by simply allowing gas to flow through the gap between the rotor and stator (rather than being routed through the rotating rotor).

In some embodiments, the vacuum pump is a vacuum booster pump. The vacuum booster pump provides increased capacity for the vacuum pump system and this is achieved by rotating at a high rate. In the event that the system it pumps has sudden pressure changes, then the pressure differential across the booster pump can change very suddenly and this causes significant stress on the booster pump. Such pumps are therefore particularly suitable for pressure relief valves which help to protect them from the high forces that occur in the event of sudden pressure changes.

While the pump may have several different forms, it may comprise a two-rotor roots pump. These pumps generally form vacuum booster pumps and are particularly suited for the pressure relief valves of the embodiments.

In some embodiments, the fluid flow channel merges with the outer surface of the vane towards the centre of the rotor, thereby extending the time during which the fluid flow channel between the leading and trailing sides is open.

Where the outlet of the fluid flow channel is towards the centre of the rotor then it is unobstructed by the other rotors for most of the rotation and provides more effective pressure relief.

Although the rotors may each comprise one blade, the pump performs better generally in the presence of at least two blades.

Although not all of the vanes may include a pressure relief valve, in some embodiments, each of the vanes includes a pressure relief valve. In the case of asymmetric blades on the rotor, then there may be some imbalance, and so it may be advantageous for each blade to comprise a pressure relief valve. In addition, providing an additional pressure relief valve helps to increase the pressure relief provided.

Other particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be understood that this includes an apparatus feature that provides the function or is adapted or configured to provide the function.

Drawings

Embodiments of the invention will now be further described with reference to the accompanying drawings, in which:

FIG. 1 shows a roots pump having a pressure relief valve according to the prior art;

FIG. 2 illustrates a roots pump having a pressure relief valve according to an embodiment;

FIG. 3 shows a blade of a rotor having a pressure relief valve according to an embodiment;

FIG. 4 shows the pressure relief valve of FIG. 3 without the movable body;

FIG. 5 illustrates an alternative embodiment of a pressure relief valve according to an embodiment;

FIG. 6 schematically illustrates a pressure relief valve opening and closing at a particular speed, in accordance with an embodiment;

FIG. 7 shows a section through the valve of FIG. 6 when in operation;

FIG. 8 shows a valve similar to that of FIG. 6 with the spring in a different position;

FIG. 9 shows a section through the valve of FIG. 8 in an open position and a closed position;

FIG. 10 shows another embodiment of a pressure relief valve similar to that of FIGS. 6 and 8, but having a different flap arrangement;

FIG. 11 shows a section through the valve of FIG. 10 in an open position and a closed position;

FIG. 12 shows another embodiment in the case where the fluid flow path does not pass through a passage in the rotor; and

fig. 13 shows another embodiment of the pressure relief valve with the passage through the hollow rotor blocked by the movable rod.

Detailed Description

Before discussing the embodiments in more detail, an overview will first be provided.

An inertial pressure relief valve incorporated into the supercharger rotor (rather than being pressure actuated and therefore having to open and close each half rotation of the rotor) is actuated by inertia and therefore can remain open throughout the pressure spike event that provides its relief.

Some embodiments of the PRV are configured to respond to deceleration and acceleration of the rotor, and thus will open once at the beginning of a pressure spike event in response to the deceleration experienced by the rotor, and then close again once the conditions have subsided and the rotor has accelerated back up towards the operating rate.

Other embodiments of the PRV are configured to open in response to a predetermined speed when the rotational speed of the rotor decreases below this predetermined speed and close when it increases above this predetermined speed. Such PRVs use centrifugal force due to rotation as the closing force, e.g. provided by a spring means.

Fig. 1 shows a roots supercharger pump with a PRV according to the prior art. In this pump, there are PRVs each having a pressure-actuated movable body 20, the movable body 20 being located on a valve seat 22 within a conduit that is external to the stator and connects the outlet of the pump to its inlet. In this arrangement, in the event of excessive pressure at the outlet, the movable body 20 is pushed away from the valve seat 22 by the pressure differential between the outlet and the inlet, and there is a flow of gas through the conduit between the outlet and the inlet, which releases the pressure differential. This is an effective pressure relief valve, but as can be seen it takes up much space and requires additional components.

Fig. 2 schematically shows a pressure relief valve for a roots pump, similar to the prior art, but wherein the pressure relief valve is within the rotor of the roots booster pump. In this embodiment, each rotor blade includes a gas flow path through the blade that is closed in steady state operation by a movable body located on a corresponding valve seat. In response to a sudden pressure increase, the rotor will suddenly decelerate and the movable body will be thrown off the valve seat and the valve will open, resulting in pressure relief and gas flowing through the rotor from the higher pressure outlet to the lower pressure inlet. In this way, a pressure relief valve having operating characteristics similar to those of the prior art is provided without additional space requirements. Furthermore, as the valve is activated by deceleration, it responds to sudden changes and pressure relief occurs rapidly.

Fig. 3 and 4 show an embodiment of a PRV within a blade of a rotor. In this embodiment, the valve comprises a ball 20 of conventional appearance and a valve seat 22 drilled across the vanes of the supercharger rotor 10. Fig. 3 shows the valve in the closed position, while fig. 4 shows the valve without the ball 20, so that the valve seat 22 is shown. The outlet side of the valve is on the front side of the vane; this is contrary to the idea in which one would think of installing a pressure controlled PRV. The road 30 is angled such that its radius from the rotor is less than the radius of the valve seat. In this way, centrifugal force on the ball will cause it to roll into the valve seat, closing the valve. Furthermore, the port to the road 30 will be closer to the centre of the rotor so that it can be unobstructed by other rotors for most of the rotation. It should be noted that the angle of the road is exaggerated in the drawings for convenience of representation.

When there is excessive outlet pressure, it causes rapid deceleration of the rotor set. The main part of the force available to decelerate the ball in the valve is the centrifugal force causing it to roll "down" the ramp. The secondary portion is the pressure differential acting across the valve. If the deceleration rate of the rotor exceeds

α _max = ω ² cos θ+ SΔP/mr，

Where theta is the angle of the slope of the guide,

ω is the angular rate of rotation,

α _max is the angular deceleration that will only keep the valve closed (any faster angular deceleration will open the valve),

s is the area of the valve seat,

ap is the pressure difference across the rotor (booster between inlet and outlet),

m is the mass of the moving element of the valve, and

r is the distance of the moving element of the valve from the axis of rotation,

the ball will be thrown off the valve seat and "ramp up" and open the valve. The point at which the valve opens can be adjusted by changing the angle between the exit road and the rotor radius. The relative importance of the pressure and centrifugal terms can be adjusted by changing the ball mass or the radius of the valve seat. The bore is closed at the end to prevent the ball from flying into the swept volume.

The gas path from the outlet to the inlet is shorter and more direct than in a conventional stator PRV, which provides conduction advantages in the rotor PRV.

In a preferred embodiment, the valves are added to the rotor in a number equal to the number of each blade of the rotor (so that equilibrium is maintained when the valves are open).

In general, a faster supercharger may be desirable because it reduces product footprint. As noted with respect to fig. 2, these faster boosters are more likely to require PRV protection (as a whole rule). This design allows a PRV to be added to the supercharger without affecting the overall product footprint. In general, there is more space on the supercharger rotor than on the stator or top plate. If more than one PRV is required, it is easier to fit additional valves into the rotor (in parallel) than, for example, looking for more space in the top plate.

For some superchargers installed within an enclosure, pressure relief is required and space is still very limited. An inertial pressure relief valve may be a key enabler to achieve the required peak velocity within a given footprint.

Figure 5 shows an alternative embodiment in which the pressure relief valve is formed by a movable body in the form of a disc 24 mounted within a guide 26. When the valve is in the closed position, the disc acts to block the link between the two laterally offset

passages

30a, 30 b.

At pump start-up, the rotational rate of the rotor will increase from zero to the operating rate of the pump. During this stage, the rolling disk 24 will roll to the rear of the slot in the rotor and close the port through the rotor 10. The clearance between the disc and the slot in which it is located is small enough to prevent significant leakage across the rotor and large enough to allow free movement of the disc.

During pump operation, the disc is held in its closed position by centrifugal forces resulting from a very small gradient in the rolling surface 50 of the guide 26 around the centre of the rotor. Alternatively or additionally, the disc may be held in the closed position by a magnet (not shown), in which case the guide 26 would not need to be increased in gradient. The gas pressure acting on the disk acts perpendicular to its travel path and, therefore, as the direction of the pressure differential changes during rotation, there is a finite force variation on the disk in the direction in which it opens and closes.

When gas is dumped into the inlet of the booster pump, the gas is delivered by the rotor to the exhaust side, which causes the rotor to decelerate rapidly. As the rotor decelerates, the disk rolls to the open position. This allows the gas to pass through the rotor, which reduces the peak pressure on the downstream side.

Analysis has shown that achievable manufacturing tolerances can be exploited to produce the gradient required to allow the disk to roll under deceleration forces on the rolling radially outer surface of the guide 50.

Fig. 6-12 show an alternative embodiment of an inertial Pressure Relief Valve (PRV) within the rotor configured to operate based on rotor speed.

In this embodiment, the bore holes on either side of the seal surface of the supercharger rotor allow gas to flow through the hollow rotor. Those holes are blocked by the flap being spring-loaded into the open position, the centrifugal force of the rotating rotor acting to close the flap when the centrifugal force exceeds the spring force. This occurs at a predetermined rotational speed.

FIG. 6 shows an overview of an inertial pressure relief valve according to one embodiment. Fig. 7 shows the same pressure relief valve in both the open (right-hand drawing) and closed (left-hand drawing) positions, below and above a predetermined speed. Thus, the left-hand drawing shows the rotor rotating at a speed above the predetermined threshold speed, which is the speed at which the trigger flap moves, and at which higher rate the flap is in the closed position. The right hand figure shows the same rotor rotating at a rate below the critical speed and the flap opening. When the rotor is rotating at a higher rate, the flap is in the closed position, and when it is rotating at a lower rate, the spring force of the transverse spring 40 extending between the arms exceeds the centrifugal force due to the rotation, and the flap opens, and gas can follow the path shown by arrows 62 and 64 through the rotor. In this way, when there is a sudden pressure increase and the rotor decelerates, the valve flap will automatically open and gas flow will occur when the rotation rate drops below a predetermined threshold rate. In this case, it is not the deceleration of the rotor that actuates the valve, but it is below a certain rate. In this way, a pressure relief valve that responds accurately to a particular rate can be designed. The rate at which this occurs will depend on the weight of the flap and the strength of the spring. It should be noted that as the velocity reaches a predetermined velocity due to centrifugal force on the pressure relief valve assembly, the valve flap will gradually close.

Thus, for example, when a booster vacuum pump is subjected to an atmospheric pressure dump, the rotor will rapidly decelerate, and as the rotor is below a predetermined speed, the valve flap will begin to open, allowing gas to pass through the rotor, due to the spring force being higher than the centrifugal force acting on the flap. This reduces the peak pressure experienced by the downstream pump. The gas will continue to pass through the rotor while the velocity remains low. Once the effect of the pressure fluctuations begins to decrease, the speed of the rotor will increase and when a predetermined speed is reached, the valve flaps will close again.

Fig. 8 and 9 show alternative arrangements in which the spring 40 is in a different position. In this embodiment, a spring 40 is mounted at the pivot point of the arm holding the flap. The pressure relief valve will operate in the same manner as the embodiment in which the spring 40 is a transverse spring between the extensions of the arms. Thus, the left-hand figure shows the rotor rotating at a speed above a predetermined threshold speed, which is the speed at which the trigger flap moves, and at which higher speed the flap is in the closed position. The right hand figure shows the same rotor rotating at a rate below the critical speed and the flap opening.

Figures 10 and 11 show an alternative embodiment in which the flap is mounted about a pivot point at either edge. Again, the left-hand figure shows a state above the critical speed, with the valve flap in the closed position, and the right-hand figure shows the same rotor at a lower velocity, with the valve flap in the open position. The spring pulls the valve flaps towards each other and the valve flaps overlap in the open position. When the centrifugal force exceeds the spring force, the valve closes and the flap extends across a majority of the outer surface of the rotor, which allows the gas passage to be larger.

Fig. 12 shows an alternative embodiment, in which the moving body is a spring mounted on an arm extending towards the centre of the rotor. The moving body 20 forms a rotor tip when in the closed position and when the rotor slows down and the centrifugal force is less than the force of the spring 40 then the body will open and there will be a passage between the stator and this rotor tip for the gas to pass through. This is a convenient way of providing a passage for gas, allows simple gas flow, and does not require gas to flow through the rotor passages which rotate themselves.

Fig. 13 shows an alternative embodiment in which the PRV opens and closes in response to acceleration and deceleration (as opposed to a predetermined speed). In this embodiment, the

openings

14, 16 at the outer surface of the rotor 10 form gas flow passages in combination with the hollow space within the rotor from one side of the rotor to the other, when the

outlets

14, 16 are not obstructed by rods 20 mounted within the hollow blades. When in steady state operation, the rod 20 is thrown to a radially outer position (as shown in the figures) in which it obstructs the

openings

14, 16. As the rotor decelerates, the rods move away from the

openings

14, 16 and gas can flow through the rotor.

Although illustrative embodiments of the present invention have been disclosed in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to the precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Reference numerals

10. Rotor

12. Center of rotor

14. 16 opening

20. Movable ball valve body

22. Valve seat

24. Movable disk main body

26. Guide piece

30. Passage through the vane

30a an offset via

30b another offset via

40. Spring

50. Rolling surface for a guide

62. Gas inlet

64. And a gas outlet.

Claims

1. A vacuum pump, comprising:

at least two rotors, each comprising at least one blade, the at least two rotors being mounted within a stator;

at least one of the vanes includes an inertial pressure relief valve comprising:

a body mounted for movement between a closed position in which the body blocks a fluid flow passage between a front side and a rear side of the rotor and an open position in which the fluid flow passage is unobstructed; wherein the content of the first and second substances,

the body is configured to move between the open and closed positions in response to a change in force acting on the body triggered by a change in rotational speed of the rotor;

the body is mounted such that deceleration of the rotor provides a force biasing the body away from the closed position and acceleration of the rotor provides a force biasing the body toward the closed position.

2. A vacuum pump as claimed in claim 1, wherein the fluid flow channel comprises a passage between a rotor tip and the stator, the body comprising at least part of the rotor tip.

3. A vacuum pump as claimed in claim 1, wherein the fluid flow channels comprise passageways through the vanes.

4. A vacuum pump as claimed in claim 3, wherein the body comprises a ball mounted within the fluid flow passage, the fluid flow passage comprising a constriction comprising a valve seat into which the ball fits in the closed position, the ball being on a leading edge of the valve seat.

5. A vacuum pump as claimed in any preceding claim, wherein the body is mounted such that centrifugal force due to rotation of the rotor acts to bias the body towards the closed position.

6. A vacuum pump as claimed in claim 5, wherein the body is mounted within a guide which constrains movement of the body between the open and closed positions.

7. A vacuum pump as claimed in claim 6, wherein the fluid flow channel comprises the guide.

8. A vacuum pump as claimed in claim 6 or 7, wherein the closed position for the body is at the point radially furthest from the centre of the rotor within the guide.

9. A vacuum pump as claimed in claim 7, the fluid flow channel comprising a passage through the vane, the fluid flow channel comprising two laterally offset channels, one extending from the front side of the rotor to a connection point and the other extending from the rear side of the rotor to a connection point, a connecting path extending between the two laterally offset channels, the body being mounted so as to block the connecting path when in the closed position.

10. A vacuum pump as claimed in claim 6 or 7, wherein at least part of the body and at least part of the rotor adjacent the closed position are formed from magnetic material such that the body is biased towards the closed position by magnetic attraction between the body and the rotor.

11. A vacuum pump as claimed in claim 6 or 7, wherein the body comprises a disc operable to roll within the guide.

12. A vacuum pump as claimed in claim 11, wherein the guide comprises a ramp such that an edge of the guide closer to the trailing side comprises the closed position for the body is radially further from a centre of the rotor than an edge of the guide closer to the leading side.

13. A vacuum pump as claimed in claim 5, wherein the fluid flow channels comprise passages through the vanes, the vanes comprise hollow vanes, the at least one of the vanes comprises a passage through an outer surface of the rotor, one of the passages of the outer surface being located on the front side of the rotor and another of the passages of the outer surface being located on the rear side of the rotor, and the body comprises a lever mounted within the at least one of the vanes, the body being operable to block the passage in the closed position and to hold the passage open in the open position.

14. A vacuum pump as claimed in claim 3 or claim 4, comprising a plurality of fluid flow paths in each vane and a plurality of inertial pressure relief valves corresponding to the plurality of fluid flow paths.

15. A vacuum pump as claimed in claim 5, wherein the body is mount connected to a spring such that the spring force biases the body towards the open position and the centrifugal force biases the body towards the closed position; such that when the rotational speed exceeds a predetermined rotational speed, the body is in the closed position, and when the rotational speed is below the predetermined rotational speed, the body is in the open position.

16. A vacuum pump as claimed in claim 15, wherein the at least one vane comprises a hollow vane and the body comprises two valve flaps each mounted on a pivotable arm and configured to cover a passage through an outer surface of the rotor, the passage of one outer surface being located on the front side of the rotor and the passage of the other outer surface being located on the rear side of the rotor, the two pivotable arms being connected via a spring biased to hold the two valve flaps in position to close the passages when the rotor is rotating at a speed above the predetermined rotational speed and not to block the passages when the rotor is rotating at a speed below the predetermined rotational speed.

17. A vacuum pump as claimed in claim 2, wherein the at least one vane comprises a hollow vane and the main body comprises a curved flap mounted on a spring biasing the flap away from the stator, the flap being operable to form an outer sealing surface of the rotor when the rotor is rotating above a predetermined rotational speed.

18. A vacuum pump as claimed in any of claims 1 to 4, wherein the vacuum pump comprises a vacuum booster pump.

19. A vacuum pump as claimed in any of claims 1 to 4, wherein the vacuum pump comprises a two rotor roots pump.

20. A vacuum pump as claimed in claim 18, wherein the fluid flow channels comprise passages through the vanes, the fluid flow channels merging with the outer surfaces of the vanes towards the centre of the rotor, thereby extending the time during which the fluid flow channels between the front and rear sides are open.

21. A vacuum pump as claimed in any of claims 1 to 4, wherein each of the rotors comprises at least two vanes.

22. A vacuum pump as claimed in any of claims 1 to 4, wherein each of the vanes comprises a pressure relief valve.