EP3762611A1

EP3762611A1 - A vacuum pump with a pressure relief valve

Info

Publication number: EP3762611A1
Application number: EP19711684.1A
Authority: EP
Inventors: Matthew Richard WICKES; Neil Turner; Alan Ernest Kinnaird Holbrook; Sivabalan KAILASAM
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2018-03-09
Filing date: 2019-03-08
Publication date: 2021-01-13
Anticipated expiration: 2039-03-08
Also published as: GB2571792B; CN111801498A; CN111801498B; GB201803859D0; EP3762611B1; WO2019171075A1; EP3762611B8; GB2571792A; TW201938916A; TW201938915A; TW201940815A; WO2019171074A1; WO2019171076A1

Abstract

A vacuum pump comprising: at least two rotors each comprising at least one lobe, the at least two rotors being mounted within a stator; at least one of the lobes comprising an inertial pressure relief valve, the inertial pressure relief valve comprising: a body mounted to move between a closed position in which it obstructs a fluid flow pathway between leading and trailing sides of the rotor and an open position in which the fluid flow pathway is not obstructed; wherein the body is configured to move between the open and closed positions in response to changes in forces acting on the movable body triggered by changes in a rotational velocity of the rotor.

Description

A VACUUM PUMP WITH A PRESSURE RELIEF VALVE

FIELD OF THE INVENTION

The field of the invention relates to the field of vacuum pumps and in particular to vacuum pumps with pressure relief valves.

BACKGROUND

The application relates to vacuum pumps and in preferred embodiments to booster pumps, and to the pressure relief valves (PRVs) used in such pumps to alleviate problems that may arise when the input pressure to such a pump rises suddenly. Booster pumps are used to boost the capacity of a vacuum pump assembly. They operate at high speeds and during much of their operation there is only a small pressure difference between the inlet and outlet of the pump. If the pressure at the inlet rises suddenly, then the gas is transferred rapidly by the rotors to the pump outlet where the pressure can rise to 5 bar or even 8 bar. This pressure places large forces on the rotors, which can cause them to break or cause the shaft gears to slip. To address this PRVs are conventionally incorporated into such booster pumps. These PRVs are conventionally mounted in the stator and are pressure actuated and open when the pressure at the booster outlet exceeds the inlet pressure by more than a certain amount. This helps to reduce the pressure difference between the inlet and outlet and protects the pump.

Such conventional PRVs are generally located in a path extending the stator which recirculates gas at the outlet to the inlet, thereby raising the pressure at the inlet. Such an arrangement adds to the pump footprint by making the stator wider and/or longer than it otherwise would be. Figure 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. SUMMARY

A first aspect of the present invention provides a vacuum pump comprising: at least two rotors each comprising at least one lobe, said at least two rotors being mounted within a stator; at least one of said lobes comprising an inertial pressure relief valve, said inertial pressure relief valve comprising: a body mounted to move between a closed position in which it obstructs a fluid flow pathway between leading and trailing sides of said rotor and an open position in which said fluid flow pathway is not obstructed; wherein said body is configured to move between said open and closed positions in response to changes in forces acting on said movable body triggered by changes in a rotational velocity of said rotor.

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

Although it might seem desirable to incorporate a valve into a pump in this way conventional PRV’s respond to a pressure difference, typically opening when the pressure acting on the outlet side of the valve is sufficient to move the moveable valve element either against its own weight or against the force provided by a spring. This makes them unsuitable for use in a lobed rotor where the pressure difference across the rotor reverses depending on the location of the rotor in its rotational cycle. Thus, were such a conventional PRV to be incorporated into a vacuum pump rotor, it would try to open and close during each revolution of the rotor as the pressure difference reverses. Not only would this mean that for some of the time the gas relief pathway would not be available, but also the speed of rotation is such that the valve would not be able to open and close successfully in the time provided and might well remain in some intermediate indeterminate state. The inventors of the present invention recognised, that although a conventional PRV might not provide an acceptable valve when located within a rotor, were an inertial pressure relief valve used instead, then the desired operational characteristics could be achieved.

In this regard they recognised that an increase in pressure experienced by such a pump will slow the rotor and thus, configuring a valve to react to this rather than the pressure difference itself, would allow a valve to be provided on the rotor which would not reverse positions with the changing pressure on the different faces as the lobe rotates. An inertial valve which is configured to move between open and closed positions in response to changes in a rotational velocity of the rotor would provide the desired operational characteristics. Such a valve incorporated into the rotor takes up no additional space and yet provides relief against pressure surges.

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

In some embodiments, the body may form a portion of the rotor tip providing with the stator at least a portion of the pump sealing face when in its closed position. When in an open position the body does not extend out to the stator and a pathway is formed between the stator and rotor lobe, allowing some pressure relief and a reduction in the forces felt by the rotor.

In other embodiments, said fluid flow pathway comprises a passage through said lobe.

Alternatively the fluid flow pathway may be a passage through the rotor that is sealed by the body when in its closed position. When in an open position the body moves to open the pathway and some pressure relief is felt and the forces on the rotor are reduced. One way of viewing this is that the rotor becomes more porous when the body is in the open position and thus, the forces exerted by the fluid being pumped on the rotating lobe are reduced.

The body may have a number of different forms, provided that it is configured to move between an open position and a closed position where it seals the fluid pathway. In some embodiments, said body comprises a sphere mounted within said fluid flow pathway, said fluid flow pathway comprising a constriction comprising a valve seat into which said ball fits in said closed positon, said ball being on said leading side edge of said valve seat.

Providing a ball on the leading side of the valve seat is not the conventional way to configure a pressure relieve valve. However, when mounted 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 that pressure relief is required. Thus, having a valve that reacts to deceleration to open provides a suitable pressure relief function. Furthermore, once the conditions that required pressure relief subside and the pressure difference between inlet and outlet start to reduce, then the rotor will start to accelerate and the acceleration forces will cause the body to move to the closed position and normal pumping operations will be resumed.

In some embodiments, said body is mounted such that a centrifugal force due to said rotor rotating acts to bias said body towards said closed position.

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

In some embodiments, said body is mounted within a guide, said guide constraining movement of said body between said open and said closed positions. In order to control the movement of the body between an open and closed position it may be desirable to mount it within some sort of guide such that its motion is constrained along a particular path allowing a more predictable movement of the body and a more controllable pressure relief operation.

In some embodiments, said fluid flow pathway comprises said guide.

In some cases the fluid flow path may act as the guide and in particular where the fluid flow path comprises the valve seat then the body may move towards and away from the valve seat within the fluid flow path depending on velocity changes of the rotor.

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

As noted previously, it may be desirable if centrifugal force due to the rotor rotating acts to bias the body towards the closed position and one simple way to provide such a function is to provide the closed position at a position that is radially furthest from the centre of the rotor in the guide. Thus, where the body moves along a guide it will be thrown out to this closed position due to centrifugal force when the rotor is rotating.

In some embodiments, said fluid flow path comprises two laterally offset pathways, one running from said leading side of said rotor to a connecting point and the other from said trailing side of said rotor to a connecting point, a connecting passageway running between said pathways, said body being mounted so as to obstruct said connecting passageway when in said closed position.

In some cases, rather than having the moveable body within the fluid flow pathway it may be within a connecting path between two offset fluid flow pathways. One advantage of this is that the pressure difference on either side of the rotor lobe acts perpendicular to the body and does not push it either towards or away from the open or closed positions. Rather the guide that the body moves in can be designed such that the only significant forces on the body are due to the motion of the rotor and with suitable angles and weights of body the desired characteristics for opening and closing the valves can be achieved.

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

In some cases, the only biasing force towards 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 a magnetic force is used to bias the body towards the closed position then it may be that in steady state operation no centrifugal force is required to retain the body in the closed position and in this case the guide need not be angled to provide the closed position at a radially furthest point. In this case acceleration and deceleration of the rotor may be used to provide the motion of the body, the body being retained in the closed position by a magnetic force. In other embodiments, it may be a combination of centrifugal and magnetic forces that hold the body in the closed position.

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

As noted previously, the body may have a number forms and it may comprise a disc mounted to roll within the guide. It may in other embodiments be a ball rolling within a guide or a rod rolling within a guide.

In some embodiments, said guide comprises a slope such that an edge of said guide closer to said trailing side is radially further from a centre of said rotor than an edge of said guide closer to said leading side, said edge of said guide closer to said trailing side comprising said closed position for said body.

The guide may be arranged such that the closed position is radially furthest along the guide such that the centrifugal force acts to close it. Having it towards the trailing side means that deceleration will provide forces on the body that act to open the valve while acceleration will provide forces acting to close the valve.

In some embodiments, said lobes comprise hollow lobes, said at least one of said lobes comprises passages through an outer surface of said rotor one of said passages being located on said leading side and one of said passages being located on said trailing side of said rotor and said body comprising a rod mounted within said at least one of said lobes, said body being operable to obstruct said passages in said closed position and to leave said passages open in said open position.

In some cases the lobes of the rotor may be hollow and this has advantage of reducing the weight of the rotors and therefore the power required to rotate them. Where the fluid flow path is a passage through the lobe then the hollow lobe may form a portion of this passage with the outer surfaces of the lobe having passages located on the leading and the trailing sides that is on either side of the sealing surface such that there is a fluid flow pathway across the lobe. In some cases the body may comprises a rod mounted within at least one of the hollow lobes, the body moving in response to inertial forces to obstruct the passages in the closed position and leave them open in the open position. In some cases the rod may be loose within the hollow lobe and in other cases there may be some sort of guide mechanism that it rolls within between the open and closed positions.

In some embodiments, the vacuum pump comprises a plurality of fluid flow passageways and a corresponding plurality of inertial pressure relief valves in each lobe. There may be just a single fluid flow passageway in one lobe or a single fluid flow passageway in each lobe of each rotor. Alternatively, a lobe may have a plurality of fluid flow passageways arranged in parallel. The advantage of this is that the effect due to these passageways being open is increased as the number of passageways increases and thus, the pressure relief provided is also increased.

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

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

In some embodiments, said body is mounted connected to a spring such that a spring force biases said body towards said open position and said centrifugal force biases said body towards said closed position; such that when said speed of rotation exceeds said predetermined velocity said body is in said closed position and when said speed of rotation is lower than said predetermined velocity said body is in said open position.

Alternatively, the body may be connected to a spring and the spring may bias the body towards the open position while the centrifugal force biases it towards the closed position. In such a case, it is the speed of rotation that determines whether it is in a closed or an open position. This is an alternative method and although the reaction of the pressure relief valve to changes in pressure may be slower than when it reacts to deceleration, it does provide operation that is predictable. This predictable nature of the spring’s operation may enable the pump to be configured to operate in a green mode. When operating in green mode, pumps are often run slowly to reduce power consumption. The valve spring could be configured to open the PRV at green mode speed and avoid any compression in the booster at this speed. This would reduce power even further. During normal higher power modes of operation, the valve will remain closed until the speed drops to these slower speeds due to deceleration caused by a pressure increase of the gas being pumped.

In some embodiments, said at least one lobe comprises a hollow lobe and said body comprises two flaps each mounted on a pivotable arm and configured to cover passages through an outer surface of said rotor, one passage being located on said leading side and one passage being located on said trailing side of said rotor, said two arms being connected via a spring biased to hold said two flaps in a position to close said passages when said rotor is rotating above said predetermined rotational velocity and to not obstruct said passages when said rotor is rotating below said predetermined rotational velocity.

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

Some embodiments may provide a movable body at the outer edge of the lobe, allowing pressure relief in a simple manner by allowing gas simply to flow through a gap between the rotor and the stator and rather than being routed through the rotating rotor. In some embodiments the vacuum pump is a vacuum booster pump. Vacuum booster pumps provide increased capacity to a vacuum pump system and do this by rotating at high speeds. Where the system they are pumping has a sudden change in pressure then the pressure difference across the booster pump may vary very suddenly and this causes much stress on the booster pump. Thus, such pumps are particularly suitable for pressure relief valves which help protect them from the high forces that occur with sudden changes in pressure.

Although the pump may have a number of different forms it may comprise a two rotor Roots pump. These pumps often form vacuum booster pumps and are particularly suitable the pressure relief valve of embodiment.

In some embodiments, said fluid flow pathway meets an outer surface of said lobe towards a centre of said rotor thereby prolonging a time during which said fluid flow pathway between said leading and trailing sides is open.

Where the outlet of the fluid flow pathway is towards the centre of the rotor then it is not obscured by the other rotor for much of the rotation and more effective pressure relief is provided.

Although the rotors may comprise one lobe each, generally pumps perform better where there are at least two lobes.

Although, not all of the lobes may comprise pressure relief valves in some embodiments each of the lobes comprise a pressure relief valve. Where the lobes on the rotor are not symmetrical then there may be some imbalance and therefore it may be advantageous if each lobe comprises a pressure relief valve. Furthermore, providing additional pressure relief valves helps increase the pressure relief effect provided.

Further 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 appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 shows a Roots pump with pressure relief valves according to the prior art;

Figure 2 shows a Roots pump with pressure relief valves according to an embodiment;

Figure 3 shows a lobe of a rotor having a pressure relief valve according to an embodiment;

Figure 4 shows the pressure relief valve of Figure 3 without the moveable body; Figure 5 shows an alternative embodiment of a pressure relief valve according to an embodiment;

Figure 6 schematically shows a pressure relief valve that opens and closes at particular velocities according to an embodiment;

Figure 7 shows cross-sections through the valve of Figure 6 when in operation; Figure 8 shows a valve similar to that of Figure 6 with the spring in a different location;

Figure 9 shows a cross-section through the valve of Figure 8 in the open and closed positions;

Figure 10 shows a further embodiment of the pressure relief valve similar to that of Figures 6 and 8 but with different flap arrangements;

Figure 11 shows a cross-section through the valve of Figure 10 in and open and closes position;

Figure 12 shows a further embodiment where the fluid flow path is not through a passage in the rotor; and Figure 13 shows a further embodiment of a pressure relief valve where the passage through a hollow rotor is obscured by a movable rod.

DESCRIPTION OF THE EMBODIMENTS

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

An inertial pressure relief valve incorporated into a booster rotor which instead of being pressure actuated, and therefore having to open and close every half turn of the rotor, is actuated through inertial effects, and can therefore stay open throughout the pressure spike event that it relieves is provided.

Some embodiments of the PRV are configured to respond to the deceleration and acceleration of the rotor, and therefore will open once at the start of the pressure spike event, in response to deceleration felt by the rotor and then close again once the condition has subsided and the rotor accelerates back up towards operational speeds.

Other embodiments of the PRV are configured to respond to a predetermined velocity, opening when the rotational velocity of the rotor falls below this predetermined velocity and closing when it rises above it. Such PRVs use the centrifugal force due to rotation as the closing force, an opening force being provided by a spring means for example.

Figure 1 shows a Roots booster pump with PRVs according to the prior art. In this pump, there are PRVs each having a pressure actuated moveable body 20 sitting on a valve seat 22 within conduits which are located outside of the stator and connect the outlet of the pump to its inlet. In this device, in the event of excessive pressure at the outlet moveable body 20 is pushed off the valve seat 22 by the pressure difference between outlet and inlet and there is a flow of gas through the conduits between the outlet and the inlet which relieves the pressure difference. This is an effective pressure relief valve but as can be seen it takes up a lot of space and requires additional parts.

Figure 2 schematically shows a pressure relief valve for a Roots pump similar to that of the prior art but with the pressure relief valves being within the rotor of the Roots booster pump. In this embodiment, each rotor lobe comprises gas flow passages through the lobe which in steady state operation are closed by a moveable body sitting on a corresponding valve seat. In response to a sudden increase in pressure, the rotors will decelerate suddenly and the moveable body will be thrown off the valve seat and the valve will open causing pressure relief and gas to flow from the higher pressure outlet to the lower pressure inlet through the rotor. In this way, a pressure relief valve with similar operational

characteristics to that of the prior art is provided without the additional space requirements. Furthermore, as the valve is activated by deceleration, it is responsive to sudden changes and the pressure relief happens promptly.

Figures 3 and 4 show one embodiment of a PRV within a lobe of a rotor. In this embodiment the valve consists of a conventional-looking ball 20 and valve seat 22 bored across the lobe of the booster rotor 10. Figure 3 shows the valve in closed position, while Figure 4 shows the valve without the ball 20, such that the valve seat 22 is shown. The outlet side of the valve is on the leading side of the lobe; this is the opposite sense in which you would think of installing a pressure- controlled PRV. The channel 30 is angled so that the radius at which it breaks out of the rotor is smaller than the radius of the valve seat. In that way the centrifugal force on the ball will roll it into the seat, closing the valve.

Furthermore, the ports to the channel 30 will be nearer the centre of the rotor enabling them not to be obstructed by the other rotor for much of the rotation. It should be noted that the angle of the channel is exaggerated in the Figure for ease of representation.

When an excessive outlet pressure is present, it causes rapid deceleration of the rotor set. The major component of the force available to decelerate the ball in the valve is the centrifugal force rolling it“down” the ramp. The minor component is the pressure difference acting across the valve. If the rate of deceleration of the rotor exceeds amax = w² cos Q+ SAP/mr

where Q is the angle of the slope of the guide,

w is the angular speed of rotation

amax is the angular deceleration that will just leave the valve closed (any faster will open the valve)

S is the area of the valve seat,

DR 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. then the ball will be flung off the seat and roll“uphill” and open the valve. The point at which the valve opens can be adjusted by changing the angle between the outlet channel and the radius of the rotor. 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 hole is closed at the end, to stop the ball from flying into the swept volume.

The gas path from outlet to inlet is shorter and more direct than in conventional stator PRV’s which provides conductance advantages in a rotor PRV.

In preferred embodiments valves are added to rotors in equal numbers to each lobe of the rotor (so that balance is maintained when the valves open).

In general faster boosters are desirable as they reduce product footprint. As noted with respect to Figure 2, these faster boosters are, as a general rule, more likely to require PRV protection. This design allows PRV’s to be added to boosters without affecting the overall product footprint. In general there is more room on the booster rotor than there is on the stator or headplate. If more than one PRV is required it is easier to fit the extra valves into the rotor (in parallel) than finding more space in a headplate (for example).

For some boosters that are mounted within enclosures pressure relief is required and yet space is very limited. The inertial relief valve could be a key enabler to achieving a required peak speed within a specified footprint.

Figure 5 shows an alternative embodiment where the pressure relief valve is formed of a movable body in the form of disc 24 mounted within a guide 26. The disc acts to obstruct a link between two laterally offset passages 30a, 30b when the valve is in the closed position.

At pump start up, the rotor’s rotational speed will increase from zero to the running speed of the pump. During this phase, the rolling disc 24 will roll to the back of the slot in the rotor and close the port through the rotor 10. The clearance between the disc and the slot that it is located in is sufficiently small to prevent significant leakage across the rotor and large enough to allow free movement of the disc.

During pump operation the disc is kept in its closed position by centrifugal force from a very small gradient in the rolling surface 50 of guide 26 about the rotor’s centre. Alternatively or additionally, the disc could be held in the closed position by a magnet (not shown) in which case the guide 26 would not need the increase in gradient. The gas pressure acting upon the disc acts perpendicular to its path of travel and thus, as the direction of pressure difference changes during rotation there is limited change in force on the disc in the direction in which it opens and closes.

When gas is dumped into the inlet of the booster pump, the gas is transferred by the rotors to the exhaust side, which causes the rotor to decelerate rapidly. As the rotor decelerates, the disc rolls to the open position. This allows gas to pass through the rotor, which reduces the peak pressure on the downstream side. Analysis has shown that the gradient required on the rolling radial outer surface of guide 50 to allow the disc to roll under the deceleration forces can be produced with achievable manufacturing tolerances.

Figures 6 to 12 show alternative embodiments of an inertia pressure relief valve (PRV) within the rotor configured to operate based on rotor speed.

In this embodiment, drilled holes on either side of the booster rotor’s sealing surface allow gas to flow through the hollow rotor. Those holes are blocked with flaps which are spring loaded into an open position, the centrifugal force of the rotating rotor acting to close the flaps when the centrifugal force exceeds the spring force. This occurs at a predetermined rotational velocity.

Figure 6 shows an overview of an inertia pressure relief valve according to one embodiment. Figure 7 shows the same pressure relief valve in an open (right hand figure) and closed (left hand figure) positon, below and above the predetermined velocity. Thus, the left hand figure shows the rotor rotating above the predetermined critical velocity, this being the velocity which triggers the flap to move, and at this higher speed the flap is in the closed position. The right hand figure shows the same rotor rotating at a speed below the critical velocity and the flap is open. When the rotor is rotating at a higher speed the flap is in the closed position whereas when it is rotating at a lower speed, the spring force of lateral spring 40 which extends between the arms. exceeds that of the centrifugal force due to rotation and the flap opens and gas can pass through the rotor following the path shown by arrows 62 and 64. In this way, when there is a sudden increase in pressure and the rotor decelerates, when the rotational speed falls below the predetermined critical speed the flap will open automatically and gas flow will occur. In this case, it is not the deceleration of the rotor that actuates the valve but rather it passing below a particular speed. In this way, a pressure relief valve which accurately responds to a particular speed can be designed. The speed at which this occurs will depend on the weight of the flap and on the strength of the spring. It should be noted that the flap will close gradually as the speed reaches the predetermined speed due to the centrifugal force on the pressure relief valve assembly.

Thus, when the booster vacuum pump is subjected to, for example, an atmospheric pressure dump, the rotor will decelerate rapidly and as the rotor passes below the predetermined velocity, the flap will start to open due to the spring force being higher than the centrifugal force acting on the flaps allowing gas to pass through the rotor. This reduces the peak pressure felt by the downstream pump. Gas will continue to pass through the rotor while the speed remains low. Once the effects of the pressure surge start to diminish the speed of the rotor will increase and when the predetermined speed is reached the flaps will close again.

Figures 8 and 9 show an alternative arrangement where the spring 40 is in a different position. In this embodiment, the spring 40 is mounted at the pivot points of the arms holding the flaps. The pressure relief valve will operate in the same way as for the embodiment where the spring 40 was a lateral spring between the extended portions of the arms. Thus, the left hand figure shows the rotor rotating above the predetermined critical velocity, this being the velocity which triggers the flap to move, and at this higher speed the flap is in the closed position. The right hand figure shows the same rotor rotating at a speed below the critical velocity and the flap is open.

Figures 10 and 11 show an alternative embodiment where the flaps are mounted about pivot points at either edge. Again the left hand figures shows the condition above the critical velocity with the flap in the closed position and the right hand figure shows the same rotor at a lower speed with the flap in the open position. A spring pulls the flaps towards each other and they overlap in the open position. When the centrifugal force exceeds the spring force, the valve closes and the flaps extend across a large portion of the outer surface of the rotor which allows the gas passages to be larger. Figure 12 shows an alternative embodiment where the moving body is spring mounted on an arm extending towards the centre of the rotor. The moving body 20 forms the rotor tip when in the closed position and when the rotor slows and a centrifugal force is less than the spring 40 force, then the body will open and there will be a passage for gas to pass between the stator and this rotor tip. This is a convenient way of providing a passage for gas allowing a simple gas flow and not requiring the gas flow to pass through a rotor passage which is itself rotating.

Figure 13 shows an alternative embodiment where the PRV opens and closes in response to acceleration and deceleration as opposed to a predetermined velocity. In this embodiment, openings 14, 16 at the rotor 10 outer surface form a gas flow passage in conjunction with the hollow space within the rotor from one side to the other of the rotor when the outlets 14, 16 are not obscured by rod 20 mounted within the hollow lobe. When in steady state operation the rod 20 is thrown to a radially outer position (as shown in the figure) where it obscures the openings 14, 16. On deceleration of the rotor the rod moves away from the openings 14, 16 and gas can flow through the rotor.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can 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 SIGNS

10 Rotor

12 Centre of rotor

14, 16 Openings

20 Moveable ball valve body 22 Valve seat

24 Moveable disc body 26 Guide

30 Passage through lobe

30a One offset passage 30b Other offset passage 40 spring

50 Rolling surface for guide 62 gas in

64 gas out

Claims

1. A vacuum pump comprising:

at least two rotors each comprising at least one lobe, said at least two rotors being mounted within a stator;

at least one of said lobes comprising an inertial pressure relief valve, said inertial pressure relief valve comprising:

a body mounted to move between a closed position in which it obstructs a fluid flow pathway between leading and trailing sides of said rotor and an open position in which said fluid flow pathway is not obstructed; wherein

said body is configured to move between said open and closed positions in response to changes in forces acting on said body triggered by changes in a rotational velocity of said rotor.

2. A vacuum pump according to claim 1 , wherein said fluid flow pathway comprises a passage between a rotor tip and said stator, said body comprising at least a portion of said rotor tip.

3. A vacuum pump according to claim 1 , wherein said fluid flow pathway comprises a passage through said lobe.

4. A vacuum pump according to claim 3, wherein said body comprises a sphere mounted within said fluid flow pathway, said fluid flow pathway comprising a constriction comprising a valve seat into which said ball fits in said closed positon, said ball being on said leading side edge of said valve seat.

5. A vacuum pump according to any preceding claim, wherein said body is mounted such that a centrifugal force due to said rotor rotating acts to bias said body towards said closed position.

6. A vacuum pump according to any preceding claim, wherein said body is mounted within a guide, said guide constraining movement of said body between said open and said closed positions.

7. A vacuum pump according to claims 4 and 6, wherein said fluid flow pathway comprises said guide.

8. A vacuum pump according to claim 6 or 7 when dependent on claim 5, wherein said closed position for said body is located at a point within said guide that is radially furthest from a centre of said rotor.

9. A vacuum pump according to claim 7 or 8, when dependent on claim 3, said fluid flow pathway comprising two laterally offset pathways, one running from said leading side of said rotor to a connecting point and the other from said trailing side of said rotor to a connecting point, a connecting passageway running between said pathways, said body being mounted so as to obstruct said connecting passageway when in said closed position.

10. A vacuum pump according to any one of claims 6 to 9, wherein at least a portion of said body and at least a portion of said rotor adjacent to said closed position are formed of magnetic material, such that said body is biased towards said closed position by a magnetic attraction between said body and said rotor.

11. A vacuum pump according to any one of claims 6 to 10, wherein said body comprises a disc operable to roll within said guide.

12. A vacuum pump according to claim 11 , wherein said guide comprises a slope such that an edge of said guide closer to said trailing side is radially further from a centre of said rotor than an edge of said guide closer to said leading side, said edge of said guide closer to said trailing side comprising said closed position for said body.

13. A vacuum pump according to any one of claims, 5, 6, 8 or 10 when dependent on claim 3, wherein said lobes comprise hollow lobes, said at least one of said lobes comprises passages through an outer surface of said rotor one of said passages being located on said leading side and one of said passages being located on said trailing side of said rotor and said body comprising a rod mounted within said at least one of said lobes, said body being operable to obstruct said passages in said closed position and to leave said passages open in said open position.

14. A vacuum pump according to any one of claims 3 or claims 4 to 12 when dependent on claim 3, comprising a plurality of fluid flow passageways and a corresponding plurality of inertial pressure relief valves in each lobe.

15. A vacuum pump according to any preceding claim, wherein said body is mounted such that a deceleration of said rotor provides a force biasing said body away from said closed position and an acceleration of said rotor provides a force biasing said body towards said closed position.

16. A vacuum pump according to any one of claims 1 to 14, wherein said body is mounted connected to a spring such that a spring force biases said body towards said open position and said centrifugal force biases said body towards said closed position; such that when said speed of rotation exceeds said predetermined velocity said body is in said closed position and when said speed of rotation is lower than said predetermined velocity said body is in said open position.

17. A vacuum pump according to claim 16 wherein said at least one lobe comprises a hollow lobe and said body comprises two flaps each mounted on a pivotable arm and configured to cover passages through an outer surface of said rotor, one passage being located on said leading side and one passage being located on said trailing side of said rotor, said two arms being connected via a spring biased to hold said two flaps in a position to close said passages when said rotor is rotating above said predetermined rotational velocity and to not obstruct said passages when said rotor is rotating below said predetermined rotational velocity.

18. A vacuum pump according to claims 2 and 16, wherein said at least one lobe comprises a hollow lobe and said body comprises a curved flap mounted on a spring that biases said flap away from said stator, said flap being operable to form an outer sealing surface of said rotor when said rotor is rotating above said predetermined velocity.

19. A vacuum pump according to any preceding claim, wherein said vacuum pump comprises a vacuum booster pump.

20. A vacuum pump according to any preceding claim, wherein said vacuum pump comprises a two rotor Roots pump.

21. A vacuum pump according to claim 19 when dependent on claim 3, wherein said fluid flow pathway meets an outer surface of said lobe towards a centre of said rotor thereby prolonging a time during which said fluid flow pathway between said leading and trailing sides is open.

22. A vacuum pump according to any preceding claim, wherein each of said rotors comprises at least two lobes.

23. A vacuum pump according to any preceding claim, wherein each of said lobes comprises a pressure relief valve.