FIELD OF THE INVENTION

The present invention relates to a surge protection device, a multi-stage vacuum pump, and a method of surge protection.

BACKGROUND

A multi-stage vacuum pump comprises at least two pumping stages connected in series, such that the outlet from one stage is connected to the inlet of the next. The inlet of the multi-stage pump is connected to the inlet of the first pumping stage, and the outlet of the final pumping stage is connected to the outlet of the multi-stage pump. Sometimes all of the stages of the pump are collected within a single machine, but in other designs the stages can be distributed across two or more connected machines. It is generally true that the pumping capacity (displacement) of an earlier stage of the pump is larger than the pumping capacity (displacement) of a later stage.

A multi-stage vacuum pump may be used to evacuate a chamber from a high pressure (low vacuum) state to a low pressure (high vacuum) state. The high-pressure state may be atmospheric pressure. A problem with such multi- stage pumps is that during initial operation when the chamber is in a high pressure state, the earlier stages of the pump can transfer a large volume of gas, at a rate greater than the later stages of the pump can cope with. This can lead to a build-up of gas, and therefore pressure, in the fluid path between the stages. The built-up pressure may be greater than atmospheric pressure and may exert stress on pump components, causing them to wear more quickly. This initial pressure build up may be described as an overload condition.

After the initial operation phase the chamber is partially evacuated and the multi-stage pump operates to evacuate the partially evacuated chamber. During evacuation of the partially evacuated chamber the earlier pumping stage transfers fluid at a rate that appropriately feeds the later stage. A common arrangement of a multi-stage vacuum pump is a combination vacuum pump. A combination vacuum pump comprises a mechanical booster in series with a backing pump. The backing pump may comprise a single pumping stage or multiple pumping stages. The backing pump is fed by the mechanical booster. In such a combination vacuum pump, the mechanical booster generally has a considerably higher pumping capacity (displacement) than the next pumping stage (being the first stage of the backing pump) and the problem of the initial overload condition can be particularly acute.

It is possible to engineer the components of the combination vacuum pump, such as the rotor and/or stator components and the bearing system, so that they become robust to the overload condition during initial operation. However, such engineering results in a combination vacuum pump that is over engineered for much of its operation.

A number of solutions have been proposed to address the flow disparity between the mechanical booster and the backing pump of a combination vacuum pump during an initial period of operation. These solutions include:

• Recirculation valves that open under high pressure and return the gas from the mechanical booster outlet to the inlet.

• Blow-off valves that dump the high-pressure gas from the mechanical booster directly into the exhaust stream of the multi-stage pump so that the backing pump poses no restriction to the flow, and the gas is not accumulated.

• A mechanical anti-surge valve with a moving plate which occludes the mechanical booster inlet and presents a much smaller opening when subjected to the momentum of inrushing gas.

However, these solutions require additional moving parts in the form of valves and often springs, which have detrimental consequences for the reliability of the multi-stage vacuum pump and/or require increased maintenance. SUMMARY OF INVENTION

There is thus a need for surge protection in a multi-stage vacuum pump.

Accordingly, there is provided a surge protection device for a multi-stage vacuum pump arranged to evacuate a fluid from a vessel, the surge protection device comprising: an inlet for receiving the fluid; an outlet for connection to an input of the multi-stage vacuum pump; and at least one chamber. Each chamber has an opening directed towards the inlet, each chamber arranged to provide a re-circulation path for at least a portion of fluid passing from the inlet to the outlet of the surge protection device. The surge protection device is arranged to reduce the flow rate of the fluid when the vessel is at an initial pressure, and to provide progressively less resistance as the vessel is evacuated.

The opening receives fluid passing through the surge protection device, the fluid is turned around in the re-circulation path and directed back through the opening and back towards the inlet. Each chamber thus operates to reverse the direction of flow of a portion of the fluid passing through the surge protection device, thus offering resistance to fluid flow through the surge protection device. The resistance offered by the surge protection device is greater when the fluid is at higher pressure as compared to when the fluid is at lower pressure. The surge protection device thus tends to address the initial overload condition with little effect on the subsequent low pressure evacuation operation.

The surge protection device may further comprise a lozenge fixed within each chamber, each lozenge arranged to divide an opening of a chamber into an input section and an output section. The input section may receive fluid passing through the surge protection device, such that the fluid is turned around in a path between a wall of the chamber and the lozenge and directed, via the output section, back towards the inlet.

The surge protection device may comprise a plurality of chambers. Adjacent chambers may be arranged on opposite sides of a flow path. The plurality of chambers may be spaced along a primary flow axis of the surge protection device. The plurality of chambers may be distributed circumferentially around a primary flow axis of the surge protection device. The primary flow axis of the surge protection device is a path from the inlet to the outlet.

The multi-stage vacuum pump may be a combination vacuum pump comprising a mechanical booster and a backing pump arranged in series. The initial pressure may be atmospheric pressure. The mechanical booster may be a roots pump. A roots pump is a positive displacement lobe pump which operates by pumping a fluid with a pair of meshing lobes resembling a set of stretched gears. Fluid is trapped in pockets surrounding the lobes and carried from an intake side to an exhaust side. The backing pump may be a dry pump or a wet pump. The backing pump may be a positive displacement pump such as a rotary vane pump, a diaphragm pump, a piston pump, a scroll pump or a screw pump. Both the mechanical booster and the backing pump may be roots pumps. A multi-stage vacuum pump may comprise a plurality of different types of pump, a plurality of the same pumps, a plurality of the same types of pumps but configured to have different pumping characteristics, or some combination thereof. For example, the multi-stage vacuum pump may comprise two roots pumps arranged in series. The first roots pump arranged to have a higher pumping capacity than the second roots pump. The surge protection device described herein may be provided at the input of the first roots pump. During operation of the multi-stage pump, the surge protection device reduces the peak pressure between the first and second roots pumps.

There is further provided a combination vacuum pump comprising: a mechanical booster; a backing pump fed by the mechanical booster; and a surge protection device as described herein, the surge protection device connected to the input of the mechanical booster.

There is further provided a method of surge protection for a multi-stage vacuum pump arranged to evacuate a fluid from a vessel, the method comprising: providing at least one chamber along an input path to the multi stage vacuum pump, each chamber having an opening directed towards the inlet, each chamber arranged to provide a re-circulation path for at least a portion of a fluid passing along the input path; and reducing the flow rate of the fluid when the vessel is at an initial pressure, and to provide progressively less resistance as the vessel is evacuated.

The method may further comprise providing a lozenge fixed within each chamber, each lozenge arranged to divide an opening of a chamber into an input section and an output section.

The method may further comprise providing a plurality of chambers. Adjacent chambers may be arranged on opposite sides of a flow path. The plurality of chambers may be spaced along a primary flow axis of the surge protection device. The plurality of chambers may be distributed circumferentially around a primary flow axis of the surge protection device.

The multi-stage vacuum pump may be a combination vacuum pump comprising a mechanical booster and a backing pump arranged in series. The initial pressure may be atmospheric pressure. BRIEF DESCRIPTION OF DRAWINGS

Figure 1A-B are schematic illustrations (not to scale) of conventional multi-stage vacuum pumps;

Figure 2 is a schematic illustration (not to scale) of an embodiment of a passive surge protection device as presented herein; Figures 3A-C are schematic illustrations (not to scale) of embodiments of surge protection devices;

Figures 4A-C are schematic illustrations (not to scale) of embodiments of surge protection devices;

Figures 5A and 5B are schematic illustrations (not to scale) of embodiments of surge protection devices; and

Figure 6 is a process flow chart showing certain steps of a method of surge protection for a multi-stage vacuum pump arranged to evacuate a fluid from a vessel. DETAILED DESCRIPTION

Figure 1 A is a schematic illustration (not to scale) of a conventional multi stage vacuum pump 100 having two stages. This multi-stage vacuum pump may be described as a two-stage vacuum pump. The multi-stage vacuum pump 100 has two stages, comprising a first stage 110 and a second stage 120. The first stage 110 is connected to the second stage 120 by a conduit 115. The conduit 115 provides a fluid path from the first stage 110 to the second stage 120. The multi-stage vacuum pump 100 further comprises an inlet 105 and an exhaust 125.

Figure 1 B is a schematic illustration (not to scale) of a conventional multi stage vacuum pump 102. The multi-stage vacuum pump 102 is a combination vacuum pump. The combination vacuum pump 102 comprises a mechanical booster 110 as the first stage and a backing pump 130 having a plurality of stages (120, 121 , 122). The input stage of the backing pump is the second stage 120 of the combination vacuum pump. The mechanical booster 110 is connected to the backing pump 130 by the conduit 115. The conduit 115 provides a fluid path from the mechanical booster 110 to the input stage 120 of the backing pump 130. The combination vacuum pump 102 further comprises an inlet 105 and a conduit 135.

Where the backing pump 130 is the final stage of the multi-stage vacuum pump 102, conduit 135 connects to an exhaust. Alternatively, where the multi stage vacuum pump 102 comprises further pumping stages in addition to the mechanical booster 110 and the backing pump 130, then the conduit 135 connects to the further pumping stages and the final stage of the further pumping stages connects to an exhaust.

In an alternative arrangement, a two-stage vacuum pump comprises a mechanical booster having a single stage and a backing pump having a single stage. In a further alternative arrangement, a multi-stage vacuum pump comprises a mechanical booster having a single stage and a backing pump having a single stage, wherein the backing pump is connected to at least one further stage of a multi-stage vacuum pump. In operation, a multi-stage vacuum pump such as the combination vacuum pump 102 may be used to evacuate a gas, such as air, from a chamber. The inlet 105 is connected to the chamber and the combination vacuum pump 102 is activated. The mechanical booster 110 is provided to increase the flow rate of evacuation of the chamber as the pressure within the chamber drops towards a vacuum. Flowever, when the chamber is at atmospheric pressure, the mechanical booster 110 tends to provide too much fluid flow to the backing pump 130, that is, fluid is delivered faster than the backing pump 130 can pump it. This causes a pressure increase in the conduit 115.

The mechanical booster 110 may be a roots pump. A roots pump is a positive displacement lobe pump which operates by pumping a fluid with a pair of meshing lobes resembling a set of stretched gears. Fluid is trapped in pockets surrounding the lobes and carried from an intake side to an exhaust side. The backing pump 130 may comprise a dry pump or a wet pump. The backing pump 130 may comprise a positive displacement pump such as at least one of a rotary vane pump, a diaphragm pump, a piston pump, a scroll pump, a screw pump, or a combination of these. Both the mechanical booster 110 and the backing pump 130 may be roots pumps. Figure 2 is a schematic illustration (not to scale) of an embodiment of a passive surge protection device 250 as presented herein. The surge protection device 250 has a primary flow path from an inlet 205 to an exhaust 295. The surge protection device is illustrated connected to a multi-stage vacuum pump, which is illustrated as a combination vacuum pump 200. The combination vacuum pump 200 comprises a mechanical booster 210 and a backing pump 220. The mechanical booster 210 is connected to the backing pump 220 by a conduit 215. The conduit 215 provides a fluid path from the mechanical booster 210 to the backing pump 220. The surge protection device 250 may be connected to any multi-stage vacuum pump, for example the multi-stage vacuum pump 100 illustrated in figure 1A, the multistage vacuum pump 102 illustrated in figure 1 B, or any other vacuum pump having more than one stage.

The exhaust 295 is connected to the inlet of the mechanical booster 210. The surge protection device 250 is for a combination vacuum pump 200 arranged to evacuate a fluid from a vessel. The surge protection device 250 comprises an inlet 205 for receiving the fluid; an outlet 295 for connection to an input of the combination vacuum pump; and at least one chamber. Each chamber has an opening directed towards the inlet 205, and each chamber is arranged to provide a re-circulation path for at least a portion of fluid passing from the inlet to the outlet of the surge protection device. The surge protection device is arranged to reduce the flow rate of the fluid when the vessel is at an initial pressure, and to provide progressively less resistance as the vessel is evacuated. In operation, the opening of each chamber receives fluid passing through the surge protection device, the fluid is turned around in the re-circulation path and directed back through the opening and back towards the inlet. Each chamber thus operates to reverse the direction of flow of a portion of the fluid passing through the surge protection device, thus offering resistance to fluid flow through the surge protection device. The resistance offered by the surge protection device is greater when the fluid is at higher pressure as compared to when the fluid is at lower pressure.

The surge protection device may further comprise a lozenge fixed within each chamber, each lozenge arranged to divide an opening of a chamber into an input section and an output section. The input section may receive fluid passing through the surge protection device, such that the fluid is turned around in a path between a wall of the chamber and the lozenge and directed, via the output section, back towards the inlet.

The surge protection device may comprise a plurality of chambers. Adjacent chambers may be arranged on opposite sides of a flow path. The plurality of chambers may be spaced along a primary flow axis of the surge protection device. The plurality of chambers may be distributed circumferentially around a primary flow axis of the surge protection device. The primary flow axis of the surge protection device is a path from the inlet to the outlet.

Figure 3A is a schematic illustration (not to scale) of an embodiment of a surge protection device 351. The surge protection device 351 comprises a single chamber 360 having a lozenge 370 therein. The surge protection device 351 further comprises an inlet 305 and an outlet 395. The chamber 360 has an opening 365 and a recirculation path 368 around the lozenge 370. The chamber 360 comprises a generally ovoid shape with a narrow end opening into a generally straight body of the surge protection device 351 that extends between the inlet 305 and the outlet 395.

Figure 3B is a schematic illustration (not to scale) of another embodiment of a surge protection device 352. The surge protection device 352 comprises two chambers 360 each without a lozenge. The surge protection device 352 further comprises an inlet 305 and an outlet 395. The chambers 360 each have an opening 365 and a recirculation path 368 around the periphery of lozenge 370.

Figure 3C is a schematic illustration (not to scale) of another embodiment of a surge protection device 353. The surge protection device 353 which comprises three chambers 360 each having a lozenge 370 therein. The surge protection device 353 further comprises an inlet 305 and an outlet 395. The chambers 360 each have an opening 365 and a recirculation path 368 around the lozenge 370.

Figure 4A is a schematic illustration (not to scale) of another embodiment of a surge protection device 451. The surge protection device 451 comprises a single chamber 460. The surge protection device 451 further comprises an inlet 405 and an outlet 495. The chamber 460 has an opening 465 and a recirculation path 468 around the periphery thereof. The chamber 460 comprises a generally ovoid shape with a narrow end opening into the primary flow path of the surge protection device 451 that extends between the inlet 405 and the outlet 495. The primary flow path of the surge protection device curves around the chamber 460. The primary flow path of the surge protection device makes a dog leg deviation around the chamber 460.

Figure 4B is a schematic illustration (not to scale) of another embodiment of a surge protection device 452. The surge protection device 452 comprises two chambers 460, each having a lozenge 470 therein. The surge protection device 452 further comprises an inlet 405 and an outlet 495. Each chamber 460 has an opening 465 and a recirculation path 468 around the periphery thereof. Each chamber 460 comprises a generally ovoid shape with a narrow end opening into the primary flow path of the surge protection device 452 that extends between the inlet 405 and the outlet 495. Each chamber 460 is provided on a different side of the surge protection device 452. The primary flow path of the surge protection device 452 curves around each chamber 460. The primary flow path of the surge protection device weaves either side of adjacent chambers 460.

Figure 4C is a schematic illustration (not to scale) of another embodiment of a surge protection device 453. The surge protection device 453 comprises two chambers 460. The surge protection device 453 further comprises an inlet 405 and an outlet 495. Each chamber 460 has an opening 465 and a recirculation path 468 around the periphery thereof. Each chamber 460 comprises a generally ovoid shape with a narrow end opening into the primary flow path of the surge protection device 453 that extends between the inlet 405 and the outlet 495. Adjacent chambers 460 are provided on different sides of the surge protection device 453. The primary flow path of the surge protection device 453 curves around each chamber 460. The primary flow path of the surge protection device weaves either side of adjacent chambers 460.

Figure 5A is a schematic illustration (not to scale) of another embodiment of a surge protection device 551. The surge protection device 551 comprises a pair of non-identical chambers 560, 561. Each pair of chambers 560, 561 are arranged on opposite sides of a primary flow path of the surge protection device 551. The surge protection device 551 further comprises an inlet 505 and an outlet 595. The chamber 560 has an opening 565 and a recirculation path 568 around the periphery thereof. Each chamber 560, 561 comprises a generally ovoid shape with a narrow end opening into the primary flow path of the surge protection device 551 that extends between the inlet 505 and the outlet 595. The second chamber 561 is deeper than the first chamber. The primary flow path of the surge protection device curves away from each chamber 560, 561 so as to encourage fluid flow into each chamber when in use. Figure 5B is a schematic illustration (not to scale) of another embodiment of a surge protection device 552. The surge protection device 552 comprises two pairs of non-identical chambers 560, 561, each containing a lozenge 570. Each pair of chambers 560, 561 are arranged on opposite sides of a primary flow path of the surge protection device 552. The surge protection device 552 further comprises an inlet 505 and an outlet 595. The chamber 560 has an opening 565 and a recirculation path 568 around the periphery thereof. Each chamber 560, 561 comprises a generally ovoid shape with a narrow end opening into the primary flow path of the surge protection device 552 that extends between the inlet 505 and the outlet 595. The second chamber 561 is deeper than the first chamber. The primary flow path of the surge protection device curves away from each chamber 560, 561 so as to encourage fluid flow into each chamber when in use. In operation, each of the surge protection devices described herein reduces a flow rate of a gas into a mechanical booster of a multi-stage vacuum pump. The flow rate reduction is increased with the gas flowing through the surge protection device is at higher pressure. Accordingly, when applied to a multi-stage vacuum pump, the surge protection device reduces the flow rate during the initial evacuation of a vessel, which tends to reduce an initial surge that can result in an increased pressure in a conduit between the mechanical booster and the backing pump.

The surge protection devices illustrated herein are of a planar form having a substantially square or rectangular inlets and outlets and each chamber comprising an oval prism. Accordingly, the surge protection devices described herein have a constant cross section along their length, that cross section illustrated herein. The principles described herein could equally be applied to a three-dimensional arrangement, such that chambers are spheroid and arranged equally around the circumference of a main fluid path having a circular cross section.

There is further provided a multi-stage vacuum pump having at least a first stage and a second stage. The multi-stage vacuum pump may comprise a combination vacuum pump, the combination vacuum pump comprising: a mechanical booster; a backing pump fed by the mechanical booster; and a surge protection device as described herein, the surge protection device connected to the input of the combination vacuum pump.

Figure 6 is a process flow chart showing certain steps of a method 600 of surge protection for a multi-stage vacuum pump arranged to evacuate a fluid from a vessel, the method comprising: step S1, providing 610 at least one chamber along an input path to the multi-stage vacuum pump, each chamber having an opening directed towards the inlet, each chamber arranged to provide a re-circulation path for at least a portion of a fluid passing along the input path; and step S2, reducing 620 the flow rate of the fluid when the vessel is at an initial pressure, and to provide progressively less resistance as the vessel is evacuated.

The method may further comprise providing a lozenge fixed within each chamber, each lozenge arranged to divide an opening of a chamber into an input section and an output section.

The method may further comprise providing a plurality of chambers. Adjacent chambers may be arranged on opposite sides of a flow path. The plurality of chambers may be spaced along a primary flow axis of the surge protection device. The plurality of chambers may be distributed circumferentially around a primary flow axis of the surge protection device.

It should be noted that certain of the process steps depicted in the flowchart of Figure 6 and described above may be omitted or such process steps may be performed in differing order to that presented above and shown in Figure 6. Furthermore, although all the process steps have, for convenience and ease of understanding, been depicted as discrete temporally sequential steps, nevertheless some of the process steps may in fact be performed simultaneously or at least overlapping to some extent temporally.

The multi-stage vacuum pump may have at least a first stage and a second stage. The multi-stage vacuum pump may comprise a combination vacuum pump, the combination vacuum pump comprising a mechanical booster and a backing pump arranged in series. The multi-stage vacuum pump may include a backing pump having a plurality of stages. The initial pressure may be atmospheric pressure.

The surge protection device used herein is described in use with a multi- stage vacuum pump. The multi-stage vacuum pump may have only two stages. The multi-stage vacuum pump may comprise a single pump, wherein the single pump has a chamber that a damaging amount of pressure can build up in if the initial flow into the pump is not managed. That chamber sits at the boundary between the first and second stages of the pump.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.

REFERENCE NUMERAL KEY

100, 102 Multi-stage vacuum pump

105 Inlet 110 First stage

115 Conduit 120 Second stage 121 , 122 Stages of a Backing Pump

125 Exhaust 130 Backing Pump

135 Conduit

200 Combination vacuum pump 205 Inlet

210 Mechanical Booster 215 Conduit

220 Backing Pump 225 Exhaust

250 Surge Protection Device 295 Exhaust 351 , 352, 353 Surge Protection Devices

305 Inlet 360 Chamber 365 Opening 368 Recirculation Path 370 Lozenge

395 Outlet 451 , 452, 453 Surge Protection Devices 405 Inlet 460 Chamber 465 Opening 468 Recirculation Path

470 Lozenge 495 Outlet

551 , 552, 553 Surge Protection Devices 505 Inlet 560 Chamber

565 Opening 568 Recirculation Path 570 Lozenge 595 Outlet