CONTROL APPARATUS AND METHOD FOR SUPPLYING PURGE GAS

Abstract

Aspects of the present invention relate to method of controlling a supply of purge gas to reduce leakage past a dynamic seal (10). The dynamic seal (10) comprising at least a first sealing member (12-1) having opposing first and second sides (12-1A, 12-1B). A pressure differential (ΔP) is determined between a first operating pressure (P1) on the first side of the first sealing member (12-1) and a second operating pressure (P2) on the second side of the first sealing member (12-1). The supply of purge gas to the second side of the first sealing member (12-1) is controlled in dependence on the determined pressure differential (ΔP). Aspects of the invention relate to a controller (23) for controlling a supply of purge gas; a dynamic sealing system (1) for a vacuum system (2); and a vacuum system (2).

Claims

1: A method of controlling a supply of purge gas to reduce leakage past a dynamic seal for sealing a vacuum chamber, the dynamic seal comprising at least a first sealing member having opposing first and second sides, the method comprising: determining a first operating pressure on the first side of the first sealing member; determining a second operating pressure on the second side of the first sealing member; determining a pressure differential (ΔP) between the first operating pressure (P1) on the first side of the first sealing member and the second operating pressure (P2) on the second side of the first sealing member; controlling supply of purge gas to the second side of the first sealing member in dependence on the determined pressure differential (ΔP); determining a rate of change (dP1/dt) of the first operating pressure (P1); and identifying when the determined rate of change (dP1/dt) is less than a predefined rate of change threshold; wherein the method comprises reducing or inhibiting the supply of purge gas when the determined rate of change (dP1/dt) of the first operating pressure (P1) is less than the predefined rate of change threshold.

2: The method as claimed in claim 1, wherein controlling the supply of purge gas comprises: increasing the supply of purge gas when the determined pressure differential (ΔP) is such that the second operating pressure (P2) is less than or equal to the first operating pressure (P1).

3: The method as claimed in claim 1, wherein controlling the supply of purge gas comprises at least one of the following: increasing the supply of purge gas when the determined pressure differential (ΔP) is less than or equal to a predefined first pressure differential (ΔP) threshold; or decreasing the supply of purge gas when the difference between the first operating pressure (P1) and the second operating pressure (P2) is greater than a predefined second pressure differential (ΔP) threshold.

4: The method as claimed in claim 1, wherein the determining the pressure differential (ΔP) comprises subtracting the first operating pressure (P1) from the second operating pressure (P2).

5: The method as claimed in claim 1, comprising controlling the supply of purge gas in dependence on the determined pressure differential (ΔP) when the second operating pressure (P2) is less than or equal to a predefined operating pressure threshold.

6. (canceled)

7: The method as claimed in claim 1, wherein controlling the supply of purge gas comprises opening a purge gas supply valve to increase the supply of purge gas and closing the purge gas supply valve to decrease the supply of purge gas.

8: A controller for controlling a supply of purge gas to reduce leakage past a dynamic seal for sealing a vacuum chamber, the dynamic seal comprising at least a first sealing member having opposing first and second sides, the controller (23) being configured to: receive a first operating pressure signature from a first operating pressure sensor disposed on the first side of the first sealing member, the first operating pressure signal indicating a first operating pressure; receive a second operating pressure signal from a second operating pressure sensor disposed on the second side of the first sealing member, the second operating pressure signal indicating a second operating pressure; determine a pressure differential (ΔP) between a first operating pressure (P1) on the first side of the first sealing member and a second operating pressure (P2) on the second side of the first sealing member; and control the supply of purge gas to the second side of the first sealing member in dependence on the determined pressure differential (ΔP); determine a rate of change (dP1/dt) of the first operating pressure (P1); and identify when the determined rate of change (dP1/dt) is less than predefined rate of change threshold; wherein the controller is configured to output a control valve signal to control a control valve to reduce or inhibit the supply of purge gas when the determined rate of change (dP1/dt) of the first operating pressure (P1) is less than the predefined rate of change threshold.

9: The controller as claimed in claim 8, wherein the controller is configured to increase the supply of purge gas when the determined pressure differential (ΔP) indicates that the second operating pressure (P2) is less than or equal to the first operating pressure (P1).

10: The controller as claimed in claim 8, wherein the controller is configured to perform at least one of the following: to increase the supply of purge gas when the determined pressure differential (ΔP) is less than or equal to a predefined first pressure differential (ΔP) threshold; and to decrease the supply of purge gas when the difference between the first operating pressure (P1) and the second operating pressure (P2) is greater than a predefined second pressure differential (ΔP) threshold.

11: The controller as claimed in claim 8, wherein the controller is configured to determine the pressure differential (ΔP) by subtracting the first operating pressure (P1) from the second operating pressure (P2).

12: The controller as claimed in claim 8, wherein the controller is configured to control the supply of purge gas in dependence on the determined pressure differential (ΔP) when the second operating pressure (P2) is less than or equal to a predefined operating pressure threshold.

13. (canceled)

14: The controller as claimed in claim 8, wherein the controller is configured to open a purge gas supply valve to increase the supply of purge gas, and to close the purge gas supply valve to reduce or inhibit the supply of purge gas.

15: A vacuum system comprising the controller as claimed in any one claim 8.

16: A dynamic sealing system for a vacuum system, the dynamic sealing system comprising: at least a first sealing member for sealing a vacuum chamber, the first sealing member having opposing first and second sides; a purge gas supply system for supplying a purge gas to the second side of the first sealing member; and a controller as claimed in claim 8 for controlling the purge gas supply system, the controller being configured to control the purge gas supply system to control supply of purge gas to the second side of the first sealing member in dependence on the determined pressure differential (ΔP); and to output a control valve signal to control a control valve to reduce or inhibit the supply of purge gas when the determined rate of change (dP1/dt) of the first operating pressure (P1) is less than the predefined rate of change threshold.

17: The dynamic sealing system as claimed in claim 16, wherein the controller is configured to perform at least one of the following: to increase the supply of purge gas when the second operating pressure (P2) is less than or equal to the first operating pressure; or to reduce the supply of purge gas when the first operating pressure (P1) is greater than the second operating pressure (P2).

18: The dynamic sealing system as claimed in claim 16, wherein the controller is configured to determine the pressure differential (ΔP) by subtracting the first operating pressure (P1) from the second operating pressure (P2).

19. (canceled)

20: The dynamic sealing system as claimed in claim 16, wherein the controller is configured to increase the supply of purge gas when the determined pressure differential (ΔP) is less than or equal to a predefined first pressure differential (ΔP) threshold.

21: The dynamic sealing system as claimed in claim 16, wherein the controller is configured to control the supply of purge gas when the second operating pressure (P2) is less than or equal to a predefined operating pressure threshold.

22: A vacuum system comprising the dynamic sealing system as claimed in claim 16.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0070] FIG. 1 shows a schematic representation of a vacuum system incorporating a dynamic sealing system in accordance with an embodiment of the present invention;

[0071] FIG. 2 shows a block diagram representing operation of a controller for the dynamic sealing system shown in FIG. 1;

[0072] FIG. 3 shows a first chart illustrating operation of a vacuum system without the dynamic sealing system according to the present invention; and

[0073] FIG. 4 shows a first chart illustrating operation of the vacuum system incorporating the dynamic sealing system according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0074] A dynamic sealing system 1 for a vacuum system 2 in accordance with an embodiment of the present invention is described herein with reference to the accompanying FIG. 1.

[0075] The vacuum system 2 comprises a vacuum pump 3 operable to create a vacuum in a vacuum (process) chamber 4. The vacuum pump 3 comprises a rotor shaft 5 which is supported in a housing 6. The rotor shaft 5 is rotatable about a longitudinal axis X and is supported at a first end by a first shaft bearing 7. A second shaft bearing (not shown) is provided for supporting a second end of the rotor shaft 5. A (high vacuum) oil box 9 for containing a lubricant is provided for lubricating the first shaft bearing 7.

[0076] The dynamic sealing system 1 is configured to form and maintain a seal around the rotor shaft 5 between the vacuum chamber 4 and the first shaft bearing 7. The dynamic sealing system 1 comprises a lip seal (also known as a radial shaft seal) denoted generally by the reference numeral 10; and a purge gas supply system 11. The lip seal 10 in the present embodiment comprises a first sealing member 12-1 and a second sealing member 12-2. The first sealing member 12-1 is disposed in an inboard position relative to the vacuum chamber 4; and the second sealing member 12-2 is disposed in an outboard position relative to the vacuum chamber 4. The first and second sealing members 12-1, 12-2 each comprise an annular flange which, in use, contacts an outer surface of the rotor shaft 5 to form a seal. The first and second sealing members 12-1, 12-2 are spaced apart from each other along the rotational axis X of the rotor shaft 5. An annular chamber 13 is formed between the first and second sealing members 12-1, 12-2. The first and second sealing members 12-1, 12-2 have substantially the same configuration. The first sealing member 12-1 has opposing first and second sides 12-1A 12-1B; and an annular tip 12-1C for contacting the rotor shaft 5 to form a seal. The first side 12-1A is an inboard side in fluid communication with the vacuum chamber 4. The second side 12-1B is an outboard side isolated from the vacuum chamber 4.

[0077] The purge gas supply system 11 is configured to supply a purge gas, such as nitrogen (N2) to the annular chamber 13 at a nominal flow rate. The purge gas supply system 11 comprises a reservoir 14 for storing the purge gas; a regulated supply line 16; a controlled supply line 17; a first operating pressure sensor 18; and a second operating pressure sensor 19. The regulated supply line 16 comprises a flow restrictor 20 for restricting the flow of purge gas through the regulated supply line 16. The regulated supply line 16 provides a metered, steady-state flow rate of purge gas to the annular chamber 13. The controlled supply line 17 comprises a control valve 21, such as a solenoid valve, which is operable selectively to open and close the controlled supply line 17. The control valve 21 is configurable in at least a first open position and a second closed position. The control valve 21 may be configurable in one or more intermediate positions to provide further control of the supply of purge gas through the controlled supply line 17. The control valve 21 may be continuously variable between the first open position and the second closed position. The regulated supply line 16 and the controlled supply line 17 are arranged in parallel to each other and are both connected to a common supply line 22 which is in fluid communication with the annular chamber 13. A continuous supply of purge gas is provided by the regulated supply line 16 to the annular chamber 13. The first operating pressure sensor 18 measures the pressure in the vacuum chamber 4; and the second operating pressure sensor 19 measures the pressure in the annular chamber 13.

[0078] A controller 23 in the form of an electronic control unit (ECU) is provided for controlling operation of the control valve 21. The controller 23 comprises a processor 24 and a system memory 25. A set of computational instructions is stored on the system memory 25. When executed, the computational instructions cause the processor 24 to perform the method(s) described herein. The controller 23 is connected to a power supply 26, as shown in FIG. 1. As described herein, the processor 24 is configured to generate a control valve signal S1 for selectively opening and closing the control valve 21. The controller 23 is in communication with the first and second operating pressure sensors 18, 19. In particular, the controller 23 receives a first pressure signal SP1 from the first operating pressure sensor 18; and a second pressure signal SP2 from the second operating pressure sensor 19. The first pressure signal SP1 represents a first operating pressure P1 in the vacuum chamber 4. The second pressure signal SP2 represents a second operating pressure P2 in the annular chamber 13. The controller 23 is configured to compare the first and second operating pressures P1, P2 to determine a pressure differential ΔP across the first sealing member 12-1. In the present embodiment, the processor 24 is configured to determine the pressure differential ΔP by subtracting the first operating pressure P1 from the second operating pressure P2 (ΔP=P1−P2). The controller 23 may optionally also receive a pump status signal PSS1 from a pump controller 27. indicating a current status of the vacuum pump 3, for example to indicate that the vacuum pump 3 is currently activated (PUMP ON) or deactivated (PUMP OFF). The pump status signal PSS1 could optionally indicate a current operating speed of the vacuum pump 3.

[0079] The processor 24 is configured to generate the control valve signal S1 in dependence on the comparison of the first and second operating pressures P1, P2. If the second operating pressure P2 is less than the first operating pressure P1, the resulting pressure gradient across the first sealing member 12-1 will tend to promote the flow of process gases from the vacuum chamber 4 past the first sealing member 12-1 and into the annular chamber 13 (and potentially also into the oil box 9). As outlined above, the processor 24 determines the pressure differential ΔP by subtracting the first operating pressure P1 from the second operating pressure P2. When the operating conditions are such that the second operating pressure P2 is less than the first operating pressure P1, the determined pressure differential ΔP is a negative variable (−ve); this is referred to herein as an “adverse pressure differential ΔP” since it promotes leakage past the first sealing member 12-1 from the first side 12-1A to the second side 12-1B. (It will be understood that the pressure differential ΔP could be calculated by subtracting the second operating pressure P2 from the first operating pressure P1 and a pressure differential ΔP which is a positive variable (+ve) would indicate an “adverse pressure differential ΔP”.)

[0080] In order to suppress or prevent the adverse pressure differential being established across the first sealing member 12-1, the processor 24 is configured to control the control valve 21 to increase the supply of purge gas to the annular chamber 13 when the comparison of the first and second operating pressures P1, P2 determines that the second operating pressure P2 is less than or equal to the first operating pressure P1 (P2≤P1). The processor 24 generates an open control valve signal S1-OP at least partially to open the control valve 21 to increase the supply of purge gas through the controlled supply line 17. The increased supply of purge gas to the annular chamber 13 increases the second operating pressure P2. Conversely, the processor 24 is configured to control the control valve 21 to reduce the supply of purge gas to the annular chamber 13 when the comparison of the first and second operating pressures P1, P2 determines that the second operating pressure P2 is greater than the first operating pressure P1 (P2>P1). The processor 24 generates a close control valve signal S1-CL at least partially to close the control valve 21 to reduce or to inhibit the supply of purge gas through the controlled supply line 17. Closing the control valve 21 may result in a decrease in the second operating pressure P2. If the control valve 21 is already in the closed configuration, the control valve 21 is maintained in the closed configuration in response to the close control valve signal S1-CL. In the present embodiment, the regulated supply line 16 is substantially unaffected by the operation of the control valve 21 and, in use, the purge gas continues to be supplied to the annular chamber 13 via the regulated supply line 16 irrespective of the operating state of the control valve 21.

[0081] The processor 24 continuously monitors the first and second operating pressures P1, P2 at least substantially in real time. The controller 23 provides active control of the second operating pressure P2, thereby reducing or preventing establishment of an adverse pressure gradient P. The controller 23 controls the control valve 21 to control the supply of purge gas to the annular chamber 13 while the vacuum pump 3 is operating. Alternatively, or in addition, the controller 23 may control the control valve 21 for a period of time after deactivation of the vacuum pump 3, for example as part of a shutdown procedure of the vacuum system 2. When the vacuum pump 3 is deactivated, air is typically drawn into the vacuum chamber 4 for example through an exhaust port (not shown) of the vacuum pump 3. The first operating pressure P1 increases and returns to atmospheric pressure. Since the dynamic sealing system 1 establishes a seal around the rotor shaft 5, the second operating pressure P2 tends to increase at a slower rate than the first operating pressure P1. Thus, the second operating pressure P2 may return to atmospheric pressure more slowly than the first operating pressure P1. The second operating pressure P2 may be less than the first operating pressure P1 following deactivation of the vacuum pump 3. In a prior art vacuum system which does not control the supply of purge gas to the lip seal 10, an adverse pressure differential ΔP could be established following deactivation of the vacuum pump 3. However, the controller 23 according to the present embodiment is configured to control operation of the control valve 21 to reduce or inhibit an adverse pressure differential ΔP across the first sealing member 12-1 following deactivation of the vacuum pump 3. The controller 23 may continue to control the supply of purge gas to the annular chamber 13 for a predetermined time period. Alternatively, or in addition, the controller 23 may continue to control the supply of purge gas until the first operating pressure P1 has at least substantially stabilised. The controller 23 may, for example, continue to control the purge gas supply until it is determined that changes in the first operating pressure P1 are less than a predefined pressure change threshold over a predefined time period. The predefined time period may, for example, be thirty (30) seconds, sixty (60) seconds, ninety (90) seconds, one hundred and twenty (120) seconds, or longer. By way of example, the controller 23 may maintain the purge gas supply until changes in the first operating pressure P1 are less than 100 mbar in a rolling time period of two (2) minutes. Upon determining that changes in the first operating pressure P1 are less than 100 mbar over a time period of two (2) minutes, the controller 23 is configured to close the control valve 21. The controller 23 may determine a rate of change of the first operating pressure (dP1/dt) and identify when the rate of change is less than a predefined rate of change threshold. When the rate of change of the first operating pressure P21 is less than the predefined rate of change threshold, the controller 23 may close the control valve 21 to prevent the supply of the purge gas. The rate of change in the first operating pressure P1 (dP1/dt) may be determined over a predefined time period.

[0082] The processor 24 of the dynamic sealing system 1 is configured to generate the open control valve signal S1-OP to open the control valve 21 when the second operating pressure P2 is less than or equal to the first operating pressure P1 (P2≤P1). The processor 24 may be modified to generate the open control valve signal S1-OP when the pressure differential ΔP calculated by subtracting the first operating pressure P1 from the second operating pressure P2 (ΔP=P2−P1) is less than or equal to a predefined pressure differential threshold. The predefined pressure differential threshold may be a positive variable (+ve). Thus, the supply of purge gas may be increased even if the second operating pressure P2 is greater than the first operating pressure P1, albeit with a margin corresponding to the predefined pressure differential threshold. (It will be understood that the predefined pressure differential threshold may be a negative variable (−ve) if the pressure differential ΔP is calculated by subtracting the second operating pressure P2 from the first operating pressure P1.)

[0083] In a variant, the controller 23 could be configured to increase the supply of purge gas through the controlled supply line 17 to the annular chamber 13 only when the adverse pressure differential ΔP has a magnitude greater than or equal to a predefined pressure differential threshold. If an adverse pressure differential ΔP is detected having a magnitude greater than the predefined pressure differential threshold, the processor 24 is configured to generate the open control valve signal S1-OP to open the control valve 21 to increase the supply of purge gas through the controlled supply line 17. Conversely, the processor 24 is configured to decrease the supply of purge gas through the controlled supply line 17 to the annular chamber 13 (or continue to inhibit the supply of purge gas through the controlled supply line 17 to the annular chamber 13) when an adverse pressure differential ΔP is detected having a magnitude less than the predefined pressure differential threshold. If an adverse pressure differential ΔP is detected having a magnitude less than the predefined pressure differential threshold, the processor 24 generates the close control valve signal S1-CL to close the control valve 21 to reduce the supply of purge gas through the controlled supply line 17. In use, the pressure differential ΔP across the first sealing member 12-1 is maintained positive, thereby reducing a risk of reverse flow which may transport process materials from the vacuum chamber 4 past the seal lip seal 10 potentially causing contamination of the first shaft bearing 7 and other drive components, such as the oil box 9.

[0084] The operation of the dynamic sealing system 1 will now be described with reference to a block diagram 100 shown in FIG. 2. The vacuum system 2 is activated and the vacuum pump 3 operates to establish a vacuum in the vacuum chamber 4. The controller 23 is activated (BLOCK 110). A check is performed to determine if an operating status of the vacuum pump 3 (BLOCK 115). If the controller 23 determines that the operating status of the vacuum pump 3 changes from activated (i.e. running) to deactivated (i.e. not running), the controller 23 monitors a shutdown period. A check is performed to determine if the shutdown period has expired (BLOCK 120). If the shutdown period has not expired, the controller 23 opens the control valve 21 (BLOCK 125). The controller 23 continues to check if the shutdown period has expired. If the controller 23 determines that the shutdown period has expired, the control valve 21 is closed (BLOCK 130); and the process ends (BLOCK 135). In a variant, rather than determine whether a shutdown period has expired (BLOCK 120), the controller 23 can monitor the first operating pressure P1 to determine when the first operating pressure P1 has stabilised sufficiently to enable the control valve 21 to be closed. The controller 23 may determine that the first operating pressure P1 is stable when a pressure gradient corresponding to a change within a predetermined pressure range, for example 100 mbar, within a rolling predefined time period, for example 30 seconds. The controller 23 may open the control valve 21 until the first operating pressure P1 has stabilised.

[0085] If the controller 23 determines that the operating status of the vacuum pump 3 has not changed from activated to deactivated (BLOCK 115), i.e. the vacuum pump 3 is still running, the controller 23 reads the first and second pressure signals SP1, SP2 from the first and second operating pressure sensors 18, 19 indicating the first operating pressure P1 in the vacuum chamber 4 and the second operating pressure P2 in the annular chamber 13 respectively. The controller 23 determines if the first operating pressure P1 is greater than a predefined pressure threshold (BLOCK 140). If the first operating pressure P1 is less than or equal to the predefined pressure threshold, the controller 23 closes the control valve 21 (BLOCK 160). If the first operating pressure P1 is greater than the predefined pressure threshold, the controller 23 determines the pressure differential ΔP across the first sealing member 12-1 by subtracting the first operating pressure P1 from the second operating pressure P2 (ΔP=P2−P1). The controller 23 compares the pressure differential ΔP to a predefined pressure setpoint SP1 (BLOCK 145). If the pressure differential ΔP is greater than the pressure setpoint SP1, the controller 23 closes the control valve 21 (BLOCK 160). If the pressure differential ΔP is less than the pressure setpoint SP1, the controller 23 checks whether a pulse ON duration period for the vacuum pump 3 has expired (BLOCK 150). If the pulse ON duration has not expired, the controller 23 opens the control valve 21 (BLOCK 155). This process continues until the pulse ON duration period has expired. Upon expiry of the pulse ON duration period, the controller 23 closes the control valve 21 (BLOCK 160). After closing the control valve 21, the controller 23 implements a check to determine if a pulse OFF duration has expired (BLOCK 165). When the pulse OFF duration expires, the controller 23 reverts to monitoring the operating status of the vacuum pump 3 (BLOCK 115).

[0086] A first chart 200 representing operation of the dynamic seal system 1 without operating the control valve 21 to control the supply of purge gas is shown in FIG. 3. A first plot 210 represents the first operating pressure P1 measured in the vacuum chamber 4; and a second plot 220 represents the second operating pressure P2 measured in the annular chamber 13. A third plot 230 represents the instantaneous pressure differential (ΔP=P2−P1). A fourth plot 240 represents the pump status signal PSS1 indicating the current status of the vacuum pump 3. The fourth plot 240 is a digital signal with a “0” value indicating that the vacuum pump 3 is not operating (PUMP OFF); and a “1” value indicating that the vacuum pump 3 is operating (PUMP ON). A fifth plot 250 represents the desired state of the control valve 21. The fifth plot 250 is a digital signal with a “0” value indicating that the control valve 21 is closed; and a “1” value indicating that the control valve 21 is open. The vacuum chamber 4 may be cycled rapidly by opening and closing a release valve (not shown). During such a cycling operation (represented in a first region R1), the first operating pressure P1 in the vacuum chamber 4 increases to atmospheric pressure before returning to the vacuum conditions. The first plot 210 has a generally square waveform as the first operating pressure P1 responds substantially instantaneously. There is a corresponding increase in the second operating pressure P2, but the seal formed between the first sealing member 12-1 and the rotor shaft 5 results in the second operating pressure P2 increasing at a slower rate than the first operating pressure P1. As a result, cycling of the vacuum chamber 4 causes the second plot 220 to follow a saw-tooth profile. The peak second operating pressure P2 is less than the peak first operating pressure P1 during this operation. Consequently, an adverse pressure differential ΔP is established across the first sealing member 12-1, as represented by the third plot 230. During steady-state operation of the vacuum system 2 (represented in a second region R2), the pressure differential ΔP is greater than zero (0). Following shutdown of the vacuum pump 3 (represented in a third region R3), the first operating pressure P1 increases to atmospheric pressure more quickly than the second operating pressure P2 and the pressure differential ΔP becomes less than zero (0). Consequently, an adverse pressure differential ΔP is established across the first sealing member 12-1.

[0087] A second chart 300 representing operation of the dynamic sealing system 1 is shown in FIG. 4 by way of example. A first plot 310 represents the first operating pressure P1 measured in the vacuum chamber 4; and a second plot 320 represents the second operating pressure P2 measured in the annular chamber 13. A third plot 330 represents the instantaneous pressure differential (ΔP=P2−P1). A fourth plot 340 represents the pump status signal PSS1 indicating the current status of the vacuum pump 3. The fourth plot 340 is a digital signal with a “0” value indicating that the vacuum pump 3 is not operating (PUMP OFF); and a “1” value indicating that the vacuum pump 3 is operating (PUMP ON). A fifth plot 350 represents the operating state of the control valve 21. The fifth plot 350 is a digital signal with a “0” value indicating that the control valve 21 is closed; and a “1” value indicating that the control valve 21 is open. As represented by the third plot 330, the supply of purge gas to the annular chamber 13 ensures that the instantaneous pressure differential ΔP remains greater than zero (0). During cycling of the vacuum chamber 4 (first region R1), the first and second operating pressures P1, P2 both have a generally square waveform, as represented by the first and second plots 310, 320. The supply of purge gas to the annular chamber 13 results in the second operating pressure P2 having a peak value which is greater than the peak first operating pressure P1. Consequently, an adverse pressure differential ΔP is prevented. Instead, the pressure differential ΔP remains greater than zero (0). Similarly, during shutdown (third region R3), the pressure differential ΔP remains greater than zero (0) following deactivation of the vacuum pump 3. In the illustrated example, the pressure differential ΔP is greater than 100 mbar (ΔP>100 mbar). The controller 23 is configured to control operation of the control valve 21 in dependence on a predefined pressure setpoint SP1. If the pressure differential ΔP is less than the pressure setpoint SP1, the controller 23 is configured to close the control valve 21. If the pressure differential ΔP is greater than the pressure setpoint SP1, the controller 23 is configured to open the control valve 21 (or maintain the control valve 21 in an open state). The pressure setpoint may be defined as 50 mbar, 100 mbar, 200 mbar, for example.

[0088] The embodiment described above comprises a controller 23 having a processor 24 for comparing the first and second operating pressures P1, P2. In a further embodiment, a mechanical valve may be used to control the supply of purge gas to the annular chamber 13. The mechanical valve may be configured to determine the pressure differential between the first and second operating pressures P1, P2. The mechanical valve may, for example, comprise a differential pressure regulator for performing a direct comparison of the first and second operating pressures P1, P2. The differential pressure regulator may, for example, be in fluid communication with the vacuum chamber 4 and the annular chamber 3. The differential pressure regulator may, for example, comprise a diaphragm which is displaced in dependence on the first and second operating pressures P1, P2 acting on opposing faces thereof. The displacement of the diaphragm may selectively seat and unseat a valve to control the supply of purge gas. When the second operating pressure P2 is less than or equal to the first operating pressure, the pressure-differential valve may be configured to open so as to supply the purge gas to the annular chamber 13. A shut-off valve, for example a solenoid valve, may be provided to close the controlled supply line 17. The shut-off valve may be configured to close the controlled supply line 17 upon expiry of a predetermined time period following deactivation of the vacuum pump 3. The purge gas supply system 11 may thereby be isolated from the vacuum chamber 3.

[0089] It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

[0090] The controller 23 for controlling the purge gas supply system 11 has been described herein as a standalone unit. It will be understood that the controller 23 could be incorporated into another control unit, for example an on-board controller for the vacuum system 2.

[0091] The controller 23 has been described herein as measuring the first and second operating pressures P1, P2. At least one of the operating pressures P1, P2 could be modelled. For example, the first operating pressure P1 could be modelled based on an operating state of the vacuum pump 3 (ON/OFF/SPEED) and/or a purge valve state (OPEN or CLOSED).

REFERENCE SIGNS

Reference Sign Component

[0092] 1 dynamic sealing system 1 [0093] 2 vacuum system 2 [0094] 3 vacuum pump 3 [0095] 4 vacuum chamber 4 [0096] 5 rotor shaft 5 [0097] 6 housing 6 [0098] 7 shaft bearing 7 [0099] 9 oil box 9 [0100] 10 lip seal 10 [0101] 11 purge gas supply system 11 [0102] 12-1 first sealing member 12-1 [0103] 12-1A first side 12-1A [0104] 12-1B second side 12-1B [0105] 12-1C annular surface 12-1C [0106] 12-2 second sealing member 12-2 [0107] 13 annular chamber 13 [0108] 14 reservoir 14 [0109] 16 regulated supply line 16 [0110] 17 controlled supply line 17 [0111] 18 first operating pressure sensor 18 [0112] 19 second operating pressure sensor 19 [0113] 20 flow restrictor 20 [0114] 21 control valve 21 [0115] 22 common supply line 22 [0116] 23 controller 23 [0117] 24 processor 24 [0118] 25 system memory 25 [0119] 29 power supply 26 [0120] 27 pump controller 27 [0121] PSS1 pump status signal