Pumping apparatus

Abstract

A conduit supplies a flow of gas to a sealed chamber surrounding the swept volume of a pump. The conduit comprises a flow impedance for limiting the rate of flow of the gas to the sealed chamber. Signals output from pressure transducers provided on either side of the flow impedance are used to detect leakage of gas from the sealed chamber into the pump swept volume, thus indicating the state of the seal surrounding the swept volume.

Description

FIELD OF THE INVENTION

The present invention relates to pumping apparatus.

BACKGROUND OF THE INVENTION

Vacuum pumps are known which are oil-free in their vacuum chambers and which are therefore useful in clean environments such as those found in the semiconductor manufacturing industry. In such an environment, if lubricant materials were present in the vacuum chamber, such materials could potentially back migrate into the semiconductor process chamber and, in so doing, may cause contamination of the product under manufacture. Such “dry” vacuum pumps are commonly multi-stage positive displacement pumps employing intermeshing rotors in the vacuum chamber of each stage of the pump. The rotors may have the same type of profile in each chamber, or the profile may change from chamber to chamber.

In either a Roots, screw or Northey (“claw”) type device, each chamber is typically defined by two separately machined stator components of the pump, with rotor components of the pump being located in the cavity defined between the stator components. It is necessary to provide a seal between the stator components in order both to prevent leakage of pumped process gas from the cavity and to prevent any ambient air from entering the cavity. An O-ring seal is typically provided to perform this sealing function. Such seals are typically formed from fluoroelastomeric material, such as Viton™ (Du Pont de Nemours, E. I & Co.)

Dry vacuum pumps are frequently deployed in applications where they are required to pump substantial quantities of corrosive fluids, particularly halogen gases and solvents. Such materials attack O-ring seals, with the result that these seals may become excessively plastic or very brittle, which can badly affect the integrity of the seal provided between the stator components.

The intensity of the attack on the seal is dependant on a number of variables including, for example, the nature of the pumped fluid, the material from which the O-ring seal is formed, and the temperature of the pump. In view of this, it is very difficult to predict the appropriate interval for replacing the seals and thus maintaining pump integrity. External inspection of the seals is seldom practical.

These problems are particularly acute when pumping reactive gases such as fluorine from semiconductor processing equipment, where gas compositions are varied by reactions in the equipment. Here, even precise knowledge regarding the gas flows admitted to the process chamber is a very poor predictor of the quantity or nature of the reactive gas entering the pump and hence the anticipated useful seal life. Recommended maintenance often includes frequent seal leak checks but this is expensive, inconvenient to do and consequently is sometimes omitted.

In principle, other types of sensor could be used to attempt to measure the integrated exposure level of the seals and hence the state of the seals. For example, a spectroscopic or chemical technique could be employed to measure gas composition. However, such techniques would require complex calibration procedures and be costly to implement.

SUMMARY OF THE INVENTION

In at least its preferred embodiment, the present invention seeks to solve these and other problems.

The present invention relates to a pumping apparatus comprising a pump adapted to from a swept volume further and a sealed chamber surrounding the swept volume, the apparatus for other comprising a conduit for supplying fluid to the chamber, the conduit comprising a flow impedance for limiting the rate of flow of fluid to the chamber, and means for determining a pressure difference across the flow impedance.

Another aspect of the present invention relates to a method of detecting a leak of fluid from a chamber surrounding a swept volume of a pump, the method comprising the steps of supplying the fluid to the chamber through a flow impedance, and monitoring a pressure difference across the flow impedance.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a front view of a stator component showing a seal assembly;

FIG. 2 is a side view of the seal assembly of FIG. 1;

FIG. 3 illustrates apparatus for monitoring the integrity of the seal assembly; and

FIG. 4 is a graph indicating the variation of pressure of a gas within the seal assembly with time.

FIG. 1 illustrates the surface 3 of a stator component 1 from an exhaust stage of a dry pump. A corresponding surface of a second stator component (not shown) is brought into contact with the surface 3 of the stator component 1 and a cavity 2 is formed between the adjacent stator components. Cavity 2 accommodates the rotor component (not shown) of the pump when the pump is assembled, and is generally referred to as a pump swept volume. A dry pump typically comprises several such cavities, each cavity 2 communicating with an adjacent cavity through a port 4.

A first O-ring seal 5 is provided around the periphery of the cavity 2. This O-ring seal 5 is preferably formed from a fluoroelastomeric material, such as Viton™, and provides a fluid tight seal between adjacent stator components so that, when the pump is in use, process or cleaning gases being pumped through the cavity 2 are prevented from leaking from the cavity 2, and ambient air is prevented from entering the cavity 2. However, as discussed above, such gases can be particularly aggressive and can readily cause damage to many parts of the pump. Typically, first O-ring seal 5 is the first component to fail in such circumstances. In view of this, a second O-ring seal 6, similar to first O-ring seal 5, is provided between first O-ring seal 5 and the periphery of cavity 2. First and second O-ring seals 5, 6 are separated by a shallow channel or groove 7 (FIG. 1) which is formed between grooves 8, 9 used to locate first O-ring seal 5 and second O-ring seal 6 between the adjacent stator components, as shown in FIG. 2. The channel 7 allows a small quantity of fluid, for example a gas such as nitrogen, to be trapped between the two adjacent stator components and the O-ring seals 5, 6, which together define a sealed chamber for the gas. The gas enters the channel 7 through port 7a from a gas reservoir 16 via a conduit 9 as indicated in FIG. 3 schematically showing an apparatus for monitoring the integrity of the seal assembly.

As shown in FIG. 3, conduit 9 includes a flow impedance 10 and a one-way valve 11. Pressure transducers 12, 13 are provided in fluid communication upstream and downstream of flow impedance 10.

Flow impedance 10 may be formed from slightly porous, sintered material that inhibits a flow of gas such that, when the flow impedance 10 is placed in conduit 9, it acts to allow only a trickle of gas to pass therethrough. Flow impedance 10 could alternatively be provided by a fine metering valve, or by creating a fine capillary hole through solid material.

One-way valve 11 prevents contamination of the supply reservoir if the pressure in the pump rises above that of the gas supply. Valve 11 also serves to minimise fluctuations in pressure in conduit 9 downstream from valve 11 in the event that the gas supply was to be temporarily interrupted or otherwise affected.

Pressure transducers 12, 13 measure the pressure P2 and P1 respectively in the conduit 9 on the upstream and the downstream side of flow impedance 10, and pass signals indicating the measured pressure to a controller 14.

The supply of gas to conduit 9 is controlled by gas module 15. In this arrangement, gas module 15 is an active manifold that regulates the supply of gas from the reservoir into conduit 9. Gas module 15 is configured to send a signal to controller 14 to indicate one or more characteristics, such as flow rate and pressure, of the gas being fed into conduit 9. Such a gas module may be used to distribute gas to different locations within the pump, for example where the gas is to be used as a purge gas for flushing impurities from the pump.

In use, pressurised gas (typically at approximately 6 psi) is passed along conduit 9, through flow impedance 10, and into channel 7 until a pressure equilibrium is established between the gas downstream from flow impedance 10, and the gas upstream of flow impedance 10. Due to the presence of pressurised gas in channel 7 downstream from flow impedance 10, during use of the pump a significant pressure difference will be experienced across second O-ring seal 6, as the pumped gas in cavity 2 (the swept volume of the pump) will be sub-atmospheric (typically 800 mbar) when the pump is under normal steady state operating conditions. In this state, when second O-ring seal 6 is new and has no defects, the signals output from pressure transducers 12, 13 will be approximately equal and non-fluctuating. However, in the event that second O-ring seal 6 should become damaged by the pumped gas so that the integrity of second O-ring seal 6 is impaired, pressurised gas can start to leak from channel 7 to cavity 2, due to the existence of a relatively higher pressure gas in channel 7 and a relatively lower pressure gas in cavity 2, which leakage will to try to equalise these pressures. Due to the presence of flow impedance 10 in conduit 9, the pressure P2 measured by the pressure transducer 12 will start to fall (as shown in FIG. 4), whilst the pressure P1 measured by the pressure transducer 13 will remain at the supply pressure. The difference in the pressures P1 and P2 is therefore indicative of a leak of gas into the cavity 2, and thus is indicative of a failure of the second O-ring seal 6. This can enable controller 14, which receives the signals output from the pressure transducers 12, 13, to output an alarm, for example, via a display, indicating the failure of the second O-ring seal 6 if the pressure difference exceeds a predetermined value.

The apparatus described above can thus provide a reliable indication of the state of critical seals inside a pump. Such an indication can allow maintenance intervals to be lengthened and costs of operation to be reduced without intrusive intervention. Since the predictability of deterioration of these critical components can be improved the probability of potentially hazardous leaks is consequently reduced.

In summary, a conduit supplies a flow of gas to a sealed chamber surrounding the swept volume of a pump. The conduit comprises a flow impedance for limiting the rate of flow of the gas to the sealed chamber. Signals output from pressure transducers provided on either side of the flow impedance are used to detect leakage of gas from the sealed chamber into the pump swept volume, thus indicating the state of the seal surrounding the swept volume.

While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the true spirit and scope of the present invention.

Claims

1. A pumping apparatus comprising a pump adapted to form a swept volume and a sealed chamber surrounding the swept volume, the apparatus further comprising a conduit for supplying fluid to the chamber, the conduit comprising a flow impedance for limiting the rate of flow of fluid to the chamber, and means for determining a pressure difference across the flow impedance.

2. The pumping apparatus as in claim 1, wherein the impedance is provided by a restriction within the conduit.

3. The pumping apparatus as in claim 1, wherein the rate of flow of fluid across the impedance is such that fluctuations in fluid pressure on one side of the impedance are not rapidly transmitted to fluid on the other side of the impedance.

4. The pumping apparatus as in claim 1, wherein the impedance is provided by one of the group of a porous, sintered material, a capillary formed within a non-porous blockage, and a valve.

5. The pumping apparatus as in claim 1, wherein the means for determining the pressure difference comprises at least one pressure transducer.

6. The pumping apparatus as in claim 5, wherein said at least one pressure transducer comprises a first pressure transducer configured to output a signal indicative of a fluid pressure in the conduit upstream of the impedance and a second pressure transducer configured to output a signal indicative of a fluid pressure in the conduit downstream of the impedance.

7. The pumping apparatus as in claim 6, comprising a controller for receiving the signals from the first pressure transducer and the second pressure transducer and for generating an alert depending on the difference in the fluid pressures indicated by the signals.

8. The pumping apparatus as in claim 7, wherein the alert is generated when the pressure difference across the impedance exceeds a predetermined value.

9. The pumping apparatus as in claim 1, wherein the conduit is connected to a reservoir for supplying fluid thereto.

10. The pumping apparatus as in claim 9, wherein the reservoir is configured to supply a purge gas to a pump of the pumping apparatus.

11. The pumping apparatus as in claim 1, wherein the sealed chamber is formed by two adjacent stator components and a first O-ring seal and a second O-ring seal therebetween.

12. The pumping apparatus as in claim 11, wherein the O-ring seals are formed from an elastomeric material.

13. A method for detecting a leak of fluid from a chamber surrounding a swept volume of a pump, the method comprising the steps of supplying the fluid to the chamber through a flow impedance, and monitoring a pressure difference in the fluid across the flow impedance.

14. The method according to claim 13, wherein the pressure difference across the flow impedance is monitored subsequent to a pressure equilibrium being established between the fluid downstream of the flow impedance and the fluid upstream of the flow impedance.

15. The method according to claim 13, wherein the monitored pressure difference is compared with a predetermined value, and an alert is generated if the monitored pressure difference exceeds the predetermined value.

16. The method according to claim 13, wherein the fluid is supplied from a reservoir.

17. The method according to claim 16, wherein the reservoir is configured to supply a purge gas to the pump.

18. The method according to claim 13, wherein the chamber is defined in part by two O-ring seals.

19. The method according to claim 18, wherein the O-ring seals are formed from an elastomeric material.