Patent Publication Number: US-2005123414-A1

Title: Pumping apparatus

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 &amp; 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  7   a  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 P 2  and P 1  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 P 2  measured by the pressure transducer  12  will start to fall (as shown in  FIG. 4 ), whilst the pressure P 1  measured by the pressure transducer  13  will remain at the supply pressure. The difference in the pressures P 1  and P 2  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.