Patent Publication Number: US-2012045322-A1

Title: Vacuum pump

Description:
The present invention relates to a vacuum pump, and particular to a vacuum pump suitable for pumping corrosive fluids. 
     A known vacuum pump  50  is shown in  FIG. 7  which comprises a pumping mechanism  52 . The pumping mechanism comprises a plurality of pumping stages  54  for pumping fluid along a fluid flow path  56  between an inlet  58  for fluid at high vacuum and an outlet  60  for fluid at low vacuum to atmospheric pressure. Five pumping stages  54  are shown. A motor  62  drives rotation of the rotors R relative to the stators S in each of the pumping stages  54 . 
     If the fluid being pumped comprises a corrosive agent, such as fluorine, corrosion is caused to the pumping mechanism  52 . Over time, corrosion causes build up of deposits on the surface of components of the pumping mechanism which causes a reduction in the running clearances between the rotors R and the stators S of the pumping stages  54 . After continued operation of the pump over many hours the corrosion can bring the rotors and stators of the pumping stages into contact causing pump failure. 
     It is possible to reduce corrosion in vacuum pumps by manufacturing the pumping mechanism from a corrosion resistant material but typically such materials are expensive. 
     The present invention provides a vacuum pump for pumping corrosive fluid, the pump comprising: a pumping mechanism comprising a plurality of pumping stages along a fluid flow path between an inlet for fluid at high vacuum and an outlet for fluid at low vacuum, and wherein the material of the pumping mechanism at a first section of said flow path is less resistant to corrosion than the material of the pumping mechanism at a second section of said flow path downstream of said first section. 
     Other preferred and/or optional aspects of the invention are defined in the accompanying claims. 
    
    
     
       In order that the present invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  shows a simplified section through a vacuum pump; 
         FIG. 2  is a graph showing corrosive build-up over time against pump reference temperature for a pump having a pumping mechanism made from a material which is relatively non-corrosion resistant; 
         FIG. 3  is a graph showing corrosive build-up over time against pump reference temperature for a pump having a pumping mechanism made from a material which is relatively corrosion resistant; 
         FIG. 4  is a graph showing corrosive build-up over time against pump reference temperature for a pump having a pumping mechanism made from a material which is relatively non-corrosion resistant and a material which is relatively corrosion resistant; 
         FIG. 5  is a table showing properties of corrosion and non-corrosion resistant materials; 
         FIG. 6  is a table showing an example of the constituents of a corrosion resistant material; and 
         FIG. 7  is a simplified section through a prior art vacuum pump. 
     
    
    
     Referring to  FIG. 1 , a multi-stage vacuum pump  10  is shown for pumping corrosive fluid. A pumping mechanism  12  comprises a plurality of pumping stages  14  for pumping fluid along a fluid flow path  16  between an inlet  18  for fluid at high vacuum and an outlet  20  for fluid at low vacuum to atmospheric pressure. Five pumping stages  14  are shown in this example. A motor  22  drives rotation of a rotor R relative to a stator S in each of the pumping stages  14 . 
     In arriving at the present invention, an analysis of the prior art pump shown in  FIG. 7  was conducted and  FIG. 2  shows a graph in which corrosion build-up over time (10,000 hours in this example) is plotted against pump reference temperature for the prior art pump. In the graph a first line shows build-up at the faces of the rotor and stator of a middle pumping stage along the flow path  56  and a second line shows build-up at the faces of the rotor and stator of a final pumping stage along the flow path  56 . The middle pumping stage in this example is the 3 rd  stage. Corrosive build-up is measured in microns and temperature is measured in degrees centigrade. The temperature used in the graph is a pump reference temperature taken at the final pumping stage. It will be appreciated that the temperature of the middle pumping stage is less than that shown in the graph but for simplicity has not been shown. The pump reference temperature increases during operation and the increase is dependent on a number of factors such as the type of fluid which is pumped and the work exerted by the pump. 
     In  FIG. 7 , the material from which the pumping mechanism is made is relatively non-corrosion resistant. An example of such a material is SG iron. It will be seen from  FIG. 2  that corrosive build-up of the final stage is considerably greater than that of the middle pumping stage, particularly when the pump reference temperature is at 200° C. At this reference temperature, corrosive build-up at the final stage is just lower than 400 μm, whereas at the middle stage, build-up is only just higher than 50 μm. 
       FIG. 3  shows an equivalent graph as shown in  FIG. 2 , except in this analysis the material from which the pump is made is relatively corrosion resistant. An example of such a material is Ni-rich SG iron. In  FIG. 3 , corrosive build-up at both the middle and final pumping stages is reduced, but build-up at the final pumping stage has been reduced by around 300 μm to just lower than 100 μm whereas build-up at the middle pumping stage has been reduced by only around 30 μm to around 20 μm. 
     When considering the pump shown in  FIG. 7 , the pressure of fluid along the flow path  56  increases from the inlet  58  to the outlet  60  as fluid is compressed by each pumping stage  54 , typically with the compression ratio increasing from one pumping stage to the next pumping stage along the flow path. The temperature of the fluid and the pumping mechanism also increases along the flow path. 
     Accordingly, if the fluid being pumped comprises a corrosive agent, such as fluorine, the amount of corrosion caused to the pumping mechanism  52  increases along the flow path  56  as temperature and pressure increase. Increased pressure increases the amount of corrosive molecules available for corroding the pumping mechanism and increased temperature increases corrosive reaction. Therefore, corrosive build-up is greater at the final pumping stage than at the middle pumping stage. Accordingly, pump failure occurs because of the reduction in running clearance at the final stage of the pumping mechanism where build-up is greatest. Whilst corrosion resistance can be increased as shown in  FIG. 3 , pump failure often occurs at the final stage of the pumping mechanism. 
     In the pump shown in  FIG. 1 , the pumping mechanism comprises a first section  24  and a second section  26 . The second section  26  is downstream of the first section  24 . 
     During operation, the temperature and pressure of the downstream section  26  is greater than the temperature and pressure of the upstream section  24 . Therefore, when pumping a corrosive fluid, corrosion of section  24  is less than corrosion of section  26 . As the build-up of corrosion deposits on the rotors and stators in section  24  is less than in section  26 , the first section  24  is required to be less resistant to corrosion than the second section  26 . The first section  24  can therefore be made from a material which is relatively less expensive than the material of the second section. 
     The first section  24  and the second section  26  comprise a respective plurality of pumping stages  14 . The first section is adjacent to the second section along the fluid flow path. In  FIG. 1 , the first section comprises 1 st , 2 nd  and 3 rd  pumping stages whilst the second section  26  comprises 4 th  and 5 th  pumping stages. In another arrangement, one of the first and the second sections may comprise a single pumping stage. For instance, the first section may comprise 1 st  to 4 th  pumping stages whilst the second section may comprise the 5 th  pumping stage. As temperature and pressure increase to the greatest extent at the final downstream pumping stage it may be desirable to manufacture this stage from a corrosion resistant material whereas stages 1 to 4 may be manufactured from a material which is less corrosion resistant. Alternatively, the first section may comprise the 1 st  pumping stage and the second section may comprise the 2 nd  to 5 th  pumping stages. 
     A graph equivalent to the graphs shown in  FIGS. 2 and 3  is shown in  FIG. 4 , which plots corrosive build-up against pump reference temperature for the pump shown in  FIG. 1 .  FIG. 4  shows at 200° C. that corrosive build-up at the final stage of the pumping mechanism has been reduced from around 400 μm as shown in  FIG. 2  to just lower than 100 μm as is the case with a corrosion resistant pump according to  FIG. 3 . However, in  FIG. 4 , corrosive build-up at the middle, or 3 rd , stage is the same as that shown in  FIG. 2  for a non-corrosion resistant pump. In this regard, the corrosive build-up of the middle stage is around 50 μm which is less than that of the final stage even though the middle stage is made from a non-corrosion resistant material (e.g. SG iron) and the final stage is made from a corrosion resistant material (e.g. Ni-rich iron). Therefore, there is no benefit to be gained from making both the first section and the second section of a pumping mechanism from a corrosion resistant material and doing so would unnecessarily add to the cost of a pump. 
     Materials should be selected for the first and second sections so that the build-up of corrosive deposits at the first section is less than or equal to the build-up of corrosive deposits at the second section. Preferably, the components of said pumping stages are fabricated from selected materials such that the build-up of corrosion deposits in each stage during pumping of corrosive gas is generally equal one stage to another stage. In this way, the materials for the various pumping stages can be selected in a cost efficient manner whilst maintaining acceptable resistance to corrosion. In  FIG. 1 , the rotor R and stator S of each pumping stage  14  of the second section  26  is made from a Nickel rich iron, whilst the rotor R and stator S of each pumping stage  14  of the first section  24  is made from an SG (spheroidal graphite) iron. 
     Examples of these materials are shown in  FIG. 5 , although other materials may be selected according to requirements. For instance, Nickel is resistant to fluorine but if the fluid being pumped contains other corrosive agents it would be desirable to select an appropriately resistant material. Further, the first section of the pumping mechanism may be made from materials other than SG iron. 
     If the pump is to be used for pumping particularly corrosive fluid, the first section may be made from a corrosion resistant material for instance NI-rich SG iron, whilst the second section may be made from a more corrosion resistant material such as cast stainless steel or nickel alloy. 
       FIG. 5  shows three examples of both SG iron and Ni-rich SG iron. It is preferable to select materials for the first and second section with similar linear expansion coefficients. In this regard, Ni-res D-5S is a preferred corrosion resistant material as its linear expansion coefficient is 12.6 m/mK which is similar to the coefficient of SG iron of 12.5 m/mK. 
     The constituents of an Ni-rich SG iron material, which exhibits good corrosion resistant properties and also good strength and stiffness in high temperature conditions are shown in  FIG. 6 . In such a material, it will be seen that the Nickel content is relatively high between 24% and 32% by weight. 
     The same or similar material is preferably used for the rotor R and stator S in each section to avoid problems associated with having components of different thermal expansion coefficients. 
     The vacuum pump  10  is shown in simplified form in  FIG. 1 . The vacuum pump may comprise a claw type pumping mechanism or roots type pumping mechanism or other type of dry pumping mechanism, particular in which running clearance between components of the pumping mechanism is required to be small to increase efficiency.