Abstract:
A piston being configured for reciprocating motion in a cylinder bore and for sealing contact with piston seals have a first end arranged for contact with a product to be processed and a second end configured for coupling to a drive means. At least a portion of the piston arranged for contact with the piston seals is coated with a low-friction material, providing a lowered friction as compared to the uncoated piston.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to pistons, in particular to a piston well adapted for use in a piston pump. 
       BACKGROUND ART 
       [0002]    A typical example where piston pumps are used, within the field of food processing, is in homogenizers. Homogenization relates to the stabilization of fat emulsion such that it is not affected by gravity separation. By homogenizing e.g. milk, cream, and other dairy products they may not separate to any significant degree during their shelf life. Homogenization may also affect flavour and other properties of the product subjected to it. A rudimentary explanation of the main purpose of homogenization is that fat globules are disrupted into smaller ones, which are not as prone to separate by gravity. 
         [0003]    The disruption of fat globules is made by application of force, and a common approach is to force a product under high pressure (homogenization pressure) through a small slit. When passing the slit the product will rapidly increase its velocity. The flow will become turbulent, generating turbulent eddies, and the elevated velocity will generate a pressure drop so substantial that the product may start to boil. The first effect contributing to homogenization is that the violent eddies will tear the fat globules apart, and the second effect is that cavitation of the imploding bubbles generated when boiling will disrupt the fat globules. 
         [0004]    The skilled person is well aware of the above, yet for the non-skilled person studying the present invention it is important to realize that homogenization is a process requiring large amount of energy. Due to a quite low efficiency a large amount of the energy applied is converted to heat, which may or may not be a problem depending on the nature of the application. 
         [0005]    In a typical homogenizer a piston pump with a piston following a reciprocating motion in a cylinder is used for pressurization purposes. In an intake stroke an inlet valve is opened and product enters the cylinder as the piston travels down to its bottom-dead-center. Following that the compression stroke commences and at a certain point an outlet valve is opened thus forming the slit through which the product is forced. The outlet valve comprises an annular valve seat and an actuated forcer (i.e. the valve). The valve seat and the valve are designed to withstand both the high pressures and the abrasive forces generated. Though the present invention in an embodiment related to homogenizers should not be limited to the stated parameters, typical homogenization pressures may reside in the range of 100-250 bar, and the annular gap forming the slit between the valve seat and the forcer has a width in the order of 0.1 mm. 
         [0006]    Piston pumps having contact with foodstuff, which may be the case for the applications of the present invention, need to have a proven performance in terms of hygiene. There may be a difficulty in separating the non-hygienic side of the piston pump, comprising a crankcase and a crankshaft connecting the piston to the crankcase, from the hygienic side, comprising the portions of the cylinder being in contact with the food stuff. By necessity the piston moves relative to the cylinder during operation of the device, and this introduces to challenges. 
         [0007]    The first challenge is that there will be an annular gap between the cylinder and the piston, which by necessity needs to be addressed since it connects the non-hygienic side and the hygienic side. This may be accounted for by arranging piston seals. The piston seals are usually annular and arranged on the cylinder wall (as oppose to piston rings being arranged on the side of the piston. A common approach is to arrange two spaced apart piston seals which are cooled and lubricated with water or steam condensate. Regular lubricant, such as lubricating oil as used on some other piston pumps cannot be used for obvious reasons, yet for some applications foodgrade oil may be used. The water may be injected in a location between the seals. Prior art is e.g. disclosed in WO2011/002376 by the present applicant, as well as in and references cited in relation to that application. 
         [0008]    The second challenge is that surface portions along the extension of the cylinder (and/or portions of the piston) will during the pump cycle be located both on the non-hygienic side and on the hygienic side or in an area therebetween, thus introducing a risk of reinfection. This challenge has been solved by injecting superheated water or steam onto the relevant surface portions. One aspect of this will be disclosed in detail in the detailed description. 
         [0009]    Piston pumps are used in other areas than homogenizers, as standalone components in a circuit, and though examples before and after will relate to homogenizers it should be apparent for the skilled person that the present invention will provide benefits for piston pumps as such. 
         [0010]    Consequently, the present invention aims at providing a solution that alleviates drawbacks in prior art by providing a new method and arrangement for piston pumps. 
       SUMMARY 
       [0011]    For the above purposes the present invention provides a piston configured for reciprocating motion in a cylinder bore and for sealing contact with piston seals. The first end of the piston is arranged for contact with a product to be processed, and the second end is configured for coupling to a piston drive, driving the reciprocating motion of the piston. The piston is characterized in that at least a portion of the piston arranged for contact with the piston seals is coated with a low-friction material. Piston is particularly well adapted for use in piston pumps in general and piston pumps used for homogenizers in particular. The invention as such may be useful in other contexts as well. The clarification of the purposes of the different ends of the piston is merely explanatory and has been added to simplify understanding. 
         [0012]    According to one or several embodiments, wherein the first end of the piston is coated with a low-friction material. In further embodiment the entire piston may be coated with a low-friction material. 
         [0013]    In embodiments relating to processing of foodstuff it is preferred that the low-friction material is a food-grade material. Some low-friction materials that may be used are PTFE or DLC. 
         [0014]    The piston described above and below may be used in piston pump, an example being a piston pump used in a homogenizer. 
         [0015]    Adding to one beneficial effect of the piston coating the piston pump may comprise at least one piston pump seal arranged in an annular slot in the cylinder wall and extending into sealing contact with the piston, wherein a radially inner surface portion of the seal may be provided with a coating of a low-friction material. In still other embodiment the entire peripheral surface of the seal is coated with a low-friction material. 
         [0016]    In one or more embodiments the cylinder of a piston pump comprising the inventive piston according to one embodiment thereof may comprise an inlet and an outlet for lubricating fluid or sterilizing fluid, such as water or other fluid in an annular slit between the cylinder wall and the piston. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1A  is a sectional view of a piston pump including a first embodiment of the present invention. 
           [0018]      FIG. 1B  is a section view of a piston, illustrating various embodiments of the present invention. 
           [0019]      FIG. 2  is a sectional view and a plan view of a seal, which may be used in an embodiment of the present invention. 
           [0020]      FIG. 3  illustrates schematically further embodiments of a seal of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0021]      FIG. 1  illustrates a portion of a piston pump of a homogenizer comprising a first embodiment of the present invention. A piston  102  is arranged in a cylinder housing  104  comprising a cylinder bore  106  (“cylinder” in the following). The piston  102  is connected to a piston drive, an actuation mechanism (not shown) for providing a drive to the reciprocating motion of the piston  102 . In the present embodiment a connection rod (not shown) connecting the piston  102  to a crank shaft (not shown) may preferably be used, yet other actuating mechanisms are not to be ruled out, such as hydraulic or pneumatic devices, cam mechanisms, or servomotors. 
         [0022]    Two annular piston seals  108 ,  110  are arranged in annular slots in the peripheral wall of the cylinder  106 , referred to as cylinder wall. Each piston seal  108 ,  110  extends into the cylinder  106  and sealingly contacts the pistons  102 . A channel  112  opens up into the cylinder  106  at a position between the seals  108 ,  110 . The channel  112  guides a sterilization and/or lubrication fluid (water, sterile water, hot sterile water or hot sterile water mixed with steam, ethanol or other sanitary fluid) to the region between the piston  102  and the cylinder wall. Another channel  114  leads the fluid away from the area. In other embodiments several such channels  112 / 114  may be arranged. It should also be mentioned that embodiments where only one piston seal is used may be relevant alternatives. 
         [0023]    In a piston pump where the seals are arranged on the cylinder wall rather than on the piston itself, the word “rod” is sometimes used for the component referred to as “piston  102 ” in the present application. 
         [0024]    In one or more embodiments the cylinder housing  104  may be formed from two parts  104 A and  104 B, which also is illustrated in  FIG. 1A . 
         [0025]    In the present embodiment portions of the piston are coated with low-friction material. The portions may be selected from the group comprising: portions being in contact with piston seals during operation of the device, portions being in contact with product during operation of the device, and portions not being in contact with piston seals or product during operation of the device, or combinations thereof. Coating portions of the piston being in contact with the piston seals will result in a friction reducing effect. The benefits emanating from a lowered friction are listed by the end of the present specification. Coating the portions of piston being in contact with the product will reduce the tendency of product sticking to the piston, and thus simplify cleaning. Also, if e.g. proteins stick to the surface of the piston it may be scraped of by the seals, thus resulting in fouling of both the piston and the seal(s), this fouling may result in a deterioration of the sealing properties. Application of a coating will also ensure a predictable contact between the product and the piston. Further, during cleaning with chemicals the piston may be protected from the quite aggressive cleaning agents, which may be used to ensure proper cleaning. Coating the rest of the piston as well will provide a combined effect as well incurring a full protection for the piston in relation to friction, product, and other fluids. Other examples include that the top and lateral sides of the piston are coated with a low-friction material, i.e. the portions particularly exposed to friction, product, or other fluids. The piston may preferably be manufactured from stainless steel, food grade, yet embodiments where the piston is fully coated enables use of other materials. Therefore aluminium or steel of other quality, etc. are also possible alternatives. 
         [0026]      FIG. 1B  is a schematic drawing illustrates some options for the extension of the coating. In 1 the entire piston is coated, in 2 the portions mainly exposed to frictional wear (the sides of the piston on its cylindrical portion) and the top of the piston have been coated, and finally, in 3 only the sides have been coated. 
         [0027]    As for the seal, which described in the following, the coating of the piston may be one and the same material, yet it may also be a combination of materials. One example would be that portions of the piston exposed to less abrasive wear are provided with a corrosion-resistant coating only, while the remaining portions of the piston are provided with low-friction material. An example would be having the entire piston is coated with coating preventing corrosion (the area  1  in  FIG. 1B ) while only the sides (area  3  in  FIG. 1B ) are coated with a low-friction material. 
         [0028]    A typical piston pump seal  210  is illustrated in  FIG. 2 , in a plan view (to the right) and a cross sectional view (to the left), and its arrangement in a piston pump is clarified e.g. in  FIG. 1  from the seals  108  and  110  respectively. The seal  210  could represent any one of the seals  108  or  110 , or a different seal not shown in the arrangement of  FIG. 1A . The side facing the piston, the radially inner side is denoted A in  FIG. 2 , while B denotes the side facing the cylinder, the radially outer side. C refers to the side facing the product, while D denotes the side facing away from the product. In use a thin film of lubricant will be generated between the side B and the piston to form a hydraulic seal. The lubricant may consist of water or product in the embodiments illustrated in the present application, yet a food-grade lubricant may also be used. In the cross section of  FIG. 2  it is worth pointing out the sealing lip  216  extending from the side C. The seal  210  is a symmetric annular construction, and the sealing lip  216  extends around the inner perimeter of the seal. In operation the pressure difference between the product side and the other side will push the sealing lip towards the piston, thus providing a proper seal. A similar sealing lip  218  may be arranged on the radially outer perimeter for the seal, so as to seal towards the cylinder wall. The cross sectional shape of the seal may vary beyond what is shown in  FIG. 2 , yet the use of sealing lip  216  or similar functional component is likely to be included in preferred embodiments. 
         [0029]    The seal  210  is formed from a base or core material, an elast, fully or partially coated by a low-friction material. Typical examples could include, but is not limited to: nitrile rubber, EPDM (ethylene propylene diene monomer) rubber, PU (polyurethane), fluoroelastomers, etc. The core may also be reinforced by woven or non-woven fibers, e.g. a cord of Nomex® or Kevlar® or other fibre-material. 
         [0030]    For the purposes of a first aspect of the present invention it may suffice that only the portions of the seal being in sealing contact with the piston is coated, i.e. the portions being exposed to frictional movement. This arrangement will have the advantage that the frictional wear as well as frictional heat generation is reduced. Yet another advantage is that the resilient or elastic properties of a base material onto which the coating is applied will be preserved, and may be utilized to the benefit of the application, e.g. by providing a good seal. 
         [0031]    It may be argued that the use of the term “low-friction material” is not clear. Within the context of the present application the term refers to a material which when applied as a coating to a first material it provides a lower (even significantly lower) friction than the first material. The coating with low-friction material provides a lowered coefficient of friction for the components on which it is arranged. Moreover, the word “low-friction material” is not merely a relative term, it is also explanatory for the skilled person in the same way as e.g. “lubricant”. 
         [0032]    In another embodiment the entire surface of the seal is coated with a low-friction material. Apart from the advantages above, this solution has an advantage in that a core material of the piston seal will never be in contact with any foodstuff. The core material may therefore not necessarily have to be approved for food contact and the sealing properties of the material may then be prioritized when selecting the material for the seal, to the benefit of the performance of the homogenizer in which it is arranged. It may sound contradictory that the fully coated core would affect the sealing properties, yet as an example a comparably soft material with good resilience may provide excellent sealing properties when coated with the low-friction material, while without coating it may not be so good, and as another example: a material with excellent sealing properties but which is sensitive for contact with the fluids present during processing may be fully coated and used. 
         [0033]    In still other embodiments the coating is applied in a non-homogenous layer. This approach may be applied for the partly coated embodiment as well as for the fully coated layer. One embodiment would comprise having a thicker coating on the portions of the seal being subjected to abrasive wear, and a thinner coating on other portions in order to maintain the properties of the base material while still protecting base material from being contacted by fluids present in the piston pump. Typical fluids include but are not limited to, lie, cleaning agents, product, steam, water. 
         [0034]    A variation of the latter embodiment includes having an axially varying thickness of the coating on the radially inner side of the seal. The reason for this may be that the pressure with which the seal is pressed towards the piston may vary with axial position, and this may be accounted for by varying the thickness of the coating. In the present context axial is the direction orthogonal to the radial direction, i.e. the direction of movement of the piston. 
         [0035]    The above embodiments are schematically illustrated in the partial cross sectional views of  FIG. 3 i   - 3   x,  where the reference numeral  220  indicates a representation of the coating in terms of extension and thickness. It should be emphasized that the illustrations are mere schematic indications for facilitating understanding of the invention as exemplified in a few embodiments thereof. 
         [0036]    For the above and further embodiments it may be beneficial that the seal is formed in one piece. Further, it may be beneficial that it lacks hidden crevices or undercut voids in order to avoid accumulation of product or other fluids during operation. 
         [0037]    Embodiments where the piston is fully or partially coated may be combined with embodiments where the seal is fully or partially coated, and vice versa, yet these two classes of embodiments may also be used in isolation. 
         [0038]    The use of a low-friction material for coating the piston seals and/or the piston may result in one or several advantages. The reduction of friction results in less generation of heat, and since heat is a major contributor to degradation of the seals, and the friction will affect the durability of the piston, the measures in isolation or in combination will add to the uptime of the piston pump and of a machine in which it is arranged. A longer uptime will add to the efficiency of the machine. In food-processing downtime is often if not always associated with a cleaning or sterilization of the machine. Cleaning a machine is time consuming and also consumes resources such as cleaning agents, water and energy. Since the time translates into a loss in productivity, the avoidance of downtime can be very rewarding. 
         [0039]    In a further aspect of the invention it relates to a seal for ensuring dynamic seal between moving parts, wherein the seal has the constructional features of the seal described in reference to the piston pump seal. 
         [0040]    An example of a potential coating material is PTFE (polytetrafluoroethylene). PTFE is what is known as GRAS material—Generally Recognized As Safe (under the Federal Food, Drug, and Cosmetic Act, U.S. Food and drug administration), and is allowed to come in contact with foodstuff. Another example could be diamond-like carbon, yet other low-friction material could be used as well, which may preferably be a food-grade material in applications where that may be an issue. “Coating” is used in the context coating in a permanent, non-removable manner, as oppose to applying a layer of lubricant onto the seal and/or piston. The coating may be worn of by abrasive wear over time, still it is considered to correspond to a permanent coating.