A piston assembly for a homogenizing apparatus

Abstract

A piston assembly (1) for a homogenizing apparatus, the piston assembly (1) comprising: - a pumping piston (2) movable back and forth in an axial direction (A-A); - a sealing package (6) surrounding a first end (2a) of pumping piston (2) at a high pressure side (8) and comprising at least two rings of a first type (4) and one ring of a second type (5) which is interposed between the two rings of the first type (4), both the rings of the first type (4) having a first thickness (t1) which is higher than a second thickness (t2) of the ring of the second type (5), the rings of the first type (4) being made of a first material that is harder than a second material constituting the ring of the second type (5); - a spring (9) interposed between the sealing package (6) and the high-pressure side (8).

Description

[0001]DESCRIPTION A PISTON ASSEMBLY FOR A HOMOGENIZING APPARATUS Technical field The present invention relates to a piston assembly for a homogenizing apparatus. The piston assembly may also be used in a high-pressure pump. The invention proposed here can be used in manufacturing fields where homogenization at high pressure is a step of the production process, such as in chemical, pharmaceutical or cosmetic industry. In this context, “high pressure” refers to pressures equal or higher than 600 bar. The invention has been developed to solve some criticalities which are mainly met in homogenization at high pressure, but it may be beneficial in applications with lower pressures as well. The invention can also be used in the food industry, in particular in the dairy sector or food and beverage, where homogenization is involved. Another application of the invention is the production of carbon-based nanostructured materials, such as graphene and carbon nanotubes or cellular breakdown of yeasts, algae, or microorganisms for the production of intracellular material. Background art As it is known, a homogenizing compression head or block comprises a high-pressure pump and a homogenizing valve assembly that act on the fluid products containing particles in order to: - crush the particles to make their dimensions uniform, reducing the average size and the variance of the distribution in order to stabilize the product and to increase its shelf-life in the case of emulsions; - break the cell membranes in order to facilitate the extraction of the active ingredients in the case of pharmaceutical applications; - modify the structure of the particles in the case of chemical applications and cellulose. Beyond the various solutions available on the market, the high-pressure pump is a volumetric pump with pistons that move with a reciprocating motion through a crankshaft (or camshaft), synchronous and phase-shifted from each other by an angle of 360° / n, where n is the number of pistons. In accordance with a known solution, the homogenizing valve, placed downstream of the piston pump, comprises a first chamber receiving the fluid at high pressure from the pump delivery and a second chamber capable of supplying outgoing homogenized fluid at low pressure. The homogenizing action is obtained by forcing the fluid to pass through an interspace or gap with reduced dimensions afforded between the first and the second chamber. The drive is obtained by means of an electric motor which drives the crankshaft through a gear reducer and a suitable kinematic reduction chain. Since the crankshaft is realized with fixed relative angles, the phase shift between the pulses of the pistons is also fixed. The Applicant has also developed a high-pressure homogenizer wherein each piston is associated with a corresponding oleodynamic cylinder, with its hydraulic circuit. An electronic control unit independently regulates the proportional valves of the circuits of each cylinder, imposing a law of motion to the individual pistons. This solution is described in WO 2014 / 097075. In high pressure homogenizing apparatuses and pumps, it is of primary importance assuring a sealing effect between the piston / plunger and the compression chamber, along the whole stroke of the piston during both delivery and suction phases. In the state-of-the-art solutions, the sealing effect is obtained thanks to a single seal element or a package of sealing elements. In addition, at both ends of a piston there may be provided rings made of metal or plastic, which serve either as guide for the piston or anti-extrusion for the packing, or both. One known solution envisages a package of rings arranged in series and having a square section obtained by a twisted rope of fibers made of the following material: graphite, glass or ceramic, vegetals (i.e., cotton, hemp, etc.), aramid fibers such as Kevlar and Nomex, synthetic materials, metals. The fibers are properly lubricated and bonded by anti-friction and anti-corrosion oils or synthetic greases. Even if this solution has good resistance properties and is simple and cheap, it is not easily cleanable, thus being unsuitable for food-grade applications. Another known solution, developed by Garlock in the late 60s, consists in a package of chevron-shaped V-rings, which are suitable for food-grade seals and high pressures. The V-rings may be made of elastomeric materials, optionally fabric-reinforced, or by plastic materials. Another known solution, used in applications where hygienic demands are high, such as pharma and VHP applications, is a sealing device comprising a slider made of UHMW-PE or PTFE which acts as sealing element on the piston, an energizing ring made of elastomeric material which surrounds the slider and pre-loads the slider against the piston in suction and delivery phases, and a backup ring made of highly thermal and mechanical resistant plastic or metal, which creates a support for the first two elements and guides the piston. Nevertheless, sealing in case of abrasive products may still be critical. Another solution, disclosed in document US 6,913,266 B2, is a sealing device comprising: - a sealing ring made of ceramic or hard metal, the sealing ring surrounds the shaft and is spaced from the shaft to form an interior gap; - a deformable pressure ring made of metal or plastic material; the pressure ring forms a static seal resting against a portion of the outer jacket of the sealing ring; - a thrust ring which generates a force in axial direction so that the pressure ring deforms. There is still felt the need to provide better sealings around the piston, in particular at the high-pressure side. As a matter of fact, critical aspects to be considered in designing these sealings are: - chemical and physical compatibility of the sealing materials with the processed fluids; - chemical and physical compatibility of the sealing materials with the cleaning and sterilizing fluids used during CIP and SIP cycles; - resistance to abrasive processed fluids; - resistance to very high pressures, i.e., 2000 bar and more; - resistance to pulsing pressures in the compression chamber; - resistance to high temperatures, for example higher than the melting point of the sealing materials; - resistance to low temperatures, for example lower than 0°C; - resistance to cavitation phenomena; - resistance to scratch which may be caused by mechanical cleaning operations in order to avoid product growth in dead zones or corners; - integrity of the material, which shall avoid dispersion of particles in the processed fluids; - dissipation of the heat generated by the mutual rubbing of the piston and sealings, which may require lubrication fluids and a temperature control thereof. Occurrence of one or more of these conditions, even if for short periods, may produce deformation, melting, rupture, and even definitive damages to the sealings, with degrading of the overall performance of the homogenizing apparatus or high-pressure pump. In practice, the sealings wear out more rapidly and must be replaced with greater frequency, with dead times for the production plants. Disclosure of the invention In this context, the object of the present invention is to provide a piston assembly for a homogenizing apparatus, which overcomes the problems of the prior art cited above. In particular, the object of the present invention is to provide a piston assembly for a homogenizing apparatus, wherein the sealing device has an increased lifetime over the prior art solutions. In particular, the object of the present invention is to provide a piston assembly for a homogenizing apparatus, wherein the maintenance operations, in particular due to substitution of worn gaskets / sealings, are reduced so that dead times for the apparatus may also be reduced. Another object of the present invention is to provide a piston assembly for a homogenizing apparatus, which can better withstand critical factors such as high pressures, pulsing pressures, high / low temperatures, cavitation phenomena, exposure to fibrous fluids or aggressive chemical products, in particular with respect to known solutions. Another object of the present invention is to propose a piston assembly for a homogenizing apparatus, which has better compatibility properties with the processed fluids and / or with the cleaning and sterilizing fluids. Another object of the present invention is to propose a piston assembly for a homogenizing apparatus, which avoids or reduces the user of lubrication fluids for heat dissipation operations. The stated technical task and specified aims are substantially achieved by a piston assembly for a homogenizing apparatus, the piston assembly comprising: - a pumping piston movable back and forth in an axial direction; - a first flange; - a sealing device surrounding a first end of the pumping piston at a high-pressure side, the sealing device being arranged between the first flange and the high-pressure side. Advantageously, the sealing device comprises: - a sealing package in turn comprising at least two rings of a first type and one ring of a second type which is interposed between the two rings of the first type, both the rings of the first type having a first thickness which is higher than a second thickness of the ring of the second type, the rings of the first type being made of a first material that is harder than a second material constituting the ring of the second type; - a spring interposed between the sealing package and the high- pressure side, wherein the spring is configured to exert a compression force on the sealing package so as to compress the sealing package against the first flange. In a preferred embodiment of the invention, the sealing package comprises more than two rings of the first type alternating with a plurality of rings of the second type. All the rings of the first type have a thickness that is the first thickness and are made by the first material. All the rings of the second type have a thickness that is the second thickness and are made by the second material. In particular, each ring of the first type is made of a metal or a metal alloy or a ceramic material, and each ring of the second type is made of a plastic or elastomeric material. According to some examples, each ring of the first type is made of one of the following materials: Alumina, Tungsten Carbide, Silicon Carbide, Silicon Nitride, Zirconia, Stellite alloys, Industrial Diamond, sintered hard materials, Bronze and Nitronic. According to some examples, each ring of the second type is made of one of the following materials: PEEK, PTFE, PVC, PVDF, polyethylene, polyamide, nylon, polyurethane, EPDM, FKM, FF1KM and NBR. Preferably, the first thickness and the second thickness are in a ratio comprised in the range 3:1 to 6:1. In particular, the spring may be a helical spring or a wave spring or a disc- conical spring. In one embodiment of the invention, the piston assembly further comprises: - a connecting element configured to integrally connect a second end of the pumping piston to actuating means; - at least a first spherical coupling member and a second spherical coupling member which are configured to establish a spherical coupling between the second end of the pumping piston and the connecting element. In particular, the spherical coupling members are adjacent and in mutual contact and are fitted between the second end of the pumping piston and the connecting element. In one embodiment of the invention, the first spherical coupling member has a flat surface configured to receive a flat surface of the second end of the pumping piston, and a rounded surface. The second spherical coupling member has a first rounded surface which is countershaped to the rounded surface of the first spherical coupling member so as to match it, and a second rounded surface which is countershaped to a rounded surface of the connecting element so as to match it. In particular, the first and the second spherical coupling members are made of a material having a low friction coefficient and anti-galling properties. According to one embodiment of the invention, the piston assembly further comprises: - a second flange which is mounted at the second end of the pumping piston and against the connecting element, and - a sleeve which surrounds the spherical coupling members. The sleeve is fitted between the second flange and the connecting element. In particular, the second flange is made of stainless steel or a metal and the sleeve is made of a plastic or elastic material. Preferably, the piston assembly further comprises: - a first annular element mounted on the second portion of the pumping piston between the second flange and the sleeve, and - a second annular element mounted on the second portion of the pumping piston and fitted between the first annular element and the second flange. More preferably, the second annular element is partly rounded so as to establish a spherical coupling with a countershaped rounded internal surface of the second flange. In particular, the first annular element is made of stainless steel or a metal and the second annular element is made of a bronze alloy or stainless steel. Brief description of drawings Further characteristics and advantages of the present invention will more fully emerge from the non-limiting description of a preferred but not exclusive embodiment of a piston assembly for a homogenizing apparatus, as illustrated in the accompanying drawings in which: - figure 1 illustrates a piston assembly for a homogenizing apparatus, in a cross-sectional view, according to the present invention; - figure 2 is an enlarged view of a part (sealing device) of the piston assembly of figure 1; - figure 3 is an enlarged view of another part (coupling with the connecting element) of the piston assembly of figure 1; - figure 4 is a variant embodiment of figure 3, in a cross-sectional view; - figure 5 illustrates a compression head for a homogenizing apparatus, with the piston assembly of figure 1, in a cross-sectional view; - figure 6 illustrates the compression head of figure 5, in a perspective view. Detailed description of preferred embodiments of the invention With reference to the figures, number 1 indicates a piston assembly 1 for a homogenizing apparatus or a high-pressure pump. The piston assembly 1 comprises a pumping piston 2 which is linearly movable back and forth in an axial direction A-A between a high-pressure side and an external side. The piston assembly 1 comprises a sealing device 3 which surrounds a first end 2a of the pumping piston 2 at the high-pressure side, indicated with number 8. The sealing device 3 is arranged between a first flange 7 and the high- pressure side 8. Advantageously, the sealing device 3 comprises a sealing package 6 applied on the first end 2a of the piston 2. The sealing package 6 comprises at least two rings of a first type, indicated with number 4, and one ring of a second type, indicated with number 5. The ring of the second type 5 is interposed between the two rings of the first type 4 so as to form the sealing package 6. More preferably, the sealing package 6 comprises a plurality of rings of the first type 4 and a plurality of rings of the second type 5 distributed according to an alternating arrangement. This means that each ring of the second type 5 is interposed between a pair of consecutive rings of the first type 4. Thus, the number of rings of the first type 4 is one more than the number of rings of the second type 5. Preferably, the number of rings of the first type 4 is from four to six, whereas the number of rings of the second type 5 is from three to five. Preferably, all the rings of the first type 4 are identical for material and dimensions. In particular, all the rings of the first type 4 are made by a first material and have a first thickness t1. Preferably, all the rings of the second type 5 are identical for material and dimensions. In particular, all the rings of the second type 5 are made by a second material and have a second thickness t2. According to one aspect of the present invention, the rings of the first type 4 differ from the rings of the second type 5 for material and dimensions, in particular for thickness. Concerning the materials, the first material is harder than the second material. According to one embodiment of the invention, the first material is a metal or a metal alloy or a ceramic material, whereas the second material is a plastic or elastomeric material. In some examples, the first material used for the rings of the first type 4 is one of the following: Alumina, Tungsten Carbide, Silicon Carbide, Silicon Nitride, Zirconia, Stellite alloys, Industrial Diamond, sintered hard materials, Bronze and Nitronic. In some examples, the second material used for the rings of the second type 5 is one of the following: PEEK, PTFE, PVC, PVDF, polyethylene, polyamide, nylon, polyurethane, EPDM, FKM, FFKM and NBR. Concerning the dimensions, the first thickness t1 is higher than the second thickness t2. Preferably, the ratio of the first thickness t1 to the second thickness t2 is comprised in the range 3:1 to 6:1. The rings of the first type 4 assure high precision in terms of dimensions, geometry, and surface finishing, whereas the rings of the second type 5 allow a uniform and optimal transmission of the pressure load along the sealing package 6. According to one embodiment of the invention, the pumping piston 2 is made of the same material chosen for the rings of the first type 4. The sealing device 3 further comprises a spring 9 which is interposed between the high-pressure side 8 and the sealing package 6. The spring 9 is a compression spring which is configured to exert a force on the sealing device 3 towards the first flange 7 so that the sealing package 6 presses against the first flange 7. This results in compressing the rings of the sealing package 6 against the first flange 7, which avoids movements during the suction phases and during the reciprocation movements of the pumping piston 2. In one embodiment of the invention, the spring 9 is a helical spring. Alternatively, the spring 9 may be a wave spring or a disc-conical spring. The arrangement of the sealing package 6 with alternating types of rings and of the spring allows to obtain a labyrinth dynamic sealing device. This means that it is tolerated a continuous but slight leakage of product, which is maintained under control by the sealing device 3. According to one embodiment of the invention, at the high-pressure side 8 is arranged a gasket comprising a double tapered hollow body 18 with a rotational symmetry. The reciprocating movement of the pumping piston 2 is obtained thanks to an integral connection of the pumping piston 2 with actuating means. In particular, the actuating means (not shown) are connected to a second 2b end of the pumping piston 2, which is opposite to the first end 2a, by means of a connecting element 10. Advantageously, the pumping piston 2 and the connecting element 10 are coupled by means of at least two spherical coupling members 11, 12 configured to obtain a spherical coupling between the second end 2b of the pumping piston 2 and the connecting element 10. The two spherical coupling members 11, 12 are adjacent and in mutual contact, and are fitted between the second end 2b of the pumping piston 2 and the connecting element 10. In particular, a first spherical coupling member 11 has: - a flat surface 11a in order to receive the flat surface of the second end 2b of the pumping piston 2, and - a rounded surface 11b. The second spherical coupling member 12 has: - a first rounded surface 12a which is countershaped to the rounded surface 11b of the first spherical coupling member 11 so as to match it, and - a second rounded surface 12b which is countershaped to a rounded surface 10a of the connecting element 10 so as to match it. In particular, the first rounded surface 12a of the second spherical member 12 is innerly rounded whereas the rounded surface 11b of the first spherical coupling member 11 is outerly rounded. Thus, the first rounded surface 12a of the second spherical member 12 receives the rounded surface 11b of the first spherical coupling member 11. The second rounded surface 12b of the second spherical member 12 is outerly rounded, whereas the rounded surface 10a of the connecting element 10 is innerly rounded. Thus, the rounded surface 10a of the connecting element 10 receives the second rounded surface 12b of the second spherical member 12. According to one aspect of the invention, the first and the second spherical coupling members 11, 12 are made of a material having a low friction coefficient and anti-galling properties. For example, the coupling members 11, 12 are made of bronze alloys or Stainless steel, such as Nitronic 60. The spherical coupling members 11, 12 so configured and the specific choice of material allow to obtain a high resistance to compressions and to avoid gripping phenomena. In addition, during the pumping phase the coupling members 11, 12 allow to dynamically compensate misalignments between the pumping piston 2 and the connecting element 10 (and the actuating means, which is also a piston), along the whole stroke of the pumping piston 2. In this context, misalignments are intended as deviations from coaxiality in the axial direction A-A, or concentricity deviations or any other offset between the pumping piston 2 and the connecting element 10 and / or the piston of the actuating means. In alternative embodiments, there could be more than two spherical coupling members for coupling the second end 2b of the pumping piston 2 and the connecting element 10. According to one aspect of the invention, a second flange 13 is mounted at the second end 2b of the pumping piston 2 and against the connecting element 10. The second flange 13 is fixed to the connecting element 10 for example via a threated coupling or by screws. The spherical coupling members 11, 12 are surrounded by a sleeve 14 which is fitted between the second flange 13 and the connecting element 10. The second flange 13 is preferably made of stainless steel or a metal. The sleeve 14 is preferably made of a plastic or elastic material and serves for simplifying the assembly of the spherical coupling members 11, 12. In order to assure a dynamical compensation of misalignments during the suction phase, the piston assembly 1 further comprises the following elements: - a first annular element 15 mounted on the second portion 2b of the pumping piston 2, in particular between the second flange 13 and the sleeve 14, and - a second annular element 16 mounted on the second portion 2b of the pumping piston 2 and fitted between the first annular element 15 and the second flange 13. Preferably, the first annular element 15 is shrink-fitted on the second portion 2b of the piston 2 or mounted by a double cone coupling joint (as in FIG.4). In particular, the second annular element 16 is partly rounded so as to establish a spherical coupling with a countershaped rounded internal surface of the second flange 13. The first annular element 15 is preferably made of stainless steel or a metal. The second annular element 16 is preferably made of a material having a low friction coefficient and anti-galling properties such as bronze alloys or Stainless steel, such as Nitronic 60. With the proposed solution, the pumping piston 2 may be fully reversible, changing the position of the first end 2a with the second end 2b if one of them is more worn than the other, especially at the high-pressure side 8. With reference to figure 5, number 100 indicates a compression head or block comprising the proposed pumping assembly 1. The characteristics of a piston assembly for a homogenizing apparatus, according to the present invention, are clear, as are the advantages. In particular, the proposed solution provides a labyrinth dynamic sealing device which allows keeping under control a slight, constant leakage, thus maintaining a good performance of the high-pressure pump / homogenizer. The specific design of the sealing package and the choice of materials allow to achieve an increased lifetime for the sealing device over the prior art solutions. In addition, the materials employed for the sealing package allows to better withstand high pressures, pulsing pressures, high / low temperatures, cavitation phenomena, exposure to fibrous fluids or to aggressive chemical products. In addition, the integral connection of the pumping piston with the actuating means is improved in order to allow a better resistance to inversion of the movements of the piston, and a better protection against gripping phenomena. In particular, compensation of axial misalignments during the pumping phase is achieved thanks to the two spherical coupling members with low friction coefficient between the pumping piston and the actuating means (in particular the connecting element). Furthermore, compensation of axial misalignments during the suction phase is assured by the two annular elements between the second flange and sleeve. In practice, this solution does not aim to avoid misalignments, which are inevitable; rather it proposes a good way to compensate them. This allows to reduce radial loads on the elements, in particular on the sealing package, thus also contributing to increasing the lifetime of the sealing device itself and of the pumping piston.

Claims

CLAIMS 1. A piston assembly (1) for a homogenizing apparatus, the piston assembly (1) comprising: - a pumping piston (2) movable back and forth in an axial direction (A-A); - a first flange (7); - a sealing device (3) surrounding a first end (2a) of the pumping piston (2) at a high-pressure side (8), said sealing device (3) being arranged between the first flange (7) and the high-pressure side (8), characterized in that the sealing device (3) comprises: - a sealing package (6) in turn comprising at least two rings of a first type (4) and one ring of a second type (5) which is interposed between the two rings of the first type (4), both the rings of the first type (4) having a first thickness (t1) which is higher than a second thickness (t2) of the ring of the second type (5), the rings of the first type (4) being made of a first material that is harder than a second material constituting the ring of the second type (5); - a spring (9) interposed between the sealing package (6) and the high- pressure side (8), wherein the spring (9) is configured to exert a compression force on the sealing package (6) so as to compress the sealing package (6) against the first flange (7).

2. The piston assembly (1) according to claim 1, wherein the sealing package (6) comprises more than two rings of the first type (4) alternating with a plurality of rings of the second type (5), all the rings of the first type (4) having a thickness that is said first thickness (t1) and being made by said first material, all the rings of the second type (5) having a thickness that is said second thickness (t2) and being made by said second material.

3. The piston assembly (1) according to claim 2, wherein each ring of the first type (4) is made of a metal or a metal alloy or a ceramic material, and each ring of the second type (5) is made of a plastic or elastomeric material.

4. The piston assembly (1) according to claim 3, wherein each ring of thefirst type (4) is made of one of the following materials: Alumina, Tungsten Carbide, Silicon Carbide, Silicon Nitride, Zirconia, Stellite alloys, Industrial Diamond, sintered hard materials, Bronze and Nitronic, each ring of the second type (5) being made of one of the following materials: PEEK, PTFE, PVC, PVDF, polyethylene, polyamide, nylon, polyurethane, EPDM, FKM, FF1KM and NBR.

5. The piston assembly (1) according to any one of the preceding claims, wherein the first thickness (t1) and the second thickness (t2) are in a ratio comprised in the range 3:1 to 6:

1.

6. The piston assembly (1) according to any one of the preceding claims, wherein the spring (9) is a helical spring or a wave spring or a disc-conical spring.

7. The piston assembly (1) according to any one of the preceding claims, further comprising: - a connecting element (10) configured to integrally connect a second end (2b) of the pumping piston (2) to actuating means; - at least a first spherical coupling member (11) and a second spherical coupling member (12) which are configured to establish a spherical coupling between the second end (2b) of the pumping piston (2) and the connecting element (10).

8. The piston assembly (1) according to claim 7, wherein said spherical coupling members (11, 12) are adjacent and in mutual contact and are fitted between the second end (2b) of the pumping piston (2) and the connecting element (10).

9. The piston assembly (1) according to claim 8, wherein the first spherical coupling member (11) has a flat surface (11a) configured to receive a flat surface of the second end (2b) of the pumping piston (2), and a rounded surface (11b), said second spherical coupling member (12) having a first rounded surface (12a) which is countershaped to the rounded surface (11b) of the first spherical coupling member (11) so as to match it, and a second rounded surface (12b) which is countershaped to a roundedsurface (10a) of the connecting element (10) so as to match it.

10. The piston assembly (1) according to any one of the claims 7 to 9, wherein the first and the second spherical coupling members (11, 12) are made of a material having a low friction coefficient and anti-galling properties.

11. The piston assembly (1) according to any one of claims 7 or 10, further comprising: - a second flange (13) which is mounted at the second end (2b) of the pumping piston (2) and against the connecting element (10), and - a sleeve (14) which surrounds the spherical coupling members (11, 12), said sleeve (14) being fitted between the second flange (13) and the connecting element (10).

12. The piston assembly (1) according to claim 11, wherein the second flange (13) is made of stainless steel or a metal and the sleeve (14) is made of a plastic or elastic material.

13. The piston assembly (1) according to claim 11 or 12, further comprising: - a first annular element (15) mounted on the second portion (2b) of the pumping piston (2) between the second flange (13) and the sleeve (14), and - a second annular element (16) mounted on the second portion (2b) of the pumping piston (2) and fitted between the first annular element (15) and the second flange (13).

14. The piston assembly (1) according to claim 13, wherein the second annular element (16) is partly rounded so as to establish a spherical coupling with a countershaped rounded internal surface of the second flange (13).

15. The piston assembly (1) according to claim 13 or 14, wherein the first annular element (15) is made of stainless steel or a metal and the second annular element (16) is made of a bronze alloy or stainless steel.

16. A compression head (100) for a homogenizing apparatus, thecompression head (100) comprising a piston assembly (1) according to any one of the preceding claims.

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