EP3080452A1

EP3080452A1 - Reciprocating compressor with composite material components

Info

Publication number: EP3080452A1
Application number: EP14808620.0A
Authority: EP
Inventors: Francesco Chiesi; Leonardo Tognarelli; Michele Sanesi; Massimo BARGIACCHI; Riccardo Bagagli
Original assignee: Nuovo Pignone SpA; Nuovo Pignone SRL
Current assignee: Nuovo Pignone SpA; Nuovo Pignone SRL
Priority date: 2013-12-09
Filing date: 2014-12-05
Publication date: 2016-10-19
Anticipated expiration: 2034-12-05
Also published as: EP3080452B1; WO2015086458A1; ITFI20130296A1

Abstract

A double-effect reciprocating compressor is described. The compressor comprises a cylinder (3) and a reciprocatingly moving arrangement comprised of a piston (7), a crosshead (37) and a piston rod (15) connecting the crosshead to the piston. A crank-shaft (31) drives the reciprocatingly moving arrangement into reciprocating motion via a connecting rod (33). At least one part of the piston and of the piston rod is at least partly made of a composite material.

Description

RECIPROCATING COMPRESSOR WITH COMPOSITE MATERIAL

COMPONENTS

DESCRIPTION

TECHNICAL FIELD

The subject matter disclosed herein generally relates to improvements to reciprocating machines. Embodiments disclosed herein specifically refer to reciprocating compressors, such as double-effect reciprocating compressors. Some embodiments of the reciprocating compressors disclosed herein comprise a reciprocatingly moving arrangement including a crosshead, a connecting rod which connects the crosshead to a crankshaft, and a piston rod connecting the crosshead to a piston reciprocatingly moving in a cylinder.

BACKGROUND ART

Reciprocating compressors are widely used in a large number of industrial applications, such as process applications including refineries, petrol chemicals, fertilizers, refrigeration and air, as well as in the gas and oil industry, for gas re-injection, gas lift, pipeline gas transmission and gas storage.

Reciprocating compressors usually comprise a cylinder wherein a piston is slidingly housed and controlled to move reciprocatingly in the cylinder. In so-called double- effect reciprocating compressors, the piston divides the cylinder into two pressure chambers, which are both connected to a suction duct and a delivery duct through automatic valves, so that in each of the two chambers formed by the cylinder and by the reciprocating piston arranged therein, gas is alternatively sucked from the suction duct, compressed and delivered through the delivery duct. The piston movement is controlled by a crankshaft, connected to the piston through a series of components. In large reciprocating compressors the reciprocating movement of the piston is transmitted by the crankshaft through a connecting rod, which connects the crankshaft to a crosshead. A rotary motion applied to the crankshaft is transformed into a reciprocating rectilinear motion of the crosshead, which is slidingly arranged in a casing. More- over, a piston rod rigidly connects the crosshead to the piston, so that the reciprocating motion is transmitted to the piston. The crosshead, the piston rod and the piston form a reciprocatingly moving arrangement. Due to the dimensions of this kind of reciprocating compressors, the mass of the reciprocatingly moving arrangement is ex- tremely large and this causes large inertial loads.

In order to withstand the high reciprocating loads generated by inertia and by the gas pressure in the cylinder, the piston rod must have a large cross-section, which contributes to the total mass and weight of the reciprocatingly moving arrangement.

The weight of the reciprocatingly moving piston and a portion of the weight of the piston rod are supported by annular sliding shoes provided around the piston and slid- ingly contacting the inner surface of the cylinder.

The sliding shoes must be dimensioned so as to meet certain requirements in terms of specific pressure and component life. The axial dimension of the sliding shoes, i.e. their extension parallel to the direction of the piston movement is determined by the need to keep the specific pressure under a certain threshold. The axial dimension of the sliding shoes adds to the total axial length of the piston. The axial length of the piston must be sufficient to accommodate the sliding shoes and the piston rings there around. The axial dimension of the piston in turn increases the overall dimension of the reciprocating machine, since the cylinder must be long enough to house the piston and to provide for a sufficiently large volume between the piston and the cylinder heads. The increased axial dimension of the piston also increases the weight thereof.

Above certain specific pressure values, lubrication of the sliding shoes is required. This increases manufacturing and operating costs of the machine, and adversely affects the machine reliability. Fig.l illustrates an exemplary embodiment of a piston and piston rod arrangement according to the current art. The arrangement is labeled 101 as a whole and comprises a piston or piston head 103 constrained to a piston rod 105. The piston rod 105 comprises a first rod end 105 A connected to the piston 103 and a second rod end 105B, which is connected to a crosshead, not shown. The piston 103 comprises a central through hole 107, where through the piston rod 105 extends. The piston 103 is constrained to the piston rod 105 by means of a screw nut 109 screwed on a threaded end of the piston rod 105, locking the piston 103 between the screw nut 109 and a shoulder 11 1 formed on the piston rod 105. The body of the piston 103 is usually hollow and has a cavity 113, in order to reduce the weight thereof. The body of the piston 103 is usually formed by a plurality of components, which are welded to one another forming an integral body. In the embodiment illustrates in Fig.l, the piston body 103 is formed by a first inner core 115, an outer cylindrical wall 117 and two intermediate annular members 119 A, 119B. In the outer cylindrical wall 117 annular seats 121 are formed, wherein respective piston rings 123 are housed. The piston rings 123 co-act with the inner surface of the cylinder, wherein the piston 103 is slidingly arranged and provide a sealing effect to separate the two variable- volume chambers, into which the inner volume of the cylinder is divided by the piston 103. On the outer surface of the outer cylindrical wall 117 fur- ther annular seats 125 are provided, housing respective sliding shoes 127. The number, the axial dimension and the position of the sliding shoes 127 depend upon the weight of the reciprocatingly moving components of the reciprocating compressor, and more specifically of the piston 101 and of the piston rod 105, and upon other design constraints and considerations. The axial dimension A of the piston must therefore be sufficient to house the required number of piston rings 123 and the required number of sliding shoes 127.

SUMMARY OF THE INVENTION

According to some embodiments of the present disclosure, a double-effect reciprocating compressor is provided, comprising a cylinder, a crankshaft and a reciprocatingly moving arrangement driven into reciprocating movement by the crankshaft. The reciprocating moving arrangement in turn comprises a piston having a piston body and slidingly moving in the cylinder, a crosshead and a piston rod connecting the cross- head to the piston. A connecting rod connects the crosshead to the crankshaft and transmits the movement of the crankshaft to the crosshead. Sliding shoes and/or piston rings are usually mounted on the piston body of the reciprocatingly moving piston, in annular grooves or seats provided around the piston body. At least one of said piston body and said piston rod is at least partly made of a composite material. In preferred embodiments, the composite material is a fiber-reinforced polymeric material. According to some embodiments, the composite material is selected from the group consisting of: a carbon-fibers reinforced polymer; a glass-fibers reinforced polymer; an aramidic-fibers reinforced polymer. A combination of different composite materials and/or of different reinforcing fibers in the same composite material can also be used. In some particularly advantageous embodiments, a reciprocatingly, double-effect compressor is provided, wherein the piston rod comprises at least a portion made of composite material and a portion made of metal and forming a rod end. The metal rod end can be the one constrained to the crosshead, or the one constrained to the piston body. In some embodiments, both ends of the piston rod can be made of metal. By providing rod end(s) made of metal, said parts of the piston rod can be designed according to current design and can be connected to existing pistons and/or cross- heads, so that a piston rod partly made of fiber-reinforced polymeric material can be replaced for a full-metal piston rod of the current art in existing machines, without modifying other machine components. The metal rod end(s) and the intermediate portion of the piston rod made of composite material can be connected or jointed to one another in several ways. In some embodiments, composite material and metal are joined by gluing. In other embodiments, connection is made by co-curing, i.e. the composite material is applied on the metal portion of the piston rod and the polymer is cured afterwards, thus forming a strong bond with the metal part of the piston rod.

In particularly advantageous embodiments, the interface between metal and composite material is in the form of a so-called scarf joint, i.e. the two materials are in mutual contact and bonded to one another along inclined surfaces, such as conical surfaces, forming a shallow angle, e.g. of around 0.5° - 10°, with the axis of the piston rod. The portion of the piston rod made of composite material has preferably a tubular structure, e.g. with a hollow inner volume, or with an inner volume filled with a core made of a different, preferably less expensive and less performing material, such as a filling expanded polymeric resin. The core can be a winding mandrel used for the manufacturing of the external tubular structure by filament winding.

In some embodiments, the piston body of the reciprocating compressor can have a top skin portion made of composite material and/or a bottom skin portion made of composite material. As used herein, the terms top skin portion and the top skin portion designate the portions of the piston body which face the two opposite chambers into which the reciprocating piston divides the inner volume of the compressor cylinder. Usually, the top skin portion designates the ceiling of the piston facing the head end chamber of the cylinder and the bottom skin portion designates the opposite piston body end, facing the crank end chamber.

According to some embodiments, the piston body further comprises a side skin por- tion made of composite material, which extends between the top skin portion and the bottom skin portion and forms at least one annular seat for at least one piston ring or a sliding shoe. In yet further embodiments the side skin portion can be combined with a side metal insert forming at least part of the side surface of the piston body, and where around annular seats for one or more piston rings and/or for one or more sliding shoes are provided.

The piston body can further comprise an inner component made of metal and bonded or jointed to the surrounding parts made of composite material. The inner metal component can be provided for connection of the piston body with the piston rod. In some embodiments, the inner metal component of the piston body can in turn form an end portion of a piston rod partly made of composite material.

Joining between the various metal and polymeric parts, sections and components of the piston body can be by gluing, co-curing or the like. In preferred embodiments, scarf joints are provided at least at some of the polymer/metal interfaces, for improving the mechanical stress distribution. Features and embodiments are disclosed here below and are further set forth in the appended claims, which form an integral part of the present description. The above brief description sets forth features of the various embodiments of the present invention in order that the detailed description that follows may be better understood and in order that the present contributions to the art may be better appreciated. There are, of course, other features of the invention that will be described hereinafter and which will be set forth in the appended claims. In this respect, before explaining several embodiments of the invention in details, it is understood that the various embodiments of the invention are not limited in their application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. As such, those skilled in the art will appreciate that the conception, upon which the disclosure is based, may readily be utilized as a basis for designing other structures, methods, and/or systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when con- sidered in connection with the accompanying drawings, wherein:

Fig. 1 illustrates a section of a piston and piston rod assembly according to the current art;

Fig.2 illustrates a section of a reciprocating double-effect compressor; Fig.3 illustrates a perspective view of a piston and piston rod assembly according to one embodiment of the subject matter disclosed herein;

Fig.4 illustrates one embodiment of an end portion of the piston rod of Fig.3;

Fig.5 illustrates an embodiment of a scarf joint between the end portion and the inter- mediate portion of a piston rod according to the present disclosure;

Figs 6 to 9 illustrate sections of alternative embodiments of a piston according to the present disclosure;

Fig. 10 illustrates an embodiment of a piston rod according to the present disclosure in a longitudinal sectional view. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

Reference throughout the specification to "one embodiment" or "an embodiment" or "some embodiments" means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase "in one embodiment" or "in an embodiment" or "in some embodiments" in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

Referring to Fig.2, in one embodiment a reciprocating compressor 1 comprises a cyl- inder 3 having an inner cylindrical cavity 5 housing a piston 7. The piston 7 is recip- rocatingly moving inside the cavity 5 according to double arrow f7. The compressor can be comprised of more than one cylinder-piston arrangement, possibly driven by a common crankshaft. Here below only one cylinder-piston arrangement will be described in detail, it being understood that the features disclosed herein after can be implemented in any number of cylinder-piston arrangements of a multi-cylinder reciprocating compressor. The piston comprises a piston body. In general terms, the piston body is the main component of the piston, supporting the mechanical stresses generated during the gas compression process. The piston body can be provided with piston rings having the function of sealingly separating two chambers into which the piston divides the cavity 5 of the cylinder 3. As will be described in greater detail below, reference being made to some exemplary embodiments, the piston body can further be provided with one or more sliding shoes.

The cavity 5 is closed at both ends by respective closure elements 9 and 11, which are constrained to a cylindrical barrel 13. The closure element 11 has a passage through which a piston rod 15 extends. Packing cups 17 provide a sealing around the piston rod 15. The piston 7 divides the inner cavity 5 of the cylinder 13 into respective first chamber 19 and second chamber 21, also named head end chamber and crank end chamber, respectively.

Each one of the first and second chambers 19 and 21 is connected through respective suction valves and discharge valves to a suction duct and a discharge duct. In some embodiments the suction valves and the discharge valves can be automatic valves, for example so-called ring valves or the like. Each valve is arranged in a valve assembly comprised of shutter rings or plates, cage and valve cover, as known to those skilled in the art. Suction valve arrangements for the first and second chambers 19 and 21 are labeled 23 and 25, respectively. A discharge valve assembly for the first chamber 19 is shown at 27, while the discharge valve assembly of the second chamber 21 is shown at 29. The number of suction and discharge valves for each one of the two chambers 19 and 21 can be different, depending upon the dimension and design of the reciprocating compressor.

The reciprocating movement of the piston 7 and of the piston rod 15 is controlled by a crankshaft 31 through a connecting rod 33. The connecting rod 33 is hinged at 35 to a crosshead 37 provided with crosshead sliding shoes 39 in sliding contact with sliding surfaces 41. The rotation movement of the crankshaft 31 is converted into reciprocating rectilinear movement of the crosshead 37 according to double arrow f37. The piston rod 15 is connected with a first end 15 A to the crosshead 37 and with a second end 15B to the piston 7 and transmits the movement from the crosshead 37 to the pis- ton 7.

According to some embodiments the piston rod 15 is at least partly made of composite material. In some embodiments, the composite material comprises a fiber- reinforced plastic material, including a polymeric material matrix or layer embedding reinforcing fibers. The reinforcing fibers can be glass fibers. In other embodiments the reinforcing fibers can be carbon fibers. In yet further embodiments the reinforcing fibers can be aramidic fibers. A combination of two or more types of fibers can also be used, for example depending upon the mechanical structural requirements and constraints. The fiber length, cross-sectional dimensions and density in the polymeric material can be adjusted or selected according to design constraints or other considera- tions.

The reinforcing fibers can be distributed in form of staple fibers, i.e. short elongated elements, in the polymeric base material. In some embodiments, the fibers can be oriented, i.e. they may have a predominant orientation in order to increase the mechanical properties of the piston rod in a specific direction. In some embodiments the fibers can be in the form of continuous filaments embedded in a polymeric base material. The continuous filaments can be helically wound around an axis of the piston rod. In some embodiments the fibers and the polymeric material can be helically wound around a forming mandrel.

The polymeric material can be cured and formed into a piston rod. Curing can be as- sisted, accelerated or promoted by using a suitable source of energy, as known to those skilled in the art of polymeric materials.

Several polymeric resins can be used to form the matrix of the composite material. Different resins can be selected based on temperature constraints. In non-limiting exemplary embodiments, suitable polymeric resins can be selected from the group con- sisting of: cyanate esters for low or high temperature applications; low-temperature epoxy resins; high-temperature epoxy resins; bis-maleimides (BMI) with post curing; polyether ether ketone (PEEK) organic polymers; polyamide. In particular polyamides for high temperature applications and cyanate esters for high temperature applications can be used up to 300°C, while cyanate esters for low temperature applications can be used where the temperature does not exceed e.g. 100°C. The remaining polymers can be used for intermediate temperature ranges.

In particularly advantageous embodiments, in order to provide a piston rod 15 which is at least partly made of a composite material, without modifying the overall design of the crosshead and of the connection between the crosshead and the piston rod, said piston rod can be comprised of a first end portion 15A made of metallic material, for example steel, such as carbon steel, and being designed according to the current art. At least part of the remaining portion of the piston rod can be made of composite material. This allows for example easy retrofitting of existing reciprocating compressors, without the need for replacing the crosshead.

In some further embodiments, a second end of the piston rod facing the piston and connected to the piston can be made of metal, such as steel. This allows retaining the current design of the piston rod/piston connection, without requiring re-designing of this component. The remaining intermediate portion of the piston rod, between two metal end portions, can be made of lighter composite material, comprised of a polymeric base material and reinforcing fibers embedded therein.

When the piston rod is made partly of composite material and partly of metal, the total length of the piston rod portion made of composite material can be selected based on the following considerations. Increasing the length the piston rod portion made of composite material and reducing the length of the portions made of metal reduces the weight and the mass of the piston rod and thus of the entire reciprocatingly moving components of the compressor, beneficially affecting the operation of the machine, since the dynamic stresses on the machine components and the specific pressure supported by the sliding shoes are reduced. On the other hand, composite materials have usually a lower stiffness than metal material, e.g. steel. A piston rod at least partly made of composite material is thus subject to higher axial deformations during operation, which increases the clearance volume and reduces the efficiency of the reciprocating compressor. The length of the piston rod portion made of composite material with respect to the total length of the piston rod is thus selected based on a compro- mise between weight and mass reduction on the one hand and maintenance of sufficient compressive stiffness on the other.

In some embodiments, different composite materials can be used for different sections or portions of the piston rod 15. For example, a composite material having a higher thermal resistance can be used for the portion of the piston rod 15 which during the reciprocating movement thereof is moved inside the cylinder 3, where higher temperatures are present due to the process of compressing the gas. Gas temperatures typically ranging between 100°C and 200°C can be reached in the reciprocating compressor chambers 19, 21. Thermally less performing and less expensive composite material can be used for the portion of the piston rod which remains outside the cylin- der.

Fig.3 illustrates a schematic perspective view of a piston 7 and relevant piston rod 15, wherein both the piston 7 and a portion of the piston rod 15 are made of composite material. The end portion 15A of the piston rod 15 is made of metal, for example steel. Exemplary embodiments of the structure of the composite material piston will be described later on.

Fig.4 illustrates a schematic section, along axis A- A, of the end of the piston rod 15 in one exemplary embodiment. The end portion 15 A can be made of steel and forms the connection member between the piston rod 15 and the crosshead 37.

At least the remaining central or intermediate portion of the piston rod 15 can be made of a tubular structure 15T of composite material. The interior of the tubular structure 15T can be hollow. In some embodiments the tubular structure 15T can be formed by filament winding around a mandrel. The mandrel can be removed after curing the tubular structure 15T. In some embodiments, a suitable coating layer can be provided on the surface of the mandrel, for easing the removal thereof from the tubular struc- ture after curing. The mandrel can be made of a light material, for example expanded polyurethane, or other closed or open cell expanded resins. In advantageous embodiments, the winding mandrel or core can remain inside the tubular structure 15T.

Especially if the winding mandrel or core remains in the tubular structure 15T, the material the mandrel is made of is preferably selected so as to have a low specific weight, preferably a specific weight lower than the composite material. In some embodiments, the composite material can have a specific weight of less than 4 kg/dm³ and preferably less than 3.5 kg/dm³. In some embodiments the material forming the winding core or mandrel can have a specific weight of less than 3, preferably less than 2 and even more preferably less than 1 kg/dm³. Filament winding is known from the art and mainly consists in winding a plurality of continuous filaments in helical fashion around the axis of a winding mandrel, thus forming one or more layers of wound continuous filaments in a matrix of polymeric material, which is subsequently cured so that a final stiff structure is obtained. The polymeric material can be dispensed together with the continuous filaments. Winding can be performed layer-by- layer, so that a thicker tubular structure is obtained by superposing a plurality of thinner individual layers of filaments and polymeric base material. A previously wound layer can be partly or entirely cured before winding the subsequent layer. In other embodiments, curing of the polymeric material forming the matrix of the structure can be performed at the end of winding of all layers. Winding can be performed with a variable winding angle. In some embodiments the winding angle can be constant along the entire layer, but a variable angle can be used for sequentially wound layers. In some embodiments at least two layers of filaments and polymeric material are wound with opposite winding angles.

The metal end portion 15 A can form part of the winding mandrel. In some embodi- ments, the mechanical connection between the tubular structure 15T of the piston rod 15 and the metal end portion 15A can be obtained by curing the polymeric material forming the composite part of the piston rod. This kind of process is known as "co- curing". In some embodiments, a layer of bonding polymeric resin, different from the polymeric resin forming the main layer of reinforced composite material, can be add- ed at the interface between the surface of the metal end portion 15A and the composite material, for improving the adhesion between the two different materials.

In other embodiments, glue can be used for connecting the tubular structure 15T of the piston rod to the end portion 15A thereof. This is particularly suitable when the tubular structure 15T is formed separately, by means of filament winding or in any other manner, and connected to the metal end portion 15A after curing.

In some embodiments the joint between the metal end portion 15A and the tubular structure 15T made of composite material forming the main body of the piston rod can be in the form of a so called scarf joint, as schematically shown in Fig.5. In this exemplary embodiment both components 15A and 15T of the piston rod 15 are hollow, but in some embodiments an inner core of light material can be provided, especially in the tubular structure 15T, which can be generated by filament winding around such core or mandrel, which is then left inside the tubular structure 15T of the piston rod 15. As shown in Fig.5, the interface between the tubular structure 15T made of composite material and the metal end portion 15A is substantially conical and labeled 45. The end portion 15A has an end frustum conical convex surface, contacting a corresponding frustum conical concave surface formed at the end of the tubular structure 15T. At the interface 45 a layer of a suitable adhesive or glue 47 can be provided. The angle a of the tapering surfaces forming the interface 45 can be for example higher than 0° and lower than 15°, preferably higher than 0° and lower than 10°. It should be understood that these values are given by way of example and should not be considered as limiting the scope of the present disclosure.

A small angle a of the scarf joint results in a better distribution of the compressive and tensile stresses generated on the piston rod 15 during operation of the compressor.

A scarf joint can be used also in cases where the tubular structure 15T and the end portion 15A are joined by co-curing, as mentioned above. Manufacturing the major portion of the piston rod 15 with a composite material, possibly with a tubular hollow or substantially hollow structure, reduces the overall weight and mass of the piston rod 15 and contributes to the reduction of the specific pressure on the sliding shoes of the piston. In some embodiments the piston can be designed according to the current art as shown in Fig.l, i.e. it can be made entirely of metal, for example steel.

According to preferred embodiments, at least part of the body of piston 7 is made of composite material. In some embodiments a piston 7 at least partly made of composite material can be combined with a piston rod 15 entirely or partly made of composite material as described herein above with reference to Figs.3, 4 and 5.

According to further embodiments, a piston 7 at least partly made of composite material can be constrained to a piston rod 15 according to the current art, i.e. entirely of metal, such as piston rod 105 of Fig.l .

Fig.6 illustrates a longitudinal section of a piston 7, the body of which is mainly made of a composite material, for example a fiber-reinforced plastic, such as a carbon- fibers reinforced polymer, or glass-fibers reinforced polymer, or aramidic-fibers reinforced polymer, or combinations thereof. In the embodiment of Fig.6 the piston 7 is mechanically constrained to a piston rod 15 designed according to the prior art, i.e. entirely made of steel or other suitable metal material. The piston 7 may have a central sleeve 51 made of metal, such as steel or the like. An outer main body 53 having a broadly cylindrical shape and made of composite material is bonded to the metal sleeve 51. An interface 55 can be provided between the main body 53 and the sleeve 51. Bonding between the sleeve 51 and the main body 53 made of composite material on interface 55 can be by gluing or co-curing. The end 15E of the piston rod 15 can be threaded and a nut 57 can be screw-engaged on the threaded end 15E of the piston rod 15 to mechanically constrain the piston 7 to the piston rod 15. The latter can be provided with a shoulder 59 and the metal sleeve 51 of the piston can be locked between the shoulder 59 of the piston rod 15 and the screwed nut 57. In some embodiments, the piston rod can be made only partly of metal, e.g. only the end portion thereof attached to the piston 7 can be made of metal, while the remaining intermediate portion thereof can be made of fiber-reinforced polymeric material as described above, forming a tubular structure, such as structure 15T. The interior of the main body 53 of the piston 7 can be hollow or can be filled with an inner core made of light material, for example an expanded resin, such as expanded polyurethane or the like. A light inner material having a specific weight lower than the specific weight of the composite material is preferred.

The piston 7 can be comprised of a top skin portion 53T forming a front planar or substantially planar surface 61 , facing the first chamber 19 of the cylinder wherein the piston 7 is slidingly housed. The main body 53 of the piston 7 can further be provided with a bottom skin portion 53B forming a substantially planar surface 63 facing the second chamber 21 of the cylinder 3.

Between the top skin portion 53T and the bottom skin portion 53B of the main body 53 of the piston 7 a side skin portion 53S can be provided. The three skin portions 53T, 53B and 53S may surround a hollow space or a central core of the main body 53 of the piston 7.

Around the substantially cylindrical side wall of the piston 7 annular seats housing one or more piston rings and one or more sliding shoes can be provided. In Fig.6 these annular seats are formed in the outer side skin portion 53 S. In other embodiments, said annular seats can be machined in the material forming the core of the body of piston 7 and the latter can be provided with only two composite material portions, namely the top skin portion 53T and the bottom skin portion 53B.

In Fig.6 reference number 65 indicates seats housing corresponding piston rings 67, i.e. sealing rings, slidingly contacting the inner surface of the cylinder wherein the piston reciprocatingly moves. The position, number and dimension of the sealing rings 67 can vary according to design requirements. In some embodiments, seats 69 are also provided on the outer surface of the piston 7, housing respective sliding shoes 71. In some embodiments two sliding shoes 71 can be provided around the body of the piston 7. The piston rings 67 can be arranged intermediate the two sliding shoes 71, as shown. A larger or smaller number of sliding shoes can be provided, if needed, depending e.g. upon the weight of the piston rod and of the piston, which is supported by the sliding shoes 71. Also the mutual position of the sliding shoes 71 and piston rings 67 can be different. In some embodiments, one or more sliding shoes can be arranged between piston rings 67. In other embodiments, one or more sliding shoes can be arranged between the piston rings, and one or more sliding shoes can be arranged outside the piston rings, between the latter and one or the other of the two opposite end surfaces formed by the top skin portion 53T and the bottom skin portion 53B.

If an inner core is provided in the main body 53 of the piston 7, the top skin portion 53T and the bottom skin portion 53B as well as the side skin portion 53S, if present, can be glued on the core. In other embodiments, connection between the components forming the piston body can be by co-curing. In some embodiments, at least one of said skin portions can be formed on the core by filament winding.

Fig.7 illustrates a further embodiment of a piston 7 made in composite material. According to this embodiment the piston 7 comprises a top skin portion 75 and a bottom skin portion 77 which can be glued or bonded in any other suitable manner to an inner core 79, made for example of an expanded resin, such as expanded polyurethane or the like, preferably having a specific weight lower than the specific weight of the composite material.

The skin portions 75 and 77 can have an annular development and a substantially C- shaped cross-section as shown in Fig.7. In some embodiments the radially inner wings of the skin portions 75 and 77, labeled 75W and 77W can be glued to a metal hub such as hub 51 in Fig.6, or co-cured therewith. In other embodiments, said wings 75W and 77W can be glued to a tubular element 80, which can be in turn connected or constrained to the piston rod 15 or form part thereof. In some embodiments, the tubular element 80 can be made of metal. In other embodiments the tubular element 80 can be made of composite material, e.g. fiber reinforced plastic. The tubular element 80 can be made as the tubular structure 15T described above.

Seats 81 for respective piston rings 83 can be formed around the piston 7. In some embodiments, seats 85 housing sliding shoes 87 are provided around the piston 7. The seats 81 and 85 can be formed directly in the material forming the core 71. The piston rings 83 and sliding shoes 87 can be constrained into the seats in direct contact with the material forming the core 79. In other embodiments, an outer composite material skin portion 81 can be provided, forming a coating or cladding of the side cylindrical surface of the piston 7 and of the seats 85 and 81. In some other embodiments, a composite material skin can be provided only inside the seats 81 and 85.

Fig.8 illustrates a longitudinal section of a piston 7 in a further embodiment. A tubular member 80 is constrained to the main body of the piston and forms part or is constrained to the piston rod 15. The piston 7 can be comprised of a core 91 made of an expanded resin, such as expanded polyurethane or the like, preferably having a specific weight lower than the specific weight of the composite material.

The core supports an outer coating or skin 93 made of composite material. In some embodiments, the composite material may be a fiber reinforced polymer, for example carbon-fibers reinforced polymer, or glass-fibers reinforced polymer, or aramidic- fibers reinforced polymer, or combinations thereof, as mentioned previously.

In some embodiments, the skin or layer 93 can be formed by filament winding around the tubular member 80 and the core 91. The composite material can form a top skin portion 93T and a bottom skin portion 93B. If the composite material layer 93 is formed by filament winding, the surfaces of the top skin portion 93T and of the bot- tom skin portion 93B can be slightly tapered, forming shallow conical surfaces. The inclination of those outer surfaces of the skin portions 93T, 93B are as small as allowed by the winding technology, to reduce clearance volume in the compressor cylinder. The side surface of the piston 7 can be formed by a side skin portion 93S, which can again be formed for example by filament winding so that the three skin portions 93 S, 93B and 93T are monolithic.

Seats 95 for piston rings 96 and further seats 97 for sliding shoes 99 can be formed in the outer side skin portion 93 S.

Fig. 9 illustrates a further embodiment of a composite material piston 7 according to the present disclosure. The body of piston 7 can be comprised of a light inner core 91 surrounded by composite material skin portions. In some embodiments a top skin portion 93T and a bottom skin portion 93B are arranged at the ends of the piston 7 form- ing the substantially planar or slightly conical top and bottom surfaces of the generally cylindrical piston 7.

The skin portions 93T and 93B can be pre-formed separately and assembled on the inner core 91. The generally cylindrical side wall of the piston 7 can be formed by a side skin portion 93 S. In the embodiment of Fig. 9 a metal insert 101 is bounded or connected to the side skin portion 93 S. In some embodiments, the metal insert 101 can be made of steel, e.g. carbon steel. The metal insert 101 can have a broadly cylindrical shape with annular piston ring seats 95 and annular sliding shoe seats 97 formed in the outer surface thereof. In the exemplary embodiment of Fig. 9 the piston ring seats 95 are arranged between sliding shoe seats 97. In other embodiments, the arrangement and the number of the annual seats can be different. For instance, the piston ring seats 95 can be placed nearer to the top and bottom surfaces of the piston 7, with the sliding shoe seats located there between.

The metal insert 101 can be joined to the side portion 93 S of the composite material skin by means of glue. In other embodiments the metal insert 101 can be bonded to the inner composite material skin portion by co-curing.

Connection of the piston 7 with the piston rod 15 can be obtained by means of a metal tubular insert or bushing 103 constrained to the body of the piston 7. The metal bushing 103 can be embedded in a sleeve 105 made of composite material, which is in turn arranged coaxially inside the body of piston 7. The metal bushing 103 can be co-cured with the sleeve 105 and therefore with the piston body. In other embodiments, the metal bushing 103 can be bonded by gluing to the composite material forming the sleeve 105. A mechanical interlocking can be provided between the metal surface of the bushing 103 and the surface of the sleeve 105, as shown in Fig. 9. Mechanical in- terlocking is formed by projections and depressions on the outer metal surface of the bushing 103, as schematically shown at 105M in the enlargement of Fig.9A. The polymeric, fiber reinforced composite material intimately contacts the irregular outer surface of the bushing 103. Curing of the polymeric material generates a strong bond between the metal surface and the polymeric, fiber-reinforced composite material. Me- chanical interlocking can be used also in the other embodiments described above, whenever a metal/composite interface is formed.

Mechanical interlocking can also be provided at an interface between two portions, both made of composite material, having different compositions or properties.

In some embodiments the inner hole of the metal bushing 103 can be threaded, as shown at 103T. The piston rod 15 can be provided with a metal end having an outer thread engaging the inner thread 103T of the metal bushing 103.

Fig. 10 illustrates a sectional view of a further embodiment of a piston rod 15 comprised of a first end portion 15 A, a second end portion 15B and an intermediate portion 15T. The end portions 15 A, 15B can be made of metal, e.g. steel, and are de- signed for connection to the crosshead (not shown) and to the piston 7, respectively. The piston body can be entirely or partly made of metal, such as steel, or entirely or partly of composite material, e.g. according to the embodiments disclosed herein above. The piston 7 can be comprised of a piston body 7B provided with piston rings 83 and sliding shoes 85 as disclosed above. The intermediate portion 15T has a tubular shape and is formed of composite material, e.g. a polymeric resin matrix reinforced with glass, carbon or aramidic fibers or the like, in the form of continuous filaments and/or staple fibers. The interior of the tubular intermediate portion 15T of the piston rod 15 can be hollow or filled with a winding core or mandrel, as mentioned above. The tubular intermediate portion 15T can be jointed to the first and second end portions 15 A, 15B by means of respective scarf joints using glue or by co-curing, i.e. by applying the composite material before curing around terminal surfaces 15 AS and 15BS of the end portions 15 A, 15B and in intimate contact therewith, followed by curing, such that the cured polymeric resin adheres to the surfaces 15 AS and 15BS.

While the disclosed embodiments of the subject matter described herein have been shown in the drawings and fully described above with particularity and detail in connection with several exemplary embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without materially departing from the novel teachings, the principles and concepts set forth herein, and advantages of the subject matter recited in the appended claims. Hence, the proper scope of the disclosed innovations should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications, changes, and omissions. In addition, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments.

Claims

CLAIMS:

1. A double-effect reciprocating compressor, comprising: a cylinder; a reciprocatingly moving arrangement comprised of: a piston having a piston body and slidingly moving in said cylinder; a crosshead; and a piston rod connecting the crosshead to the piston; a crankshaft for driving said reciprocatingly moving arrangement into reciprocating motion, a connecting rod being arranged for connecting the crosshead to the crankshaft and transmitting the movement of the crankshaft to the crosshead; wherein at least one of said piston body and said piston rod is at least partly made of a composite material.

2. The compressor of claim 1, wherein said composite material is a fiber- reinforced polymeric material.

3. The compressor of claim 1, wherein said composite material is selected from the group consisting of: a carbon-fibers reinforced polymer; a glass-fibers reinforced polymer; an aramidic-fibers reinforced polymer; or a combination thereof.

4. The compressor of any one of the preceding claims, wherein said piston comprises at least one sliding shoe mounted on said piston body and in sliding contact with an inner surface of said cylinder.

5. The compressor of any one of the preceding claims, wherein said piston comprises at least one piston ring mounted on said piston body and in sliding contact with an inner surface of said cylinder.

6. The compressor of any one of the preceding claims, wherein the piston rod comprises at least a portion made of composite material and a portion made of metal and forming a rod end, constrained to the crosshead or to the piston body.

7. The compressor of any one of claims 1 to 5, wherein the piston rod comprises a first end portion made of metal, a second end portion made of metal and a third portion made of composite material, intermediate said first end portion and second end portion; and wherein the piston rod is connected to the crosshead and to the piston body by means of said first end portion and said second end portion, respectively.

8. The compressor of claim 6 or 7, wherein said portion of the piston rod made of composite material has a tubular structure.

9. The compressor of claim 8, wherein the tubular structure is formed by filament winding.

10. The compressor of claim 8 or 9, wherein said tubular structure surrounds a winding mandrel or core.

11. The compressor of claim 10, wherein said winding mandrel or core is made of a material having a specific weight lower than a specific weight of the composite material.

12. The compressor of any one of the preceding claims, wherein said piston body comprises a top skin portion made of composite material and a bottom skin portion made of composite material.

13. The compressor of claim 12, wherein the piston body further comprises a side skin portion made of composite material, which extends between the top skin portion and the bottom skin portion and forms at least one annular seat for at least one piston ring or a sliding shoes.

14. The compressor of claim 12, wherein the piston body further comprises a side portion made of metal, intermediate said top skin portion and said bottom skin portion and coaxial to a piston axis, said side portion forming at least one annular seat for a sliding shoe or a piston ring.

15. The compressor of claim 14, wherein the side portion is bonded to a side skin made of composite material, which extends between the bottom skin portion and the top skin portion of the piston body.

16. The compressor of any one of claims 12 to 15, wherein said piston body has an inner hollow cavity between the top skin portion and the bottom skin portion.

17. The compressor of any one of claims 12 to 15, wherein said piston body comprises a core whereon the top skin portion and the bottom skin portion and optionally the side skin portion are connected.

18. The compressor of claim 17, wherein the core is made of a material having a specific weight lower than the specific weight of the composite material.

19. The compressor of any one of claims 12 to 18, wherein said piston body comprises an inner bushing made of metal, connected to the piston rod.

20. The compressor of claim 19, wherein an outer body at least partly made of composite material is constrained to said inner bushing, connecting the inner bushing with the top skin portion and the bottom skin portion of the piston body.