AND METHOD

DESCRIPTION

TECHNICAL FIELD [0001] The present disclosure concerns improvements in gas compressors. Embodi ments disclosed herein specifically relate to reciprocating compressors.

BACKGROUND ART

[0002] Pressure of a process gas can be boosted using several kinds of compressors depending, among others, by the compression ratio and flow rate required. [0003] Reciprocating compressors are typically used when low flow rate and high compression ratio are necessary. Reciprocating compressors typically include a com pressor frame, with a crankshaft supported for rotation therein. The crankshaft is driven into rotation by a driver, for instance an electric motor, or a turbine. The rotary motion of the crankshaft is converted into a reciprocating motion to control the recip- rocating sliding movement of a piston in a gas compression cylinder.

[0004] The rotary motion is converted into reciprocating motion by a connecting rod, which drivingly couples the crankshaft to a crosshead, arranged for reciprocating mo tion in a crosshead guide. The crosshead is in turn drivingly coupled to a first end of a piston rod. The opposite, second end of the piston rod is connected to the piston, which is reciprocatingly moving inside the gas compression cylinder. A distance piece is po sitioned between the crosshead guide and the gas compression cylinder. The piston rod slides through the distance piece. The distance piece has an inner volume which is maintained at ambient pressure, or at a gas pressure slightly above ambient pressure.

[0005] The reciprocating movement of the piston in the gas compression cylinder, in combination with automatic or controlled suction valves and discharge valves, cause suction of process gas at a suction pressure and discharge of compressed process gas at a discharge pressure, higher than the suction pressure.

[0006] To prevent process gas leakages from the gas compression cylinder to the distance piece, a piston rod pressure packing is positioned around the piston rod on the back side of the gas compression cylinder chamber (crank end), i.e. facing the cross head. The differential pressure across the piston rod pressure packing fluctuates ac cording to the pressure inside the compression chamber, defined by the piston and the gas compression cylinder. The maximum value is equal to the difference between the process gas delivery pressure in the gas compression cylinder and the pressure in the distance piece, which is equal or almost equal to ambient pressure.

[0007] In order to prevent leakages along the piston rod, the piston rod pressure pack ing must seal the side surface of the piston rod. The higher the differential pressure across the piston rod pressure packing, the higher is the contact pressure of the sealing rings of the piston rod pressure packing against the side surface of the piston rod. This poses a limitation on the maximum allowable discharge pressure achievable by the compressor, especially if the piston rod pressure packing is a dry pressure packing, i.e. not using a lubricant. Dry packings are applied when contamination of the compressed gas is not allowed by specific process application.

[0008] It would therefore be welcomed in the art a reciprocating compressor capable of overcoming or alleviating the limitations of the reciprocating compressors of the current art.

SUMMARY

[0009] According to embodiments disclosed herein, a reciprocating compressor is provided, including a compressor frame, with a crankshaft supported for rotation therein. A connecting rod connects the crankshaft to a crosshead, arranged for recip rocating motion in a crosshead guide, to convert rotary motion of the crankshaft into reciprocating motion of the crosshead. A piston rod is coupled at one end to the cross head and at the opposite end to a piston, which is arranged for reciprocating movement in a gas compression cylinder. At least one pressure-drop module is positioned be tween the gas compression cylinder and the crosspiece guide. The first pressure-drop module includes a pressure-drop chamber. The piston rod extends from the gas com pression cylinder through the pressure-drop chamber. In use, the pressure-drop cham ber is adapted to be purged with process gas at a pressure lower than a delivery pressure of the reciprocating compressor and higher than ambient pressure. [0010] The disclosure also concerns a method of operating the reciprocating com pressor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Reference is now made briefly to the accompanying drawings, in which: [0012] Fig.l is a sectional view of a reciprocating compressor according to the dis closure in one embodiment; and

[0013] Fig.2 is a flow chart of a method of operating the reciprocating compressor. DETAILED DESCRIPTION

[0014] In order to reduce the differential pressure across piston rod pressure packing acting on the piston rod, a pressure-drop chamber is provided between the gas com pression cylinder and the crosshead guide of a reciprocating compressor. The pressure- drop chamber is maintained at a pressure which is lower than the compressor delivery pressure and higher than ambient pressure, for instance at a pressure around the suction pressure of the reciprocating compressor. The pressure differential between the com- pressor delivery pressure and ambient pressure is thus divided on at least two piston rod pressure packings. Sealing against leakages along the piston rod can thus be achieved at a reduced pressure of the packing against the side surface of the piston rod.

[0015] Turning now to Fig.l, a reciprocating compressor 1 includes a compressor frame 3, which receives a crankshaft 5 arranged for rotation in the compressor frame 3 around a rotation axis A-A. A connecting rod 7 drivingly connects the crankshaft 5 to a crosshead 9. The crosshead 9 is arranged for slidingly reciprocating along a cross head guide 11 according to double arrow f9 when the crankshaft 5 rotates with a con tinuous motion around axis A-A.

[0016] The reciprocating compressor 1 further includes a gas compression cylinder 13, including a cylinder body 13.1, a crank end 13.2 and a head end 13.3. A piston 15 is slidingly arranged in the inner volume of the gas compression cylinder 13 and di vides the inner volume of the gas compression cylinder 13 into a first compression chamber 13 A and a second compression chamber 13B. The reciprocating compressor 1 of Fig.1 is therefore a double-acting reciprocating compressor.

[0017] The piston 15 is connected to an end of a piston rod 17, the opposite end whereof is connected to the crosshead 9 and reciprocates in the gas compression cyl inder 13.

[0018] The gas compression cylinder 13 further includes at least one suction valve for each compression chamber 13 A, 13B. The suction valves are shown at 19A and 19B, respectively, and are in fluid connection with a suction line 20. Each compression chamber 13 A, 13B is further provided with a respective discharge valve 21A, 21B. The discharge valves 21 A, 2 IB are fluidly coupled to a delivery line 22.

[0019] In some embodiments, a distance piece 23 is positioned between the cross head guide 11 and the gas compression cylinder 13. In the embodiment of Fig.1, the distance piece 23 includes a first inner volume 25 and a second inner volume 27. The first inner volume 25 can be fluidly coupled at 29 with a flare stack, where process gas leaking in the first inner volume 25 is flared. The pressure inside the first inner volume 25 is therefore slightly above ambient pressure, to allow gas to be delivered to the flare stack. The second inner volume 27 can be maintained at ambient pressure. An oil slinger 31 can be fitted on the piston rod 17 in the portion thereof crossing the second inner volume 27 of the distance piece 23.

[0020] The reciprocating compressor 1 further includes a pressure-drop module 33 positioned between the distance piece 23 and the gas compression cylinder 13. The pressure-drop module 33 includes a pressure-drop chamber 35 through which the pis ton rod 17 extends.

[0021] A first piston rod pressure packing 37 surrounding the piston rod 17 is posi tioned between the gas compression cylinder 13 and the pressure-drop chamber 35. A second piston rod pressure packing 39 is positioned between the pressure-drop cham ber 35 and the crosspiece guide 11 and more specifically between the pressure-drop chamber 35 and the distance piece 23. In the embodiment of Fig.1, the second piston rod pressure packing 39 is positioned between the pressure-drop chamber 35 and the first inner volume 25 of the distance piece 23. A further piston rod intermediate pres sure packing 41 can be arranged between the first inner volume 25 and the second inner volume 27 of the distance piece 23.

[0022] An oil wiper packing 43 can be arranged around the piston rod 17, between the second inner volume 27 of the distance piece 23 and the crosshead guide 11.

[0023] In some embodiments, the pressure-drop chamber 35 is fluidly coupled to a source of process gas at a pressure lower than the delivery pressure, but higher than the pressure in the first inner volume 25 of the distance piece 23. In the embodiment of Fig.1, the pressure-drop chamber 35 is fluidly coupled to the suction line 20 of the reciprocating compressor 1. The pressure inside the pressure-drop chamber 35 is thus substantially equal to the suction pressure of the reciprocating compressor 1. In some embodiments, not shown, the pressure in the pressure-drop chamber 35 can be adjusted to be lower than the suction pressure, e.g., by providing a pressure reducing valve in the line 26 which connects the pressure-drop chamber 35 to the suction line 20.

[0024] With the above described arrangement, the maximum differential pressure across the first piston rod pressure packing 37 is equal to the difference between the delivery pressure and the suction pressure of the reciprocating compressor 1 and the differential pressure across the second piston rod pressure packing 39 is equal to the difference between the suction pressure and the pressure in the first volume 25 of the distance piece 23, which may be almost equal to the ambient pressure.

[0025] The maximum differential pressure across a single piston rod pressure pack ing is thus reduced with respect to reciprocating compressor configurations of the cur rent art.

[0026] If desired, more than one pressure-drop module 33 can be arranged between the crank end 13.2 of the gas compression cylinder 13 and the distance piece 23, such as to split the difference between the delivery pressure and the pressure in the first inner volume 25 of the distance piece 23 across more than just two piston rod pressure packings. For instance, two or three pressure-drop modules 33 can be arranged in se quence between the crank end 13.2 and the distance piece 23. Each pressure-drop room of the plurality of pressure-drop modules can be fluidly coupled to sources of process gas at progressively lower pressure values. For instance, the first pressure-drop cham ber 35, i.e. the one adjacent the gas compression cylinder 13 can be coupled to the suction side of the reciprocating compressor 1, and a second pressure-drop chamber can be fluidly coupled to the suction side of a reciprocating compressor arranged in series upstream the reciprocating compressor 1. In other embodiments, the second pressure-drop chamber can be fluidly coupled to the suction line 20 with the interpo sition of a pressure reduction valve, such that the pressure in the second pressure-drop chamber is lower than the pressure in the first pressure drop chamber.

[0027] In the embodiment of Fig.1, a distance piece 23 divided into two parts with a first inner volume 25 and a second inner volume 27 is provided. In other embodiments, a shorter distance piece with just one inner volume can be provided, which can be fluidly coupled to the environment or to a process gas recovery line, e.g. fluidly cou pled to a flare stack.

[0028] While in Fig.1 the pressure-drop chamber 35 is fluidly coupled to the suction side of the reciprocating compressor 1, in other embodiments the pressure-drop cham ber 35 can be fluidly coupled to a different source of process gas, for instance at a pressure lower than the suction pressure. In embodiments, the source of process gas to which the pressure-drop chamber 35 is connected can be the suction side of a com pressor stage arranged upstream of the gas compression cylinder 13.

[0029] Fig.2 illustrates a flowchart, which summarizes a method for operating a re ciprocating compressor 1 according to the present disclosure The method comprises the following steps: rotating the crankshaft 5 (step 101); converting the rotary motion of the crankshaft 5 in a reciprocating motion of the piston 15 in the gas compression cylinder 13 (step 102); sequentially sucking process gas at as suction pressure in the gas compression chamber and discharging process gas at a delivery pressure from the gas compres sion chamber (step 103); and purging the pressure-drop chamber 35 with process gas at a pressure lower than the delivery pressure of the reciprocating compressor and higher than ambient pressure (step 104). [0030] The mentioned steps may be performed in any suitable order. During steady- state operation of the compressor 1, the above mentioned seps are usually performed in parallel, i.e. at the same time.

[0031] Exemplary embodiments have been disclosed above and illustrated in the ac- companying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.