Patent Description:
In a reciprocating piston compressor sealing devices are used to seal the compression chamber in cylinder against the crank case or the distance piece respectively. Such sealing devices comprise a number of packing retainers, often formed in a circular plate like fashion, in each of which retainers a retaining opening as arranged. In each retaining opening a packing ring is arranged, which during operation of the compressor cooperates with an outer circumferential surface of a piston rod in order to form a sealing barrier. During operation of the compressor, the piston rod predominantly executes a reciprocating movement relative to the sealing device in an axial direction of the cylinder. Often packing rings, which consist of multiple segments, are used. However, such segmented packing rings are typically activated only during operation by a differential pressure between a high pressure in the compression chamber and a low pressure in the crankcase in order to form the sealing barrier.

At standstill of the compressor however, such segmented rings are typically (at least partially) deactivated. This is due to the fact, that, although there is still a certain differential pressure present, the sealing surfaces of the segmented rings do not conform to the outer surface of the piston rod anymore, when the piston rod cools down after shutdown. This is essentially a result of the segmented rings and the piston rod having different thermal expansions. Therefore, in the cold state, e.g., at ambient temperature, a certain gap is created between the segmented rings and the piston rod in, which gaps form a leakage passage.

Hence, at standstill with such segmented packing rings, no sufficient sealing barrier can be established. This however is disadvantageous, because at standstill the gas, which is contained inside the compression chamber, can leak from the compression chamber in the axial direction past the sealing device in the direction of the crankcase, e.g., into the distance piece. While in some cases, such leakage is unproblematic, e.g., when air is used as the compression gas, it could lead to a variety of problems, when other gases, such as flammable or toxic gases are used. When for instance a flammable gas, such as natural gas, is compressed, leakage needs to be prevented on the one hand for safety reasons, in order to avoid a possible flammable atmosphere to be created in the distance piece or in the environment surrounding the compressor. On the other hand, leakage needs to be prevented also for environmental reasons, since the leakage of any greenhouse gases into the environment is unwanted and, in some cases, prohibited or at least limited.

In the past, a couple of different approaches were developed for solving this particular leakage problem at compressor standstill. In <CIT> a sealing device is disclosed, which uses an unsegmented sealing ring, which is activated due to the residual differential pressure after shutdown when a valve in a vent line is closed. During operation of the compressor, the valve is opened and the sealing ring is relieved due to the missing differential pressure over the ring. However, although the contact pressure during operation is low, the ring is at any time in contact with the piston rod, which leads to a rather fast wearing of the unsegmented sealing ring.

Furthermore, in order to be suitable for pressure activation, the unsegmented sealing ring needs to be made of a very soft and elastic material. Such materials however tend to wear out faster compared to rigid materials and also have lower suitability to withstand high pressures or corrosive gases. Additionally, the opening and closing of the valve needs to be coupled to the operation mode of the compressor. However, a failure of the valve control, e.g., the valve being closed during operation of the compressor, would lead to a quick wear and most likely to a destruction of the sealing ring. This would lead to an unwanted leakage of gas past the broken sealing ring and into the crankcase, since due to the closed valve no venting is possible. While the gas contained in the vent line is safely treated, e.g., sent to a burning device, the gas that accumulates in the distance piece is often untreated and might cause formation of explosive atmosphere and/or environmental damage due to release of greenhouse gas into the air.

<CIT> discloses a different approach for a standby seal. The stuffing box comprises an inflatable sealing ring, which is made of an expandable material and which is selectively in fluid communication with a pressurized fluid source. When inflated, the sealing ring expands and forms a seal interface with the outer surface of the rod. However, the control of the activation and deactivation and also the design of the sealing ring are rather complex. Further, this solution requires an external source of energy in the form of pressurized medium e.g., nitrogen which is sent to the inflatable seal.

<CIT> discloses a sealing device including a special piston, which is connected to a pressure source in order to be axially moved. When activated, the piston moves a ring in the axial direction towards a flexible sealing collar. On the ring and the collar cooperating chamfers are provided, such that when the ring is moved, the collar is pressed against the piston rod by the ring in order to form a seal. However, the structure and control of the seal are very complex and an external source of energy in the form of pressurized medium is also required. Also, this invention is known to be not very robust because, when the movable piston is retracted, sometimes the lip seal does not detach from the rod and therefore is irreparably damaged at compressor startup.

It was therefore an object of the invention to provide a sealing device for sealing a piston rod of a reciprocating compressor, which provides good sealing properties at compressor standstill, which has a simple structure, which allows for a simple handling and which requires no external source of energy.

The object is achieved with the above-mentioned sealing device in that the second packing ring is made from a material comprising a polymer, the material having a thermal expansion coefficient, which is at least two times higher than the thermal expansion coefficient of iron, wherein at or below a defined activation temperature an inner diameter of the second packing ring is smaller than an outer diameter of the piston rod to be sealed, such that in the mounted state of the sealing device in the compressor the second packing ring is prestressed in a radial direction in order to form a tight seal between the continuous inner circumferential sealing surface of the second packing ring and the outer circumferential surface of the piston rod, wherein at a given operating temperature, the inner diameter of the second packing ring is larger than the outer diameter of the piston rod, such that in the mounted state of the sealing device in the compressor the continuous inner circumferential sealing surface is detached from the outer circumferential surface of the piston rod in order to provide a leakage path between the inner circumferential surface of the second packing ring and the outer circumferential surface of the piston rod in the axial direction.

This allows for an automatic thermally dependent activation and deactivation of the sealing effect of the second packing ring. By reducing the contact pressure between the second packing ring and the piston rod to zero during operation of the compressor, wear of the second packing ring is also minimized. Further, due to the thermal activation of the second packing ring, there is no need for a certain residual pressure to be present in the compression chamber at the time of shut-off of the compressor. This allows the sealing device of the invention to be applied in essentially any compressor application, regardless of the suction pressure of the cylinder. Hence, it essentially also works with a suction pressure below ambient (as in vacuum compressors). The sealing device of <CIT> on the other hand necessarily requires a certain cylinder pressure to activate the unsegmented sealing ring, such that it cannot work on compressors whose suction pressure is the ambient pressure or below.

In a preferred embodiment, the thermal expansion coefficient of the material of the second packing ring is at least 30x10-<NUM>-<NUM>, preferably at least 60x10-<NUM>-<NUM>, in particular at least 90x10-<NUM>-<NUM>. The region 30x10-<NUM>-<NUM> and above was found to provide a suitable expansion.

The operating temperature is preferably <NUM> or above and/or the activation temperature is preferably <NUM> or below, wherein the operating temperature and the activation temperature are temperatures in the region of the piston rod, in particular a temperature of the outer circumferential surface the piston rod. This allows a good sealing effect at standstill and a sufficient expansion for detachment of the second packing ring from the piston rod.

In a preferred embodiment the second packing ring has a first axial end and an opposite second axial end, wherein the second packing ring is arranged in in the retaining opening of the second packing retainer, such that the first axial end faces towards the first axial device end of the sealing device and wherein the second packing ring comprises one of: a U-shaped cross section comprising an inner shank and an outer shank, which are spaced apart in the radial direction, wherein the inner circumferential sealing surface of the second packing ring is provided on the inner shank and a radially outer circumferential surface is provided on the outer shank of the second packing ring, wherein an open side of the U-shape faces toward the first axial end of the second packing ring in the axial direction; an L-shaped cross section comprising an axial shank and a radial shank, wherein the inner circumferential sealing surface of the second packing ring is provided on the axial shank and a radially outer circumferential surface of the second packing ring is provided on the radial shank, wherein the radial shank is arranged at the second axial end of the second packing ring; a rectangular cross section, wherein a number of openings are provided on the second packing ring, which openings in each case connect the first axial end of the second packing ring with an outer circumferential surface of the second packing ring. Thereby different preferred shapes are provided, such that the second packing ring can be flexibly adapted to different applications.

When the second packing ring has a U-shaped profile, a number of openings are preferably provided on the outer shank, wherein each of the number of openings connects an inside space of the U-shaped packing ring with the radially outer circumferential surface of the second packing ring, the inside space lying between the inner shank and the outer shank in the radial direction, wherein the openings are spaced from the opposite axial ends, of the second packing ring. When openings are provided, the number of openings preferably comprise a number of elongated holes or elliptical holes, wherein each elongated hole or elliptical hole preferably comprises a longitudinal axis, a first hole end and an opposite second hole end in the direction of the longitudinal axis, wherein the first hole end is closer to the first ring end than the second hole end. Preferably in the unmounted state of the second packing ring, a length of the inner shank in the axial direction is smaller than a length of the outer shank in the axial direction. The openings allow gas to flow past the second packing ring. Further, the openings give the second packing ring a certain flexibility in the axial direction. This permits a tight seat of the second packing ring in the retaining opening of the second packing retainer, essentially independent from axial thermal expansion.

The material of the second packing ring is preferably a fiber-reinforced composite material. This enhances the mechanical strength of the second packing ring. Additionally, or alternatively the polymer of the material of the second packing ring preferably comprises at least one of: polytetrafluoroethylene, polyphenylene sulphide, polyether ether ketone, polyimide, polyamide. Those polymers provide sufficient rigidity and have favorable tribological properties.

In a preferred embodiment, the sealing device further comprises a support passage having a first support passage end, a second support passage end and a valve for opening and closing the support passage, the support passage being configured to vent a gas, leaking from the first axial device end in the direction of the second device end past at least one of the number of first packing rings, from the first support passage end to the second support passage end. Although not essential for the invention, a support passage enhances the functionality during operation of the compressor. This is because a relatively large pressure drop over the second packing ring is avoided, that would occur without the provision of a support passage during operation.

According to a preferred embodiment, the first support passage end is connected to the retaining opening of the second packing retainer, preferably in a region radially outside of the second packing ring, or the first support passage end is located between the second packing ring and the first packing ring of the adjacent first packing retainer in an axial direction of the sealing device or the number of first packing retainers comprises at least two first packing retainers, wherein the first support passage end is located in the region of the first packing ring, which is adjacent the second packing ring or between the first packing ring, which is adjacent the second packing ring and the first sealing device end in the axial direction of the sealing device. The first two alternatives enhance the design flexibility and are essentially similar in function. The third alternative can be advantageous in two ways. On the one hand, this configuration allows the first packing ring, that is adjacent the second packing ring, to provide an additional seal during the time between a shut-off of the compressor and the activation of the second packing ring, which is delayed due to necessary time for the thermal activation. On the other hand, the pressure-drop over the first packing ring, that is adjacent the second packing ring, can be used for cooling the second packing ring, which accelerates the thermal activation. In the latter, it is particularly preferable, if the respective first packing ring comprises a metal material.

Preferably the valve comprises an electrically controllable actuator, which can be controlled by a control unit in order to open and close the valve. This allows to open and close the support passage e.g., via the compressor control unit.

Preferably the valve further comprises a sensor configured to generate a sensor value, representative for an opening state of the valve. By this a closed control loop can be created, which allows e.g., the compressor not being started, when the valve is still closed.

In a preferred embodiment, the sealing device further comprises an unobstructed vent passage having a first vent passage end and a second vent passage end, the vent passage being configured to vent a gas, leaking from the first axial device end in the direction of the second axial device end past the second packing ring, from the first vent passage end to the second vent passage end. This enhances the safety of the operation, because even in case of a breakdown of the second packing ring, gas can be prevented from flowing e.g., into the distance piece of the compressor, which could potentially create a hazardous environment, such as a combustible atmosphere.

Preferably the sealing device further comprises at least one third packing retainer including a retaining opening in which a third packing ring is arranged, wherein the at least one third packing retainer is arranged closer to the second axial device end of the sealing device than the second packing retainer. The third packing ring can further improve the safety, because an additional sealing barrier is created behind the second sealing ring.

When the above-mentioned unobstructed vent passage is provided, the first vent passage end is preferably connected to the retaining opening of the third packing retainer, preferably radially outside of the at least one third packing ring or the first vent passage end is located between the second packing ring and the third packing ring in the axial direction of the sealing device. This allows an essentially free flow of gas into the vent passage.

When an unobstructed vent passage and a support passage are provided, the second vent passage end of the unobstructed vent passage and the second support passage end of the support passage are preferably connected to a common discharge passage, which is connectable to a discharge space, preferably to a disposal system. This allows a single disposal system to be used for the gas coming from the support passage as well as the gas coming from the vent passage. The valve of the support passage in this case can of course only open/close the support passage, while the vent passage remains unobstructed.

In a preferred application, at least one sealing device according to the invention is preferably used in a reciprocating piston compressor, which comprises a number of cylinders, in each of which cylinders a piston is arranged, that is movable in a reciprocating manner. Each piston is connected to a piston rod, wherein for at least one cylinder of the number of cylinders, the sealing device according to the invention is arranged for sealing the respective piston rod, wherein the sealing device is arranged such that the first axial device end faces towards the cylinder and the second axial device end faces towards a crankcase of the compressor. If the compressor comprises more than one cylinder, preferably for each cylinder a sealing device according to the invention is arranged.

The compressor preferably comprises a compressor control unit for controlling an operation of the compressor, wherein the control unit is further configured to control the electrically controllable actuator of the valve of the support passage dependent on an operation condition of the compressor. by this, the opening/closing of the valve can be coupled to the operation condition of the compressor.

In a preferred embodiment the compressor comprises an operation condition sensor, configured to detect a sensor value, representative for an operation condition of the compressor, preferably a temperature sensor or a movement sensor, wherein the electrically controllable actuator of the valve of the support passage is configured to control the valve dependent on the sensor value or wherein the control unit is configured to control the electrically controllable actuator of the valve dependent on the sensor value. This allows to open/close the valve dependent on preferred conditions of the compressor.

Preferably the compressor also comprises a drive unit for driving the compressor, wherein the compressor control unit is configured to send a start signal to the drive unit for starting the operation of the compressor and to send an opening signal to the actuator of the valve of the support passage for opening the valve simultaneously with the start signal or a predetermined or adjustable opening lead time before the start signal, wherein the opening lead time is preferably in the range between <NUM> to <NUM> seconds. Additionally or alternatively the control unit is preferably configured to send a stop signal to the drive unit for stopping the operation of the compressor and to send a closing signal to the actuator of the valve of the support passage for closing the valve simultaneously with the stop signal or after a predetermined or adjustable closing delay time after the stop signal, wherein the closing delay time is preferably in the range between <NUM> to <NUM> seconds. In this way, the opening/closing of the valve can be directly coupled to the starting/stopping of the drive unit.

The object of the invention is also achieved with the method for operating a reciprocating piston compressor, the compressor comprising a number of cylinders, in each of which cylinders a piston is arranged, that is movable in a reciprocating manner, wherein each piston is connected to a piston rod, wherein for each cylinder of the number of cylinders, a sealing device for sealing the respective piston rod is provided, which device comprises a first axial device end a, facing toward the respective cylinder, and an opposite second axial device end, facing toward a crankcase of the compressor, wherein at least one sealing device of the number of sealing devices comprises a number of first packing retainers, each retainer including a retaining opening in which a first packing ring is arranged, a second packing retainer, including a retaining opening, in which a second packing ring is arranged, the second retainer being positioned closer to the second axial device end than the number of first packing retainers in an axial direction of the sealing device, wherein the second packing ring is an uncut ring, comprising a continuous inner circumferential sealing surface, wherein the second packing ring is made from a material comprising a polymer, the material having a thermal expansion coefficient, which is at least two times higher than the thermal expansion coefficient of iron, the method comprising: starting an operation of the compressor from standstill at or below a defined activation temperature, wherein at or below the activation temperature the second packing ring is prestressed in a radial direction to form a tight seal between the continuous inner circumferential sealing surface of the second packing ring and the outer circumferential surface a of the piston rod; operating the compressor until a predefined operating temperature is reached, wherein upon reaching the operating temperature, the continuous inner circumferential sealing surface of the second packing ring detaches from the outer circumferential surface a of the piston rod and provides a leakage path between the inner circumferential sealing surface of the second packing ring and the outer circumferential surface a of the piston rod in the axial direction.

Preferably the method further comprises the following steps: stopping the operation of the compressor until standstill; cooling down the compressor until of below the activation temperature , wherein upon reaching the activation temperature the continuous inner circumferential sealing surface of the second packing ring attaches to the outer circumferential surface of the piston rod and forms a tight seal between the continuous inner circumferential sealing surface of the second packing ring and the outer circumferential surface of the piston rod.

Preferably the at least one sealing device further comprises a support passage having a first support passage end, a second support passage end and a valve for opening and closing the support passage, the method further comprising: opening the valve of the support passage simultaneously with the starting of the operation of the compressor or an opening lead time before the start of the operation of the compressor, wherein the opening lead time is preferably set to be in the range between <NUM> to <NUM> seconds, venting a gas, leaking from the first axial device end in the direction of the second axial device end past at least one of the number of first packing rings, from the first support passage end to the second support passage end, closing the valve simultaneously with the stopping of the operation of the compressor or after an closing delay time after the stopping of the operation of the compressor, wherein the closing delay time is preferably set to be in the range between <NUM> to <NUM> seconds.

Preferably the method further comprises: venting a gas, leaking in the axial direction from the cylinder in the direction of the crankcase past the second packing ring, through an unobstructed vent passage.

The present invention is explained in more detail below with reference to <FIG>, which show exemplary, schematic, and non-limiting advantageous embodiments of the invention.

In <FIG> an exemplary reciprocating piston compressor <NUM> is shown. In the following for the sake of simplicity the word compressor is used. The compressor <NUM> comprises a compressor casing, which in this example includes a crankcase <NUM>, a distance piece <NUM> and a cylinder housing <NUM>. However, the distance piece <NUM> is only optional and the compressor <NUM> could also be designed without a distance piece <NUM>. In the crankcase <NUM> a crankshaft <NUM> is arranged, which can rotate about an axis of rotation. The crankshaft <NUM> is connected to a connecting rod <NUM>, which is in turn connected to a cross-head <NUM>. The cross-head <NUM> is mounted in the crank case <NUM> by means of a suitable bearing, such that a cross-head <NUM> can perform an axial movement. The cross-head <NUM> is connected to a piston rod <NUM>, which extends through the distance piece <NUM> and into a cylinder, <NUM> which is arranged in the cylinder housing <NUM>. In the cylinder <NUM> a piston <NUM> is arranged, which is connected to the piston rod <NUM>. The depicted compressor <NUM> is designed as a single cylinder compressor. However, the compressor <NUM> can of course also comprise multiple cylinders <NUM>, in each of which cylinders <NUM>, a piston is arranged, that is connected to the common crankshaft <NUM> by means of a piston rod <NUM>. For the sake of simplicity, the invention is described in connection with one cylinder <NUM> only.

The compressor <NUM> is designed as a double acting compressor, wherein the piston divides the cylinder <NUM> into a first compression chamber 11a facing away from the crankshaft <NUM> and a second compression chamber 11b facing towards the crankshaft <NUM>. The first compression chamber 11a is closed in the axial direction by means of a first cylinder head 12a and the second compression chamber 11b is closed in the axial direction by means of a second cylinder head 12b. For each compression chamber 11a, 11b at least one inlet valve <NUM> and at least one outlet valve <NUM> is provided in the cylinder housing <NUM>. The inlet valves <NUM> and the outlet valves are only shown in a simplified way. The valves <NUM>, <NUM> can for example be designed as automatic valves or could also comprise electrically controllable actuators for opening and closing. If the inlet valves are designed as automatic valves, valve unloaders (not shown) can additionally be provided, which are configured for keeping the respective valve in an open state independent of the pressure difference, acting on the valve. The valve unloader can comprise an electrically controllable actuator for activating the valve unloader.

A suitable drive unit <NUM> is also provided for driving the crankshaft <NUM> of the compressor <NUM>. For the sake of simplicity, the drive unit <NUM> is only shown in a schematic fashion. The drive unit <NUM> can for instance comprise a suitable electric motor, a combustion engine, or another suitable drive. In case of an electric motor, an electric power source is of course also provided for supplying electric energy (not shown). The compressor <NUM> further comprises a compressor control unit <NUM>, which is configured to control different functions of the compressor <NUM>. In the following the abbreviated wording "control unit" will be used for sake of simplicity. Via the control unit <NUM> the drive unit <NUM> can be controlled in order to start or stop the operation of the compressor <NUM> or in order to control the running speed of the compressor <NUM>. The control unit <NUM> can comprise suitable hardware and/or software. The control unit <NUM> can e.g., also be configured to control the mentioned actuators of the inlet valves <NUM> and outlet valves <NUM> or the actuators of the valve unloaders. Of course, further available functions of the compressor <NUM>, such as an oil supply to the cylinder, could also be controlled by the control unit <NUM>. Also monitoring functions could be controlled, such as processing sensor values or the like.

Further the compressor <NUM> comprises a sealing device <NUM> for sealing the piston rod <NUM>. If the compressor <NUM> comprises multiple cylinders <NUM>, of course a sealing device <NUM> is provided for the piston rod <NUM> of each cylinder <NUM>. The sealing device <NUM> comprises a first axial device end 15a, which faces towards the cylinder <NUM> and comprises an opposite second axial device end 15b which faces towards the crankcase <NUM>. In the example shown, the first device end 15a is arranged in the second cylinder head 12b the second device end 15b is arranged in the distance piece <NUM>. Of course, the position can vary depending on the design of the compressor <NUM>. The sealing device <NUM> is essentially formed cylindrically and comprises a number of packing retainers, which are essentially formed in a disc like manner. Each packing retainer can include a retaining opening, in which a packing ring can be arranged (not shown in <FIG>). The packing rings surround the piston rod <NUM> in order to form a sealing barrier. The sealing device <NUM> according to the invention will now be described in greater detail in connection with <FIG>.

<FIG> shows a section view of the sealing device <NUM> of <FIG> according to an exemplary embodiment of the invention. The sealing device <NUM> comprises a first axial device end 15a, configured to face toward a cylinder <NUM> of the compressor <NUM>, which represents a high-pressure side HP. The sealing device <NUM> comprises an opposite second axial device end 15b, configured to face toward a crankcase <NUM> of the compressor <NUM> or the distance piece <NUM> respectively. The sealing device <NUM> comprises a number of first packing retainers <NUM>, each retainer <NUM> including a retaining opening 18a, in which a first packing ring <NUM> is arranged. Each of the first packing rings is configured for sealing the piston rod <NUM> during operation of the compressor <NUM>. The retaining opening 18a is only indicated for one packing retainer <NUM> in <FIG>. The first packing retainers <NUM> can have the shape of a cylindrical disc and can be made of a suitable material such as steel or a steel alloy.

Each of the first packing rings <NUM> can for example comprise multiple ring segments in the axial direction (as indicated in <FIG>) and/or multiple ring segments in a circumferential direction (not shown). A first packing ring <NUM> can for example comprise a tangentially cut sealing ring and a radially cut sealing ring. The tangentially cut sealing ring can comprise a number of ring segments in the circumferential direction, which segments abut at cooperating sealing surfaces in the circumferential direction in order to form a seal in the radial direction. The radially cut sealing ring can comprises a number of ring segments in the circumferential direction, the segments being configured to overlap the tangential cuts of the tangentially cut ring, in order to form a seal in the axial direction. Also combined rings are known, which have both, tangential cuts and radial cuts. Further a so-called "backup-ring" could be arranged in the retaining opening 18a adjacent a first packing ring <NUM>, which backup-ring can be uncut and which is configured to prevent the ring segments of the first packing ring <NUM>, usually made from plastic from extrusion. However, backup rings are normally made of metal and do not serve for sealing the piston rod. Since those different packing rings are known in the art, no detailed description will be provided at this point. Of course, the number of first packing rings <NUM>, arranged in the number of first packing retainers <NUM>, do not necessarily need to be identical.

The sealing device <NUM> further comprises a second packing retainer <NUM>, which includes a retaining opening 20a, in which a second packing ring <NUM> is arranged. The second packing ring <NUM> is configured for sealing the piston rod <NUM> during standstill of the compressor <NUM>, as will be described in further detail below. The second packing ring <NUM> has a different design than the number of first packing rings <NUM>. In an axial direction of the sealing device (corresponding to the axial direction of the piston rod <NUM>), the second packing retainer <NUM> is positioned closer to the second axial device end 15b than the number of first packing retainers <NUM>, as can be seen in <FIG>. Between the second packing retainer <NUM> and the subsequent first of the first packing retainers <NUM>, an intermediate plate <NUM> is arranged in the shown example. The intermediate plate <NUM> delimits the retaining opening 20a of the second packing retainer <NUM> in the axial direction and also delimits the retaining opening 18a of the subsequent first packing retainer <NUM> in the axial direction, which openings face each other. The intermediate plate <NUM> therefore separates the second packing ring <NUM>, arranged in the retaining opening 20a of the second packing retainer <NUM>, from the first packing ring <NUM>, which is arranged in the retaining opening 18a of the adjacent first packing retainer <NUM>. However, the arrangement of an intermediate plate <NUM> is only optional and it could also be omitted, e.g., if the first packing retainers <NUM> are arranged mirror-inverted.

The second packing ring <NUM> is an uncut ring, comprising a continuous inner circumferential sealing surface 21a in the circumferential direction. As indicated in <FIG>. , the second packing ring <NUM> can for example have a U-shaped cross section, wherein an open side of the U-shape faces toward the first axial device end 15a of the sealing device <NUM>. The U-shape supports the ring deformation under pressure loading. For the same reason, the second packing ring <NUM> could also be L-shaped. However, also a ring with a solid cross section could generally be used as the second packing ring <NUM>. Preferred embodiments of the second packing ring <NUM> will later be described in connection with <FIG>.

According to the invention the second packing ring <NUM> is made from a material comprising a polymer, which material has a thermal expansion coefficient α, which is at least two times higher than the thermal expansion coefficient αFE of iron. In a preferred embodiment the thermal expansion coefficient α of the second packing ring <NUM> is at least α=<NUM>×<NUM>-<NUM>K-<NUM>, preferably at least α=<NUM>×<NUM>-<NUM>K-<NUM>, in particular at least α=<NUM>×<NUM>-<NUM>K-<NUM>. According to a preferred embodiment, the polymer comprises at least one of: polytetrafluoroethylene (known as PTFE), polyphenylene sulphide (known as PPS), polyether ether ketone (known as PEEK), polyimide (known as PI) or polyamide (known as PA). Of course, also a combination of different polymers is conceivable.

The second packing ring <NUM> is further designed, such that at (or below) a defined activation temperature an inner diameter d_i of the second packing ring <NUM> is smaller than an outer diameter D_a of the piston rod <NUM>, such that the second packing ring <NUM> is prestressed in a radial direction in order to form a tight seal between the continuous inner circumferential sealing surface 21a of the second packing ring <NUM> and an outer circumferential surface 8a of the piston rod <NUM>. The second packing ring <NUM> is further designed, such that at a given operating temperature, the inner diameter d_i of the second packing ring <NUM> is larger than the outer diameter D_a of the piston rod <NUM>, such that the continuous inner circumferential sealing surface 21a is detached from the outer circumferential surface 8a of the piston rod <NUM> in order to provide a leakage path between the inner circumferential surface 21a of the second packing ring <NUM> and the outer circumferential surface 8a of the piston rod <NUM> in the axial direction. In <FIG> the cold state at ambient temperature is shown. Of course, in the mounted state of the second packing ring <NUM>, the inner diameter d_i of the second packing ring <NUM> corresponds to the outer diameter D_a of the piston rod <NUM>. However, in an unmounted state at ambient temperature, the inner diameter d_i of the second packing ring <NUM> would be smaller than the outer diameter D_a of the piston rod <NUM>, such that an interference, similar to a press fit, exists.

The activation temperature, at which the second packing ring <NUM> is in its shrunk state and seals the piston rod <NUM>, is typically <NUM> or below. The operating temperature, at which the second packing ring <NUM> is in its extended state and does not seal the piston rod <NUM>, typically lies in the range of <NUM> and above. However, the activation temperature highly depends on the operating temperature of the compressor <NUM>, which in turn can vary depending on the particular design and application of the compressor <NUM>. Therefore, once the expected operating temperature is known, the design of the second packing ring <NUM> can be adapted to a desired activation temperature. The operating temperature and activation temperature are preferably temperatures in the region of the piston rod <NUM>, in particular temperatures of the surface 8a of the piston rod <NUM>.

Due to the above-described features, at (a sufficiently long) standstill of the compressor <NUM>, the temperature of the second packing ring <NUM> as well as the temperature of the piston rod <NUM> decrease from the operation temperature to a temperature equal or below the activation temperature, e.g., the ambient temperature or a temperature between the activation temperature and the ambient temperature, such that the second packing ring <NUM> forms a tight sealing barrier on the surface 8a of the piston rod <NUM>. Hence, no gas, which is still contained in the cylinder <NUM> can escape from the cylinder <NUM> in axial direction into the distance piece <NUM> and possibly further into the crankcase <NUM> or the surrounding environment.

After the compressor <NUM> is started, during operation of the compressor <NUM>, the components of the compressor <NUM> begin to heat up due to the compression work and due to friction. In particular, the second packing ring <NUM> and the piston rod <NUM> are heated up due the friction, which results from the reciprocating motion of the piston rod <NUM> relative to the second packing ring <NUM> and also from the reciprocating motion of the piston rod <NUM> relative to the number of first packing rings <NUM>. After a certain operating time, the operating temperature is reached, which essentially stays constant during further operation. Within the scope of the invention, "essentially constant" can mean, that the operating temperature has a certain variation, which can be in the range of around ±<NUM>. Due to the above-described features, upon reaching the operating temperature, the continuous inner circumferential sealing surface 21a of the second packing ring <NUM> detaches from the outer circumferential surface 8a of the piston rod <NUM>, such that a leakage path between the inner circumferential surface 21a of the second packing ring <NUM> and the outer circumferential surface 8a of the piston rod <NUM> in the axial direction is created.

When the compressor <NUM> is again shut down, the reverse effect occurs. This means, that the components of the compressor <NUM> gradually cool down until the activation temperature is reached. Upon reaching of the activation temperature the second packing ring <NUM> is shrunk back onto the rod (d_i ≤ D_a), such that a tight sealing barrier is reestablished. Thus, it can be seen, that an automatic, essentially temperature-dependent, sealing effect is reached, without any control intervention being necessary. However, further advantageous features can be applied to the sealing device, in order to improve its performance, which features will be described in further detail below.

As shown in <FIG>, the second packing ring <NUM> preferably has a U-shaped cross section comprising an inner shank <NUM> an outer shank <NUM> in the radial direction, which shanks are spaced in the radial direction. A preferred embodiment of such a U-shaped second packing ring <NUM> is shown in detail in <FIG>, wherein <FIG> shows a cross section, <FIG> shows a plan view and <FIG> shows an isometric view of the second packing ring <NUM>. The second packing ring <NUM> has an outer diameter d_a, which is smaller than the diameter of the retaining opening 20a of the second packing retainer <NUM>, in which the second packing ring <NUM> is to be arranged, as can be seen in <FIG>. Thus, a ring space is formed between the outer circumferential surface 21b of the second packing ring <NUM> and an inner circumferential surface of the cylindrical retaining opening 20a.

Further, in the example shown in <FIG> a number of openings <NUM> are provided on the outer shank <NUM> of the second packing ring <NUM>, wherein each of the number of openings <NUM> connects an inside space <NUM> of the U-shaped packing ring <NUM> with the radial outer circumferential surface 21b of the second packing ring <NUM>. The inside space <NUM> is formed between the opposite shanks <NUM>, <NUM> of the U-shape in the radial direction. The openings <NUM> therefore extend through the outer shank <NUM> and are spaced from opposite axial ends 21c, 21d of the second packing ring <NUM>. The number of openings <NUM> can essentially have any suitable form, e.g., cylindrical drillings or groove-shaped millings. If the second packing ring <NUM> is for instance 3D-printed, also more complex forms could be used.

Preferably the number of openings <NUM> are shaped as elongated holes or elliptical holes, the elongated holes being shown in <FIG>. Each elongated hole comprises a longitudinal axis L and a first hole end 22a and an opposite second hole end 22b in the direction of the longitudinal axis L, wherein the first hole end 22a is preferably closer to the first ring end 21c than the second hole end 22b, which is closer to the second axial ring end 21d. Thus, the elongated holes <NUM> are essentially inclined relative to the front surfaces of the second packing ring <NUM>, which are formed at the axial ring ends 21c, 21d. This shape is advantageous, because it makes the second packing ring <NUM> more flexible in the axial direction.

Further, the outer shank <NUM> of the U-shape has a length b_a in the axial direction which is preferably larger than a length b_i of the inner shank <NUM> in the axial direction. In the unmounted state, the length b_a of the outer shank <NUM> is preferably slightly larger than an axial length of the retaining opening 20a of the second packing retainer <NUM>. In the example according to <FIG>, the axial length of the retaining opening 20a of the second packing retainer <NUM> is formed by a distance between an axial end face of the retaining opening 20a and the opposing axial end face of the intermediate plate <NUM>, as can be seen in <FIG>. Due to the openings <NUM>, the outer shank <NUM> has a certain structural elasticity, such that the outer shank <NUM> can be elastically deformed in the axial direction, which results in a tight seat inside the retaining opening 20a. The number of openings <NUM> as well as the arrangement of the openings <NUM> on the second packing ring <NUM> depend on the size of the ring and can vary. In the circumferential direction, the openings <NUM> are preferably equally distributed, in order to provide an elasticity in the circumferential direction, which is as uniform as possible.

In the radial direction however, the second packing ring <NUM> does not have such a structural elasticity, but is particularly rigid, such that even in the heated state at the operating temperature, essentially no (or negligibly small) elastic deformation due to a differential pressure occurs. Thereby, it is guaranteed, that at operating temperature, the inner diameter d_i of the second packing ring <NUM> is always larger than the outer diameter D_a of the piston rod <NUM>, which results from the thermal expansion, as explained above. In order to enhance the structural rigidity also at higher temperatures, it can be advantageous, that the material of the second packing ring <NUM> is a fiber-reinforced composite material.

The embodiment according to <FIG> is of course not limiting. As mentioned above, there are also other embodiments possible, some which are described below in connection with <FIG>.

<FIG> shows a U-shaped second packing ring <NUM>, which is essentially similar to the ring shown in <FIG>. Therefore, only the differences will be described in detail. The second packing ring <NUM> according to <FIG> additionally comprises an O-ring <NUM>, which has a relatively high elasticity, in particular much higher than the material of the second packing ring <NUM>. The O-ring is arranged on a shoulder, which is formed by a circumferential groove <NUM>. The circumferential groove <NUM> is positioned adjacent the outer circumferential surface 21b and adjacent to the surface of the second packing ring <NUM> at the second axial end 21d. As can be seen in <FIG>, a diameter of the O-ring <NUM> is slightly larger than the axial depth of the groove <NUM> in the unmounted state. In the mounted state of the second packing ring <NUM>, the O-ring <NUM> essentially acts like a spring. The O-ring <NUM> helps sealing in the radial direction (between the second axial end 21d of the second packing ring <NUM> and the axial wall of the retaining opening 20a - see <FIG>), when the second packing ring <NUM> is in its cold state, since in the cold state, the axial length of the second packing ring <NUM> is smaller than in the hot state due to thermal expansion, which also acts in the axial direction. In the hot state, when the second packing ring <NUM> is thermally expanded in the axial direction, the O-ring <NUM> is compressed and compensates the axial change in length. Thus, the mechanical stress on the outer shank <NUM> of the second packing ring <NUM> can be decreased.

<FIG> shows a cross section of another preferred embodiment of the second packing ring <NUM>. As can be seen, the second packing ring <NUM> has an L-shaped cross section. This allows for a very simple manufacturing, since no openings <NUM> are necessary in this embodiment. Like the U-shaped ring of <FIG> and <FIG>, the second axial end 21d of the L-shaped second packing ring <NUM> of <FIG> also faces in the direction of the first axial device end 15a (or the cylinder <NUM> respectively - see <FIG>) of the sealing device <NUM> or the high-pressure side HP respectively (see <FIG>). Of course, an O-ring (not shown), similar as in <FIG>, could additionally be provided on the L-shaped packing ring <NUM> of <FIG>.

<FIG> shows a cross section of another preferred embodiment of the second packing ring <NUM>. In this embodiment, the second packing ring <NUM> comprises a rectangular cross section. In order to allow gas to flow from the first axial end 21c (cylinder side) to the ring's outer circumferential surface 21b, a number of openings <NUM> are provided, which in each case connect the first axial end 21c of the second packing ring <NUM> with the outer circumferential surface 21b of the of the second packing ring <NUM>. The openings <NUM> can essentially have any suitable form, e.g., inclined drillings, as shown in <FIG>, or intersecting and preferably perpendicular axial and radial openings.

The sealing device <NUM> of the embodiment shown in <FIG> further comprises a support passage <NUM> having a first support passage end 28a and a second support passage end 28b, as indicated in <FIG>. For the sake of simplicity, a part of the support passage <NUM> is only shown schematically in <FIG>. In the shown preferred embodiment, the first support passage end 28a of the support passage <NUM> is connected to the retaining opening 20a of the second packing retainer <NUM> in a region radially outside of the second packing ring <NUM>. In particular, the first support passage end 28a opens into a space, which is formed between the outer circumferential surface 21b of the second packing ring <NUM> and the inner circumferential surface of the retaining opening 20a, in which the second packing ring <NUM> is arranged. However, this is only an exemplary embodiment and the first support passage end 28a of the support passage <NUM> does not necessarily need to be connected to the retaining opening 20a. It could as well be located between the second packing ring <NUM> and the subsequent first packing ring <NUM>, for instance on an inner circumferential surface of the intermediate plate <NUM>. However, if the second packing retainer <NUM> was designed in another fashion, the first support passage end 28a of the support passage <NUM> could for instance also be located at an inner circumferential surface of the second packing retainer <NUM>, next to the retaining opening 20a towards the first axial device end 15a. Further a valve <NUM> for opening and closing the support passage <NUM> is provided between the first support passage end 28a and the second support passage end 28b.

As depicted in <FIG>, a first part of the support passage <NUM> is integrally formed with the second packing retainer <NUM>, while and a second part of the support passage <NUM> lies outside of the second packing retainer <NUM>. The second part of the support passage <NUM> can e.g., be implemented as a conduit, which is connected to the second packing retainer <NUM> by means of a suitable connector. In the shown embodiment, the connector is arranged on an axial face of the second packing retainer <NUM>, facing towards the second device end 15b of the sealing device <NUM>. However, also another position is possible, e.g., on a radial outer circumference, provided, that there is sufficient space for connecting the second part of the support passage <NUM>.

In the shown example, the second packing retainer <NUM> also serves as a mounting flange for mounting the sealing device <NUM> on the compressor <NUM>. For the sake of simplicity, mounting elements, such as screws or the like, are not shown in <FIG>. Of course, this is not necessary and the second packing retainer <NUM> could also be provided in the form of a separate component in addition to the mounting flange, in particular adjacent the mounting flange in the direction of the first axial device end 15a. In this case the first part of the support passage <NUM> (adjacent the first passage end 28a) can be integrally formed with the second packing retainer <NUM>, a second (intermediate) part of the support passage <NUM> can be integrally formed with the flange and a third part of the support passage <NUM> (adjacent the second passage end 28b) can again be implemented as a conduit. Of course, a suitable sealing element, such as an O-ring, can be arranged between the flange and the second packing retainer <NUM>, in order to provide a seal between the first part of the support passage <NUM> (inside the second packing retainer <NUM>) and the second part of the support passage <NUM> (inside the flange).

The support passage <NUM> has essentially two functions. During operation of the compressor <NUM> (when the operating temperature is reached and the second packing ring <NUM> is detached from the piston rod <NUM>) the function of the support passage <NUM> is to provide an essentially free leakage path for the gas, which flows from the cylinder <NUM> past the number of first packing rings <NUM> in the direction of the second device end 15b of the sealing device <NUM>. Thus, the support passage <NUM> essentially prevents an undesired, relatively large, pressure drop, which would otherwise (without the passage <NUM>) occur across the second packing ring <NUM> due to the relatively small gap (similar to a throttle), created between the inner circumferential sealing surface 21a of the second packing ring <NUM> and the outer circumferential surface 8a of the piston rod <NUM> in the detached state. Although, the invention would also work without the support passage <NUM>, the support passage <NUM> is advantageous in order to allows for an essentially free flow of gas coming from the cylinder <NUM>.

On the other hand, in the sealing state of the second packing ring <NUM> at or below the activation temperature and at standstill of the compressor <NUM>, a tight sealing barrier is formed between the second packing ring <NUM> and the piston rod <NUM>, as was described earlier. In order for the second packing ring <NUM> to unfold its desired sealing effect, the valve <NUM> of the support passage of course needs to be in its closed position, because otherwise the gas would not be contained and would flow through the support passage <NUM> in the direction of the second support passage end 28b. In order to automatically control the opening/closing of the valve <NUM>, it is advantageous, that the valve <NUM> comprises an electrically controllable actuator <NUM>, which can be controlled by a control unit (e.g., the compressor control unit <NUM>, as shown in <FIG>) in order to open and close the valve <NUM>.

In the shown advantageous embodiment according to <FIG>, the sealing device <NUM> further comprises an unobstructed vent passage <NUM>, comprising a first vent passage end 31a and an opposite second vent passage end 31b. The vent passage <NUM> is configured to vent a gas, leaking from the first axial device end 15a (or the cylinder <NUM> respectively) in the direction of the second axial device end 15b (or the distance piece <NUM> respectively) past the number of first packing rings <NUM> and past the second packing ring <NUM> (in particular between the inner circumferential surface 21a of the second packing ring <NUM> and the outer circumferential surface 8a of the piston rod <NUM>), from the first vent passage end 31a to the second vent passage end 31b. This enhances the safety at compressor standstill, because gas, leaking past the second packing ring <NUM>, e.g., due to the second packing ring <NUM> being worn out, damaged or completely broken, can at any time be safely vented through the unobstructed vent passage <NUM>.

Preferably the sealing device <NUM> further comprises at least one third packing retainer <NUM> including a retaining opening 32a, in which a third packing ring <NUM> is arranged, as shown in <FIG>. The at least one third packing retainer <NUM> can essentially be designed in a similar fashion as the number of first packing retainers <NUM>. However, as can be seen in <FIG>, different types of packing rings can be used for the third packing ring <NUM> and for the first packing ring <NUM>. Therefore, the size of the retaining opening 32a of the third packing retainer <NUM> can be different than the size of the retaining opening 18a of a first packing retainer <NUM>.

The third packing retainer <NUM> is arranged closer to the second axial device end 15b of the sealing device <NUM> than the second packing retainer <NUM>. Similar as described above in connection with the intermediate plate <NUM>, the retaining opening 32a of the third packing retainer <NUM> is delimited by the axial front face of the second packing retainer <NUM>, which front face faces towards the second device end 15b of the sealing device <NUM>. Similar to the first packing rings <NUM>, the at least one third packing ring <NUM> can for instance comprise multiple ring segments in the axial direction and/or multiple ring segments in the circumferential direction. In the shown example in <FIG>, a single third packing retainer <NUM> is provided and the third packing ring <NUM>, which is arranged in the retaining opening 32a of the third packing retainer <NUM>, comprises three sealing rings consecutively arranged in the axial direction, wherein each of the sealing rings comprises multiple ring segments in the circumferential direction. Such rings are known in art, as already mentioned above in connection with the first packing ring <NUM>.

Similar to the first packing retainers <NUM> and the second packing retainer <NUM>, also the third packing retainer <NUM> can either be made from one piece, in which the retaining opening 32a is arranged, e.g., milled, or can be assembled from more than one piece. For instance, the third packing retainer <NUM> can comprise a face plate and an adjacent piece, in which the third packing ring <NUM> is arranged. The face plate can e.g., be a cylindrical plate such as the intermediate plate <NUM> and the adjacent piece can e.g., be a hollow cylinder.

If a third packing retainer <NUM> is provided in the sealing device <NUM>, the first vent passage end 31a of the unobstructed vent passage <NUM> can be directly connected to the retaining opening 32a of the third packing retainer <NUM>, preferably to a space, which is formed radially outside of the third packing ring <NUM>, as shown in the example according to <FIG>. Again, as described in connection with the support passage <NUM>, a first part of the vent passage <NUM>, adjacent the first vent passage end 31a, can be integrally formed with the third packing retainer <NUM> and a second part of the vent passage <NUM>, adjacent the second vent passage end 31b, can be formed by means of a conduit. However, <FIG> only shows an exemplary embodiment and similar to the first support passage end 28a of the support passage <NUM>, the first vent passage end 31a of the vent passage <NUM> could as well be arranged at a different location. For example, the first vent passage end 31a could be located between the third packing ring <NUM> and the second packing ring <NUM> in the axial direction of the sealing device <NUM>, e.g. on the inner circumferential surface of the second packing retainer <NUM>.

As can be depicted from <FIG>, the second vent passage end 31b of the unobstructed vent passage <NUM> and the second support passage end 28b of the support passage <NUM> are preferably connected to a common discharge passage <NUM>, which is connectable to a discharge space <NUM>. Due to the vent passage <NUM> being unobstructed, at any time gas, which leaks past the second packing ring <NUM> can be safely discharged into the discharge space <NUM>. The discharge space <NUM> can for instance be a reservoir for storing the gas, such as a tank, or it could also be a disposal system for disposing the gas, such as a flare device for burning the gas. This allows for a very safe operation, because even in case of a breakdown of the second packing ring <NUM> gas is safely discharged through the vent passage <NUM> and does therefore not enter into the distance piece <NUM> at standstill of the compressor <NUM>, in which the valve <NUM> is in its closed position.

In the shown example, in which the valve <NUM> of the support passage <NUM> can be actuated (opened/closed) by means of an electrically controllable actuator <NUM>, the actuator <NUM> can preferably be controlled dependent on an operation condition of the compressor <NUM>. For instance, a logic, according to which the actuator <NUM> of the valve <NUM> is controlled dependent on a start signal S_start and/or stop signal S_stop for the drive unit <NUM>, can be implemented in the control unit <NUM>. The control unit <NUM> can for instance be configured to send a start signal S_start to the drive unit <NUM> for starting the operation of the compressor <NUM>, as indicated in <FIG>. Additionally, the control unit <NUM> can send an opening signal S_open to the actuator <NUM> of the valve <NUM> of the support passage <NUM> for opening the valve <NUM>. The opening signal S_open can either be sent simultaneously with the start signal S_start, such that the valve <NUM> is immediately opened upon startup. Alternatively, the opening signal S_open can be sent a predetermined or adjustable opening lead time before the start signal S_start is sent, such that the valve <NUM> is opened a certain time prior to startup. The starting lead time can preferably be in the range between <NUM> to <NUM> seconds. By this, it is guaranteed, that valve <NUM> is open at the moment the compressor starts.

The control unit <NUM> can also be configured to send a stop signal S_stop to the drive unit <NUM> for stopping the operation of the compressor <NUM> and can further send a closing signal S_close to the actuator <NUM> of the valve <NUM> of the support passage <NUM> for closing the valve <NUM>. Again, the closing signal S_close can be sent simultaneously with the stop signal S_stop or after a predetermined or adjustable closing delay time after the stop signal S_stop has been sent. The closing delay time is preferably also in the range between <NUM> to <NUM> seconds. Although a simultaneous sending is possible, this is not necessary, since according to the invention the second packing ring <NUM> is thermally activated and not by the differential pressure. A certain closing delay time can e.g., be advantageous in order to make sure, that the piston rod <NUM> is at a complete standstill, before the valve <NUM> is closed, such that a contact and possible wear of the second packing ring <NUM> can in any case be avoided.

The valve <NUM> preferably also comprises a sensor <NUM> for detecting an opening state of the valve <NUM>. The sensor <NUM> is configured to transmit a sensor value X, representative for the opening state (e.g., opened or closed position) to the control unit <NUM>. The control unit <NUM> can then process the sensor value X and can for instance send the start signal S_start to the drive unit <NUM>, only when a sensor signal X, representative for an open state of the valve <NUM>, is received. The compressor <NUM> is therefore only started, when the valve <NUM> is open and a start can be prevented as long as the valve <NUM> is closed. This prevents the compressor <NUM> to start with the valve <NUM> closed and therefore a high differential pressure acting on the second packing ring <NUM>, which could possibly push the packing ring <NUM> too hard onto the piston rod <NUM>, which might create overheat, excessive wear or possible mechanical failure.

According to another preferred embodiment, the compressor <NUM> can comprise a suitable operation condition sensor (not shown), which is configured to detect a sensor value, representative for an operation condition of the compressor <NUM>. The operation condition sensor can e.g., be a temperature sensor, which is configured to sense the operation temperature. The temperature sensor can be arranged at a suitable position in order to sense the operating temperature e.g., in the region of the second packing ring <NUM> and/or in the region of the piston rod <NUM>. The operation condition sensor can be connected to the control unit <NUM> for submitting the sensor value to the control unit <NUM>. The control unit <NUM> can further be configured to control the valve <NUM> of the support passage <NUM> dependent on the sensor value, in particular the operating temperature. On the other hand, valve <NUM> could also be directly controlled by the operation condition sensor, such that no control by the control unit <NUM> is necessary. In this case, for instance a thermally actuated valve can be used as the valve <NUM>, which is essentially a combination of the valve <NUM>, the actuator <NUM> and the operation condition sensor. Thus, the actuator <NUM> of the valve <NUM> can open and close the valve <NUM> automatically dependent on the detected sensor value, without a control intervention of the control unit <NUM> being required.

According to an alternative embodiment, the operation condition sensor could e.g., be a movement sensor, which is configured to detect a sensor value, representative of a movement of a moving part of the compressor <NUM>. The movement sensor can for instance be a speed sensor for detecting a rotational speed of the crankshaft or a translational speed of the piston rod <NUM> or the like. The movement sensor can again be connected to the control unit <NUM>, wherein the control unit <NUM> is configured to control the actuator <NUM> of valve <NUM> depending on the sensor value of the movement sensor. On the other hand, the movement sensor can again be implemented in the valve <NUM> together with the actuator <NUM>, such that the valve <NUM> is directly actuated, independently from the compressor control unit <NUM>. Of course, the temperature sensor and the movement sensor are only examples and other sensors, which are suitable to detect relevant sensor values, which are representative for the operation condition of the compressor <NUM> could also be used. In a preferred embodiment, the activating values of the measured sensor value, e.g., temperature, rotational speed, etc., at which the valve <NUM> is to be closed or opened, can be adjusted, e.g., via the control unit <NUM> or directly at the valve <NUM> or the actuator <NUM>.

Although not shown in the example according to <FIG>, further packing retainers could additionally be arranged in the sealing device <NUM>. For instance, it would be conceivable to add a fourth packing retainer <NUM> adjacent the third packing retainer <NUM>, the fourth packing retainer <NUM> being closer to the second axial device end 15b, than the third packing retainer <NUM>. An exemplary embodiment is shown in <FIG>. The fourth packing retainer <NUM> includes a retaining opening 42a, in which a wiper ring <NUM> is arranged, which wiper ring <NUM> serves for wiping residual oil from the outer circumferential surface 8a of the piston rod <NUM>. A wiper ring <NUM> can comprise one or more wiping edges 43a on the inner circumferential surface. The wiper ring <NUM> can further comprise one or more discharge channels 43b, connecting an inner circumferential surface of the wiper ring <NUM> with an outer circumferential surface. Through the discharge channel(s) 43b wiped oil can be transferred to an oil collecting space, provided inside the retaining opening 42a arranged radially outside of the wiper ring <NUM>. The discharge space can be connected to an oil reservoir <NUM> by means of a suitable oil return line <NUM>. Since such wiper rings <NUM> are known in the art, there will be no further detailed description at this point. Such an embodiment of the sealing device <NUM>, that includes a wiper ring <NUM>, can for instance be used in a compressor <NUM>, which does not have a distance piece <NUM> like the compressor <NUM> shown in <FIG>. Such a sealing device <NUM> is often referred to a so-called combined sealing device.

Similar as the embodiment according to <FIG>, the sealing device <NUM> according to <FIG> also comprises a support passage <NUM> including a valve <NUM> and an unobstructed vent passage <NUM>. Since the function of the support passage <NUM> and the function of the vent passage <NUM> were already described in detail in connection with <FIG>, only the differences between the two embodiments will be described in the following. Unlike in <FIG>, in the embodiment of <FIG> the first vent passage end 31a of the vent passage <NUM> is located between the third packing ring <NUM> and the second packing ring <NUM> in the axial direction of the sealing device <NUM> (or the piston rod <NUM> respectively). A first part of the vent passage <NUM> is integrally formed in the second packing retainer <NUM>, in which the second packing ring <NUM> is arranged. The first vent passage end 31a in this case is located on the inner circumferential surface of the second packing retainer <NUM>. In the shown example, the second packing retainer <NUM> also serves as a mounting flange for mounting the sealing device <NUM> on the compressor <NUM>. Of course, suitable mounting means (not shown), such as bolts or screws, can be provided on the flange. The function of the vent passage <NUM> is however the same, as in <FIG>. Such a design is advantageous, because a standardized packing retainer (without an integrated passage) can be used the third packing retainer <NUM>.

In the embodiment according to <FIG> the location of the first support passage end 28a of the support passage <NUM> also differs from the embodiment according to <FIG>. In <FIG>, the first support passage end 28a is directly connected to the retaining opening 20a of the second packing retainer <NUM>. Contrary to that, in <FIG> the first support passage end 28a is located between the first packing ring 19_1, that is arranged adjacent the second packing ring <NUM>, and the subsequent first packing ring 19_2 in the direction of the first axial device end 15a of the sealing device <NUM>. This design has essentially two advantages. On the one hand, this configuration allows the first packing ring 19_1, that is adjacent the second packing ring <NUM>, to provide an additional seal during the time between a shut-off of the compressor <NUM> and the thermal activation of the second packing ring <NUM>, which activation is delayed due to necessary time for cooling down and shrinking. Therefore, until the point, at which the sealing effect of the second packing ring <NUM> unfolds entirely, the first packing ring 19_1 can provide an additional sealing effect due to the remaining pressure in the cylinder <NUM> (<FIG>).

In order to provide such an additional sealing effect, it is advantageous, that the first packing ring 19_1, which is adjacent the second packing ring <NUM>, is designed differently than the subsequent first packing ring 19_2 (and differently than the further first packing rings 19_3, which are arranged adjacent the first packing ring 19_2 towards the first axial device end 15a in <FIG>). In particular, the first packing ring 19_1 is preferably designed as a so-called "double-acting" ring, which is configured to provide a sealing effect also in absence of a differential pressure over the ring. A double-acting ring can for example comprise a ring assembly with two tangentially cut rings, e.g., two identical tangentially cut rings. Another suitable ring would e.g., be a so-called "tangential to rod" ring. Such rings are known in the art. The first sealing ring 19_2 and the subsequent first sealing rings 19_3 can for example be identical and can e.g., each comprise a ring assembly with a radially cut ring and a tangentially cut ring, as already mentioned in connection with the first packing rings <NUM> in <FIG>. On the other hand, the pressure-drop over the first packing ring 19_1, that is adjacent the second packing ring <NUM>, can be used for cooling the second packing ring <NUM>, which accelerates the cooling down and hence the thermal activation. In this case it is particularly preferable, if the respective first packing ring 19_1 comprises a metal material. For instance, the above-mentioned backup ring, made of metal, can be used as a part of the first packing ring 19_1.

Another preferred embodiment of the sealing device <NUM> of the invention will be explained in connection with <FIG>. The number of first packing retainers <NUM> of the sealing device <NUM> of the shown embodiment comprises two first packing retainers <NUM>, each having a retaining opening 18a, in which a first packing ring <NUM> is arranged. The first packing rings <NUM> can again be designed, as was described earlier. Of course, a larger number of first packing retainers <NUM> could also be provided, similar as shown in <FIG>+2b. The sealing device <NUM> further comprises a second packing retainer <NUM>, having a retaining opening 20a, in which the second packing ring <NUM> is arranged. The second packing retainer <NUM> is arranged closer to the second device end 15b than the first packing retainers <NUM>. An exemplary U-shaped second packing ring <NUM> is shown. However, the second packing ring <NUM> could have a different design, as was described in connection with Flg. Adjacent the second packing retainer <NUM>, an intermediate plate <NUM> is arranged, in which a first part of the unobstructed vent passage <NUM> is arranged. As can be seen, the first vent passage end 31a of the vent passage <NUM> is located between the second packing ring <NUM> and the subsequent first packing ring <NUM> in the direction of the second device end 15b, similar as in the embodiment according to <FIG>.

The sealing device <NUM> further comprises a fifth packing retainer <NUM> having a T-like cross section and comprising two retaining openings 46a, which are separated by a central part of the T-like cross section in the axial direction. In each of the retaining openings 46a a fifth packing ring <NUM> is arranged. The fifth packing rings <NUM> are each formed as a so-called single-acting SLP-ring assembly, which comprises three consecutive rings in the axial direction, wherein each of the three rings being a cut ring, comprising multiple ring segments in the circumferential direction. The central ring and one of the outer rings each comprise a chamfer, wherein the chamfer of the central ring faces towards the outside in the radial direction and the chamfer on the outer ring faces towards the inside in the radial direction. The chamfers face each other and are in contact.

As indicated in <FIG>, the two fifth packing rings <NUM> are arranged mirror-inverted, such that the outer chamfered rings face each other in the axial direction and are in contact with the central part of the T-like cross section of the fifth packing retainer <NUM>. In the mounted state of the sealing device <NUM> in the compressor <NUM>, a pressure chamber 46b is formed in a space, which is located between the two fifth packing rings <NUM> in the axial direction and between the central part of the T-like cross section of the fifth packing retainer <NUM> and the piston rod <NUM> in the radial direction. Further a flange F for mounting the sealing device <NUM> on the compressor <NUM> is arranged in the sealing device <NUM>. The flange F is arranged on the very left and comprises the second axial device end 15a of sealing device <NUM>, that is configured to face towards the crankcase <NUM> (<FIG>). As can be seen in <FIG>, the Flange F delimits the left retaining opening 46a of the fifth packing retainer <NUM>, which is closer to the second device end 15b and the intermediate plate <NUM> delimits the right retaining opening 46a of the fifth packing retainer <NUM>, which is closer to the first device end 15a.

Further a purge passage <NUM> is provided on the sealing device <NUM>, the purge passage <NUM> having a first purge passage end 48a and a second purge passage end 48b. The first purge passage end 48a is located on the inner circumferential surface of the on the central part of the T-like cross section of the fifth packing retainer <NUM> between the two fifth packing rings <NUM> in the axial direction. A first part of the purge passage <NUM>, adjacent the first purge passage end 48a, is integrally formed in the fifth packing retainer <NUM>, a second intermediate part of the purge passage <NUM> is integrally formed in the flange F and a third part of the purge passage <NUM>, adjacent the second purge passage end 48b, is formed as a conduit, which is suitably connected to the flange F. The second purge passage end 48b is connected to a source <NUM> of pressurized gas, preferably comprising nitrogen. A valve (not shown) for opening/closing the purge passage <NUM> could also be provided.

When the pressurized gas from the source <NUM> is introduced into the pressure chamber 46b, a pressure is generated inside the pressure chamber 46b, which presses the respective outer chamfered ring of each fifth packing ring <NUM> onto the respective central ring in the axial direction. Due to the interacting chamfers, the central chamfered rings are pressed onto the piston rod <NUM> in the radial direction in order to form a tight seal. Gas, leaking from the first device end 15a (or the compression chamber <NUM> respectively - <FIG>) past the first packing rings <NUM> and past the second packing ring <NUM> in the direction of the second device end 15b, can thus be safely vented through the unobstructed vent passage <NUM>. This is not only due to the sealing effect of the fifth packing rings <NUM>, but also due to the pressure in the pressure chamber 46b preferably being higher than the pressure in the region of the first vent passage end 31a.

<FIG> shows a different design of the sealing device <NUM> including a fifth packing retainer <NUM>. In this case the fifth packing retainer <NUM> has an L-shaped cross section and only comprises a single retaining opening 46a. The retaining opening 46a faces in the direction of the second axial device end 15b of the sealing device <NUM> and is delimited by the flange F in the axial direction. In the retaining opening 46a only one fifth packing ring <NUM> is arranged, which is designed differently than the fifth packing ring <NUM> of <FIG>. The fifth packing ring <NUM> in this case is a so-called double acting DSLP-ring assembly. The DSLP-ring assembly is essentially a combination of the above-mentioned SLP-ring assembly, in which the respective outer chamfered rings are combined to a single central ring, having chamfers on both sides in the axial direction. Hence, the fifth packing ring <NUM> of <FIG> comprises five consecutive rings in the axial direction, each ring having multiple ring segments in the circumferential direction.

The first purge passage end 48a of the purge passage <NUM> is connected to a space inside the retaining opening 46a, located radially outside of fifth packing ring <NUM>, which space forms the pressure chamber 46b. The axial position of the first purge passage end 48a is preferably in the center of the retaining opening 46a, however also another position would be possible. Again, a first part of the purge passage <NUM>, adjacent the first purge passage end 48a, is integrally formed in the fifth packing retainer <NUM> and a second intermediate part of the purge passage <NUM> is integrally formed in the flange F. The third part of the purge passage <NUM>, adjacent the second purge passage end 48b, lies outside of the flange F and can be implemented as a conduit. The conduit is connected to a source of pressurized gas (not shown). Further, in the embodiment according to <FIG>, the unobstructed vent passage <NUM> also passes through the fifth packing retainer <NUM> as well as through the flange F.

The purge passage <NUM> is spaced from the vent passage <NUM> in the circumferential direction, in order to not interfere with the vent passage <NUM>. Like in <FIG>, the first vent passage end 31a of the vent passage <NUM> is located between the second packing ring <NUM> and the fifth packing ring <NUM> in the axial direction of the sealing device <NUM>. However, no intermediate plate <NUM> as in <FIG> is necessary in the embodiment of <FIG>. The overall axial length of the sealing device <NUM> according to <FIG> can therefore be slightly smaller than the length of the sealing device according to <FIG>. The second packing ring <NUM> is designed according to the embodiment of <FIG>. However, any other of the described designs would of course be possible as well.

Claim 1:
Sealing device (<NUM>) for sealing a piston rod (<NUM>) of a reciprocating compressor (<NUM>), the sealing device (<NUM>) comprising a first axial device end (15a), configured to face toward a cylinder (<NUM>) of the compressor (<NUM>), and an opposite second axial device end (15b), configured to face toward a crankcase (<NUM>) of the compressor (<NUM>), a number of first packing retainers (<NUM>), each retainer including a retaining opening (18a) in which a first packing ring (<NUM>) is arranged, a second packing retainer (<NUM>), including a retaining opening (20a) in which a second packing ring (<NUM>) is arranged, the second retainer (<NUM>) being positioned closer to the second axial device end (15b) than the number of first packing retainers (<NUM>) in an axial direction of the sealing device (<NUM>), wherein the second packing ring (<NUM>) is an uncut ring, comprising a continuous inner circumferential sealing surface (21a), wherein the second packing ring (<NUM>) is made from a material comprising a polymer, characterized in that the material has a thermal expansion coefficient (α), which is at least two times higher than the thermal expansion coefficient (αFE) of iron, wherein at or below a defined activation temperature an inner diameter (d_i) of the second packing ring (<NUM>) is smaller than an outer diameter (D_a) of the piston rod (<NUM>) to be sealed, such that in the mounted state of the sealing device (<NUM>) in the compressor (<NUM>) the second packing ring (<NUM>) is prestressed in a radial direction in order to form a tight seal between the continuous inner circumferential sealing surface (21a) of the second packing ring (<NUM>) and the outer circumferential surface (8a) of the piston rod (<NUM>), wherein at a given operating temperature, the inner diameter (d_i) of the second packing ring (<NUM>) is larger than the outer diameter (D_a) of the piston rod (<NUM>), such that in the mounted state of the sealing device (<NUM>) in the compressor (<NUM>) the continuous inner circumferential sealing surface (21a) of the second sealing ring (<NUM>) is detached from the outer circumferential surface (8a) of the piston rod (<NUM>) in order to provide a leakage path past the second packing ring (<NUM>) in the axial direction.