Method and system for reducing seal gas consumption and settle-out pressure reduction in high-pressure compression systems

A compressor is disclosed having a shaft seal assembly and system that allows a high-pressure compressor to settle-out at a lower pressure level during shutdown. The seal assembly may be disposed about a portion of the shaft and define a blow-down seal chamber, the seal assembly including one or more gas seals in fluid communication with the blow-down seal chamber. A blow-down line is communicably coupled to the blow-down seal chamber to reference the blow-down seal chamber to a low pressure reference, such as a separate centrifugal compressor, or the like. Referencing the blow-down seal chamber to the low pressure reference reduces the required sealing pressure of the one or more gas seals. A valve may be disposed in the blow-down line and configured to regulate a flow of process gas leakage through the blow-down line.

BACKGROUND

During the shut-down process of a high-speed compressor, such as a centrifugal compressor, and after the compressor remains idle at zero speed, the pressure inside the compressor casing eventually reaches what is known as “settle-out” pressure. At settle-out pressure, the pressures inside the compressor and any process piping connected thereto reach an equilibrium that will typically remain until the system is either vented or restarted. Current seal technology implemented in high-pressure compressors is limited as to how high the settle-out pressure can reach before exceeding the current state of the art of internal seals, such as gas seals, and thereby risking potential seal failure.

One common method of circumventing this occurrence is to overdesign the machine and accompanying process system such that the projected settle-out pressure is always within the design range of the seal system. Oftentimes, this can involve designing and installing an expensive and inexperienced prototype seal. Unfortunately, prototype seals often have unknown reliability and are designed only for the application at hand. Another common method of reducing the effects of settle-out pressure is to design the process system to favor the suction (i.e., low pressure) volume of the compressor system, and minimizing the volume of the high pressure system isolated by the shut down valve. This frequently results, however, in larger and more costly process systems.

What is needed, therefore, is a system and method configured to allow a high-pressure compressor to settle-out at reduced pressure levels while simultaneously reducing blow-down seal leakage during normal operation.

SUMMARY

Embodiments of the disclosure may provide a compressor. The compressor may include an inlet for receiving a process gas, and an outlet for discharging a high-pressure process gas, and a shaft extending from a first end of the compressor to a second end of the compressor, the shaft having one or more compression stages disposed about the shaft and rotatable therewith, wherein the one or more compression stages are configured to receive and compress the process gas from the inlet and discharge the high-pressure process gas via the outlet. The compressor may further include a seal assembly disposed about a portion of the shaft and defining a blow-down seal chamber, the seal assembly including at least one gas seal in fluid communication with the blow-down seal chamber, and a blow-down line communicably coupled to the blow-down seal chamber to reference the blow-down seal chamber to a low pressure reference and thereby reduce a pressure which the at least one gas seal must seal against. In one embodiment, a valve is disposed in the blow-down line and configured to regulate a flow of process gas leakage through the blow-down line.

Embodiments of the disclosure may further provide a method of operating a compressor. The method may include progressively compressing a process gas in one or more compression stages disposed about a rotatable shaft, sealing the process gas within the compressor with a seal assembly disposed about the shaft and defining a blow-down seal chamber, the seal assembly including at least one gas seal in fluid communication with the blow-down seal chamber, and referencing the blow-down seal chamber to a lower-pressure machine via a blow-down line. The method may further include regulating a flow of process gas leakage through the blow-down line with a valve disposed in the blow-down line.

Embodiments of the disclosure may further provide a shaft seal system. The shaft seal system may include a seal assembly disposed about a rotatable shaft and defining a blow-down seal chamber, the seal assembly including at least one gas seal in fluid communication with the blow-down seal chamber, and a blow-down line communicably coupled to the blow-down seal chamber to reference the blow-down seal chamber to a low-pressure compressor to reduce a pressure which the at least one gas seal must seal against. The shaft seal system may further include a valve disposed in the blow-down line and configured to regulate a flow of process gas leakage through the blow-down line, and control logic communicably coupled to the valve and configured to adjust the valve in response to pressures detected in the blow-down line.

DETAILED DESCRIPTION

FIG. 1illustrates an exemplary compressor100, according to one or more embodiments disclosed. The compressor100may be a turbomachine, such as a high-pressure centrifugal compressor, having a shaft101extending longitudinally from one end of the compressor100to the other. The shaft101may be configured to rotate about a longitudinal axis X. For simplicity, the portions of the compressor100located below the longitudinal axis X of the shaft101are omitted, whereas the portions above the shaft101axis X are depicted in some detail.

The compressor100may have an inlet102configured to receive a process gas and deliver the process gas to the compressor100for processing. In one or more embodiments, the process gas may include a hydrocarbon gas, such as natural gas or methane derived from a production field or via a pressurized pipeline. In other embodiments, the process gas may include CO2, H2S, N2, methane, ethane, propane, i-C4, n-C4, i-C5, n-C5, and/or combinations thereof. The pressure of the incoming process gas will oftentimes depend on the type of process gas being compressed, and/or the state of the production field where a hydrocarbon process gas is being compressed.

The compressor100may further include an outlet104configured to discharge a high-pressure compressed gas. In one or more embodiments, the compressor100may be capable of compressing the process gas to pressures reaching about 8000 psi to about 9000 psi, or even higher. As can be appreciated, however, embodiments contemplated herein include compressors100that are capable of compressing process gases to higher or lower pressures for varying applications, without departing from the scope of the disclosure.

As illustrated, the compressor100may be a straight through-type compressor, including successive, axially-spaced gas compression stages or impellers106(e.g., stages106aand106b). Each compression stage106may be coupled to or otherwise attached circumferentially about the shaft101and configured for rotation therewith.FIG. 1shows, by way of example, a first and a second compression stage106aand106b, but it is understood that any number of such stages or impellers may be used without departing from the scope of the disclosure. For instance, embodiments contemplated herein include compressors having between one and ten gas compression stages. In operation, the compression stages106a,bprogressively compress the incoming process gas and discharge the high-pressure gas from the compressor100via the outlet104.

The compressor100may further include a balance piston labyrinth seal108disposed axially-adjacent the second or last impeller106band adapted to separate the high-pressure process gas from an adjacent balance chamber110. In one or more embodiments, the balance chamber110may be maintained at or near the inlet102pressure by referencing the balance chamber110to the compressor inlet102via a pressure equalization line112. Consequently, the outboard side of the balance piston labyrinth seal108may be subjected to a lower pressure emanating from the inlet102and thereby creating a pressure differential opposite the direction of the impellers106a,band opposite the net axial forces resulting from the impellers106a,b.

In order to enclose or otherwise contain the circulating process gas within the compressor100, and prevent process gas leakage into the surrounding environment, the compressor100may include a series or an assembly of seals114disposed circumferentially about the shaft101on either side of the impellers106a,b. Each seal assembly114may include, in at least one configuration, a blow-down labyrinth seal116, a seal balance labyrinth seal118, an inner labyrinth seal120, and a gas seal122, each seal116,118,120,122being axially-spaced along the length of the shaft101. It will be appreciated by those skilled in that art that the number and type of seals may vary depending on the application or pressure demands. During operation, any process gas leakage escaping via the minute gaps defined between each adjacent seal116,118,120is progressively minimized as it advances toward the gas seal122. So that each seal balance labyrinth seal118experiences, or “sees,” the same or substantially the same pressure, a seal balance line126may be included to reference these seals118to each other.

The gas seal122may be a dry gas seal as known in the art, and may be a single or tandem gas seal with an accompanying gas seal panel123. Each gas seal122may be configured to receive a seal gas124adapted to maintain a high-pressure sealing effect and prevent the further progression of any process gas leakage. In at least one embodiment, the seal gas124may include a cleaned or otherwise filtered portion of the high-pressure process gas. In other embodiments, however, the seal gas124may include a pressurized inert gas, such as nitrogen or argon, derived from an external source, such as a small reciprocating compressor. In yet other embodiments, the seal gas124may be air. In operation and in order to block further process gas leakage, the seal gas124may be injected at each gas seal122at a pressure higher than the pressure of the preceding inner-areas of the compressor100. For example, the seal gas124may be injected at a pressure higher than the pressure seen by each inner labyrinth seal120, thereby forcing any process gas leakage back across the inner labyrinth seal120, as indicated by the arrows.

A blow-down seal chamber128may interpose the blow-down seal116and the seal balance labyrinth seal118. The blow-down seal chamber128may be communicably coupled or otherwise referenced to a low pressure reference132via a blow-down line130. In one or more embodiments, the low pressure reference132may be any machine, device, or pressurized cavity having a pressure that is generally lower than the pressures generated by the high-pressure compressor100. For instance, the low pressure reference132may be a separate compression unit, such as a low-pressure centrifugal or reciprocating compressor. As used herein, the term “low-pressure compressor” indicates a compression unit that is configured to compress a process gas to pressures less than what the compressor100is capable of. In other embodiments, the low pressure reference132may include a pressurized chamber. In yet other embodiments, the low pressure reference132may be an intermediate compression stage of the compressor100.

In one embodiment, the blow-down seal chamber128is referenced to the low pressure reference132in order to reduce the overall pressure seen by the gas seal122. As will be appreciated, this may prove advantageous in applications where the gas seal122is unproven or otherwise unable to withstand the sealing pressures during normal operation and/or settle-out during shut down procedures of the compressor100. Accordingly, the implementation of the blow-down line130may circumvent the need to overdesign the gas seals122and accompanying gas seal panel123, to a higher pressure rating which may be costly and ultimately ineffective.

In one or more embodiments, where the low pressure reference132is a separate compression unit, the blow-down line130may reference the blow-down seal chamber128to, for example, the inlet or suction side of the low pressure reference132. In other embodiments, the blow-down line130may reference the blow-down seal chamber128to the discharge side of the low pressure reference132, such as upstream of a discharge shut down valve on the separate compression unit. Where the separate compression unit has several compression stages, the blow-down seal chamber128may be referenced to an intermediate compression stage106of the low pressure reference132. In yet other embodiments, the blow-down seal chamber128may be referenced to an intermediate compression stage106of the compressor100itself, especially in embodiments where there are more than two compression stages106a,band a variety of pressure ranges able to be referenced to.

Referring now toFIG. 2, depicted is another exemplary compressor200, according to one or more embodiments described. The compressor200may include several components that are similar to the compressor100ofFIG. 1. ConsequentlyFIG. 2may be best understood with reference toFIG. 1, where like numerals represent like components that will not be described again in detail. Unlike the compressor100ofFIG. 1, however, the compressor200ofFIG. 2may include a back-to-back compressor arrangement as known in the art, where the impellers or compression stages106(e.g.,106aand106b) are situated on the shaft101so that the incoming process gas is progressively compressed toward the middle of the shaft101on either side. Although only two compression stages106a,bare depicted, it will be again appreciated that any number of compression stages106may be used in the compressor without departing from the scope of the disclosure.

In operation, a process gas may be introduced into the compressor200via the inlet102to be compressed by the first impeller or compression stage106aand generate a compressed process gas. The compressed process gas is then discharged from the first compression stage106a(or any number of succeeding compression stages where there are more than two compression sages106) and subsequently injected into the second impeller or compression stage106bvia a second compressor inlet202. The second compression stage106bmay be configured to further increase the pressure of the compressed process gas and eventually discharge a high-pressure process gas via the compressor outlet104.

Because of the back-to-back configuration, the compressor200does not necessarily require a balance piston labyrinth seal108, as described with reference toFIG. 1. Instead, the compressor200may include, for example, a gas balance labyrinth seal204disposed axially-adjacent and outboard from the last impeller106band adapted to separate the high-pressure process gas within the compressor200from the balance chamber110. The gas balance line112, seal balance line126, and blow-down line130may function substantially similar to the embodiments disclosed with reference toFIG. 1and, therefore, will not be discussed again in detail.

Referring now toFIG. 3, depicted is another embodiment of the compressor100ofFIG. 1, shown and embodied as compressor300inFIG. 3. As such,FIG. 3may be best understood with reference toFIG. 1where like numerals represent like components that will not be described again in detail. At least one notable difference between the compressor100ofFIG. 1and the compressor300ofFIG. 3is the implementation of a valve302in the blow-down line130. Without the valve302during normal operation, the compressor300(including compressors100and200) may be constantly recycling process gas leakage via the blow-down line130to the low pressure reference132where the process gas may ultimately be recompressed back up to the high-pressures previously experienced. Consequently, horsepower is lost, and an overall increase in power consumption is required to offset this loss.

The valve302, however, may be used to selectively provide low pressure reference when necessary in lieu of continuous leakage recycle thereby minimizing process gas leakage to the low pressure reference132referenced downstream by the blow-down line130. Furthermore, the valve302may be implemented in applications where it is necessary to change the seal reference pressure of the compressor300beyond a predetermined range of pressures where the gas seals122and accompanying gas seal panel123, are designed to safely operate. Consequently, instead of redesigning or reconfiguring the gas seals122and gas seal panel123for pressure anomalies, the pressures seen by the gas seals122and gas seal panel123may be adjusted in real-time via the valve302, thereby effectively expanding the operating range of the compressor300.

During normal operation the valve302may be closed and adjusted only when needed. During shutdown events, when settle-out pressures may potentially exceed the design pressure of the gas seals122, the valve302may be opened to relieve or reference the pressure of the blow-down seal chamber128to the low pressure reference132. Referencing the blow-down seal chamber128to the low pressure reference132reduces the sealing pressure of the gas seals122to a pressure that can be safely and reliably handled.

Although the valve302may be adjusted manually when desired, in one or more embodiments, the valve302may also be controlled or otherwise regulated via control logic304communicably coupled to the valve302. Accordingly, the valve302may include one or more servos or other mechanical devices (not shown) configured to selectively open and close the valve302in response to a command received from the control logic304. Moreover, the valve302, or the blow-down line130adjacent the valve302, may include one or more pressure transducers, transmitters, senders, indicators and/or piezometers or manometers (not shown) configured to sense the pressure in the blow-down line130and transmit said pressure readings to the control logic304for processing. The control logic304may be programmed with a predetermined pressure range within which the compressor300and its accompanying gas seals122may safely operate. Such predetermined pressure ranges may be exceeded or otherwise breached during compressor300settle-out or an increase in discharge pressure via the outlet104, as generally described above. If the pressure in the blow-down line130exceeds the predetermined pressure range, the control logic304may react by either commanding the servos to adjust the valve302, or alert the operator to the pressure range anomaly and thereby induce manual adjustment of the valve302.

Accordingly, the valve302may be used in the blow-down line130to mitigate a potential failure of the gas seals122by maintaining a reference pressure of the gas seals122within safe design limitations. Moreover, the valve302may allow the gas seals122and gas seal panel123to be sized more cost effectively, since the gas seals122may not have to withstand the extreme pressure rating limitations for extreme pressure anomalies. As can be appreciated, this can result in a significant cost savings.

In at least one embodiment, the valve302may be partially or fully open during normal operation of the compressor300. For example, the valve302may be opened during normal operation when a higher compressor300discharge pressure is required, but the gas seals122are not prepared to receive such an increase. Accordingly, the valve302may be opened and adjusted so that the gas seals122experience pressures within its design limitations so that the compressor300may operate at higher operating pressures without having to completely redesign the gas seals122and gas seal support systems (e.g., the gas seal panel123)

The valve302may also prove advantageous in embodiments where the process gas is derived from a hydrocarbon field where field conditions gradually change over time. For example, field pressures may progressively decline over time such that the design of the compressor300may become out of date for what pressure ranges it was originally designed for. As the field conditions change, the valve302may be adjusted correspondingly to compensate for the increased or decreased pressure demand that may be required of the gas seals122.

Referring now toFIG. 4, depicted is another embodiment of the back-to-back compressor200ofFIG. 2, shown and embodied as compressor400inFIG. 4. As such,FIG. 4may be best understood with reference toFIG. 2, where like numerals represent like components that will not be described again in detail. The compressor400may include the valve302, as generally described above with reference toFIG. 3. The valve302disposed in the blow-down line130may allow multiple types of high-pressure compressors (e.g., straight through, back-to-back, etc.) to operate efficiently over a broader range of operating pressures, and also protect the gas seals122from damage or failure during settle-out.

Referring now toFIG. 5, illustrated is a flowchart of a method500of operating a compressor. The method500may include progressively compressing a process gas in one or more compression stages disposed about a rotatable shaft, as at502. The process gas may be sealed within the compressor using a seal assembly that is disposed about the shaft and defines a blow-down seal chamber, as at504. A blow-down labyrinth seal may form part of the seal assembly and be disposed adjacent the blow-down seal chamber. The blow-down seal chamber may be referenced to a low pressure reference, such as a separate centrifugal compressor or pressurized cavity, via a blow-down line, as at506. Consequently, the blow-down labyrinth seal may also be referenced to the low pressure reference via the blow-down line. Process gas leakage through the blow-down line may then be regulated using a valve disposed in the blow-down line, as at508. During normal operation, the valve may be in a closed position and would only open during the shutdown of the compressor to allow the high-pressure compressor to settle-out at a lower pressure level.