Patent Publication Number: US-2022235789-A1

Title: Compressor arrangement and method of operating a compressor

Description:
TECHNICAL FIELD 
     The subject-matter disclosed herein relates to gas compressor arrangements and to methods for operating a compressor, in particular to an arrangement and a method making use of process gasses containing hydrocarbons such as methane, ethane and butane. 
     BACKGROUND ART 
     A compressor arrangement comprises at least a compressor, for example a centrifugal compressor, fluidly connected to a suction duct and a discharge duct. In order to avoid surges in the compressor, the suction duct and the discharge duct are fluidly connected through a recycle duct controlled by an anti-surge valve. The recycle duct creates a loop between the compressor outlet and the compressor inlet and allows protecting the compressor from surge through the anti-surge valve. 
     In order to perform some maintenance, some repair operations or any other prolonged stop due to plant operations, the compressor is stopped and depressurized. The final portion of the suction duct, the initial portion of the discharge duct and the recycle duct are also depressurized. 
     A common practice for depressurizing the inner volume of a compressor and the ducts connected to it consists in releasing the process gas directly into the atmosphere or to burn it with a flare stack. However this practice leads to the release of greenhouse gasses in the atmosphere, which constitutes both a loss of a valuable good and an emission of potent greenhouse gasses (for example methane has 28 to 34 more greenhouse power than carbon dioxide over 100 years). 
     In addition, some compressors currently employed in the industry cause other emissions of hydrocarbon gasses. They may have mechanical dry gas seals which, in order to avoid contact between moving parts, tolerate a slow and constant leakage of process gas which is vented in the atmosphere or flared. Also, dry gas seals of compressors comprise a stand-by filter, which is kept in reserve and ready to replace the operating filter. In order to prevent condensations when the stand-by filter is put into use, the stand-by filter and the gas inside it are kept warm by spilled process gas. The spilled gas is then vented in the atmosphere or flared. 
     Additionally, compressors are often driven by a gas turbine and the process gas, thanks to its pressure, may also be used to initiate rotation of the gas turbine before starting combustion; in this case, (un-combusted) process gas at the outlet of the turbine is released in the atmosphere or flared. 
     The gas turbine driving the compressor benefits from heating of the turbine fuel inlet duct prior to startup of the turbine in order to avoid condensation of the propellant. Such heating is also performed by spilling fuel gas, which is vented in the atmosphere or flared afterwards. 
     SUMMARY 
     According to an aspect, the subject-matter disclosed herein relates to a compressor arrangement comprising: at least one main compressor having a main inlet and a main outlet; an additional compressor having an additional inlet and an additional outlet; a piping system arranged to supply gas to the main inlet and to collect gas from the main outlet; one or more components emitting depressurized gas, each component having a collector arranged to collect the depressurized gas; wherein the additional inlet is fluidly coupled with one or more of the collectors; and wherein the additional inlet is fluidly coupled with the piping system and arranged to extract gas from the piping system when the main compressor is shut down. 
     According to another aspect, the subject-matter disclosed herein relates to a method for operating a compressor, comprising the steps of: collecting a depressurized gas from the compressor while the compressor is running or starting up; pumping the depressurized gas into a pressurized duct; shutting the compressor down; collecting a process gas from the compressor while the compressor is not running, and pumping the process gas into the pressurized duct. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  shows a schematic view of a first embodiment of a compressor arrangement according to the subject-matter disclosed herein; 
         FIG. 2  shows a schematic view of a second embodiment of a compressor arrangement according to the subject-matter disclosed herein, wherein some elements are not shown for simplicity; 
         FIG. 3  shows a flow chart of an embodiment of a control method according to the subject-matter described herein. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The subject matter herein disclosed relates to compressor arrangements and a methods of operating a compressor. 
     A compressor arrangement, in particular for Oil &amp; Gas applications, is arranged to receive a flow of hydrocarbon gas, process it, and discharge it at a higher pressure. In these types of applications, the incoming gas flow is already pressurized upstream of the compressor arrangement, i.e. it is at a high pressure for example at 40 bar. The compressor arrangement processes the incoming gas flow by increases its pressure at an even-higher level, for example at 80 bar. 
     Such compressor arrangement comprises a main compressor, in particular a centrifugal compressor, and a piping system which is fluidly connected to the inlet and the outlet of the main compressor. The piping system includes at least a suction duct, a discharge duct and preferably a recycle duct arranged to create a loop between the compressor inlet and the compressor outlet. 
     The compressor arrangement disclosed herein further includes an additional compressor, in particular a reciprocating compressor, fluidly connected to the piping system. During shutdown of the main compressor, the piping system is substantially isolated and a substantial amount of process gas remains trapped in the piping system and inside the main compressor. A purpose of the additional compressor is to pump the process gas out of the piping system after shutdown so that then it is possible to inspect, maintain or repair the main compressor without discharging any substantial amount of process gas into the atmosphere or flaring it. 
     In particular, the additional compressor is arranged to collect the process gas trapped in the piping system and to pump it in a suction header or a pressurized duct upstream of the piping system. The additional compressor is therefore configured to increase the pressure of the trapped gas up to the pressure inside the suction header (for example 40 bar). 
     It is another purpose of the additional compressor to recycle depressurized un-combusted gas lost by the compressor arrangement for example through leakage and venting. In fact, one or more components of the compressor arrangement may emit depressurized hydrocarbon gas. For example, the main compressor may have mechanical dry gas seals which, while operating, cause by design a continuous leakage of process gas and are therefore a source of depressurized gas. In addition, such dry gas seals may comprise filters which are preferably kept warm while not operating. In order to keep the non-operating filters and the gas they contain warm, the compressor assembly may comprise a spilled gas system which circulates (warm) process gas inside the filter of the main compressor and constitutes an additional source of depressurized gas. 
     According to some embodiments, the compressor arrangement comprises a gas turbine driving the main compressor and other components, related to the gas turbine, emitting depressurized gas. For example, the gas turbine may have a pneumatic starter which employs (pressurized) process gas for starting the gas turbine and emits depressurized gas. Additionally, the gas turbine has a fuel duct which requires heating prior to starting up the turbine to prevent condensation in the gas fuel. Such heating may be accomplished by flowing (warm) process gas, which is then emitted as depressurized gas. 
     In order to prevent the depressurized gas to be discharged or flared into the atmosphere, the compressor arrangement comprises one or more collectors arranged to collect the depressurized gas emitted from one or more of the above-mentioned components. Such collector is fluidly coupled with the additional compressor in order to pressurize and recycle the collected depressurized gas. 
     According to preferred embodiments, the compressor arrangement comprises an accumulation vessel positioned downstream of the collector in order to store the depressurized gas collected from the gas emitting components and the additional compressor is fluidly connected with the accumulation vessel. 
     The accumulation vessel and the additional compressor can be sized and configured to perform the task of emptying the piping system in a predetermined amount of time after the shutdown of the main compressor. The additional compressor configured in such way is oversized for the task of recycling the depressurized gas during the operation of the main compressor resulting from leakages. The accumulation vessel allows the additional compressor the work in intermittent runs and the depressurized gas is stored in the accumulation vessel between the runs. 
     Reference now will be made in detail to embodiments of the disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the disclosure, not limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
     When introducing elements of various embodiments the articles “a”, “an”, “the”, and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including”, and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     According to one aspect and with reference to  FIG. 1 , the subject-matter disclosed herein provides a compressor arrangement  1 . The compressor arrangement  1  is arranged to be used in Oil &amp; Gas applications and is configured receive a flow of hydrocarbon gas at a pressure higher than atmospheric pressure, for example 40 bar, process it, and discharge it at a pressure higher than the suction pressure, for example 80 bar. 
     The compressor arrangement  1  comprises at least one main compressor  100 , in particular a centrifugal compressor. Depending on the design requirements of the compressor arrangement  1 , the latter may comprise two or more main compressors  100 , arranged in series and/or in parallel. 
     The main compressor  100  has a main inlet  101  arranged to receive a flow of hydrocarbon gas to be processed and a main outlet  106  arranged to discharge the processed flow. The main compressor  100  further comprises one or more mechanical seals  125 , in particular a dry gas seal, interposed between the shaft and the outer body of the main compressor  100  itself. 
     Such dry gas seals rely on continuous gas spilling from the main compressor  100  in order to maintain a buffer of flowing gas between its moving parts. The mechanical seal  125  has a gas inlet arranged to collect spilled process gas from the compressor arrangement  1  and a gas outlet arranged to emit leakage depressurized gas. Inside the seal, the gas flows from the gas inlet to the gas outlet and creates buffer between its moving parts. Preferably, the mechanical seals  125  comprise a collector  126  arranged to collect the depressurized gas emitted at the gas outlet. With the expression “depressurized gas” it is intended gas containing hydrocarbons emitted at a pressure lower than the pressure of the process gas upstream of the main compressor  100 . 
     The compressor arrangement  1  further comprises filters for the buffer gas upstream of the mechanical seals  125  in order to prevent liquids, particles and other solid matter having a diameter above a predetermined limit from entering the seal and deteriorating it. At least one operational filter is used for filtering the buffer gas while at least one clean stand-by filter is kept in reserve to be switched with the operational filter in order to avoid a stop of the main compressor  100  when the operational filter is dirty. The compressor arrangement  1  comprises a stand-by filter warm-up system  127  arranged to warm-up the filter kept in reserve with spilled process gas, which has a temperature comprised between 70° C. and 95° C. The stand-by filter warm-up system  127  is configured to keep the gas inside the stand-by filter warm in order to avoid condensations when the stand-by filter is activated. The spilled gas is emitted by the stand-by filter warm-up system  127  after circulation in the stand-by filter and constitutes another source of leaked depressurized gas. The stand-by filter warm-up system  127  preferably comprises a collector  128  arranged to collect the depressurized gas downstream of the stand-by filter. 
     The main compressor  100  is fluidly coupled with a piping system  110  arranged to supply gas to the main inlet  101  and to collect gas from the main outlet  106 . The piping system  110  has a system inlet  111  configured for a fluid connection with an upstream gas source and a system outlet  116  configured for a fluid connection with a downstream gas receiving device. A suction header may be arranged at the system inlet and a discharge header may be arranged at the system outlet. The piping system  110  comprises an inlet duct  112  extending from the system inlet  111  to the main inlet  101  and an outlet duct  117  extending from the main outlet  106  to the system outlet  116 . A suction isolation valve  113  is positioned at the system inlet  111  and is arranged to open or close a fluid connection between the inlet duct  112  and the upstream gas source. A discharge isolation valve  118  is positioned at the system outlet  116  and is arranged to open or close a fluid connection between the outlet duct  117  and the downstream gas receiving device. 
     The piping system  110  further comprises at least one return duct  120  fluidly connecting the main outlet  106  with the main inlet  101 . An anti-surge valve  121  is installed in the return duct  120  and is arranged to control a recycle flow through the return duct  120  in order to prevent surges in the main compressor  100  and/or to equalize the pressures in case of an emergency shutdown. 
     As shown in  FIG. 2 , the compressor arrangement  1  further comprises a driver arranged to drive the main compressor  100 . In a preferred embodiment, the driver is a gas turbine  130  mechanically coupled with the main compressor  100 . A fuel duct  131  is fluidly coupled with the gas turbine  130  and arranged to supply the gas turbine  130  with fuel gas. In the embodiment of  FIG. 2 , the fuel duct  131  is arranged to draw process gas from the piping system  110  or upstream of the system inlet  111  in order to use it as fuel gas. In a possible alternative embodiment, the source of the fuel gas of the gas turbine  130  is different from the process gas. 
     Preferably, the compressor arrangement  1  comprises a heating system  132  arranged to circulate spilled process or fuel gas in the fuel duct  131  prior to a start-up of the gas turbine  130 . The heating system  132  prevents condensation of the fuel gas at entering the gas turbine  130  caused by convection with the fuel duct  131  itself. Such spilled gas constitutes a source of depressurized gas and the heating system  132  preferably comprises a collector  133  arranged to collect it. 
     In a possible embodiment, also shown in  FIG. 2 , the compressor arrangement  1  further comprises a pneumatic starter  135  for the gas turbine  130 . The pneumatic starter  135  is arranged to collect process gas (which is pressurized normally at around 40 bar) and convert its pressure into mechanical energy for spinning the gas turbine  130  during its start-up. The pneumatic starter  135  emits depressurized gas during the start-up of the gas turbine  130  and comprises a collector  136  arranged to collect such depressurized gas. 
     The annexed  FIG. 1 , shows an embodiment of the compressor arrangement  1  having only the collector  126  for collecting depressurized gas from the mechanical seal  125  of the main compressor  100 . 
     The annexed  FIG. 2  shows an embodiment of the compressor arrangement  1  comprising: the collector  126  for collecting depressurized gas from the mechanical seal  125 , the collector  128  for collecting depressurized gas from the stand-by filter warm-up system  127 , the collector  133  for collecting depressurized gas from the heating system  132  and the collector  136  for collecting depressurized gas from the pneumatic starter  135 . In  FIG. 2  some components such as the return duct  120  and the additional compressor  150  have been omitted for simplicity. 
     Preferably, the compressor arrangement  1  comprises an accumulation vessel  140  fluidly coupled with one or more of the collectors described above in order to receive and store the depressurized gas flowing from the components emitting it. The compressor arrangement  1  may comprise other collectors fluidly coupled with the accumulation vessel  140  and arranged to collect depressurized gas emitted from any component of the compressor arrangement  1 . 
     In the embodiment of  FIG. 1  the accumulation vessel  140  is fluidly coupled with the collector  126  through a duct having a collector valve  141  that can be opened and closed. In the embodiment of  FIG. 2  the accumulation vessel  140  is fluidly coupled with the collectors  126 ,  128 ,  133  and  136  through respective ducts having respective collector valves  141 . In an alternative possible embodiment, the compressor arrangement  1  comprises multiple accumulation vessels, each fluidly coupled to a respective collector for depressurized gas. The accumulation vessel  140  is essentially a tank having an inner chamber for storing gas at pressures between 1 bar and 20 bar, preferably between 1 bar and 5 bar. Preferably, the accumulation vessel  140  has a storing volume comprised between 3 m 3  and 500 m 3 . More preferably, the accumulation vessel  140  has a storing volume comprised between 5 m 3  and 30 m 3 . 
     The compressor arrangement  1  further comprises an additional compressor  150 , preferably a reciprocating compressor, having an inlet for receiving gas hereby called “additional inlet  151 ” and an outlet for emitting gas hereby called “additional outlet  156 ”. 
     The additional inlet  151  is fluidly coupled with the piping system  110  through a first duct  152  which houses a piping valve  153  which can be opened and closed. Alternatively, the additional inlet  151  may be fluidly coupled with the inner chamber of the main compressor  100 , which also fluidly communicates with the piping system  110 . 
     The additional inlet  151  is also fluidly coupled with the accumulation vessel  140  thorough a second duct  154 . A valve may be installed in the second duct  154  for opening and closing it. 
     By selecting the position of the collector valve(s)  141  and the piping valve  153 , the additional compressor  150  may be configurable to receive gas from the accumulation vessel  140  or from the piping system  110 . 
     An atmospheric vent  145  controlled by a valve is fluidly coupled with the collector(s) and configured to release the depressurized gas in the atmosphere when the accumulation valves  141  are closed, which can happen when the piping valves  153  are open because the additional compressor  150  is extracting fluid from the piping system  110 . An additional venting valve (not illustrated in the attached drawings) may be fluidly coupled with the piping system  110  and arranged to release in the atmosphere the gas contained in the piping system  110  and in the main compressor  100 . Such additional valve may be opened in case there is a need to depressurize the compression arrangement  1  and the piping valve  153  cannot be opened or the additional compressor  150  cannot be activated. A flare stack may be arranged to burn flammable gasses released by the atmospheric vent  145  and/or by the additional venting valve. 
     The additional outlet  156  of the additional compressor  150  is either fluidly coupled with the system inlet  111  upstream of the suction isolation valve  113  or with the system outlet  116  downstream of the discharge isolation valve  118 . In the embodiment of  FIG. 1 , which is preferred due to the lower pressure upstream of the compressor arrangement  1 , the additional outlet  156  is fluidly coupled with the system inlet  111  through a connecting duct  157 . 
     The additional compressor  150  is able to extract the process gas trapped in the piping system  110  after the main compressor  100  is shut down and the suction and discharge isolation valves  113  and  118  have been closed. Such gas is then pumped upstream or downstream of the piping system  110  and is prevented from being released or flared into the atmosphere. 
     Preferably, the additional compressor  150  is configured to extract the gas from the piping system  110  in order to lower the pressure in the piping system  110  from an operating pressure of around 60 bar (at the shutdown of the main compressor  100 ) to a final pressure equal or lower than 10 bar, preferably equal or lower than 3 bar, in an interval of time comprised between 15 minutes and 20 hours, preferably between 2 hours and 10 hours. In a preferred embodiment, the additional compressor  150  has a power comprised between 10 kW and 150 kW and a flow rate comprised between 100 Nm 3 /hr and 2000 Nm 3 /hr. 
     The additional compressor  150  configured as described above is able to extract the depressurized gas accumulated in the accumulation vessel  140  and to pump it upstream or downstream of the piping system  110  during the operation of the main compressor  100 , thereby preventing the release of the depressurized gas into the atmosphere. 
     In a possible embodiment, the accumulation vessel  140  is fluidly coupled with the piping system  110  through a valve and can be arranged to receive gas from the piping system  110  after the shutdown of the compressor  100 , before extracting the gas through the additional compressor  150 . 
     The additional compressor  150  configured as described is oversized for the continuous pumping of depressurized gas, therefore the accumulation vessel  140  allows the temporary accumulation of depressurized gas so that the additional compressor  150  can be activated intermittently to empty the accumulation vessel  140  when it has reached a certain pressure. 
     Preferably, the compressor arrangement  1  comprises a control unit configured to turn on and off the additional compressor  150  in order to maintain the pressure in the accumulation vessel  140  between a minimum predetermined value, for example 1.1 bar, and a maximum predetermined value. The maximum predetermined value is preferably lower than 20 bar and even more preferably lower than 6 bar. In a preferred embodiment the maximum predetermined value is around 3 bar. 
     In an alternative embodiment of the compressor arrangement  1 , the compressor arrangement  1  doesn&#39;t have an accumulation vessel  140  and the additional inlet  151  is directly connected with one or more of collectors  126 ,  128 ,  133  and  136 . Preferably, in this embodiment the additional compressor  150  is a variable speed compressor which is able to adapt its flow rate to the rate of the emissions of depressurized gas and is also able to provide the required flow rate to empty the piping system  110  in an interval of time comprised between 15 minutes and 20 hours, preferably between 2 hours and 10 hours. 
     Preferably, the compressor arrangement  1  further comprises a by-pass valve  158  fluidly coupling the additional inlet  151  with the additional outlet  156 , which allows to by-pass the additional compressor. Such by-pass valve  158  may be opened when, after shutting off the main compressor  100 , the gas pressure upstream of the suction isolation valve  113  is lower than the pressure in the piping system  110 . This allows the process gas to naturally flow outside of the piping system  110 . 
     According to a second aspect and with reference to  FIG. 3 , the subject-matter disclosed herein provides a method for operating a compressor, in particular for operating the main compressor  100  of the compressor arrangement  1 . 
     While the compressor  100  is running or starting up, the method comprises a step A 1  (block  210  in  FIG. 3 ) of collecting a depressurized gas, in particular the depressurized gas collected by the collectors  126 ,  128 ,  133  and  136  respectively of the embodiment of  FIG. 2 . 
     Preferably, step A 1  (block  210  in  FIG. 3 ) comprises one or more of the following sub-steps. 
     A 11 ) (block  211  in  FIG. 3 ) Collecting depressurized buffer gas from a mechanical seal  125  of the compressor  100 , in particular through the collector  126 . 
     A 12 ) (block  212  in  FIG. 3 ) Collecting spilled gas used for warming up the gas volume inside a filter of a mechanical seal  125  of the compressor  100 , in particular through the collector  128 . 
     A 13 ) (block  213  in  FIG. 3 ) Collecting depressurized gas from a pneumatic starter  135  of a gas turbine  130  driving the compressor  100  during a start-up of the gas turbine  130 , in particular through the collector  136 . 
     A 14 ) (block  214  in  FIG. 3 ) Collecting spilled gas used for heating a fuel duct  131  of a gas turbine  130  driving the compressor  100 , in particular through the collector  133 . 
     Preferably, step A 1  (block  210  in  FIG. 3 ) further comprises accumulating the depressurized gas inside an accumulation vessel  140 . 
     The method further comprises step A 2  (block  220  in  FIG. 3 ) of pumping the depressurized gas into a pressurized duct, in particular to a duct fluidly coupled with the piping system  110  described above, preferably upstream of the suction isolation valve  113 . Preferably, step A 2  (block  220  in  FIG. 3 ) comprises pumping the depressurized gas out of the accumulation vessel  140  after the pressure in the accumulation vessel  140  has reached a maximum predetermined value. The maximum predetermined value is preferably equal or lower than 20 bar and even more preferably equal or lower than 6 bar. Preferably, the step A 2  (block  220  in  FIG. 3 ) is performed through a reciprocating compressor, in particular through the additional compressor  150  described above. 
     The method further comprises a step A 9  (block  290  in  FIG. 3 ) of shutting the compressor  100  down. After the shutdown of the compressor  100 , the method comprises a step B 0  (block  300  in  FIG. 3 ) of sealing the suction isolation valve  113  and the discharge isolation valve  118 . 
     After step B 0  (block  300  in  FIG. 3 ), the method comprises a step B 1  (block  310  in  FIG. 3 ) of collecting a process gas from the compressor  100 , in particular from the piping system  110 . In a preferred embodiment, step B 1  (block  310  in  FIG. 3 ) is performed through the first duct  152  described above. 
     The method further comprises a step B 2  (block  320  in  FIG. 3 ) of pumping the process gas into the pressurized duct, performed by the same reciprocal compressor as step A 2  (block  320  in  FIG. 3 ). In a possible embodiment, the process gas coming from the piping system  110  may be temporarily stored in the accumulation vessel  140  prior to being pumped into the pressurized duct.