DRY GAS SEALS

DESCRIPTION

The present disclosure relates to a fluid pumping system, particularly but not exclusively for pumping carbon dioxide (CO2), comprising a plurality of pumps and a dry gas seals protecting apparatus for protecting the integrity of dry gas seals of said pumps, preferably centrifugal pumps, during standby conditions.

More in particular, the present disclosure relates to a pumping system which allows to protect the integrity of the dry gas seals of a centrifugal pump during standby conditions of said pump for CO ₂ applications, e.g . the re-injection, transportation and sequestration of pure CO ₂ and CO ₂ +hydrocarbons in oil and gas recovery plants.

The application of dry gas seals to centrifugal pumps for the shaft seal ing instead of wet seals offers several benefits, e.g . reducing power consumption, reducing seals system footprint, increasing the reliability and eliminating maintenance costs related to barrier fluid refill ing .

The many benefits of dry gas seals at the running conditions of centrifugal pumps hide problems associated with the use of dry gas seals on centrifugal pumps at other operating conditions such as in the standby condition, as it will be more clearly explained in the following .

Gas, and in particular CO ₂ , leakage across the primary seal is normal because also in the standby condition the pump is pressurized and ready to start.

Therefore, the gas pressure inside the pump is higher than the external, atmospheric, pressure. Downstream of the primary seal there is a pressure established by a buffer fluid, typically nitrogen or air available at a pressure of four to six bar. Further, the higher pressure and un-treated process gas permeates the primary dry gas seal, transporting particulate and liquid contamination.

This problem is emphasized with carbon dioxide (CO2) as the process flow, and or with any other fluid which may change phase (ice or liquid) due to the fluid expansion through the seal. The carbon dioxide (CO2) expansion through the tight tolerances of the dry gas seal rings can form ice on the seal rings. Subsequently, when the pump returns to normal operating conditions, the contamination between the dry gas seal rings results in premature wear and failure of the dry gas seal.

With reference to Figures from 1 to 3, a known configuration comprising a single centrifugal pump utilizing a dry gas seal is shown.

Figure 1 discloses a detailed diagram of a related art configuration of a dry gas seal (DGS) system 100 for a carbon dioxide (CO2) pump. It should be noted in this configuration that any fluid in a supercritical state can be used as a barrier fluid in place of the exemplary carbon dioxide (CO2).

The configuration of Figure 1 reflects the behavior of the dry gas seal during operating conditions and includes a CO2 pump 102 with its associated area to be sealed, a primary (inboard) seal 104 of a dry gas seal, a secondary (outboard) seal 106 of the dry gas seal, a process fluid filter 108, a process fluid heater 110, a valve and control element 120 for controlling the flow to a flare-safe area, an intermediate buffer gas filter 114, intermediate buffer gas 116, barrier fluid 118, pressure reduction valve 120, a primary dry gas seal chamber 122 and a secondary dry gas seal chamber 124.

In the related art configuration of Figure 1 is shown a process fluid, e.g. carbon dioxide, from the pump discharge being used as a barrier fluid. The pressure of the barrier fluid is reduced by a valve 120 and heated by a heater 110. The barrier fluid is then filtered by filters 108 and injected into the primary dry gas seal chamber 122.

The pressure of the barrier fluid is higher than the suction pressure of the pump and therefore it prevents the entry of any untreated process gas into the primary seal 104.

The barrier fluid (carbon dioxide) flows partly into the pump through the inner labyrinth and partly to the primary vent through the primary dry gas seal. Next in the related art configuration, the carbon dioxide (CO2) that flows into the pump reaches a suction pressure that is higher than the critical pressure for carbon dioxide (CO2) and accordingly no problems of icing will occur. Further in the considered configuration, the carbon dioxide (CO2) that flows through the primary seal to the primary vent expands from P1 to a value established by the buffer gas (typically N2/air at 4-6 bar). It should be noted that in the known configuration the temperature of the carbon dioxide (CO2) barrier fluid must be maintained, by a heater, to a value high enough to avoid, during the expansion, the risk of icing.

An intermediate buffer gas 116, e.g. nitrogen or dry air is filtered by filters 114 and injected into the secondary dry gas seal chamber 124. It should be noted in the known configuration that other gases than nitrogen or air can also be used as a buffer gas. The pressure of the intermediate buffer gas 116 is higher than the pressure of the barrier gas passing through the primary seal 104 and prevents the barrier gas from reaching the secondary seal 106. In the known configuration, the mixture of barrier gas 118 and intermediate buffer gas 116 in the secondary dry gas seal chamber 124 passes through a valve 112 and flows to a flare-safe area.

Figure 2 shows the same diagram of Figure 1 when the pump is in a standby condition. In this condition the discharge pressure from the pump is equal to the pressure in the area to be sealed 1 02. When the pump is in a standby condition, the pressure into the pump reaches a uniform value very close to the suction pressure.

It should be noted that in the related art configuration the result of the standby condition is the process fluid from the pump discharge can no longer act as a barrier fluid to prevent the flow of untreated process fluid, from the area to be sealed 1 02, into the primary seal 1 04. Furthermore, the untreated process fluid is not heated or filtered and therefore contaminates can enter the primary seal 1 04 and icing can occur in the primary seal 1 04.

Due to the fact that the pump is pressurized also in the standby condition, there is a natural carbon dioxide (CO2) leakage across the primary seal when the pump is in standby condition .

According to the state of the art, the reference is to Figure 3, in order to avoid risks of damages and icing when the pump is in standby condition additional boosters, not shown in the drawing, are provided for the barrier fluid 1 1 8 to maintain the barrier gas at the conditions provided during running condition of the pump. This solution requires the similar treatment of the process fluid with respect to filtering and heating to prevent contamination of the dry gas seal .

SUMMARY

Plants for carbon dioxide (CO ₂ ) applications, e.g . the re-injection, transportation and sequestration of pure carbon dioxide and carbon dioxide + hydrocarbons in oil and gas recovery plants, are here considered .

In said plants, at least one centrifugal pump is provided, preferably at least two centrifugal pumps working alternatively are provided, so that when a first pump is in an operative/running condition the second pump is in a standby condition .

Switching between the pumps allows reduction in the maintenance costs due to replacing the barrier fluid and increase in the MTBF (Mean Time Between Failures).

Having more centrifugal pumps, each pump may work for less time, lengthening the expected time between a fault and the other (MTBF).

Furthermore, the use of more centrifugal pumps avoids the plant shutdown, being able to operate a second pump when it is necessary to perform maintenance on a first pump.

The object of the present disclosure is to provide a pumping system suitable to achieve, among others, the advantages listed above.

Further details and specific embodiments will refer to the attached drawing, in which : - Figure 1 is a schematic view of a dry gas seal and the associated gas support system when the pump is in an operating/running condition ;

- Figure 2 is a schematic view of a dry gas seal and the associated gas support system when the pump is in a dangerous standby condition (risk of damages and icing); - Figure 3 is a schematic view of a dry gas seal and the associated gas support system when the pump is in a safe standby condition (no risk of damages and icing);

- Figure 4 is a schematic view of the pumping system according to the present disclosure; - Figure 5 represents a flowchart of the steps of the method according to the present disclosure. DETAILED DESCRIPTION

The following description of exemplary embodiments refers to the accompanying drawings. The following detailed description does not limit the disclosure. Instead, the scope of the invention is defined by the appended claims.

The present disclosure relates the oil and gas recovery plants, i.e. on off-shore oil and gas implants. More in details, the present disclosure concerns industrial plants for carbon dioxide (CO2) applications.

Some of the most shared applications of carbon dioxide, are the re- injection, transportation and sequestration of pure carbon dioxide and carbon dioxide + hydrocarbons in oil and gas recovery plants.

In said plants, at least one centrifugal pump is provided, preferably at least two centrifugal pumps working alternatively are provided, so that when a first pump is in an operative/running condition the second pump is in a standby condition .

Switching between the pumps allows reduction in the maintenance costs due to replacing the barrier fluid and increase in the MTBF (Mean Time Between Failures).

Having more centrifugal pumps, each pump may work for less time, lengthening the expected time between a fault and the other (MTBF).

Furthermore, the use of more centrifugal pumps avoids the plant shutdown, being able to operate a second pump when it is necessary to perform maintenance on a first pump.

The object of the present disclosure is to provide a pumping system suitable to achieve, among others, the advantages listed above.

The pumping system 200 according to the present disclosure comprises at least a first pump 300 and a second pump 400, preferably the first and the second pump are centrifugal pumps, at least one dry gas seal being associated to each of the first and second pump.

The pumping system 200 according to the present invention comprises a process shared fluid delivery circuit 150 in turn comprising a first barrier fluid delivery circuit 301 which fluidly connects the pump discharge 330 of the first pump 300 to at least one dry gas seal of the second pump 400 to deliver a barrier fluid to said at least one dry gas seal of the second pump 400.

The shared process fluid delivery circuit 150 according to the present invention comprises a second barrier fluid delivery circuit 401 which fluidly connects the pump discharge 430 of the second pump 400 to at least one dry gas seal of the first pump 300 to deliver a barrier fluid to said at least one dry gas seal of said first pump.

According to a preferred embodiment of the present disclosure shown in Figure 4, the first 301 and second 401 barrier fluid delivery circuits of said shared delivery circuit 150 merge in a shared branch line 500 on which a reduction valve 512 suitable to reduce the pressure of the barrier fluid coming from the pump discharges 330, 430, a filter 514 and a heater 513 are provided. A shared header 515 is further provided on said shared branch 500 downstream to said heater 513 with respect to the flow direction. From said shared header 515, the shared branch 500 splits into two return branch lines: a first return branch line 517 which fluidly connects said shared header 515 to the dry gas seals of the first pump 300, said first return line 517 in turn comprising a first 517a and a second 517b barrier fluid delivery sections, each of said sections of said first return line 517 being fluidly connected to one of said dry gas seals of the first pump 300, and a second return branch line 518 which fluidly connects said shared header 515 to the dry gas seals of the second pump 400, said second return branch line 518 in turn comprising a first 518a and a second 518b barrier fluid delivery sections, each of said first 518a and second sections 518b of said second return line 518 being fluidly connected to one of the dry gas seals of the second pump 400.

With reference to Figure 4, the process fluid, e.g. carbon dioxide (CO2), coming to the shared header 515 from the first pump discharge 330 of the first pump 300 is used as a barrier fluid for the dry gas seals of the second pump 400 when, in an operative condition, said first pump 300 is in a running condition and said second pump 400 is in a standby condition. In the same way, the process fluid coming to the shared header 515 from the second pump discharge 430 of the second pump 400 is used as a barrier fluid for the dry gas seals of the first pump 300 when, in an operative condition, said second pump 400 is in a running condition and said first pump 300 is in a standby condition. The pressure of the barrier fluid is reduced by the reduction valve 512 and filtered by the filter 514 and then heated by the heater 513. The barrier fluid is then injected into the dry gas seal chamber of the pump in the standby condition.

To give an example, when the first pump 300 in in a running condition, the process fluid is bled from the discharge 330 of the first pump 300 through the first barrier fluid delivery circuit 301 and enters the shared branch line 500.

The pressure of the process fluid is then reduced in the shared branch 500 by the reduction valve 512, and then the fluid is filtered by the filter 514 and heated by the shared heater 513.

The process fluid, ready to be used as barrier fluid for the gas seals, then flows into the shared header 515 where all the fluctuations, due to discontinuous operations, are smoothed and the fluid properties are stabilized. In this way the most fragile components of the dry gas seals, that are the sealing faces, are protected from abrupt pressure changes.

Downstream to the shared header 515 the process fluid flows through the second return branch line 518 to the dry gas seals of the second pump 400, which is in a standby condition.

A first check valve 302 is advantageously provided on said first barrier fluid delivery duct 301, and a second check valve 402 is provided on said second barrier fluid delivery duct 401.

The pumping system 200 according to the present disclosure as above described, is therefore adapt to feed a flow of fluid, for example carbon dioxide (CO2), from the pump discharge of a first running pump 300 and to flush the dry gas seal or seals of the second pump 400 when said second pump is in standby condition.

In the same way, when the first pump 300 is switched to a standby condition and the second pump 400 to a running condition, the pumping system 200 according to the present disclosure is apt to feed a flow of process fluid from the pump discharge of the second pump 400 ant to flush the dry gas seal or seals of the first pump 300.

Thanks to the pumping system according to the present disclosure, it is not necessary to provide highly reliable, and therefore also highly expensive, auxiliary booster compressors to provide barrier fluid to the dry gas seals of the pump which is in a standby condition the whole time the pump is in a standby condition. Eventually, in case that the pumping system comprises only two pumps and both of them are in a standby condition at the same time, it may be enough to provide the system with simpler and cheaper boosters. In fact, with the pumping system according to the present disclosure, said booster compressors will be switched on only in the rare case and for the short period of time in which all the pumps of the pumping system are simultaneously in a standby condition .

Therefore, the pumping system according to the present disclosure allows installing more economical booster compressors and, because said booster compressors are rarely switched on, the system is very reliable and energy efficient.

The pumping system 200 according to the present invention allows switching of the pump function from a running condition to a standby condition . When the first pump 300 is in a running condition, the pump discharge 330 supplies the barrier fluid through the first barrier fluid delivery circuit 301 to the shared header 51 5 and, finally, to the dry gas seals of the second pump 400, which is in standby condition .

When it is required by the circumstances, for example when maintenance operations are necessary on the first pump, or when it is advantageous to operate the two pumps alternatively for shorter periods, e.g . to increase the MTBF of the pumps, the first pump 300 is turned in a standby condition, while the second pump 400 is turned in an operative condition . The pumping system according to the present disclosure is highly versatile with respect to the possibility to switch the operative cond itions of the two pumps, so that the end user has a high number of possibilities to operate the pumps in the best way in view of the circumstances and of the results to be achieved . Thanks to the second barrier fluid delivery circuit 401 , the second pump 400, which now is the running pump supplies the barrier fluid through the second barrier fluid delivery circuit 401 to the first pump, which is in standby condition .

It will be obvious to those skilled in the art that the pumping system may advantageously comprise more than two pumps, e.g . three or four pumps. In these possible configurations, the pumping system will comprise a corresponding number of barrier fluid delivery circuits.

The pumping system 200 according to the present invention may advantageously comprise a control unit configured to switch the operative conditions of the pumps and to control the operative conditions of the devices provided on the barrier fluid delivery circuits.

Among the others, the pumping system according to the present disclosure achieves the results consisting of reduction of maintainability costs, increase of MTBF and cost reduction of booster compressors, which are no longer necessary.

The present disclosure also concerns a method for providing a barrier fluid to a dry seal of a pump.

The method according to the present disclosure allows to provide barrier fluid to a dry seal of a pump when said pump is in a standby condition and without the need to provide additional boosters. In order to achieve this result, a first running pump is fluidly connected, by means of a shared process fluid circuit, to said standby pump.

The method comprises at least the following steps, disclosed in the flowchart of Figure 5. A first step 1 consisting of receiving a barrier fluid from a discharge port of a first pump into a shared process fluid recovery circuit coupled with the discharge port.

A second step 2 consisting of reducing pressure of the barrier fluid;

A third step 3 consisting of directing the reduced pressure barrier fluid to a dry seal of a second pump coupled with the shared process fluid recovery circuit. The method here above described advantageously further comprises a further step 2a consisting of filtering and warming the barrier fluid in said shared process fluid recovery circuit after reducing its pressure and before directing the reduced pressure barrier fluid to the dry seal of the second pump.

According to the method herein disclosed, when the first pump is in a running condition and the second pump is in a standby condition, and the barrier fluid coming from the running condition pump is provided to the dry seal of the standby pump. At any time, the functioning conditions of the two pumps can be switched so that the first pump is switched to a standby condition and the second pump is switched to a running condition. Thanks to the shared process fluid recovery circuit, the barrier fluid coming from the second pump, now in a running condition, is provided to the dry seal of the first pump, now in standby condition.

Reference throughout the specification to "one embodiment" or "an embodiment" means that a 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 phrases "in one embodiment" or "in an embodiment" in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.