System for pumping a fluid and method for its operation

Abstract

A method for operating a pump includes establishing a pump limit characteristics diagram by mapping a first system parameter (P1) as a function of a second system parameter (P2) to identify a permissible operating region of the pump; for each first system parameter value (P1.sub.0), identifying a minimum allowable second system parameter value (P2.sub.0); monitoring the first system parameter (P1) and identifying a minimum allowable second system parameter value (P2.sub.0) corresponding to the monitored first system parameter value (P1.sub.m); monitoring the second system parameter (P2) and comparing the monitored second system parameter value (P2.sub.m) with the identified minimum allowable second system parameter value (P2.sub.0); and regulating a control valve that controls fluid flow through a return line connecting the suction and discharge sides of the pump so that the monitored second system parameter value (P2.sub.m) does not fall below the minimum allowable second system parameter value (P2.sub.0).

Claims

1. A method of operating a system for pumping a fluid, the system including a pump comprising a suction side and a discharge side, a motor for driving the pump, the motor being drivingly connected to the pump via a shaft, a return line providing a feed-back conduit for the fluid from the discharge side to the suction side, and a control valve for controlling the flow of the fluid through the return line, the method comprising: establishing a pump limit characteristics diagram by mapping a first system parameter (P1) as a function of a second system parameter (P2) to identify a permissible operating region of the pump, wherein the first system parameter (P1) is a function of a differential pressure across the pump and the second system parameter (P2) is a function of a pump torque; for each first system parameter value, identifying a minimum allowable second system parameter value (P2.sub.0); monitoring the first system parameter (P1) and identifying a minimum allowable second system parameter value (P2.sub.0) corresponding to the monitored first system parameter value (P1.sub.m); monitoring the second system parameter (P2) and comparing the monitored second system parameter value (P2.sub.m) with the identified minimum allowable second system parameter value (P2.sub.0); and regulating the control valve based on a relationship between the monitored second system parameter value (P2.sub.m) and the minimum allowable second system parameter value (P2.sub.0) such that the monitored second system parameter value (P2.sub.m) does not fall below the minimum allowable second system parameter value (P2.sub.0); wherein the system comprises a variable speed drive for operating the motor, and wherein the step of monitoring the second system parameter (P2) comprises sampling the second system parameter (P2) from the variable speed drive; wherein the step of identifying a minimum allowable second system parameter value (P2.sub.0) comprises compensating the minimum allowable second system parameter value (P2.sub.0) for at least one of mechanical losses in at least one of the motor and the pump and electrical losses between the variable speed drive and the motor; and wherein said mechanical losses are determined based on a rotational speed of the pump and said electrical losses are determined based on a power of the pump and the rotational speed of the pump.

2. The method according to claim 1, wherein the first system parameter (P1) is a differential pressure across the pump.

3. The method according to any one of claims 1 and 2, wherein the second system parameter (P2) is a torque (T) of the pump or a current (I) in windings of the motor.

4. The method according to claim 1, wherein the step of regulating the control valve comprises opening the control valve when the monitored second system parameter value (P2.sub.m) is within a predetermined range of the minimum allowable second system parameter value (P2.sub.0).

5. The method according to claim 1, wherein said fluid is a hydrocarbon fluid.

6. The method according to claim 1, wherein said step of establishing a pump limit characteristics diagram comprises establishing a pump limit characteristics curve for the pump, and wherein said step of identifying a minimum allowable second system parameter value (P2.sub.0) for each first system parameter value comprises establishing a pump operation curve in the permissible operating region, said pump operation curve being positioned at a predetermined distance from the pump limit characteristics curve.

7. A system for pumping a fluid, comprising: a pump comprising a suction side and a discharge side; a motor for driving the pump, the motor being drivingly connected to the pump via a shaft; a return line providing a feed-back conduit for the fluid from the discharge side to the suction side; a control valve for controlling the flow of the fluid through the return line; a first sensor device for monitoring a first system parameter (P1) which is a function of the differential pressure across the pump; a second sensor device for monitoring a second system parameter (P2) which is a function of a torque of the pump; and a control unit which is arranged to: receive monitored first system parameter values (P1.sub.m) from the first sensor device and, for each monitored first system parameter value (P1.sub.m), identify a minimum allowable second system parameter value (P2.sub.0); receive monitored second system parameter values (P2.sub.m) from the second sensor device and, for each monitored second system parameter value (P2.sub.m), compare the monitored second system parameter value (P2.sub.m) with the identified minimum allowable second system parameter value (P2.sub.0); and regulate the control valve based on a relationship between the monitored second system parameter value (P2.sub.m) and the minimum allowable second system parameter value (P2.sub.0) such that the monitored second system parameter value (P2.sub.m) does not fall below the minimum allowable second system parameter value (P2.sub.0); wherein the system comprises a variable speed drive for operating the motor, and wherein the control unit is arranged to monitor the second system parameter (P2) by sampling the second system parameter (P2) from the variable speed drive; wherein the control unit is arranged to compensate the minimum allowable second system parameter value (P2.sub.0) for at least one of mechanical losses in at least one of the motor and the pump and electrical losses between the variable speed drive and the motor; and wherein said mechanical losses are determined based on a rotational speed of the pump and said electrical losses are determined based on a power of the pump and the rotational speed of the pump.

8. The system according to claim 7, wherein the minimum allowable second system parameter value (P2.sub.0) is identified using a pump limit characteristics diagram which maps the first system parameter (P1) as a function of the second system parameter (P2) to identify a permissible operating region of the pump.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 discloses a DP-Q diagram conventionally used to illustrate the operational range of a pump in a fluid pumping system.

(2) FIG. 2 discloses a diagram of an alternative, novel way of illustrating the operational range of a pump in a fluid pumping system.

(3) FIG. 3 discloses a hydrocarbon fluid pumping system according to an embodiment of the invention.

(4) FIG. 4 is a block diagram schematically illustrating a method of regulating a hydrocarbon pumping system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(5) FIG. 1 discloses a conventional pump limit characteristics diagram 1 for a hydrocarbon pump where the differential pressure DP across the pump is mapped as a function of the volumetric flow Q through the pump. This type of diagram is conventionally referred to as a DP-Q diagram. The diagram discloses a first pump limit characteristics curve 2 for a first gas volume fraction, GVF1, a second pump limit characteristics curve 3 for a second gas volume fraction, GVF2, and a third pump limit characteristics curve 4 for a third gas volume fraction, GVF3, of the hydrocarbon fluid, where GVF1<GVF2<GVF3. Each pump limit characteristics curve 2-4 comprises a minimum flow curve section 5, a minimum speed curve section 6 and a maximum speed curve section 7 defining a permissible operation region 8 and an impermissible operation region 9 of the pump. When the GVF is increased, it is necessary to increase the pump speed (and flow) in order to maintain the same torque. As is shown in the diagram 1, the operational point of the pump should be shifted when the gas volume fraction changes from GVF1 to GVF2 and then further to GVF3, as is indicated by the arrow 10.

(6) FIG. 2 discloses an alternative pump limit characteristics diagram 11 for the pump where the differential pressure across the pump, DP, is mapped as a function of the pump torque T.

(7) The manner of establishing a pump limit characteristics diagram as disclosed in FIG. 2 is beneficial since it has been revealed that the minimum pump torque required to uphold a sufficient differential pressure across the pump is valid for different gas volume fractions and fluid densities. Consequently, instead of requiring pump limit characteristics curves for different GVFs or densities, only one pump limit characteristics curve 12 needs to be established and stored in the system. Therefore, the pump limit characteristics curve 12 defines second system parameter values below which the pump may experience a pumping instability or surge, independent of the gas volume fraction and density of the fluid. The curve 12 separates a permissible operating region 13 from an impermissible operating region 14 of the pump. Consequently, for every differential pressure value, DP.sub.0 (P1.sub.0), it is possible to identify an allowable, desired torque value, T.sub.0 (P2.sub.0), thus establishing a pump operation curve 15 in the permissible operating region13 positioned at a predetermined, safe distance from the pump limit characteristics curve 12. Consequently, for each differential pressure value DP.sub.0 (P1.sub.0), the torque value T.sub.0 (P2.sub.0) may be used as a setpoint or target value for the torque, or as a minimum allowable torque value.

(8) During normal operation of the pump, the motor current of the motor driving the pump, i.e. the current flowing in the windings of the pump motor, will generally be proportional to the pump torque. Consequently, instead of mapping the differential pressure against the torque, the differential pressure may alternatively be mapped against the winding current of the pump motor, I, as is indicated in FIG. 2.

(9) The method of operating a fluid pumping system according to the invention comprises the step of establishing a pump limit characteristics diagram 11 of the type disclosed in FIG. 2 by mapping a first system parameter P1 as a function of a second system parameter P2 identifying a permissible operating region 13 of the pump, wherein the first system parameter P1 is a function of a differential pressure across the pump, and wherein the second system parameter P2 is a function of the torque acting on the pump shaft. As discussed above, the first parameter P1 may be the differential pressure measured across the pump, and the second system parameter P2 may be the torque T acting on the pump shaft or, alternatively, the motor current of the pump motor.

(10) The method further comprises the step of identifying a minimum allowable second system parameter value P2.sub.0 for each first system parameter value P1.sub.0. The set of minimum allowable values P2.sub.0 may be defined by the above-discussed pump operation curve 15. The set of minimum allowable second system parameter values P2.sub.0 may, for example, comprise a minimum allowable pump shaft torque value, T.sub.0, or a minimum allowable pump motor current value I.sub.0 for every differential pressure value DP.sub.0, as is indicated in FIG. 2.

(11) Once established, the set of minimum allowable second system parameter values P2.sub.0 are stored in the system to provide reference values during its operation.

(12) FIG. 3 discloses a hydrocarbon fluid pumping system 16 according to a preferred embodiment of the invention. The system comprises a pump 17 having a suction side 18 and a discharge side 19. The pump 17 may advantageously be a helicoaxial (HAP) or centrifugal type pump. The system 16 further comprises an electrical motor 20 for driving the pump 17 via a shaft 21. The motor 20 is a variable speed motor which is controlled by a variable speed drive, VSD 22. The system 16 also comprises a return line 23 providing a feed-back conduit for the hydrocarbon fluid from the discharge side 19 to the suction side 18 of the pump 17, and a control valve 24 controlling the flow of the hydrocarbon fluid through the return line 23. The system further comprises a control unit 25 providing control signals for the control valve 24 via a signal conduit 26.

(13) In order to monitor the first parameter P1, i.e. the parameter indicative of the differential pressure across the pump 17, the system 16 comprises a first measuring or sensor device 27. This sensor device 27 may be a pressure sensor arranged to monitor the differential pressure DP across the pump 17.

(14) Also, in order to monitor the second parameter P2, i.e. the parameter indicative of the pump torque, the system 16 comprises a second measuring or sensor device 28. The second sensor device 28 may be a torque sensor arranged to monitor the torque T acting on the shaft 21 or, alternatively, a current sensor arranged to monitor the motor current I.

(15) The monitored first and second system parameter values are conveyed from the sensor devices 27, 28 to the control unit 25 via signal conduit 29.

(16) When monitoring the second parameter P2, the most accurate parameter value is obtained by measuring the pump torque directly at the shaft 21. In subsea applications, however, this may not be a viable option since surface signal conduits may have bandwidth ratings ruling out efficient transfer of the torque signal. Therefore, it may be advantageous to sample the second parameter P2 from the variable speed drive 22. In the variable speed drive 22, signals indicative of the shaft torque are readily available. For example, the pump torque can easily be calculated from the power and the pump speed with the following function:
T=(P.Math.60000)/(2.Math..Math.N)
where the torque T is given in Nm, the power P in kW and the pump speed N in rotations per minute.

(17) Also, the signals of the variable speed drive 22 are sampled with a relatively high sampling frequency which makes it possible to realise a responsive control system. Furthermore, in subsea pumping systems, the variable speed drive is generally more accessible than the pump-motor assembly since the variable speed drive is normally positioned topside, i.e. above sea level.

(18) If the second system parameter P2 is sampled from the variable speed drive 22, the monitored second system parameter values are advantageously conveyed from the variable speed drive 22 to the control unit 25 via signal conduit 30.

(19) In the following, a method of operating the system 16 will be discussed with reference to FIG. 4. The method comprises the step of monitoring the first system parameter P1 and, for each monitored first system parameter value P1.sub.m, identifying the minimum allowable second system parameter value P2.sub.0, e.g. using the above-discussed pump operation curve 15 (cf. FIG. 2). In FIG. 4, this step is illustrated by reference numeral 31. As discussed above, the first system parameter P1 may advantageously be a function of the differential pressure across the pump and the second system parameter value may advantageously be a function of the pump torque. The minimum allowable second system parameter value P2.sub.0 may for example relate to the pump torque T.sub.0 or to the motor current I.sub.0, depending on which parameter is chosen as the second system parameter.

(20) The method further comprises the step of monitoring the second system parameter P2 and, for each monitored second system parameter value P2.sub.m, comparing the value with the previously identified minimum allowable second system parameter value P2.sub.0. In FIG. 4, this step is illustrated by reference numeral 32. As is indicated by the dashed path in FIG. 4, the monitored second system parameter value P2.sub.m may be compared directly with the minimum allowable second system parameter value P2.sub.0. However, if the second system parameter P2 is sampled from the variable speed drive 22, mechanical losses in the motor and electrical losses in cables and transformers between the variable speed drive and the motor may advantageously be compensated for prior to the step of comparing the monitored second system parameter value P2.sub.m with the minimum allowable second system parameter value P2.sub.0. For example, mechanical losses in the motor 20 and/or the pump 17 may be calculated based on the rotational speed N of the pump, as is illustrated by reference numeral 33, and electrical losses may be calculated based on the power P and the pump speed N, as is illustrated by reference numeral 34.

(21) The method finally comprises the steps of calculating a control valve control signal S.sub.valve based on the difference between the monitored second system parameter P2.sub.m and the minimum allowable second system parameter value P2.sub.0, and using the control valve control signal S.sub.valve to regulate the control valve 24 such that the monitored second system parameter does not fall below the minimum allowable second system parameter value. In particular, the control valve control signal S.sub.valve is set to open the control valve 24 when the monitored second system parameter value P2.sub.m approaches the minimum allowable second system parameter value P2.sub.0, thus preventing the second system parameter from undercutting the minimum allowable second system parameter value P2.sub.0.

(22) As previously discussed, the differential pressure over the pump 20 normally varies relatively slowly due to large volumes of hydrocarbon fluid upstream and downstream of the pump. However, the gas volume fraction and/or the density of the hydrocarbon fluid may change quickly, e.g. due to gas and/or liquid slugs in the system. Consequently, the pump torque may also changes relatively quickly. Therefore, in order to enable the system to react quickly to a change in the gas volume fraction and/or the density of the fluid, it may be advantageous to sample the second system parameter P2 using a higher sampling frequency than the first system parameter P1.

(23) In the preceding description, various aspects of the invention have been described with reference to the illustrative embodiment. For purposes of explanation, specific numbers, systems and configurations were set forth in order to provide a thorough understanding of the invention and its workings. However, this description is not intended to be construed in a limiting sense. Various modifications and variations of the illustrative embodiment, as well as other embodiments of the apparatus, which are apparent to persons skilled in the art to which the disclosed subject matter pertains, are deemed to lie within the scope of the present invention.