Pump protection method and system

Abstract

Protecting a hydrocarbon pump from excessive flow rates in a hydrocarbon fluid system comprising an electrical motor for driving the pump. For each of a plurality of gas volume fraction values of the hydrocarbon fluid, establishing a maximum torque limit for the pump by mapping the maximum allowable torque of the pump as a function of the differential pressure, thereby creating a plurality of maximum torque curves, each representing the maximum torque limit for a unique gas volume fraction value. Establishing a master maximum torque curve which represents the maximum torque limit for all gas volume fraction values. Monitoring the torque of the pump and the differential pressure across the pump. Based on the monitored differential pressure and using the master maximum torque curve, establishing a maximum allowable torque for the pump. Taking action if the monitored torque exceeds the established maximum allowable torque.

Claims

1. A method of protecting a hydrocarbon pump from excessive flow rates in a system for pumping a hydrocarbon fluid, which system comprises said pump and an electrical motor for driving the pump, the method comprising the steps of: for each of a plurality of gas volume fraction values of the hydrocarbon fluid, establishing a maximum torque limit for the pump by mapping a maximum allowable torque of the pump as a function of a differential pressure across the pump, thereby creating a plurality of maximum torque curves, each representing the maximum torque limit for a unique gas volume fraction value; from the plurality of maximum torque curves, establishing a master maximum torque curve which represents the maximum torque limit for all said gas volume fraction values; monitoring a torque of the pump and a differential pressure across the pump; based on the monitored differential pressure (DP) and using the master maximum torque curve, establishing a maximum allowable torque (T) for the pump; and taking a predetermined action if the monitored torque exceeds the established maximum allowable torque (T).

2. The method according to claim 1, wherein the step of taking the predetermined action comprises at least one of raising an alarm and shutting down the system.

3. The method according to claim 1, wherein the step of taking the predetermined action comprises regulating the system such that the monitored torque is reduced.

4. The method according to claim 1, wherein the step of monitoring the torque of the pump comprises monitoring a power and a speed of the pump and calculating the torque of the pump based on the monitored power and speed.

5. The method according to claim 4, wherein the step of monitoring the power and the speed of the pump comprises sampling an output power from a variable speed drive controlling said motor.

6. The method according to claim 4, wherein the step of calculating the torque of the pump comprises compensating for at least one of mechanical and electrical losses in the system.

7. The method according to claim 1, wherein the master maximum torque curve, for each differential pressure value (DP), has a lower torque value T than the corresponding torque values of the maximum torque curves.

8. The method according to claim 1, wherein the step of establishing the master maximum torque curve comprises positioning the master maximum torque curve adjacent to and on the permissible operating side of the maximum torque curves.

9. The method according to claim 1, wherein the step of establishing the master maximum torque curve comprises applying one of a linear or a second degree polynomial approximation algorithm to said plurality of maximum torque curves.

10. A system comprising a hydrocarbon pump and an electrical motor for driving the hydrocarbon pump, the system being configured to protect the hydrocarbon pump from excessive flow rates by performing the following steps: for each of a plurality of gas volume fraction values of the hydrocarbon fluid, establishing a maximum torque limit for the pump by mapping a maximum allowable torque of the pump as a function of a differential pressure across the pump, thereby creating a plurality of maximum torque curves, each representing the maximum torque limit for a unique gas volume fraction value; from the plurality of maximum torque curves, establishing a master maximum torque curve which represents the maximum torque limit for all said gas volume fraction values; monitoring a torque of the pump and a differential pressure across the pump; based on the monitored differential pressure (DP) and using the master maximum torque curve, establishing a maximum allowable torque (T) for the pump; and taking a predetermined action if the monitored torque exceeds the established maximum allowable torque (T).

11. The system according to claim 10, wherein the system is the subsea hydrocarbon fluid pumping system.

12. The system according to claim 10, wherein the step of taking the predetermined action comprises at least one of raising an alarm and shutting down the system.

13. The system according to claim 10, wherein the step of taking the predetermined action comprises regulating the system such that the monitored torque is reduced.

14. The system according to claim 10, wherein the step of monitoring the torque of the pump comprises monitoring a power and a speed of the pump and calculating the torque of the pump based on the monitored power and speed.

15. The system according to claim 14, wherein the step of monitoring the power and the speed of the pump comprises sampling an output power from a variable speed drive controlling said motor.

16. The system according to claim 14, wherein the step of calculating the torque of the pump comprises compensating for at least one of mechanical and electrical losses in the system.

17. The system according to claim 10, wherein the master maximum torque curve, for each differential pressure value (DP), has a lower torque value T than the corresponding torque values of the maximum torque curves.

18. The system according to claim 10, wherein the step of establishing the master maximum torque curve comprises positioning the master maximum torque curve adjacent to and on the permissible operating side of the maximum torque curves.

19. The system according to claim 10, wherein the step of establishing the master maximum torque curve comprises applying one of a linear or a second degree polynomial approximation algorithm to said plurality of maximum torque curves.

Description

DESCRIPTION OF THE DRAWINGS

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

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

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

DETAILED DESCRIPTION OF THE INVENTION

(4) 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 for different gas volume fractions of the fluid being pumped. This type of diagram is conventionally referred to as a DP-Q diagram. The diagram discloses a plurality of pump limit characteristics curves 1a-1e for different gas volume fraction values. The curve 1a represents the maximum flow limit for a first gas volume fraction, GVF.sub.a, the curve 1b represents the maximum flow limit for a second gas volume fraction, GVF.sub.b, etc., where GVF.sub.a<GVF.sub.b<GVF.sub.c<GVF.sub.d<GVF.sub.e, and where the curves 1a-1e define an impermissible operating region 2 and a permissible operating region 3 of the pump. As is indicated by the arrow A, for a given differential pressure value DP the pump limit characteristics curves 1a-1e shift towards higher flow values when the gas volume fraction increases. Consequently, in order to establish the pump limit characteristics curve for a multiphase fluid in a DP-Q diagram, the flow rate as well as the gas volume fraction of the fluid need to be measured which, as was discussed above, requires the use of complex and expensive multiphase flow meters.

(5) FIG. 2 discloses an alternative, novel way of illustrating the operational range of a pump. In FIG. 2 the differential pressure across the pump, DP, is mapped as a function of the pump torque T for the same gas volume fraction values as in FIG. 1, thus forming a set of pump limit characteristics curves in the form of maximum torque lines or curves 4. As with the curves 1a-1e in FIG. 1, the maximum torque curves 4 define an impermissible operating region 17 and a permissible operating region 18 of the pump. As is apparent from FIG. 2, the maximum torque lines or curves 4 are concentrated to a more restricted region than are the pump limit characteristics curves 1a-1e in FIG. 1. In other words, the maximum torque curves 4 do not shift much when the gas volume fraction of the fluid varies.

(6) Consequently, if the differential pressure across the pump is mapped as a function of the pump torque T instead of the flow rate Q, it is possible to establish a master maximum torque line or curve 5 which is representative for all gas volume fractions of the fluid, as is indicated by the dotted line in FIG. 2. In other words, based on the maximum torque curves 4, a master maximum torque curve 5 can be established which represents the maximum flow limit for all gas volume fractions of the fluid.

(7) The master maximum torque curve 5 may be established by mapping the differential pressure DP across the pump as a function of the pump torque T for a set of different gas volume fraction values, thus obtaining a cluster of maximum torque curves 4, and then positioning the master maximum torque curve 5 adjacent to and on the permissible operating side 18 of the maximum torque curves 4. For example, it may be advantageous that the master maximum torque curve 5 is positioned as close as possible to but on the permissible operating side of the cluster of maximum torque curves 4. However, for any given differential pressure value DP, the master maximum torque curve 5 should be positioned at a lower torque value T than for the corresponding torque values of the maximum torque curves 4, as is illustrated in FIG. 2. Given this criteria, a linear or second degree polynomial approximation algorithm can be used to establish the master maximum torque curve 5 from the cluster of maximum torque curves 4.

(8) When choosing said set of different gas volume values, it is advantageous that the set covers the intended or expected range of gas volume fraction values, i.e. gas fraction volumes representing the whole operational range of the pump.

(9) FIG. 3 discloses a hydrocarbon fluid pumping system in which the method according to the invention can be realised. The system comprises a pump 6 mounted on a hydrocarbon fluid conduit 7. The pump 6 has a suction side 8 and a discharge side 9. The pump 6 may advantageously be a helicoaxial (HAP) or centrifugal type pump. The system further comprises an electrical motor 10 for driving the pump 6 via a shaft 11. The motor 10 is advantageously a variable speed motor which is controlled by a variable speed drive, VSD 12.

(10) In order to monitor a parameter indicative of the differential pressure across the pump 6, the system comprises a first measuring or sensor device 13. This sensor device 13 may advantageously comprise one or a plurality of pressure sensors arranged to monitor the differential pressure across the pump 6, e.g. a first pressure sensor 13a positioned upstream of the pump 6 and a second pressure sensor 13b positioned downstream of the pump 6.

(11) The system further comprises a control unit 14 which is connected to the variable speed drive 12 and to the sensor device 13 via control conduits 15 and 16, respectively.

(12) Using this system, the method according to the invention comprises the steps of establishing, for each of a plurality of gas volume fraction values of the hydrocarbon fluid in the conduit 7, a maximum torque limit for the pump 6 by mapping the maximum allowable torque of the pump 6 as a function of the differential pressure across the pump 6, thereby creating a plurality of maximum torque curves 4 (cf. FIG. 2), each representing the maximum torque limit for a unique gas volume fraction value of the hydrocarbon fluid.

(13) From the plurality of maximum torque curves 4, a master maximum torque curve 5 is established, which master maximum torque curve 5 represents the maximum torque limit for all gas volume fraction values. Consequently, the master maximum torque curve 5 will define the rightmost delimiting border, or edge, of an allowable envelope or operating region of the pump 6 which is to be valid for all gas volume fractions of the hydrocarbon fluid. The master maximum torque curve 5 is established as an approximation for the cluster of maximum torque curves 4, e.g. as has been described above in relation to FIG. 2.

(14) Once the master maximum torque curve 5 is established, it is stored in the system, e.g. as a look-up table in the control unit 14.

(15) During operation of the system, the differential pressure across the pump 6 is monitored using the sensor device 13.

(16) Also, the motor torque is monitored, e.g. by monitoring the power and the speed of the pump 6 and calculating the torque of the pump 6 based on the monitored power and speed. Advantageously, the step of monitoring the power and the speed of the pump 6 comprises sampling output power and pump speed from the variable speed drive 12.

(17) 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 rounds per minute.

(18) If the torque of the pump 6 is calculated based on the output from the variable speed drive 12, it may be advantageous if due account is taken to estimated mechanical and/or electrical losses in the system, i.e. electrical losses in the motor 10 and in the energy supply system of the motor 10 and mechanical losses to the pump shaft 11, such that the calculated torque reflects the true torque at the pump 6.

(19) In subsea pumping systems, it may be particularly advantageous to sample the variable speed drive 12 for the pump torque as the variable speed drive is generally easily accessible topside, i.e. above sea level.

(20) The monitored differential pressure signal is sent to the control unit 14 via the signal conduit 16, and using the stored master maximum torque curve 5 stored therein, a maximum allowable torque T corresponding to the monitored differential pressure DP is established (cf. FIG. 2). Likewise, the monitored motor torque is sent to the control unit 14 via the signal conduit 15. In the control unit 14, the established maximum allowable torque T is compared to the monitored torque, and if the monitored torque exceeds the maximum allowable torque T, a predetermined action is taken, e.g. the raising of an alarm and/or shutting down the system.

(21) In the preceding description, various aspects of the invention have been described with reference to the illustrative figures. 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 as defined by the following claims.