Pump protection method and system

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.