System for pumping a fluid and method for its operation

1. A method of operating a system for pumping a fluid, the system 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 recirculation conduit providing a fluid path for the fluid from the discharge side to the suction side of the pump; and

a control valve controlling the flow of the fluid through the recirculation conduit;

wherein the method comprises the steps of:

mapping a plurality of minimum torque curves for the pump, wherein each minimum torque curve identifies a minimum allowable torque of the pump as a function of an operational parameter of the pump;

from said plurality of minimum torque curves, identifying a minimum torque curve best representing the current operation of the pump;

monitoring said operational parameter of the pump and, from the minimum torque curve best representing the current operation of the pump, identifying a minimum allowable torque value (T 0 ) corresponding to a monitored value (DP m ) of said operational parameter of the pump;

monitoring a torque of the pump and comparing the monitored torque value (T m ) with the identified minimum allowable torque value (T 0 ); and

regulating the control valve such that the monitored torque value (T m ) does not fall below the minimum allowable torque value (T 0 ).

2. The method according to claim 1 , wherein said operational parameter of the pump is a differential pressure over the pump and said monitored value of said operational parameter of the pump is a monitored differential pressure value (DP m ) of the pump.

3. The method according to claim 1 , wherein each minimum torque curve represents a unique combination of a suction pressure and a rotational speed of the pump.

4. The method according to claim 3 , wherein each minimum torque curve defines mapping points for a predetermined suction pressure value and a predetermined rotational speed value of the pump.

5. The method according to claim 4 , wherein the step of mapping each minimum torque curve comprises the sub-step of:

for the predetermined suction pressure value and the predetermined rotational speed value, defining said plurality of mapping points by mapping differential pressure over the pump as a function of torque of the pump for different gas-volume fraction (GVF) values.

6. The method according to claim 5 , wherein the step of mapping each minimum torque curve comprises the sub-step of:

establishing a minimum torque curve from said mapping points by interpolating between the mapping points.

7. The method according to claim 5 , wherein said step of identifying the minimum torque curve best representing the current operation of the pump comprises:

monitoring suction pressure and rotational speed of the pump; and

choosing the minimum torque curve to represent the current operation of the pump based on the monitored suction pressure and torque.

8. The method according to claim 7 , wherein said step of choosing the minimum torque curve to represent the current operation of the pump based on the monitored suction pressure and torque comprises:

choosing, for each monitored suction pressure value and rotational speed value, the minimum torque curve having the next lower suction pressure value and the next higher rotational speed value.

9. 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 recirculation conduit providing a fluid path for the fluid from the discharge side to the suction side of the pump;

a control valve controlling the flow of the fluid through the recirculation conduit;

a first sensor device adapted to monitor an operational parameter of the pump;

a second sensor device adapted to monitor or estimate the torque of the pump;

a control unit in which is stored a plurality of minimum torque curves for the pump, wherein each minimum torque curve identifies a minimum allowable torque of the pump as a function of said operational parameter of the pump;

wherein the control unit is configured to:

identify, from said plurality of minimum torque curves, a minimum torque curve best representing the current operation of the pump;

monitor said operational parameter of the pump and, from the minimum torque curve best representing the current operation of the pump, identify a minimum allowable torque value (T 0 ) corresponding to a monitored value (DP m ) of said operational parameter of the pump;

monitor a torque of the pump and compare the monitored torque value (T m ) with the identified minimum allowable torque value (T 0 ); and

regulate the control valve such that the monitored torque value (T m ) does not fall below the minimum allowable torque value (T 0 ).

10. The system according to claim 9 , wherein said operational parameter of the pump is the differential pressure over the pump and said monitored value of said operational parameter of the pump is a monitored differential pressure value (DP m ) of the pump.

11. The system according to claim 9 , wherein the control unit is adapted to:

monitor suction pressure and rotational speed of the pump; and

identify, at predetermined points in time, the minimum torque curve of said plurality of minimum torque curves which best corresponds to the monitored suction pressure and rotational speed values.