Actuator for a valve

1. An actuator for a valve in a subsea installation, the valve having a valve element which is movable between a first position and a second position and a spring which biases the valve element toward the first position, the actuator comprising:

a hydraulic cylinder which includes a movable piston, the piston being mechanically connectable to said valve element;

a fluid pump which is connected to said hydraulic cylinder through a first hydraulic line, said fluid pump being adapted to supply fluid to the hydraulic cylinder through the first hydraulic line at a sufficient pressure to move the piston and thereby move the valve element to the second position;

a reservoir for storing a quantity of hydraulic supply fluid at a pressure that is less than said sufficient pressure;

a second hydraulic line which provides a fluid passage between the first hydraulic line and said reservoir;

a bypass control unit which is located in said second hydraulic line and which comprises a movable element that operates to open or close said fluid passage;

first and second force generating means, both of which bias the movable element to a position to close the fluid passage; and

a third force generating means which biases the movable element to a position to open the fluid passage, the third force generating means being configured to operate independently of both the first force generating means and the second force generating means.

2. The actuator according to claim 1 , wherein during use the third force generating means comprises pressurized fluid from the first hydraulic line.

3. The actuator according to claim 2 , wherein the first force generating means comprises a solenoid.

4. The actuator according to claim 3 , wherein the second force generating means comprises a spring.

5. The actuator according to claim 1 , wherein the movable element comprises a piston which is arranged in a casing.

6. The actuator according to claim 5 , wherein the bypass control unit comprises a seal which is positioned around the piston when the fluid passage is closed and around a retainer element within the casing when the fluid passage is open, the retainer element being configured to retain the seal in position in the casing when the fluid passage is open.

7. The actuator according to claim 6 , wherein the retainer element abuts the piston when the fluid passage is closed and follows the movements of the piston until a stop element on the retainer element comes into contact with a stop surface in the casing, wherein the fluid passage comprises an axial gap formed between the retainer element and the piston, which fluid passage extends through an outlet in the casing.

8. The actuator according to claim 6 , wherein the retainer element abuts the piston when the fluid passage is closed and is adapted to follow the movements of the piston, and wherein the retainer element is formed with at least one radial through-going opening forming a part of the fluid passage when the fluid passage is open.

9. The actuator according to claim 1 , wherein the second force generating means provides a force on the movable element which is greater than the force provided by the first force generating means on the movable element.

10. The actuator according to claim 1 , wherein the third force generating means comprises pressurized fluid in a third hydraulic line which is connected to the second hydraulic line.

11. The actuator according to claim 1 , wherein the control unit comprises a permanent magnet which is adapted to apply a force overriding the force of the first and second force generating means to thereby force the fluid passage open.

12. The actuator according to claim 1 , wherein the bypass control unit is a two-way valve which includes the movable element.

13. The actuator according to claim 12 , wherein the second force generating means comprises a spring which is adapted to move the movable element to close the fluid passage, and wherein the force of the spring is less than the force generated by the third force generating means.

14. The actuator according to claim 13 , wherein, in a first position, a lever acts on the movable element to close the fluid passage and, in a second position, the lever enables the pressure in a third hydraulic line which is connected to the first hydraulic line to move the movable element to open the fluid passage.

15. The actuator according to claim 14 , wherein the lever is held in the first position by an electro-magnet.

16. The actuator according to claim 1 , wherein the fluid pump is driven by an electric motor.

17. The actuator according to claim 1 , wherein the piston is releasably connected to the valve element.

18. The actuator according to claim 1 , wherein the actuator further comprising a one-way valve in said first hydraulic line to enable fluid to flow in only one direction from the fluid pump to the hydraulic cylinder.

19. The actuator according to claim 1 , wherein the actuator further comprises a fourth fluid line connecting said hydraulic fluid supply reservoir to the fluid pump.