GB2383627A

GB2383627A - Fail safe valve actuator

Info

Publication number: GB2383627A
Application number: GB0131115A
Authority: GB
Inventors: James Brian Wilson
Original assignee: ABB Offshore Systems Ltd
Current assignee: Baker Hughes International Treasury Services Ltd
Priority date: 2001-12-31
Filing date: 2001-12-31
Publication date: 2003-07-02
Also published as: AU2002353229A1; WO2003056221A1; GB0131115D0

Abstract

An actuator for use in underwater systems and does not require reverse drive comprises a linear actuator assembly fitted with a lost motion device 8 which comprises in a pre-primed state a closed valve 6, a de-energised actuator 15, an uncompressed spring 12, a cylinder 13 having a notch 14 and a latching ball 21 which allows free movement of an input rod 10. In a primed state (fig 4) input rod 10 is lifted, spring 12 is compressed, actuator 15 is energised, latch ball 21 moves into a notch 22 in rod 10 and cylinder 13 is lifted by actuator 15 to create a solid link between input rod 10 and output rod 19 via cylinders 18 and 11. The solid link allows rod 10 to be pushed in an opposite direction and valve 6 opened (fig 5). Power failure of actuator 15 releases the ball latch allows spring 12 to lift cylinder 11, rod 19 and close valve 6. To slow the closing speed of large valves oil in cylinder 11 may be included. Rod 10 may be depressed to close the valve 6 by locating spring 12 the other side of piston 9. A remote operating vehicle (fig 6) may be provided having a depressible rod 23 through the middle of input rod 10 and piston 9, and contacts cylinder 11 which abuts spring 12 for manual operation of valve 6.

Description

LINEAR ACTUATORS The present invention relates to linear actuators.

Linear electric actuators, i. e. electric actuators with push-pull outputs, are becoming employed in fluid extraction installations as a replacement for the traditional hydraulic linear actuators, typically employed to operate valves. One of the features of such an actuator, particularly for a subsea installation, is that the device operated by the actuator should return to a required position in the event of a failure, such as a loss of electrical control or a mechanical failure. For example, if the actuator operates a valve, then the valve must revert to a closed position, or, more rarely, to an open position, in the event of a failure. There are many actuators available on the market most of which employ an electric motor which drives, via a gearbox, a rotary to linear mechanism such as a screw drive and a small percentage of them have a fail-safe mechanism built in. Those that are available as fail-safe employ an integral mechanism that'rewinds'the actuator back to its original position in the event of a failure of electric power. The actuator motor winds or compresses a spring when it is powered, so that on power failure the spring returns the actuator to its original position. Typically, the motor drives the linear mechanism to an electrically powered mechanical latch to fully operate a valve, and on failure of the power supply to the latch, the spring returns the linear mechanism to its original position.

A hydraulic actuator normally comprises a simple piston and cylinder and has a fail-safe mechanism provided by the compression of a coil spring so that failure of the hydraulic power source results in the actuator reverting to its initial position by virtue of the potential energy in the spring returning the piston to its original position. Such a mechanism is very simple and reliable and is thus attractive to the fluid extraction contractor, which is one reason why hydraulic actuators have been popular.

The disadvantage of an electric actuator as described above is that the fail-safe mechanism is not simple and has to reverse drive the actuator through its relatively complicated mechanism, which includes the motor, gearbox and rotary to linear mechanism. Furthermore, any failure of the relatively complicated drive mechanism

involving seizing or jamming will also result in failure of the fail-safe feature.

According to the present invention, there is provided a combination of a linear actuator and a device operated by the actuator, including a lost motion arrangement for causing the device to fail-safe in the event of a fault associated with the actuator.

The actuator may comprise a rotary motion producer and means for converting rotary motion to linear motion.

The lost motion arrangement could be coupled between the actuator and the device. In this case, the lost motion arrangement could be such that it can be put, by power from the actuator, into a first, primed condition in which the actuator can put the device in a first condition, the combination being such that with the lost motion arrangement in said primed condition, a fault associated with the actuator results in the lost motion arrangement moving from its primed condition, which movement causes the device to be put into a fail-safe condition.

Said device could comprise a valve.

Said device could be a device for use in controlling an operation of an underwater hydrocarbon production system.

The actuator could be a linear electric actuator.

The present invention also comprises an underwater hydrocarbon production system including a combination according to the present invention.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which :- Fig. 1 is a schematic diagram of a known arrangement of a linear actuator and a valve;

Fig. 2 is a schematic diagram of an example of a combination according to the present invention; Figs. 3-5 are diagrams of an example of a lost motion arrangement for use in the combination of Fig. 2 in different conditions; and Fig. 6 is a diagram of a modified form of the arrangement of Figs. 3-5.

Fig. 1 illustrates, diagrammatically, a typical arrangement of a linear electric actuator operating a flow control valve. An electric motor 1 is attached to a gearbox and rightangled drive 2 (e. g. a worm and pinion gear). In most cases, particularly in subsea applications, some part of the rotary drive is extended via item 3 to provide a facility to operate the mechanism manually. The primary purpose of this is to permit operation of the valve during installation and commissioning, i. e. for testing purposes. In subsea applications, this manual drive output is designed to be easily operated by a remoteoperating vehicle (ROV). A fail-safe'rewind'spring is generally housed in or attached to the gearbox and right-angled drive 2. The rotary output from the gearbox and rightangled drive 2 is coupled to a bevel gearbox 4, whose rotary output is in turn coupled to a rotary to linear device 5, which is generally a lead screw mechanism, producing a push or a pull action on a valve 6. Since a typical operating time for a valve from closed to open is some 30 seconds, the overall gear ratio of the system is large. This is why any fail-safe mechanism tends to be applied to the motor drive end of the mechanism, since the forces required to return the valve to its initial state are much less than if they were applied to any other stage of the mechanism. Indeed, many rotary to linear mechanisms cannot be driven in reverse. Many commercially available actuators are supplied as modules, which can be bolted together by flanges to form the complete assembly. Fig. 1 shows a motor 1 and gearbox and right-angled drive 2 module bolted to a bevel gearbox module 4, which is bolted to a rotary to linear mechanism module 5, the couplings being via flanges 7.

Fig. 2 illustrates diagrammatically an example of a combination according to the present invention, which employs a'lost motion'linear fail-safe device (LM device) 8, fitted to the linear actuator assembly of Fig. 1 but excluding any'integral spring powered, fail-

safe mechanism', thus resulting in a fail-safe linear actuator with major advantages over the integral-spring method. A requirement of the'lost motion'device 8 is that, under normal operating conditions, the linkage between the linear output of the rotary to linear device 5, through the lost motion device 8, to the valve 6, should be a solid link. In the arrangement shown, the valve 6 is closed when the actuator operates in the direction of the arrow'A'and is open when the actuator operates in the direction of the arrow'B'.

Also, in the event of electric power failure, the output of the device 8 must drive in the direction of the arrow'A'to close the valve 6 without any movement of the output of the rotary to linear device 5 and in the event of any failure of the actuator drive mechanism 1,2, 4 and 5, the output of the LM device 8 must be able to return the valve to the required fail-safe state.

Fig. 3 illustrates an example of a mechanism for achieving the above requirements. A piston 9, attached to an input rod 10 of the LM device 8, is housed in a cylinder 11, with a compression spring 12 being located between the piston 9 and the end of the cylinder 11. A latch mechanism is fitted, for example a ball type as illustrated, to enable latching of the position of the LM device input rod 10. This comprises a sliding cylinder 13 with a notch 14 in its inner wall, operated by an electric actuator 15, typically a solenoid or motor, via an operating rod 16. The electric actuator 15 is mounted on a flange 17, which is attached to a cylinder 18, which in turn is attached to the cylinder 11. The cylinder 11 is also attached to an output rod 19 of the LM device 8 to operate the valve 6. The whole LM device is housed in a container 20, fitted with flange 7 to permit its attachment to a conventional commercial linear actuator. Fig. 3 shows the mechanism in its'pre-primed'or'expanded'state. Thus, the electric actuator 15 is not powered, the spring 12 is not compressed and the cylinder 13 is in the nonlatching position, so that a latching ball 21 allows free movement of the input rod 10. It should be noted that the valve 6 is in the closed position so that its operating shaft is fully lifted in the direction of the arrow'A'but the input rod 10 (and thus the linear to rotary device that the LM device is normally connected to) is in the fully'down'position - in the direction of the arrow'C'.

Fig. 4 shows the mechanism in the'primed'or'compressed'state. The input rod 10 has been lifted in the direction of the arrow'A' (by the linear actuator that it is normally

attached to), resulting in compression of the spring 12, since the cylinder 11 is unable to move any further in the direction of the arrow'A'because the valve 6 is already fully closed. The electric actuator 15 is powered so that once the position of the input rod 10 has reached that shown in the figure, the latch ball 21 can move into a notch 22 in the rod 10 and permit the cylinder 13 to be lifted by the electric actuator 15. As long as the electric actuator is powered, the latch between the input rod 10 and the cylinder 18 ensures a solid link between the input rod 10 and the output rod 19 via the cylinders 18 and 11 and the storage of energy in the compressed spring 12. It should be noted that the linear actuator provides the energy stored in the spring, which has to be sufficient to operate the valve under failure conditions and that the electric actuator 15 merely has to operate the sliding cylinder 13 and can thus be a low power consumption device. The lost motion device 8 is thus in the'primed'state.

An additional beneficial feature of the mechanism is that the peak power requirement of the actuator motor is reduced by typically between 10 and 20%. In a conventional device, the winding or compressing of the fail-safe mechanism spring is effected simultaneously with the operation of the valve. In this example of the invention, the compression of the spring 12 and the opening of the valve 6 are two separate actions, i. e. the compression of the spring is the priming process described above with the valve 6 already closed, following which operation of the valve is permitted without further compression of springs.

Fig. 5 shows the primed lost motion device 8 operating as a simple solid link between its input rod 10 and its output rod 19 when the valve is operated to the open position. Thus, the input rod 10 has been driven in the direction of the arrow'C'and this motion is transmitted directly to the valve 6 to open it.

If there is a failure of the electric power to the electric actuator 15 when the mechanism is in the state shown in Fig. 5, i. e. the valve 6 is open, then the cylinder 13 will fall (although it may be necessary to provide a little assistance rather than rely on gravity, by a small compression spring in the electric actuator). This releases the ball latch allowing the spring 12 to lift the cylinder 11, and thus lift the output rod 19 and close the valve 6. The final state of the mechanism after this process is that shown in Fig. 3.

It should be noted that the mechanism would also return the valve to its safe state from any intermediate position, not just from the state shown in Fig. 5, i. e. from a failure during the process of opening or closing the valve 6.

In the case of large valves used in subsea fluid extraction, it is generally beneficial to the life of the valve to close it over a short period of time, rather than virtually instantaneously. Filling the cylinder 11 with a fluid, such as hydraulic oil, accommodates this need. On release of the latch mechanism, the progression rate, with time, of the piston 9, down the cylinder 11, can be controlled by an appropriate choice of fluid viscosity and clearance between the piston 9 and the cylinder 11 in relation to the required tension in the spring 12.

Although in the example of the invention the valve 6 has its actuating rod lifted to close it, the opposite mode of operation, i. e depressing the actuating rod to close it, can easily be accommodated in the mechanism described. Simply locating the spring 12 between the piston 9 and the bottom face of the cylinder 11 reverses its action when the electric actuator 15 is released.

The present invention comprises a combination of a lost motion device with a linear actuator to achieve a far more reliable fail-safe actuator, for example for subsea fluid extraction from wells, than existing'fail-safe'actuators. The present invention removes the need to reverse drive the actuator mechanism and because the LM device is located between the linear actuator output and the device it operates (in the above, a valve) it provides fail-safe operation of that device independently of any mechanical failure of the actuator. The LM device mechanism described is only an illustration of how the invention can be implemented.

The reason why the fail-safe mechanism described is even more reliable than a hydraulic device is that the release of hydraulic fluid, under electric power failure, to return a hydraulic mechanism to the required safe position, is achieved by operation of a directional control valve (DCV) in the hydraulic line. DCV's are effectively solenoid operated valves, which carry some risk of sticking in the wrong position due to their use of high-pressure seals and close tolerance moving parts. Typically, mechanisms of

present invention do not require close tolerance parts or high pressure seals and are thus free of these failure risks.

Once the LM device has been primed by the first operation of the linear actuator, with electric power supplied to the electric actuator 15, it becomes a solid link between the actuator and the valve. However, when a new production fluid well is being commissioned, it is common practice to operate valves using a ROV. The mechanical operation of the actuator by access to the drive, item 13 of Fig. 1, is provided for this purpose. It can be seen that in the absence of electric power to the electric actuator 15 in Fig. 3, there is no solid link between the linear actuator and the valve and so ROV operation of the valve by means of item 13 of Fig. 1 is not possible. However, there are a number of methods of overcoming this problem. One is illustrated in Fig. 6 in which a rod 23 is passed through a hole through the middle of the input rod 10 and the piston 9 so that it comes into contact with the bottom of the cylinder 11. The rod 23 can be extended through the linear actuator to protrude through the whole assembly to be accessible to a ROV (not shown). Depression of the rod 23 by the ROV in the direction of the arrow'D'depresses the cylinder 11 against the spring 12 thus operating the valve 6. Removal of the depressing force by the ROV on the rod 23 results in the valve 6 returning to its original position by virtue of the spring 12. Thus, the need to operate valves without electric power during commissioning can be facilitated by the suitable design of the LM device.

Claims

1. A combination of a linear actuator and a device operated by the actuator, including a lost motion arrangement for causing the device to fail-safe in the event of a fault associated with the actuator.

2. A combination according to claim 1, wherein the actuator comprises a rotary motion producer and means for converting rotary motion to linear motion.

3. A combination according to claim 1 or 2, wherein said lost motion arrangement is coupled between the actuator and the device.

4. A combination according to claim 3, wherein the lost motion arrangement is such that it can be put, by power from the actuator, into a first, primed condition in which the actuator can put the device in a first condition, the combination being such that with the lost motion arrangement in said primed condition, a fault associated with the actuator results in the lost motion arrangement moving from its primed condition, which movement causes the device to be put into a fail-safe condition.

5. A combination according to any preceding claim, wherein said device comprises a valve.

6. A combination according to any preceding claim, wherein the actuator comprises a linear electric actuator.

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7. A combination according to any preceding claim, wherein said device is a device for use in controlling an operation of an underwater hydrocarbon production system.

8. A combination of a linear actuator and a device operated by the actuator, substantially as herein described with reference to Figs. 2-6 of the accompanying drawings.

9. An underwater hydrocarbon production system including a combination according to claim 7 or 8.