GB2480423A

GB2480423A - Valve system

Info

Publication number: GB2480423A
Application number: GB201004240A
Authority: GB
Inventors: Paul Ward; David Talbot
Original assignee: Jena Rotary Technology Ltd
Current assignee: Jena Rotary Technology Ltd
Priority date: 2010-03-15
Filing date: 2010-03-15
Publication date: 2011-11-23
Also published as: GB201004240D0

Abstract

A valve system 10 comprises a valve 12 positioned in a conduit 14 of a power generation system for inhibiting gas flow 16 through the conduit, and a device 20 for moving the valve 12 in either direction between its closed and open conditions. The device 20 comprises an electro-mechanical actuator 21 which may be a ball-screw, a part of which is connected to the valve 12 and is linearly movable by an electric motor 25 to open or close the valve 12.

Description

Title: A valve system

Description of Invention

This invention relates to a valve system. More particularly, this invention has been devised in relation to valve systems for use in power generation systems which generate electricity.

Power generation systems utilising a turbine, which is caused to rotate by flow of gas, e.g. steam, include at least one conduit (e.g. a pipework) for transporting gas to or away from the turbine. Each conduit includes at least one valve system for regulating the amount of gas flowing through the conduit or for preventing any gas from passing through the conduit. The valve system therefore includes a valve (e.g. a butterfly, gate or any other appropriate valve) which is operated by an actuator to move the valve in either direction between its closed and open conditions.

Prior art power generation systems have, at least for the past 50 years, relied on hydraulically operated actuators to move the valve, because the required operational forces (which can often be high, particularly when initially opening the valve) can easily be reached. However, hydraulic actuators suffer from a number of safety and performance issues. Hydraulic actuators tend to leak hydraulic fluid during their in-service life, because of the mechanical integrity of the seals therein, which gives rise to significant fire risks. It is not uncommon for leaked hydraulic fluid to catch fire, as a result of leaked hydraulic fluid contacting hot pipework, which can then cause fire damage to other nearby equipment. In addition, it is not uncommon for hydraulic actuators to fail catastrophically, which can be potentially fatal to operators. These problems can often result in a reduced performance of the power generation system, which can give rise to significant financial losses.

In addition, hydraulic actuators provide little to no positional feedback. For example, it is very difficult to determine the position (between its closed and open conditions) of a valve which is moved by a hydraulic actuator, without an operator visually monitoring the valve or implementing an auxiliary (e.g. separate) measurement system. Essentially, all the operator can tell, remotely, is whether the valve appears to be fully open or fully closed, because the hydraulic actuator (e.g. a piston/cylinder configuration) is no longer moving. However, a problem with relying on such an analysis is that the piston may have stopped moving because the valve has become stuck at a position between its open and closed conditions, or because a linkage between the actuator and the valve has failed. It is therefore difficult to know whether the valve is fully closed or fully open, without a visual inspection or without relying on other measurable parameters of the power generation system (e.g. decreased performance thereof).

According to a first aspect of the invention we provide a valve system comprising:-a valve for positioning in a conduit of a power generation system for inhibiting the flow of gas through the conduit; and a device for moving a valve in either direction between its closed and open conditions, the device comprising an electro-mechanical actuator a part of which is connected to the valve and is linearly movable by an electric motor to open or close the valve.

According to a second aspect of the invention we provide a power generation system comprising:-at least one conduit for transporting gas to an electricity generator; and a valve for inhibiting flow of gas through the conduit; wherein the valve is movable in either direction between closed and open conditions by a device comprising an electro-mechanical actuator a part of which is connected to the valve and is linearly movable by an electric motor to open or close the valve.

According to a third aspect of the invention we provide a kit of parts for providing a valve system for inhibiting the flow of gas through a conduit of a power generation system, comprising:-a valve for inhibiting flow of gas through the conduit; and a device for moving the valve in either direction between closed and open conditions, the device comprising an electro-mechanical actuator a part of which is connectable to the valve and is linearly movable by an electric motor to open or close the valve.

According to a fourth aspect of the invention we provide a method of moving a valve from its closed condition to its open condition, the valve being configured to inhibit the flow of gas through a conduit and being movable in either direction between its closed and open conditions by a device comprising an electro-mechanical actuator a part of which is connected to the valve and is linearly movable by an electric motor to open or close the valve, the method comprising the steps of: providing an electrical power supply to the electric motor; increasing the electrical power supply to the electric motor so as to increase an output torque of the motor, until the valve starts to move from its closed condition; and decreasing the electrical power supply to the motor as the valve approaches its open condition so as to decrease the speed of opening of the valve.

According to a fifth aspect of the invention we provide a method of moving a valve from its open condition to its closed condition, the valve being configured to inhibit the flow of gas through a conduit and being movable in either direction between its closed and open conditions by a device comprising an electro-mechanical actuator a part of which is connected to the valve and is linearly movable by an electric motor to open or close the valve, the method comprising the steps of: providing an electrical power supply to the electric motor; increasing the electrical power supply to the electric motor so as to increase an output torque of the motor, until the valve starts to move from its open condition; and decreasing the electrical power supply to the motor as the valve approaches its closed condition so as to decrease the speed of closing of the valve.

According to a sixth aspect of the invention we provide electro-mechanical actuator comprising a housing and a first part which is linearly moveable relative to the housing by an electric motor, wherein the actuator comprises means for manually effecting linear movement of the first part relative to the housing.

According to a seventh aspect of the invention we provide a ballscrew shaft for a ball screw assembly having a formation at one end thereof with which an operating member can be engaged so that a user can manually rotate the ballscrew shaft.

According to an eighth aspect of the invention we provide a method of monitoring the operation of a valve system, the valve system including a valve which is movable in either direction between its closed and open conditions by a device, the method including the steps of:-during the course of movement of the valve in either direction between its closed and open conditions measuring a physical characteristic associated with the valve system; and comparing the measured physical characteristic with a predetermined user-selected range of values of the physical characteristic.

Further features of the first to eighth aspects of the invention are set out in the claims appended hereto.

Embodiments of the inventions will now be described by way of example only, with reference to the accompanying drawings, of which:-Figure 1 is a side illustrative view of a valve system in accordance with the present invention, with a valve thereof in a closed condition; Figure 2 is a side illustrative view of a valve system in accordance with the present invention, with a valve thereof in a partially open condition; Figure 3 is a side illustrative view of a valve system in accordance with the present invention, with a valve thereof in a fully open condition; and Figures 4 to 9 are graphs showing graphs of various plots of values of measured physical characteristics associated with a valve system during the course of movement of the valve between its closed and open conditions.

Referring to figures 1 to 3 a valve system 10 includes a valve 12 which is positioned in a conduit 14. In this example the conduit 14 is part of a power generation system and the purpose of the valve 12 is to inhibit/prevent the flow of gas 16 through the conduit 14. The valve 12 may be used to regulate the flow of gas 16 (e.g. steam) through the conduit 14, or it may be utilised as a safety shut-off or override valve. As can be seen in the figures, the valve 12 used is a gate valve, which is moved linearly in either direction between its closed and open conditions. However, it should be noted that any appropriate valve could be utilised, e.g. a butterfly valve which is rotate about an axis in either direction transversely across the conduit 14 between its closed and open conditions.

The valve system 10 includes a device 20 for moving the valve 12 in either direction between its closed and open conditions. The device 20 comprises an electro-mechanical actuator 21 a part 22 of which is connected to the valve 12 by an appropriate coupling 13. The part 22 is linearly movable by an electric motor 25 to open or close the valve 12, and includes a bi-directional load cell 23 for measuring a load experienced by the part 22. The device 20 is supported relative to the conduit 14 by a support frame 30.

In the present example the electro-mechanical actuator 21 is a ball screw assembly 21 and the linearly moveable part 22 is a part which is connected to a ball nut of the assembly 21. The ball nut (not shown), as is usually the case in a ball screw assembly 21, is rotatably supported by a ballscrew shaft (not shown) and the ballscrew shaft is driven by the motor 25. As the ballscrew shaft rotates, it effects linear movement of the ball nut relative to a housing of the assembly 21.

The motor 25 in this example is an electro servo motor and is connected to the ballscrew shaft of the assembly 21 through a gearbox 26 (which may be a planetary gearbox) via a pulley system including a pair of pulleys 27, 29 which are connected by an endless flexible member 28 (e.g. a belt). The device 20 includes a brake 33 to effect emergency braking of the motor 25, an encoder 32 for determining the position of the valve between its open and closed conditions, and a connection 31 for an electrical power supply.

As can be seen in the figures, the gearbox 26 is positioned in-line with the motor 26. However, it should be appreciated that the motor 25 and gearbox 26 can be positioned elsewhere. For example, the gearbox could be positioned in-line with the assembly 21.

The device 20 comprises means 40 for manually effecting linear movement of the linearly moveable part (the ball screw shaft) of the assembly 21 relative to its housing. In this example, the means 40 is provided as a formation connected to an axle which is drivingly connected to the second rotatable part (the ball nut) of the assembly 21. The formation 40 may alternatively be an integral part of the second rotatable part (the ball nut) which extends rearwardly out of the housing of the assembly 21. The formation 40, which may be for example a male or female socket connection, can be engaged by an operating member (e.g. a socket lever) so that the user can manually rotate the ballscrew shaft in either direction. If a component part of the valve system 10 is damaged, meaning that valve 12 cannot be opened/closed by the motor 25, the present invention permits a user to manually rotate the ball nut so as to move the ballscrew shaft to open/close the valve 12. This is highly advantageous, especially in a power generation system where a long delay in opening a valve can be costly and can cause damage to component parts of the system, e.g a turbine of the system.

As an alternative, and depending on the positioning of the motor 25, gearbox 26 and assembly 21, the formation 40 may be provided on an input to or output from the gearbox 26. If the gearbox 26 and the assembly 21 are in-line, the mechanical advantage of the gearbox 26 can be used if the formation 40 drives and input to the gearbox 26, which then drive the second rotatable part 22 of the assembly 21.

In figure 1, the valve 12 is shown in a fully closed condition. In this condition little or no gas 16 can pass along the conduit 14. In figure 2, the valve 12 is shown just after it has been moved from its closed condition. In this condition a regulated amount of gas 16 can pass along the conduit 14 past the vale 12.

Finally, in figure 3, the valve 12 is shown fully open. In this condition a maximum flow of gas 16 is permitted along/through the conduit 14.

The valve system 10 is controlled by a controller (not shown) to fully or partially open or fully or partially close the valve 12 depending on the operational requirements of the power generation system. In more detail, when it is desired to move the valve 12, electrical power is supplied to the electric motor 25 which in turn effects movement of the part 22 of the assembly 21 (in either direction of the arrow A, depending on the direction of rotation of the motor 25). The electrical power supply to the electric motor 25 is then increased so as to increase an output torque of the motor 25, until the valve 12 starts to move from its current position.

If the valve 12 is being moved from its fully closed condition, a high motor torque will be required to achieve the required opening (or cracking' force) force. This is because the pressure across the valve 12 in the fully closed condition is relatively high. After the valve 12 has started to move the power supply to the motor 25 is further increased (preferably gradually) to a maximum torque so as to increase the speed of opening of the valve 12 to a maximum speed. As the valve 12 approaches its fully open condition (which is known from the encoder 32) the electrical power supplied to the motor is decreased so as to decrease the speed of closing of the valve 12. This ensures that minimal impact to a valve stop, at the fully open position, is achieved. The electrical power supply to the motor 25 is withdrawn once the valve 12 reaches its fully open condition.

If the valve 12 is being moved from its fully open condition, the initial motor torque will not usually be as high as for opening. This is because the pressure across the valve 12 in the fully open condition is minimal. After the valve 12 has started to move the power supply to the motor is increased (preferably gradually) to a maximum to increase the speed of closing of the valve 12 to a maximum speed. As the valve 12 approaches its fully closed condition the electrical power supplied to the motor is increased (preferably gradually) so as to overcome the increased pressured differential across the valve 12. The electrical power supply to the motor 25 is withdrawn once the valve 12 reaches its closed condition.

The valve system 10 of the present invention also provides a feedback system for assessing whether the valve system 10 is operating within desired limits.

To achieve this feedback functionality, during the course of movement of the valve 12 in either direction between its closed and open conditions a microprocessor, e.g. a computer, measures and records certain physical characteristic associated with the valve system 10. Such physical characteristics may be, for example, one or more of the following:-load on the linearly moveable part of the actuator; electric motor torque; current drawn by the electric motor; rotational speed of a rotor of the electric motor; position of the valve between its open and closed conditions; acceleration of the rotor of the electric motor; ambient temperature; temperature of the electric motor; velocity of the linearly moveable part of the assembly 21; acceleration of the linearly moveable part of the assembly 21; electric motor following error; and/or time to complete opening or closing of the valve.

These physical characteristics are measured a plurality of times during the course of movement of the valve 12 in either direction between its closed and open conditions. For example, these parameters may be measured at predetermined time intervals, e.g. every 0.1 seconds, and the results stored in a suitable memory for later analysis. A typical time for the valve to close/open would be 3 to 4 seconds and thus this represents 30 to 40 data points.

During the movement of the valve 12 the microprocessor compares each measured (and recorded) physical characteristic with a predetermined user-selected range of values of the physical characteristic. In other words, a user programmes into the microprocessor his/her desired operational limits for each physical characterised -an upper threshold and a lower threshold -essentially providing a tolerance band for satisfactory' operation of the valve system 10.

If during the course of movement of the valve 12 in either direction between its closed and open conditions, one of the measured physical characteristics falls outside the predetermined user-selected range, an output is generated, e.g. an audible or visual alarm, to alert the operator to that fact. The operator can then, if he/she deems necessary, investigate why the alarm has been set off.

As the microprocessor stores all measured physical characteristics in a memory, it is possible to consult the recorded values after the alarm has been set off. In one example, the present invention is able to provide a graph plotting each measured physical characteristic against valve position (between its closed and open conditions). An operator can then visually inspect these graphs and determined whether a physical inspection of the valve system 10 is required. Alternatively, software could be utilised for interpreting the graph plots, and for providing an output to the operator informing him/her of the likely fault with the valve system 10. The valve position is provided by the encoder 32 on the motor 25, which calculates the expected position of the valve 12 by the number of rotations of a rotor of the motor 25, but any other appropriate positioning means could be used, e.g. a sensor on the valve itself.

Figure 4 to 9 shows various plots of measured physical characteristics associated with the valve system 10. Figure 4 shows expected values for motor speed, motor acceleration, motor torque/current drawn, force recorded by load cell and the following error of the electric motor. This plot may be achieved by one or more test runs of the valve system 10 during/shortly after its installation. This graph will provide base' plots with which later graphs can be compared.

Figure 5 shows the test' measured load cell values and test' measured motor speed values for a valve system in accordance with the invention. Upper and lower thresholds have been selected by a user to define a tolerance band within which the alarm will not be set off. Corresponding ranges are also set for the other physical characteristic measured. As an example, the tolerances may be selected as positive and negative percentages of the test' measurements. For example the upper threshold may be set to +15% and the lower threshold may be set to -5%. Once these ranges have been set by the user, the microprocessor will, when the valve 12 is subsequently opened/closed, compare the measure values with these set limits and will set off the alarm if the limits are exceeded, either positively or negatively.

Figure 6 to 9 shows graphs plotting measured values which are indicative of certain faults with the valve system 10. In these figures, the solid lines are the values as measured/recorded during the movement of the valve 12 where the alarm was set off and the dotted lines are the expected or test' values with which they are being compared. The user selected ranges are not shown, but in all cases, the alarm was set off, because a measured value fell short afar exceeded the respective user-selected range.

In figure 6, the motor speed is well below the expected values throughout the opening of the valve 12. The measured force is about equal to the expected value. However, the measured motor torque/current drawn is much higher than expected, with the motor following error also higher. Such a plot would indicate to an operator that the ballscrew shaft, ball nut or gearbox are worn and will likely require replacement soon, because it is taking a much longer time to open the valve and the motor is having to work much harder during opening.

In figure 7 the motor speed, force and motor torque/current drawn plots closely follow the expected or test' plots. However, roughly halfway through the opening/closing of the valve, the motor speed decrease, the force increases and the motor torque/current drawn increases. All three variables return roughly to the expected levels either side of this halfway point. Such a plot would indicate to an operator that the valve has a stiff/sticking point halfway through its opening/closing movement. It may be that a servicing of the valve rectifies the problem. It may be a worn bearing and thus needs replacing or servicing.

In figure 8 the measured motor speed is on par with the expected value.

However, the force, the motor following error and motor torque/current drawn are all much lower than expected. Such a plot would indicate to an operator that the drive belt connecting the motor to the assembly 21 has failed.

Finally, in figure 9, during opening of the valve, the force, motor torque/current drawn and motor speed are initially on par with the expected values.

However, shortly after the initial opening of the valve, the force drops dramatically, as does the motor torque/current drawn value. Such a plot would indicate to an operator that a component part of the valve system (e.g. the coupling connecting the assembly 21 to the valve) has failed (e.g. broken) during initial opening of the valve.

The assembly 21 described in the above examples is a ballscrew assembly.

However, it should be noted that other types of electro-mechanical actuator could be utilised for the present invention, e.g. a lead screw actuator or a planetary roller screw. All that is necessary is for the actuator to be able to translate rotation provided by the electric motor into linear movement of a part of the actuator so that that part can effect opening/closing of the valve.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS1. A valve system comprising:-a valve for positioning in a conduit of a power generation system for inhibiting the flow of gas through the conduit; and a device for moving a valve in either direction between its closed and open conditions, the device comprising an electro-mechanical actuator a part of which is connected to the valve and is linearly movable by an electric motor to open or close the valve.
2. A power generation system comprising:-at least one conduit for transporting gas to an electricity generator; and a valve for inhibiting flow of gas through the conduit; wherein the valve is movable between closed and open conditions by a device comprising an electro-mechanical actuator a part of which is connected to the valve and is linearly movable by an electric motor to open or close the valve.
3. A power generation system according to claim 2 further including at least one turbine which utilises a flow of gas through the conduit to effect movement thereof to generate electricity.
4. A kit of parts for providing a valve system for inhibiting the flow of gas through a conduit of a power generation system, comprising:-a valve for inhibiting flow of gas through the conduit; and a device for moving the valve between closed and open conditions, the device comprising an electro-mechanical actuator a part of which is connectable to the valve and is linearly movable by an electric motor to open or close the valve.
5. A valve system, a power generation system or a kit of parts according to any one of claims 1 to 4 wherein the electro-mechanical actuator is a ballscrew assembly.
6. A valve system, a power generation system or a kit of parts according to any one of claims 1 to 5 wherein the electro-mechanical actuator comprises a load cell for measuring a load on the actuator when opening/closing the valve.
7. A valve system, a power generation system or a kit of parts according to claim 6 wherein the load cell measures a load on the linearly moveable part of the actuator.
8. A method of moving a valve from its closed condition to its open condition, the valve being configured to inhibit the flow of gas through a conduit and being movable in either direction between its closed and open conditions by a device comprising an electro-mechanical actuator a part of which is connected to the valve and is linearly movable by an electric motor to open or close the valve, the method comprising the steps of: providing an electrical power supply to the electric motor; increasing the electrical power supply to the electric motor so as to increase an output torque of the motor, until the valve starts to move from its closed condition; and decreasing the electrical power supply to the motor as the valve approaches its open condition so as to decrease the speed of opening of the valve.
9. A method according to claim 8 including the step of, after the valve has started to move, further increasing the electrical power supply to the electric motor so as to increase the speed of opening of the valve.
10. A method according to claim 8 or claim 9 wherein, after the valve is moved from its closed condition, the electric power supply to the motor is increased until the valve is moving at a maximum speed.
11. A method according to claim 8, claim 9 or claim 10 wherein as the valve approaches its open condition, the electrical power supply to the motor is reduced.
12. A method according to claim 11 wherein the electrical power supply to the motor is gradually reduced to a predetermined value.
13. A method according to any one of claims 8 to 12 wherein the electrical power supply to the motor is withdrawn once the valve reaches its open condition.
14. A method of moving a valve from its open condition to its closed condition, the valve being configured to inhibit the flow of gas through a conduit and being movable in either direction between its closed and open conditions by a device comprising an electro-mechanical actuator a part of which is connected to the valve and is linearly movable by an electric motor to open or close the valve, the method comprising the steps of: providing an electrical power supply to the electric motor; increasing the electrical power supply to the electric motor so as to increase an output torque of the motor, until the valve starts to move from its open condition; and decreasing the electrical power supply to the motor as the valve approaches its closed condition so as to decrease the speed of closing of the valve.
15. A method according to claim 14 including the step of, after the valve has started to move, further increasing the electrical power supply to the electric motor so as to increase the speed of closing of the valve.
16. A method according to claim 14 or claim 15 wherein, after the valve is moved from its open condition, the electric power supply to the motor is increased until the valve is moving at a maximum speed.
17. A method according to claim 14, claim 15 or claim 16 wherein as the valve approaches its closed condition, the electrical power supply to the motor is increased.
18. A method according to any one of claims 14 to 17 wherein the electrical power supply to the motor is gradually increased to a predetermined value.
19. A method according to any one of claims 14 to 18 wherein the electrical power supply to the motor is withdrawn once the valve reaches its closed condition.
20. An electro-mechanical actuator comprising a housing and a first part which is linearly moveable relative to the housing by an electric motor, wherein the actuator comprises means for manually effecting linear movement of the first part relative to the housing.
21. An electro-mechanical actuator according to claim 20 wherein the electro-mechanical actuator comprises a second part which is supported for rotation by the housing and which is drivingly connected to the first part, the second part being rotatable by the motor such that rotation of the second part effects linear movement of the first part, wherein said manual means permits the second part to be rotated manually by a user.
22. An electro-mechanical actuator according to claim 20 or claim 21 wherein the actuator includes a formation, which is drivingly connected to the second part, with which an operating member can be engaged so that a user can manually rotate the second part.
23. An electro-mechanical actuator according to claim 20, claim 21 or claim 22 wherein the electro-mechanical actuator comprises a gearbox which is connected to an output of the motor and to an input to the second part so as to transfer rotational drive from the motor to the second part.
24. An electro-mechanical actuator according to claim 23 wherein said manual means permits an input to the gearbox to be rotated manually by a user.
25. An electro-mechanical actuator according to claim 24 including a formation, which is drivingly connected to said input to the gearbox, with which an operating member can be engaged so that a user can manually rotate said output.
26. An electro-mechanical actuator according to claim 23 wherein said manual means permits an output from the gearbox to be rotated manually by a user.
27. An electro-mechanical actuator according to claim 26 including a formation, which is drivingly connected to said output from the gearbox, with which an operating member can be engaged so that a user can manually rotate said output.
28. An electro-mechanical actuator according to any one of claims 23 to 27 wherein the gearbox is provided in-line with the motor.
29. An electro-mechanical actuator according to any one of claims 23 to 27 wherein the gearbox is provided in-line with the second, rotatable, part.
30. An electro-mechanical actuator according to any one of claims 20 to 29 wherein the electro-mechanical actuator is a ballscrew assembly.
31. An electro-mechanical actuator according to claim 30 wherein the first, linearly moveable, part is a ball nut and the second, rotatable, part is a ballscrew shaft.
32. A ballscrew shaft for a ball screw assembly having a formation at one end thereof with which an operating member can be engaged so that a user can manually rotate the ballscrew shaft.
33. A method of monitoring the operation of a valve system, the valve system including a valve which is movable between its closed and open conditions by a device, the method including the steps of:-during the course of movement of the valve in either direction between its closed and open conditions measuring a physical characteristic associated with the valve system; and comparing the measured physical characteristic with a predetermined user-selected range of values of the physical characteristic.
34. A method according to claim 33 wherein said physical characterised is measured a plurality of times during the course of movement of the valve in either direction between its closed and open conditions.
35. A method according to claim 34 including the step of providing an output to an operator if, during the course of movement of the valve in either direction between its closed and open conditions, the measured physical characteristic falls outside the predetermined user-selected range.
36. A method according to claim 35 wherein the output indicate to the operator where during the course of movement of the valve in either direction between its closed and open conditions the measured physical characteristic fell outside the predetermined user-selected range.
37. A method according to any one of claims 34 to 36 wherein the method includes measuring two or more physical characteristics associated with the valve system and comparing each measured physical characteristic with a respective predetermined user-selected range.
38. A method according to any one of claims 34 to 37 including the initial step of selecting a predetermined range of values for each physical characteristic to be measured.
39. A method according to any one of claims 34 to 38 wherein the device comprises an electro-mechanical actuator a part of which is connected to the valve and is linearly movable by an electric motor to open or close the valve, and wherein the physical characteristic to be measured is one or more of the following:-load on the linearly moveable part of the actuator; electric motor torque; current drawn by the electric motor; rotational speed of a rotor of the electric motor; position of the valve between its open and closed conditions; acceleration of the rotor of the electric motor; ambient temperature; temperature of the electric motor; velocity of the linearly moveable part of the actuator; acceleration of the linearly moveable part of the actuator; electric motor following error; and/or time to complete opening or closing of the valve.
40. A valve system, a valve, a power generation system, an electro-mechanical actuator, a ballscrew shaft for a ball screw actuator or a method substantially as hereinbefore described with reference to and/or as shown in the accompanying drawings.
41. Any novel feature or novel combination of features described herein and/or in the accompanying drawings.