FIELD OF INVENTION

The present invention relates to a valve actuator for positional control of a valve. Particularly, but not exclusively, the invention relates to a valve actuator including a partial stroke testing mode.

BACKGROUND OF INVENTION

Industrial valve actuators are often required to be used in safety critical systems such as for control of emergency shut down valves. Such valves may be rarely used but must be reliable in the event of an emergency. As such, partial stroke testing is a known and tested method for valves and valve actuators. In particular, following the Piper Alpha disaster it became a legal requirement to partial stroke test safety critical valves in the oil and gas industry. Partial stroke testing allows the user to test a percentage of the possible failure modes of a shut down valve without the need to physically close the valve. Typically a partial stroke test moves the valve actuator (and associated valve) by a predetermined portion of the stroke of the actuator without fully closing (or fully opening) the valve. During a partial stroke test, the valve is only partially closed (or partially opened), so that flow is always maintained (or restricted) with minimal impact to operations.

Despite increasingly strict regulatory safety requirements, a large cause of dangerous undetected failures remains a low valve test frequency, or in some cases, not testing the valves at all. The ability to have higher frequency partial stroke testing would significantly reduce this risk. However, there are several reasons why the partial testing of safety systems may not be carried out sufficiently or routinely in the oil and gas industry. If the valve is not tested sufficiently or regularly, there is a risk that the valve could become "stuck", and thus require additional force to overcome the frictional stiction forces. There is then a risk that when the valve becomes "unstuck", the additional force applied could cause the valve to overshoot.

It will be appreciated by those in the art that valve actuators which may be used in safety critical applications will have a "safety factor" selected appropriate to the required function. The safety factor is the ratio between the torque which the actuator provides and the torque required to activate the valve. A typical valve actuator for a shut down valve may, for example, comprise a spring providing a force in a first direction and a pressurised cylinder for operating the valve in the opposing direction which must act against the spring force. When the safety factor of the actuator is 2 the pressure must drop by ½ to allow partial stroke to commence (under the force of the spring). The valve will move by 30-40% before the air sufficiently re-compresses to stop movement. As such, actuators with a safety factor of less than 4 will overshoot during partial stroke testing which can be undesirable. When an actuator has a safety factor of 1 the pressure will need to be zero to enable movement of the valve (this ensures there is sufficient spring force to move the valve), as such partial stroke testing will be expected to result in full closure (or full opening) of the valve.

Whilst partial stroke testing does not normally close (or open) a valve there remains a risk that testing may cause an unintended shut down. For example this may be the result of overshoot of a valve during testing. Such issues may also cause an unintended trip of further plant safety systems. It will be appreciated that unintended downtime results in significant disruption and potential economic loss - particularly, it will be appreciated that in the oil and gas industry, any downtime on a drilling platform can result in very significant economic losses.

It is therefore an object of embodiments of the present invention to provide a valve actuator which overcomes one or more of the above identified problems.

SUMMARY OF INVENTION

According to a first aspect of the present invention there is provided a valve actuator for positional control of a valve, the actuator comprising:

an actuation piston comprising:

a housing

a piston,

a first fluid chamber defined between a first end of the housing and a first face of the piston; and,

a second fluid chamber defined between a second end of the housing and a second face of the piston;

a first fluid control valve for supplying pressurised fluid to the first fluid chamber;

a second fluid control valve for supplying pressurised fluid to the second fluid chamber; and

a controller adapted to control the first and second fluid control valves to supply fluid to the first and second chambers; and wherein

the controller is configured to include a partial stroke test mode, in which the valve actuator is actuated to a predetermined partial position corresponding to a predetermined partial stroke position of a valve to verify performance of the valve, and in which: the first chamber is supplied with fluid at a first positive pressure;

the second chamber is supplied with fluid at a second positive pressure; and wherein the controller relatively varies the first and second pressures to actively control the differential pressure across the piston.

In a preferred embodiment, during a partial stroke test, the controller fixes the first pressure in the first chamber, and varies the second pressure in the second chamber, to provide a required differential pressure. The required differential pressure relates to the predetermined partial stroke position of the valve.

In an exemplary embodiment, the controller may control the pressure differential between the first chamber and second chamber to provide a pressure corresponding to the required valve actuator position. The controller may adjust the second pressure in response to the pressure in the first chamber and the movement of the actuator. The controller may actively adjust the second pressure during the partial stroke test mode. The controller may detect stiction during partial stroke testing and actuate the second fluid control valve to apply a second positive pressure to the second chamber.

In embodiments, the valve may be a quarter turn valve. The valve may be an industrial valve. The valve may be an emergency shut down valve.

The valve actuator may further comprise a spring against which the piston acts. The valve actuator may be a substantially zero bleed valve. The actuator may be an actuator for high integrity emergency shutdown valves. The actuator may be a pneumatic actuator. Alternatively, the actuator may be a hydraulic actuator.

The controller may comprise a screen, such as an organic light emitting diode (OLED) screen, which may locally display real-time data regarding the partial stroke test. The actuator and/or controller may further comprise a memory to store the real-time data for each partial stroke test.

The valve actuator may further comprise at least one low power and high-speed solenoid valve. The valve actuator may also comprise integrated filter boosters.

The valve actuator may be retrofitted into existing systems.

According to a further aspect of the invention there may also be provided a method of partial stroke testing a valve actuator comprising one or more features described herein.

The method may for example comprise actuating a valve actuator to a predetermined partial position corresponding to a predetermined partial stroke position of a valve to verify performance of the valve/actuator. The method may comprise:

supplying a first chamber with fluid at a first positive pressure;

supplying a second chamber with fluid at a second positive pressure; and

varying the first and second pressures to control the differential pressure across the piston. The method may comprise fixing the first chamber pressure and varying the second chamber pressure to provide the required partial pressure. Whilst the invention has been described above, it extends to any inventive combination set out above, or in the following description or drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be performed in various ways, and an embodiment thereof will now be described by way of example only, reference being made to the accompanying drawings, in which:

Fig. 1 shows a valve actuator in partial stroke test mode with the valve fully open (or closed) with no back pressure, in accordance with embodiments of the present invention;

Fig. 2 shows the valve actuator partial stroke test mode with the valve fully open (or closed) with increased back pressure, in accordance with embodiments of the present invention;

Fig. 3 shows the valve actuator partial stroke test mode with the valve at 80% open (or 80% closed), in accordance with embodiments of the present invention; and

Fig. 4 shows a valve actuator display presenting data from a partial stroke test.

DETAILED DESCRIPTION OF THE DRAWINGS

Figs 1 - 3 show a valve actuator 10 for positional control of a valve (not shown). The valve would normally be located beneath the board 20 at the base of the actuator. The valve can either be a normally open valve, or a normally closed valve. The valve actuator 10 is an emergency shut down valve of a general type commonly utilised in oil and gas safety applications. The valve actuator may be configured to have a typical safety factor of 2 and comprises a fluid activated actuation piston 30 (for example a otork "zero bleed" actuator) and return spring (not shown) within the spring housing 15. The actuation piston 30 and spring are arranged on opposing sides of a valve stem/connection and act against one another in use.

To aid understanding of the invention, it is worth noting that in a conventional (prior art) actuator a chamber on one side of the actuation piston would be pressurised against the force provided by the return spring. An initial pressure would be required (for example to 3 bar) and would hold the valve in the 100% position (fully closed or fully open) against the spring force. If the pressure against the actuation piston is reduced the valve will move with the position varying in proportion to the pressure (for example a 50% position may correspond to 2.0 bar pressure and 0% position may be achieved when the pressure is less than 1.5 bar). The design values for the pressure and position may be used to test the performance of the actuator and valve during a partial stroke test. In the event of valve stiction, the pressure in the actuator may need to decrease below the expected/design pressure before the valve/actuator will commence movement. This can then result in valve overshoot. The applicants have found that during such valve stiction it may be necessary for the pressure within the actuator to be decreased significantly in order to commence movement. Once the valve does move it will then compress the fluid within the actuator until the pressure and actuator position balance. This will typically be at a greater position than the original target/desired position for the partial stroke test (i.e. it will cause overshoot of the valve) and may even result in unintended tripping of safety systems. In contrast to the above, the present invention utilises an active control of pressure supplied to the actuator to avoid overshoot during partial stroke testing.

In accordance with embodiments of the invention the actuation piston 30 comprises a housing 32 and a piston 34. The actuation piston 30 also comprises a first fluid chamber 36 defined between a first end 31 of the housing 32 and a first face 33 of the piston 34; and a second fluid chamber 38 defined between a second end 35 of the housing 32 and a second face 37 of the piston 34. The first fluid chamber 36 is provided with an associated first fluid control valve 41 for supplying pressurised fluid to the first fluid chamber 36. Likewise, the second fluid chamber 38 comprises a second fluid control valve 42 for supplying pressurised fluid to the second fluid chamber 38. A controller 50 is adapted to control the first and second fluid control valves 41, 42 for supplying fluid to the first and second chambers 36, 38, in use.

The controller 50 is configured to include a partial stroke test mode, in which the valve actuator 10 is actuated to a predetermined partial position of the piston 34 corresponding to a predetermined partial stroke position of the valve. The partial stroke test verifies the performance of the valve and valve actuator 10. It will be appreciated that the intended position of the valve will correspond to a pressure applied to the actuation piston acting against the spring force of the return spring. For example, for a particular actuator/valve combination a 100% valve position could correspond to a 2.5 bar operating pressure of the actuator and a partial stroke test may target an 80% which would be expected to correspond to a 1.6 bar pressure in the actuator. As will be described more in Fig. 4, the controller 50 comprises a screen 51 which locally displays real-time data relating to the partial stroke test.

In use, the actuator 30 applies a torque to the valve stem to position the valve. The applied torque is determined by the pressure applied to the piston 34 providing a force which acts against the spring force in the return direction.

Prior to commencement of the partial stroke test mode, the pressure differential is set such that the return spring force is completely overcome and the valve is held fully open (or fully closed), this initial position is shown in Fig. 1. The piston 34 is positioned such that the valve is at a 100% position (which may typically correspond to the 100% open position). In this example the 100% position requires a pressure of 2.5 bar to be applied to the piston 34 to hold the valve position against the return springs spring force. The first fluid control valve 41 supplies pressurised fluid to the first chamber 36 until the pressure reaches 3 bar (the "main" pressure). The pressure in the second chamber 38 (the "back" pressure) is at zero bar. The differential pressure is 3 bar, which is greater than the 2.5 bar of pressure required to hold the valve fully open (or fully closed). To close (or open) the valve, the differential pressure applied by the actuator must be reduced. To close (or open) the valve partially, such as to a predetermined partial position, as would be done during a partial stroke test, the differential pressure must be reduced by a set amount. For example, the partial stroke test may require that the valve is open by 85% (that is it must be 15% closed). In another example, the partial stroke test may require that the valve is closed by 85% (that is it must be 15% open).

During the partial stroke testing the controller in accordance with embodiments of the present invention causes the first chamber 36 to be supplied with fluid at a first positive pressure and the second chamber 38 to be supplied with a fluid at a second positive pressure. The controller 50 relatively varies the first and second pressures to actively control the differential pressure across the piston 34. In contrast to the prior art configuration described above the invention applies a pressure to both sides of the piston and uses the pressure differential to provide an effective pressure on the piston 34 which sets the desired position (rather than allowing the second chamber to vent and adjusting the pressure in only the first chamber 36). As will be explained in more detail below, this provides advantages in the event of stiction. As shown in Figure 2, the controller in accordance with embodiments of the invention sets the pressure in the first ("main") chamber 36 at a constant value, for example 3 bar. In other words, the pressure in the first chamber is "locked in" at the start of the partial test procedure. The controller 50 adjusts the pressure differential applied by the piston 30 to counter the spring return force by increasing the pressure in the second chamber 36. For example, to provide a pressure differential of 0.8 bar at the piston 34 the back pressure in the second chamber 38 may be increased to 2.2 bar whilst the main chamber pressure is locked at 3 bar.

As commonly experienced in valve testing the valve may be initially stuck, for example due to increased friction, so does not move to the intended position. Therefore, the effective pressure on the piston 34 (i.e. the differential pressure) must be adjusted beyond the pressure corresponding to the desired position in order to increase spring force applied to the valve, to overcome the stiction. This further reduction in the differential pressure is achieved by the controller further increasing the pressure in the second chamber 38 whilst maintaining the locked in pressure in the first chamber 36. Eventually, this overcomes the stiction and the valve (and therefore piston 34) is freed. The movement of the piston 34 when the stiction is overcome then causes the volume of the first chamber 36 to reduce, thus compressing the fluid and increasing the pressure in the first chamber 36, for example to 4 bar. Likewise, the volume of the second chamber 38 increases, which in turn reduces the back pressure, for example to 1.7 bar. The differential pressure between the main and back pressures is now 2.3 bar, which corresponds to the valve being 80% open (or 80% closed). The valve therefore has only overshot by 5%, which is negligible, compared to conventional valve actuators.

Advantageously, embodiments of the invention can quickly and accurately move the valve to a predetermined partial position, with negligible or no overshoot, despite having a safety factor of 2 (it is widely known in the oil and gas industry that valves with a safety factor less than 4 are liable to overshoot).

Negligible overshoot can help to mitigate the risk of spurious trips (unintentional shut downs), which in turn helps to minimise economic losses, which is particularly relevant in the oil and gas industry. Mitigating the risk of spurious trips can help to encourage more frequent partial stroke testing, which would itself help to reduce the occurrence of valves becoming "stuck", thus further reducing the risk of overshoots and spurious trips. Furthermore, the actuator of the present invention can be used with on/off valves, such as emergency shut down valves, therefore providing on/off valves with accurate positional control for partial stroke tests. In contrast, conventional valve actuators control the piston, and the valve, by supplying and/or venting pressure in the first chamber during use. However, there is no back pressure provided to actively balance the main pressure. When the pressure in the first chamber is too high, a vent opens to release some of the pressure. For example, for a conventional actuator with a safety factor of 2, in order to provide a differential pressure of 0.8 bar for overcoming stiction of the valve, the main pressure in the first chamber must be vented until a pressure of 0.8 bar is reached. Once stiction has been overcome, the piston rapidly moves to maximally compress the fluid in the first chamber, thus the volume of the first chamber is reduced and the pressure is increased to 1.9 bar (the back pressure remains at zero bar). The differential pressure is thus 1.9 bar, which corresponds to the valve being 40% open (or 40% closed). The valve has therefore overshot the desired value (by around 45%), which is incredibly problematic.

Instead of venting the pressure, the present invention senses when the pressure is high, and utilises the high-speed and zero bleed solenoids 41, 42 to control the pressure in both the first and second chambers 36, 38 (such as increasing the back pressure in the second chamber, and maintaining the pressure in the first chamber). Conventional high speed solenoids leak, and are therefore not suitable for controlling the valve position. Fig. 4 shows the screen 51 of the controller 50 in more detail. As the partial stroke test is run, the display plots a graph in real-time of the valve position against the actuator pressure difference. It can be seen in the graph that as the valve closes (as it moves from 100% open/closed to 83.2% open/closed), the pressure difference drops as described above. Once the valve has reached the predetermined partial position, the valve then returns to a fully opened (or fully closed) position by increasing the pressure difference. The screen 50 also displays the safety factor at various positions of the valve during the partial stroke test (at 95% open/closed, 90% open/closed, and 85% open/closed). The safety factors in this example are very high because in this particular test no valve was attached to the actuator. During a normal partial stroke test, a valve would be attached to the actuator, and the safety factor values will be much lower. In this example, the predetermined valve position was 85% open (or 85% closed), and the minimum position of the valve was 83.2% open (83.2% closed). Thus the valve overshot by 1.8%, which is negligible. Although the invention has been described above with reference to an exemplary embodiment, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the statements of invention.

For example, whilst in the embodiment above utilises a pressure lock in the main chamber and a back pressure in the second chamber for all partial stroke testing, the skilled person will appreciate that such an approach could be selectively applied. For example, the actuator could initially commence testing in the manner of a conventional actuator and only apply a back pressure if/when stiction or "out of standard" movement of the valve is detected.