WO1995022027A1

WO1995022027A1 - Safety device for a valve actuator

Info

Publication number: WO1995022027A1
Application number: PCT/GB1995/000268
Authority: WO
Inventors: Michael Arnold Calvert; Christopher Paul Shrive
Original assignee: Prime Actuator Control Systems Limited
Priority date: 1994-02-11
Filing date: 1995-02-10
Publication date: 1995-08-17
Also published as: GB9402691D0

Abstract

A safety device for a valve actuator which converts linear motion of a linear rod into rotary or angular motion of a body within the actuator, comprising an electric motor (1); a magnetic particle clutch (2) selectively engageable with the motor and means for transmitting power from the motor to the linear rod (6), and spring force means (11) mounted within the actuator compressible by the action of the linear rod during rotation of the body.

Description

SAFETY DEVICE FOR A VALVE ACTUATOR

This invention relates to a safety device for stored energy transmission systems and more particularly, provides a fail-safe control arrangement for use in a Valve Actuator.

Valve Actuators are widely used and are required for, remote control of Valve opening and closing functions. Generally, Valve Actuators are powered by either pneumatic, hydraulic, gas or electric means. This fail-safe electric device can be used in place of pneumatic, hydraulic or gas functions. More particularly, a usage would be to remove complexity and cost by removing or reducing the need for hydraulic system applications. The hydraulic fluid used in Actuator Control Systems may cause operational difficulties due to contamination and further, may present a fire risk in the event that a hydraulic fluid leak occurs from a pipe, hose, joint or Actuator seal rupture. Furthermore, hydraulic operating equipment may have to be installed in conjunction with the Va.lve Actuator to provide the hydraulic pressure source and controlling functions. This type of equipment for critical applications tends to be bulky and expensive. Nevertheless, hydraulic Actuators remain a well-proven concept and a popular choice in Valve Actuator Control Systems.

Fail-safe systems are provided by the Actuators for use in the case of an emergency such as the sudden loss of pneumatic or hydraulic pressure to the Actuator or by removal of a signal voltage or pressure causing the Valve to move to a pre-determined fail position. This may be valve open or valve closed depending upon the system application. The Actuator concept described here can be assembled to provide either direction of rotation. It is an established principle to use a spring force integral to the Actuator as the stored energy source to generate the fail movement stroke, thus typically the valve is driven against the counter-acting force of the spring force upon receipt of an operating signal for the system whereby either by the loss of the operating signal by accident or function, a release of the compressing force from the Actuator occurs allowing the stored energy device, in this case, the spring to cause the Actuator to travel to the end of the fail-safe stroke.

In order to obviate risks and disadvantages associated with hydraulic systems one might consider alternative power- driven systems such as electric motors but transmission of power to the Actuator mechanism becomes problematic when considering the fail-safe aspects required. Thus, although electrically powered Valve Actuators have been proposed, wherein the valve is powered by an electric motor, this generally provides slow closure only being limited by practicable motor size for fast valve emergency operation and has no direct in-built fail-safe device for such Actuators. Common practice at this time is to add the fitment of an air motor and air storage vessel using this device to drive the Actuator to the valve fail-safe position upon loss of electrical supply. It can be considered this system is cumbersome, expensive and prone to moisture content, icing, etc., in operation. The problem of providing rapid response to emergency circumstances remains unsolved.

A satisfactory system requires that quick valve operation must be maintained even in an emergency situation when it is most likely that electrical power supplies may fail. Considering the possibilities for driving the Actuator from an electric motor, it becomes apparent that a drive train may be necessary to convert the generally low torque output/high rotational speed of the electric motor into the high force torque application required for Actuator/Valve control . Typically a quarter turn or less takes the valve from fully open to fully closed, however, this device is not limited to an approximation of 90 degrees of rotation and can also be used for non-rotary, i.e. linear application. Power transformation can readily be accomplished by a variety of mechanical reduction gear mechanisms, however, it will be appreciated that in a shut-down or an emergency situation with loss of electrical power having occurred, the valve may be held fully open or perhaps partially closed by the impotent static condition of the mechanical components in the drive train. In such case the stored energy in the counter acting spring loaded valve return mechanism would be inaccessible.

Possible solutions currently available would include providing back-up power to the motor or devising an alternative override facility. Override systems involving manual controls must be considered with great caution and are not normally used for emergency shut-down systems. The release of the return force must be carefully controlled otherwise the Actuator could return to its rest position with great force causing injury to personnel or damage to the equipment. Furthermore, if the valve is closed too quickly by an uncontrolled release of the stored energy forces, a shock- wave may be generated in the main pipeline which again can lead to installation damage.

The likelihood of this happening is normally obviated by the pipeline designer specifying a specific valve stroking time during the fail-safe operation. An object of this invention is to obviate or mitigate the aforesaid disadvantages of known systems and potential difficulties in the operation of electrically-powered Actuators.

According to one aspect of the present invention there is provided a safety device for a valve actuator which converts linear motion of a linear rod into rotary or angular motion of a body within the actuator, comprising an electric motor; a magnetic particle clutch selectively engageable with the motor and means for transmitting power from the motor to the linear rod, and spring force means mounted within the actuator compressible by the action of the linear rod during rotation of the body. It will be appreciated that the arrangement of this invention is particularly advantageous in that in the event of an emergency when the spring force has to be released, the clutch is gradually de-energised thereby allowing the spring force to return to a preset rest position without injury to the user or damage to the actuator.

Preferably limit switches are provided within the actuator to monitor the position of the rotary body and pass signals to the electric motor to control operation of the motor.

Alternatively, the limit switches may be replaced by torque or current measurement switches.

Conveniently, the rotary body may be a scotch yoke which is rotated by a spigot mounted on the linear rod, the spigot being received within the arms of the scotch yoke.

Alternatively, the rotary body may be in the form of a toothed gear wheel rotated by corresponding teeth provided on the linear rod.

Advantageously, the transmission means comprises a plurality of meshing spur gears, the shaft of one of which engages a rack provided on the linear rod to advance the rod within the actuator.

Alternatively, the transmission means comprises a worm and worm wheel, the shaft of the worm wheel engaging the rack provided on the rod.

In a further alternative, the shaft of the worm wheel is provided with a re-circulating screw to advance the linear rid within the actuator.

Embodiments of the present invention will now be described with reference to and as shown in the accompanying drawings in which:-

Figure 1 is the perspective schematic review of one embodiment of the present invention; Figure 2 is an enlarged view of a portion of the safety device according to a further embodiment of the invention;

Figure 3 is an enlarged view of the embodiment of figure 2 with an alternative linear device, and

Figure 4 is a perspective schematic view of the safety device of Figure 1 in which the "Scotch Yoke" is replaced by a gear wheel which is directly driven by the Actuator rod.

Referring now to the Figures, Figure 1 is a perspective schematic view of one embodiment of the present invention. The safety device is installed in a Valve Actuator which in this embodiment converts linear motion into angular motion. The conversion is provided by a "Scotch Yoke" mechanism which has been used extensively in the Valve Actuator industry for many years.

The pneumatic or hydraulic power sources associated with Valve Actuators is replaced by an electric motor 1 which powers the Actuator through a magnetic particle clutch 2. A suitable braking mechanism may be provided on or by the electric motor 1.

The clutch 2 comprises separate input and output shafts 3,4 supported in bearings in a housing. The gap between the shafts is filled with a free-flowing stainless steel or similar powder or other small particle content and a coil is wound around the gap. Means for energising the coil (not shown) , are provided in a known manner. An electrical/electronic control unit (not shown) , controls the operation of the clutch 2 by controlling the voltage and current signatures supplied to the clutch.

The clutch is selectively engageable with a plurality of spur gears 5, one of which forms a rack and pinion arrangement with a linear rod 6 in the Actuator. The spur gears may be provided with braking means if required. A stop member 7 is provided at a pre-determined position along the rod 6, the stop member being received within the arms 8 of the Scotch Yoke 9 of the Actuator. The other end of the rod 6, remote from the motor 1 is associated with a piston 10 which compresses a spring force 11 within the Actuator housing.

Limit switches or torque/current measurement switches (not shown) may be provided within the Actuator which monitor the angular position of the Scotch Yoke 9 and pass controlling signals to the electric motor 1.

In operation of the device, the electric motor 1 is activated and the motor shaft is rotated. A current is applied to the clutch 2 and torque is transmitted proportional to the voltage applied. Upon energising the coil a magnetic field is created which causes the "Powder" particles to form chains along the magnetic field lines thereby linking the disc to the housing and providing rotation to the output shaft of the clutch. If the torque is higher than that set by the voltage, the clutch will slip until the torque falls to the set value or below.

The spur gears 5 are rotated by the output shaft 4 of the clutch and the shaft on the final gear engages the rack provided on the linear rod 6 in the Actuator, thus advancing the rod into the housing of the Actuator.

As the rod is advanced, the stop member 7 abuts against the Scotch Yoke 9 which is rotated within the house of the Actuator. The piston 10 provided at the remote end of th rod 6 compresses the spring 11 of the safety device during rotation of the Scotch Yoke 9. At the end of the compression stroke, a signal from the limit switch or current measurement switch shuts off the current supply to the electric motor 1 and activates the braking control function for the motor. As the gear train 5 of the Actuator is fully reversible the full spring load 11 is held by the motor brake. As an alternative an anti-back drive device may be installed to accept spring force loadings outwith the motor 1.

The magnetic particle clutch 2 provides a range of functions in the Actuator including a normal clutching operation of selectively engaging and disengaging the output shaft of the electric motor from the spur gears 5, a soft- start option where the clutch is gradually engaged with the spur gears thereby reducing the mechanical stress placed on the components in the driving assembly. In this option, the clutch initially slips until the motor achieves full speed, thereby gradually building up the motor load. Furthermore, the clutch provides an over-torque protection for the driving ^■ assembly as in the event of high or stall load due to unusual demand in this case the clutch slips and motor load is limited to the maximum torque value.

As described above, during normal operation of the Actuator, the spring force 11 is compressed and released during each stroke of the Actuator. In the event that an emergency situation occurs during which power to the motor 1 is lost during a compression stroke of the Actuator the forces stored in the spring must be released in a controlled manner to return the linear rod 6 to a predetermined starting position.

With the spring 11 held in compression by the motor brake or the anti back-drive device the magnetic field of the clutch 2 is gradually de-energised in a controlled manner, thus removing the particle chains between the input and output shafts of the clutch. As the clutch slips the spring force is gradually returned to the rest position, thus releasing the forces stored in the Actuator without injury to the user or damage to the Actuator or installation. As an alternative a mechanical retardation device may be fitted outwith the motor to control Actuator speed.

A further embodiment of the safety device is shown in Figure 2 in which the spur gears 5 are replaced with a worm and wheel reduction arrangement 12. This may also be used in conjunction with a spur gear reduction. The electric motor turns the worm and wheel arrangement 12 and the magnetic particle clutch 2 is mounted on the wheel 13. This embodiment avoids the need for a separate braking mechanism on the electric motor as the worm and wheel arrangement is not capable of back-drive and therefore this embodiment allows better flexibility in the choice of electric motors.

In the embodiment shown in Figure 3 the rack and pinion arrangement is replaced by a re-circulating ball screw 14 which is attached to the wheel 13 of the worm and wheel arrangement 12. A separate threaded nut which may be of the recirculating type (not shown) engages the threads of the ball screw 14 and is used to provide drive to the Scotch Yoke 9 during operation of the Actuator. The ball screw is capable of being back driven.

In the embodiment shown in Figure 4, the Scotch Yoke 9 is replaced by. a toothed gear wheel 15 the teeth of which have been removed in this Figure. The pitch of the teeth on the wheel correspond to the pitch of the teeth on the linear rod 6 of the Actuator and the gear wheel 15 is directly rotated by actuation of the linear rod 6.

Claims

1 A safety device for a valve actuator which converts linear motion of a linear rod into rotary or angular motion of a body within the actuator, comprising an electric motor; a magnetic particle clutch selectively engageable with the motor and means for transmitting power from the motor to the linear rod, and spring force means mounted within the actuator compressible by the action of the linear rod during rotation of the body.

2 A safety device according to claim 1, wherein limit switches are provided within the actuator to monitor the position of the rotary body and pass signals to the electric motor to control operation of the motor.

3 A safety device according to claim 1 or 2, wherein the rotary body is a scotch yoke which is rotated by a spigot mounted on the linear rod, the spigot being received within the arms of the scotch yoke.

4 A safety device according to claim 1 or 2, wherein the rotary comprises a gear wheel rotated by means mounted on the linear rod.

5 A safety device according to any one of the preceding claims, wherein the transmission means comprises a plurality of meshing spur gears, the shaft of one of which engages a rack provided on the linear rod to advance the rod within the actuator.

6 A safety device according to any one of claims 1 to 4, wherein the transmission means comprises a warm wheel, means being provided on the shaft of the warm wheel to engage a rack provided on the linear rod and thereby advance the rod within the actuator.