Monitor and control circuit for electric surface controlled subsurface valve system

What is claimed is:

1. A control system for applying power to a solenoid operated valve in a well completion within a borehole comprising:

programmable power supply means mounted within a surface control unit for selectively applying electric power at a first selected higher current value and a second selected lower current value to a cable connected to supply operating current to the solenoid coil of said valve;

means located in said valve and responsive to said first higher value of current for operating said solenoid to move the valve to an open state and responsive to the second lower value of current for maintaining the open state of operation of the solenoid to hold the valve open and responsive to interruption of all current to the solenoid for closing said valve;

means for continuously monitoring the state of actuation of said valve and providing an indication thereof at the well surface; and

means within said programmable power supply means and responsive to said monitoring and indicating means for interrupting said higher value of current and applying said lower value of current in response to an indication that said valve has opened and for interrupting all current after a predetermined period of time following application of electric power at said higher current value and failure to receive an indication that said valve has opened.

2. A control system as set forth in claim 1 wherein said system includes means mounted within said surface control unit for measuring the current flow down the cable to the solenoid coil and controlling the voltage produced by said programmable power supply to produce said selected values of current.

3. A control system as set forth in claim 1 wherein said programmable power supply includes a constant current source.

4. A control system as set forth in claim 1 wherein said means for monitoring the state of actuation of said safety valve includes means for measuring the inductance of the solenoid coil to detect whether the armature thereof is in a position to open the valve or close the valve.

5. A control system as set forth in claim 1 which also includes means for measuring the value of current supplied to the solenoid coil of said valve.

6. A control system for applying power to a solenoid operated valve in a well completion within a borehole comprising:

means mounted within a surface control unit for selectively producing a programmable value of voltage said means being capable of supplying a constant value of current;

an electrical cable connecting said voltage producing means to a solenoid valve located downhole;

means connected in the circuit with said electrical cable for measuring the value of electric current flowing from said voltage producing means to the solenoid valve;

means located in the valve and responsive to a selected value of electric current for changing the state of said solenoid and opening said valve and responsive to interruption of electric current for closing said valve; and

means mounted within said surface control unit and responsive to said electric current value measuring means for varying the value of voltage produced by said programmable voltage producing means to produce said selected value of electric current to said solenoid for a selected period of time and to interrupt the electric current to said solenoid in response to failure of said solenoid to change states and open said valve within said selected period of time.

7. A control system as set forth in claim 6 wherein said electric power consists of DC current.

8. A control system as set forth in claim 6 wherein said solenoid operated valve is a safety valve.

9. A control system as set forth in claim 6 wherein said solenoid operated valve is a gas lift valve.

10. A control system as set forth in claim 6 wherein said voltage producing means includes a constant current source.

11. A control system for applying power to a solenoid operated valve in a well completion within a borehole comprising:

means mounted within a surface control unit for selectively producing a programmable value of voltage said means being capable of supplying a constant value of current;

an electrical cable connecting said voltage producing means to a solenoid valve located downhole;

means connected in the circuit with said electrical cable for measuring the value of electric current flowing from said voltage producing means to the solenoid valve;

means located in the valve and responsive to a selected value of electric current for changing the state of said solenoid and opening said valve and responsive to interruption of electric current for closing said valve;

means mounted within said surface control unit and responsive to said electric current value measuring means for varying the value of voltage produced by said programmable voltage producing means to produce a selected value of electric current to said solenoid; and

means for detecting whether the armature of the solenoid is in the valve open or valve closed condition.

12. A control system as set forth in claim 11 in which said detecting means includes means for measuring the inductance of the solenoid coil.

13. A control system as set forth in claim 11 which also includes:

means responsive to detection that the armature of the solenoid is in the valve open condition for reducing the value of electric current to said solenoid to a value less than that of said selected value and holding the state of said solenoid in such condition.

14. A circuit for controlling the operation of an electric solenoid operated valve within a borehole between an open and a closed condition, comprising:

a surface control unit including means for selectively producing a programmable value of voltage and means for selecting a valve open or valve closed condition;

means located in the valve and responsive to a first selected value of electric current for actuating the solenoid to operate the valve into an open condition, a second selected value of electric current, less than said first value, for holding the solenoid in an actuated condition, and a third selected value, less than said second value, for deactuating the solenoid and operating the valve into a closed condition;

means for detecting whether the armature of the solenoid is in the valve open or valve closed condition;

an electrical cable connecting the programmable voltage producing means within the surface control unit with the electric current responsive means within the valve;

means for measuring the value of the electric current flowing through said electrical cable;

means responsive to the selection of a valve open condition for increasing the value of voltage produced by said surface control unit until the measured value of current reaches said first selectet value;

means responsive to a detection that the solenoid armature is in a valve open state for reducing the value of voltage produced by said surface control unit until the measured value of current reaches said second selected value;

means responsive to a failure to detect that the solenoid armature is in a valve open state within a preselected period of time following said selection of a valve open condition for reducing the value of voltage produced by said surface control unit until the measured value of current reaches said third selected value; and

means responsive to the selection of a valve closed condition for decreasing the value of voltage produced by said surface control unit until the measured value of current reaches said third selected value and the valve is operated into a closed condition.

15. A circuit for controlling the operation of an electric solenoid operated valve within a borehole between an open and a closed condition as set forth in claim 14 wherein said means for detecting whether the armature of the solenoid is in the valve open or valve closed condition includes means for measuring the inductance of the solenoid coil.

16. A circuit for controlling the operation of an electric solenoid operated valve within a borehole between an open and a closed condition as set forth in claim 14 wherein said means for selectively producing a programmable value of voltage includes means for producing a constant value of current.

17. A circuit for controlling the operation of an electric solenoid operated valve within a borehole between an open and a closed condition as set forth in claim 14 which also includes:

means for reestablishing a current flow of said first selected value for a preselected period of time to attempt to actuate the solenoid and operate the valve into an open condition.

18. A circuit for controlling the operation of an electric solenoid operated valve within a borehole between an open and a closed condition as set forth in claim 14 which also includes:

monitor means within said surface control unit for displaying to an operator an indication that the valve is in an open condition and that the valve is in a closed condition; and

means responsive to a detection that the solenoid armature is in a valve closed state for actuating said valve closed condition indication display means and to a detection that the solenoid armature is in a valve open state for actuating said valve open condition indication display means.

19. A circuit for controlling the operation of an electric solenoid operated valve within a borehole between an open and a closed condition as set forth in claim 18 wherein said monitor means includes a valve open indication lamp and valve closed indication lamp.

20. A circuit for controlling the operation of an electric solenoid operated valve within a borehole between an open and a closed condition as set forth in claim 18 wherein said monitor means includes a computer interface having a display screen.

21. A circuit for controlling the operation of an electric solenoid operated valve within a borehole between an open and a closed condition as set forth in claim 18 which also includes monitor means within said surface control unit for displaying to an operator an indication of the measured value of the electrical current flowing through said electrical cable.

22. A circuit for controlling the operation of an electric solenoid operated valve within a borehole between an open and a closed condition, comprising:

a surface control unit including means for selectively producing a programmable value of voltage and means for selecting a valve open or valve closed condition;

means located in the valve and responsive to a first selected value of electric current for actuating the solenoid to operate the valve into an open condition, a second selected value of electric current, less than said first value, for holding the solenoid in an actuated condition, and a third selected value, less than said second value, for deactuating the solenoid and operating the valve into a closed condition;

means for detecting whether the armature of the solenoid is in the valve open or valve closed condition;

an electrical cable connecting the programmable voltage producing means within the surface control unit with the electric current responsive means within the valve;

means for measuring the value of the electric current flowing through said electrical cable;

means responsive to the selection of a valve open condition for increasing the value of voltage produced by said surface control unit until the measured value of current reaches said first selected value;

means responsive to a detection that the solenoid armature is in a valve open state for reducing the value of voltage produced by said surface control unit until the measured value of current reaches said second selected value;

means responsive to the selection of a valve closed condition for decreasing the value of voltage produced by said surface control unit until the measured value of current reaches said third selected value and the valve is operated into a closed condition;

means associated with said programmable voltage producing means and responsive to said detection means to attempt to move the armature of said solenoid into the valve open condition if said valve fails to open after a predetermined period of time;

means for establishing preselected values of temperature, pressure, and fluid flow rate;

means for monitoring values of downhole temperature, pressure and fluid flow rate through the valve; and

means responsive to selected relationships between said monitored values and said preselected values for selecting a valve open or a valve closed condition.

23. A circuit for controlling the operation of an electric solenoid operated valve within a borehole between an open and a closed condition as set forth in claim 22 which also includes:

computer means for changing said preselected values.

24. A circuit for controlling the operation of an electric solenoid operated valve within a borehole between an open and a closed condition, comprising:

a surface control unit including means for selectively producing a programmable value of voltage and a selected constant value of current;

means located in the valve and responsive to a first selected value of electric current for actuating the solenoid to operate the valve into an open condition, a second selected value of electric current, less than said first value, for holding the solenoid in an actuated condition, and a third selected value, less than said second value, for deactuating the solenoid and operating the valve into a closed condition;

an electrical cable connecting the programmable voltage producing means within the surface control unit with the electric current responsive means within the valve;

means for detecting whether the armature of the solenoid is in the valve closed state or the valve open state; and

means responsive to a detection that the solenoid armature is in a valve closed state for reducing the value of voltage produced by said surface control unit until value of current through said current responsive means reaches said second selected value.

25. A circuit for controlling the operation of an electric solenoid operated valve within a borehole between an open and a closed condition as set forth in claim 24 wherein said detecting means includes means for measuring the inductance of the coil of the solenoid.

26. A method of controlling the operation of a solenoid actuated valve within a well completion located in a borehole which includes an electrically conductive path from the surface of the borehole to the coil of the solenoid, said method comprising:

increasing the value of the electric current flowing through the path to said solenoid coil;

ceasing said increasing and maintaining the current value constant in response to said value reaching a first preselected value;

initializing a first timer in response to said current reaching said first preselected value;

determining whether or not said valve has opened in response to said first preselected value of current flow through the solenoid coil thereof; and

decreasing the value of the electric current flowing through the path to said solenoid to a second preselected value, less than said first preselected value, in response to either the expiration of a first preselected period of time following initialization of said first timer or the opening of said valve.

27. A method of controlling the operation of a solenoid actuated valve within a well completion located in a borehole as set forth in claims 26 Which includes the additional steps of:

initializing a second timer in response failure of said valve to open prior to expiration of said first preselected period of time; and

repeating said increasing and ceasing steps in response to the expiration of a second preselected period of time following initialization of said second timer.

28. A method of controlling the operation of a solenoid actuated valve within a well completion located in a borehole as set forth in claims 27 which includes the additional steps of:

maintaining the value of the electric current flowing through the path to said solenoid at said second preselected value, less than said first preselected value, in response to opening of said valve; and

interrupting all current flow through the path to said solenoid to close said valve.

29. A method of controlling the operation of a solenoid actuated valve within a well completion located in a borehole as set forth in claim 27 which includes the additional step of:

providing an indication to an operator of the open and closed states of said valve.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to solenoid operated valves for petroleum production wells and, more particularly, to a control and monitor arrangement for an electrical solenoid operated safety valve system.

2. History of the Prior Art

Oil and gas wells, and in particular those located off-shore, are frequently subject to wellhead damage which may be produced by violent storms, collisions with ships and numerous other disastrous occurrences. Damage to the wellhead may result in the leakage of hydrocarbons into the atmosphere producing the possibility of both the spillage of the petroleum products into the environment as well as an explosion and fire resulting therefrom. In addition to off-shore production wells, another environment in which damage to a wellhead may have disastrous effects is that of producing wells located in urban areas. Moreover, in such urban production wells, it is generally a specific legal requirement that there be some downhole means of terminating the flow of petroleum products from the well in the event of damage to the wellhead. In such instances, the safety valve system must be responsive to a dramatic increase in flow rate from the well so as to close down and terminate production flow from the well. For these reasons, sub-surface safety valves located downhole within a borehole have long been included as an integral part of the operating equipment of a petroleum production well.

Various types of petroleum production flow safety valve systems have been provided in the prior art. Each system includes a valve means for controlling the flow of petroleum products up the tubing from a point down in the borehole from the wellhead. Safety valve systems also include sensing means which are responsive to wellhead damage, a dramatic increase in production flow, or some other emergency condition requiring that the flow from the well be terminated by the valve.

One type of operating mechanism used to actuate a safety valve within a well includes an electrical solenoid employed to hold the safety valve in an open condition and a spring means to return it to a normally closed condition in response to interruption in the flow of current to the solenoid. Numerous such systems have been proposed, for example, U.S. Pat. No. 4,002,202 to Huebsch et al., U.S. Pat. No. 4,161,215 to Bourne, Jr. et al., and U.S. Pat. No. 4,566,534 to Going III. Each of these systems provide a solenoid actuated operating mechanism for the safety valve which is responsive to a DC electric current supplied from surface equipment. Such solenoids generally require a fairly high level surge of initial operating current to cause the solenoid to operate and change states and then a smaller level of current to hold the solenoid in its operated condition. These large actuating current surges require heavy electrical conductors in order to carry such current downhole for any substantial distance and still maintain a voltage level sufficient to operate the solenoid. Moreover, such solenoids are usually supplied with current from a conventional power supply at the surface which produces a fixed voltage output signal. This limits the depth to which the solenoid can be used and still operate with a particular power supply configuration. Use of the same solenoid actuate safety valve in deeper wells requires a change in the power supply circuit in order to supply sufficient current to operate it.

Prior art solenoid actuated safety valve systems have also dealt with the design constraints of high downhole pressures and corrosive borehole fluid in a relatively conventional manner. For example, large values of downhole pressure have required that the pressure resisting walls of the parts of the valve isolating the coil from well pressure be relatively thick in order to swerve as a load bearing member of the valve assembly and protect the valve components inside. Thick walls both increase the diameter of the overall valve structure for a given pressure rating as well as limit the thickness of the magnetic armature of the valve and hence, restricts its magnetic responsiveness to a given value of solenoid actuation current. Similarly, prior art solenoid actuated safety valves have also relied upon the precise machining of valve parts and the presence of high pressure resilient seals, such as O-rings, in order to protect the internal electrical components of the value, such as the solenoid coil, from borehole fluids. Such fluid sealing components increase the cost of the safety valve and are subject to failure under use. The structure and construction techniques of the valve systems of the present invention overcome many of these disadvantages of prior solenoid actuated safety valve systems.

The inherent disadvantages of providing several different power supply circuits for different depths of operation of a solenoid actuated safety valve is obviated by the system of the present invention which provides means for coupling a constant value of current from the surface down the electrically conductive path interconnecting that current to the windings of a solenoid actuated safety valve. The system provides an optimum value of current for actuation of the solenoid and control of the safety valve regardless of the voltage required to deliver that current to the solenoid at the particular depth of the safety valve. In addition, the solenoid actuated safety valve of the present invention also allows construction of a less expensive and more reliable valve which is of a smaller overall diameter for a particular pressure rating of the valve. In addition, the safety valve of the present invention is more magnetically responsive for a given value of operating current delivered to the solenoid coil.

The system of the present invention overcomes many of the disadvantages of the prior art electrically operated solenoid actuated safety valve systems.

SUMMARY OF THE INVENTION

In one aspect, the present invention includes a control system for applying power to a solenoid operated valve in a well completion within a borehole in which a programmable power supply is mounted within a surface control unit for selectively applying electric power at a first selected higher current value and a second selected lower current value to a conductive path connected to supply operating current to the solenoid coil of the valve. The valve includes means responsive to the first higher value of current for changing the state of the solenoid and responsive to the second lower value of current for maintaining the state of the safety valve and responsive to interruption of all current for closing the safety valve. The surface monitoring and control unit includes means for continuously monitoring the state of actuation of the safety valve and providing an indication thereof at the well surface.

In a further aspect, the present invention encompasses a system for controlling the operation of a solenoid actuated valve within a well completion located in a borehole which includes an electrically conductive path from the surface of the borehole to the coil of the solenoid. The value of the electric current flowing through the path to the solenoid soil is first increased and then maintained at a constant current value in response to the current reaching a first preselected value. A first timer is also initialized in response to the current reaching the first preselected value. It is determined whether or not the valve has opened in response to the first preselected value of current flowing through the solenoid coil and then the value of the electric current flowing through the path to the solenoid is decreased to a second preselected value, less than the first preselected value, in response to either the expiration of a first preselected period of time following initialization of the first timer or the opening of the valve. A second timer is initialized in response to failure of the valve to open prior to expiration of the first preselected period of time. Thereafter, the current may again be increased to the first selected value following the expiration of a second preselected period of time following initialization of the second timer.

In a still further aspect, the present invention includes a control system for applying power to a solenoid operated valve in a well completion within a borehole. A programmable power supply is mounted within a surface control unit for selectively applying electric power at a first selected higher current value and a second selected lower current value to a cable connected to supply operating current to the solenoid coil of the valve. Control circuits are responsive to the first higher value of current for operating the solenoid to move the valve to an open state and responsive to the second lower value of current for maintaining an open state of operation of the solenoid to hold the valve open and responsive to interruption of all current to the solenoid for closing the valve. The state of actuation of the valve is continuously monitored and an indication thereof is provided at the well surface. The higher value of current is discontinued and the lower value of current is applied in response to an indication that the valve has opened and all current is interrupted after a predetermined period of time following application of electric power at the higher current value and failure to receive an indication that the valve has opened.

BRIEF DESCRIPTION OF THE DRAWINGS

For an understanding of the present invention and for further objects and advantages thereof, reference can now be had to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic drawing of a well completion including an illustrative cross-sectional view of one embodiment of an electrically operated solenoid actuated safety valve system which is related to that which is constructed in accordance with the teachings of the present invention;

FIG. 2 is a schematic diagram of the electrical circuitry of one embodiment of the electrically operated solenoid actuated safety valve system of the present invention;

FIG. 2A is a flow chart describing the operation of one embodiment of a control circuit constructed in accordance with the present invention;

FIGS. 3A-3D are longitudinal cross-section drawings of the embodiment of the solenoid operated safety valve assembly shown in FIG. 1;

FIG. 4 is an electrical schematic diagram of another embodiment of the electrically operated solenoid actuated safety valve system of the present invention;

FIG. 4A is a flow chart describing the operation of another embodiment of a control circuit constructed in accordance with the present invention;

FIG. 5 is a schematic drawing of a well completion including an illustrative cross-sectional view of a electrically operated solenoid actuated safety valve system constructed in accordance with the preferred embodiment of the system of the present invention; and

FIGS. 6A-6D are longitudinal cross-section drawings of the solenoid actuated safety valve assembly of the preferred embodiment of the system of the present invention shewn in FIG. 5.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, there is shown a schematic cross-sectional illustration of one embodiment of a well completion incorporating a related embodiment of the electrically operated solenoid actuated safety valve system of the present invention. This embodiment is set forth and claimed in U.S. paten application Ser. No. 169,814 filed Mar. 18, 1988, a predecessor of the present application. In that application, the principal emphasis was on the manner in which current was delivered to the valve for actuation of the solenoid. However, it will be discussed here because of the relationship between the valve structure shown in that embodiment and the preferred embodiment of the present invention discussed below.

Referring to FIG. 1, a casing 11 is positioned along the borehole 12 formed in the earth and extending from a wellhead 13 located at the surface down into the petroleum producing geological formation. The wellhead 13 includes a typical Christmas tree production flow control configuration 14 having an output line 15 leading to storage facilities (not shown) for receiving production flow from the well. A wellhead support flange 16 is formed of conventional conductive metal material and is mechanically and electrically connected to the case 11 extending down the borehole 12. A tubular production conduit 17 extends from the output line 15 co-axially through the wellhead support flange 16 and includes an outwardly flared radially extending flange region 18 at its lower end. The flange region 18 of the production conduit 17 extends into and is physically coupled with the open end of a tubing head 19 but is electrically insulated therefrom by an electrically insulative shield 20 which surrounds the radially flared flange 18 to mechanically connect it to the tubing head 19 but electrically insulate it. The cylindrical outer periphery of the tubing head 19 is also covered with electrically insulative material 22 so that in the event there is mechanical contact between the outer walls of the insulator 22 and the inner walls of the casing 11 no electrical conduction will take place.

A wellhead monitoring and control circuit 25 is connected to a source of AC electric current by means of a cable 26 and includes means for rectifying current from that source and producing a positive DC voltage on a first power cable 27 and a negative DC voltage on a second power cable 28. The negative potential on the second cable 28 is electrically connected to the wellhead support flange 16 which is, of course, electrically connected to the casing 11 and the earth potential of the borehole. The positive potential on the first cable 27 passes through an insulator 31 extending through the sidewalls of the wellhead support flange 16 and is electrically connected to the upper end of the tubing head 19 which is electrically insulated from both the wellhead support flange 16, by means of the insulator 22, and from the tubular production conduit 17 by means of the insulator 20.

The tubing head 19 is mechanically and electrically connected in conventional fashion to additional elongate sections of tubing 32 which extend coaxially down the casing 11. Insulative tubing centralizers 33 are longitudinally spaced from one another along the tubing 32 to support the tubing near the central axis of the casing 11 and to prevent any electrically conductive contact therewith.

At the lower end of the tubing 32 there is positioned a solenoid safety valve assembly 35 which is coupled to the lower end of the tubing by means of an assembly support flange 36 which threadedly engages the lower end of the tubing 32. The safety valve assembly includes an elongate housing 41 formed of a conventional electrically conductive magnetic material having a generally cylindrical outer configuration and recesses formed therein for receiving the components of the solenoid operated safety valve. The assembly support flange 36 includes a threaded tubular upper end 40 and a lower end having a radially extending flange portion 42 which is mechanically attached to but electrically insulated from the inner walls of the housing 41 by means of an electrically insulative upper adaptor 43. The adaptor 43 electrically isolates the positive electrical potential on the tubing 32 from the negative potential of the housing 41. The housing 41 includes an axially extending central bore 44 for receiving an operator tube 45 adapted for axial movement therein. The operator tube 45 may preferably be formed of several cylindrical sections of different thickness and mass as well as of materials having different magnetic permeability. At the upper end of the operator tube 45 there is a relatively thin walled upper section 46 formed of relatively less magnetic material, such as 9CR-1MOLY steel. An intermediate armature portion 47 is constructed of a highly magnetic material such as 1018 low carbon alloy steel and forms a central portion of the operator tube 45 while an elongate thin walled lower section 48 is formed of the less magnetic material such a 9CR-1MOLY steel. The bottom section 50 located at the lower end of the operator tube 45 is also of relatively less magnetic material and includes a radially extending circumferential flange member 49 which is received within a radially extending cavity 51 formed in the inner walls of the housing 41. A helical spring 52 surrounds the lower end 48 of the operator tube 45 and normally biases the tube in the upward direction by a force exerted against the circumferential flange 49.

A lower cavity 53 in the housing 41 receives a valve flapper member 54 which is pivotally mounted to the sidewall of the housing 41 by a hinge 55 which is spring biased toward the closed position, as shown. A sufficient force against the upper side of the valve flapper 54 will cause it to pivot about the hinge 55 and move into the side walls of the cavity 53 thereby opening the interior axial passageway 44 through the housing 41 to allow the flow of borehole fluids lower down in the borehole up the tubing to the wellhead. The lower end of housing 41 is mechanically and electrically connected to well packer 61 by an additional portion of production conduit 17 therebetween. Packer 61 include radially extending seal elements 62 which form a fluid barrier with the inside wall of casing 11. Packer 61 directs the flow of well fluids between wellhead 13 and a downhole formation (not shown) via production conduit 17 and safety valve 35. Slips 63 carried by packer 61 form a series of toothed engagements with the inside wall of casing 11 to anchor packer 61 at a selected downhole location. Slips 63 mechanically and electrically engage packer 61 with casing 11 to form a positive electrical contact between casing 11 and housing 41 of safety valve assembly 35. If desired, one or more conventional tubing centralizers (not shown) with bow springs or other contacting means could be installed in the portion of production conduit 17 between safety vale 35 and well packer 61. The bow springs on such centralizers can provide additional electrical contact with casing 11.

The assembly support flange 36 is electrically connected to a conductive cable 71 which extends through an opening in the insulative upper adaptor 43 down through a passageway formed in the side wall of the casing 41 to electrically connect with one end of an electrical solenoid 72 in a cavity formed in the inner side walls of the housing 41. The solenoid coil 72 comprises a plurality of helically wound turns of a conductor. The other end of the winding of the solenoid coil 72 is electrically connected to the body of the housing 41 by means of a set screw 73 to thereby indirectly form an electrical connection with the casing 11.

The coil 72 is positioned within the body of the housing 41 so that the highly magnetic armature portion 47 of the operator tube 45 is located near the upper ends of the coil 72 when there is no current flow through the coil and the tube 45 is in its upwardly spring biased position. A cylindrical magnetic stop 60 is positioned within the central bore 44 near the lower end of the solenoid coil 72 so that the lower portion 48 of the operator tube 45 is axially movable there through. A mechanical stop 56 is formed on the lower inside edges of the cavity 53 to limit the extent of the downward movement by the operator tube 45. When the lower edge of bottom section 50 of the operator tube 45 abuts the mechanical stop 56, the lower edge of the armature portion 47 is spaced by a small but distinct air gap from the upper edges of the magnetic stop 60. The highly magnetic stop 60 creates a low reluctance path for magnetic flux generated by the solenoid coil 72 so that the armature 47 of the operator tube can be held adjacent thereto by a relatively low value of current flow through the coil 72. The air gap, for example on the order of 0.050 inch, is provided to insure that the operator tube 45 will return to its upper position in response to the force generated by the bias spring 52 when current is removed from the coil 72 and not be retained in its lower position by residual magnetism due to physical contact between the operator tube 45 and the magnetic stop 60.

When an actuation current of a first value flows through the winding of the solenoid coil 72 the magnetic flux generated thereby causes the armature 47 to move downwardly toward the center of the coil 72. As the lower edges of the operator tube