Dual filter isolation block

A dual filter isolation block for isolating a fluid stream between a fluid source and a user device and including a main body, a first fluid path provided in the main body between the source and the user device and a first filter provided in the first fluid path. A second fluid path is also provided in the main body between the source and the user device and a second filter is included in the second fluid path. A pair of spools are slidably disposed in the main body and intersect the first fluid path and the second fluid path, respectively, for selectively isolating the first fluid path and the first filter from the second fluid path and the second filter. In a specific application the dual filter isolation block selectively isolates a pair of filters for filteringg an operating fluid such as hydraulic oil or fluid between an actuator and a servo valve to protect the servo valve from contaminants in the operating fluid. The filters are each designed for separate removal and replacement while the hydraulic fluid flows through the other filter, to avoid interrupting operation of the user device. A method for maintaining a flow of operating fluid between a fluid source and a user device while continuously filtering the operating fluid is also included.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to the protection of control devices and mechanisms
 such as servo valves from contaminated operating fluid and more
 particularly, to a dual filter isolation block for positioning between a
 source of operating fluid such as hydraulic fluid or oil and the user
 mechanism or device receiving the operating fluid, wherein the dual filter
 isolation block includes a main body having a pair of fluid paths, a pair
 of filters provided in the fluid paths, respectively, and a pair of spools
 disposed for sliding operation in the main body of the isolation block and
 intersecting the fluid paths, respectively, such that the operating fluid
 is selectively diverted through one of the fluid paths and the associated
 filter, while the remaining fluid path and filter remain free of operating
 fluid. The latter filter can then be removed and replaced without
 interrupting operation of the user device because of the constant flow of
 operating fluid through the first fluid path and filter to the user
 device.
 In a specific embodiment the dual filter isolation block of this invention
 is designed to isolate hydraulic oil or fluid flowing between an actuator
 and a servo valve, which actuator operates steam valves on a turbine and
 the servo valve serves to control operation of the actuator. A pair of
 distinct fluid paths are provided in the main body of the isolation block
 to selectively receive operating hydraulic oil or fluid flowing under
 pressure from a storage tank through the actuator and the isolation block,
 into the servo valve. Each of these fluid paths includes a removable
 filter and a pair of spool valves operate in sliding concert in the
 isolation block and intersect the respective fluid paths, such that the
 hydraulic operating oil or fluid can be directed from a tank through the
 actuator and through a selected one of the fluid paths and filters into
 the servo valve and back from the servo valve through the isolation block
 and the actuator to the tank, while the other filter is removed and
 replaced. This operation eliminates the necessity for discontinuing or
 disrupting operation of the servo valve while typically removing a
 conventional filter which serves the conventional single fluid path
 between the actuator and the servo valve.
 2. Description of the Prior Art
 Various mechanisms are known in the art for diverting fluid flow from one
 point to another in various types of devices. U.S. Pat. No. 3,521,673,
 dated Jul. 28, 1970, to Gruner, et al, details a constant flow fluid
 diverting valve which has six fluid ports and is used in four pipe
 temperature conditioning systems, with a cylindrical plunger
 longitudinally movable in a valve body to selectively connect to the
 appropriate ports. U.S. Pat. No. 4,271,020, dated Jun. 2, 1981, to Van
 Meter, details a valve for a filter device, wherein the valve assembly
 includes a rotatable valve spindle removable with respect to a valve
 housing and having first and second channels communicating with the
 filter. A bypass channel is also provided to bypass the fluid filter.
 Fluid may flow through the respective channels, including the bypass
 channel, responsive to rotation of the spindle into a selected position.
 U.S. Pat. No. 4,469,131, dated Sep. 4, 1984, to Paul L. Traylor, details a
 spool valve including a valve stem mounting a pair of valve heads
 removable in a body and cooperating with various valve seats to direct
 fluid along respective paths through the fluid body. U.S. Pat. No.
 4,501,295, dated Feb. 26, 1985, to Williams, details a transfer valve
 having a valve casing with a closed bottom, a closed top and a separator
 plate dividing the interior of the valve casing into an upper chamber and
 a lower chamber. Inlet and outlet ports communicate with the upper and
 lower chambers and two additional ports open into both the upper chamber
 and the lower chamber. Valves are provided in the upper and lower chamber
 and a control rod extends through the device to simultaneously rotate the
 valve and channel fluid through the respective ports. U.S. Pat. No.
 4,683,914, dated Aug. 4, 1987, to Brisland, details a slide valve having a
 valve body with a slide mounted therein for controlling opening
 interconnection and closing of various valve ports in the body. U.S. Pat.
 No. 5,152,320, dated Oct. 6, 1992, to Zimmerly, details a diverter valve
 which includes a valve body with two identical sections having valve seats
 and a valve stem extending through the valve body. A valve actuator lifts
 the valve stem, closing a pair of plugs in the valves and selectively
 allowing fluid to flow from various ports communicating with the valve
 body. U.S. Pat. No. 5,184,643, dated Feb. 9, 1993, to Raymond, details a
 valve sleeve assembly, typically having a valve sleeve defining a
 generally truncated, triangular, upraised land formed by complimentary
 shaped, adjacent recesses, when the sleeve is used as shown in a section
 taken along a radial plane perpendicular to the axis of the spool bore.
 It is an object of this invention to provide a new and improved dual filter
 isolation block or selectively filtering an operating fluid from a source
 to a user apparatus or device and facilitating removal and replacement of
 one of the filters in the isolation block without discontinuing or
 interrupting operation of the user device.
 Another object of the invention is to provide a dual filter isolation block
 for sandwiching between a source of operating fluid and a user or control
 device receiving that fluid, which isolation block includes a pair of
 distinct fluid paths extending through the isolation block and serving a
 pair of filters, respectively, with a pair of spools operating in sliding
 concert in the isolation block and extending through the fluid paths,
 respectively, such that the operating fluid can be selectively diverted
 through one of the fluid paths and its corresponding filter into the user
 or control device, thus leaving the second fluid path and filter free of
 operating fluid to facilitate changing the second filter without the
 necessity of interrupting the flow of operating fluid from the source to
 the user or control device.
 Still another object of the invention is to provide a dual filter isolation
 block and method of use for protecting a user device or mechanism from
 contaminants in operating oil or fluid flowing from a source, which dual
 filter isolation block is inserted between the source and the user device.
 The dual filter isolation block is characterized by a pair of fluid paths,
 each fitted with a filter and a pair of slidably disposed spools
 intersecting the respective fluid paths to facilitate selective isolation
 of one of the fluid paths and the filter from the other fluid path and
 filter by slidable operation of the spools in the isolation block.
 Yet another object of this invention is to provide dual filter isolation
 block and method for isolating an operating oil or fluid such as hydraulic
 fluid between an actuator and a control device such as a servo valve in
 order to protect the servo valve from contaminants in the hydraulic fluid.
 The isolation block includes a main body; a pair of separate fluid paths
 provided in the main body extending from the actuator to the servo valve
 and a pair of filters provided in the fluid paths, respectively; and a
 pair of spools having areas of reduced diameter for fluid flow, the spools
 slidably disposed in the main body of the isolation block and intersecting
 the respective fluid paths, such that the flow of hydraulic fluid from the
 actuator into the servo valve is isolated in and directed through a
 selected one of the fluid paths, the areas of reduced diameter in the
 spools and the associated filter and the opposite filter can be replaced
 without interrupting the flow of hydraulic fluid between the actuator and
 the servo valve.
 still further object of the invention is to provide a method for protecting
 an end user mechanism or device such as a servo valve from contaminants in
 an operating fluid flowing from a source to the end user device, which
 method includes providing a dual filter isolation block between the
 operating fluid source and the end user device; providing a pair of fluid
 paths in the isolation block, both of which fluid paths include a
 removable filter; providing a pair of slidably-operated spools having
 areas of reduced diameter in the isolation block for intersecting the
 fluid paths, respectively; and operating the spools in concert to direct a
 flow of operating fluid from the fluid source through one of the fluid
 paths and its associated filter and spool in the isolation block, to the
 user device, thus facilitating removal and replacement of the second
 filter without interrupting the flow of operating fluid from the source to
 the user device.
 SUMMARY OF THE INVENTION
 These and other objects of the invention are provided in a new and improved
 new filter isolation block and method for isolating an operating fluid
 such as hydraulic fluid between a control device such as an actuator and
 an end user control mechanism or device such as a servo valve, as in a
 turbine electricity generating system, which isolation block includes a
 pair of fluid paths and a pair of filters provided in the fluid paths,
 respectively, along with a pair of spool valves having fluid flow
 cavities, slidably seated in the isolation block and intersecting the
 fluid paths, respectively, wherein operating hydraulic fluid is allowed to
 flow through the actuator and through a selected one of the fluid paths
 and the spool cavities in the filter isolation block, into the servo valve
 for operating the servo valve responsive to first and second selected
 positions of the spool valve, and the opposite filter is isolated from the
 flow of the operating fluid and may be removed from the opposite or second
 fluid path without interrupting the flow of operating fluid to and from
 the servo valve.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIGS. 1 and 13 of the drawings illustrate a conventional turbine operating
 system wherein steam valves (not illustrated) are used to control the
 speed of a turbine (not illustrated) and an actuator-servo valve
 combination is used to control the steam valves. A servo valve 28 is
 connected to an actuator 30 in functional configuration. The actuator 30
 controls the steam valve (not illustrated), which in turn controls the
 speed of a turbine (not illustrated) in an electricity-generating system,
 according to the knowledge of those skilled in the art. The servo valve 28
 is typically attached directly to the actuator 30 such that an inlet fluid
 flow 47 flows from the actuator 30 into the servo valve 28 and returns by
 means of a return fluid flow 48, as illustrated in FIG. 13. A control
 fluid flow 49 serves to control operation of the actuator 30 responsive to
 a pilot fluid flow 50 through the actuator 30 and into the servo valve
 pilot mechanism 28a of the servo valve 28, further according to the
 knowledge of those skilled in the art. A servo valve filter 28b is
 typically provided in the pilot fluid flow 50 for filtering the pilot
 fluid prior to entry into the servo valve pilot mechanism 28a.
 Referring now to FIGS. 2 and 14 of the drawings, a dual filter isolation
 block 1 is inserted between the servo valve 28 and the actuator 30 and
 includes a valve 42, illustrated in schematic form in FIG. 14, which
 represents in schematic a pair of spool valves 5 and 6 (FIG. 3) which
 control the flow of control fluid through the main body 2 of the dual
 filter isolation block 1 and selectively, through a top filter 15 and a
 bottom filter 31, as hereinafter more particularly described. As in the
 case of the conventional servo valve 28-actuator 30 combination
 illustrated in FIG. 13, a control fluid flow 49 is provided from the
 actuator 30 through the main body 2 of the dual filter isolation block 1
 and into the servo valve 28. Similarly, an inlet fluid flow 47 extends
 from a storage tank or vessel (not illustrated) through the actuator 30
 and the main body 2 of the dual filter isolation block 1 and into the
 servo valve 28, while a return fluid flow 48 extends from the servo valve
 28 through the main body 2 of the dual filter isolation block, into the
 actuator 30 and back to the storage tank or vessel.
 Referring to FIGS. 3-6 of the drawings, the dual filter isolation block 1
 is characterized in a preferred embodiment by a generally rectangular main
 body 2, having a flat servo valve face 2a and actuator face 2b (FIG. 6)
 for receiving and mounting the servo valve 28 and the actuator 30,
 respectively. A bar slot 34a is provided on each end of the main body 2
 and a bar slot opening 34b extends from the bottom of the bar slot 34a
 downwardly through the main body 2. A spool lock bar 34 element of a lock
 assembly 39 is designed to removably and selectively seat in the
 respective bar slots 34a and extend beneath a synchronizing bar or plate
 12, attached to the ends of the inlet spool 5 and outlet spool 6 by means
 of clips 40, as illustrated in FIGS. 5 and 6. Accordingly, the lock
 assembly 39 serves to secure the spool lock bar 34 in place in the
 respective bar slots 34a for a purpose which is hereinafter described. A
 filter access plug 19 is threaded in a corresponding top filter cavity 14
 and is sealed therein by means of a sealing washer 19a that fits in a
 sealing washer seat 19b, as further illustrated in FIG. 4. A top filter 15
 is inserted in the top filter cavity 14, and the sealing washer 19a is
 fitted in the sealing washer seat 19b as the filter access plug 19 is
 threaded into the top segment of the top filter cavity 14. As further
 illustrated in FIGS. 5 and 6, a fluid inlet port 22 and a fluid return
 port 23, as well as a first shifting port 44 and a second shifting port 45
 are typically provided in both the servo valve face 2a and the actuator
 face 2b of the main body 2. It will be appreciated by those skilled in the
 art that either three or four of these ports may be used in any typical
 installation, depending upon the design of the servo valve 28 and the
 actuator 30. For example, as illustrated in FIG. 14, three of the four
 ports, the fluid inlet port 22, the fluid return port 23 and either the
 first shifting port 44 or the second shifting port 45, are utilized to
 accommodate the inlet fluid flow 47, return fluid flow 48 and the control
 fluid flow 49 in the main body 2 and connecting with the servo valve 28
 and the actuator 30, with the unused one of the shifting port 44 or the
 shifting port 45 closed or "blinded" against the servo valve 28 and the
 actuator 30. Mount bolt holes 20 extend through the main body 2 from the
 servo valve face 2a to the actuator lace 2b for receiving mount bolts (not
 illustrated) and mounting the dual filter isolation block 1 between the
 servo valve 28 and the actuator 30.
 Referring now to FIG. 7 of the drawings the dual filter isolation block 1
 is illustrated in exploded view and it will be appreciated that the spool
 lock bar 34 can be slidably inserted in the corresponding bar slot 34a on
 either the top or bottom of the main body 2, depending upon whether the
 top filter 15 or the bottom filter 31 is to be locked inside the main body
 2. When the spool lock bar 34 is slidably inserted in the selected bar
 slot 34a, the corresponding filter access plug 19 is blocked and the grip
 lock pin 36 of a lock bar grip 35 in the lock assembly 39 is extended
 through a bar opening 34c provided in the spool lock bar 34, and further
 into the underlying and registering bar slot 34b in the bar slot 34a, to
 secure the spool lock bar 34 in the bar slot 34a over the filter access
 plug 19. In a preferred embodiment a spring-loaded ball 37 is provided in
 the bottom end of the grip lock pin 36 and is caused to extend from and
 recess into the grip lock pin 36 by depression and release, respectively,
 of the push button 38 fitted in the lock bar grip 35. Accordingly, the
 spring-loaded ball 37 can be caused to retract in the grip lock pin 36
 upon application of pressure to the push button 38, and the grip lock pin
 36 then extended through the bar opening 34c in the spool lock bar 34 and
 into the bar slot 34b, where it is locked in place by release of pressure
 from the push button 38 as the spring-loaded ball 37 extends into a slot
 or depression (not illustrated) provided in the main body 2 at the base of
 the bar slot 34b. Similarly, in the lock assembly 39 can be assembled on
 the bottom side of the main body 2 in a corresponding bar slot 34a (not
 illustrated) as illustrated in phantom in FIG. 7, under circumstances
 where it is desired to facilitate operation of the dual filter isolation
 block 1 with the inlet spool 5 and the outlet spool 6 locked in a selected
 alternative operational mode, as hereinafter further described.
 Referring again to FIG. 7 of the drawings the inlet spool 5 and outlet
 spool 6 are each characterized by clip seats 41 at each end that receive
 clips 40 to facilitate mounting of the respective synchronizing bars 12 on
 each end of the inlet spool 5 and outlet spool 6, as illustrated.
 Additionally, corresponding sets of O-ring grooves 27 are provided in the
 inlet spool 5 and outlet spool 6 inwardly of the clip seats 41, for
 accommodating wiper O-rings 8 and sealer O-rings 9, respectively.
 Moreover, an inlet spool cavity 5a is provided near the center of the
 inlet spool 5 and a corresponding, somewhat longer, outlet spool cavity 6a
 is provided near the center of the corresponding outlet spool 6. The inlet
 spool 5 is slidably seated in a corresponding inlet spool cavity bore 5b,
 while the outlet spool 6 is similarly slidably mounted in a parallel
 outlet spool cavity bore 6b, each provided in the main body 2, as
 illustrated. It will be appreciated that the inlet spool 5 and outlet
 spool 6 are constrained to slide in concert in the respective inlet spool
 cavity bore 5b and outlet spool cavity bore 6b, by operation of the two
 synchronizing bars 12, each having bar openings 12a and seated on each end
 of the inlet spool 5 and outlet spool 6 by means of the clips 40. As
 further illustrated in FIG. 7, each of the top filter 15 and bottom filter
 31 are seated and sealed in the corresponding top filter cavity 14 and
 bottom filter cavity 31a, respectively, by means of top and bottom filter
 seal O-rings 13, respectively.
 Referring now to FIGS. 8 and 9 of the drawings, the main body 2 is
 characterized by a pair of fluid paths, one of which accommodates the top
 filter 15 and the other, the bottom filter 31. As illustrated in FIG. 8,
 the top filter inlet port 16 is typically created by drilling a hole
 transversely through the main body 2 from the actuator face 2b and
 terminating at the top filter cavity 14. A weld 18 is provided in the main
 body 2 at the top filter inlet port 16 to seal the top filter inlet port
 16 and a top filter inlet port leg 16a extends from the top filter inlet
 port 16 and terminates at the outlet spool 6. A top filter outlet port 17
 is drilled from the servo valve face 2a of the main body 2 downwardly at
 an angle and connects to the bottom of the top filter cavity 14. Another
 weld 18 closes the entrance end of the top filter outlet port 17 and a top
 filter outlet port leg 17a extends from the top filter outlet port 17 to
 the inlet spool 5.
 Similarly, as further illustrated in FIG. 9, a bottom inlet port 32 extends
 from the actuator face 2b of the main body 2 to the bottom filter cavity
 31a, which houses the bottom filter 31. Another weld 18 closes the
 entrance end of the bottom filter inlet port 32. A bottom filter inlet
 port leg 33a joins the bottom filter inlet port 32 to the outlet spool
 cavity 6a of the outlet spool 6. A bottom filter outlet port 33 extends
 from the top end of the bottom filter cavity 31a to the servo valve face
 2a face of the main body 2 and is typically drilled from that face in
 upwardly angular relationship into the bottom filter cavity 31a, as
 illustrated. A weld 18 closes the entrance drill bore of the bottom filter
 outlet port 33 and a bottom filter outlet port leg 33a extends from the
 bottom filter outlet port 33 to the inlet spool cavity 5a of the inlet
 spool 5.
 Referring again to FIG. 8 of the drawings, an O-ring seat 7 is typically
 provided in the outlet pilot pressure port 26a, provided in the actuator
 face 2b of the main body 2, to seal this port against a corresponding
 fluid flow aperture in the actuator 30. The inlet pilot pressure port 26
 extends to the outlet spool cavity 6a of the outlet spool 6, while the
 outlet pilot pressure port 26a extends to the inlet spool cavity 5a of the
 inlet spool 5, as illustrated. Accordingly, when the dual filter isolation
 block 1 is in the configuration illustrated in FIG. 8, an operating fluid
 such as hydraulic oil or fluid introduced under pressure from the actuator
 30 into the inlet pilot pressure port 26 at the actuator face 26, flows
 downwardly through the annulus created by the outlet spool cavity 6a and
 the outlet spool bore 6b, to the bottom filter inlet port leg 32a and from
 there into the bottom filter inlet port 32 and through the bottom filter
 31 and the top of the bottom filter cavity 31a, to the bottom filter
 outlet port 33. The hydraulic fluid then flows from the bottom filter
 outlet port 33 through the bottom filter outlet port leg 33a, to the
 annulus created between the inlet spool cavity 5a and the inlet spool bore
 5b and from there upwardly, to the pilot pressure port 26a at the servo
 valve face 2a and into the servo valve 28.
 It will be understood that while the working oil or fluid is constrained to
 flow through the bottom filter 31 as indicated above in the flow
 configuration illustrated in FIG. 8, it is not permitted to flow
 simultaneously through the top filter 15, since the top filter inlet port
 leg 16a is closed against the outlet spool 6 at the outlet spool bore 6b
 above the outlet spool cavity 6a, to prevent hydraulic fluid from flowing
 past that point. Accordingly, since hydraulic working oil or fluid is not
 introduced into the top filter inlet port 16, the top filter 15 is
 isolated from the operating hydraulic fluid and can be removed and
 replaced by simply unthreading the top filter access plug 19 and removing
 and replacing the top filter 15. It is further significant that this
 operation in no way hinders the flow of operating hydraulic fluid through
 the bottom filter 31 and to the servo valve 28, as above described. The
 inlet spool 5 and outlet spool 6 are locked into the filter 31 flow
 configuration illustrated in FIG. 8 and access to the bottom access plug
 19 is blocked by means of the spool lock bar 34 and the lock bar grip 35
 elements of the lock assembly 39.
 Referring now to FIG. 9 of the drawings, under circumstances where it is
 desired to isolate the bottom filter 31 and facilitate a flow of operating
 hydraulic fluid through the top filter 15, the lock assembly 39 is removed
 from the bottom end of the inlet spool 5 by depressing the push button 38
 on the lock bar grip 35 and the inlet spool 5 and outlet spool 6 are
 shifted upwardly in concert to the position illustrated in FIG. 9, thereby
 also shifting the relative positions of the inlet spool cavity 5a and
 outlet spool cavity 6a internally in the main body 2. The lock assembly 39
 is then replaced on the top end of the inlet spool 5 and outlet 5 spool 6,
 to block access to the top filter access plug 19, as illustrated.
 Consequently, operating hydraulic fluid introduced into the inlet pilot
 pressure port 26 extends to the annulus created between the outlet spool
 cavity 6a and the corresponding outlet spool bore 6b and the hydraulic
 fluid is caused to flow through that annulus into the top filter inlet
 port leg 16a and from there into the top filter inlet port 16, through the
 top filter 15 and from the bottom end of the top filter cavity 14, through
 the top filter outlet port 17 and the top filter outlet port leg 17a, to
 the annulus created by the inlet spool cavity 5a and the inlet spool bore
 5b. From that annulus, the operating hydraulic fluid flows through the
 servo valve 28 from the outlet pilot pressure port 26a. While the
 hydraulic operating fluid is flowing through the top filter 15 as
 described above, it will be appreciated that it is unable to flow through
 the isolated bottom filter 31, since the bottom filter inlet port leg 32a
 is blinded against the inlet spool 5 at the inlet spool bore 5b and is not
 in alignment with the outlet spool cavity 6a. Furthermore, the bottom
 filter outlet port leg 33a is similarly blinded or closed against the
 inlet spool 5. Accordingly, the hydraulic fluid cannot flow through the
 bottom filter 31 and the bottom filter 31 may be quickly and easily
 removed from the bottom filter cavity 31a by removing the bottom filter
 access plug 19 as described above with respect to the top filter 15,
 without interrupting the flow of hydraulic fluid from the actuator 30 to
 the servo valve 28 through the top filter 15.
 Referring now to FIGS. 8, 9 and 10 of the drawings, hydraulic oil or fluid
 is caused to enter the inlet pilot pressure port 26 under pump pressure
 from a tank (not illustrated) and from the actuator 30, typically by means
 of a pilot service line 29 (FIG. 10) which extends from the fluid inlet
 port 22 to the inlet pilot pressure port 26. Furthermore, FIG. 10 also
 illustrates the bar slot 34a in both the top and bottom ends of the main
 body 2 for accommodating the spool lock bar 34 element of the lock
 assembly 39, as heretofore described, and the mount bolt holes 20 are also
 illustrated for mounting the main body 2 to the servo valve 28 and the
 actuator 30 using suitable mount bolts 21, as illustrated in FIGS. 2-4.
 Referring now to FIGS. 8, 9 and 11 of the drawings, the inlet spool 5 and
 outlet spool 6 are shown in section, more particularly illustrating the
 inlet spool cavity 5a and corresponding outlet spool cavity 6a, as well as
 the inlet pilot pressure port 26 and the outlet pilot pressure port 26a.
 The welds 18 serve to blind off the top filter outlet port 17 and the
 outlet pilot pressure port 26a, as further heretofore described.
 Referring to FIGS. 8, 9 and 12 of the drawings, in similar fashion the
 inlet spool 5 and outlet spool 6 with accompanying inlet spool cavity 5a
 and outlet spool cavity 6a are illustrated, with the connecting bottom
 filter inlet port 32 and the bottom filter inlet port leg 32a, as well as
 the bottom filter outlet port 33 and the bottom filter outlet port leg
 33a, one end of each of which bottom filter inlet port 32 and bottom
 filter outlet port 33 is terminated by welds 18 to facilitate a flow of
 hydraulic fluid through the respective ports as described above.
 In operation, and referring again to the drawings, under circumstances
 where it is desired to facilitate a flow of operating hydraulic oil or
 fluid from the actuator 30 to the servo valve 28 and back to the actuator
 30 through the main body 2 of the dual filter isolation block 1, with the
 flow path extending through the bottom filter 31, the inlet spool 5 and
 outlet spool 6 are initially shifted downwardly in the main body 2, as
 illustrated in FIG. 8. Secure positioning of the inlet spool 5 and outlet
 spool 6 in the configuration illustrated in FIG. 8 and blocking of the
 bottom filter access plug 19 is assured by sliding the spool lock bar 34
 into the corresponding bar slot 34a in the bottom side of the main body 2,
 depressing the push button 38 and projecting the grip lock pin 36 through
 the corresponding bar opening 34c in the spool lock bar 34 and into the
 bar slot 34b provided in the bottom of the bar slot 34a. The push button
 38 is then released to facilitate extension of the spring-loaded ball 37
 outwardly into a slot (not illustrated) provided in the main body 2 to
 lock the spool lock bar 34 securely in the bar slot 34a beneath the lower
 synchronizing bar 12. This action prevents shifting of the inlet spool 5
 and outlet spool 6 from the position illustrated in FIG. 8 to the position
 illustrated in FIG. 9. Accordingly, hydraulic oil or fluid introduced from
 the actuator 30 into the fluid inlet port 22 illustrated in FIGS. 7 and
 10, also flows under pressure through the pilot service line 29 into the
 inlet pilot pressure port 26 and through the bottom filter 31 as
 heretofore described, where it exits the outlet pilot pressure port 26a
 and flows into the servo valve 28. In this flow configuration, as
 heretofore described, access to the bottom filter 31 is blocked and the
 hydraulic oil or fluid cannot flow through the top filter 15 and the top
 filter 15 may therefore be removed and replaced, as further heretofore
 described.
 Under circumstances where it is desired to facilitate a flow of hydraulic
 oil or fluid through the newly installed top filter 15 and change the
 bottom filter 31, the push button 38 on the lock bar grip 35 is depressed
 and the lock bar grip 35 removed from contact with the spool lock bar 34
 to facilitate sliding the spool lock bar 34 from beneath the bottom
 synchronizing bar 12 and from the bottom bar slot 34a. This action
 facilitates shifting of the inlet spool 5 and the outlet spool 6 in
 concert to the position illustrated in FIG. 9, where the spool lock bar 34
 is again slipped into position in the top bar slot 34a beneath the top
 synchronizing bar 12 and the spool lock bar 34 again locked into position
 to block access to the top filter 15 by operation of the push button 38
 and spring-loaded ball 37, as heretofore described. The inlet spool 5 and
 outlet spool 6 cannot therefore be inadvertently shifted back into the
 position illustrated in FIG. 8 due to the presence of the spool lock bar
 34. Under these circumstances, hydraulic oil or fluid introduced into the
 fluid inlet port 22 is also introduced under pressure into the inlet pilot
 pressure port 26 as heretofore described and flows through the top filter
 15 and from the outlet pilot pressure port 26a, into the servo valve 28.
 The isolated bottom filter 31 can then he removed and exchanged as desired
 and as heretofore described.
 While the preferred embodiments of the invention have been described above,
 it will be recognized and understood that various modifications may be
 made in the invention and the appended claims are intended to cover all
 such modifications which may fall within the spirit and scope of the
 invention.