US3138207A - Pressure balanced actuating device - Google Patents

Description

June 23, 1964 J. M PEPPERS 3,138,207

PRESSURE BALANCED ACTUATING DEVICE Filed June 20,

2 Sheet -Sheet 1 I I6 4O 48 INVENTOR.

5594M ewm June 23, 1964 J. M PEPPERS PRESSURE BALANCED ACTUATING Filed June 20, 1960 DEVICE 2 Sheets-Sheet 2 amen 6 $9M;

United States Patent 3,133,207 PRESURE BALANCED ACTUATENG DEVECE James M. Peppers, Houston, Tern, assignor to Halliburton Company, a corporation of Delaware Filed June 20, 1960, Ser. No. 37,431 6 Claims. (Cl. l66--63) This invention generally relates to apparatus utilizing fluid pressure to operate equipment and more particularly relates to improved devices for operating equipment while situated within elevated fluid pressures, such as within well bores or the like.

Devices of this type are well developed in the art, as may be seen by reference to Nordone No. 2,177,044; Broyles No. 2,189,937; Miller No. 2,334,428; Baker No. 2,373,006; or Spangler No. 2,326,404; for example.

A primary problem in the design of actuating devices utilizing combustible propellants is the burning rate of the propellant. If too fast, undue shock and inertia loading places great stress on the actuating equipment, causing improper operation of the driven equipment and oftentimes damage to the actuation device. The burning or combustion rate of a given propellant varies as a function of the ambient pressure in the combustion space during its combustion and also as a function of its temperature. When such actuating devices (herein termed setting tools, for convenience) are used in well bores, the temperature of the propellant is usually elevated by the heat of the well bore and accordingly burns at an increased rate. Inasmuch as such a setting tool, in acting, must buck or overcome the forces exerted on its work output or actuating member by ambient hydrostatic pressure in the bore hole, the tool must necessarily generate pressure sufiicient to overcome this force due to hydrostatic pressure before it can bring any net force to bear in performing its designed function, i.e., to operate interconnected equipment. This force utilized merely to overcome the resistance of bore hole pressure is wasted in that no useful work is accomplished. The hydrostatic head and bore temperature in a particular well bore may be as high as 10,000 p.s.i. and 350 B, respectively. Such an illustrative environment often causes detonation of the propellant and improper operation of the setting tool for the foregoing reasons.

To reduce the rapid burning rate and shock attendant thereto in such actuating devices, slowburning propellants have been developed, such as shown by Norclone No. 2,299,464 and Baker No. 2,640,547 for example. However, in the presence of sufficiently high temperature and pressure even the best controlled of these slowburning propellants have been found to burn at unduly high rates. Such propellants are'also very expensive.

Another approach aimed at enabling a slow and orderly operation of the associated equipment, with lessor no regard to burning rate of the propellant, has been to provide hydraulic damping means associated with the propellant pressure and the actuating equipment. Apparatus of this type may be found in Banning, Jr. No. 2,426,335; Baker No. 2,618,343; and Baker No. 2,675,877; for example.

Although provision of controlled propellants and hydraulic damping means with such setting tools has greatly enhanced the operational reliability for such tools, the problem of high environmental temperatures and pressure still exists as substantial factors in the design and operation of such tools.

As previously mentioned, presently available setting tools require that resisting forces of the surrounding or environmental pressure be overcome before any useful work is derived from the tool. Therefore, in these presently available tools, excess pressure is always required 3,138,207 Patented June 23, 1964 to overcome the hydrostatic head before these tools can do any useful work, e.g., for setting a plug, packer, etc. This excess pressure may be greatly in excess of that actually required to perform the useful work. Further, this excess pressure, in effectively raising the pressure in the combustion chamber and hence on the propellant, may unduly increase the burning rate of even the slowest burning propellant and be instrumental in causing undesirable operation of such. tools.

Some presently available tools provide no means to relieve the dangerous high combustion pressures created in the combustion chambers of such devices until the tool is partially or completely disassembled. As disassembly is performed manually there is the obvious attendant danger to maintenance personnel who must clean and reload such tools.

It is also seen that if a gas stored under pressure, e.g., bottled gas, is'used for motive power, the surrounding pressure may be sufficiently great that the operation of a tool, so powered, may never occur in such a high pressure environment.

It is therefore a general object of the present invention to eliminate the variation in combustion pressure on such actuation devices as may be caused by elevated environmental pressures.

It is another object of this invention to provide an actuation device which creates only such force as may actually be required to actuate its interconnected equipment. 1

It is the further object of this invention to provide actuating apparatus adapted to automatically vent the motive combustion gases atthe end of its operational cycle.

Other objects will become apparent from consideration of the embodiments shown in the accompanying drawings and forming a part of the present specification. These embodiments will be described in detail but it is to be understood that such description is to be taken in a sense limited only by the appended claims.

Referring to the drawings:

FIGURE 1 is a longitudinal section through an apparatus illustrating one embodiment of the invention;

FIGURE 2 is a cross-sectional view taken along line 2-2 of FIGURE 1;

FIGURE 3 is a longitudinal section through an apparatus illustrating another embodiment of the invention;

FIGURE 4 is a cross-sectional view taken along the line 4-4 of FIGURE 3;

FIGURE 5 is a longitudinal cross-sectional view of the apparatus of FIGURE 3 showing a portion thereof in a post-actuation condition;

FIGURE 6 is a longitudinal sectional view of an alternate embodiment of a portion of the apparatus illustrated in FIGURE 3; and I FIGURE 7 is an enlarged sectional view of the fluid meter portion of the embodiment shown in FIGURE 6. 7

filled chamber containing a second piston is also pro A vided. The first and second pistons are in connection by means of a rod. The piston face of each piston that is connected to the rod is exposed to any surrounding arnin the bient pressure. The arrangement provides that the force exerted by ambient pressure against an actuating rod connected to the second piston and extending from the device will be balanced by a compensating force exerted against the second piston in excess of that exerted against the first piston.

A device of the invention, as disclosed in the various drawings, is useful, e.g., for running and setting a well packer at a desired depth in a well casing 12 disposed within a. bore hole. The result of this operation is that the packer is anchored in packed-off condition with respect to the well casing 12. One form of setting tool in accord with this invention is generally designated by reference numeral 14. The setting tool 14 is shown suspended from the lower end of a wire line 16 by means of which the tool is manipulated and controlled in the bore hole. Electrical current for tool actuation initiation may be passed down the wire line 16 from the surface of the earth to setting tool 14.

Various commonly known types of well packers are used for plugging well casing. An exemplary packer of one general variety of such well packers usable in connection with the present invention is illustrated as packer 10 in the drawings. As shown, packer 10 includes a main body 18 having a central bore which may be plugged if desired. Packer 10 is adapted to be anchored against upward and downward movement in the well casing by a plurality of upper and lower segmental slips 20. The inter surfaces of slips 20 are tapered for sliding engagement with correspondingly tapered upper and lower conical expander members 22. Each expander member 22 is disposed adjacent a resilient packing sleeve 24 which is disposed about the body 18 and interjacent said expander members 22. The sleve 24 is adapted when compressed longitudinally by the base ends of said conical expanders, to expand radially to contact and seal off a casing bore.

Initially, the slips 20 and the expander members 22 are supported in retracted position with respect to main body 18 by means of shear screws or other suitable connecting elements such that the bases of the expanders do not compress the sleeve 24 any appreciable amount. An upward force directed on body 18 with respect to the upper slips 20 causes sequential shearing of such screws or connecting elements (not shown) and further causes the slips 20 to move outwardly and downwardly along the mating tapered surfaces of conical expander member 22 to set the saw tooth edges (as shown) of the upper slips in anchoring engagement with the inter surface of casing 12 in response to the lateral movement of these slips, further force causes the upper and lower expander members 22 to move into compressed relation with respect to sleeve 24. Continued force operates to set the lower slips 20 in a manner similar to that described for the setting of the upper slips, supra. The application of compressive force to sleeve 24, as brought out above, expands the sleeve radially. The relationship of the parts of packer 10 of FIG. 1 are shown in their set and packed condition in packer 10 of FIG. 5. When completely set, the packer 10 is supported against vertical movement by the slips 20 which are in locked anchored engagement with the inter surface casing 12. This locking holds the sleeve 24 in longitudinally compressed and radially expanded in sealed engagement with the bore of casing 12.

A perhaps fuller description of the structure and mode of operation of a packer of the above-referred-to general variety may be found in United States Patent No. 2,373,- 006, previously referred to herein.

The setting of packer 10 is accomplished by force exerted by setting tool 14 derived from gas pressure generated therein. In the embodiments shown, the pressure is developed within a combustion chamber disposed in the upper portion of a cylinder 26. Chamber 30 is closed at its upper end by a head 28 which is connected to wire line 16. Disposed in chamber 30 is a propellent charge 32 and an electrical igniter 33. Igniter 33 is in electrical connection with cable 16. Disposed in slidable relation within the lower end of cylinder 26 and in communication with combustion chamber 30 is an upper driving piston 34.

A lower cylinder 36 is also disposed in tool or body 14 below cylinder 26. A balance piston 38 is disposed in slidable relation within cylinder 36 and is connected to piston 34 by means of a piston rod 40. A piston rod 42 is connected at its upper end to piston 38 and extends at its lower end through a sealing means 44. Sealing of pistons 34 and 38 is respectively accomplished within cylinders 26 and 36 by sealing means 46, such as O-rings. Disposed in tool or body 14 are exhaust vents 48, which permit communication of the interior of cylinder 26 with the exterior of tool or body 14 upon complete excursion of piston 34 to its lowermost position.

Piston rod 42 is connected at its lower end to a compression sleeve 50 which abuts, at its lower end, against upper slips 20. Tool or body 14 is connected through a yoke arrangement to a tension member 52. The yoke arrangement of sleeve 50 and tension member 52 is more clearly shown by joint reference to FIGS. 1 and 2. The cross-section of the yoke arrangement, illustrated in FIG. 1, is taken at line 1-1 of FIG. 2.

Downward force, exerted by lower piston rod 42 with respect to tool or body 14, exerts an upward force on tension member 52 with respect to compression sleeve 51). Tension member 52 is connected to the packer body 18 by means of a releasable connective member illustrated here as being a shear pin 54.

Upon such force being supplied between tension member 52 and compression sleeve 50, the packer member 10 is actuated from the position shown in FIG. 1, through the previously described sequence of operation, to its set position as shown in FIG. 5. Further force of predetermined magnitude shears the shear pin 54, releasing the device from the packer 10, where upon the tool or body 14, together with the tension member 52 and compression sleeve 50, may be withdrawn from the well casing 12 by means of wire line 16.

As provided by the present invention, the pressure developed within the combustion chamber 30 will exert a force between tension member 52 and compression sleeve 50 commensurate with the cross-sectional area of piston 34, the effect of hydrostatic fluid pressure which may be found within casing 12 having been nulled out insofar as the operation of the device of the invention is concerned. This feature is provided as follows:

Assume, for purposes of illustration, that the tool or body 14 is emersed in a hydrostatic fluid environment having an ambient pressure intensity P; further assume that piston 34 has an effective area, i.e., the cross-sectional area of piston 34 minus the cross-sectional area of the piston rod 40, at its lower surface equal to A area units; further assume that the piston 38 has an effective area, i.e., the cross-sectional area of the piston 38 minus the cross-sectional area of the piston rod 40, at its upper surface equal to A area units; and further assume that the piston rod 42 has an effective cross-sectional area, at the location defined by sealing member 44, of A area units. Hydrostatic pressure of intensity P will be admitted through the vent 48 and act upwardly on piston 34 with an upward force equal to PA and downwardly on piston 38 with a downward force equal to FA This same ambient pressure intensity P is exerted upwardly on the effective cross-sectional area, i.e., the exposed end, of rod 42 with an upward force equal to PA Therefore, if the aforementioned effective areas have a relative size relationship such that A +A =A then the sum of the upward forces, PA and PA equals the downward force PA So long as such an upward and downward force equality exists, the mechanical elements 34, 40, 38 and 42 will remain in equilibrium at all positions within the cylinders 26 and'3'6 insofar as the surrounding hydrostatic pressure is concerned. Itis, of course, obvious that any air pressure beneath piston 38 will increase during downward travel of this piston. But this increase in air pressure is negligible in view of the relatively high combustion and hydrostatic pressures in volved.

In operation, assuming the tool has been lowered into a bore hole to a desired depth, an electrical firing signal is transmitted from the surface over the wire line 16 to the tool and ignites the igniter 33. The ignition of the igniter accomplishes the initiation of burning of the propellant charge 32, which, in turn, evolves combustion gases and pressurizes cylinder 26. This pressure is exerted on the upper surface of piston 34 and causes the downward displacement thereof together with elements interconnect therewith, i.e., piston rod 453, piston 38, piston rod 42, and compression member Sit. This movement is not interfered with, as has been elaborately pointed out above, by the effects of hydrostatic pressure because of the hydrostatic pressure balancing feature of this invention. The tool or body 14 has aflixed at its lower'end a tension member 52 which, relatively speaking, moves upwardly with respect to compression member 50. The

tension member 52 exerts an upward force on the body 18 of the packer and compression member 5% exerts a downward force on the upper slips 20 of the packer it). These parts of the packer are forced by these forces to move relatively to one another to accomplish the setting thereof. The operation and setting of the particular packer shown for purpose of illustration has been described in sufficient detail for understanding by anyone having ordinary skill in the art. After setting, continued application of force by the relative moving tension and compression members operates to shear the shear pin 54, thus disconnecting the tool from the set packer to thereby enable the withdrawal of the tool or body 14 from the bore hole. a

As the piston 34 passes vent 48 toward the lower end of its actuation excursion or stroke, pressure within chamber 38 is allowed to equalize through this vent with the pressure which may exist in the surrounding well bore.

The embodiment of tool or body 14' as shown in FIG. 3, is similar in structure and operation to the tool shown in FIG. 1, and, for simplicity in description, similar elements bear similar reference numerals with appropriate prime notations added. The primary difference of the tool of PEG. 3 from that of FIG. 1 is that the piston 38' is larger in effective area with respect to the piston rod 42' with the result that the hydrostatic pressure intensity P is utilized to add to the force obtainable by virtue'of the pressure generated in the chamber 39'. This has the effect of increasing the total force obtainable at the piston rod 42' for its actuating function. Also, the embodiment of FIG. 3 is seen to employ pistons 34' and 38' which are considerably larger in diameter than the connected piston rods 49 and 42'. Also, additional vents 49 are provided below the vents 48' (which are similar in function to the vents 48 previously described). Vents 49 remain open to the interior of cylinder 36 above piston 38 after piston 34 has passed the vents 48'.

In the embodiment of FIG. 3, the compression sleeve 5% is connected to the lower piston rod 42' by means of a transverse pin 51. Pin 51 is adapted to slide through longitudinal slots in tension members 52 during extension of piston rod 42' from the tool or body 14'. This alternate construction is further illustrated in FIG. 4. y

it is to be understood that the relationship of the effective area of piston rod 42' and the pistons 34 and 38' of PEG. 3 may conform to the pressure balance relationship set forth for their counterparts in the description of FIG. 1 or the relationship may be such that the hydrostatic pressure intensity P will add to the net downward force as desired. With either of such relationships provided, the setting tool 14 f FIG. 3 may function in predetermined response to pressures created withinthe combustion chamber 30', either unimpeded by or aided by surrounding hydrostatic pressure.

FIG. 6 illustrates an alternate embodiment of the apparatus of PEG. 3 wherein a liquid damping means is incorporated. As shown, a piston rod 40" is connected to a piston rod 33 which rests on an incompressible power transfer fluid such as water or oil as indicated within cylinder 36,. Below the fluid, is a fluid meter means 56. The transfer fluid is communicated through the fluid meter 56 to the upper face of a piston 38 within a cylinder 3%. Piston 38 is connected to a piston rod 42", similar in function to the piston rod 42' described with reference to FIG. 3.

A detailed section of the improved fluid meter 56 is shown in FIG. 7. As shown, the path for transfer fluid flow through the meter 56 is first through a first longitudinal port 58 into a bore or port 60, provided substantially transversed to the axis of tool or body 14". The path for transfer fluid flow then continues through a selected portion of bore or port 60 and then through a second port 62 into a chamber above piston 38,,. Inserted within bore or port 69 is a metering pin member 64 disposed on the end of an insertion plug 66. A seal 68, such as an O-ring, confines the power transfer fluid within the tool or body 14".

Variations in size of the pin 64 relative to the bore or port 6% provide variations in the flow area available for passage of the power transfer fluid therethrough. The dimensions of pin 64 may be selected such as to merely smooth out variations in combustion of the propellent charge 32 or to provide for thecomplete setting of the packer 10 long after complete combustion of the propellent charge. Further, the pin member 64 may be readily replaced by removing insertion plug 66 without any other I disassembly of the setting tool or body 14".

The general operation of the device of FIG. 3 with or without the embodiment of FIG. 6 is substantially the same of that described with respect to the device of FIG. 1.

In the operation of the embodiment illustrated in FIG. 6, the piston rod 40 responsive to the pressure of combustion gas on an upper piston (not shown) depresses piston 38,, through piston rod 40" which forces the transfer fluid through fluid meter 56 into the chamber above piston 38 The ambient pressure of the bore hole is admitted through vents 43 and 49 between piston 38,, and the one thereabove (not shown) to accomplish the principal teaching of this invention. The pressure of the power transfer fluid moves piston 38 thus driving the piston rod 42" associated therewith downwardly. The relative dimensions of meter pin 64 and bore 60 and the pressure developed on the transfer fluid in cylinder 36,, determine the time intervalwhich is required to transfer the power transfer fluid through the meter 56 and thus the time to fully depress the piston 38,, (acted on by the power transfer fluid) and to set the packer 10. In the embodiment of FIG. 6, it is seen that the same pressure balance relationship with respect to hydrostatic fluid pressure has been accomplished as has been previously described with reference to the embodiment of FIGS. 1 and 3.

Accordingly,while preferred embodiments of the invention are herein shown and described, no limitation is placed on any of the details shown or described, except as set forth in the appended claims.

What is claimed is:

1. An actuator for use within a pressurized fluid environment comprising: a body; an actuation fluid pressure source in said body; a bore in said body communicating said source with the exterior of said body; said bore including a bore enlargement intermediate the ends thereof; first and second interconnectedly spaced pistons in sealing slidable engagement respectively with said bore intermediate said source and said bore enlargement and with said bore enlargement; an actuator rod in force transmitting relation to said second piston and in sealing slidable engagement with said bore and having one end area effectively exposed to said fluid invironment; said rod being extendable from said bore in response to displacement to said pistons by pressure applied from said source; said first and second interconnectedly spaced pistons having confronting effective areas of different size such that their area difference is substantially equal to the effective area of said actuator rod exposed to said fluid environment; and means admitting fluid from said environment to said bore intermediate said confronting effective piston areas whereby the pressure of said environment exerts a net force on said interconnectedly spaced pistons which is substantially equal and opposite the force exerted on said actuator rod by the pressure of said fluid environment.

2. An actuator device for use within a pressurized fluid environment comprising: a body; an actuation fluid pressure source chamber in said body; a bore in said body communicating said chamber with the exterior of said body; said bore including a bore enlargement intermediate the ends thereof; first and second spaced pistons in sealing slidable engagement respectively with said bore intermediate said chamber and said bore enlargement and with said bore enlargement; an actuator rod in force transmitting relation to said second piston and in sealing slidable engagement with said bore; said rod being extendable from said bore and having one end ,area effectively exposed to said fluid environment; said first and second spaced pistons having confronting effective areas of different size such that their area difference is substantially equal to the effective area of said actuator rod exposed to said fluid environment; first and second passageways in said body disposed to communicate said bore to said fluid environment at either side of said first piston when said actuation rod is extended within said pressurized fluid environment; said first passageway adapted to vent said actuation fluid pressure source chamber and said second passageway adapted to admit fluid pressure of said fluid environment to said bore intermediate said confronting areas of first and second spaced pistons, whereby the pressure of said environment exerts a net force on said spaced pistons which is substantially equal and opposite the force exerted on said exposed area of said actuator rod by the pressure of said fluid environment.

3. An actuator device for use within a pressurized fluid environment comprising: a body; an actuation fluid pressure source chamber in said body; a bore in said body communicating said chamber with the exterior of said body; said body including a bore enlargement intermediate the ends thereof; first and second interconnectedly spaced pistons in sealing slidable engagement respectively within said bore intermediate said chamber and said bore enlargement and with said bore enlargement; a third piston spaced apart from said second piston in said bore enlarge ment by an interposing incompressible fluid and by an intervening wall defining first and second compartments within said bore enlargement; means incorporated in said body for metering said incompressible fluid from said first to said second compartment as said second and third pistons are displaced in said bore enlargement; an actuator rod in force transmitting relation to said third piston and in sealing slidable engagement with said bore; said rod being extendable from said bore and having one end area effectively exposed to said fluid environment; said first and second interconnectedly spaced pistons having confronting effective areas of different size such that their area difference is substantially equal to the effective area of said actuator rod exposed to said fluid environment; and means admitting fluid from said environment to said confronting areas, whereby the pressure of said environment exerts a net force on said interconnectedly spaced pistons which is substantially equal and opposite the force exerted on said actuator rod by the pressure of said fluid environment.

4. The device of claim 3 wherein said metering means comprises a passageway communicating with the exterior of said body and having first and second ports spaced apart therealong which respectively communicate with said first and second compartments, a metering pin disposed in said passageway providing a metering clearance therewith and means associated with said pin, body and passageway for removably retaining said pin in said body.

5. A balanced actuating device for operation while situated within elevated fluid pressure comprising: a body; a motive fluid pressure source within said body; a first cylinder disposed within said body in communication with said pressure source; a first piston means in sealed slidable engagement within said first cylinder to provide a first chamber; a second cylinder disposed within said body and communicating with said first cylinder; a second piston means in sealed slidable engagement Within said second cylinder; a first piston rod interconnecting said first and second piston means; said first and second piston means having confronting effective areas of different size; means permitting entry of surrounding fluid into said body to said mutually confronting effective piston areas; a second piston rod connected to said second piston means and extending in sealed slidable engagement through a wall of said body and exposed to the surrounding fluid pressure; said second cylinder, said second piston means and said second piston rod forming a second chamber filled with compressible medium; said confronting effective areas of said piston means having a net total effective area such that the pressure of surrounding fluid thereon produces a net force which is substantially equal and opposite the force of said surrounding fluid on said second rod, whereby the total force effect of said surrounding fluid on said device is substantially nullified.

6. A balanced actuating device for operating a mechanism While situated within a pressurized fluid environment comprising: a body; an actuation fluid pressure source in said body; a bore in said body communicating said source with the exterior of said body; said bore including a bore enlargement intermediate the ends thereof; first and second spaced pistons in sealing slidable engagement respectively with said bore intermediate said chamber and said bore enlargement and with said bore enlargement; an actuator rod in force transmitting relation to said second piston and in sealing slidable engagement within said bore; said rod being extendable from said bore in response to force exerted from said fluid pressure source and having one end area effectively exposed to said fluid environment; said first and second pistons having confronting effective areas of different size such as that their area difference is substantially equal to the effective area of said actuator rod exposed to said fluid environment; and means admitting fluids from said environment to said bore intermediate said confronting piston areas whereby the pressure of said fluid environment exerts a net force on said spaced pistons which is substantially equal and opposite the force exerted on said rod by the pressure of said fluid environment.

References Cited in the file of this patent UNITED STATES PATENTS Conrad June 19,

Claims (1)

1. AN ACTUATOR FOR USE WITHIN A PRESSURIZED FLUID ENVIRONMENT COMPRISING: A BODY; AN ACTUATION FLUID PRESSURE SOURCE IN SAID BODY; A BORE IN SAID BODY COMMUNICATING SAID SOURCE WITH THE EXTERIOR OF SAID BODY; SAID BORE INCLUDING A BORE ENLARGEMENT INTERMEDIATE THE ENDS THEREOF; FIRST AND SECOND INTERCONNECTEDLY SPACED PISTONS IN SEALING SLIDABLE ENGAGEMENT RESPECTIVELY WITH SAID BORE INTERMEDIATE SAID SOURCE AND SAID BORE ENLARGEMENT AND WITH SAID BORE ENLARGEMENT; AN ACTUATOR ROD IN FORCE TRANSMITTING RELATION TO SAID SECOND PISTON AND IN SEALING SLIDABLE ENGAGEMENT WITH SAID BORE AND HAVING ONE END AREA EFFECTIVELY EXPOSED TO SAID FLUID INVIRONMENT; SAID ROD BEING EXTENDABLE FROM SAID BORE IN RESPONSE TO DISPLACEMENT TO SAID PISTONS BY PRESSURE APPLIED FROM SAID SOURCE; SAID FIRST AND SECOND INTERCONNECTEDLY SPACED PISTONS HAVING CONFRONTING EFFECTIVE AREAS OF DIFFERENT SIZE SUCH THAT THEIR AREA DIFFERENCE IS SUBSTANTIALLY EQUAL TO THE EFFECTIVE AREA OF SAID ACTUATOR ROD EXPOSED TO SAID FLUID ENVIRONMENT; AND MEANS ADMITTING FLUID FROM SAID ENVIRONMENT TO SAID BORE INTERMEDIATE SAID CONFRONTING EFFECTIVE PISTON AREAS WHEREBY THE PRESSURE OF SAID ENVIRONMENT EXERTS A NET FORCE ON SAID INTERCONNECTEDLY SPACED PISTONS WHICH IS SUBSTANTIALLY EQUAL AND OPPOSITE THE FORCE EXERTED ON SAID ACTUATOR ROD BY THE PRESSURE OF SAID FLUID ENVIRONMENT.

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