Mud solids immune clean fluid compensating system
This is a continuation of application Ser. No. 07/292,701 filed 01/03/89 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The subject matter of the present invention relates to well tools, and more particularly to a mud solids immune clean fluid compensating system for use in such tools. 
     Downhole well tools typically comprise comparatively intricate flow passages in which operating mechanisms displace working fluids into and out of the tool. During such periods when the working fluids are being displaced into/out of the tools by the operating mechanisms, mud solids enter the tool and clog the operating mechanisms, preventing the mechanisms from displacing the working fluids. As a result, an alternate design is needed to allow the operating mechanisms to perform their displacement functions without also allowing mud-solids to enter the tool and clog the operation of such mechanisms. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the present invention to design a mud solids immune clean fluid compensating system for use with a well tool which will allow the operating mechanism of such tool to displace working fluid into and out of the tool without simultaneously allowing mud solids to enter such tool and clog the functioning of the operating mechanism. 
     In accordance with these and other objects of the present invention, an operating piston is relocated to a position external to the tool housing, and a piston sleeve is mounted around the piston, one end of the piston sleeve overlapping the piston by a predetermined amount thereby forming an open-ended cavity bounded on three sides by the piston, the piston sleeve, and the tool housing. Working fluid is disposed within the tool and is displaced through intricate flow passages within the tool when the piston moves relative to the tool housing. Mud solids enter the open-ended cavity when the piston moves; however, since the cavity is open-ended, that is, bounded on three sides only, the mud solids fall out of the open end portion of the cavity in response to either or both of the force of gravity and a downward movement of the piston, thereby preventing the operating mechanisms within the tool from becoming clogged and inoperative. 
     Further scope of applicability of the present invention will become apparent from the detailed description presented hereinafter. It should be understood, however, that the detailed description and the specific examples, while representing a preferred embodiment of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become obvious to one skilled in the art from a reading of the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the present invention will be obtained from the detailed description of the preferred embodiment presented hereinbelow, and the accompanying drawings, which are given by way of illustration only and are not intended to be limitative of the present invention, and wherein: 
     FIGS. 1a-1b illustrate prior art well tools wherein a piston was located internal to a tool housing thereby allowing mud solids to enter the housing when the piston moves relative to the housing while displacing working fluids; and 
     FIGS. 2a-2b illustrate the well tool design of the present invention wherein the piston is relocated to a position external to the tool housing. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1a to 1b, a prior art well tool is illustrated. 
     In FIG. 1a, a piston 10 is biased against a spring 12 and is movable within a chamber 14 bounded by a housing 15. A working fluid enters channel 16 and applies a pressure against a working surface 18 of the piston 10. If the hydrostatic pressure of the working fluid against the working surface 18 of piston 10 is greater than the pressure of the spring 12 against an opposite surface 20 of piston 10, the piston 10 moves downwardly, as shown in the drawings of FIGS. 1a and 1b. 
     Very often, mud solids exist in a borehole when the tool of FIGS. 1a and 1b is disposed downhole in the borehole. Because the piston 10 is disposed internally to the housing 15, when the piston 10 moves downwardly as shown in FIG. 1b, mud solids enter the channel 16 and rest against the working surface 18 of piston 10. When the piston attempts to move upwardly in response to the bias force of spring 12, as shown in FIG. 1a, the mud solids on the working surface 18 of piston 10 block the upward movement of piston 10 and thereby clog the operation of piston 10 when moving upwardly in chamber 14. As a result, the well tool of FIGS. 1a and 1b must be re-designed to avoid the clogging problem. 
     Referring to FIGS. 2a-2b, a well tool in accordance with the present invention is illustrated. 
     In FIGS. 2a-2b, the well tool comprises a piston 30 which is disposed externally to the housing 15, but internally to a piston sleeve 32. The piston 30 is bounded on three sides by housing 15, piston sleeve 32, and a portion 34 of the tool. A mandrel 36 is disposed inside the tool, defining a chamber 14 between the housing 15 and mandrel 36. The mandrel 36 is adapted to moves along a longitudinal axis of the tool in response to fluid pressure placed on surface 36a of mandrel 36. Since the piston 30 is not bounded on a fourth side, the piston 30 is disposed in an open-ended cavity joined by three wall surfaces consisting of housing 15, piston sleeve 32, and the portion 34 of the tool. The piston 30 communicates with chamber 14 via a channel 16. A working fluid is disposed in channel 16 and chamber 14. The piston 30 is adapted for moving within the open-ended cavity joined by the said three wall surfaces (15, 32, and 34). The tool of FIGS. 2a and 2b has a side (a), and a side (b), side (a) being disposed above side (b) in the borehole of an oil well. In FIG. 2b, mud solids 38 are disposed in the open-ended cavity. 
     In operation, referring to FIGS. 2a and 2b of the drawings, the tool of FIGS. 2a and 2b is disposed in the borehole of an oil well with side (a) of the tool being disposed above side (b) of the tool in the borehole. As a result, the open-end of the open-ended cavity on the tool, formed of the housing 15, the piston sleeve 32, and the tool portion 34, faces downwardly, into the borehole. When the piston 30, of FIG. 2a, moves from right to left in the figure, working fluid in channel 16 is displaced into chamber 14. A mandrel 36 begins to move in response to displacement of the working fluid from channel 16 into chamber 14. As shown in FIG. 2b, mandrel 36 has moved (from left to right) in response to movement of piston 30. However, as shown in FIG. 2b, mud solids 38 have entered the open-ended cavity bounded by piston sleeve 32, tool portion 34, and housing 15. Since the side (b) of the tool is disposed below side (a) of the tool, when the piston 30 moves in the opposite direction, from left to right as shown in FIG. 2a, the piston 30 pushes the mud solids 38 out of the open-ended cavity. The mud solids 38 exit the open-ended cavity in response to movement of piston 30 and in response to the downward force of gravity on the mud solids 38. Therefore, since the piston 30 is disposed external to the housing 15 of the tool (in lieu of internally, as in the prior art tool) and within an open-ended cavity, the contents of which are responsive to the downward force of gravity, the mud solids do not clog the operation of piston 30 when such piston 30 moves down and up in the borehole (left and right in the drawing figures), while displacing the working fluids within the channel 16 and chamber 14 of the tool. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the are intended to be included within the scope of the following claims.