Patent Publication Number: US-8534369-B2

Title: Drill string flow control valve and methods of use

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional patent application No. 61/294,402, filed Jan. 12, 2010, the entire disclosure of which is incorporated herein by reference. 
     This application is related to U.S. provisional patent application No. 60/793,883, filed Apr. 21, 2006; U.S. utility patent application Ser. No. 11/788,660, filed Apr. 20, 2007, now U.S. Pat. No. 7,584,801; U.S. utility patent application Ser. No. 12/432,194, filed Apr. 29, 2009; and U.S. utility patent application Ser. No. 12/609,458, filed Oct. 30, 2009, the entire disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     This disclosure generally relates to drill string flow control valves and more particularly, drill string flow control valves for prevention of u-tubing of fluid flow in drill strings and well drilling systems. 
     Managed Pressure Drilling (MPD) and Dual Gradient Drilling are oilfield drilling techniques that often utilize a higher density of drilling mud inside the drill string and a lower density return mud path on the outside of the drill string. 
     In Dual Gradient Drilling, an undesirable condition called “u-tubing” can result when the mud pumps for a drilling system are stopped. Mud pumps are commonly used to deliver drilling mud into the drill string and to extract return mud from the wellbore and a return riser (or risers). In a typical u-tubing scenario, fluid flow inside a drill string may continue to flow, even after the mud pumps have been powered down, until the pressure inside the drill string is balanced with the pressure outside the drill string, e.g., in the wellbore and/or a return riser (or risers). This problem is exacerbated in those situations where a heavier density fluid precedes a lighter density fluid in a drill string. In such a scenario, the heavier density fluid, by its own weight, can cause continued flow in the drill string even after the mud pumps have shut off. This u-tubing phenomenon, can result in undesirable well kicks, which can cause damage to a drilling system. For this reason, it is desirable that when mud pumps in a drilling system are turned off, the forward fluid flow be discontinued quickly. 
     Drill string flow control valves or flow stop valves are sometimes used to control flow in a downhole tubular, which may be, or form part of, a drill string. Some drill string flow control valves utilize the pressure differential between certain pressure ports positioned along the primary flow path of the valve to apply pressure to a valve sleeve within a valve housing to cause actuation of the valve sleeve. Movement of the valve sleeve, in turn, opens or closes the main drilling fluid flow ports within the valve. In prior art valves, at least two know drawbacks exist. First, to open the sleeve, significant forces maintaining the sleeve in a closed position must initially be overcome. Second, a rapid opening of the sleeve can cause a significant pressure drop in the valve. Thus, in some flow control valves, in order to overcome the significant forces maintaining the sleeve in a closed position, a solid piston is used to slowly initiate movement of the valve sleeve. As the valve sleeve of a prior art flow control valve is initially urged into the open position by the solid piston, flow through the main flow ports of the flow control valve begins. With respect to pressure drops within the valve, those skilled in the art will understand that because the main flow ports are relatively large, as they begin to open, just a small amount of movement of the valve sleeve can cause a drop in pressure as the ports open. For this reason, the solid piston described above is also desirable because it permits the valve sleeve to be opened slowly, thereby minimizing pressure drop. However, by slowly opening the main flow ports utilizing such a solid piston, the fluid flow passing through the ports is maintained at a high pressure, thereby causing potential washout of the flow ports, i.e., the high velocity of the fluid passing through the partially-open main flow ports will corrode or wash away the steel from which such flow control valves and main flow ports are typically fabricated. 
     SUMMARY 
     This disclosure generally relates to drill string flow control valves and more particularly, drill string flow control valves for prevention of u-tubing of fluid flow in drill strings and well drilling systems. 
     One example of a drill string flow control valve utilizes a piston with a flow passage therethrough to initiate movement of a valve sleeve within a flow control valve. The flow passage communicates fluid through the piston and into the interior of the valve sleeve, thereby bleeding off pressure from the fluid passing through the primary flow ports as the valve sleeve is initially opened. Thus, initially, drilling fluid flow through the valve sleeve is via the bore through the piston. As the valve sleeve continues to crack open, flow through the main flow ports begins. This permits a greater degree of control of flow through the main flow ports and minimizes the pressure drop associated with the prior art. In one preferred embodiment, part or all of the piston components are formed of a material, such as tungsten carbine, that is harder than, i.e., a higher Rockwall hardness factor, the material used to fabricate the rest of the valve (usually steel). 
     In one embodiment of the invention, a ball valve is disposed to control flow through the flow passage of the piston. Preferably, the ball valve comprises a ball and a ball seat disposed between a piston pressure port and a piston pressure surface. As pressure on the ball is increased, the ball engages the piston pressure surface and urges the piston against the valve sleeve, thereby initiating “opening” of the valve sleeve and main flow ports. At the same time, flow past the ball through the flow passage and into the interior of the valve sleeve reduces pressure at the primary sleeve flow ports. A biasing element may be used to urge the ball valve into the valve seat, i.e., the closed position. Those skilled in the art will appreciate that by altering the force of the biasing element on the ball, pressure at which movement of the ball initiates, and hence, operation of the overall flow control valve, can be adjusted as desired. Increasing pressure urges the ball out of the seat, and flow passes around the ball into the bore of the piston. Because the ball has a comparatively small surface area and there is little friction on the ball, a lower pressure can be used to open the ball valve. 
     The ball seat can simply be a ring with a bore therethrough and edges chamfered or otherwise shaped to mate with the profile of the ball. A snap ring may be used to secure the ball seat in place within the port used to direct a portion of the flow through the piston. 
     In one embodiment, a plug body with an axial bore has the piston axially mounted in the plug body. The ball seat mounts in the axial bore of the plug. The axial bore forms the flow port to the piston. 
     In one embodiment, a filter type lockdown nut is used to secure the ball seat in place within the port. The lockdown nut has a bore therethrough which opens to the end of the nut. A first end of the nut is provided with a plurality of apertures to allow flow into the bore. 
     In any event, the arrangement of the invention permits a slow, controlled increase in the flow rate through the small piston to create sufficient pressure differential to begin to open the main flow ports of the valve sleeve. 
     In one example, a drill string flow control valve comprises a valve housing characterized by a wall defining a valve interior, wherein the valve housing has an internal housing flow path formed therein with a housing outlet flow port disposed along said internal housing flow path; a valve sleeve disposed at least partially in the interior of the valve housing, the valve sleeve characterized by a first end and a second end and a wall defining a sleeve interior, a first sleeve flow port defined within the valve sleeve wall, and a second sleeve flow port defined within the valve sleeve wall adjacent said first end, wherein the valve sleeve is axially movable within the valve housing between a closed position and an open position, such that the valve sleeve wall substantially impedes fluid flow from the housing outlet flow port to the first sleeve flow port when the valve sleeve is in the closed position and wherein the first sleeve flow port and the housing outlet flow port are in substantial alignment when in the open position; wherein the valve sleeve has an upper pressure surface defined thereon so as to provide a first surface area upon which a first fluid pressure from the internal housing flow path may act to provide a downward force on the valve sleeve and wherein the valve sleeve has a lower pressure surface defined thereon so as to provide a second surface area upon which a second fluid pressure may act to provide an upward force on the valve sleeve; a spring wherein the spring biases the valve sleeve to the closed position by exertion of a biasing force on the valve sleeve; an upper pressure port in fluid communication with said internal housing flow path, said upper pressure port disposed to allow the first fluid pressure to act upon the upper pressure surface; a lower pressure port that allows the second fluid pressure to act upon the lower pressure surface; a piston having a first end and a second end and axially movable within the valve housing, said piston further characterized by a flow passage therethrough, wherein the second end of the piston is adjacent one end of the valve sleeve to permit fluid communication between said piston flow passage and said second sleeve flow port and wherein the first end of the piston has a piston pressure surface characterized by a piston surface area; and a piston pressure port in fluid communication with the internal housing flow path that allows a fluid pressure internal to the valve to act upon the piston pressure surface, said piston pressure port in fluid communication with said piston flow passage The drill string flow control valve may include a ball and a ball seat disposed between the piston pressure port and the piston pressure surface. A biasing element, such as a spring, may be disposed to urge the ball into contact with the ball seat. Another example of a drill string flow control valve comprises a valve housing, wherein the valve housing is characterized by a cylindrical wall extending from a first end to a second end and defining a valve interior, wherein the valve housing has an internal housing flow path channel formed between said first and second ends with a housing outlet flow port disposed along said flow path channel; a valve sleeve disposed at least partially in the valve housing, the valve sleeve characterized by a valve sleeve wall defining a valve sleeve interior, said valve sleeve having a first sleeve flow port defined within said wall and a second sleeve flow port defined within said wall, wherein the valve sleeve is axially movable within the valve housing between a closed position and an open position, such that fluid flow between said housing outlet flow port and said first sleeve flow port is substantially impeded when the valve sleeve is in the closed position and wherein the first sleeve flow port and the housing outlet flow port are substantially aligned when in the open position; wherein the valve sleeve has a first pressure surface defined thereon so as to provide a first surface area upon which a first fluid pressure from the housing flow path channel may act to provide a downward force on the valve sleeve, and wherein the valve sleeve has a second pressure surface defined thereon so as to provide a second surface area upon which a second fluid pressure may act to provide an upward force on the valve sleeve; a biasing mechanism wherein the biasing mechanism biases the valve sleeve to the closed position; a first pressure channel that allows the first fluid pressure to act upon the first pressure surface; a second pressure channel that allows the second fluid pressure to act upon the second pressure surface; an elongated piston having a first end, an internal bore and a second end open to said internal bore, said piston axially movable within the valve housing, wherein said second open end is in fluid communication with said second sleeve flow port; and a piston pressure in fluid communication with the internal housing flow path, said piston pressure port in fluid communication with said internal bore of said piston. 
     An example of a method for controlling flow in a downhole tubular comprises restricting flow through the downhole tubular by closing a flow stop valve when a difference between a first fluid pressure outside the downhole tubular and a second fluid pressure along a primary flow path within inside the downhole tubular at the flow stop valve is below a threshold value; and permitting flow through along the primary flow path of the downhole tubular by opening the flow stop valve when a difference between the first fluid pressure outside the downhole tubular and the second fluid pressure inside the downhole tubular at the flow stop valve is above a threshold value, wherein said flow stop valve is opened by: introducing drilling fluid into the valve to induce a pressure applied to the pressure surface of a piston, thereby causing said piston to urge a valve sleeve from a closed position; directing a portion of said drilling fluid through said piston and into the interior of said valve sleeve to establish initial flow through said valve; directing another portion of said drilling fluid against said valve sleeve to apply a fluid pressure on the valve sleeve; and increasing the fluid pressure upon the valve sleeve so as to cause the valve sleeve to axially move against the biasing direction of a spring, thereby increasing fluid flow through said valve sleeve. 
     Another example of a method for controlling flow in a downhole tubular comprises providing a valve housing, wherein the valve housing is characterized by a tubular wall extending from a first end to a second end and defining a valve interior, wherein the valve housing has an internal housing flow path formed between said first and second ends with a housing outlet flow port disposed along said internal flow path; providing a valve sleeve disposed at least partially in the valve housing, the valve sleeve having at least two pressure surfaces and axially movable within the valve housing between a closed position and an open position, providing a piston having a flow passage therethrough within the valve housing and bearing against the valve sleeve; biasing the valve sleeve under a biasing force in a first direction against the piston so as to close the valve; introducing drilling fluid into the valve housing to induce a first fluid pressure therein; applying said first fluid pressure to the piston pressure surface, thereby causing said piston to urge the valve sleeve in a second direction opposite the first direction; directing a portion of the drilling fluid to flow through said piston flow passage and into the interior of said valve sleeve to initiate flow; applying a fluid pressure from within the valve housing to a first surface of the valve sleeve to generate a first force to urge the valve sleeve in the second direction; applying a second fluid pressure derived from downstream of said first fluid pressure to a second surface of the valve sleeve to generate a second force to urge the valve sleeve in the first direction; maintaining a drilling fluid flow through the valve sleeve so that the first force is greater than the biasing spring force plus the second force; and decreasing the fluid flow through the valve sleeve so as to allow the biasing force to shift the valve sleeve in the first direction, thereby urging the valve into a closed position. 
     An example of a drill string flow control valve system comprises a valve housing, wherein the valve housing is characterized by a tubular wall extending from a first end to a second end and defining a valve interior, wherein the valve housing has an internal housing flow path formed between said first and second ends with a housing outlet flow port disposed along said internal flow path; a valve sleeve disposed at least partially in the valve housing, the valve sleeve having a first end and a second end and characterized by a valve sleeve wall extending between said first and second ends to define a valve sleeve interior, said valve sleeve having a first flow port disposed in said valve sleeve wall and a second flow port at said first end, wherein the valve sleeve is axially movable within the valve housing between a closed position and an open position, such that fluid flow between said housing outlet flow port and said first flow port is substantially impeded when the valve sleeve is in the closed position and wherein the first flow port and the housing outlet flow port are substantially aligned when in the open position; wherein the valve sleeve has an upper pressure surface defined thereon so as to provide a first surface area upon which a first fluid pressure from the internal housing flow path may act to provide a downward force on the valve sleeve, and wherein the valve sleeve has a lower pressure surface defined thereon so as to provide a second surface area upon which a second fluid pressure may act to provide an upward force on the valve sleeve; a spring, wherein the spring biases the valve sleeve to the closed position by exertion of a biasing force on the valve sleeve; an upper pressure port disposed internally to said valve housing between said sleeve flow port and the second end of said valve sleeve, said upper pressure port in fluid communication with the upper pressure surface, said upper pressure port disposed to allow the first fluid pressure to act upon the upper pressure surface, wherein the first fluid pressure is measured from adjacent the first end of the valve housing; a lower pressure port disposed internally to said valve housing so as to allow the second fluid pressure to act upon the lower pressure surface, wherein the second fluid pressure is measured from adjacent the second end of the valve housing; an upper pressure port that allows the first fluid pressure to act upon the first pressure surface; a lower pressure port that allows the second fluid pressure to act upon the second pressure surface; an elongated piston having a first end, an internal bore and a second end open to said internal bore, said piston axially movable within the valve housing, wherein the second end of the piston is adjacent an end of the valve sleeve and in fluid communication with the second flow port of said valve sleeve, and wherein the first end of the piston has a piston pressure surface characterized by a piston surface area; and a piston pressure port in fluid communication with said internal housing flow path that allows a fluid pressure internal to the valve to act upon the piston pressure surface, said piston pressure port in fluid communication with said piston internal bore, wherein the valve sleeve further comprises a flow restriction in the valve sleeve interior, wherein said lower pressure port is disposed in the wall of the valve sleeve below the flow restriction and the upper pressure port is disposed in the wall of the valve sleeve above the flow restriction. 
     Another example of a drill string flow control valve system comprises a valve housing formed of a tubular member extending from a first end to a second end and characterized by an external surface, said tubular member having a first flow path internally disposed therein; a valve sleeve slidingly mounted in the valve housing, said valve sleeve having a first end, a first flow port, a second flow port, a valve sleeve interior and a second end; a piston having a first end, an internal piston bore and a second open end in fluid communication with said piston bore, said piston slidingly mounted in the valve housing between said first end of the tubular member and said valve sleeve, wherein the second end of the piston is disposed to urge the valve sleeve axially relative to the valve housing, wherein said second open end of said piston is in fluid communication with the second flow port of said valve sleeve; a piston pressure port in fluid communication with said first internal housing flow path, said piston pressure port also in fluid communication with the piston bore; a ball and ball seat disposed along said piston pressure port; a first biasing mechanism disposed to urge said piston against said ball and to urge said ball into contact with said ball seat; a second biasing mechanism for biasing the valve sleeve against the piston; a first pressure port in the valve sleeve, said first pressure port in fluid communication with said internally disposed first flow path, said first pressure port in fluid communication with a first surface of the sleeve to provide a pressure acting on the first surface of the sleeve; and a second pressure port in fluid communication with a second surface of the sleeve to provide a second fluid pressure acting on the second surface of the sleeve, said second fluid pressure derived from adjacent the second end of said valve housing. 
     An example of a drill string flow stop valve comprises a tubular housing having an external surface and a first flow path internally disposed therein and an internal flow port disposed along said flow path; a hollow tubular section slidingly mounted in the valve housing and movable between a first position and a second position thereby establishing a second flow path in the interior of the hollow tubular section, wherein the hollow tubular section substantially impedes fluid flow through the internal flow port to an interior of the hollow tubular section when the valve sleeve is in the first position and wherein fluid flow through the internal flow port to the interior of the hollow tubular section is permitted when the valve sleeve is in the second position; a biasing mechanism for biasing the hollow tubular section toward the first position; a first vent in fluid communication with the internally disposed first flow path, said first vent in fluid communication with a first pressure chamber; a second vent in fluid communication with a second pressure chamber which is separate from the first pressure chamber, said second vent in fluid communication with the second flow path; an elongated piston having a first end, an internal bore and a second end open to said internal bore, wherein said second open end is in fluid communication with the interior of said hollow tubular section; and a third vent in fluid communication with the internally disposed first flow path, said third vent in fluid communication with said internal bore of said elongated piston. 
     In another improvement over the prior art, it has been found that flow control valves that utilize a jet or flow restriction disposed within the valve sleeve can position the first pressure channel (or upper pressure port or first pressure port) in the wall of the valve sleeve above the flow restriction as opposed to locating the first pressure channel outside the valve sleeve. A second pressure channel (or lower pressure port or second pressure port) is located downstream of the flow restriction. Although not necessary for use with embodiments of a flow control valve utilizing a small piston as described above, this arrangement is particularly beneficial in embodiments of a flow control valve utilizing a small piston since the initial flow through the small piston establishes fluid flow through the valve sleeve and restriction. The fluid has a first pressure above the restriction and a second pressure below the restriction. This pressure difference can be utilized to continue to open the valve as described in the prior art. However, the need for separate or complicated flow channels formed outside the valve sleeve, such as in the mandrel of the flow control valve, is eliminated. For fabrication purposes and simplification of manufacture and costs thereof, it is much easier to create flow ports that simply extend through the wall of the valve sleeve. 
     An example of a drill string flow control valve system comprises a valve housing, wherein the valve housing is characterized by a tubular wall extending from a first end to a second end and defining a valve interior, wherein the valve housing has an internal housing flow path formed between said first and second ends with a housing outlet flow port disposed along said internal flow path; a valve sleeve disposed at least partially in the valve housing, the valve sleeve having a first end and a second end and characterized by a valve sleeve wall extending between said first and second ends to define a valve sleeve interior, said valve sleeve having a first flow port disposed in said valve sleeve wall and a second flow port at said first end, wherein the valve sleeve is axially movable within the valve housing between a closed position and an open position, such that fluid flow between said housing outlet flow port and said first flow port is substantially impeded when the valve sleeve is in the closed position and wherein the first flow port and the housing outlet flow port are substantially aligned when in the open position; wherein the valve sleeve has an upper pressure surface defined thereon so as to provide a first surface area upon which a first fluid pressure from the internal housing flow path may act to provide a downward force on the valve sleeve, and wherein the valve sleeve has a lower pressure surface defined thereon so as to provide a second surface area upon which a second fluid pressure may act to provide an upward force on the valve sleeve; a spring, wherein the spring biases the valve sleeve to the closed position by exertion of a biasing force on the valve sleeve; an upper pressure port disposed internally to said valve housing between said sleeve flow port and the second end of said valve sleeve, said upper pressure port in fluid communication with the upper pressure surface, said upper pressure port disposed to allow the first fluid pressure to act upon the upper pressure surface, wherein the first fluid pressure is measured from adjacent the first end of the valve housing; a lower pressure port disposed internally to said valve housing so as to allow the second fluid pressure to act upon the lower pressure surface, wherein the second fluid pressure is measured from adjacent the second end of the valve housing; an upper pressure port that allows the first fluid pressure to act upon the first pressure surface; a lower pressure port that allows the second fluid pressure to act upon the second pressure surface; an elongated piston having a first end, an internal bore and a second end open to said internal bore, said piston axially movable within the valve housing, wherein the second end of the piston is adjacent an end of the valve sleeve and in fluid communication with the second flow port of said valve sleeve, and wherein the first end of the piston has a piston pressure surface characterized by a piston surface area; and a piston pressure port in fluid communication with said internal housing flow path that allows a fluid pressure internal to the valve to act upon the piston pressure surface, said piston pressure port in fluid communication with said piston internal bore, wherein the valve sleeve further comprises a flow restriction in the valve sleeve interior, wherein said lower pressure port is disposed in the wall of the valve sleeve below the flow restriction and the upper pressure port is disposed in the wall of the valve sleeve above the flow restriction. The system may further have an elongated piston having a first end, an internal bore and a second end open to said internal bore, the piston axially movable within the valve housing, wherein the second end of the piston is adjacent an end of the valve sleeve and in fluid communication with the second flow port of said valve sleeve, and wherein the first end of the piston has a piston pressure surface characterized by a piston surface area; and a piston pressure port in fluid communication with said internal housing flow path that allows a fluid pressure internal to the valve to act upon the piston pressure surface. In this embodiment, the piston pressure port is in fluid communication with the piston internal bore. 
     In another embodiment, the flow restriction or jet can be interchangeable so as to permit the flow rate and the desired pressure drop across the flow restriction to be adjusted (and thereby adjust operating pressures for the valve). For example, a restriction may be formed by providing a ring with a bore through the ring that narrows from one end to the other end of the ring. The dimensions of the bore can be altered to adjust the pressure drops. The ring may be interchangeable with others and secured in place within the annulus of the valve sleeve by a snap ring or similar fastener. As described above, while most beneficial in flow stop valves utilizing a small piston that engages a valve sleeve, the arrangement of a flow restriction in a valve sleeve bounded by an upper and lower pressure port would also be beneficial in flow stop valves without such a piston. 
     The features and advantages of this disclosure will be apparent to those skilled in the art. While numerous changes may be made by those skilled in the art, such changes are within the spirit of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of this disclosure and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying figures, wherein: 
         FIG. 1  illustrates a cross-sectional view of a drill string flow control valve according to an exemplary embodiment, the drill string flow control valve being in a closed position and including a valve housing, a plug and a lockdown nut. 
         FIG. 2  illustrates an elevational view of a portion of the drill string flow control valve of  FIG. 1 , according to an exemplary embodiment, the portion omitting the valve housing of  FIG. 1 . 
         FIG. 3  illustrates a top plan view of the portion of the drill string flow control valve of  FIG. 2 , according to an exemplary embodiment. 
         FIG. 4A  illustrates an enlarged view of a portion of  FIG. 1 , according to an exemplary embodiment. 
         FIG. 4B  illustrates an enlarged view of another portion of  FIG. 1 , according to an exemplary embodiment. 
         FIG. 5  illustrates a perspective view of the plug of  FIG. 1 , according to an exemplary embodiment. 
         FIG. 6  illustrates a cross-sectional view of the plug of  FIG. 5 , according to an exemplary embodiment. 
         FIG. 7  illustrates a perspective view of the lockdown nut of  FIG. 1 , according to an exemplary embodiment. 
         FIG. 8  illustrates a cross-sectional view of the lockdown nut of  FIG. 7 , according to an exemplary embodiment. 
         FIG. 9  illustrates a view similar to that of  FIG. 1 , but depicts the drill string flow control valve of  FIG. 1  in an open position, according to an exemplary embodiment. 
         FIG. 9A  illustrates an enlarged view of a portion of  FIG. 9 , according to an exemplary embodiment. 
         FIG. 10  illustrates a cross-sectional view of a portion of a drill string flow control valve, according to another exemplary embodiment. 
     
    
    
     While this disclosure is susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the appended claims. 
     DETAILED DESCRIPTION 
     This disclosure generally relates to drill string flow control valves and more particularly, drill string flow control valves for prevention of u-tubing of fluid flow in drill strings and well drilling systems. 
     Drill string flow control valves are provided herein that, among other functions, can be used to reduce and/or prevent u-tubing effects in drill strings. 
     To facilitate a better understanding of this disclosure, the following examples of certain embodiments are given. In no way should the following examples be read to limit, or define, the scope of the disclosure. 
     For ease of reference, the terms “upper,” “lower,” “upward,” and “downward” are used herein for convenience only to identify various components and refer to the spatial relationship of certain components, regardless of the actual orientation of the flow control valve. The term “axial” refers to a direction substantially parallel to the drill string in proximity to a drill string flow control valve. 
     In an exemplary embodiment, as illustrated in  FIGS. 1 ,  2  and  3 , a drill string flow control valve is generally referred to by the reference numeral  10  and includes a mandrel or valve housing  12  having an upper end  12   a  and a lower end  12   b , and is characterized by a housing wall  12   c  extending therebetween so as to define an interior  14  of the valve  10  extending from the upper end  12   a  to the lower end  12   b . The valve housing  12  has an internal housing flow path  16  formed therein for the flow of drilling fluids and the like through the valve  10 . The valve housing  12  further includes an internal threaded connection  12   d  proximate the upper end  12   a , and an internal threaded connection  12   e  proximate the lower end  12   b . It will be appreciated that flow path  16  includes a primary portion, which is the path along which the largest volume of fluid flows when valve  10  is fully open. 
     A plug  18  having a varying-diameter tubular wall  18   a  is disposed within the interior  14 . A plurality of axially-extending flow bores  18   b  are defined in a flanged portion  18   aa  of the tubular wall  18   a . A plurality of housing outlet flow ports  19  is defined in the tubular wall  18   a . Although the valve housing  12  and the plug  18  are shown here as two or more components, in several exemplary embodiments, these components may be formed as one integral piece such that the plug  18  is simply a part of the valve housing  12 . Moreover, the plug  18  may be considered to be part of the valve housing  12 , regardless of whether the valve housing  12  and the plug  18  are formed as one integral piece or are two or more components. In this particular embodiment, a plug is preferred because it obviates the need to bore internal flow channels in the valve housing. Rather, internal flow channels, such as internal housing flow path  16 , can be defined between or by the engagement of plug  18  and valve housing  12 , such as by an annulus that may be defined when plug  18  is engaged with valve housing  12 . In any event, the axially-extending flow bores  18   b  and the housing outlet flow ports  19  form part of the flow path  16 . A lockdown nut  20  is connected to the upper end portion of the plug  18 . In an exemplary embodiment, the lockdown nut  20  is a filter-type lockdown nut. A lock nut  22  is engaged with the lower end portion of the plug  18 . 
     A valve sleeve  24  is disposed within the interior  14 . The valve sleeve  24  is axially slidable or movable within the valve housing  12 . In an exemplary embodiment, the valve sleeve  24  may be partially disposed within a portion of the plug  18 , as shown in  FIG. 1 . The valve sleeve  24  is characterized by an upper end  24   a  and a lower end  24   b , and a valve sleeve wall  24   c  extending therebetween and defining a sleeve interior  24   d . The sleeve interior  24   d  forms part of the flow path  16 . A plurality of sleeve flow ports  24   e  is defined in the valve sleeve wall  24   c . The sleeve flow ports  24   e  form part of the flow path  16 . In an exemplary embodiment, the sleeve flow ports  24   e  are substantially radially formed in the valve sleeve wall  24   c . A sleeve flow port  24   f  is defined in the valve sleeve wall  24   c  adjacent the upper end  24   a . In an exemplary embodiment, the sleeve flow port  24   f  is substantially axially formed in the valve sleeve wall  24   c . A flange  24   g  may be formed on valve sleeve  24 . The flange  24   g  defines thereon an first pressure surface  24   h  so as to provide a surface area upon which a fluid pressure from the flow path  16  may act to provide a downward force on the valve sleeve  24 , under conditions to be described below. The flange  24   g  also defines thereon a second pressure surface  24   i  so as to provide another surface area upon which a fluid pressure may act to provide an upward force on the valve sleeve  24 , under conditions to be described below. An annular portion  24   j  extends radially inwardly from the valve sleeve wall  24   c . While flange  24   g  is described as a single component, those skilled in the art will appreciate that separate projections or surfaces extending from sleeve  24  may be utilized so long as they provide the pressure surfaces as described herein. One or more sealing elements  24   l , such as o-rings and o-ring grooves, may be positioned along the length of sleeve  24  so as to form a seal between sleeve  24  and valve housing  12  (and/or plug  18 , as the case may be). 
     A jet or flow restriction  26  may be disposed within the sleeve interior  24   d . Although flow restriction  26  may be located anywhere along the interior  24   d  of sleeve  24 , in a preferred embodiment, flow restriction  26  is positioned adjacent the lower end of the annular portion  24   j  of the valve sleeve  24 . A snap ring  28  is disposed within the sleeve interior  24   d  and is engaged with the valve sleeve wall  24   c . The flow restriction  26  is axially positioned between the annular portion  24   j  and the snap ring  28 . In an exemplary embodiment, the flow restriction  26  may be formed by providing a ring with a bore therethrough that narrows from one end to the other end of the ring. In several exemplary embodiments, the flow restriction  26  may be interchangeable with other jets or flow restrictions and secured in place within the sleeve interior  24   d  by the snap ring  28 , other snap ring(s), or similar fastener(s). 
     An external threaded connection  30   a  at one end of a sub  30  is engaged with the internal threaded connection  12   e  of the valve housing  12 , thereby connecting the sub  30  to the valve housing  12 . The sub  30  defines an upper end surface  30   b , and an interior  30   c , which, in several exemplary embodiments, forms part of the flow path  16 . The sub  30  further includes an external threaded connection  30   d  at the other end thereof, and an internal shoulder  30   e.    
     A variable-volume pressure chamber  32  is defined adjacent pressure surface  24   i . In one embodiment, pressure chamber  32  is an annular region formed between the inside surface of the valve housing wall  12   c  of the valve housing  12 , and the outside surface of the valve sleeve wall  24   c  of the valve sleeve  24 . The annular region  32  is axially defined between the lower pressure surface  24   i  of the valve sleeve  24 , and a location at least proximate the upper end surface  30   b  of the sub  30 . A coil sleeve spring  34  is disposed within the annular region  32  so that the valve sleeve wall  24   c  extends through the sleeve spring  34  and the coils of the sleeve spring  34  extend circumferentially about the valve sleeve wall  24   c . The valve sleeve  24  is biased upwards by the sleeve spring  24 . In several exemplary embodiments, instead of, or in addition to, the coil sleeve spring  34 , one or more other biasing mechanisms may be disposed in the annular region  32  to thereby bias the valve sleeve  24  upwards. 
     One or more pressure fluid ports or vents  36  are in fluid communication the flow path  16 . The pressure fluid ports  36  are preferably bled off from an upper portion of flow path  16 . In an exemplary embodiment, as shown in  FIG. 1 , the upper pressure fluid ports  36  are formed in the valve sleeve wall  24   c . Pressure fluid ports  36  are positioned above flow restriction  26  in those embodiments in which a flow restriction  26  is provided. A variable-volume pressure chamber  38  is defined adjacent pressure surface  24   h . In one embodiment, pressure chamber  38  is an annular region defined between the inside surface of the valve housing wall  12   c  of the valve housing  12 , and the outside surface of the valve sleeve wall  24   c  of the valve sleeve  24 . The annular region  38  is axially defined between the lower end of the lock nut  22  and the upper pressure surface  24   h  of the valve sleeve  24 . Via the upper pressure fluid ports  36 , the annular region  38  is in fluid communication with the sleeve interior  24   d  and thus with the flow path  16 . 
     At least one lower pressure fluid port or vent  40  is in fluid communication with the sleeve interior  24   d  and thus with the flow path  16 . In an exemplary embodiment, the lower pressure fluid port  40  is formed in the valve sleeve wall  24   c . Via the lower pressure fluid port  40 , the annular region  32  is in fluid communication with the sleeve interior  24   d  and thus with the flow path  16 . In several exemplary embodiment, instead of, or in addition to, the lower pressure fluid port  40 , one or more other lower pressure fluid ports identical to the lower pressure fluid port  40  may be formed in the valve sleeve wall  24   c  below the lower pressure surface  24   i  of the valve sleeve  24  at different axial positions therealong. 
     A piston  42  is disposed within the plug  18  and thus within the interior  14 . The piston  42  is axially slidable or movable within the plug  18  and thus within the valve housing  12 . In an exemplary embodiment, as show in  FIG. 1 , at least a portion of the piston  42  engages the valve sleeve  24 . The valve  10  further includes a piston spring  44 , which is adapted to engage each of the piston  42  and the valve sleeve  24 . The piston  42  and the piston spring  44  will be described in further detail below. 
     In an exemplary embodiment, as illustrated in  FIGS. 4A and 4B  with continuing reference to  FIGS. 1 ,  2  and  3 , the piston  42  has an upper end  42   a  and a lower end  42   b , and is characterized by a piston flow passage  42   c  therethrough. The lower end  42   b  of the piston  42  is adjacent the upper end  24   a  of the valve sleeve  24  to permit fluid communication between the flow passage  42   c  and the sleeve flow port  24   f . The upper end  42   a  of the piston  42  has a piston pressure surface  42   d  characterized by a piston surface area. In an exemplary embodiment, the piston pressure surface  42   d  is a concave surface, as shown in  FIG. 4A . In an exemplary embodiment, the piston surface area of the piston pressure surface  42   d  is smaller than the surface area of the upper pressure surface  24   h  of the valve sleeve  24 . The piston  42  includes an elongated, cylindrical body  42   e  through which the flow passage  42   c  is formed. The cylindrical body  42   e  extends between the upper end  42   a  and the lower end  42   b . A flange  42   f  extends radially outwardly from, and thus circumferentially about, the cylindrical body  42   e . A lower surface  42   g  is defined by the flange  42   f . Axially-extending bores  42   h  are formed through the flange  42   f . The piston  42  is axially slidable or movable within the plug  18  and thus within the valve housing  12 . Flow ports  42   i  are formed in upper end  42   a  of the piston  42  to communicate with flow passage  42   c . One or more sealing elements  42   k , such as o-rings and o-ring grooves, may be positioned along the length of piston  42  so as to form a seal between piston  42  and plug  18 . 
     As shown in  FIG. 4B , an annular region  46  is defined around the outside surface of the cylindrical body  42   e  of the piston  42 . In one preferred embodiment, annular region  46  may be formed by an inside surface of the valve sleeve wall  24   c  of the valve sleeve  24 , and specifically, annular region  46  is axially defined between the lower pressure surface  42   g  of the flange  42   f  of the piston  42 , and an inside shoulder  24   k  formed in the valve sleeve wall  24   c  of the valve sleeve  24  at the end  24   a  thereof. In another embodiment, annular region  46  may be formed by an inside surface of plug  18  such that piston  42  simply abuts a shoulder  24   k  of valve sleeve  24 . Bores  42   h  permit flange  42   f  to slide within region  46  without impedance by fluid disposed in the interior of valve sleeve  24 . In any event, piston spring  44  is disposed within the annular region  46  so that the cylindrical body  24   e  extends through the piston spring  44  and the coils of the piston spring  44  extend circumferentially about the cylindrical body  24   e . Piston spring  44  may be a coil spring. The piston  42  is biased upwards by the piston spring  44 . In several exemplary embodiments, instead of, or in addition to, the piston spring  44 , one or more other biasing mechanisms may be disposed in the annular region  46  to thereby bias the piston  42  upwards. As shown in  FIG. 4B , the valve sleeve wall  24   c , and thus the valve sleeve  24 , is characterized by an outer diameter, and the cylindrical body  42   e  of the piston  42  is characterized by an outer diameter, which is smaller than the outer diameter of the valve sleeve  24 . 
     As shown in  FIG. 4A , a ball seat  48  is disposed within the plug  18 . A ball  50  is disposed within the plug  18  and between the ball seat  48  and the piston pressure surface  42   d . Since the piston  42  is biased upwards by the piston spring  44 , the piston spring  44  is thus disposed to urge the ball  50  into contact with the ball seat  48 . In an exemplary embodiment, the ball seat  48  includes a ring with a bore therethrough and edges chamfered or otherwise shaped to mate with the profile of the ball  50 . In an exemplary embodiment, a snap ring may be used to secure the ball seat  48  in place within the plug  18 . 
     In an exemplary embodiment, as illustrated in  FIGS. 4A ,  4 B,  5  and  6  with continuing reference to  FIGS. 1 ,  2  and  3 , the tubular wall  18   a  of the plug  18  further includes an upper end portion  18   ab  extending upward from the flanged portion  18   aa , a neck portion  18   ac  extending downward from the flanged portion  18   aa , and a body portion  18   ad  extending downward from the neck portion  18   ac . The plurality of housing outlet flow ports  19  is defined in the body portion  18   ad  of the tubular wall  18   a  of the plug  18 . A piston bore  18   c  is formed in plug  18  and thus through at least the upper end portion  18   ab , the flanged portion  18   aa , and the neck portion  18   ac . Piston bore  18   c  is disposed for receipt of a portion of cylindrical body  42   e , which is slidingly disposed therein. An axially-extending region  18   d , which may be part of the piston bore  18   c , is formed in the body portion  18   ad , and defines an upper surface  18   e  and an upper internal shoulder  18   f . A lower end  18   g  of the plug  18  engages the lock nut  22 . 
     As shown in  FIGS. 4A ,  5  and  6 , a piston pressure port or vent  52  is defined at the upper end portion  18   ab  of the plug  18 . The piston pressure port  52  is in fluid communication with the flow path  16  and is configured to allow a fluid pressure internal to the valve housing  12  and thus the valve  10  to act upon the piston pressure surface  42   d , under conditions to be described below. The piston pressure port  52  is in fluid communication with the piston flow passage  42   c . The ball seat  48  and the ball  50  are disposed between the piston pressure port  52  and the piston pressure surface  42   d , with the ball seat  48  being disposed between the piston pressure port  52  and the ball  50 , and the ball  50  being disposed between the ball seat  48  and the piston pressure port  52 . 
     In an exemplary embodiment, as illustrated in  FIGS. 7 and 8  with continuing reference to  FIGS. 1 ,  2 ,  3 ,  4 A,  4 B,  5  and  6 , the lockdown nut  20  includes a body  20   a  having an upper end  20   b , an internal bore  20   c  formed in the body  20   a , and a lower end  20   d  open to the internal bore  20   c . The lockdown nut  20  further includes a plurality of apertures  20   e  adjacent the upper end  20   b  and in fluid communication with the internal bore  20   c . An external threaded connection  20   f  is adjacent the lower end  20   d . As shown in  FIG. 4A , the lockdown nut  20  is disposed adjacent the piston pressure port  52  and secures the ball seat  48 . Apertures  20   e  permit fluid flow from the flow path  16  into piston flow passage  42   c.    
     In an exemplary embodiment, in order to resist the high pressure and flow rates that can cause wash out of sleeve flow ports  24   e , part or all of the piston  42  is formed of a material, such as tungsten carbide, that is harder than, i.e., has a Rockwell hardness factor that is higher than, the material used to fabricate the remainder of the valve  10  (usually steel). In an exemplary embodiment, the valve housing  12  or the valve sleeve  24  is manufactured of a material having a Rockwell hardness and the piston  42  is manufactured of another material having a Rockwell hardness higher than the Rockwell hardness of the material used to manufacture the valve housing  12  or the valve sleeve  24 . In an exemplary embodiment, the valve housing  12  and the valve sleeve  24  are manufactured of steel and the piston  42  is manufactured of tungsten carbide. 
     In operation, in an exemplary embodiment, with continuing reference to  FIGS. 1 ,  2 ,  3 ,  4 A,  4 B,  5 ,  6 ,  7  and  8 , the valve  10  is part of a downhole tubular, tubular string or casing, or drill string. A threaded end of a tubular support member (not shown) that defines an internal passage may be connected to the internal threaded connection  12   d  of the valve housing  12  so that the internal passage of the tubular support member is in fluid communication with the flow path  16 . Similarly, a threaded end of another tubular member (not shown) that defines an internal passage may be connected to the external threaded connection  30   d  of the sub  30  so that the internal passage of the other tubular member is in fluid communication with the flow path  16 . The valve  10  operates to control flow in the downhole tubular or drill string of which the valve  10  is a part, and can prevent u-tubing in the downhole tubular or drill string. 
     More particularly, the drill string of which the valve  10  is a part is positioned within a preexisting structure such as, for example, a wellbore that traverses one or more subterranean formations, thereby defining an annular region between the inside wall of the wellbore and the outside surface of the drill string. At this time, the valve  10  and thus the valve sleeve  24  may be in a closed position as shown in  FIGS. 1 ,  4 A and  4 B. 
     When the valve  10  and thus the valve sleeve  24  are in the closed position as shown in  FIGS. 1 ,  4 A and  4 B, the sleeve spring  34  biases the valve sleeve  24  upwards by exertion of a biasing force on the valve sleeve  24  so that the sleeve flow ports  24   e  are axially offset from the housing outlet flow ports  19 . As a result, in the closed position, the valve sleeve wall  24   c  covers the housing outlet flow ports  19  and thus substantially impedes any fluid flow from the housing outlet flow ports  19  to the corresponding sleeve flow ports  24   e . As another result, in the closed position, the upper end  24   a  of the valve sleeve  24  contacts or is at least proximate the internal shoulder  18   f  of the plug  18 . Moreover, in the closed position, the piston spring  44  biases the piston  42  upwards. As a result, in the closed position, the ball  50  is seated against the ball seat  48 . As another result, in the closed position, the flange  42   f  of the piston  42  is at least proximate the upper surface  18   e  of the plug  18 , as shown in  FIG. 4A . 
     In an exemplary embodiment, during or after the positioning of the drill string of which the valve  10  is a part within the wellbore, fluid flow through the valve  10  is restricted by placing the valve  10  and thus the valve sleeve  24  in the closed position described above, that is, closing the valve  10 , when a difference between a fluid pressure on the upper and lower pressure surfaces is below a threshold value. This difference in pressure causes the valve sleeve  24  to remain in the closed position, thereby substantially impeding any fluid flow from the housing outlet flow ports  19  to the corresponding sleeve flow ports  24   e , and vice versa. And this difference in pressure causes the piston  42  to remain upwardly biases, thereby urging the ball  50  upwards to seat the ball  50  against the ball seat  48  and substantially impeding any fluid flow past the ball  50 . 
     In an exemplary embodiment, during or after the positioning of the drill string of which the valve  10  is a part within the wellbore, fluid flow through the valve  10  is permitted by opening the valve  10 , that is, placing the valve  10  and thus the valve sleeve  24  in an open position from the above-described closed position, when a difference between the fluid pressure between the upper and lower pressure surfaces is above a threshold value. To so open the valve  10 , drilling fluid is introduced into the valve  10 , with the drilling fluid initially flowing downward past the upper end  12   a  of the valve housing  12 . As a result of introducing drilling fluid into the valve  10 , a pressure applied to the piston pressure surface  42   d  is induced, thereby causing the piston  42  to urge the valve sleeve  24  from the closed position. 
     As the pressure applied to the piston pressure surface  42   d  increases, the ball  50  is urged out of the ball seat  48 . In particular, the ball  50  pushes downward against the piston pressure surface  42   d , which causes the piston  42  to overcome the biasing force exerted by the piston spring  44 , thereby urging the piston  42  downward. In an exemplary embodiment, a relatively low pressure can be used to urge the ball  50  out of the ball seat  48  because the ball  50  has a comparatively small surface area and there is little friction on the ball  50 . Via the piston pressure port  52 , a portion of the drilling fluid is directed through the piston  42  and into the sleeve interior  24   d  of the valve sleeve  24 , thereby establishing an initial flow through the valve  10 . In particular, the portion of the drilling fluid flows through the apertures  20   e  of the lockdown nut  20 , through the bore  20   c , through the piston pressure port  52 , past the ball seat  48  and the ball  50 , through the flow ports  42   i  of the piston  42 , through the flow passage  42   c  of the piston  42 , and into the sleeve interior  24   d . Thus, initially, drilling fluid flow through the valve sleeve  24  occurs past the ball  50  and through the piston  42 . The flow of the drilling fluid through the apertures  20   e  filters the drilling fluid before the drilling fluid flows past the ball seat  48 , blocking any relatively large particles from flowing into or past the ball seat  48 . 
     Another portion of the drilling fluid flows through the upper pressure fluid ports  36  from the flow path  16 , entering the annular region  38  and contacting upper pressure surface  24   h  of the valve sleeve  24 . As a result, a downwardly-directed fluid pressure is applied on the upper pressure surface  24   h  of the valve sleeve  24 . 
     In an exemplary embodiment, as illustrated in  FIGS. 9 and 9A  with continuing reference to  FIGS. 1 ,  2 ,  3 ,  4 A,  4 B,  5 ,  6 ,  7  and  8 , once fluid flow has been initiated, the fluid pressure on the valve sleeve  24  is increased so as to cause the valve sleeve  24  to axially move against the biasing direction of the sleeve spring  34 , thereby increasing fluid flow through the valve sleeve  24 . In particular, as the downwardly-directed fluid pressure applied on the upper pressure surface  24   h  increases, the valve sleeve  24  moves axially downward, overcoming the biasing force exerted by the sleeve spring  34 . As the valve sleeve  24  continues to crack open, at least respective portions of the sleeve flow ports  24   e  increasingly overlap with respective portions of the housing outlet flow ports  19  and thus flow through the partially open flow ports  19  and  24   e  begins. In particular, as respective portions of the sleeve flow ports  24   e  increasingly overlap with respective portions of the housing outlet flow ports  19 , drilling fluid (off which the drilling fluid flowing through the piston  42  is split) flows along the primary portion of flow path  16 , that is, axially downward through the flow bores  18   b , between the outside surface of the neck portion  18   ac  of the plug  18  and the inside surface of the housing wall  12   c  of the valve housing  12 , between the outside surface of the body portion  18   ad  of the plug  18  and the inside surface of the housing wall  12   c  of the valve housing  12 , through the partially open flow ports  19  and  24   e , through the sleeve interior  24   d , through the flow restriction  26 , and through the interior  30   c  of the sub  30 . The foregoing permits a greater degree of control of fluid flow through the flow ports  19  and  24   e  and minimizes pressure drop. Moreover, by splitting the fluid flow so that a portion of the fluid flows through the piston  42  and another portion flows through the ports  19  and  24   e , the velocity of the fluid flowing through the partially open ports  19  and  24   e  is reduced, thereby reducing the risk that the partially open ports  19  and  24   e  will experience potential washout, i.e., the corroding or washing away of the material (such as steel) from which the housing  12 , the plug  18  and the sleeve  24  are typically fabricated. In accordance with the foregoing, in an exemplary embodiment, the flow rate of the drilling fluid flow through the piston  42  may be slowly increased to create a sufficient pressure differential to open the ports  19  and  24   e.    
     As shown in  FIGS. 9 and 9A , the valve sleeve  24  continues to axially move against the biasing direction of the sleeve spring  34 , thereby increasing fluid flow through the valve sleeve  24 , until the end  24   b  of the valve sleeve  24  contacts or, is at least proximate, the internal shoulder  30   e  of the sub  30 . At this point, the valve  10  and thus the valve sleeve  24  are in the open position in which the sleeve flow ports  24   e  and the corresponding housing outlet flow ports  19  are in substantial alignment, as shown in  FIGS. 9 and 9A . 
     In an exemplary embodiment, once fluid flow has been initiated, a fluid pressure, derived downstream of the fluid pressure applied to the upper pressure surface  24   h , is applied to the valve sleeve  24  to generate a force to urge the valve sleeve  24  upward. In particular, drilling fluid flows through the lower pressure fluid port  40 , entering the annular region  32  and contacting lower pressure surface  24   i  of the valve sleeve  24 . As a result, an upwardly-directed fluid pressure is applied on the lower pressure surface  24   i  of the valve sleeve  24 . When the valve  10  and thus the valve sleeve  24  are in the open position, the drilling fluid flow through the valve  10  is maintained so that the force urging the valve sleeve  24  downward is greater than the upwardly-directed biasing force exerted by the sleeve spring  34  plus the upwardly-directed force exerted by the fluid pressure against the lower pressure surface  24   i.    
     In an exemplary embodiment, whether or not flow control valve  10  includes a piston  42  as described herein, the upper pressure fluid ports  36  are positioned upstream of flow restriction  26  and the lower pressure port  40  is positioned downstream of flow restriction  26 . As a result, during the flow of the drilling fluid along the flow path  16 , the pressure differential across the flow restriction  26  can be utilized to facilitate control of valve sleeve  24 . In several exemplary embodiments, the dimensions of the flow restriction  26  can be altered to adjust pressure drops. If the flow restriction  26  includes a ring with a bore formed therethrough, the dimensions of the bore can be altered to adjust pressure drops, and the ring may be interchangeable with others and secured in place with the snap ring  28  or similar fastener. 
     In an exemplary embodiment, the valve  10  and thus the valve sleeve  24  may be placed back into the closed position shown in  FIGS. 1 ,  4 A and  4 B from the open position shown in  FIGS. 9 and 9A  by decreasing the downwardly-directed fluid flow through the valve  10  so as to allow the biasing force exerted by the sleeve spring  34  to shift the valve sleeve  24  upwards, thereby urging the valve sleeve  24  and thus the valve  10  into the closed position described above. 
     In an exemplary embodiment, as illustrated in  FIG. 10  with continuing reference to  FIGS. 1 ,  2 ,  3 ,  4 A,  4 B,  5 ,  6 ,  7 ,  8 ,  9  and  9 A, the lockdown nut  20  is omitted from the valve  10 . Additionally, a lock ring  54  is disposed in the piston pressure port  52 , and is connected to the plug  18 . The lock ring  54  secures the ball seat  48  in place. The operation of the valve  10  without the lockdown nut  20  but with the lock ring  54  is substantially identical to the above-described operation of the valve  10  with the lockdown nut  20 , except that, due to the omission of the lockdown nut  20 , the drilling fluid is not filtered by the lockdown nut  20  before flowing past the ball seat  48 . 
     In several exemplary embodiments, and as illustrated in at least  FIGS. 1 ,  2 ,  4 A,  4 B,  5 ,  6 ,  9 ,  9 A and  10 , optional seals are provided at the indicated locations to prevent or at least resist unwanted leakage of fluid and to prevent or at least resist unwanted communication of fluid pressures to undesired sites. In several exemplary embodiments, such optional seals may include annular grooves formed in outside surfaces of tubular walls and corresponding annular sealing elements disposed in the annular grooves, with the sealing elements sealingly engaging inside surfaces of tubular walls within which the tubular walls having the annular grooves respectively extend. Examples of such optional seals are referred to by the reference S in  FIG. 10 . 
     Although drill pipe threads have been depicted herein in several embodiments, it is explicitly recognized that the drill string flow control valves, the joints of drill pipe, and other drill string components herein may be attached to one another by any suitable means known in the art including, but not limited to, drill pipe threads, ACME threads, high-torque shoulder-to-shoulder threads, o-ring seals, welding, or any combination thereof. 
     While the foregoing has been described in relation to a drill string and is particularly desirable for addressing u-tubing concerns, those skilled in the art with the benefit of this disclosure will appreciate that the drill string flow control valves of this disclosure can be used in other fluid flow applications without limiting the foregoing disclosure. 
     Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present invention. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee.