A device for limiting the flow of drilling fluid through a section of drill string includes a body with a hole in the periphery. Flow enters the device through one axial end, at least a portion of the flow exits through the other axial end. Some of the fluid flow can be diverted through the peripheral hole. A spring-biased axial piston has an approximately constant force throughout its range of travel. The piston moves axially in response to the changing fluid flow rate to ensure that a constant amount of flow exiting the axial end of the tool is achieved while diverting away excess flow through the side.

This application claims priority to and the benefit of U.S. Provisional Patent App. No. 61/690,346, filed Jun. 25, 2012, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Disclosure 
     The present invention relates in general to drill strings and, in particular, to a system, method and apparatus for regulating fluid flow through a drill string. 
     2. Description of the Related Art 
     Conventional oil and gas drilling typically includes pumping a quantity of fluid through a pipe or drill string to a drill bit for cutting the hole in the rock. The fluid is then circulated back up though the wellbore in the annular or outer section of the hole. Drilling fluid is beneficial to the drilling process since it clears away pieces of rock that have been cut from the bottom of the wellbore. Without this cleaning action the cut pieces of rock would accumulate near the drill bit and interfere with further drilling. 
     In general, the higher level of fluid flow that a drilling operation can achieve, the better that cut pieces of rock or “cuttings” are cleared from the bottom of the wellbore. However, there are several factors that limit the fluid flow level. One of these factors is the amount of pressure that it takes to pump a large amount of fluid. As the drill string becomes longer or narrower, the resistance to pumping a given amount of fluid increases, which increases the need for higher pressure. With any fluid pump set up there is a limit to the amount of pressure that can be overcome in order to make the fluid flow. Accordingly, the size or type of pump can limit the available flow rate. 
     Another limiting factor is the capability of the downhole mud motor. Mud motors are used to make the rock cutting drill bit rotate faster than the drill pipe that it is connected to. For example, a drilling operator may desire to drill while holding the drill string stationary, or may want to rotate the drill bit faster to achieve a higher rate of rock penetration. The mud motor works in a manner similar to a turbine in that the mud that flows through the motor turns a rotor that is connected to the drill bit. Energy from the pressure of the fluid flow is converted into rotational work by the drill bit. Mud motors are usually designed such that there is a maximum amount of flow that the motors are designed to handle. Forcing excess fluid through a mud motor can damage the motor and inhibit the drilling process. 
     The desire to flow higher volumes of drilling fluid through the well and the need to limit the volume flow rate due to the constraints of the motor can be conflicting. It would be desirable to flow as much fluid as is desired while ensuring that the motor did not experience a rate of flow higher than its design criteria. 
     A conventional solution to this problem is to form annular ports in the drill string above the mud motor. By choosing the size of the ports, the amount of flow that exits through the ports and the amount of flow that continues on through the drill string into the mud motor can be approximated. 
     A problem with this technique is that the amount of fluid that exits through the ports varies depending on the back pressure from the mud motor. The back pressure from the mud motor is a factor of the torque that it delivers. Thus, the more torque that is needed or generated by the motor, the higher the back pressure from the motor, which diverts more fluid through the ports in the sides of the drill string. More diverted flow means less fluid is transferred down through the motor. Less fluid to the motor reduces its torque and power, which can induce a situation where the motor stalls and needs more torque to overcome its bound condition. Conversely, an off-bottom situation where there is relatively low amounts of back pressure generated by the motor because there is no drilling torque resistance can result in a higher amount of fluid passing through the motor and a lower amount of fluid exiting the drill string. This too is problematic since a low torque situation causes the motor to spin faster at a given flow rate. Increased amounts of flow will only exacerbate this situation. 
     Some motor manufacturers attempt to solve this problem by drilling a hole through the rotor of the mud motor so that some fluid may pass through the tool without generating torque or causing damage to the motor. Unfortunately, since the drilled hole is static and does not change its shape to account for differing flow or pressure conditions, it is subject to the same limitations as the previously described method. Thus, improvements in controlling drill string fluid flow continue to be of interest. 
     SUMMARY 
     Embodiments of a system, method and apparatus for controlling fluid flow through a drill string are disclosed. For example, an apparatus may include a housing having an axis, a radial wall with a bore extending axially therethrough, and an aperture formed in the radial wall. The aperture is in fluid communication with the bore. A piston may be located inside the housing and have an orifice configured to permit axial fluid flow through the housing. A spring may be located in the housing and be configured to axially bias the piston to a closed position. 
     In some embodiments, the piston is movable from the closed position wherein the piston is configured to close the aperture in the housing to substantially block radial fluid flow therethrough when axial fluid flow through the orifice is insufficient to overcome a spring force of the spring, and an open position wherein the piston is configured to permit radial fluid flow through the aperture when axial fluid flow through the orifice is sufficient to overcome the spring force of the spring and axially move the piston. 
     In other embodiments, a method of controlling fluid flow through a drill string may include operating the drill string to drill a hole in an earthen formation; pumping fluid through the drill string to a mud motor such that substantially all of the fluid is flows axially to the mud motor and substantially none of the fluid is radially diverted out of the drill string; and then increasing a flow rate of the fluid such that some of the fluid is radially diverted out of the drill string before reaching the mud motor, and a remainder of the fluid is flows axially to the mud motor. 
     In still other embodiments, a method of controlling fluid flow through a drill string may include operating a drill string to drill a hole in an earthen formation; pumping fluid through the drill string; closing a piston in the drill string to direct substantially all of the fluid to a mud motor; and then changing a parameter of the drill string such that the piston moves to an open position allowing at least a portion of the fluid to be diverted away from the mud motor. 
     The foregoing and other objects and advantages of these embodiments will be apparent to those of ordinary skill in the art in view of the following detailed description, taken in conjunction with the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features and advantages of the embodiments are attained and can be understood in more detail, a more particular description may be had by reference to the embodiments thereof that are illustrated in the appended drawings. However, the drawings illustrate only some embodiments and therefore are not to be considered limiting in scope as there may be other equally effective embodiments. 
         FIG. 1  is a sectional side view of an embodiment of drill string assembly. 
         FIGS. 2-4  are sectional side views of an embodiment of a system, method and apparatus for limiting fluid flow through a drill string, illustrating a closed position, a partially open position, and a fully open position, respectively. 
         FIGS. 5 and 6  are isometric and side views, respectively, of an embodiment of a sleeve. 
         FIG. 7  is an exploded isometric view of an embodiment of a tool assembly. 
     
    
    
     The use of the same reference symbols in different drawings indicates similar or identical items. 
     DETAILED DESCRIPTION 
     Embodiments of a system, method and apparatus for controlling fluid flow through a drill string are disclosed. For example,  FIG. 1  depicts an embodiment of a downhole tool assembly  11  for drilling a well bore  10 . The downhole tool assembly  11  may comprise a variety of configurations. In one embodiment, the downhole tool assembly  11  may include an axis  12 , a plurality of drill pipes  13 , measurement while drilling (MWD) equipment  15 , a fluid flow control tool  17 , a mud motor  19  and a drill bit  21 . The order or sequence of these components may be varied depending on the application. For example, the MWD equipment  15  may be located above or uphole from the drill bit  21 . In some embodiments, the MWD equipment  15  may be axially relatively close (e.g., within about 100 meters) to the drill bit  21 . Likewise, the MWD equipment  15  may be located above but axially relatively close to fluid flow control tool  17 , such that fluid flow control tool  17  is relatively close to the drill bit  21  as well. 
       FIGS. 2-4  are enlarged views of fluid flow control tool  17 . Each drawing depicts a piston  23  in a closed position ( FIG. 2 ), a partially open position ( FIG. 3 ) and a fully open position ( FIG. 4 ). The fluid flow control tool  17  includes a housing  25  having an aperture  27  extending through a radial wall thereof. The aperture  27  may comprise one or more holes, slots, etc. In the illustrated embodiment, a sleeve  29  that is stationary is mounted to the inner bore  31  of the housing  25 . Sleeve  29  has a sleeve aperture  33  that corresponds with aperture  27  in housing  25 . In some embodiments, the sleeve aperture  33  is smaller than and complementary in shape to the aperture  27 . In some versions, the sleeve  29  and sleeve aperture  33  are configured to take the brunt of fluid erosion damage away from the housing  25  and aperture  27 . Sleeve  29  may be more readily replaced in fluid flow control tool  17  than housing  25 . Sleeve  29  may be affixed to housing  25  such that it can be considered to be part of the housing  25 . 
     Embodiments of the piston  23  also comprise an element  35  having an inner axial orifice. As fluid  37  flows through the orifice of element  35  it may create a pressure drop and thus a downward force on piston  23 . As long as the flow rate of fluid  37  is low enough, the resultant downward force by the fluid on piston  23  does not exceed the upward force of a spring  41 . Under such conditions ( FIG. 2 ), a shoulder  42  on the piston  23  will remain against an upper stop  43  located on an inner surface of sleeve  29 . In addition or alternatively, the upward axial travel of piston  23  may be limited by landing a lower shoulder  53  of piston  23  on an upper shoulder  51  of sleeve  29 . 
       FIG. 3  illustrates the same tool with the fluid flow rate increased such that the downward force that the fluid exerts on piston  23  is equivalent to or exceeds the upward force of spring  41 . Under these conditions, the piston  23  moves axially downward to the “partially open” position shown in  FIG. 3 . The shoulder  42  on piston  23  is located axially below upper stop  43  on sleeve  29 . As the top  45  of piston  23  moves below the top of the sleeve aperture  33  in sleeve  39  (and, thus, the top of aperture  27  in housing  25 ), a flow path begins to open such that some of the fluid  47  escapes out the radial side of the tool  17 . Fluid  47  escapes to the wellbore annulus  49  ( FIG. 1 ) located between the outer surface of downhole tool assembly  11  and the wellbore  10 . The piston  23  finds an axial equilibrium between the downward pressure from fluid  37  through the orifice of element  35  and the upward force from spring  41 . In some versions, the spring rate of the spring  41  may be selected such that the balancing force is substantially constant throughout the axial range of travel of the piston  23 . 
       FIG. 4  shows the piston  23  in a “fully open” position when it is subjected to an even larger fluid flow rate than that of  FIG. 3 . The fluid flow is divided between fluid  47  through the apertures  33 ,  27  in the side of the tool  17 , and the fluid  37  flowing through the center of the tool  17 . In the fully open position, the fluid flow completely overcomes the spring force of spring  41  and pushes piston  23  completely open. In this condition, fluid flow through apertures  33 ,  27  may be completely unobstructed by piston  23 . In addition or alternatively, the downward axial travel of piston  23  may be limited by landing a lower shoulder  55  ( FIG. 7 ) of piston  23  on an upper shoulder  57  of a sub  13 . 
     In some embodiments, the apparatus or tool  17  may comprise a housing  25  having an axis  12 , a radial wall with a bore  31  extending axially therethrough, and an aperture  27  formed in the radial wall. In some versions, the housing  25  may have has an axial length of about 3 feet to about 12 feet, and an outer diameter of about 3.5 inches to about 8 inches. 
     The aperture  27  may be in fluid communication with the bore  31 . The aperture  27  in the housing  25  may comprise a plurality of apertures  27 . The aperture  27  may comprise an elongated slot, such as the teardrop shape of sleeve aperture  33  in sleeve  29  shown in  FIGS. 5 and 6 . The sleeve aperture  33  (and, similarly, aperture  27 ) may include an upper leading edge  28  that is not greater than about 0.030 inches wide in a circumferential direction with respect to the axis  12 . The sleeve aperture  33  (and, similarly, aperture  27 ) may increasingly taper in width, such as toward a trailing edge thereof, at not greater than about 15° with respect to the axis  12 . In addition, the sleeve aperture  33  (and, similarly, aperture  27 ) may be skewed with respect to the axis  12 , as shown. 
     A piston  23  may be located inside the housing  25  and have the element  35  configured to permit axial fluid flow through the housing  25 . A spring  41  may be located in the housing  25 . The spring  41  may be configured to axially bias the piston  23  to a closed position ( FIG. 2 ). 
     The piston  23  may be movable from the closed position wherein the piston  23  is configured to close the aperture  27  in the housing  25  to substantially block radial fluid flow therethrough when axial fluid flow  37  through the orifice of element  35  is insufficient to overcome a spring force of the spring  41 . In an open position (which may include any position other than the closed position), the piston  23  may be configured to permit radial fluid flow  47  through the aperture  27  when axial fluid flow  37  through the orifice of element  35  is sufficient to overcome the spring force of the spring  41  and axially move the piston  23 . In the open position, the piston  23  may be configured to permit substantially unobstructed radial fluid flow through the aperture  27 . 
     Embodiments of the piston  23  may further comprise a partially open position, located between the closed position and the open position, wherein the piston  23  may be configured to reach a force equilibrium between the axial fluid flow  37  and the spring force such that the aperture  27  is only partially obstructed to radial fluid flow  47  by the piston  23 . 
     The piston  23  may be configured to generate a pressure differential as fluid  37  flows through the orifice of element  35  so that the piston  23  pushes against the spring  41 . The element  35  may be replaceable within a body of the piston  23 , such that the body is configured to be reusable after the element  35  is replaced within the body. In some versions, the orifice of element  35  may have an inner diameter in a range of about 0.75 inches to about 1.5 inches. In addition, the piston  23  may be formed from a single material, or formed from at least two materials, one of which is harder (e.g., tungsten carbide) than the other (e.g., steel). 
     Embodiments of the apparatus  17  may further comprising a sleeve  29  located between the bore  31  of the housing  25  and the piston  23 . The sleeve  29  may be stationary with respect to the housing  25 . The piston  23  may be movable with respect to the sleeve  29  and housing  25 . In some versions, both axial ends of the sleeve  29  may be sealed with respect to the bore  31  of housing  25 . 
     The sleeve  29  may be consumable. The sleeve  29  may comprise a material that is harder than a material of the housing  25 . For example, the housing may be some form of steel, and the material of sleeve  29  may comprise at least one of tungsten carbide, a ceramic, stabilized zirconia, alumina, and silica. Like the sleeve  29 , the element  35  may be consumable and comprise a material that is harder than a material of the housing, and the orifice material comprises at least one of those same materials. 
     The piston  23  and the sleeve  29  may include a shoulder  42  and upper stop  43 , respectively, that abut each other in the closed position ( FIG. 2 ). The shoulder  42  and upper stop  43  may be axially spaced apart in the open position ( FIG. 3 or 4 ). The shoulder  42  and upper stop  43  may comprise at least one of upper shoulders and lower shoulders. In some versions, the piston  23  may have a range of axial travel in a range of about 1 inch to about 6 inches. 
     In addition, embodiments of the sleeve  29  may comprise a sleeve aperture  33  that registers with the aperture  27  in the housing  25 . The sleeve aperture  33  may be smaller than the aperture  27  in the housing  25 . 
     In some versions, at least some fluid leakage through the aperture  27  is permitted when the piston  23  is in the closed position. In other words, the aperture  27  is not necessarily sealed to stop fluid leaks when the piston is in the closed position. For example, up to about 5% of the fluid entering the apparatus  17  may be permitted to leak through the aperture  27  when the piston  23  is in the closed position. 
     The apparatus  17  may further comprise a labyrinth seal  65  ( FIG. 7 ) between the housing  25  (or sleeve  29 , if present) and the piston  23 . The labyrinth seal  65  may be formed on an exterior of the piston  23 , or could be on the inner surface of housing  25  or sleeve  29 , if present. 
     Embodiments of the spring  41  may have a spring rate and may be configured to apply a force that is substantially constant over a range of axial movement of the piston. For example, the spring  41  may have a spring rate in a range of about 10 lb/in to about 70 lb/in. Examples of the spring  41  may comprise t least one of a coil spring, a Belleville spring stack and a polymer spring. In some embodiments, there is a frictional force between the housing  25  (or sleeve  29 , if present) and the piston  23 . The spring  41  may have a compression preload, such that the frictional force is less than about 5% of the compression preload. 
     The apparatus may further comprise a wash pipe  61  mounted to the piston  23 . The spring  41  may be located between the bore  31  of the housing  25  and the wash pipe  61 . Embodiments of the wash pipe  61  may be sealed to the piston  23  at one axial end US ( FIGS. 5 and 6 ) and with a seal S ( FIG. 7 ) to the housing  25  (e.g., a sub or drill pipe  13 ) at the other axial end. The wash pipe  61  may comprise at least one hole  63  for communicating fluid to and from the spring  41 . Pressure generated by fluid flow through the hole  63  is configured to act as a damper for the axial motion of the piston  23 . 
     In some embodiments, the spring rate may be sufficiently low and the spring  41  is preloaded such that the force provided by the spring  41  is substantially constant over its operating range. In addition, the spring force may be sufficiently high such that at least about 95% of the resistance to downhole movement of the piston  23  may be provided by the spring  41  and not by unpredictable forces like friction. 
     In other embodiments of the tool  17 , the amount of fluid flow through the center (i.e., the orifice of element  35 ) of the tool  17  is substantially constant regardless of the fluid pressure, flow rate, fluid density, etc. The spring rate may be selected such that it is between about 10% and about 15% of the compression preload on the spring  41 . Such a spring  41  may have a relaxed length that is about 2.5 times its compressed length. For example, a spring  41  having a spring rate of 25 lb/in may be compressed to provide a spring force or pre-load of 250 lbs in the compressed state (i.e., when the tool  17  is in the closed position). In order to move the piston  23  a distance of 1.5 inches, the spring force increases by 1.5 times the spring rate. In this example, 250 lbs+(1.5 in ×25 lb/in)=282 lbs. Since the fluid pressure difference through the orifice of element  35  increases with the square of the flow rate, the axial fluid flow rate through the orifice of element  35  of the tool  17  can be considered to be substantially constant. The actual amount of increase in flow rate at the point where the piston moves to the point where the apertures are fully open can be calculated as increasing by a factor of the square root of the ratio of spring force on the piston in the open position to the spring force on the piston in the closed position, or:
 
Flow(open)=Flow(closed)×sqrt(282/250)
 
Flow(open)=Flow(closed)×1.06.
 
     So, even though the spring force increases by 13% (282/250) as the piston moves into an open position, the flow that is allowed to pass axially through the tool only increases by 6%. 
     Should the tool be configured such that the rate was 15% of the preload, the preceding calculation would be done as follows:
 
Flow(open)=Flow(closed)×sqrt(306.25/250)
 
Flow(open)=Flow(closed)×1.10.
 
     Therefore, in the case where the spring rate is configured to be 15% of the preload value, with a 1.5″ axial movement of the piston the axial flow through the tool increases by 10%. 
     In other embodiments, a method of controlling fluid flow through a drill string may comprise operating the drill string to drill a hole in an earthen formation; pumping fluid through the drill string to a mud motor such that substantially all of the fluid is flows axially to the mud motor and substantially none of the fluid is radially diverted out of the drill string; and then increasing a flow rate of the fluid such that some of the fluid is radially diverted out of the drill string before reaching the mud motor, and a remainder of the fluid is flows axially to the mud motor. The valve opening may be proportional to the fluid flow rate. Pumping may comprise insufficient fluid pressure to overcome a mechanical force biasing a valve to a closed position. In some versions, increasing the flow rate may comprise opening a valve with fluid pressure that overcomes a mechanical force biasing the valve to a closed position. In other versions, increasing the flow rate may comprise variably controlling an amount of fluid that is radially diverted and the remainder of the fluid flowing axially to the mud motor. 
     Embodiments of a method of controlling fluid flow through a drill string may comprise operating a drill string to drill a hole in an earthen formation; pumping fluid through the drill string; closing a piston in the drill string to direct substantially all of the fluid to a mud motor; and then changing a parameter of the drill string such that the piston moves to an open position allowing at least a portion of the fluid to be diverted away from the mud motor. 
     When operating the tool, the impact of tool  17  that will be noticed at the surface of the well is that once the flow rate is increased to the point that the tool opens, the stand pipe pressure (or surface operating pressure) will increase more slowly with any further flow rate increases. Thus, once the piston in the tool begins to open (i.e., from one of the partially open positions to the fully open position), the fluid pressure does not substantially increase even with an increase in fluid flow rate. This is due to the fact that pressure of the fluid at the surface is a function of the drilling fluid flow rate through the surface piping, the drill pipe, and the bottom hole assembly (BHA, or MWD, mud motor, drill bit, etc.). As fluid flow opens the tool, an increasing amount of fluid bypasses the BHA through the radial aperture. Thus, even though the fluid flow rate may increase, the fluid pressure through the BHA is substantially constant. Increases in fluid pressure can originate from more fluid flow through the surface piping and the drill string. 
     For example, the tool  17  may be configured with the following constants. The ID of most of the tool components is about 2 inches, which will be the number used in flow calculations for Bernoulli's equation. The piston/orifice combination may be considered a single part for these purposes. Further, for the purposes of calculation it can be thought of as a toroid (donut) shape with a cross-sectional area that is a function of its ID and OD and will, in conjunction with the orifice pressure drop (delta P), determine the downward force that the piston applies to the spring. The OD of the piston may be 3 inches. The ID of the orifice may be determined based on flow rate. 
     In this example, the spring has a spring rate of 25 lb/in and is compressed (preloaded) in the closed state such that it applies a force of 200 lb on the piston. The spring may be compressed 8 inches for this example. Incidentally, and not considered in this calculation, the force on the piston increases slightly as it moves downwards. If the pistons moves down by one inch the force will increase by 25 lbs to 225 lbs. 
     In one example, the tool may be set up so that only 250 gpm of fluid will go axially through the tool and that any increase in flow rate will be allowed to exit through the radial apertures. A flow rate of 250 gallons per minute is equivalent to 962.5 cubic inches per second. In this example, the density of the fluid flowing through the tool can be about 10 ppg (pounds per gallon), or 6.9 slugs/cubic ft. 
     This may comprise an iterative calculation (where the orifice diameter determines the pressure drop at a given flow rate, but it also can determine the cross sectional area over which the pressure is applied. Thus, the calculation could be performed many times. However, the ID does not drastically affect the area as much as it affects pressure drop. Accordingly, a good starting estimate for orifice size is sufficient to bring the calculation to a satisfactory conclusion. 
     For example, if the orifice ID may be estimated at 1.2 inches. If the piston has an OD of 3.00 inches, then the cross sectional area is: 
     A=pi*((Piston OD/2)squared−(Orifice ID/2)squared)=5.93 sqin. This is the area that the delta P acts on to push against the spring. 
     With this area, the pressure drop (delta P) that will start to move the spring is: 
     deltaP=preload force/cross sectional area. 
     So, delta P=200 lb/5.93 sqin=33.7 psi. Or, 4853 lbs/square foot. 
     The velocity of the fluid may be determined as it goes through the 2″ ID section of the tool. If the design goal is 250 gpm, velocity may be calculated as V=Q/A where Q is the volume flow rate. For consistent units, the calculation in feet per second is: for flow rate 962.5 cubic inches per second, and area is 3.14 sq in, the inlet velocity is 306.4 in/second or 25.5 ft/second. 
     Bernoulli's equation for pressure drop across an orifice is:
 
Delta  P =(density×(orifice fluid velocity)squared)/2−density×(inlet fluid velocity)squared)/2
 
     The delta P and inlet velocity are known, and the equation may be configured for orifice velocity.
 
Orifice Velocity=sqrt((2*delta P/density)+(inlet fluid velocity)squared)
 
Thus, Orifice velocity=sqrt((2*4853/(6.9))+(25.4)squared)
 
Orifice Velocity=45.3 ft/s
 
     Converted to in/s, velocity is 543.6 in/s
 
And back calculating an orifice area,  A=Q/V , so  A= 962.5/543.6=1.77 sqin.
 
And finally, the orifice diameter becomes sqrt(4*Area/pi)=sqrt(4*1.77/3.14159)
 
     Diameter=1.50 inches. 
     This calculation provides an orifice diameter of 1.50 inches gives a pressure drop of 33.7 psi at a flow rate of 250 gallons per minute. This calculation is slightly different from the original estimate of 1.20 inches. The area difference that this equates to is 5.3 inches squared as opposed to the original estimate of 5.93 inches, which is a difference of 0.63 square inches or 10%. The formula may be recalculated with this new estimate to yield a more precise value. With a new estimate of a 1.5 inch orifice, recalculating the numbers provides an orifice value of 1.48 inches. A value of 1.48 inches is sufficiently close to the previous iteration value of 1.50 that the calculation can be considered to be complete. 
     Embodiments of the tool described herein solves the problems described above with a piston assembly that moderates the amount of flow that exits the tool. The holes in the sides of the tool can be partially closed to change their size. As the holes are made smaller, a larger portion of the flow is directed downward through the motor. As the holes are enlarged, more of the flow is directed radially outward to bypass the motor and yet still aid in the hole cleaning process. The moderation of hole size can be done very quickly, typically in a fraction of a second. Rapid hole size selection addresses issues such as motor stalls and stick-slip, which can occur and can be resolved very quickly. 
     In some embodiments, the piston assembly comprises a sleeve that slides axially to open or close one or more holes in the tool. The holes may comprise a variety of shapes, such as axially elongated shapes. An orifice is attached to the sleeve to generate a pressure difference across the orifice that depends on the amount of fluid flow. Pushing the sleeve and orifice upwards is a spring with a spring rate that is as low as is reasonable given the other mechanical constraints of the tool. The spring may be preloaded such that a high amount of force is required to make the sleeve initially move from the seated position, but relatively low additional force may be required to push the sleeve down to its fully open position. Thus, the position of the piston may be correlated with the amount of fluid flow that exits through the side of the tool, rather than the amount of flow that is directed down hole to the motor. Accordingly, the spring may have a relatively constant force over its range of travel. The downward force from the fluid is generated by flow through the orifice. Since the downward force balances with the upward spring force, the flow through the orifice may remain relatively constant as well. Fluid flow that is in excess of an amount required to push the sleeve down may be directed out the side of the tool. 
     A motor “stalls” when its rotor stops turning and fluid flow is backstopped such that the fluid stops flowing through the motor. With the embodiments described herein, motor stalls are avoided since pressure drops through the orifice allow the sleeve to move upward to close the radial holes and direct more fluid down through the orifice to the motor where it is needed to correct the stall. 
     Change in the size of the radial holes or slots may be effected through the use of piston that is constructed of a hard material (e.g., tungsten carbide) and fits snugly inside of the housing. The tungsten carbide piston may be coupled with a tungsten carbide housing to resist fluid erosion even with very abrasive mud types. 
     This written description uses examples to disclose the embodiments, including the best mode, and also to enable those of ordinary skill in the art to make and use the invention. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 
     Note that not all of the activities described above in the general description or the examples are required, that a portion of a specific activity may not be required, and that one or more further activities may be performed in addition to those described. Still further, the order in which activities are listed are not necessarily the order in which they are performed. 
     In the foregoing specification, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of invention. 
     As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of features is not necessarily limited only to those features but may include other features not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive-or and not to an exclusive-or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). 
     Also, the use of “a” or “an” are employed to describe elements and components described herein. This is done merely for convenience and to give a general sense of the scope of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims. 
     After reading the specification, skilled artisans will appreciate that certain features are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. Further, references to values stated in ranges include each and every value within that range.