Abstract:
A multiplex fluid pump assembled from a plurality of pump bodies connected side by side between opposing end plates with a plurality of fasteners tightened to compress the pump bodies between the end plates. Raised surfaces on opposite exterior side surfaces of each pump body are engaged with an adjacent end plate or an adjacent pump body to apply a pre-compressive force at the raised surfaces and thereby inhibit the initiation of fatigue cracks.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of and priority to provisional application U.S. 61/233,709, filed Aug. 13, 2009. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable 
       THE NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0003]    Not applicable 
       INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
       [0004]    Not applicable 
       BACKGROUND OF THE INVENTION 
       [0005]    (1) Field of the Invention 
         [0006]    The current application is related in general to wellsite surface equipment such as fracturing pumps and the like. 
         [0007]    (2) Description of Related Art including information disclosed under 37 CFR 1.97 and 1.98 
         [0008]    Reciprocating pumps such as triplex pumps and quintuplex pumps are generally used to pump high pressure fracturing fluids downhole. Typically, the pumps that are used for this purpose have plunger sizes varying from about 7 cm (2.75 in.) to about 16.5 cm (6.5 in.) in diameter and may operate at pressures up to 144.8 MPa (21,000 psi). In one case, the outer diameter of the plunger is about 9.5 cm (3.75 in) and the reciprocating pump is a triplex pump. 
         [0009]    These pumps typically have two sections: (a) a power end, the motor assembly that drives the pump plungers (the driveline and transmission are parts of the power end); and (b) a fluid end, the pump container that holds and discharges pressurized fluid. 
         [0010]    In triplex pumps, the fluid end has three fluid cylinders. For the purpose of this document, the middle of these three cylinders is referred to as the central cylinder, and the remaining two cylinders are referred to as side cylinders. A fluid end may comprise a single block having cylinders bored therein, known in the art as a monoblock fluid end. Similarly, a quintuplex pump has five fluid cylinders, including a middle cylinder and four side cylinders. 
         [0011]    The pumping cycle of the fluid end is composed of two stages: (a) a suction cycle: During this part of the cycle a piston moves outward in a packing bore, thereby lowering the fluid pressure in the fluid end. As the fluid pressure becomes lower than the pressure of the fluid in a suction pipe (typically 2-3 times the atmospheric pressure, approximately 0.28 MPa (40 psi)), the suction valve opens and the fluid end is filled with pumping fluid; and (b) a discharge cycle: During this cycle, the plunger moves forward in the packing bore, thereby progressively increasing the fluid pressure in the pump and closing the suction valve. At a fluid pressure slightly higher than the line pressure (which can range from as low as 13.8 MPa (2,000 psi) to as high as 144.8 MPa (21,000 psi) the discharge valve opens, and the high pressure fluid flows through the discharge pipe. In some cases, the pump is operated at 12,000 psi. In some other cases, the pump is operated at 15,000 psi. In some further cases, the pump is operated at 20,000 psi. 
         [0012]    Most commercially available reciprocating pumps for fracturing jobs are rated at least 300 RPM, or 5 Hz. Given a pumping frequency of 2 Hz, i.e., 2 pressure cycles per second, the fluid end body can experience a very large number of stress cycles within a relatively short operational lifespan. These stress cycles may induce fatigue failure of the fluid end. Fatigue involves a failure process where small cracks initiate at the free surface of a component under cyclic stress. The cracks may grow at a rate defined by the cyclic stress and the material properties until they are large enough to warrant failure of the component. Since fatigue cracks generally initiate at the surface, a strategy to counter such failure mechanism is to pre-load the surface under compression. 
         [0013]    Typically, this is done through an autofrettage process, which involves a mechanical pre-treatment of the fluid end in order to induce residual compressive stresses at the internal free surfaces, i.e., the surfaces that are exposed to the fracturing fluid, also known as the fluid end cylinders. US 2008/000065 is an example of an autofrettage process for pretreating the fluid end cylinders of a multiplex pump. During autofrettage, the fluid end cylinders are exposed to high hydrostatic pressures. The pressure during autofrettage causes plastic yielding of the inner surfaces of the cylinder walls. Since the stress level decays across the wall thickness, the deformation of the outer surfaces of the walls is still elastic. When the hydrostatic pressure is removed, the outer surfaces of the walls tend to revert to their original configuration. However, the plastically deformed inner surfaces of the same walls constrain this deformation. As a result, the inner surfaces of the walls of the cylinders inherit a residual compressive stress. The effectiveness of the autofrettage process depends on the extent of the residual stress on the inner walls and their magnitude. 
         [0014]    It remains desirable to provide improvements in wellsite surface equipment in efficiency, flexibility, reliability, and maintainability. 
       BRIEF SUMMARY OF THE INVENTION 
       [0015]    The present application in one embodiment applies pre-compressive forces to raised surfaces on pump bodies to inhibit initiation of fatigue cracks in the fluid end of a multiplex pump. 
         [0016]    In one embodiment, a method comprises connecting a plurality of pump bodies side by side between opposing end plates with a plurality of fasteners to form a pump assembly. Each pump body comprises a piston bore, an inlet bore, an outlet bore and at least one pump body comprises a raised surface on an opposite exterior side surface thereof. The raised surface engages with an adjacent end plate or an adjacent pump body. In another embodiment, each pump body comprises a raised surface on an opposite exterior side surface thereof. The method also comprises tightening the fasteners to compress the pump bodies between the end plates. In this manner, a pre-compressive force can be applied at the raised surfaces on each of the pump bodies. 
         [0017]    In one embodiment, a fluid pump assembly comprises a plurality of pump bodies connected side by side between opposing end plates with a plurality of fasteners tightened to compress the pump bodies between the end plates. Each pump body comprises a piston bore, an inlet bore, an outlet bore and raised surfaces on opposite exterior side surfaces thereof. The raised surfaces engage with an adjacent end plate or an adjacent pump body to apply a pre-compressive force at the raised surfaces on each of the pump bodies. 
         [0018]    In one embodiment, a method is provided to inhibit fatigue cracks in a fluid pump assembly comprising a plurality of pump bodies comprising a piston bore, an inlet bore and an outlet bore. This method in an embodiment comprises: (a) providing raised surfaces on opposite exterior side surfaces of the plurality of pump bodies; (b) forming the pump assembly by connecting the plurality of pump bodies side by side between opposing end plates with a plurality of fasteners, wherein the raised surfaces engage with an adjacent end plate or an adjacent pump body; and (c) tightening the fasteners to compress the plurality of pump bodies between the end plates, whereby a pre-compressive force is applied at the raised surfaces on each of the pump bodies. 
         [0019]    In the various embodiments, the pump bodies can also be optionally autofrettaged. 
         [0020]    In the various embodiments, the fasteners can be or include tie rods extending through bores aligned through the pump bodies. 
         [0021]    In the various embodiments, the raised surfaces can engage with an adjacent end plate or the raised surface of an adjacent pump body. 
         [0022]    In the various embodiments, the pre-compressive force can be applied at a predetermined location of each of the pump bodies. 
         [0023]    In the various embodiments, the raised surfaces can be adjacent an intersection of the piston bore, the inlet bore, and the outlet bore. 
         [0024]    In the various embodiments, the pre-compressive force can extend the operational life of the assembly by reducing stress adjacent an intersection of the piston bore, the inlet bore, and the outlet bore. 
         [0025]    In the various embodiments, the pump assembly can be operated to reciprocate a piston in the piston bore and cycle between relatively high and low fluid pressures in the inlet and outlet bores, wherein the compression of the pump bodies between the end plates inhibits, delays, or postpones the initiation of fatigue cracks. 
         [0026]    In the various method embodiments, the method can further comprise disassembling the fluid pump assembly to remove one of the pump bodies exhibiting fatigue crack initiation, and reassembling the fluid pump assembly with a replacement pump body without fatigue cracks. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0027]      FIG. 1  is a fluid end perspective view of a triplex pump fluid end assembly according to an embodiment of the application. 
           [0028]      FIG. 2  is another fluid end perspective view of the triplex pump fluid end assembly of  FIG. 1  according to an embodiment of the application. 
           [0029]      FIG. 3  is a power end perspective view of the triplex pump fluid end assembly of  FIGS. 1-2  according to an embodiment of the application. 
           [0030]      FIG. 4  is a partially disassembled view of the triplex pump fluid end assembly of  FIGS. 1-3  according to an embodiment of the application. 
           [0031]      FIG. 5  is a perspective view of one of the pump body portions of the triplex pump fluid end assembly of  FIGS. 1-4  according to an embodiment of the application. 
           [0032]      FIG. 6  is a side sectional view of the pump body of  FIG. 5  according to an embodiment of the application. 
           [0033]      FIG. 7  is a perspective view, partially cut away, of the pump fluid end assembly of  FIGS. 1-4  according to an embodiment of the application. 
           [0034]      FIG. 8  is another fluid end perspective view of the triplex pump fluid end assembly of  FIGS. 1-3  according to an embodiment of the application. 
           [0035]      FIG. 9  is a perspective view of the bore configuration of the pump body of  FIGS. 5-6  according to an embodiment of the application. 
           [0036]      FIG. 10  is an exploded view of the triplex pump fluid end assembly of  FIGS. 1-3  according to an embodiment of the application. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    Referring now to all of the Figures, there is disclosed a pump body portion or fluid end, indicated generally at  100 . The pump body portion  100  comprises a body  102  that defines an internal passage or piston bore  104  for a receiving a pump plunger (best seen in  FIG. 7 ). The pump body portion  100  may further define an inlet port  106  and an outlet port  108 . The inlet port  106  and the outlet port  108  may be substantially perpendicular to the piston bore  104 , forming a conventional crossbore body portion  100 , best seen in  FIG. 6 . The piston bore  104  may comprise a pair of bores, such as that shown in  FIG. 9 . The intersection of the piston bore  104  and the inlet and outlet ports  106  and  108  defines at least one area  110  of stress concentration that may be a concern for material fatigue failure. In addition to the stress concentration, the area  110  is subject to operational pressure of the pump discussed hereinabove, which may further increase its fatigue failure risk. Those skilled in the art will appreciate that the pump body portion  100  may comprise bores formed in other configurations such as a T-shape, Y-shape, in-line, or other configurations. 
         [0038]    According to some embodiments, three pump body portions  100  are arranged to form a triplex pump assembly  112 , best seen in  FIG. 1 . Those skilled in the art will appreciate that the pump body portions  100  may also be arranged in other configurations, such as a quintuplex pump assembly comprising five pump body portions  100  or the like. 
         [0039]    A raised surface  114  extends from an exterior surface  116  of the pump body portions  100 , best seen in  FIG. 5 . The raised surface  114  may extend a predetermined distance from the exterior surface  116  and may define a predetermined area on the exterior surface  116 . In one embodiment, at least one pump body comprises a raised surface on an opposite exterior side surface of the pump body. In another embodiment, each pump body comprises a raised surface on the opposite exterior side surface of the pump body. While illustrated as circular in shape in  FIG. 5 , the raised surface  114  may be formed in any suitable shape. 
         [0040]    An end plate  118  is fitted on each of the outer or side pump body portions  100  to aid in assembling the body portions  100  into the pump fluid end assembly, such as the triplex pump fluid end assembly  112  shown in  FIG. 1 . The end plates  118  are utilized, in conjunction with fasteners  120 , to assemble the pump body portions  100  to form the pump fluid end assembly  112 . The end plates  118  may further comprise a raised surface  119 , best seen in  FIG. 10 , similar to the surface  114  on the pump body portions  100  for engaging with the raised surfaces  114  of the pump body portions  100  during assembly. 
         [0041]    The bores  104 ,  106 , and  108  of the pump body portions  100  may define substantially similar internal geometry as prior art monoblock fluid ends to provide similar volumetric performance. When the pump fluid end assembly  112  is assembled, the three pump body portions  100  are assembled together using, for example, four large fasteners or tie rods  120  and the end plates  118  on opposing ends of the pump body portions  100 . At least one of the tie rods  120  may extend through the pump body portions  100 , while the other of the tie rods  120  may be external of the pump body portions  100 . 
         [0042]    As the tie rods  120  are torqued (via nuts or the like) to assemble the pump fluid end assembly  112 , the raised surfaces  114  on the pump body portions  100  and raised surfaces  119  on the end plates  118  engage with one another to provide a pre-compressive force to the areas  110  of the pump body portions  100  adjacent the intersection of the bores  104 ,  106 , and  108 . The pre-compressive force is believed to counteract the potential deformation of the areas  110  due to the operational pressure encountered by the bores  104 ,  106 , and  108 . By counteracting the potential deformation due to operational pressure, stress on the areas  110  of the pump body portions  100  is reduced, thereby increasing the overall life of the pump bodies  100  by reducing the likelihood of fatigue failures. Those skilled in the art will appreciate that the torque of the fasteners  120  and the raised surfaces  114  and  119  cooperate to provide the pre-compressive force on the areas  110 . 
         [0043]    Due to the substantially identical profiles of the plurality of pump body portions  100 , the pump body portions  100  may be advantageously interchanged between the middle and side portions  100  of the assembly  112 , providing advantages in assembly, disassembly, and maintenance, as will be appreciated by those skilled in the art. In operation, if one of the pump bodies  100  of the assembly  112  fails, only the failed one of the pump bodies  100  need be replaced, reducing the potential overall downtime of a pump assembly  112  and its associated monetary impact. The pump body portions  100  are smaller than a typical monoblock fluid end having a single body with a plurality of cylinder bores machined therein and therefore provides greater ease of manufacturability due to the reduced size of forging, castings, etc. 
         [0044]    An attachment flange  122 , best seen in  FIG. 3 , may extend from the pump body portion  100  for guiding and attaching a power end (not shown) to the plungers (see  FIG. 7 ) and ultimately to a prime mover (not shown), such as a diesel engine or the like, as will be appreciated by those skilled in the art. 
         [0045]    The preceding description has been presented with reference to present embodiments. Persons skilled in the art and technology to which this disclosure pertains will appreciate that alterations and changes in the described structures and methods of operation can be practiced without meaningfully departing from the principle, and scope of this application. Accordingly, the foregoing description should not be read as pertaining only to the precise structures described and shown in the accompanying drawings, but rather should be read as consistent with and as support for the following claims, which are to have their fullest and fairest scope.