Abstract:
A pressure relief system for a reciprocating pumping system that deintensifies pressure from the pumping system and controls relief operation with the deintensified pressure. The relief system includes a relief valve connected to the pumping system and a control system that selectively opens the relief valve when an overpressure is sensed in the pumping system. To keep the relief valve in a closed position, the control system maintains a backpressure on the relief valve using the deintensified pressure. A dump valve in the control system selectively vents the backpressure so the relief valve can open. A charging system can be used for charging the control system and for reseating the relief valve.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of co-pending U.S. Provisional Application Ser. No. 61/138,795, filed Dec. 18, 2008, the full disclosure of which is hereby incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to valves and in particular to an improved pressure relief valve that relieves a pressure of a system. The relieving set pressure is maintained by a dedicated hydraulic circuit. 
       DESCRIPTION OF RELATED ART 
       [0003]    In oil field operations, reciprocating pumps are often used for various purposes. Some reciprocating pumps, generally known as “service pumps,” typically pump service fluids used for downhole operations such as cementing, acidizing, or fracing a well. These service pumps may typically operate for relatively short periods of time, but on a frequent basis such as several times a week. Often they are mounted to a truck or a skid for transport to various well sites. 
         [0004]    Pressure within the pump discharge circuit may unexpectedly increase, due to a line blockage, excess heating, or other reason. Pressure relief systems can be included with the discharge circuit for relieving unacceptable pressure buildup in the discharge circuit. The pressure relief system generally includes a valve having a side in fluid communication with the discharge circuit, and another side in fluid communication with a relief circuit. Opening the relief valve vents the discharge circuit to the relief thereby allowing the overpressured circuit to vent before damaged by overpressure. The relief valve can open directly in response to overpressure, one example is where a prestressed spring seats the valve until a discharge circuit reaches a set pressure. Optionally, sensors can monitor pressure, and when an unacceptable high pressure is sensed, a signal sent to a controller may actuate the valve open enabling the discharge circuit to relieve. 
       SUMMARY OF THE INVENTION 
       [0005]    Disclosed herein is an example of a method of relieving pressure from a pumping circuit that includes providing a relief valve that has a body, an axial passage in the body, a discharge port formed in the body in fluid communication with a portion of the passage, and a piston axially moveable within the passage and having a high pressure end a low pressure end on a side opposite the high pressure end. The method of, this example may further include providing fluid communication between the pumping circuit and the high pressure end of the piston, urging the piston into a closed position by maintaining a backpressure on the low pressure side of the piston so that the high pressure side of the piston seats into a closed position to block fluid communication between the pumping circuit and the discharge port, providing a selectively openable dump valve in pressure communication with the low pressure side of the piston, and opening the dump valve, when the pressure in the pumping circuit reaches a set point, by communicating to the dump valve a fraction of the pumping circuit pressure, so that the backpressure on the piston vents through the dump valve, the pumping circuit pressure unseats the piston, and fluid in the pumping circuit flows into the relief valve and out the discharge port. 
         [0006]    Also disclosed herein is a pressure relief system for use with a pumping circuit. In one example the pressure relief system includes a relief valve, a body, an axial passage in the body, a discharge port formed in the body in fluid communication with a portion of the passage, a piston axially moveable within the passage and having a high pressure end in pressure communication with the pumping circuit and a low pressure end on a side opposite the high pressure end, and a back pressure space within the portion of the passage between the low pressure end of the piston and the body. The pressure relief system also may include a control system operating at a pressure that is a fraction of the pumping circuit operating pressure and having a selectively openable dump valve in a flow path between the back pressure space and a vent. 
         [0007]    In yet another example, disclosed herein is a relief valve for a pumping circuit, the relief valve can include a body, an inlet port, a backpressure space in the body, a high pressure surface having a cross sectional area in pressure communication with the pumping circuit, a low pressure surface having a cross sectional area that is greater than the cross sectional area of the high pressure surface and in pressure communication with the backpressure space, a coupling between the high pressure surface and the low pressure surface, so that when the pumping circuit is operating, the backpressure space is at a pressure having a value at least the value of the pumping circuit pressure multiplied by the ratio of a quotient of the high pressure surface cross sectional area divided by the low pressure surface cross sectional area, a resultant force within the coupling urges the high pressure surface into sealing engagement with the inlet port to block flow from the pumping circuit into the relief valve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Some of the features and benefits of the present disclosure having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
           [0009]      FIG. 1  is a side schematic view of a subterranean fracturing system in the process of fracturing a well. 
           [0010]      FIG. 2  is a side partial sectional view of a relieving system. 
           [0011]      FIG. 3  is an example of a control system for use with the relieving system of  FIG. 2 . 
           [0012]      FIG. 4  illustrates the control system of  FIG. 3  in an alternate configuration. 
           [0013]      FIG. 5  illustrates the control system of  FIG. 4  in an alternate configuration. 
       
    
    
       [0014]    While the subject device and method will be described in connection with the preferred embodiments but not limited thereto. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the present disclosure as defined by the appended claims. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The described method and system may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. 
         [0016]      FIG. 1  is schematic partial sectional view illustrating an example of a fracing system  1  delivering high pressure fluid into a subterranean formation. Example uses of the fluid include fracturing a formation, treating the well, or cementing casing. The system  1  includes a manifold  2  connected to pumps  3  that pressurize fluid for delivery to the subterranean formation. The pumps  3  each have a suction line shown drawing a fluid from a reservoir  4 . The reservoir  4  may include injection ports (not shown) for additive injection, such as a proppant, to the fluid. The pumps  3  discharge into a discharge line  11  shown extending to wellhead assembly  6 . After delivering fluid to the wellhead  6 , the fluid can be ported into the wellbore  7  as well as the formation  8 . A pressure relief system  10  is provided on the discharge line  11  to prevent over-pressuring of the discharge line  11 . In the embodiment shown, the pressure relief system  10  includes a relief valve  12  connected to the discharge line  11 , and a control system  25  also connected to the discharge line  11  and to the relief valve  12 . Examples of maximum operating pressure in the discharge line can include around 5,000 pounds per square inch, about 15,000 pounds per square inch, and values between. While blockage in the line  5  or within the wellbore  7  can cause over pressure, the inherent nature of the fracturing fluids and its constituents therein can also cause pressure excursions in the system  1  that can exceed a maximum desired pressure. 
         [0017]    With reference now to  FIG. 2 , a side partial sectional schematic of a relieving system  10  for use in high pressure fluid is shown. In this embodiment, discharge line  11  is shown with an attached relief valve  12 . As noted above, the discharge line  11  may include a pressurized fracturing fluid from a pump, such as from one or more of the pumps  3  in  FIG. 1 . The relief valve  12  illustrated includes a valve body  13  with an attached annular inlet  14 . The annular inlet  14  is shown threaded within the valve body  13  on one end, and the other end of the inlet  14  is shown flangedly connected to the discharge line  11 . An annular seating collar  51  is provided on the end of the inlet  14  projecting within the valve body  13 . The inlet  14  is ported along an axis A x  of the valve  12  and in open fluid communication with the discharge line  11 ; thereby providing fluid communication between the valve body  13  and the discharge line  11 . A passage  16  is shown formed through the valve body  13  oriented along the axis A x . A disk  24  is coaxially set in the passage  16  distal from the inlet  14 . A lip  39  extends radially from an end of the disk  24  and is shown resting on a shoulder  41  formed within the valve body  13  where the passage  16  extends radially outward. 
         [0018]    A piston  15  shown provided within the passage  16  and axially moveable therein. The passage  16  radius reduces at a transition  43  shown in the valve body  13  between the disk  24  and the inlet  14 . A portion of the piston  15  has a radius that is correspondingly reduced so that the reduced portion of the piston  15  is axially moveable within the passage  16  below the transition  43 . A seal  45  is shown disposed on the outer circumference of the piston  15  and on its larger diameter portion. Seals  47  are also illustrated along the inner surface of the passage  16  below the transition  43 . The seals  45 ,  47  can form a pressure barrier between the piston  15  and the passage  16  to prevent pressure communication along the length of the piston  15 . An annular relief port  19  is shown threadingly attached to the valve body  13  between the inlet  14  and the shoulder  43 . An open space through the relief port  19  provides communication from a portion of the passage  16  to outside of the valve body  13 . The pressure barrier provided by the seals  45 ,  47  isolates the open space in the relief port  19  from the portion of the passage  16  above the piston  15 . 
         [0019]    A valve element  17  is shown depending from the end of the piston  15  distal from the disk  24 . In the embodiment of  FIG. 2 , the valve element  17  includes a head portion  50  with a smaller diameter stem portion  49  projecting from the head portion  50 . The valve element  17  is attached to the piston  15  by insertion of the stem portion  49  into the lower end of the piston  15 . The head portion  50  is shown having a seating surface  20  on its side opposite where the stem portion  49 . The side of the head portion  50  having the seating surface  20  is shown beveled along its outer periphery. In the example of  FIG. 2 , the valve element  17  is shown in sealing contact with the seating collar  51  and blocking fluid flow through the inlet  14  and into the valve body  13 . The beveled portion of the seating surface  20  is shown sealingly mating with a correspondingly beveled surface  52  provided where the seating collar  41  opens into the valve body  13 . 
         [0020]    A control line  18  is shown in fluid communication with the portion of the passage  16  through a backpressure port  74  formed through the valve body  13  between the disk  24  and piston  15 . The control line  18  further communicates to a control system  25 . As will be described in further detail below, the control system  25  maintains a particular back pressure on the upper end of the piston  15  so that the valve  12  sustains a substantially precise discharge pressure. Preferably, cross sectional area of the side of the piston  15  facing the disk  24  exceeds the cross sectional area of the seating element  20 . The area differential on opposing sides of the piston  15  and valve element  17  assembly produces a resultant force urging the seating element  20  against the seating collar  51 ; this can occur even when pressure in the discharge line  11  exceeds pressure in the portion of the passage  16  above the piston  15 . The area differential thus can provide for lower pressures in the control line  18  and the control system  25 , which maintaining the valve element  17  in a seated configuration. One advantage of lowering pressure requirements in the control system  25  and its associated hardware, is that smaller and less expensive control elements may be used. 
         [0021]    In one example of use, the control system  25  maintains a pressure in the control line  18  and the portion of the passage  16  above the piston  15  of sufficient magnitude so that a resultant force is formed urging the valve element  17  into sealing engagement with the seating collar  51 . In one example however, the pressure maintained by the control system  25  is maintained at a value so that if the discharge line  11  pressure exceeds a set relieving pressure, the valve element  17  will unseat. Examples of set relieving pressures include up to about 5,000 pounds per square inch (psi), up to about 7,500 psi, up to about 10,000 psi, up to about 15,000 psi, and values therebetween. When the force applied on the valve element  17  from the fluid pressure in the discharge line  11  exceeds the force applied on the piston  15  from the fluid pressure in the control line  18 , the direction of the resultant force will reverse thereby sliding the valve element  17  and piston  15  will upwards away from the inlet  14 . Moving the valve element  17  away from the inlet  14  unseats the seating element  20  from the seating collar  51  to open fluid communication between the inlet  14  and the relief port  19 . The outlet port  19  is shown connected to an optional drain line  53 , so that any relieving flow from the discharge line  11  exiting the relief port  19  may be directed to desired collection site (not shown). 
         [0022]    A sensing unit  27  is shown attached to and in fluid communication with the discharge line  11 . In one example, the sensing unit  27  is disposed proximate the relief valve  12 . The sensing unit  27  as shown includes an outer housing  28  having an end in fluid communication with the discharge line  11  and another end in fluid communication with the control system  25 . A differential piston  29  is illustrated within the housing  28  having a high pressure head  55  on one end and a low pressure head  57  on an opposite end. The high pressure head  55  is shown having a cross sectional area less than the cross sectional area of the low pressure head  57 . The inner circumference of the housing  28  is sized to substantially match the outer profiles of both the high and low pressure heads  55 ,  57  and to maintain a sealing surface between the housing  28  and outer profiles of the high and low pressure heads  55 ,  57  with movement of the piston  29  axially within the housing  28 . Seals  56 ,  58  are shown respectfully provided on the outer peripheries of the high and low pressure heads  55 ,  57 . 
         [0023]    A surface of the high pressure head  55  is shown facing and in pressure communication with the discharge line  11 . An oppositely facing surface on the low pressure head  57  is in pressure communication with the control system  25  via a sensing line  31  shown connecting the housing  28  and control system  25 . As noted above, the disparate cross sectional areas of the high and low pressure heads  55 ,  57  define an area ratio between the high and low pressure heads  55 ,  57 . The area ratio requires the pressure in the discharge line  11  exceed the pressure on the low pressure head  57  by a factor substantially equal to the area ratio. 
         [0024]    The control system  25  includes a discharge or drain line  33  communicating the control system  25  to a drain  35 . The control system  25  may be hydraulically pressurized by a pump  21  shown having a discharges coupled with a charging line  22 . The charging line  22  extends from the discharge of the pump  21  and connects to the control system  25 . An accumulator  23  may optionally be included that is in pressure and/or fluid communication with the charging line  22 . In one example of use, the pump  21  pressurizes a fluid that is discharged into the charging line  22  and flows to the control system  25 . An example fluid pressurized by the pump  21  includes hydraulic fluid, incompressible liquids, and other liquids. The accumulator  23  is illustrated as a closed container with a space therein in which a volume of fluid can be stored. Some of the fluid being discharged into the charging line  22  may be diverted into the accumulator  23  and stored therein. A resilient member (not shown) can be disposed in the head space of the accumulator  23 . The resilient member can be a compressed gas, such as air, nitrogen, helium, and the like. Alternatively, a compressible bladder, a seal with a spring, or a sealed foam member can be used as the resilient member. Thus fluid from the pump  21  that is stored in the accumulator  23  can be maintained under pressure for a period of time. A line  30  also connects to the charging line  22  shown branching to the housing  28 . A check valve  37  may be provided in the line  30  to prevent backflow from the housing  28  to the charging line  22 . Thus any fluid urged from the housing is diverted to the control system  25 . 
         [0025]    The pump  21  can operate until pressure in the lines  22 ,  30 , accumulator  23 , and control system  25  is at a pre-determined pressure. This value can vary depending on design parameters of devices in communication with the pump  21 , but can be determined by those skilled in the art without undue experimentation. In one example of use, the pump  21  will charge the system to a pressure relating to the maximum pressure allowable in the discharge line  11  and the area ratio between the cross sectional area of the upper surface of the piston  15  and cross sectional area of the sealing element  20 . Optionally, the area ratio of the high and low pressure heads  55 ,  57  can be correlated to the maximum allowable pressure in the discharge line  11 . Alternatively, both area ratios can dictate the charging pressure of the pump  21 . Once charging is complete, the pump  21  can be deactivated. Pressurizing and sealing fluid in the charging line  22 , accumulator  23 , and control system  25  creates a pressurized system. In the event of some leakage in the system, pressure stored in the accumulator  23  can maintain a backpressure in the lines  22 ,  30  and control system  25 . 
         [0026]    The pressure in the discharge line  11  can be monitored by measuring fluid pressure in line  30 , line  31 , or the control system  25  and multiplying the measured pressure by the area ratio of the high and lower pressure heads  55 ,  57 . Depending upon the desired set or maximum pressure in the discharge line  11 , the control system  25  is configurable to actuate the relieve valve  12  upon sensing an established pressure via the sensing line  31 . In one example, when the pressure for relieving is detected in the sensing lines  30 ,  31  by the control system  25 , the control system  25  can provide fluid communication between the pressure line  18  and discharge line  33 . The discharge line  33  vents to atmosphere and thus is at ambient pressure; fluid in the portion of the passage  16  above the piston  15  is maintained at some pressure above ambient, and in some instances well above ambient. Thus fluid in passage  16  above the piston  15  will readily enter the pressure line when the control system  25  communicates the pressure line  18  to the discharge line  33 . Evacuating fluid from above the piston  15  removes the force for urging the valve element  17  downward. When the downward force on the valve element  17  is removed, pressure in the discharge line  11  unseats the seating element  20  from the seating collar  51  to allow fluid communication between the inlet  14  and relief port  19 . As fluid in the discharge line  11  flows through the inlet  14 , past the valve element  17 , and out the relief port  19 , it can be routed to a designated location via the drain line  53 . Directing the flow from the discharge line  11  and through the drain line  53  depressurizes the discharge line  11 . 
         [0027]    In  FIG. 3 , one example of a control system  25  is shown in a schematic view. In this embodiment, a sequence valve  32  is provided in line with the sensing line  31 . The sequence valve  32  may be selectively adjustable to open at a pre-determined pressure at the side in fluid communication with the sensing unit  27  (shown in phantom view). A pressure indicator  60  may be used in conjunction with the sequence valve  32  to establish the opening pressure. The embodiment of the control system  25  of  FIG. 3  also includes a selector valve  36 . The selector valve  36  is selectively moveable between different positions that in turn selectively directs flow across the selector valve  36 , then to a particular destination depending on the position of the selector valve  36 . Further illustrated in the embodiment of  FIG. 3 , a line  34  connects between the side of the sequence valve  32  opposite the sensing unit  27  with the selector valve  36 . A control line  38  is provided shown connected to an activation port  72  formed in a side of the selector valve opposite line  34  and extending into connection with a counter balance valve  42 . The counter balance valve  42  illustrated in  FIG. 3  includes a piston  44  that includes a flanged portion  64  with a reduced diameter body portion  66  projecting downward. The body portion  66  of the piston  44  is shown inserted into a cylinder  46 . An annular space  68  is illustrated provided within the counter balance valve  42  that circumscribes the piston  44  in the space adjacent where the body portion  66  joins the flanged portion  64 . A spring  62  provides a force to urge the piston  44  downward to maintain the body portion  66  within the cylinder  46 . The upper surface of the piston  44  shown facing the spring  62  is in pressure communication with the drain  35  via a drain line  48 . Thus the spring  62  and weight of the piston  44  are the only forces that need to be overcome to lift the piston  44 . 
         [0028]    Further illustrated in this example, the pressure line  18  connects to a return port  70  formed through a side of the counter balance valve  42 ; the discharge line  33  connects to a location on the lower end of the counter balance valve  42 . As shown, when the reduced diameter portion of the piston  44  is inserted within the cylinder  46 , fluid communication between the pressure line  18  and the discharge line  33  is blocked by the body portion  66 . Thus in the embodiment of  FIG. 3 , the counter balance valve  42  is in a closed configuration. 
         [0029]    For the non-limiting purpose of illustration herein, the position of the sequence valve  32  depicted in  FIG. 3  is referred to as the normal operating position. When in the normal operating position, flow from the sensing unit  27  and passing through the sequence valve  32 , is directed through the selector valve  36 , through the control line  38 , and to the counter-balance valve  42 . If the fluid from the sensing unit  27  in the line  31  exceeds the pre-determined pressure selected in the sequence valve  32 , the sequence valve  32  will open. When the sequence valve  32  is open, pressure and/or flow in the sensing unit  27  can communicate through the sequence valve  32  and selector valve  36  into the counter-balance valve  42  and to the annular space  68 . When the force exerted onto the piston  44  from pressure in the control line  38  exceeds the weight of the piston  44  and the force applied by the spring  62 , the piston  44  is moved upward thereby removing the body portion  66  from its pressure blocking position between the pressure line  18  and discharge line  33 . Communicating the pressure line  18  with the discharge line  33  provides an outlet for fluid in the pressure line  18  and above the piston  15 . Thus fluid within the valve body  13  above the piston  15  exits, to form a pressure imbalance across the piston  15  and allowing it to be urged upward towards the disk  24  by the higher pressure discharge fluid  11 . As discussed above, this positions the relief valve  12  in a relieving configuration to allow fluid from the discharge line  11  to enter the drain line  53 . 
         [0030]    The pressure from the discharge line  11  on the differential piston  29  decreases as the discharge line  11  flow is relieved. The relief valve  12  can remain in the open position and can be reset by repositioning the piston  44  to block flow between the pressure line  18  and the discharge line  33 . Shown in  FIG. 4  is an example of resetting the counter balance valve  42 . In this example the selector valve  36 A configuration is switched so that line  34  communicates with drain line  40  through the selector valve  36 A. In addition to resetting the counter balance valve  42 , this configuration of the selector valve  36 A could be used when recharging the control circuit  25 . This diverts the flow from the sensing unit  27  that provides the force for lifting the piston  44 . Optionally, fluid in the annular space  68  is directed to the drain line  48 , via the selector valve  36 , so the body portion  66  of the piston  44  can reinsert into the cylinder  46  and block communication between the pressure line  18  and the discharge line  33 . An advantage of the presently disclosed system is the selector valve  36  can require manual actuation before the relief valve  12  can be reset into operational mode. Thus in an embodiment, the relief valve  12  does not automatically reseat. Optionally, a relief valve (not shown) can be included in the charging line  22  to prevent the pump  21  from overpressuring this line or the components in this line. Moreover, in another alternative embodiment, a bleed valve can be included in one of line  31  or line  34  for venting of excess hydraulic fluid or other fluid in the lines  31 ,  34 . 
         [0031]    In one example of use, the piston  15  has a 3-to-1 surface area ratio between its upper end and the valve element  17 . Therefore, in situations where it is desired to have a set pressure at around 15,000 pounds per square inch, continuously maintaining pressure in the passage  16  above the piston  15  at 5,000 pounds per square inch can provide an adequate seating pressure. In one example, the side of the piston  15  and or differential piston  29  in fluid communication with the discharge line  11  is about one third that of the piston side in communication with the control system  25 . Optionally, the sequence valve  32  may be adjusted so the valve  12  opens at a pressure in the discharge line  11  at or below 15,000 pounds per square inch. It is within the capabilities of those skilled in the art to determine a sequence valve  32  setting that causes the valve  12  to open at a particular pressure. 
         [0032]    Referring now to  FIG. 5 , an alternate embodiment of a pressure relieving system  10 A is illustrated in a side schematic view. Further included in this embodiment is a flow control valve  76  shown having a port connected to the charging line  22  and to a control line  80 . The control line  80  is illustrated branching from pressure line  18 , and thus is in fluid and pressure communication with the space  16 . A piston  78  is shown provided within the flow control valve  76  that is selectively moved axially within the flow control valve  76  for blocking/enabling fluid communication between the charging line  22  and the control line  80 . In the example of  FIG. 5 , the piston  78  is moved upward and out of the flow path  79  between the charging line  22  and the control line  80 , thus allowing fluid from the charging line  22  to flow through the flow control valve  76 , the control line  80 , and space  16 . A backpressure line  82  represents pressure communication from the control line  80  to above the piston  78  for maintaining the piston  78  in a closed position. When the pressure line  18  is pressurized and not being vented to the discharge line  33 , the pressure on the piston  78  applied through backpressure line  82  urges the piston  78  into a closed position. However, after the pressure and/or fluid in the pressure line  18  is evacuated, and fluid flow from the pump  21  and charging line  22  is applied to the flow control valve  76 , the piston  78  can be urged upward to open the valve and allow flow therethrough. Pressure drops through the flow control valve  76  and disparate cross sectional areas above and below the piston  78  results in a resultant upward force on the piston  78  when fluid flows from the charging line  22  to the control line  80 . An optional spring  77  is shown above the piston  78  for providing a downward urging force onto the piston  78  which can be used to close the flow control valve  76 , such as when no flow and/or pressure is applied to the flow control valve  76 . 
         [0033]    While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.