Abstract:
A fluid pump having a pump housing that includes at least one expansion joint, provides volume compensation, as needed, to adjust for changes in pressure in the pump housing. Various embodiments automatically and passively reduce static pressure in the pump housing associated with a freezing event, thereby preventing damage to the pump head. Volume compensation is achieved by employing, in each expansion joint, a dynamic seal that allows relative movement of two portions of the pump housing, and a bias that provides a selected counter-force to the movement of the housing portions. The subject pumps are particularly suited for use in automotive and other rugged applications, in which fluid pumps may experience recurring freezing events.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This claims the benefit of U.S. Provisional Patent Application No. 61/360,835, filed on Jul. 1, 2010, which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     This disclosure pertains to, inter alia, gear pumps and other pumps configured to operate in a substantially primed condition to urge flow of a fluid. The subject pumps and pump heads include various types having one or more rotary pumping members, such as meshed gears, or at least one pumping member that operates continuously in a cyclic manner. More specifically, the disclosure pertains to pumps and pump heads capable of accommodating a change in internal volume in the pump head caused by, for example, a freezing event, a pressure fluctuation, or the like involving fluid in the pump head. 
     BACKGROUND 
     Several types of pumps are especially useful for pumping fluids with minimal back-flow and that are amenable to miniaturization. An example is a gear pump. Another example is a piston pump. A third example is a variation of a gear pump in which the rotary pumping members have lobes that interdigitate with each other. Gear pumps and related pumps have experienced substantial acceptance in the art due to their comparatively small size, quiet operation, reliability, and cleanliness of operation with respect to the fluid being pumped. Gear pumps and related pumps also are advantageous for pumping fluids while keeping the fluids isolated from the external environment. This latter benefit has been further enhanced with the advent of magnetically coupled pump-drive mechanisms that have eliminated leak-prone hydraulic seals that otherwise would be required around pump-drive shafts. 
     Gear pumps have been adapted for use in many applications, including applications requiring extremely accurate delivery of a fluid to a point of use. Consequently, these pumps are widely used in medical devices and scientific instrumentation. Developments in many other areas of technology have generated new venues for accurate pumps and related fluid-delivery systems. Such applications include, for example, delivery of liquids in any of various automotive applications. 
     Automotive applications are demanding from technical, reliability, and environmental viewpoints. Technical demands include spatial constraints, ease of assembly and repair, and efficacy. Reliability demands include requirements for high durability, vibration-resistance, leak-resistance, maintenance of hydraulic prime, and long service life. Environmental demands include internal and external corrosion resistance, and ability to operate over a wide temperature range. 
     A typical automotive temperature range includes temperatures substantially below the freezing temperature of water and other dilute aqueous liquids. These temperatures can be experienced, for example, whenever an automobile is left out in freezing winter climate. A property that is characteristic of water and most aqueous solutions is that they tend to expand as they undergo a phase change from liquid to solid (ice). As is well known, household plumbing systems exposed to sub-freezing temperatures may develop static pressures produced by freeze-expansion that are sufficiently high to fracture pipes. Thus, these pressures can cause substantial damage to a pump that is coupled, in a primed condition, to a hydraulic circuit exposed to a sub-freezing temperature. 
     In view of the above, a simple solution is to add anti-freeze to the liquid or to constitute the liquid with sufficient solute to depress its freezing point. Unfortunately, changing the liquid in these ways changes the composition and possibly other important properties of the liquid, which may render the liquid ineffective for its intended purpose. 
     U.S. Pat. No. 8,323,008, (hereinafter the “&#39;008 patent”), discloses pumps and pump heads comprising internal pressure-absorbing member(s) for alleviating at least some of a pressure increase occurring inside the pump head. The pressure-absorbing member is located inside the pump housing at a non-wearing location and contacts the fluid being pumped by the pump head. The pressure-absorbing member has a compliant property to exhibit a volumetric compression when subjected to a pressure increase in the fluid contacting the pressure-absorbing member. Pumps and pump heads as disclosed herein take a different approach to alleviating pressure inside the pump head. 
     SUMMARY 
     Generally provided herein are disclosures of pumps and pump heads that, when primed, can volumetrically compensate for, or at least partially offset changes in, internal volume so as to nullify or at least reduce corresponding changes of internal pressure in the pump head that otherwise would be caused by the internal-volume changes. The change in internal pressure can be static, as in a freezing event, or it can be dynamic. 
     The term “fluid” is meant to encompass liquids and other substances, such as, for example, gels, pastes, slurries, high-viscosity liquids, and the like, that share at least some properties of liquids. The devices, systems, and methods described herein may, in certain instances, be applicable to gaseous-type fluids. 
     The subject pumps and pump heads operate in a substantially primed condition. Because liquids are substantially non-compressible, conventional pumps operating in a primed condition are vulnerable to pressure damage if liquid in the pumps is allowed to freeze and possibly undergo freeze-expansion. In a conventional primed pump, it may be very difficult or impossible for the liquid to find additional hydraulic space for expansion as the liquid freezes. Pumps and pump heads as disclosed herein are equipped with expansion features that automatically provide additional hydraulic space, as needed, to accommodate these pressure increases. This provision of additional hydraulic space may occur repeatedly over an indefinite time period and can be maintained in a static manner, which is effective for reducing pressure increases within the pump that accompanying freezing of the liquid in the pump. 
     The various embodiments are particularly effective for reducing static pressure accompanying events such as freezing events. The events may occur occasionally or regularly (such as every night in a freezing cold external environment). The reduction in pressure is achieved by the pump housing or portion thereof expanding a corresponding amount in a defined direction. The expansion is automatic and passive, occurs without external leaks, and is automatically reversible as external conditions change. In addition, any of the embodiments disclosed herein can include at least one internal pressure-absorbing member as disclosed in the &#39;008 patent cited above. Such a combination of an expansion joint and a pressure-absorbing member is particularly effective for alleviating both dynamic and static pressures. 
     Various embodiments of a pump comprise a pump housing defining a pump cavity that has at least one inlet, and at least one outlet. The pump includes a movable pumping member situated in the pump cavity. The pumping member, when driven to move, urges flow of the liquid from the inlet through the pump cavity to the outlet. The pump exhibits volumetric (and hence pressure) compensation, but in a manner that is different from the manner discussed in the patent cited above. Specifically, the pump housing in this embodiment comprises walls that can be termed “pressure-boundary” walls. Pressure compensation is provided by the pump housing correspondingly changing the area of at least one of (or a portion of) its pressure boundary walls in response to a pressure change inside the pump housing. For example, the pump housing has first and second portions, wherein the second portion is movable in a particular direction relative to the first portion in a way that increases or decreases the volume inside the pump housing. This movement occurs without the pump head “breaking prime,” by means of a dynamic seal. An increased volume inside the housing causes a corresponding pressure decrease inside the housing. In the &#39;008 patent, in contrast, the area of the housing walls is kept substantially fixed while, inside the housing, a pressure-absorbing member changes its volume in response to a pressure increase in the housing. It is understood that the internal force necessary to expand the housing must be less than the burst strength of the housing. Otherwise, the housing could burst during a freezing event before the dynamic seal releases movement of the housing portions. 
     In the subject embodiments, the internal pressure-absorbing member can be omitted because the housing wall, by making pressure-responsive changes in surface area, achieves the desired corresponding reduction of pressure inside the housing. In other embodiments, however, the features of embodiments described herein may be used in conjunction with features disclosed in the &#39;008 patent. 
     In certain embodiments of the pump, the movable pumping member comprises a rotatable pumping member, such as at least one gear. These gear-including embodiments typically have at least one “driving” gear and at least one “driven” gear that contra-rotate about their respective axes in the usual manner of gear pumps. In other embodiments the movable pumping member comprises at least one piston that typically undergoes a reciprocating motion. 
     The operable part of a pump, aside from the “mover” used to actuate the pump, is often referred to as a “pump head.” Pump heads can be manufactured and distributed as modular units that can be coupled to various movers. Example movers are any of various types of motors that can be coupled directly or indirectly to the movable pumping member in the pump head. Actuation of the mover causes corresponding motion of the movable pumping member in a pump cavity. An example mover includes a magnet coupled to the movable pumping member, and a magnet driver magnetically coupled to the magnet to move the magnet (e.g., rotate it about its axis) and thus move the pumping member in a pump cavity. Pumps including magnetic movers are generally termed “magnetically actuated” pumps. Such pumps are advantageous because they do not require dynamic seals such as shaft seals, which are prone to leaks. Alternatively, the mover can include a mechanical, rather than magnetic, coupling to the movable pumping member such as, for example, a direct coupling to the armature of an electrical motor. 
     Any of various embodiments of the pump can further include one or more sensors in fluid communication with the liquid in the pump housing. Example sensors include, but are not limited to, pressure sensors, temperature sensors, flow sensors, chemical sensors, and the like. 
     This disclosure pertains to gear pump heads as well as to gear pumps. Each of several embodiments of a gear pump head comprise a pump housing that defines a gear-cavity, at least one inlet hydraulically coupled to the gear-cavity, at least one outlet hydraulically coupled to the gear-cavity, and at least one interior non-wearing location that contacts fluid in the pump housing. At least one driving gear and one driven gear are enmeshed with each other in the gear-cavity. The housing of the gear pump head can further include a cup-housing (also termed a “magnet cup”). The magnet cup defines a magnet-cup-cavity in hydraulic communication with the gear-cavity. The magnet-cup-cavity contains the liquid and a rotatable driven magnet that is coupled to the driving gear such that rotation of the driven magnet about its axis causes corresponding rotation of the driving gear and thus of the driven gear. These embodiments can impart rotation to the magnet by magnetically coupling the magnet to a second magnet, called a “driving” magnet mounted on the armature of a motor. Alternatively, rotation of the driven magnet can be caused by placing a stator in coaxial surrounding relationship to, but outside of, the magnet cup. The stator is magnetically coupled to the driven magnet so as to cause, whenever the stator is electrically energized, rotation of the driven magnet. This latter embodiment eliminates the need for a driving magnet. 
     This disclosure also pertains to hydraulic circuits such as those used in automobiles and other vehicles. An exemplary hydraulic circuit comprises a pump, such as any of the embodiments disclosed herein, a liquid source hydraulically connected upstream of the pump to the pump inlet, and a liquid-discharge port hydraulically connected downstream of the pump to the pump outlet. The pump can be, by way of example, a gear pump or a piston pump, but it will be understood that these specific pumps are not intended to be limiting. It is contemplated that various other specific types of pumps can readily include a volume-compensation feature as discussed herein. 
     This disclosure also pertains to methods, in the context of a method for pumping a liquid using a substantially primed pump, for preventing a fluid cavity of the pump from experiencing at least a threshold magnitude of pressure increase within the fluid cavity. The threshold magnitude can be, for example, a pressure condition generated in the fluid cavity if the liquid in the fluid cavity became at least partially frozen and experienced a corresponding increase in volume. Alternatively or in addition, the threshold magnitude may be a pressure condition generated in the fluid cavity as a result of a pressure fluctuation of the liquid in the fluid cavity accompanying operation of the pump. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a prior art magnetically driven gear pump. 
         FIG. 2  is an orthogonal end view of the gear pump shown in  FIG. 1 , in which the pump head is visible. 
         FIG. 3  is an orthogonal end view of the gear pump shown in  FIG. 1 , in which the end-plate and electrical connections opposite the pump head are visible. 
         FIG. 4  is a cross-sectional view of the magnetically driven gear pump shown in  FIG. 1 . 
         FIG. 5  is a detail view of the cross-section shown in  FIG. 4 , in which the magnet-cup portion of the pump is magnified. 
         FIG. 6  is a schematic diagram of a first exemplary embodiment of a pump head equipped with a volume-compensation feature. 
         FIG. 7  is a schematic diagram of a second exemplary embodiment of a pump head equipped with a volume-compensation feature different from that shown in  FIG. 6 . 
         FIG. 8  is a cross-sectional view of a magnetically driven gear pump similar to that shown in  FIGS. 1-5 , equipped with an expansion joint biased with spring coils to provide volume compensation. 
         FIG. 9  is a cross-sectional view of a magnetically driven gear pump similar to that shown in  FIGS. 1-5 , equipped with an expansion joint that uses a spring-loaded clamp ring to provide volume compensation. 
         FIG. 10  is a cross-sectional view of a magnetically driven gear pump equipped with an expansion joint and a bellows. 
         FIG. 11  is a block diagram of a hydraulic circuit comprising a pump equipped with a volume-compensation feature. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, the singular forms “a,” “an,” and “the” include the plural forms unless the context clearly dictates otherwise. Additionally, the term “includes” means “comprises.” Further, the term “coupled” encompasses mechanical as well as other practical ways of coupling or linking items together, and does not exclude the presence of intermediate elements between the coupled items. 
     The devices, systems and methods described herein should not be construed as being limiting in any way. Instead, this disclosure is directed toward all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The disclosed devices, systems and methods are not limited to any specific aspect or feature or combinations thereof, nor do the disclosed devices, systems and methods require that any specific advantages be present or problems be solved. 
     Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed devices, systems and methods can be used in conjunction with other devices, systems and methods. Additionally, the description sometimes uses terms like “produce” and “provide” to describe the disclosed methods. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms may vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art. 
     In the following description, certain terms may be used such as “up,” “down,”, “upper,” “lower,” “horizontal,” “vertical,” “left,” “right,” and the like. These terms are used, where applicable, to provide clarity of description when dealing with relative relationships. But, these terms are not intended to imply absolute relationships, positions, and/or orientations. For example, with respect to an object, an “upper” surface can become a “lower” surface simply by turning the object over. Nevertheless, it is still the same object. 
     Certain general features of an exemplary gear pump  10  are depicted in  FIGS. 1-5 . The gear pump  10  may be magnetically driven. It will be understood that “gear” as used herein encompasses rotary members configured as conventional pump gears as well as any of various other rotary members having lobes, teeth or the like that interdigitate with the same of a second such member to produce fluid flow, when contra-rotated relative to each other. 
     With reference to  FIGS. 1-3 , the pump  10  comprises an actuator portion  12  and a pump-head portion  14 , which are symmetric about an axis  15 . The actuator portion  12  comprises an outer casing  16 , a first end-plate  18 , and a second end-plate  20 . The actuator portion  12  contains a “mover” for the pump-head portion  14 , as described below. The end-plate  18  may be attached to the casing  16  by hexagonal bolts  21 . The pump-head portion  14  includes a fitting block  24  that defines a fluid-inlet port  25   a  and a fluid-outlet port  25   b  (only the fluid-outlet port  25   b  is visible in  FIG. 1 , but the fluid-inlet port  25   a  is shown in  FIG. 2 ). As shown in  FIG. 3 , the second end-plate  20  includes a pair of threaded electrical connectors  22 . 
     With reference to  FIGS. 4 and 5 , the pump-head portion  14  also includes a cup-housing  28  that contains a rotatable magnet  30  mounted to a shaft  32 . The shaft  32  is mounted to a driving gear  34  that rotates and that is interdigitated (meshed) with a driven gear  36 . The gears  34 ,  36  are situated in a gear-cavity  38  (a portion of the “pump cavity” that also includes the interior surfaces of the inlet and outlet ports). The gear-cavity  38  and the interior of the cup-housing  28  (“cup-cavity”) are wetted by liquid being pumped by the pump  10 . The magnet  30  has multiple magnetic poles that are magnetically coupled, in this embodiment, through the wall of the cup-housing  28 , to a stator  40  contained within the outer casing  16 . The stator  40  comprises wire windings  42  associated with a ferrous core  44  that surrounds, and is co-axial with, the cup-housing  28 . The windings  42  are selectively energized by electronics  46  also contained within the outer casing  16 . Power is supplied to the electronics  46  via the connectors  22 . Thus, energization of the stator  40  causes axial rotation of the magnet  30 , which rotates the driving gear  34 , which in turn rotates the driven gear  36 . This contra-rotation of the gears  34 ,  36  urges flow of liquid through the cavity  38 . For improved operation with certain liquids, the cavity  38  optionally may include a suction shoe (not detailed). 
     The fitting block  24  defines passageways leading to and from the cavity  38  and connecting the cavity  38  to the inlet and outlet ports  25   a  and  25   b . If desired or required, the fitting block  24  also includes a pressure transducer  26  (that can be hydraulically connected to the outlet port  25   b , for example). The pressure transducer  26  includes an electrical connector  27 , permitting electrical connection of the pressure transducer  26  in a manner that establishes, for example, feedback control of energization of the stator  40 . The pressure transducer  26  and the electrical connector  27  may be skewed with respect to axis  15 . 
     As shown in  FIG. 5 , the fitting block  24  is coupled to the end-plate  18  and is sealed against the rim of the cup-housing  28  to establish, within the cup-housing  28 , a cup-cavity  52 . The cup-cavity  52  is sealed using a static seal  54  (e.g., an O-ring). The cup-cavity  52  is in hydraulic communication with the gear-cavity  38 , and hence both are wetted by the pumped liquid, as noted above. Also, during normal operation, at least the cup-cavity  52  and gear-cavity  38  are substantially primed with the liquid being pumped. 
     The gear pump  10  can be made of any of various materials that are inert to the particular fluid to be pumped. For example, a high performance organic polymer thermoplastic such as polyether ether ketone (PEEK) may be used to fabricate the gears  34 ,  36  and the cup-housing  28 . 
     The range of candidate pump heads is not limited to heads for gear pumps. An exemplary alternative type of pump head is a valveless piston pump. A valveless piston pump is disclosed in, for example, U.S. Patent Publication No. 2007-0237658, incorporated herein by reference. See particularly FIG. 11 of the &#39;658 reference and the accompanying discussion on pages 9-14 thereof. 
     The embodiment now to be described is directed to a pump head having a housing that provides volumetric compensation without the need for an internal pressure-absorbing member. The basic concepts of this embodiment are: (1) the housing comprises multiple (at least two) portions that are conjoined in such a way that at least one portion can move relative to another portion (or multiple portions can move relative to each other) to produce an alleviating volumetric response to a pressure change, such as a pressure increase inside the housing; (2) at least two portions of the housing are connected together at a housing expansion joint; (3) the expansion joint constrains relative motion of the housing portion(s) to a desired direction(s); (4) the expansion joint has a dynamic seal; and (5) the expansion joint has a bias (e.g., is spring-loaded). 
     A key feature in maintaining the seal integrity of the pump is the use of a dynamic seal that engages in the direction(s) that are constrained, while allowing at least one of the housing portions to move in one or more other directions (or axes) without leaking or breaking prime, thereby providing an expansion or contraction in housing volume in response to pressure inside the housing. The bias provides a restoring force that allows the expansion joint to be self-resetting. Alleviating a pressure increase can be sufficient to prevent freeze-expansion damage to the pump, and/or can be sufficient to reduce pressure fluctuations in the pumped liquid, such as at the outlet of the pump. Alleviation of pressure fluctuations is further facilitated by the ability of the movable portion of the pump housing to exhibit a volumetric contraction when subjected to a pressure decrease in the housing. 
     According to the present embodiment, volumetric (and hence pressure) compensation is achieved by the housing itself correspondingly changing the area of at least one of its pressure boundary walls or portion thereof. To illustrate, consider a pump housing such as any of the housings in the embodiments described above. The wall in substantially any part of the housing represents a pressure boundary, and hence is a pressure-boundary wall. (If there were no pressure difference across the wall, there would be little to no pumping action produced by the pump. This happens, for example, when a pump head loses prime.) The wall constitutes a pressure boundary because the pressure inside the housing is different (usually greater) than the pressure outside the wall. By definition, pressure is force per unit area, so a change in surface area of a pressure-boundary wall yields a corresponding change in pressure within the housing. As a portion of the pressure-boundary wall expands to increase the volume inside the housing it produces a corresponding increase in the surface area of the pressure-boundary wall, and in turn a corresponding pressure decrease inside the housing. 
     In contrast, in the pump heads disclosed in the &#39;008 patent publication, the area of the pressure boundary is kept substantially constant as a pressure-absorbing member(s) inside the housing is compressed. Thus, the pressure-absorbing member(s) exhibit a reduction in thickness and an increase in surface area in response to the pressure increase. In the embodiments disclosed herein, in contrast, internal pressure-absorbing members can be omitted because the housing wall, by making pressure-responsive changes in surface area, achieves the desired corresponding reduction of pressure inside the housing. 
     Reference is now made to  FIG. 6 , depicting a pump head  600 . The pump head  600  includes a housing  602 , an inlet  604 , an outlet  606 , and a pump element  608  (e.g., a rotor, piston, or set of pump gears). As the pump element  608  moves, fluid enters the pump head  600  through the inlet  604 , passes through the housing  602 , and exits through the outlet  606 . The pump head  600  normally operates in a primed condition, and the pressure inside at least most portions of the housing  602  is normally greater than the pressure outside the housing. The housing  602  comprises a first portion  610  and a second portion  612 . The second portion  612  is fitted to (e.g., slip-fitted in) the first portion  610  such that the second portion engages the first portion  610  in a manner allowing the second portion  612  to move relative to the first portion  610  in the horizontal direction  614  shown in  FIG. 6 . Thus, motion of the second portion  612  is constrained in substantially all but the horizontal direction  614 . Meanwhile, motion of the first portion  610  is constrained by a fixed structure  616  from moving in any direction. The first and second portions  610 ,  612  are conjoined at a housing expansion joint  619 , which may comprise compliance means such as, for example, a dynamic seal  618  (e.g., a ring seal such as an O-ring), or a bellows. The interior of the housing  602  remains sealed from the external environment regardless of motion of the second portion  612  relative to the first portion  610 . Motion of the second portion  612  desirably is against a bias  620  (e.g., a compression spring) secured against stationary structure  622 . The bias  620  and actuation of the pump element  608  establish a nominal pressure inside the housing  602 . If the internal pressure increases, the second portion  612  moves to the left in  FIG. 6 , relative to the first portion  610 , to increase the volume inside the housing  602  and thereby reduce the internal pressure. Likewise, if the internal pressure decreases, the second portion  612  automatically moves to the right in  FIG. 6 , relative to the first portion  610 , to decrease the volume inside the housing  602  and thereby increase the internal pressure. 
     A variation of the general configuration is shown in  FIG. 7 , showing a pump head  750  including a housing  752 , an inlet  754 , an outlet  756 , and a pump element  758  located inside the housing. The housing  752  includes a first portion  760  and a second portion  762 . The second portion  762  is movable relative to the first portion  760  (see arrow  764 ). A structure  766  substantially immobilizes the first portion  760 , allowing the second portion  762  to move relative to the first portion  760  in response to a pressure change inside the housing  752 . Movement of the second portion  762  desirably is against a bias  770  (e.g., a compression spring) held by a stationary structure  772 . The second portion  762  is connected to the first portion  760  at an expansion joint  769  including a dynamic seal  768  (e.g., an O-ring). 
     The bias  770  and actuation of the pump element  758  establish a nominal pressure inside the housing  752 . If the internal pressure increases, the second portion  762  automatically moves downward in  FIG. 7 , relative to the first portion  760 , to increase the volume inside the housing  752 , thereby reducing the internal pressure so as to prevent the pump head from fracturing or developing cracks. If the internal pressure decreases, the second portion  762  automatically moves upward in  FIG. 7 , relative to the first portion  760 , to decrease the volume inside the housing  752  and thereby increase the internal pressure. 
     A more specific configuration is shown in  FIG. 8 , which is a cross-section of a gear pump  802  featuring a pair of expansion features  803 . The gear pump  802  is driven by a rotating magnet  804  mounted to a spring-loaded shaft  805  surrounded by a magnet cup  806 . A pump housing  810  comprises three portions: the magnet cup  806 , a portion  808  that extends proximally from the magnet cup, and a pump block  814 . The portion  808  encloses pump elements such as gears  812 . Respective axles for the gears  812  and for the magnet  804  are secured in the pump block  818  along an axis  815 , about which the gear pump  802  is generally symmetric. The magnet cup  806  and portion  808  are contiguous with each other, but the portion  808  as shown in  FIG. 8  has a greater diameter than the magnet cup  806  to accommodate the pump elements  812 . A pump block  818  having a cylindrical outside surface  816  defines a pump inlet  817  and a pump outlet  818 . The housing portions  806 ,  808 , and pump block  818  collectively constitute a pressure vessel of the pump  802  and collectively establish a pressure boundary of the pump  802 . The magnet cup  806  and magnet  804  are coaxial with and surrounded by a stator (not shown) located inside the housing  810 . The housing  810  and the stator are located outside the pressure boundary of the pump. 
     The housing portions  806 ,  808  and the pump block  818  collectively define the pump housing. The portions  806 ,  808  can be regarded as a first housing portion that is slidable as a unit relative to the pump block  818 , which can be regarded as a second housing portion. The first and second housing portions are in hydraulic communication with each other and are both wetted by the pumped fluid. Note arrows  821  in  FIG. 8 , indicating a volumetric expansion of the housing by upward movement of the portions  806 ,  808  relative to the pump block  818 . Meanwhile, pressure integrity inside the pump housing is maintained, despite such movement, by a sliding dynamic seal  819  (e.g., a radial O-ring) located between the inside wall of the housing portion  808  and the outside wall of the pump block  818 . As shown in  FIG. 8 , the sliding seal  819  allows for axial movement of the pressure boundary (portions  806 ,  808 , collectively) so as to alleviate a substantial increase in pressure that otherwise would occur if, for example, the primed liquid contents of the pump housing became frozen. As the portions  806 ,  808  move, they impart a corresponding displacement of a connecting ring  822  against a bias provided by springs  824 . The springs  824  are held in place by screws  820  extending through the connecting ring  822 , and through the housing  810 , and threaded into a securing ring  826  attached to the pump block  818 . As the portions  806 ,  808  move, the shaft  805  also moves, such that when the springs  828  are compressed, the shaft spring  832  is correspondingly released, and vice versa. At the end opposite shaft spring  832 , motion of the shaft  805  is constrained by the magnet cup  806 . 
     The sliding dynamic seal  819  extends circumferentially around the pump block  818 . The sealing area is against an inside-diameter surface  828  of the first housing portion  806 ,  808 . As the first housing portion  806 ,  808  is allowed to move in the axial direction against the spring bias, the seal  819  retains its sealing integrity. The seal  819 , situated in a circumferential gland  830 , defined in the cylindrical outside surface of the pump block  818 , allows the portion  808  to slide relative to it. This sliding motion generally does not affect the immediate environment or action of the pump gears  812 , so the pumping action is generally unaffected, adversely or otherwise, by movement of the first housing portion  806 ,  808  relative to the second housing portion  818 . 
     Thus, compensation for pressure increases in the pump housing (which could be due, for example, to expansion of freezing liquid inside the pump housing) is achieved by increasing the volume inside the pressure boundary by expanding a selected area of the housing walls. This represents a different approach than the configurations discussed in the &#39;008 patent in which the pressure boundary of the housing is kept fixed, and fluid-volume expansions are compensated by decreasing the volume of a pressure-absorbing member located inside the pressure boundary. It will be understood that the embodiment of  FIG. 8  (and of  FIG. 9  discussed below) can include at least one internal pressure-absorbing member as discussed in the &#39;008 patent publication, incorporated herein by reference. 
     Another volume-compensating configuration is shown in  FIG. 9 , in which a gear pump  902  is equipped with a different type of expansion joint  903 , similar to the embodiment of the expansion joint  803  shown in  FIG. 8  except that, in the  FIG. 9  embodiment, the springs  824  (serving as the bias) are replaced by a clamp ring  906  that is integrally spring-loaded and essentially functions as a spring washer. The embodiment of  FIG. 9  is one example of a manner in which the spring(s) can be replaced by a combination of materials and/or structures to achieve a desired bias, or restoring force. In the depicted embodiment the clamp ring  906  both holds the portion  808  in place and provides the desired bias. Thus, the clamp ring has a shape that provides spring-loading on the portion  808  in the axial direction (vertical direction in the figure). 
     Another exemplary embodiment of a gear pump is a bellows gear pump  912 , as shown in  FIG. 10 . Bellows gear pump,  912  is similar to the gear pump  902  shown in  FIG. 9 , with the addition of a bellows  914  located at the magnet-cup end of the shaft  805 . As shown, the bellows  914  provides a further bias to absorb expansion of the pump housing  810  through the expansion joint  903 . An alternative configuration of a bellows gear pump may incorporate a bellows as a compliance means in place of the sliding seal  819 , to provide elastic coupling within expansion joint  903 . 
     An advantage of the foregoing embodiments is that their performance of pressure relief is done automatically and passively, simply in response to pressure conditions inside the pump housing. As the pressure increases, the volume inside the housing increases, and as the pressure decreases, the volume inside the housing decreases. 
     A hydraulic circuit  1000  comprising a pump, such as any of the specific embodiments described above, is shown in  FIG. 11 , which includes a pump and pressure sensor  1020  having an inlet  1040  and an outlet  1060 . The inlet  1040  is situated downstream of a filter  1080 , which is situated downstream of a tank  1100  serving as a reservoir for liquid to be pumped by the pump  1020 . The outlet  1060  is hydraulically connected to a downstream injector  1120  or other component from which pumped liquid is discharged from the circuit  1000 . If desired, the circuit  1000  can include a return line  1140  for returning liquid to the tank  1100  that is not actually discharged from the injector  1120 . The circuit  1000  in  FIG. 11  represents a circuit as used in an automotive application, in which at least the pump and pressure sensor  1020  is located in an environment that experiences episodes of freezing. Since the pump  1020  includes a pressure-relieving feature as described above, freeze-expansion of liquid inside the pump  1020  is accommodated, and pump damage is prevented.