Abstract: “The disclosure relates to a pump at least for conveying a fluid, comprising at least one conveyor device which has at least one conveyor chamber, at least one dimensionally stable conveyor chamber element that at least partly delimits the conveyor chamber, and at least one elastically deformable conveyor element that together with the conveyor chamber element delimits the conveyor chamber and is arranged on the conveyor chamber element; at least one drive unit for acting on the conveyor device; and at least one housing for receiving the conveyor device, wherein the housing is formed at least separately from the conveyor chamber element of the conveyor device, in particular from the conveyor device as a whole, in particular such that the conveyor chamber element, in particular the conveyor device, can be removed as a whole from the housing.

Abstract: A system includes a tubular body, a pressure actuated circulation valve, a first power section, an isolation valve, and a housing. The pressure actuated circulation valve is connected to the tubular body. The first power section is configured to rotate when a fluid is pumped from the pressure actuated circulation valve. The isolation valve is connected to the first power section. The isolation valve comprises a plurality of openings each having a different area and the fluid is exposed to a plurality of different flow path areas. The housing is connected to the isolation valve and comprises a set of springs connected to a piston. The set of springs are configured to pulsate the piston in response to the fluid being pumped through the plurality of different flow path areas and pulsation of the piston creates an axial force acting on the tubular body.

Abstract: Tapered stator designs are engineered in a positive displacement motor (PDM) power section to relieve stator stress concentrations at the lower (downhole) end of the power section in the presence of rotor tilt. A contoured stress relief (i.e. a taper) is provided in the stator to compensate for rotor tilt, where the taper is preferably more aggressive at the lower end of the stator near the bit.

Abstract: A stator element of a progressive cavity pump having a reinforcement tube having a longitudinal axis, an inner face (22), and an outer face, and an elastomer liner fixed to the inner face of the reinforcement tube, wherein a portion of the reinforcement tube has a substantially constant thickness (e) and in that said portion of the reinforcement tube is deformed such that it comprises at least a first relief pattern and a second relief pattern, the first relief pattern having the shape of a helical strip that is right-handed relative to the longitudinal axis, the second relief pattern having the shape of a helical strip that is left-handed relative to the longitudinal axis, the first and second relief patterns meeting in at least one section.

Abstract: A progressing cavity device operates as a motor to impart torque to a bit. A stator of the device defines an internal profile having uphole stages with a first dimension being less than a second dimension of downhole stage. A rotor has an external profile with a constant outer dimension along its length. Disposed in the stator, the rotor defines cavities with the stator and is rotatable with pumped fluid progressing in the cavities from the uphole to downhole to transfer torque to the drive toward the downhole end. Although the rotor is subjected at the downhole end to a reactive torque from the bit, the interference fit of the rotor's constant dimension with the stator's downhole stages is less than with the uphole stages, which can mitigate issues with heat buildup in the downhole stages. The device can also operates as a progressing cavity pump.

Abstract: A method for operating a device for dosed supply of a liquid, having a pump to deliver the liquid. The pump has an inlet and an outlet. An eccentric is arranged on the pump housing and a deformable diaphragm is arranged between a pump housing and the eccentric. The deformable diaphragm and the pump housing delimit a delivery path from the inlet to the outlet. The seal can be displaced along the delivery path by movement of the eccentric. A pressure sensor is connected to the outlet of the pump. A liquid is delivered by the pump. A time curve of the pressure at the outlet of the pump is monitored during delivery by the at least one pressure sensor. An angle position of the eccentric of the pump is detected using at least one characteristic feature of the time curve at the outlet.

Abstract: Embodiments disclosed herein relate to a composition useful for forming a stator or a portion thereof. The composition may include: a curable elastomer; a fiber or fibrous compound; a fiber dispersion compound; and optionally carbon black.

Abstract: A stator for a helical gear device includes a first section having first helically convoluted chamber with a set of radially inwardly extending lobes and a second section adjacent to the first section. The second section includes a stack of cutter disks. Each cutter disk includes a front surface, a rear surface, an interior surface defining a central opening extending from the front surface to the rear surface, a forward cutting edge, and a rearward cutting edge. The interior surface forms a same number of lobes for the central opening as the set of radially inwardly extending lobes in the first section. Each cutter disk is aligned along a common centerline, and each cutter disk is rotated slightly relative to each other to form a second helically convoluted chamber with a same pitch as the first helically convoluted chamber. The second helically convoluted chamber exposes, to materials passing through, portions of the forward cutting edge or the rearward cutting edge of each cutter disk.

Abstract: Techniques involve a motor assembly including a rotor and a stator. The stator includes a contact surface for contacting an outer surface of the rotor. The contact surface includes a rigid material. The motor assembly also includes at least one constraint disposed along a length of the motor assembly, where the constraint constrains a radial and/or tangential movement of the rotor relative to the stator. The at least one constraint may be disposed at one or more proximate ends of the motor assembly, and/or along the length of the motor assembly. The contact surface of the stator may have a profile including peaks and valleys, and in some embodiments, the contact surface may be treated to reduce friction and/or wear.

Abstract: A progressive cavity positive displacement motor having a solid metal stator and a rotor having an elastomeric seal layer on its outer surface, as well as a method of manufacturing the motor. The elastomeric seal layer on the rotor can be formed by extruding the uncured elastomer, applying the extrusion to the metal rotor core and machining the cured elastomer to produce a uniform thickness seal layer. The elastomer can be made from a high molecular weight elastomer compound. Graphene additives can further enhance the performance characteristics of the elastomer.

Abstract: The invention relates to a cooling system for a computer system, said computer system comprising at least one unit such as a central processing unit (CPU) generating thermal energy and said cooling system intended for cooling the at least one processing unit and comprising a reservoir having an amount of cooling liquid, said cooling liquid intended for accumulating and transferring of thermal energy dissipated from the processing unit to the cooling liquid. The cooling system has a heat exchanging interface for providing thermal contact between the processing unit and the cooling liquid for dissipating heat from the processing unit to the cooling liquid. Different embodiments of the heat exchanging system as well as means for establishing and controlling a flow of cooling liquid and a cooling strategy constitutes the invention of the cooling system.

Abstract: A method for operating a device configured to provide a dosed supply of a liquid. The device has: a pump delivering the liquid and having a pump housing having an inlet and an outlet, an eccentric on the pump housing, and a deformable diaphragm arranged between the pump housing and the eccentric. The deformable diaphragm and the pump housing delimit a delivery path from the inlet to the outlet and form at least one seal of the delivery path, the seal being displaceable along the delivery path by a movement of the eccentric to deliver the liquid. The method comprises: detecting a demanded dose amount of the liquid; activating the pump to deliver the liquid by the pump; stopping operation of the pump when the delivered amount of the liquid corresponds to the demanded dose amount; deactivating the pump when the eccentric is situated in a predefined park position.

Abstract: A method for producing modular down hole, hydraulic motor components involving the formation of replaceable stator slugs to be collectively housed within a stator housing to form a stator assembly, including, in some embodiments, replaceable lobe components for the stator slugs for altering the interference with a selected rotor for such motor.

Abstract: A motor assembly includes a power section extending between a first end and a second end. A bearing assembly section is disposed downhole from the second end of the power section. The outer diameter of the power section is smaller than an outer diameter of the bearing assembly section.

Abstract: A tool (100) for working the internal bore of a tube comprises a motor (10), having a housing with a rotary output; a housing sleeve (14a) and shaft (22); a first bearing housing (16), having a rotatably journalled first spindle (102), mounted on the housing sleeve; and an end sleeve and shaft connected to a tool head. The motor output drives, along a common axis (1), the housing shaft, first bearing spindle, end shaft and tool head. A change mechanism (56,58) is in the tool head to change direction of said drive to transverse said common axis. An output (60) is adapted to receive a tool bit. Gauge means (18,18?) is disposed on said tool head and is adapted, in use, to bear against the bore of the tube and maintain the tool head located radially with respect to the tube axis; and support means (24,24?) is disposed on one or more of said housing shaft, first bearing housing and end sleeve adapted, in use, to bear against the bore of the tube and support the tool.

Abstract: Methods for improving adhesion or bonding between materials used in forming components of directional drilling assemblies, such as rotors and stators, are provided. A surface of a component may be treated, such as through cleaning, etching and/or activating a surface. The use of plasma treatment may enhance the adhesion between the surfaces and/or materials to be bonded, thereby reducing the degradation or mechanical failure of these materials in oilfield applications.

Abstract: Techniques are generally described for a progressive cavity pump and methods for making and using a progressive cavity pump. In an example, a progressive cavity pump may include a stator, a rotor and/or a bearing. The stator may include a helical shaped interior with a first pitch. The rotor may be inside the stator. The rotor may include a helical shaped exterior with a second pitch. The second pitch may be different from the first pitch. The rotor may be effective to rotate with respect to the stator and effective to move eccentrically within the stator to define at least one cavity inside the stator. At least one bearing may be between the rotor and the stator. The bearing may be sized and shaped so that the rotor is effective to rotate inside the bearing and the bearing is effective to move eccentrically along with the rotor.

Abstract: An apparatus for use in a wellbore may include a stator having a bore and a rotor disposed in the bore. Either or both of the rotor and the stator may include a layer that has an asymmetrical material property profile along at least a portion of a circumference of that layer.

Abstract: A stator casing of an eccentric screw pump with an elastomer stator inner part including a plurality of shell-like casing segments and a production method for such a stator casing, with which the outlay for maintenance work can be reduced. According to the invention, casing segments adjacent in the longitudinal direction of the stator casing are disposed offset with respect to one another around the longitudinal axis of the stator casing, wherein each casing segment includes connection means and at least four casing segments engage with one another and form a composite. For the production of the stator casing, the casing segments are acted upon by an axial force component as a result of clamping, wherein the casing segments engage with one another, as a result of which a radial force component is generated and the composite is formed.

Abstract: A progressive cavity type motor or pump including a stator insert with a reinforcing agent dispersed in a manner to improve properties of the stator insert. The reinforcing agent may be a fiber, nanotube, metal, ceramic, or polymer. A dispersing agent may be used to obtain a homogenous distribution. A magnetic reinforcing agent may be incorporated into a stator insert. The stator insert is subjected to a magnetic field to orient the magnetic reinforcing agent in a particular orientation. The magnetic field may also reposition the magnetic reinforcing agent within the stator insert. The stator insert may be formed by injection molding, transfer, or compression molding among other methods.

Abstract: A downhole drilling motor for well drilling operations includes a tubular housing and a stator disposed in the tubular housing. The stator defines an internal cavity passing therethrough, wherein the stator includes one or more lobes defining at least a portion of the cavity. A rotor is operatively positioned in the internal cavity to cooperate with the one or more lobes of the stator. At least a portion of the stator or of the rotor comprises a memory material adapted to expand or contract when heat is applied by a localized heating module to the memory material. A fluid escape gap between the rotor and stator is adjusted by applying heat to the rotor and/or stator. At least one controller is adapted to receive input data and provide output signals increasing and/or decreasing electrical current applied to the at least one localized heating module.

Abstract: In an aspect, the disclosure provides an apparatus for use downhole. In one aspect the apparatus includes a rotor with lobes disposed in stator with lobes, wherein at least one of the contours of the rotor lobe and the contour of the stator lobe is asymmetric.

Abstract: A tool (100) for working the internal bore of a tube comprises a motor (10), having a housing with a rotary output; a housing sleeve (14a) and shaft (22); a first bearing housing (16), having a rotatably journalled first spindle (102), mounted on the housing sleeve; and an end sleeve and shaft connected to a tool head. The motor output drives, along a common axis (1), the housing shaft, first bearing spindle, end shaft and tool head. A change mechanism (56, 58) is in the tool head to change direction of said drive to transverse said common axis. An output (60) is adapted to receive a tool bit. Gauge means (18, 18?) is disposed on said tool head and is adapted, in use, to bear against the bore of the tube and maintain the tool head located radially with respect to the tube axis; and support means (24, 24?) is disposed on one or more of said housing shaft, first bearing housing and end sleeve adapted, in use, to bear against the bore of the tube and support the tool.

Abstract: The present invention recites a method of fabricating a stator for a downhole motor, the method comprising the steps of providing a stator tube having an interior surface and applying a bonding agent to the interior surface of the stator tube. Additionally, a mandrel is positioned within the stator tube, the mandrel having an outer geometry that is complimentary to a desired inner geometry for the stator. Furthermore, a reinforcing material is introduced into the stator tube to fill space between the mandrel and the interior surface of the stator tube and subsequently solidified to bond the reinforcing material to the interior surface of the stator tube, The mandrel is then removed from the bonded stator tube and reinforcing material such that a stator is fabricated.

Abstract: An eccentric-screw pump has an axially split stator having axially opposite intake and output ends and an elastomeric liner defining an axially throughgoing passage. Respective intake and output housings are releasably secured to the intake and output ends of the stator. An axially extending screw rotor in the passage has an end in the intake housing. A drive includes a rotatable universal joint having a pair of relatively swivelable parts in the intake housing. Respective generally complementary formations on the end of the rotor and on one of the universal-joint parts can fit axially together to rotationally couple the one universal-joint part with the rotor. A latch secures the formations axially together for angular force transmission between the one universal-joint part and the rotor.

Abstract: A method for use in producing a stator for a progressing cavity apparatus which includes the use of electroforming to produce the stator tube. A stator tube for a progressing cavity apparatus which is produced using electroforming and a stator for a progressing cavity apparatus which includes a stator tube produced using electroforming.

Abstract: The present application discloses a progressive cavity motor or pump component, either stator or rotor, which provides a high glass transition temperature polymeric surface on the component which becomes resilient at or below the expected operating temperature of the motor or pump, but which remains solid at ambient temperatures, along with a method of fabricating either a stator or a rotor with such surface characteristics. Since the surface becomes resilient, the progressive cavity pump operates efficiently at temperatures above the glass transition temperature of the selected polymeric surface.

Abstract: A progressing cavity pump assembly utilizes a reinforced upper section of the stator rubber helix. The pump stator is located at the end of a string of tubing. The reinforced upper section of the stator rubber helix is strong enough, whether reinforced by mechanical means or by strengthening the rubber matrix chemically, so as not to allow the passage of a stop located above or on the top of the pump rotor. The rotor is lowered on a string of rods until the stop above or on top of the rotor lands onto the reinforced upper section of the stator rubber helix. The rotor is then repositioned away from the stator helix a predetermined distance before pump operation begins with rod string rotation.

Abstract: The claimed invention relates to rotary pumps. The claimed rotary pump comprises: an hollow housing having a side wall and end-face walls; a shaft rotatably positioned in the housing essentially in parallel with the side wall, the distance between the housing side wall and the shaft is variable; deformable rollers that are arranged and adapted to move, when the shaft rotates, between the housing side-wall and the shaft, while moving their deformation being maximal in the region of the minimal distance between the housing side-wall and the shaft; and sealed cavities, each of which cavities being defined by two contiguous rollers, the housing's side wall and end-face walls; the sealed cavities being adapted to communicate with the suction port as their volume increases, and communicate with the delivery port as their volume decreases.

Abstract: A skinned rotor 201 or skinned stator 305 of a progressive cavity apparatus is described. A rotor 201 can be skinned by threading a sleeve 210 with a profiled helical outer 212 and profiled helical inner 214 surface onto a core 202 with a profiled helical outer surface 204. A rotor (1301, 1401) can also be skinned by inserting a non-helical core (1302, 1402) into a non-helical longitudinal bore (1314, 1414) of a sleeve (1310, 1410) with a profiled helical outer surface (1312, 1412). A stator 305 can be skinned by threading a tubular liner 310 with profiled helical inner 314 and profiled helical outer 312 surfaces into a profiled helical bore 308 of a tube 306. A stator (2405, 2505) can also be skinned by inserting a tubular liner (2410, 2510) with a non-helical outer surface (2412, 2512) into a non-helical bore (2408, 2508) of a tube (2406, 2506).

Abstract: The present invention is to provide a screw rotor including a resin rotor formed around a metallic shaft without generation of cracks. Spiral chamfers are formed on surfaces of metallic shafts around which resin rotors are formed. Preferably the surfaces of the shafts may be sandblasted, and after the surfaces of the shafts are preliminarily coated with resin and then the rotors may be molded.

Abstract: A rotary positive displacement pump (1) that includes an impeller (3) that defines, within a pumping chamber (20), a plurality of successive chambers (21) with variable volume through which a fluid is mechanically conveyed from an inlet (7) to an outlet (8). The chambers (21) with variable volume are defined, at axially opposite ends, by a pair of rotational surfaces (40, 50) that close the pumping chamber (20) and are arranged to rotate about mutually inclined axes. Also described is a method of manufacturing the pump.

Abstract: A Moineau device includes a stator that interfaces to a rotor whereby fluid flows through cavities between the stator and rotor that progress axially as the rotor is rotated relative to the stator. At least one of the stator and the rotor is realized from a nanocomposite that includes a polymeric matrix with carbon nanotubes dispersed therein.

Abstract: The present invention provides a positive displacement rotary vane mixing pump in combination with a positive displacement rotary vane fluid pump. The mixing pump having an air inlet, an inlet for foamable fluid from the fluid pump, and an outlet from a discharge sector of the pump for discharging a mixture of air and liquid to a foam generator.

Abstract: An eccentric screw pump with an annular outer part (10; 40; 74) and an inner part (12; 42; 72) arranged therein has an interior of the outer part (10; 40; 74) and an exterior of the inner part (12; 42; 72) tapering in a complementary manner towards an axial end (16; 46; 70). In the axial direction (X, W), the inner part (12; 42; 72) and the outer part (10; 40; 74) are movably received in relation to each other and the inner part (12; 42; 72) and/or the outer part (10; 40; 74) are configured in such a manner that pressure applied to the pressure side of the eccentric screw pump generates a force that acts upon the inner part (12; 42; 72) axially to the direction in which the inner part (12; 42; 72) tapers and/or a force that acts upon the outer part (10; 40; 74) in an opposite axial direction.

Abstract: The invention relates to a rotor for a positive displacement rotary pump. The rotor comprises a hub with an aperture and a center axis through aperture and at least 2 lobes radially extending from hub and in a direction essentially perpendicular to the axis of hub. The rotor comprises a core of a first polymer material and a cover of a second polymer material, which second polymer material has a hardness which is lower than the first polymer material. The first and the second polymer materials are connected to each other. The hub or the major part of the hub is preferably provided by the core, whereas at least a part of the lobes is provided by the cover. The invention also relates to a method of providing the rotor and a pump comprising the rotor.

Abstract: The invention relates to a stator casing for eccentric worm pumps comprising an elastic lining, the cylindrical stator casing having a surface on the inner side, along the longitudinal axis whereof grooves are incorporated.

Abstract: The invention relates to an eccentric screw pump and a method for its operation. Unlike the previously known prior art, where changes to the internal geometry of the pump stator caused by wear were always remedied with radially acting clamping measures, the invention is based on bringing this about by a change in the length of the lining.

Abstract: A Moineau style stator includes a helical reinforcement component that provides an internal helical cavity. A resilient liner is deployed on an inner surface of the helical reinforcement component. The helical reinforcement component includes a solder or braze material and is typically metallurgically bonded to an inner wall of a stator tube. In exemplary embodiments, the helical reinforcement component includes a composite mixture of solder and aggregate. Exemplary embodiments of this invention address the heat build up and subsequent elastomer breakdown in the lobes of prior arts stators by providing a helical reinforcement component. Solder reinforced stators tend to be less expensive to fabricate than reinforced stators of the prior art.

Abstract: The invention relates to a low pressure pump/vacuum pump for gaseous media, comprising two interacting combs the teeth of which are configured as cycloidal components, the duplicate compressor stages being interconnected. A power unit of the pumps/compressors is driven and the power unit of the other pump/compressor, which is arranged coaxially, is entrained via a rotational connection.

Abstract: A rotary device comprises a stator and a rotor. The stator has a cam surface. The rotor has a sleeve and a hub. There is a plurality of vanes reciprocatingly mounted on the rotor. There is a plurality of first longitudinal edge seals disposed on the sleeve. The first longitudinal edge seals each seal at least a portion of a first longitudinal edge of one of the vanes against the sleeve. There is a plurality of second longitudinal edge seals disposed on the hub. The second longitudinal edge seals each seal at least a portion of a second longitudinal edge of one of the vanes against the hub. There is a plurality of end edge seals. The end edge seals each seal a respective end edge of one of the vanes against the cam surface. The end edge seals each are in sealing engagement with respective ones of the first and second longitudinal edge seals.

Abstract: The rotor of a downhole motor includes a mandrel having at least one radial lobe, and an elastomeric tubular sleeve compressed about the mandrel so as to establish frictional engagement therebetween. The sleeve is compressed about the mandrel through one of various processes, including heat shrinking, vacuum shrinking, and stretching.

Abstract: A Moineau style stator includes a helical cavity component having reinforced helical lobes. The lobes are reinforced with a three-dimensional network of physically bonded aggregate particles. The network of bonded aggregate provides a porous skeletal-like structural reinforcement. Pore volume between the bonded aggregate particles may optionally be partially or substantially filled with an elastomer. An elastomer liner is typically deployed on an inner surface of the helical lobes to promote a rotational interference fit with a rotor.

Abstract: The invention relates to a clamping device which clamps the pump stator with the pump housing and an additional pump end part. The width of the clamping bars is at least 20% of the cross section of the pump stator in order to stabilise the clamping device and the simplify handling. All parts which are required to clamp the pump end part remain on the clamping bars.

Abstract: A method for producing a stator segment for a segmented stator of an eccentric screw pump or an eccentric screw motor, as well as a stator segment and a stator. A stator segment in an initial state is processed or machined with a linear cutter that removes material to provide the stator segment with a helical inner segment surface.

Abstract: A stator and method of producing a stator for an eccentric screw pump or an eccentric screw motor, including an outer stator tube, at least one stator segment disposed within the outer stator tube, and at least one tube end portion that completes one end of the outer stator tube. A region of the tube end portion that in an axial direction follows the outer stator tube has a thickness that is greater than the thickness of the outer stator tube. The outer stator tube is radially deformed to produce a connection between the at least one stator segment and outer stator tube.

Abstract: A displacement machine has at least two adjacent working chambers which are defined by: a) a common first wall section made of an elastic material; b) a common second wall section; and c) a movable element which is movable with respect to the first wall section, which movable element has a press-on section that presses the first wall section against the second wall section to form a common sealing region between the two adjacent working chambers, which common sealing region moves with the movement of the press-on section. At least one of the first and second wall sections is provided with a non-smooth profile on the inner side of the working chambers.

Abstract: Disclosed is a progressive cavity device. In some embodiments, the device includes a stator with an inner surface having a number of lobes and a rotor disposed within the stator and having a different number of lobes. The stator lobes define a major diameter and a minor diameter, where the major diameter circumscribes the stator lobes and the minor diameter inscribes the stator lobes. A rotor-stator, defined as the major diameter divided by the minor diameter, is selected from the group consisting of 1.350 or less for a progressive cavity device with a stator having two lobes, 1.263 or less for three lobes, 1.300 or less for four lobes, 1.250 or less for five lobes, 1.180 or less for six lobes, 1.175 or less for seven lobes, 1.150 or for eight lobes, 1.125 or less for nine lobes, and 1.120 or less for ten lobes.

Abstract: A pump is formed by a housing (10) having an inlet (11) for connection to a source of fluid and an outlet (12) for pumped fluid. A rotor (15) is rotatable within the housing and the inlet (11) and the outlet (12) are spaced apart around the path of the rotor (15) in the housing. The rotor (15) has surfaces (16a, 16b, 16c, 16d) that form, with the housing (10), closed chambers (18a, 18b, 18c, 18d) which travel around the housing (10) to convey fluid from the inlet (11) to the outlet (12). The housing (10) carries a seal (14) that is located between the inlet (11) and the outlet (12) in the direction of travel of the rotor (15). The seal (14) co-operates with the rotor surfaces (16a, 16b, 16c, 16d) as the surfaces (16a, 16b, 16c, 16d) pass between the outlet (12) and the inlet (11) to prevent the formation of a chamber during said passage and so prevent fluid flow from the outlet (12) to the inlet (11). Such a pump is easily and cheaply produced and is particularly useful in medical applications.

Abstract: A pump is formed by a housing (10) having an inlet (11) for connection to a source of fluid and an outlet (12) for pumped fluid. A rotor (15) is rotatable within the housing and the inlet (11) and the outlet (12) are spaced apart around the path of the rotor (15) in the housing. The rotor (15) has surfaces (16a, 16b, 16c, 16d) that form, with the housing (10), closed chambers (18a, 18b, 18c, 18d) which travel around the housing (10) to convey fluid from the inlet (11) to the outlet (12). The housing (10) carries a seal (14) that is located between the inlet (11) and the outlet (12) in the direction of travel of the rotor (15). The seal (14) co-operates with the rotor surfaces (16a, 16b, 16c, 16d) as the surfaces (16a, 16b, 16c, 16d) pass between the outlet (12) and the inlet (11) to prevent the formation of a chamber during said passage and so prevent fluid flow from the outlet (12) to the inlet (11). Such a pump is easily and cheaply produced and is particularly useful in medical applications.