APPARATUS AND METHOD FOR SENSING DIAPHRAGM FAILURES IN RECIPROCATING PUMPS

Abstract: Apparatus for monitoring the diaphragm integrity in a diaphragm pump. A single continuous circuit trace (29) is supported on a pump diaphragm (12). An electrical circuit is connected to the continuous circuit trace (29) for measuring both the continuity and a potential ground fault condition between the circuit trace and pumping liquid. The circuit trace is formed in a configuration which resists the effects of strain induced by continuous flexing of the diaphragm (12).

Description: The present invention relates to fluidic devices which employ diaphragms. Specifically, apparatus for monitoring the integrity of a diaphragm in a diaphragm pump is disclosed. The diaphragm pump utilizes a piston acting upon a volume of liquid bounded on one side by a flexible diaphragm. The other side of the diaphragm forms a boundary with pumped liquid to a pumping chamber. Reciprocating motion of the piston causes alternating flexure of the diaphragm, transmitting a change in volume to the liquid in the pumping chamber. As the piston retracts, pumping chamber volume increases, reducing pressure, so that the discharge check valve remains closed while a volume of liquid equal to piston displacement is admitted through the inlet check valve. As the piston advances, pressure is increased, causing the inlet check valve to close and expel an equal volume of liquid through the discharge check valve. A major advantage of the diaphragm pump is the transmission of hydraulic power through a flexible membrane, or diaphragm, without the reciprocating or rotating liquid seals required for most pumping devices. Consequently, it is essentially leak-free under normal operating conditions, making it a preferred device for the transfer or metering of chemically or biologically hazardous liquids. It is likewise a preferred device for handling liquids of a required level of purity which might be contaminated by inward leakage from exterior sources. Various methods for detecting diaphragm rupture have been employed in the past. These include the use of two diaphragm halves which have a vacuum drawn between them. The rupture of one or the other diaphragm half results in the loss of vacuum which may be detected by monitoring the pressure of the space between halves. In another technique, two diaphragms are employed, separated by an intermediate liquid. A pair of electrodes are immersed in the intermediate liquid. A change in electrical resistance between the two electrodes may be used to sense the presence of the pumped liquid indicating the rupture of one of the diaphragms. The presence of a hydraulic liquid may also be detected when the other of the diaphragms ruptures which also results in a corresponding change in resistance between the electrodes. Another technique which has heretofore been limited to rupture disc assemblies is disclosed in US-A-4,342,988. This patent describes the use of a conductive loop which is monitored to determine the integrity of a deformable member supporting the loop. Any change in the conductive loop is sensed as a change in the deformable member integrity. In US-A-4,569,634 there is disclosed an apparatus for determining a pump diaphragm failure wherein a conductor is supported by the diaphragm and insulated from the pumping liquid. The conductor forms part of an electrical circuit, and an indicator responsive to the output of the circuit indicates a failure of the diaphragm whenever the circuit senses a change in the electrical resistance between the conductor and the pumping liquid representing a ground fault condition. A problem with this arrangement is that it does not readily detect an imminent failure of the diaphragm. According to one aspect of the present invention there is provided an apparatus for determining a pump diaphragm failure, the apparatus comprising: a conductor supported by the diaphragm and insulated from a pumping medium, and an indicator responsive to the output of an electrical circuit including the said conductor for indicating a failure of the diaphragm, characterised in that the conductor is formed as a single continuous circuit trace traversing substantially the entire surface area of the diaphragm, and the electrical circuit includes means for sensing a voltage produced by a current flow through the trace, first failure detection means responsive to a predetermined change in the sensed voltage in one direction for detecting an open-circuit in the circuit trace, and second failure detection means responsive to a predetermined change in the sensed voltage in the opposite direction for detecting current leakage from the trace to the pumping medium whereby a diaphragm failure is detected when either the circuit trace electrical continuity is broken or the pumping medium comes into contact with the continuous circuit trace. In a preferred embodiment of the invention, the conductive circuit trace is formed as a printed circuit trace, and the substrate bearing the trace is bonded to the diaphragm surface. The trace is formed as a plurality of radial spokes. The radial spokes include circumferential segments interconnected to form a single conductive circuit trace which changes direction a plurality of times as it traverses the distance from the periphery to the center of the diaphragm. This configuration reduces the level of strain applied to the circuit to a level significantly lower than that applied to the diaphragm material itself. The behaviour of the circuit is analogous to that of a spring, which because of its geometrical configuration, can withstand large deformations and long-term flexure without failure. The circuit trace continuity is maintained for the life of the diaphragm or greater, and is lost only in response to diaphragm failure. According to a further aspect of the present invention there is provided a method for detecting a failure of a diaphragm separating two liquid mediums, the method comprising: disposing a conductor over the diaphragm, insulating the conductor from the liquid mediums, and passing a current through the conductor, characterised by disposing the conductor as a continuous trace over a major portion of the diaphragm, sensing a voltage dependent on the magnitude of the current flow through the circuit trace, and detecting a first predetermined change in the sensed voltage when the continuity of the trace is broken and a second predetermined change in the sensed voltage when current leaks from the trace to one of the liquid mediums. In the accompanying drawings, by way of example only: Figure 1 illustrates a typical diaphragm pump structure incorporating apparatus embodying the invention. Figure 2 illustrates one form of a diaphragm sensing circuit trace which is resistant to strain and fatigue. Figure 3 illustrates another form of a diaphragm sensing circuit trace which is resistant to strain and fatigue. Figure 4 is a cross-sectional view of the diaphragm with a respective sensing circuit trace. Figure 5 shows circuitry for detecting a failure condition. Figure 6 is a cross-sectional view of a plastic or elastomer diaphragm having a sensing circuit trace. Figure 7 illustrates an alternative diaphragm pump incorporating a non-metallic diaphragm. Figure 8 illustrates a detail of the non-metallic diaphragm used in the pump of Figure 7. Figure 9 illustrates the detail for connecting the circuit trace to the lead conductors. Referring now to Figure 1, there is shown a conventional diaphragm pump which employs a diaphragm equipped with a failure detection apparatus embodying the present invention. Those familiar with diaphragm pumps will recognize the reagent head assembly 15 which includes a pumping chamber 17 connected at both ends to an inlet 18 and outlet 19. The inlet 18 and outlet 19 include respective ball valves 20 and 21 for providing one-way liquid flow through the pumping chamber 17. Pump head assembly 7 includes a cylinder bore 6. The diaphragm 12 is clamped between the peripheral portions of the reagent head 16 and the dishplate 13. A pair of O-ring seals 9, 11 are between the diaphragm 12 and the reagent head 16 and the dishplate 13, respectively. Facing the curved dish surface 16a is a diaphragm 12, shown to be metal which may be grade 316 stainless steel in the pump arrangement of Figure 1. The metal diaphragm incorporates a continuous circuit trace insulated from the diaphragm and disposed on the diaphragm side which is normally not in contact with the pumped liquid. The diaphragm pump of Figure 1 includes, on the remaining side of diaphragm 12, another O-ring seal 11, which seals the diaphragm with a curved dish plate 13. Dish plate 13 is a removable part of the pump head assembly 7. Additional O-ring 8 seals the removable dish plate 13 with the pump head assembly. A reciprocating piston 10 is shown which will force a hydraulic liquid medium through the openings 13a of the dish plate, thereby flexing the diaphragm 12 in a direction to pressurize the pumping chamber 17. The curved surface of dish plate 13 is substantially concave. Surface 16a of the dish of the reagent head assembly is similarly curved. Surface 16a and dishplate 13 protect the diaphragm from excessive displacement which would tear or rupture the diaphragm. The operation of the pump can be briefly described beginning with the reciprocating piston 10, which is driven linearly in the chamber 6. The piston forces a hydraulic liquid through channels 13a against the diaphragm 12. Flexure of diaphragm 12 will transmit a change in volume to the liquid in the pumping chamber 17 via channels 16b, forcing liquid through outlet valve 21 while closing valve 20. As the piston 10 retracts, pumping chamber 17 volume increases, reducing pressure, so that outlet valve 21 closes, permitting a volume of liquid 20 equal to the piston 11 displacement to enter the pumping chamber 17. A conduit connector 22 protects a pair of conductors 28 which are connected to a continuous circuit trace supported on the diaphragm 12. Conductors 28 exit through the conduit connector 22, and via an armored cable 24, are connected to failure detector circuit 34. When a tear or rupture in the diaphragm 12 occurs, the failure detector will provide an enable signal to an alarm 35. Alarm 35 may be an audible or visual alarm indication that a failure in diaphragm 12 has occurred or is imminent. Additionally, the enable signal may be used to stop pump operation. The metal diaphragm 12 shown in Figure 1 can be configured in accordance with Figures 2 and 3. In both Figures 2 and 3, a single continuous circuit trace 29 is supported on the diaphragm 12 insulated therefrom, facing the hydraulic liquid, and which includes insulating layers 45 and 48, as shown more particularly in the section view of Figure 4. The single circuit trace 29 is shown in Figure 2 as a double spiral. The center of the double spiral coincides with the center of the diaphragm 12. The two ends of each spiral are joined together at the center. The double circuit trace is then wound in a spiral until substantially all of the surface area of the diaphragm is covered by the circuit trace. The spacing of the circuit trace 29, which is desired to be small in order to enhance detection resolution, has been selected to be approximately 0.015 inches (15 mils) as determined by the present economics and state of the art of the flexible printed circuit manufacturing processes. An extending tab 14 for the diaphragm provides support for the exit leads 30 and 31 of the spiral circuit trace 29, and supports a connector 22 in hole 39. The insulating substrate supporting the spiral 29 necks down at 32 so that eyelets 37 and 38 are supported over the hole 39. Eyelets 37 and 38, connected to exit leads 30 and 31 receive the conductors 28 of Figure 1 and are soldered thereto or otherwise electrically bonded to eyelets 37 and 38. An improved version of the diaphragm with a continuous sensing circuit trace is shown in Figure 3. It has been found that, although the diaphragm of Figure 2 will work under many conditions, the diaphragm of Figure 3 provides a circuit trace 29 which experiences lower levels of strain during operation of the diaphragm. The circuit trace 29 is a single continuous circuit trace formed as a plurality of 24 radial segments or spokes which decrease in size towards the center of the diaphragm. Each radial segment is comprised of a plurality of circumferential segments 40, decreasing in size, the ends of each circumferential segment connected to an adjacent circumferential segment. The radial segments are interconnected at both ends to provide a continuous circuit trace alternating in direction as it extends from the periphery to the center of the diaphragm 12. The three identified radial segments 41, 42 and 43 commence with their outer circumferential segments connected to an adjacent radial segment. The individual circumferential segments shown generally as 40 decrease in length to define a taper for each radial segment. The ends of each radial segment are connected to an adjacent radial segment in a manner which will provide a continuous single circuit trace covering substantially the entire surface area of the diaphragm 12. Variations of the embodiment of Figure 3 may be perfected without departing from the principle of having multiple circumferential segments disposed on the diaphragm 12 and connected such that the single circuit trace formed from the segments changes direction frequently, thus reducing the effects of flexure of the diaphragm on conductor fatigue. Thus, by providing a continuous single circuit trace 12, as shown in Figure 3, it is possible to extend the life of the continuous circuit trace to at least that of the metal diaphragm 12. A section view of the metal diaphragm 12, taken from Figure 2, which is applicable to the construction of Figure 3, is shown in Figure 4. Figure 4 illustrates the metal diaphragm 12 bonded along line 47 to a substrate 46. The insulating substrate 46 is a printed circuit material which may be the well-known DuPont polyimide film, referred to in the trade as KAPTON, having bonded thereto metal foil. The metal foil is photochemically etched to derive the required conductor circuit trace 29. The conductor circuit trace 29 is further encapsulated with bonded layers 45, 48, which may also be polyimide film to provide insulation between the conductor circuit trace 29 and the hydraulic liquid which would necessarily contact that side of the diaphragm bearing the conductor trace 29. The layers 45 and 48 are approximately 2 mils and 5 mils thick, bonded along line 44 with a B-staged modified acrylic adhesive, as well as being bonded to the circuit trace 29 along line 49, which has a thickness of 3 mils on a 2 mil substrate 46. Diaphragm 12 and substrate 46 are similarly bonded together along line 47 with the same adhesive. The continuous circuit trace of the embodiments of Figures 2 and 3 provide for two separate modes of failure detection. Turning now to Figure 5, there is shown a circuit 34 which will provide failure sensing in two distinct modes. The continuous circuit trace 29 is shown as a resistive element in Figure 5. Each of the conductor leads 30, 31, connected to leads 28, is terminated in first and second resistances 50 and 51, respectively. Resistances 50 and 51 are in turn connected across a DC voltage supply represented by VCC and a standard ground symbol. The DC voltage will provide a small but measurable current flowing through resistor 50, continuous circuit trace 29 and resistor 51. The circuit of Figure 5 will detect two types of failures. The first is a break in continuity of the single conductor circuit trace 29. To detect the break in the circuit trace, a first high limit comparator 54 is provided. Comparator 54 has a reference threshold voltage set by resistor network 52. If the circuit trace 29 should be broken due to an imminent diaphragm failure, the potential in the voltage comparator 54 inverting input would rise significantly, triggering an OPEN signal. Alternatively, an imminent diaphragm failure may be sensed by a ground current formed when conductor 29 comes in contact with either the pumping liquid or the pump head 15, or other component of the diaphragm pump due to an insulation failure. This ground fault current would result in a lowering of the potential V1 such that the voltage comparator 55 would sense the ground fault condition. The reference voltage provided to comparator 55 is supplied by a resistor network 53, set to a level indicating the presence of a ground fault with the circuit trace 29. The detection of a break in the circuit trace can sometimes be masked by the presence of a conductive pumping liquid such as strongly acidic or basic substances. The ground fault detection mode will, or course, sense the presence of such liquid, permitting rapid alternative failure mode detection. This method of failure detection is especially useful to detect incipient failure, prior to full rupture across the thickness of the diaphragm. Each of the conditions representing failure of a diaphragm are supplied to an OR gate 56 which will provide a logic signal for operating an audible or visual alarm. Although not shown in Figure 5, it is possible to monitor each input of OR gate 56. Different fault conditions represented by each input of OR gate 56 can be utilized to indicate the type of corrective action to be taken. It is clear that the foregoing technique for detecting failure of a diaphragm in the presence of an electrically conductive liquid provides increased reliability by using multiple detection modes. Furthermore, in the case of an imminent failure causing circuit exposure to a conductive liquid prior to circuit breakage, it provides reduced response time. Additionally, by employing the advantageous configurations of Figure 3 and its obvious variants, the diaphragm circuit trace 29 is not subject to excessive strain, such that the sensing circuit trace 29 suffers a failure earlier in time than the diaphragm itself. Two prominent features of this invention, both of which afford it significant advantages over conventional devices, are that it is independent of the characteristics of the liquids adjacent to either side of the diaphragm (particularly viscosity and electrical conductivity) and that the spacing between adjacent conductors establishes the resolution of detection, that is, it establishes the largest rupture than can occur without detection. The foregoing description of a metal diaphragm pump is, of course, equally applicable to non-metallic diaphragms. The plastic diaphragms made of PTFE (polytetrafluoroethylene), and other well known diaphragm materials shown in Figure 6 may include an imbedded continuous circuit trace 29 such as is shown in Figures 2 and 3. The conductor may be a stainless steel conductor 29 of 3 mils thickness which is corrosion-resistant and having high flexural fatigue strength. A substrate 58 of 3-5 mils thickness supports the steel conductor trace etched to the required configuration. The conductor material may be bonded to the substrate by any of several known industrial processes. The substrate may be a thermoplastic fluorocarbon, such as fluorinated ethylene propylene generically referred to as FEP or perfluoroalky-oxytetrafluoroethylene, generally referred to as PFA. The etched conductor and supporting substrate is bonded between two layers of diaphragm material 60, 61 such as PTFE polytetrafluoroethylene which are bonded together, as shown in Figure 6, by a combination of heat and pressure, forming a laminated structure. PTFE is not a thermoplastic material, but it will form effective surface bonds to FEP and PFA at temperatures below the point at which its sintered structure is destroyed. Still other common industrial processes may be utilized to implement the conductor circuit trace in a non-metallic diaphragm. For instance, an unsupported circuit can be photochemically etched by a process called chemical milling. The resulting circuit trace may be positioned and encapsulated between layers of PTFE bonded to one another by means of thermoplastic PFA or FEP layers, applied in the form of film or powder below and/or above the circuit traces. Heat and pressure are applied sufficient to bond the structure together. Alternatively, a circuit trace may be fused with heat and pressure between layers of like thermoplastic materials, such as the foregoing, to provide a simple, homogeneous matrix encapsulating the entire circuit trace. As used throughout this specification, the word diaphragm is not restricted to disks or initially flat surfaces, but is construed to mean any shape suitable to perform the functions of a diaphragm. A practical embodiment of the plastic diaphragm incorporating the present invention is shown in Figure 7. Figure 7 illustrates the common diaphragm pump as illustrated in Figure 1, modified to receive a plastic diaphragm 65. Diaphragm 65 has an internal construction as demonstrated in Figure 6. Additionally, a flange 66 is incorporated on the diaphragm which is received in a like facing channel of the reagent head 16. The flange 66 is advantageous to prevent the diaphragm 65 from slipping between the reagent head 16 and the dish plate 13, as well as provide a seal for reagent head 16 analogous to an O-ring seal against leakage of the pumped fluid. Additionally, a tab 69 extends form the periphery of the diaphragm 65, supporting the ends of the spiral circuit trace. A bulkhead connector 22 is shown threaded into a portion of the reagent head 16 at a location clear of the pumping chamber. The threaded bulkhead connector 22 supports the armor 24 and conveys conductors 28 to the tab 69 for connection to the circuit trace winding. A detail of the plastic diaphragm is shown in Figure 8 illustrating the flange 66 which is integral to the diaphragm layer 60. A substrate 58 bearing the required circuit trace is encapsulated between diaphragm material 60 and 61, in accordance with the foregoing methods. A plastic sleeve 70 containing conductors 28 is fused to layer 60 and the bare conductor ends of conductors 28 are soldered into two eyelets in the substrate 58 bearing each end of the circuit trace. The plastic insulating 70 provides a hermetic seal with the diaphragm material 60. The sleeve 70 and connections of conductors 28 to eyelets 37 and 38 are effected before layer 61 is bonded or fused in place. As shown in Figure 9, the substrate 58 has an extension bearing the ends 30 and 31 of the conductor trace. Two eyelets 37 and 38 can receive the conductors 28. Thus, it is seen that the foregoing invention may be implemented in a plastic diaphragm as well as in the metal diaphragm of the embodiments of Figures 1-5. The teaching of the modified spiral trace of Figure 3 is, of course, applicable to the plastic diaphragm as well as the metal diaphragm. Thus, having described our invention in terms of the preferred embodiments, those skilled in the art will recognize yet other embodiments defined more particularly by the claims which follow.

Claims: An apparatus for determining a pump diaphragm (12) failure, the apparatus comprising: a conductor (29) supported by the diaphragm (12) and insulated from a pumping medium, and an indicator (35) responsive to the output of an electrical circuit (34) including the said conductor (29) for indicating a failure of the diaphragm, characterised in that the conductor (29) is formed as a single continuous circuit trace (29) traversing substantially the entire surface area of the diaphragm (12), and the electrical circuit (34) includes means (54, 55) for sensing a voltage produced by a current flow through the trace (29), first failure detection means (52, 54) responsive to a predetermined change in the sensed voltage in one direction for detecting an open-circuit in the circuit trace (29), and second failure detection means (53, 55) responsive to a predetermined change in the sensed voltage in the opposite direction for detecting current leakage from the trace (29) to the pumping medium whereby a diaphragm failure is detected when either the circuit trace (29) electrical continuity is broken or the pumping medium comes into contact with the continuous circuit trace. The apparatus of claim 1 wherein the continuous circuit trace (29) is disposed in a plurality of radial spokes (40, 41, 42, 43) on the diaphragm (12), beginning at the periphery of the diaphragm and extending to the centre (28) thereof, each spoke being connected to an adjacent radial spoke to form the single continuous circuit trace (29). The apparatus of claim 2 wherein the radial spokes comprise a plurality of circumferential segments (40, 41, 42, 43) which decrease in size towards the diaphragm centre (28). The apparatus of claim 2 wherein the single continuous circuit trace (29) is a conductive trace formed on a printed circuit substrate (46), the substrate (46) being bonded to the diaphragm (12), and an insulative film (45, 48) being formed over the trace (29). The apparatus of claim 1 wherein the continuous circuit trace (29) changes direction a plurality of times while traversing the diaphragm surface. The apparatus of claim 1 wherein the circuit trace (29) comprises a continuous metallic trace (29) disposed on a substrate (58) between first and second diaphragm halves (60, 61) forming a laminated structure. The apparatus of claim 1 wherein the diaphragm (12) is of stainless steel, and the single continuous trace (29) is supported on a substrate (46) bonded to the stainless steel diaphragm. The apparatus of claim 7 further comprising a coating (45, 48) over the continuous circuit trace (29) and insulating the trace from the pumping medium. The apparatus of claim 1 wherein the circuit trace (29) is disposed on a substrate (58) between two plastic diaphragm halves (60, 61) insulating the circuit trace from a fluid on one side of the diaphragm. The apparatus of claim 9 wherein one of the diaphragm halves (60, 61) includes an integral annular flange (66). An apparatus according to any one of the preceding claims wherein the electrical circuit (34) comprises a voltage supply (Vcc) connected through a first resistance element (50) to one end of the circuit trace (29), and connected through a second resistance element (51) to a second end of the circuit trace, whereby a current flow through the circuit trace is established; and wherein the voltage sensing means comprises a window comparator circuit (54, 55) connected to compare the voltage potential on one of the circuit trace ends with first and second threshold levels. The apparatus of claim 11 wherein the continuous circuit trace (29) is in the form of a double spiral (30, 31) beginning at the centre (28) of the diaphragm. A method for detecting a failure of a diaphragm (12) separating two liquid mediums, the method comprising: disposing a conductor (29) over the diaphragm (12), insulating the conductor (29) from the liquid mediums, and passing a current through the conductor (29), characterised by disposing the conductor (29) as a continuous trace over a major portion of the diaphragm (12), sensing a voltage dependent on the magnitude of the current flow through the circuit trace (29), and detecting a first predetermined change in the sensed voltage when the continuity of the trace (29) is broken and a second predetermined change in the sensed voltage when current leaks from the trace (29) to one of the liquid mediums. The method of claim 13 further comprising: disposing the circuit trace (29) between two halves (60, 61) of the diaphragm (12) to insulate the trace from the liquid mediums, and fusing the said two halves (60, 61) and circuit trace (29) together to form a single diaphragm structure. The method of claim 13 wherein the circuit trace (29) is etched on a substrate (46) fixed to the diaphragm.

Inventors: FRAWLEY, JAMES JOHN, MEARNS, RICHARD LEE
Applicants: PULSAFEEDER INC.
Application year: 1988
Main classification: F04B43
IPC codes: F04B43