FLOW MEASUREMENT AND MONITORING SYSTEM FOR POSITIVE-DISPLACEMENT PUMPS AND PUMPS EQUIPPED WITH THIS SYSTEM

Abstract: A system for measuring the flow rate of fluids discharged from positive-displacement pumps, and real-time monitoring of these pumps. The system comprises pressure and proximity sensors and a microcomputer. The pressure sensors transmit the pressure in the discharge chambers of the pumps to the microcomputer in the form of signals. The proximity sensors provide a reference in the operating cycle of the pumps. The computer monitors sensor status and processes the data received from the sensors to provide monitoring of the pump. It transmits the results via a serial communications bus. In the event of a malfunction, an alarm is transmitted.

Description: This invention consists of an automatic system for the measurement of flow rate and monitoring of positive-displacement pumps, according to the preamble of claim 1. Existing measurement systems are generally adequate for the measurement of fluids having at least one constant characteristic. There is no all-purpose flow-meter for accurate measurement without re-calibration of all of the types of fluids likely to be handled by a positive-displacement pump. The characteristics of these fluids may vary widely; they may be highly viscous or fluid, conductors of electricity or not, have solid particles in suspension or not, be liquid or gaseous etc... Flow may also be laminar or turbulent. There are many flowmeters suited to the measurement of specific fluids but none which provides accurate measurement for all of these types of fluid. The Invention provides for measurement of flow rate of any fluid discharged from a positive-displacement pump. The nature of the flow, whether laminar or turbulent, does not affect the accuracy of measurement. The invention makes use of the volumetric pumping characteristics of positive-displacement pumps. A technique commonly used is to count the number of pump strokes and to multiply this number by the theoretical volume discharged by one stroke. This method of measurement remains accurate so long as the pump and pumping conditions remain good. However, if either of these conditions deteriorates, such systems may become highly inaccurate. Consider an extreme case in which the pumping conditions are so bad that the fluid to be pumped does not even reach the pump. This will not prevent the pump from running as though conditions were normal. The flowmeter will still indicate a flow proportional to the speed of the pump even though no fluid is actually discharged. Such an indication is totally false. This is an extreme example, however conditions under which the fluid does not entirely fill the chambers during the suction phase are met frequently. Under these conditions, the counting of the number of strokes method is erroneous as the volume actually pumped is less than the theoretical volume which should be discharged. Document EP - A - 0183 295 describes a pump equipped with internal sensors. The present invention represents an improvement over EP '295 in that an original monitoring system is proposed leading to an innovative set of oilfield use capabilities. Document US-A-3 779 457 describes a monitoring system as well as Document US-A-4526 513. The invention uses the above described technique of counting the number of pump strokes and corrects it by measuring the volume of fluid actually discharged at each stroke. In this way correct measurement of the flow rate is obtained regardless of pumping conditions or the condition of the pump. In order to measure the volume of fluid actually discharged by the pump, the operational state of the hydraulic part of the pump must be known. Constant monitoring of pumping conditions and operational condition of the pump are provided. In the event of damage to a valve resulting in a leak or if a valve spring should break, the system will measure the leak and correct the flow rate value accordingly. To our knowledge there are no existing systems which monitor by detection and correction for leaks at the valves or cylinder sleeves: leaks or spring-breaks are usually detected by the operator, alerted by noise from the pumps or vibrations on the fluid circulation lines. The system resulting from this invention will carry out permanent and automatic monitoring of such malfunctions. EP-A-0183 295 describes a positive displacement pump equipped with sensors. The preferred sensors are a sensor allowing to give information on the end position of at least one piston, and a sensor allowing to give information on the opening and/or closing time of at least a valve (generally, the outlet valve) in the same chamber. The present invention describes an improved monitoring system to be used in connection with at least one of so equipped pumps. Monitoring is performed by a microcomputer. If certain parameters reach or exceed pre-determined values, the microcomputer will make the required calculations to monitor correct pump operation. It checks the sensors and then checks over several cycles that the fault is real. If the fault is confirmed the microcomputer transmits the data and takes the measurements required for flow correction. The data transmitted are generally the values for the flow and the volume of fluid actually discharged from the pump, as well as a value given by a "pumping conditions and state-of-pump" indicator. The latter is in fact the volumetric efficiency of the pump, i.e. the ratio between the volume actually discharged, over the volume theoretically discharged under perfect pumping conditions with a perfect pump. This indicator is extremely useful for observing the reactions of the pump to variations in pumping conditions. The operator of a pump knows in real time if the pumping conditions have been improved or worsened due to his actions or to external actions. Valve or sleeve leaks, spring-breaks and sensor malfunctions are also transmitted. Figure 1 shows an example of connection of the invented system. Figure 2 is a cross-section through the compression chamber of an example of a positive-displacement pump (piston pump),substantially as described in EP 0183295. Figure 3 is a working drawing for the construction of the microcomputer. Figure 4 is a sample of curves obtained from an operating pump with the aid of pressure and displacement sensors and by calculation. Figure 5 shows the pressure curves for 2 chambers in a triplex pump and the moments when the moving parts are at rest. (The same numerical references indicate the same part on the various figures). In figure 1, item 1 is a central display and checking unit providing real-time monitoring of a set of positive-displacement pumps and recording the pumping operations. Items 2 are the local monitoring elements intended for use by the pump operators. Items 3 are microcomputer units, part of the invention. Depending on their configurations, these microcomputers can be connected to one or several positive-displacement pumps. In figure 1, they are connected successively from left to right to two triplex pumps 4, a quintuplex pump 5 and then to two triplex pumps 4 again. The use of a multipoint serial data bus between parts 1 and 2 simplifies the addition or removal of a particular equipment item. A similar bus is used between parts 2 and 3, allowing for connection of other sensors in series with microcomputer 3, plus the use of a single line to local monitoring unit 2. The number of pressure sensors 6 connected to microcomputer 3 is equal to the sum of the number of discharge chambers 7 of pumps 4 or 5 to which microcomputer 3 is connected. The number of proximity sensors 8 is equal to the number of pumps 4 or 5 connected. In other words, there must be a pressure sensor 6 for each discharge chamber 7 and a proximity sensor 8 per pump 4 or 5. A preferential mode for realisation of the Invention consists of sensor 8 detecting the passage of a ring (B) attached to the piston and providing a position reference. In a known manner, the nature of the reference will be selected to suit the sensor; the preferred example would be a steel ring detected by an inductive proximity sensor 8. Another example consists of an optical sensor associated with an optical reference on the piston or a Hall-effect sensor associated with a reference consisting of a magnet. The pump phase reference can also be obtained, for example, by detecting passage of a referenced tooth on a piston drive wheel or similar part mechanically linked to the piston, or by a sensor as described above. Figure 2 is the cross-section through a discharge chamber 7 of an example of a positive-displacement pump 4 or 5. The principal characteristic of positive-displacement pumps 4 or 5 is that discharge chamber 7 is filled by the alternating action of slide 9, and then evacuated into discharge circuit 10. The direction of fluid flow is established by valve 11, known as the suction valve, and valve 12, known as the discharge valve. Movement of valves 11 and 12 is determined by the action of suction valve spring 13 and discharge valve spring 14, and by the forces exerted by the moving fluid and the pressures in discharge circuit 10, the discharge chamber and suction circuit 15. The preferred configuration is with pressure sensor 6 mounted on the inner side of flap P to chamber 7. In this way the sensor does not weaken the pump body. However, if this solution is technically too complex, the sensor may be installed flush on another flat part of the chamber. Normal pump operation is as follows : when slide 9 advances into discharge chamber 7 from its stationary position (point which corresponds to maximum withdrawal), the fluid in the chamber is firstly expelled into suction circuit 15 until suction anti-return valve 11 closes, cutting off the fluid flow. The fluid is then compressed into discharge chamber 7 until the forces exerted on discharge valve 12 by the pressure in chamber 7, become greater than the forces on this same valve 12 by the pressure in discharge circuit 10 plus spring 14. At this moment, discharge valve 12 opens and the fluid is expelled into the discharge circuit. The volume of fluid delivered into discharge circuit 10 is equal to the volume displaced by slide 9 as the latter advances into chamber 7 from the position it occupied at the moment discharge valve l2 opened, up to its stationary position corresponding to maximum penetration into chamber 7. For many positive-displacement pumps this calculation is not sufficient : when slide 9 withdraws from discharge chamber 7 from its stationary (maximum-penetration) point, discharge valve l2 is not necessarily closed, especially when the pump is running at high speed. A certain volume of fluid therefore flows back into discharge chamber 7 until valve 12 closes. This volume must be deducted from the volume expelled by the pump into discharge circuit 10; it is equal to the volume displaced by slide 9 when it withdraws from its stationary (maximum-penetration) point in discharge chamber 7, before closure of discharge valve 12. Figure 3 is a block diagram of a microcomputer unit. Item 15 is a microprocessor system with its clock, bus and memories. A safeguarded memory 16 provides for storage of a certain quantity of data, in particular the calibration values of pumps 4 and 5 which are connected to the microcomputer. These values allow in particular for the calculation of the volumes displaced by slide 9 betweem its stationary position and its position at the moments of opening and closing of discharge valve 12. Items 17 are connecting parts providing links with the multipoint serial bus. In addition, pressure sensors 6 are connected to microcomputer 15 via adapters 18. Similarly, proximity detectors 8 are connected to microprocessor system 15 by adapters 19. Items 6, 18, and 19 are sufficient in number to provide for a pressure sensor 6 and an adapter 18 per discharge chamber 7, and for a proximity detector 8 and an adapter 19 per pump 4 or 5. Figure 4 shows three curves plotted against time. Curve 21 shows the variations in the output signal from a discharge sensor which measures the position of discharge valve 12. At the origin point, valve 12 is at rest on its seat: the curve is at maximum. As the curve begins to drop this indicates that valve 12 is moving away from its seat. The fluid then begins to be discharged into the discharge circuit 10. Knowing the moment of origin when slide 9 is in the stationary position corresponding to maximum withdrawal from chamber 7, plus the moment at which valve 12 begins to leave its seat, it is possible to calculate the volume displaced by slide 9 between these two moments. Curve 22 represents the signal from a sensor 6 placed in discharge chamber 7 corresponding to discharge valve 12 whose position is observed. Curve 23 is the derivative in relation to time, of curve 22. Research during development of the Invention showed that use of the derived curve brought technical improvements. Part of the Invention consists in using a pressure sensor 6 to detect opening and closing of discharge valves 12: use of movement sensor is not always suitable for meas ment of the movement of valve 12 inside the pump, whereas a pressure sensor has no moving parts and resists the pressures created by the pumps. Furthermore, pressure sensor 6 gives more information on the operational state of the pump than would a movement sensor measuring the movement of discharge valve 12. The highest point of curve 23 corresponds to the exact moment of opening of valve 12: this is used in the software of microcomputer 3 to determine the moment of opening of valve 12 from the form of the signal representing the pressure in the chamber. The moment of closing of discharge valve 12 is calculated in a similar way. Another technique used to determine the moment of opening and closing of discharge valves 12 in another configuration of the Invention makes use of the comparison between the signals from a pressure sensor 6 in discharge chamber 7 and a pressure sensor of the same type in discharge circuit 10: when the signals are equal, discharge valve 12 is open. If the pressure in the discharge chamber is lower than the pressure in discharge circuit 10, discharge valve 12 will be closed. Some difficulties are encountered with this technique as it requires the use of pressure sensors which are sufficiently accurate to allow for comparison. (The use of correlating algorithms allows for correction and real-time comparison of the signals from the sensors even where the latter are not very accurate. However, use of such algorithms may prove to be too long in relation to the real-time requirements of the application). For certain applications, pumping is regular: any changes in the volumetric efficiency of the pump take place slowly in relation to the operating speed of the pumps plus the calculations performed by microcomputer 3. In such cases it is often the case that the volumetric efficiency of the pump does not vary during several pumping cycles. It is therefore necessary to perform the efficiency calculations every n cycles only, and to connect several pumps to a given microcomputer 3. Microcomputer 3 calculates the volumetric efficiency of each pump in turn. This value is stored in memory and used as often as necessary, (e.g, every second), along with the pump operating speed, for flow-rate calculation for each pump (the volumetric efficiency value being assumed to be constant since it was calculated for the last time). Figure 5 shows signals 24 and 25 from the pressure sensors 6 in two discharge chambers 7. The pressure sensor 6 whose signal is represented by curve 24 is located in a discharge chamber 7 whose discharge valve 12 is in good working order. On the other hand the pressure sensor 6 whose signal is represented by curve 25 is in a discharge chamber 7 whose discharge 12 is defective so that there is a leak from discharge circuit 10 to discharge chamber 7 when discharge valve 12 is at rest on its seat. The pressure in discharge circuit 10 is greater than the pressure in suction circuit 15. The vertical lines represent the moments when the respective discharge chamber slides 9 are stationary. The invention is partly based on the observation that whereas curve 24 shows that the pressure in discharge chamber 7 does not increase before immobilising of slide 9, the pressure in discharge chamber 7 with a faulty discharge valve 12 does increase before the slide stops. An observation of the same type may be made for faults occuring at intake valves 11, the piston sleeves or for breakage of springs 13 and 14. These observations are used by the microcomputer 3 software to determine the state of the valves, sleeves and springs. When a leak is detected (and provided the discharge pressure is high enough), it is possible to measure the quantity of fluid leaked by analysing the development of the pressure-increase curve in chamber 7. When the system is in operation the micro-computer runs a program stored in memory which contains a number of tasks which may be as listed below (but not necessarily in the given order) : Energizing and initialisation of microprocessor (15) Acquisition of data from pressure sensors 6 and proximity detectors 19. Calculation of moments of opening and closing of discharge valve 12 of each discharge chamber 7 by one of the methods indicated above. Detection of the state of the pump (running or stopped), and calculation of operating speed according to the case. Calculation of moments of immobilisation of slides 9 of each discharge chamber 7 by analysis of signals from proximity detectors 8. Calculation of volumes of fluid actually discharged and volumes re-introduced into each discharge chamber 7. Comparison of values calculated with values determined so as to initiate certain calculations for checking of condition and correct operation of various (pump) parts, plus calculation of any leaks. Calculation of volumetric efficiency of each pump. Calculation of cumulative flow and volume for each pump. Transmission of data via data bus. In the event of request from a local monitoring unit, running of test programs or special calibration programs, storing in permanent memory or communication of certain parameters. Microcomputer 3 can perform numerous other calculations and run other programs; those listed above are given by way of example. The system resulting from the invention is designed to be sufficiently flexible in application to allow it to be used with different types of positive-displacement pumps. Appended Figure 6 represents the display panel for the unit, per the invention. This shows more clearly the progress represented by the invention, since in addition to accurate and precise measurement of flow and volume, it provides a direct reading of volumetric efficiency and indicates operating faults. The "Chamber" window equipped with LEDs indicates the chamber in which the fault has appeared as well as the valve concerned. This allows an operator to intervene immediately and with maximum effectiveness which is not possible with presently existing systems. FIG. 6 1. FLOW 2. VOLUMETRIC EFFICIENCY 3. VOLUME 4. CHAMBER 5. MOTOR 1 6. METRIC 7. NO 8. UNITS 9. 1/MIX 10. 2/DIAMETER 11. SUM 12. LOCK 13. MOTOR 2 14. 100 RPM

Claims: Real-time monitoring system and flow-measurement of positive displacement pump(s), said pump(s) being equipped of at least one piston (9) and a respective discharge chamber (7) and a discharge valve (12) and at least one sensor (8) capable to detect the position of the piston (9) in at least one of the discharge chambers (7) and at least one sensor (6) capable to detect the exact closing/opening time of at least one of the discharge valves (12) by observation of the pressure of the respective discharge chamber(s) (7), versus the time, characterized in that the sensors signals coming from each one or from several of the pumps are sent to microcomputer units (3), that uses the derivative curve versus time (23) of the discharge valve (12) position curve versus time (22) obtained from the signal of the discharge chamber pressure sensor 6), to detect the opening and closing times of discharge valve (12), or uses to the same purpose the comparison between the signal from a discharge chamber pressure sensor (6) and that from a discharge circuit (10) pressure sensor, and can calculate the volumetric efficiency of the pump, each unit (3) is connected to a local monitoring unit (2) for use by the operator of the pumps, and units (2) are connected to a central display and monitoring unit (1). System according to claim 1, characterized in that the connection between the local monitoring units (2) and the central unit (1) is performed through a multipoint serial data bus. System according to claim 2, characterized in that the units (3) are connected to the local units (2) through a multipoint serial data bus. System according to anyone of claims 1 to 3, characterized in that it comprises a proximity sensor 8 (and adapter 19) for the detection of the end position(s) of the piston in a considered chamber. System according to claim 4, characterized in that the proximity sensor (8) signal and the pressure sensor (6) signal are analysed to provide the volumetric efficiency of the pump, then the volumetric efficiency value is used to accurately correct the theoretical flow rate of the pump in order to get the actual flow rate in the discharge line.

Inventors: DANCER, PAUL, MONCHARMONT, JEAN
Applicants: PUMPTECH N.V.
Application year: 1987
Main classification: F04B51
IPC codes: F04B51