Abstract:
A pump system using multiple reciprocating positive displacement pumps which phase shift is controlled by a phase shift controller. The phase shift controller uses a virtual master pump inside the phase shift controller which is used as a phase reference against which the phase shifts of the individual pumps is calculated. The phase shift controller adjusts the speed reference set-point for the variable speed drives of the individual pumps such that a desired phase shift is obtained and maintained. The operation of multiple reciprocating pumps using phase shift control can significantly reduce the pressure pulsation levels in the pump system. The use of a virtual master pump eliminates master slave scheduling and increases system reliability and availability as the operating of the phase control is not depending on the reliability of a real master pump.

Description:
[0001]    PRIORITY CLAIM 
         [0002]    This patent application is a U.S. National Phase of International Patent Application No. PCT/NL2011/050230, filed 5 Apr. 2011, which claims priority to U.S. Provisional Patent Application No. 61/321,601, filed 7 Apr. 2010, and Dutch Patent Application No. 2004979, filed 28 Jun. 2010, the disclosures of which are incorporated herein by reference in their entirety. 
     
    
     FIELD 
       [0003]    Disclosed embodiments relate generally to pumps and more particularly to multiple reciprocating positive displacement pumps for handling mineral slurries. 
       BACKGROUND 
       [0004]    Reciprocating positive displacement pumps are used for pumping fluid against relatively high pressure, when compared to single stage centrifugal pumps, for example. Further characteristics of such positive displacement pumps include high efficiency and an accurate flow output, but a relatively low flow capacity when compared to centrifugal pumps. When the flow requirements of a typical application cannot be met with a single pump, more positive displacement pumps can be arranged in parallel such that their suction and/or discharge connections are connected into a single suction and/or discharge line. This means that the sum flow of the individual pumps can meet the total flow requirements of the application. The combination of the individual pumps and the interconnecting suction and discharge lines forms a pump system. 
         [0005]    In reciprocating pumps, a displacement element such as a piston or plunger makes a reciprocating motion inside a cylinder liner enabling the positive displacement the fluid to be pumped. In a particular disclosed embodiment of the reciprocating pump, the reciprocating motion of the displacement element is generated by a mechanism which transfers the rotating motion of the pump drive into a reciprocating motion of the displacement element. Particular disclosed embodiments of this mechanism may include crankshaft, eccentric shaft, camshaft or cam disc mechanisms. 
       SUMMARY 
       [0006]    Disclosed embodiments provide a solution for the described shortcomings of the phase shift control systems of the prior art crankshaft-driven positive displacement pumps. In the prior art systems, a real pump is used as a master in a master/slave control scheme for controlling the phase shift between the master and slave pump. The drawbacks included the complex master/slave scheduling procedures, the reduced reliability of the pump system as it depends on the reliability of a single master pump, and the reduced performance of the entire pump system in case of an unstable master pump operation. 
         [0007]    Disclosed is a pump system using multiple-reciprocating, positive displacement pumps which phase shift is controlled by a phase shift controller. The phase shift controller uses a virtual master pump inside the phase shift controller which is used as a phase reference against which the phase shifts of the individual pumps is calculated. The phase shift controller adjusts the speed reference set-point for the variable speed drives of the individual pumps such that a desired phase shift is obtained and maintained. The operation of multiple reciprocating pumps using phase shift control can significantly reduce the pressure pulsation levels in the pump system. The use of a virtual master pump eliminates master slave scheduling and increases system reliability and availability as is the operating of the phase control is not depending on the reliability of a real master pump as is the case in prior art phase shift controllers. 
         [0008]    The virtual master pump creates a phase reference signal within the phase shift controller based on a single pump system reference speed set-point just as a real master pump would do. All the real pumps in the pump system act as slaves in the phase shift controller. The phase of each individual pump is compared to the phase of the virtual master pump inside the controller which is then used as an input for the phase shift control. In  FIG. 4  a control flow diagram for the virtual master phase shift controller is shown. 
         [0009]    The use of a virtual master pump can provide some operational improvements over the known prior art crankshaft-driven, positive displacement phase shift control systems. The slave pumps are always referenced against the same virtual master pump, hence no scheduling is required. The virtual master pump is considered to be available at all times as it does not require maintenance and has a much higher reliability than a real mechanical pump. Furthermore, the speed of the master pump is stable at all times since it is not influenced by the performance of a single master pump, which is especially useful when a positive displacement pump is used for pumping abrasive slurries in the mining and mineral processing industry. 
         [0010]    The disclosure is not limited to triplex single acting positive displacement pumps but applies to all single or multi cylinder single and double acting positive displacement pumps. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Notwithstanding any other forms which may fall within the scope of the apparatus, disclosed embodiments will now be described, by way of example, and with reference to the accompanying drawings in which: 
           [0012]      FIG. 1  illustrates a schematic cross section of a prior art triplex single acting positive displacement pump, also showing a disclosed embodiment using an intermediate fluid and an additional flexible displacement element; 
           [0013]      FIG. 2  illustrates a triplex single acting positive displacement pump flow pulsation of the prior art; 
           [0014]      FIG. 3  illustrates a prior art control flow diagram of reciprocating pump phase control with a master-slave control scheme using a real pump as master; and 
           [0015]      FIG. 4  illustrates a control flow diagram of reciprocating pump phase control with a master-slave control scheme using a virtual master, in accordance with disclosed embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    In the following description only the embodiment of the crankshaft type is disclosed, further referred to as a crankshaft driven positive displacement pump. In  FIG. 1  a schematic cross section of a 3-cylinder or triplex single acting crankshaft driven positive displacement pump is shown. The displacement element can directly displace the pumped fluid or displace an intermediate fluid which displaces a flexible displacement element which displaces the pumped fluid, such as an abrasive slurry. The disclosure applies to an embodiment of a positive displacement pump, but as the improvement is of particular interest to positive slurry pumps as described below, the embodiment using an intermediate fluid and flexible displacement as specifically shown in  FIG. 1 . 
         [0017]    A typical characteristic of the crankshaft-driven positive displacement pump is the non-constant reciprocating velocity of the displacement element. Crankshaft-driven positive displacement pumps therefore inherently generate a non-constant flow or flow pulsation each crankshaft revolution. In  FIG. 2  a typical flow pulsation generated during one crankshaft revolution or pump cycle of a triplex single-acting positive displacement pump is shown. Depending on the hydraulic response of the connected system these flow pulsations can result in pressure pulsations in the pumped fluid which can result in vibration of the piping and its support structure through which the fluid is flowing, and the pressure pulsations can create an unbalanced load in the piping system. 
         [0018]    When more than one crankshaft-driven positive displacement pump is connected to a single suction and/or discharge inlet or outlet, an interaction between the flow pulsations generated by the individual pumps can occur. This interaction can cancel out or increase the total level of flow and pressure pulsations in the pump system, again depending on the hydraulic response of the connected system. Also, hydraulic resonances present in the pump system can be excited by the flow pulsations generated by each individual pump. An important parameter which determines the total flow and pressure pulsation in a given pump system is the phase shift between the crankshafts of the individual pumps. Controlling this phase shift can therefore help in controlling the flow and pressure pulsation in a given pump system using crankshaft-driven positive displacement pumps. 
         [0019]    This phase shift control, also referred to as pump synchronization, is described below and shown in  FIG. 3 . The phase shift control requires pumps equipped with variable speed drives (VSD) which can be used to adjust and maintain the phase shift between the pumps by speed adjustments of the individual drives. Furthermore the individual pump and/or their drives are equipped with a phase sensor which indicates the position of the pump cycle of the individual pump, further referred to as phase of the individual pump. This phase information is then used by the phase shift calculator to calculate the phase shifts between the individual pumps, which then is used by the phase shift controller to adjust the speed of the individual pumps such that the phase shift is adjusted towards or maintained at the desired phase shift. 
         [0020]    In the known prior art, one pump in the pump system is assigned as the master pump. This master pump follows the pump system speed reference set-point without any adjustments for phase shift control. The other pumps are assigned as slaves who have to follow the master pump. The phase shift controller calculates the phase difference between the master and each slave pump and generates a speed set-point for each individual slave pump which is based on the phase shift between the master and the individual slave pump, such that a the constant and desired phase shift between the master and slave pump is obtained and maintained. 
         [0021]    This approach has several shortcomings:
       1. The system operator has to decide which pump is going to operate as the master pump before starting the pump system, after which the phase shift of the slave pumps with respect to the selected master pump is determined. This can result in complex master/slave and phase shift scheduling procedures which can also be dependent on the particular system.   2. The phase shift control is lost when the master pump trips or has to be shut down. Depending on the specific embodiment of the phase shift control, it may be required to shut down the complete pump system because the master and slave initialization might need to be re-initialized from start up. The reliability of the phase shift control for the complete pump system is thus dependant on the reliability of a single pump which is assigned as the master pump.   3. When the operating of the master pump is unstable, for example by a malfunction of suction and/or discharge valves, speed oscillation of the master pump can occur. The resulting unstable operation of the master pump has the result of creating an unstable operation in all of the other pumps in the pump system, and thus an unstable operation of the entire pump system.       
 
         [0025]    These shortcomings are a particular concern with crankshaft-driven positive displacement pumps used in the mining and mineral processing industry, in which highly abrasive slurries are pumped. The applications in the mining and mineral processing industry require continuous operation of the pump system without unexpected stops. Furthermore the shortcomings of the known arrangements become of even greater concern in high flow rate applications, which are also typical for the mining and mineral processing industry. 
         [0026]    Embodiments known and used in prior art are normally limited to three or four pumps per pump system, and for which the master/slave scheduling procedures are relatively easy. Furthermore the total flow rate of prior art pump systems with phase shift control are limited so that the system can still operate reliably because unbalanced loads generated by the pressure pulsations are relatively low and can still be acceptable in some applications. 
         [0027]    However, in high volume slurry applications in the mining and mineral processing industry, a considerably higher number of pumps in a single pump system may be used. Known examples typically use up to 10 pumps in a single pump system, making the master/slave scheduling very complex. The increased size of the pump systems used in the mining and mineral processing industry can result in unbalanced loads generated by the pressure pulsations in the pump system in the connected piping, being of such magnitude that phase shift control is a prerequisite for reliable pump system operation. 
         [0028]    Furthermore it should be noted that as a result of the abrasive characteristics of pumped slurry which result in higher wear rates of pump components, the time between maintenance of positive displacement slurry pumps can be relatively short in comparison with non-slurry applications. Each time maintenance on the master pump is required, a new master has to be assigned which might require a pump system shutdown, which in turn greatly influences the availability of the entire pump system in which continuous operation is optional. 
         [0029]    The present disclosure includes several embodiments for the individual parts of the phase shift controller. For completeness, a listing of some embodiments is given: 
       Variable Speed Drive 
       [0030]    The disclosure is not limited to a particular embodiment of the used variable speed drive, however the following embodiments are mentioned in particular:
       1. AC electric drives   2. DC electric drives   3. Diesel drives   4. Hydraulic drives       
 
       Pump Cycle Phase Sensor 
       [0035]    The disclosure is not limited to a particular embodiment of the used phase sensor, however, the following embodiments are mentioned in particular:
       1. The sensor embodiment can generate absolute phase information on the pump cycle   2. The sensor embodiment can generate relative phase information on the pump cycle which is combined with a zero point reference of the pump cycle phase   3. The sensor embodiment can generate phase information on the pump cycle based on the angular position of the main rotating component in the pump which transfers the rotating motion of pump drive into a reciprocating motion of the displacement elements, such as a crankshaft.   4. The sensor embodiment can generate phase information on the pump cycle based on the linear position of one ore more displacement elements in the pump   5. The sensor embodiment can generate phase information on the pump cycle based on the angular position of the variable speed drive which can be directly coupled or coupled via speed reduction device with known reduction ratio to the main rotating component in the pump.   6. The sensor embodiment can generate phase information on the pump cycle based on a single pulse generated at a predetermined position of the pump cycle.   7. The sensor embodiment can generate phase information on the pump cycle based on a multiple pulses generated at a predetermined positions of the pump cycle   8. The sensor embodiment can generate phase information on the pump cycle based on a multiple pulses generated at a predetermined positions of the pump cycle such that the number of pulses per pump cycle is equal to the number of displacement elements in the pump   9. The sensor embodiment can be composed of any combination of sensor embodiments as described above       
 
       Phase Shift Controller 
       [0045]    The disclosure is not limited to a particular embodiment of the phase shift controller, however, the following embodiments are mentioned in particular:
       1. Analogue electronic control circuit   2. Digital electronic control circuit based on solid state electronics   3. Programmable controller using microprocessor technology   4. Programmable logic controller   5. Embedded micro controller       
 
         [0051]    In the foregoing description of disclosed embodiments, specific terminology has been resorted to for the sake of clarity. However, the disclosure is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar technical purpose. Some terms are used as words of convenience to provide reference points and are not to be construed as limiting terms. 
         [0052]    The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavor to which this specification relates. 
         [0053]    Finally, it is to be understood that various alterations, modifications and/or additional may be incorporated into the various constructions and arrangements of parts without departing from the spirit or scope of the invention.