Patent Publication Number: US-8529214-B2

Title: Variable speed progressing cavity pump system

Description:
The present invention is directed to a pump system, and more particularly, to a variable speed progressing cavity pump system and a method for varying the speed of a progressing cavity pump. 
     BACKGROUND 
     Progressing cavity pumps are often used in settings in which the speed and operation of the pump must be carefully controlled. For example, when a progressing cavity pump is used in a down-hole operation, the speed and torque of the pump are often manually controlled to ensure that the pump is operating efficiently, and is not running in the pumped-off condition. 
     SUMMARY 
     In one embodiment, the present invention is a progressing cavity pump system in which the output or production of the pump is monitored to ensure that the pump is not operated in the pumped off condition, while the pump is simultaneously monitored and/or controlled to maximize pump efficiency. More particular, in one embodiment, the invention is a pump system including a pump and a sensor system for monitoring a fluid production of the pump and providing an output indicative of the fluid production. The sensor system is configured to use at least two discreet methodologies for measuring the fluid production. The pump system further includes a control system operatively coupled to the sensor system and to the pump and configured to automatically vary the speed of the pump at least partially based upon the output of the sensor system. 
     In another embodiment, the invention is a pump system including a pump and a sensor configured to sense a fluid production of the pump and provide an output indicative of the fluid production. The pump system further includes a control system operatively coupled to the pump for automatically varying the speed of the pump at least partially based upon the output of the sensor system. The control system is configured to increase the speed of the pump after a predetermined period of time has elapsed, or at a predetermined time, and to subsequently decrease the speed of the pump after the increase if the output of the sensor indicates that the pump is not producing fluid at a sufficient rate. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view of one embodiment of the pump system of the present invention; 
         FIG. 2  is a schematic representation of a flow meter usable in the pump system of  FIG. 1 ; 
         FIG. 3  (broken into  FIGS. 3A and 3B ) is a flow chart illustrating an algorithm for controlling a pump system such as the system of  FIG. 1 ; 
         FIG. 4  is a graph illustrating various parameters of a pump system operated via the algorithm of  FIG. 3  under a first set of operating conditions; and 
         FIG. 5  is a graph illustrating various parameters of a pump system operated via the algorithm of  FIG. 3  under a second set of operating conditions. 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIG. 1 , in one embodiment, the pump system  10  includes a progressing cavity pump  12  positioned in or adjacent to a formation  14  which contains a substance desired to be extracted, such as oil, methane, natural gas, etc. The pump system  10  includes a well casing  16  which receives production tubing  18  disposed therein, with an annulus  20  formed therebetween. The progressing cavity pump  12  is positioned down hole in or adjacent to the production tubing  18  to pump fluid from the annulus  20  upwardly to the surface  22 . Pumped fluid passes through an outflow line  24  and then downstream for further processing as desired. 
     A flow meter, flow sensor or sensor system  26  is positioned in the outflow line  24  to measure the velocity and/or flow rate of pumped fluid. A controller  28 , such as a CPU, microprocessor, processor, computer or the like is operatively coupled to the flow meter  26 . The controller  28  is, in turn, operatively coupled to the pump  12  to control speed and operation of the pump  12 . In the illustrated embodiment, the pump  12  is hydraulically driven and includes a hydraulic power plant or hydraulic pump  30  coupled to a hydraulic pump head, or hydraulic motor  32 , by a hydraulic fluid line  34 . In this manner, the controller  28  can control the pump  12  by sending command signals to the hydraulic pump  30 . 
     In order to operate the pump system  10 , the hydraulic pump  30  provides pressurized hydraulic fluid (pressurized by a driver motor (not shown) which can be a gasoline motor, an electric motor, etc.) to the hydraulic motor  32 . In order to vary the speed and/or torque of the motor  32 , and thereby the pump, in one embodiment a control valve or swash plate  36  is provided at the output of the hydraulic pump  30  to restrict the volume and/or pressure of the hydraulic fluid which necessarily restricts the speed of the hydraulic motor  32  and pump  12 . The open/closed state of the valve or swash plate  36  on the hydraulic pump  30  thus controls the speed and/or torque of the hydraulic motor  32 . In this manner, the speed of the hydraulic motor  32  can be regulated from 0% to 100% of the hydraulic motor&#39;s capable speed. The controller  28 , control valve/swash plate  36  and flow meter  26  may be powered by the driver motor, by service power, by a battery associated with the hydraulic motor  30 , or by some other source. 
     The driver motor/hydraulic pump  30  may run at a constant speed, and in this case the swash plate  36  is utilized to vary the output of the hydraulic pump  30 . In an alternate embodiment, however, the speed of the driver motor/hydraulic pump  30  may be directly varied and in this case the swash plate  34  may not be utilized. Moreover, it should be understood that the pump system  10  need not necessarily be hydraulically driven, and could also be operated by other means, such as by an electric motor including a variable speed electric motor or the like, which would replace the hydraulic pump  30  and hydraulic motor  32  in a well-known manner. 
     In one embodiment, the progressing cavity pump  12  includes a generally cylindrical stator tube  38  having a stator  40  located therein. The stator  40  has a opening or internal bore  41  extending generally axially or longitudinally therethrough in the form of a double lead helical nut to provide an internally threaded stator  40 . The pump  12  includes an externally threaded rotor  42  in the form of a single lead helical screw rotationally received inside the stator  40 . The rotor  42  may include a single external helical lobe, with the pitch of the lobe being twice the pitch of the internal helical groove of the stator  40 . The rotor  42  and stator  40  can be made of any of a wide variety of materials, including metals and/or elastomers that are chemically inert and wear resistant. 
     The rotor  42  fits within the stator bore  41  to define a plurality of cavities therebetween. The rotor  42  is rotationally coupled to a drive shaft  44  which is rotationally coupled to the motor  32 . When the motor  32  rotates the drive shaft  44 , the rotor  42  is rotated about its central axis and thus eccentrically rotates within the stator  40 . As the rotor  42  turns within the stator  40 , the cavities progress from the inlet or suction end of the pump  46  to an outlet or discharge end  48  of the pump  12 , causing fluids adjacent to the pump inlet  46  to be pumped upwardly to the production tubing  18 , and ultimately to the outflow line  24 . 
     By way of example, the pump system  10  disclosed herein may be used in the production of methane gas from in situ coal seams. Such pumping operations typically require the removal of water from the coal formation. Once the water is removed, methane gas will be released from the producing region. A low pressure source, or suction source (not shown), may be applied to the annular space  20  of the pump system  10  (via tubing  50 , for example) to extract methane gas from the formation, or the methane may naturally rise out of the annulus  20  and be captured. 
     When the pump  12  is pumping liquids, such as water, therethrough, such liquids provide cooling and lubrication to the pump  12 . Water evacuated from the well/annulus  20  is typically replaced with water flowing into the annulus  20  from the surrounding formation. If the pump  12  evacuates water at a rate that exceeds the rate that water reenters the producing region/annulus  20 , eventually the liquid in the annulus  20  will be sufficiently evacuated such that the pump  12  will start pumping gas. Excessive gas in the pump  12  can lead to overheating of, and potential damage to, the pump  12 . Moreover, pumping of gas by the pump  12  prevents capture of the pumped gas, and can damage equipment due to the rapid expansion of gas as it is raised from the well to surface atmospherical pressure. Accordingly, optimal production is achieved is when water is removed from the producing region/annulus  20  at the same rate that water enters the annulus  20 . 
     In addition, it is generally desired to extract water at the maximum practical rate (without extracting gas) since extraction of water tends to also accelerate the production of gas. Extraction of the water depressurizes the extraction site, and allows the gas to be released into the pump  12 . Thus, it is desired to operate the pump  12  to maintain the lowest liquid level possible in the annulus  20 , yet without pumping any gas. 
     The flow meter  26  is configured to monitor the output/production of the pump  12 , such as fluid velocity and/or rate of fluid being extracted by the pump  12 . As noted above, the output of the flow meter  26  is provided to the controller  28  to aid in controlling operation of the pump  12 . In one embodiment, the flow meter  26  is a dual mode flow meter which utilizes two discreet methodologies for monitoring the fluid output. 
     For example, in one particular embodiment, the dual mode sensor  26  utilizes both ultrasonic and Doppler flow measurement methods. Ultrasonic flow measurement methods are typically most effective in measuring liquid that lacks significant amounts of gas and/or solids. On the other hand, Doppler flow measurement methods may be more effective in measuring liquid that has gas and/or solids dissolved, mixed or carried therein. In the extraction of water for recovering methane gas, solids such as paraffin, sand or other impurities, along with gasses such as methane, air, etc., may be included in the pumped liquid. Thus, as shown in  FIG. 2 , the dual mode flow meter  26  can include an ultrasonic flow meter portion  52  and a Doppler flow meter portion  54  to best accommodate both types of flow. 
     The ultrasonic flow meter portion  52  may include a pair of ultrasonic sources  56   a ,  56   b , and a pair of ultrasonic detectors  58   a ,  58   b , spaced along the length of the outflow pipe  24 . Ultrasonic source  56   a  provides a output pulse sent in the flow direction which is sensed by ultrasonic detector  58   a . Conversely, ultrasonic source  56   b  provides an output pulse in a direction opposite to the fluid flow which is sensed by ultrasonic detector  58   b . The time required for each pulse to travel from the source  56   a ,  56   b  to the associated detector  58   a ,  58   b  is tracked. The difference between the travel time for the pulses can be determined to measure the speed of the fluid. The ultrasonic flow meter portion  52  shown in  FIG. 2  includes ultrasonic sources  56   a ,  56   b  and detectors  58   a ,  58   b  positioned internally of the outflow pipe  24 . However, if desired, non-intrusive or “clamp-on” ultrasonic flow meters may be utilized and mounted on the outside of the outflow pipe  24 . 
     The Doppler flow meter portion  54  can take the form of an acoustic Doppler velocimeter (“ADV”) which records instantaneous velocity components at a single point. The Doppler flow meter portion may include a probe head with one transmitter  58  and a number of receivers  60   a ,  60   b ,  60   c ,  60   d . A beam of electromagnetic radiation, such as a laser, an ultrasonic beam or the like, is emitted from the transmitter  58 , and reflects off solid particles or bubbles in the fluid, turbulence in the fluid, etc. The frequency of the reflected beams is then sensed by the receivers  60   a ,  60   b ,  60   c ,  60   d , and the velocity of the fluid can be calculated by determining the Doppler shift effect. 
     As shown in  FIG. 2 , the flow meter  26  may be installed at a relatively low point of the pipe  24  to ensure the pipe  24  is typically completely filled with liquid during operation. Each flow meter portion  52 ,  54  may measure the speed of the liquid, and the flow rate can then be determined when the cross sectional area of the outflow pipe portion  24  is known. The sensor  26  may be configured to detect the presence of gas when a lack of fluid, or lack of a fluid velocity, is detected. Although not shown in  FIGS. 1 and 2 , the dual mode sensor  26 , and the individual sensor components, can be threaded into the pipe  24  such that the dual mode sensor  26 , or parts thereof, can be easily replaced as desired. 
     The signal to-noise ratio or signal output strength for both flow meter portions/methodologies  52 ,  54  may be tracked. In one case, both fluid sensing methodologies/flow meter portions  52 ,  54  are utilized simultaneously, and their outputs are monitored. If it appears that both sensor portions  52 ,  54  are providing reliable outputs (i.e., based upon their signal-to-noise ratio), the output of one or both sensor portions  52 ,  54  may be utilized (i.e., the readings of the sensors  52 ,  54  may be averaged, weighted, etc.). Alternately, the output of one of the sensors  52 ,  54  may be utilized until its output becomes unreliable. For example, in some scenarios Doppler fluid velocity measurements may be deemed more reliable, and therefore the output of the Doppler sensor portion  54  may be utilized as the default output of the sensor  26  until its output is deemed unreliable. At that point, the output of the ultrasonic fluid flow sensor portion  52 , or some combination, average or weighted average of the sensor portions  52 ,  54 , may be utilized. 
     As noted above, the use of the dual flow meter  26  allows the speed of the fluid to be determined by the single best source for a particular fluid flow, or by a combinations of sensors. In this manner, the system can achieve an optimum measurement of fluid speed and flow rate across a spectrum of flow conditions. Although the dual mode meter  26  is described in conjunction with ultrasonic and Doppler fluid speed measurement sensors/methodologies, it should be understood that the flow sensor  26  is not necessarily limited to those particular sensors/methodologies, and other sensors/methodologies for determining fluid flow, such as turbines, flowmeters, heat/temperature sensors (such as hot wire anemometers), pressure sensors (such as pressure anemometers, pitot tubes, venturi tubes, etc.), force sensors, particle image velocimetry, electromagnetic flowmeters, positive displacement flowmeters, coriolis flowmeters, etc., can be used without departing from the scope of the invention. In addition, more than two sensor portions or fluid methodologies, such as three, four, or more, may be used in the flow meter  26 . 
     If desired, the pump system  10  may be operated manually. In this scenario, the pump operator may monitor the operating characteristics of the pump  12 , such as its speed, torque, amount of fluid output, fluid output rate, and adjust the speed/torque accordingly. In this case, the controller  28  may have a manual mode allowing such manual control. The controller  28  may also have an automatic mode in which operation of the pump  12 /pump system  10  is controlled automatically, such as by the algorithm described below and shown in  FIG. 3  which can be implemented in the controller  28  in the form of computer-readable instructions embodied in software, hardware, or some combination thereof. 
     In general, the algorithm of  FIG. 3  seeks to first ramp up operation of the pump  12  (during the pump&#39;s start up phase), and then seeks to operate the pump  12  to ensure that at least a minimum flow of liquid is being generated. The operation of the pump  12  is periodically boosted to its absolute maximum speed, while the amount of fluid output by the pump  12  is monitored. If it appears that the pump  12  is extracting fluid too rapidly after the speed boost (or at any time during operation), then the pump speed is decreased until sufficient fluid flow is generated. 
     In particular, with reference to  FIG. 3 , it can be seen that at step  62  the pump  12  is started, and at step  64  the speed of the pump  12  is increased by step amounts until the pump  12  reaches its start up speed, as examined at step  66 . The start up speed can vary depending upon the nature of the pump  12 , the nature of the pump system  10 , the desires of the pump/well operator, the material being pumped, ambient conditions, etc. However, for the case of one particular illustrative example, which is further described below in association with  FIG. 3 , the start-up speed can be between about 50-200 rpm. 
     Next, at step  68 , the fluid pump up timer is started, and, at step  70 , the fluid flow rate is compared to a minimum fluid flow rate (also termed the “feedback target” flow rate). In one case, the fluid flow rate is determined by the flow meter, such as the dual mode flow meter  26  described above. However, it should be understood that the fluid flow rate can be determined by any of a variety of fluid flow meters, not necessarily a dual-mode flow meter. The minimum fluid flow rate is, again, set by a wide variety of factors. However, continuing with the illustrative example, in one case the minimum fluid flow rate can be between about one and about ten gallons per minute. 
     If, at step  70 , the fluid flow is below the minimum flow rate, at step  72  it is checked whether the fluid pump up timer has expired. In the illustrative example, the pump up timer can range from about one minute to about ninety minutes, with thirty minutes expected to be a typical number in some cases. If the fluid pump up timer has not expired, the system proceeds to step  74  and waits a safety check time, typically a few minutes in the illustrative example. The system resides in the safety check time  74  to provide the pump  12  time to conduct additional pumping operations, allowing the fluid flow to stabilize a bit, and allowing any air pockets to pass through before the fluid flow rate is again checked. 
     The system then returns to step  70 . If, at step  70 , the fluid flow is still below the minimum level and, at step  72 , the fluid pump up timer has expired, then the system proceeds to step  76  wherein a “low target action” is executed. The low target action may be selected by the operator of the pump/well, and can constitute, for example, a shut down of the pump  12 /pump system  10 , running the pump  12  at some minimum speed, continuing to pump at the last or current speed of the pump  12 , etc. In some cases, the pump  12  may be shut down, although in other cases it may be desired to avoid a total shut down of the pump  12 /pump system  10  to ensure some fluid flow continues in order to avoid loss of production of the well due to freezing or certain other conditions. 
     Returning to step  70 , if the fluid flow exceeds the minimum level, then the system proceeds to step  78 , effectively exiting the start up phase. At step  78 , the reoptimize timer is started, and the reoptimize state flag is set to false. At step  80 , the system checks whether the pump  12  is operating at its maximum effective speed. As will become clear in the description below, the maximum effective speed can be considered the highest speed at which the pump  12  can operate without becoming pumped off for the current well conditions. As will be further described below, the maximum effective speed of the pump  12  can be varied to maximize the output of the pump  12  while still providing sufficient fluid flow. However, for initialization purposes a maximum effective speed may be selected based upon calculations and/or past experiences. In the illustrative case, for example, the maximum effective speed is initially set to 180 rpms. 
     If, at step  80 , it is determined the pump  12  is operating below the maximum effective speed, the system advances to step  82 , wherein the system resides for the speed adjust time, to allow the system  10 /pump  12  to stabilize after any previous adjustments. Similar to the safety check time at step  74 , the speed adjust time of step  82  can vary, but in the illustrative example typically is in the range of a few minutes. The speed adjust time can be the same as, or different from, the safety check time. 
     Next, at step  84 , the pump speed is increased by a speed adjust step. The magnitude of the speed adjust step can vary, but in one case in the illustrative example is between about 10-25 rpms. The system then advances to step  86 , wherein the rate of fluid flow is checked. At step  86 , if the fluid flow is at or above the minimum fluid flow rate, the system advances to step  88 , wherein the state of the reoptimize flag is checked. The purpose of the reoptimize flag will be described in greater detail below. However, the illustrative system begins with a reoptimize flag initially being set to “false”, as set in step  78 . Accordingly, in this case, the system returns to step  80 , where the speed of the pump  12  is checked. 
     If, at step  80 , the pump  12  is at or above the maximum effective speed, the system proceeds to step  90  to determine if the reoptimize timer has expired. The reoptimize timer is typically set to a relatively long period of time to allow short and medium fluctuations in the system  10  to dissipate. For example, in the illustrative case, the reoptimize timer is set to last several hours, days, a week or more but can be shorter or longer as desired. If the reoptimize timer has not expired, then the system proceeds to step  86 . 
     If, on the other hand, the reoptimize timer has expired, the system proceeds to step  92  where the pump speed is set to its absolute maximum speed, and the reoptimize state flag is set to “true”. In this case, then, the pump  12  is set to a high operating speed, typically at or close to the maximum speed that the pumping system can handle (such as within about 10% in one case, or about 20% in another case, or about 30% in yet another case, of the absolute maximum speed). The absolute maximum speed may be determined by certain limitations in the system  10 , for example, top speed of the motor  30 / 32  and/or the pump  12 , the ability of the various pipes and other mechanical components within the system  10  to accommodate high speed fluid flow, safety or regulatory limits, operator desires or the like. The absolute maximum speed may also, or instead, be the top speed during operation of the pump  12 ; that is, during normal operation of the pump  12  the speed which is not exceeded, or a speed which cannot be exceeded and such that the pump  12  continues operating properly for an extended period of time. In the illustrative example, the absolute maximum speed is about 300 rpm. 
     This increase in speed at step  92  can, depending upon the current operating conditions, represent a significant increase in speed of the pump  12 , and more than a simple “step up” which might be implemented in other systems. For example, in one case the increase in speed my be at least about a 30%, or at least about 50%, increase in speed over the current speed of the pump. 
     At step  94 , which is optional, the system may wait a reoptimize sense interval to allow the effects of the speed increase to be felt. The reoptimize sense interval can range, in one embodiment, between about 5 and about 60 minutes, and may be the same as, or different from, the speed adjust time, pump up timer, and/or safety check time. The rate of fluid flow is then checked at step  86  and compared to the minimum flow rate. If minimum fluid flow rate is met, and the reoptimize state flag is “true” as checked at step  88 , then the system proceeds to step  96 , and the maximum effective speed is set to the current pump speed. Thus, if the absolute maximum speed is determined to provide sufficient fluid flow, the maximum effective speed is set to the absolute maximum speed of the system. The system then returns to step  80 . 
     Returning to step  86 , if the fluid flow at step  86  is below the minimum flow rate (i.e., after the pump is set to the absolute maximum speed, or at any other appropriate time during operation of the pump), at step  98  the maximum low target timer is started. The maximum low target time may be the same as, or different from, the length of the reoptimize sense interval, speed adjust time, safety check time or pump-up timer. It is next determined, at step  100 , if the maximum low target time has expired. 
     If the maximum low target time has not expired, then at step  102 , the pump speed is decreased by the speed adjust step and the system moves to step  103  where the maximum effective speed is set to the current pump speed. At step  104  the system resides for the remedy interval. The remedy interval can be the same as, or different from, the safety check time, pump up timer, speed adjust time, reoptimize sense interval, and may be several minutes in the illustrative example. The system then advances to step  106 , wherein the fluid flow rate is compared to the minimum fluid flow rate. If the fluid flow rate is not sufficiently high, the system returns to step  100 . On the other hand, if the fluid flow rate is sufficiently high, the system proceeds to step  88 . In this manner, the system seeks to step down the speed of the system until the minimum flow rate is achieved. 
     If, at step  100  the maximum low target time has expired, then it is assumed that sufficient fluid flow has not been generated in sufficient time, and corrective action is required. In this case, the system proceeds to step  108  and the low target action is carried out. The corrective action at step  108  can be the same as, or different from, the corrective action described above in the context of step  76 . 
     An example of implementation of the algorithm of  FIG. 3  is shown in the graph of  FIG. 4 .  FIG. 4  illustrates variance of speed of the pump  12  (also known as the rod speed) and the maximum effective speed. The graph also illustrates the fluid flow rate of the pump  12 , along with the minimum flow rate. As the system progresses to point A of the graph, the pump  12  is in its start up phase and the pump speed increases in steps (or alternately, generally linearly) from 0 to approximately 150 rpms. During this phase, the fluid flow rate increases generally linearly, although the increase of the fluid flow rate may lag behind the pump speed due to the time required for fluid to reach the surface of the well. From point A to B, the fluid flow is checked to see whether it exceeds the minimum flow rate, (i.e., steps  70 ,  72  and  74  of  FIG. 3 ), allowing the fluid flow rate to increase sufficiently. 
     Once the fluid flow exceeds the minimum level (at point B), the system exits the start up phase and ramps up to the maximum effective speed, as implemented by steps  80 ,  82  and  84  of  FIG. 3 . Once the maximum effective speed is reached at point C of  FIG. 4 , the system resides at that speed until, at point D, the reoptimize interval has expired. 
     At point D, once the reoptimize interval has expired, the pump speed is set to its absolute maximum speed and the fluid flow rate correspondingly increases. Although the pump speed is set to the absolute maximum speed at point D, as can be seen, the pump  12  may not be able react instantaneously and must ramp up to the absolute maximum speed between points D and E. At point E, after the absolute maximum speed has been achieved, the fluid flow rate is above the minimum flow rate (step  86 ), and the reoptimize state is true (step  88 ) so the maximum effective speed is increased to the absolute maximum speed (i.e. at step  96  of  FIG. 3 ). 
     At point F, the fluid flow rate falls below the minimum level, and the speed of the pump and maximum effective speed are correspondingly stepped down (according to steps  102  and  103  of  FIG. 3 ) until fluid rate increases above the minimum flow rate, which occurs at point G. The system then reaches stability at point G at which the new optimum speed is discovered. 
     Thus as can be seen in this example, the reoptimize process has effectively increased the pump speed from about 180 rpms (at point D) to about 230 rpms (at point G), shown as ΔS in  FIG. 4 . The reoptimize process has also increased the flow rate, and therefore production, by the increase shown as ΔR in  FIG. 4 . Accordingly, the reoptimize process results in increased production which might otherwise not be realized. 
     The system may then reside in the state shown at point G until the reoptimize process is carried out again (unless fluid output falls below the minimum flow rate). Thus, the reoptimize process periodically increases the pump speed to its absolute maximum speed, and steps down, as necessary, to ensure maximum production is achieved, while ensuring the pump  12  is not damaged. Thus, this algorithm is different from many others in that the speed of the pump  12  is increased to the absolute maximum, and then reduced, as necessary, as opposed to stepping up to reach the maximum effective speed. Immediately increasing to the maximum speed, and then stepping down as necessary, is believed to maximize effective speed increases and therefore provide increased production. 
     The system may carry out the reoptimize process at various times. For example, as outlined above the reoptimize process may occur at regular intervals. Each time interval may be measured from a starting point, i.e. the reoptimize process may commence a certain period of time (a waiting period) after the starting point. In the scenario outlined above the starting point would be the start/end of the previous reoptimize process. However, the start point for the reoptimize timer can be set/trigged by various other events, such as, for example, a specific time (i.e. 5 am), or a time at which the reoptimize process was previously implemented, or a time at which it was previously determined that a minimum rate of fluid flow was (or was not) being generated by the pump, or a time at which it was determined that the pump was (or was not) at the maximum effective speed, etc. Alternately, the reoptimize process may be set to be carried out a predetermined times; i.e. at 3 pm every third day; at 12 pm every day, etc. The waiting period (elapsed time measured after the starting point) can be a fixed period of time or changed based upon certain operating characteristics of the pump, or based upon operator desires. 
       FIG. 5  provides a graph illustrating various parameters of a pump under differing operating conditions than those shown in  FIG. 4 . In particular, at point A of  FIG. 5 , the initial speed up of the pump  12  is completed. At point B, the minimum flow rate is exceeded and the start-up phase is exited. At point C, the pump  12  has been increased to its maximum effective speed. However, at point D, a decrease in flow rate is anticipated, thereby causing the pump  12  to decrease in speed to avoid pumped off condition, and the maximum effective speed is decreased as indicated at steps  102  and  103  of  FIG. 3 . At point E, the pump speed has been sufficiently decreased that the minimum flow rate is again exceeded, and this stepped-down speed is set as the new maximum effective speed. ΔS represents the adjustment to the maximum effective speed which is instituted to avoid the pump off condition and protect the pump  12 . 
     It is noted that  FIG. 5  illustrates a condition in which the speed of the pump is reduced (i.e., at point D) before the flow rate actually falls below the minimum flow rate. In this case, the controller  28  may implement an algorithm which can predict a decrease in fluid flow rate. In particular, if the pump  12  is pumping gas, such gas tends to expand as it is raised, and such expansion can cause a temporary increase in fluid flow rate as the gas expands and pushes towards the surface. However, such a rapid increase in fluid flow rate, with little to no change in the pump speed, can be taken as an indication that there is gas in the pump system  10 , and that the fluid flow rate will soon decrease. For example, an increase of between about 10-30%, or about 15% in one case, in the fluid flow rate over a time period of from about 20 seconds to 3 minutes, or about 60 seconds in one case, with a less than about 0.05 to 3% change, or about 2% in one case, change in the pump speed, can be taken as a sign that the pump  12  is pumping gas, allowing corrective measures to be taken. 
     Thus, if the flow or rate of flow of pumped fluids increase without a corresponding increase in speed of the pump  12 , it can be taken as a sensor failure or the presence of gas in the system  10 . For example, at step  86  and/or  106  of  FIG. 3B , rather than simply inquiring whether the fluid flow is at or above the minimum flow rate, it may be asked whether the fluid flow rate is anticipated to drop below the minimum flow rate, such as by the methodologies described above. The system may keep track of past measured flow rates and thus can look back to compare current fluid flow rates to previous fluid flow rates. The amount of head pressure that the pump  12  experiences can also effect fluid flow rate. If, for example, a valve is opened or there is a break in flow line then there is less pressure on the pump system  10  and it begins to operate at greater efficiency, with greater fluid flow. 
     Although the invention is shown and described with respect to certain embodiments, it should be clear that modifications will occur to those skilled in the art upon reading and understanding the specification, and the present invention includes all such modifications.