Abstract:
A pumping unit and method controlling a variable speed pump to output constant flow rate against varying outlet pressure using a controller with stored data curves of flow rate versus discharge pressure for a plurality of pump speeds for the variable speed pump, including a processor determining appropriate pump speed for the sensed discharge outlet pressure by extrapolating between the stored data curves.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a non-provisional application claiming priority to U.S. Provisional Application Ser. No, 62/049,839, entitled “Pump Control for Operation on a Variable Pressure Force Main” and filed Sep. 12, 2014. 
     
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       MICROFICHE/COPYRIGHT REFERENCE 
       [0003]    Not Applicable. 
       FIELD OF THE INVENTION 
       [0004]    The present invention relates to pump systems, and more particularly to pump systems which encounter fluctuating discharge pressures. 
       BACKGROUND OF THE INVENTION 
       [0005]    Constant speed centrifugal pumps may be advantageously used to pump into a pressurized pipe such as used to carry water or sewage (also known as a force main). In such environments, the discharge pressure of the pump may vary for a variety of reasons (e.g., discharge conditions and/or the presence of multiple stations discharging into a common force main). Since the pump discharge flow rate is dependent on the total head or pressure that the pump must overcome (generally, the higher the head level, the less the pump will discharge), in environments where the total head in the discharge main that the pump must overcome varies, the discharge volume from the pump will vary, with relatively high flow at low force main pressures and relatively low flow at high force main pressures. 
         [0006]    However, some pumping applications require that a consistent flow be discharged notwithstanding the variable discharge pressures, with varying flow potentially leading to issues such as pump damage, overloading of the motor, and/or failure to pump the liquid from the station leading to overflow conditions. 
         [0007]    Various pump operations have been used in an attempt to maintain a flow rate in environments where outlet pressure fluctuates. For example, U.S. Pat. No. 7,040,868 addresses air plugs encountered by fire engines (i.e., where the pump is expected to intermittently pump gasses) and uses an engine governor which controls the operating speed of the engine, with the controller overriding the governor to control the engine. 
         [0008]    Still other designs have been used for controlling pumps based on various encountered conditions, including those disclosed in U.S. Pat. Nos. 6,178,393, 7,308,906, 8,249,826, 8,417,483, 8,444,394, 8,540,493, 8,801,389 (Publ. No. US2011/0076156), and U.S. Pat. No. 8,840,376 (Publ. No. US2010/0247332). 
         [0009]    The present invention is directed toward providing a pump and method in which a constant flow rate will be reliably delivered notwithstanding changing pressure in the system. 
       SUMMARY OF THE INVENTION 
       [0010]    In one aspect of the present invention, a pumping unit adapted to pump fluid at a selected flow rate is provided, including a selected variable speed pump having a discharge outlet, a pressure sensor at the pump discharge outlet, a controller connected to the pump to control the speed of the pump, and a link providing the discharge outlet pressure sensed by the sensor to the controller. The controller includes a memory storing data curves of flow rate versus discharge pressure for a plurality of pump speeds for the selected pump, and a processor adapted to determine the pump speed which will provide the selected flow rate for the sensed discharge outlet pressure by extrapolating between the stored data curves. 
         [0011]    In one form of this aspect of the present invention, the memory stores data curves for first, second, third and fourth pump speeds, and the processor determines the pump speed by determining the two data curves closest to the sensed pressure for the selected flow rate and selecting a pump speed which is proportionately the same between the pump speed of the two closest data curves as the sensed discharge pressure is between the pressure of the two closest data curves at the selected flow rate. 
         [0012]    In another form of this aspect of the present invention, the sensor is a pressure transducer. 
         [0013]    In still another form of this aspect of the present invention, a plurality of pumping units pump into a force main, and at least one of the pumping units is a pumping unit according to this aspect of the invention. 
         [0014]    In another aspect of the present invention, a method of controlling a selected pump to pump a constant flow rate of a fluid against changing pressures is provided, including (1) storing, in a memory, a plurality of data curves of flow rate versus discharge pressure for a plurality of pump speeds for the selected pump, (2) continually sensing pressure by a sensor at the discharge outlet of the pump, (3) signaling the sensed pressure to a controller having a processor, (4) in the processor, determining the pump speed which will provide the selected flow rate for the sensed discharge outlet pressure by extrapolating between the stored data curves, and (5) controlling the selected pump to operate at the determined pump speed. 
         [0015]    In another form of this aspect of the invention, the plurality of data curves are for pump speeds spaced in uniform increments, with first, second, third and fourth data curves indicating the fluid flow rates discharged from the selected pump over a range of fluid pressures at the pump discharge for first, second, third and fourth pump speeds, respectively, and extrapolating between data curves includes determining on the processor (a) the two data curves closest to the sensed pressure for the selected flow rate, (b) the relative position of the sensed discharge pressure between the discharge pressure of the closest two data curves for the selected flow rate, and (c) a desired pump speed which is the same relative position between the pump speeds of the closest two data curves as the relative position of the sensed discharge pressure between the discharge pressure of the closest two data curves for the selected flow rate. 
         [0016]    Other objects, features, and advantages of the invention will become apparent from a review of the entire specification, including the appended claims and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a diagram of an embodiment of an advantageous pump station; 
           [0018]      FIG. 2  is a circuit diagram of a controller suitable for use with the pump station of  FIG. 1 ; and 
           [0019]      FIG. 3  is an example set of system curves for a pump, which curves may be advantageously used to program the controller. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    To overcome potential problems such as described above, a pump system is provided with an automatic pump control system that alters the pump operation based on conditions at the pump station&#39;s discharge and on actual operation of the pumps) used in the system. 
         [0021]    As illustrated in  FIG. 1 , a pump station  10  includes a suitable pump  14  (such as a constant speed centrifugal pump) having an input (suction pipe  20 ) and a discharge  24  which discharges into, for example, a force main  26  and therefore encounters the variable pressures of the force main. The pump  14  also includes a motor  28 . 
         [0022]    A control  30  is advantageously provided for the pump  14  as schematically illustrated in  FIG. 1 . Specifically, a pressure sensor  34  is provided at the pump discharge  24 , which sensor  34  provides a signal via link  36  to a controller  40  such as a programmable logic controller (PLC) reflective of the pressure sensed at the pump discharge  24 . The controller  40  includes memory  42  in which the pump system curve ( FIG. 3 ) may be stored. 
         [0023]    The controller  40  determines the operating speed required of the motor  28  to maintain a selected discharge flow rate at the sensed discharge pressure based on the pump system curve ( FIG. 3 ), and signals that speed to variable speed pump controller (variable frequency drive, or VFD)  44 . The VFD  44  in turn provides the power which then drives the pump motor  28  to operate at the desired pump speed. 
         [0024]    A PLC  40  advantageously permits the input signal to be refined based on system characteristics as further described below, where a modified analog output is sent to the variable speed pump controller  44 . 
         [0025]      FIG. 2  includes a diagram of a control  30  suitable for the above operation, illustrating a control circuit having simplified controls when a programmable logic controller (PLC)  40  is used. The top of  FIG. 2  shows the three phase power going to the circuit breaker (CB), variable frequency drive (VFD)  44  and the pump motor. The pressure sensor  34  provides the input to the PLC  40 , which outputs a control signal to the VFD  44  at the bottom right of  FIG. 2 . 
         [0026]    Generally, in order to have the pump run at a consistent desired flow rate, the pump speed (i.e., the speed of the pump motor  28 ) will be controlled to increase if the discharge (e.g., force main) pressure increases and decrease if the force main pressure decreases. More specifically, the PLC  40  may advantageously be controlled to operate based on system information including suitably determined pump performance data. Such pump performance data may be determined, for example, from the pump curve often available from the pump manufacturer, or advantageously by suitable testing of the pump. 
         [0027]    More specifically, system curves (pressure versus flow developed by the force main system) may be based on conditions at the installation. For example, the system curve may be a combination of the “static” head (i.e., the elevation difference that the pump must “lift” the liquid-level at the final pump discharge point minus the elevation of the water at the pump itself), and “dynamic” head (e.g., the friction losses developed by the fluid to traveling through the system). Friction losses are calculated based on the fluid velocity and, as the discharge flow increases, the velocity through the discharge piping and other devices (elbows, meters, valves) increases, so that the more liquid pumped, the more dynamic friction head that will be developed. 
         [0028]      FIG. 3  shows an exemplary pump system curve which may be advantageously used to program the PLC  40  in accordance with the present invention. The  FIG. 3  chart includes maximum and minimum system curves, which may be determined from a plot of total head versus flow. 
         [0029]    In  FIG. 3 , the lines labeled as  875 ,  800 ,  700 ,  600 ,  500  are rotational speeds (in rpm) for a particular impeller (La, for the same impeller diameter), and show the total head created by the pump versus flow rate for the pump running at that specific speed. The VFD may operate the pump  14  at other speeds as well using the VFD  44 , with the data for such speeds readily interpolated between those in  FIG. 3 . As can be seen from  FIG. 3 , if the pump speed is slowed down, it can pump at the same flow rate with less total head (e.g., by shifting down from the  800  rpm curve to the 700 rpm curve), or it can pump at lower flow rate with the same head when compared to the higher speed (e.g., by shifting left from the 800 rpm curve to the 700 rpm curve). 
         [0030]    The lines labeled 5, 7 ½, 10, 15 and 20 BHP in  FIG. 3  show the motor horsepower draw to run at that point on the curve(s). This information allows for determination that the motor is sufficiently sized so that it can run at all anticipated points without the horsepower draw exceeding the motor horsepower. This thus advantageously permits limiting the flow and facilitates determination of suitable pump motor  28 , enabling use of smaller pump motors than might otherwise be used. 
         [0031]    In  FIG. 3 , the vertical line  50  (at 500 gpm) represents an example minimum flow recommended for the pump system  10  being illustrated (if the flow is less than this it may be noisy and can cause damage to the pump). Such information may be included in the PLC  40  to the pump from running below this minimum flow line when the static head increases. 
         [0032]    The “max system curve”  52  and “min system curve”  54  in  FIG. 3  are plots (determined from the installation data [elevations and friction head]) of flow when under the minimum or maximum static head. There could be additional system curves that would be in between the maximum and minimum and they would have the same shape as those curves depending on the system conditions. However, the system curves are normally drawn for min and max conditions. 
         [0033]    It should be appreciated that the pump will operate at a flow rate (i.e., U.S. Gallons Per Minute) at the point in  FIG. 3  where the pump running speed line (i.e., appropriate rpm line) crosses the current system line (La, total head). Thus, if a pump is running at 700 rpm and the static head is at the maximum level, the pump would run at 500 gpm and  32 ′ total head (the point  60  where the 700 rpm line crosses the max, system curve), Conversely, if a pump is running at 700 rpm and the static head is at the minimum level, then the pump would run at about 1250 gpm at 26′ total head (the point  62  where the 700 rpm line crosses the min. system curve). 
         [0034]    In accordance with the disclosure herein, the speed is varied based on the discharge conditions. Thus, as an example using  FIG. 3 , in order to maintain a 1000 gpm flow rate with the head at the maximum level, the pump would need to produce  37 ′ total head (the head for point X in  FIG. 3 , which is where the max. system curve crosses the 1000 gpm vertical line). Comparing point X (1000 gpm and 37′ total head) to the rotational speed lines and extrapolating between the 700 and 800 rpm curves shows that approximately 795 rpm for the pump would be required to run at 1000 gpm and 37′ total head (point X being between the 700 and 800 rpm lines, but substantially closer to the 800 rpm line). 
         [0035]    Similarly, if the head is at the minimum level, then the pump would need to produce 22′ total head (the head for point Y in  FIG. 3 , where the min. system curve crosses the 1000 gpm vertical line). Comparing point Y (1000 gpm and 22′ total head) to the rotational speed lines by extrapolating between the nearest lines (i.e., for 600 and 700 rpm) shows that approximately 625 rpm for the pump would be required to run at 1000 gpm and 22′ total head (point Y being approximately ¼ of the way from the 600 rpm line to the 700 rpm line). 
         [0036]    Therefore, using the above examples, if the sensor  34  were to detect that the discharge pressure dropped from 37′ to 22′ head (16 to 9.5 psi), the controller  40  would extrapolate the pump system curve data in memory  42  to drop the operating speed of the pump from 795 rpm to 625 rpm to achieve a consistent discharge rate of 1,000 gpm. 
         [0037]    Such data can be stored and used by the controller (PLC)  40  to automatically and continuously control the rotational rate of the pump motor  28  to achieve the desired discharge rate notwithstanding varying discharge pressures. 
         [0038]    It should therefore be appreciated that the controller  40  programmed as described herein will cause the pump station  10  to discharge a consistent desired flow rate into the force main even when the force main has variable pressures caused by factors unrelated to the pump station  10 , For example, in environments in which a pump station  10  discharges into a force main having other pumps also discharging into the force main, the discharge pressure at the station  10  may frequently increase or decrease significantly based on whether the additional pumps are operating or not. The control  30  would detect any such increase or decrease in pressure and, based on the stored pump curve data, the PLC  40  would determine the speed of the motor  28  and control the variable speed pump controller  44  to cause the pump  14  to maintain the desired flow rate by appropriately increasing or decreasing the speed of the motor  28  accordingly. Thus, the pump station  10  will not operate at flow rates which are disadvantageously too high or too low and moreover will automatically adjust the pump operation to maintain a user defined flow volume from the pump  14  no matter what the discharge pressure. 
         [0039]    It should also be appreciated that inlet pressure may vary due to a change in water level in the tank or well that the pump is evacuating, and the described pump station  10  will allow for inlet pressure variations to the pump while still maintaining a constant outlet flow. Moreover, by using the pump system curves ( FIG. 3 ) specifically related to the pump  14  of the pump station and extrapolating between such curves for all conditions, ideal operation may be achieved notwithstanding constantly changing conditions. It should also be appreciated that pump stations such as described herein could advantageously be used on a booster pump where the inlet pressure varies.