Patent Publication Number: US-6213724-B1

Title: Method for detecting the occurrence of surge in a centrifugal compressor by detecting the change in the mass flow rate

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of Ser. No. 08/861,974 filed May 22, 1997, now U.S. Pat. No. 5,971,712; which claims priority from Provisional Application, Serial No. 60/017,193, filed on May 22, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention generally relates to centrifugal compressors and more particularly to an improved method for electronically detecting the occurrence of surge in a centrifugal compressor driven by an electric motor, based on the measured rates of change of the discharge pressure and motor current. 
     Surge is an unwanted phenomenon in centrifugal compressors which occurs when the fluid flow rate through the compressor is suddenly reduced. When the flow rate is reduced to a point below a predetermined required minimum flow rate, fluid collects at the compressor discharge port and as the fluid collects, the fluid pressure at the discharge port increases until surge occurs. During the occurrence of surge, the direction of fluid flow is reversed and the built up fluid is flowed back into the compressor. 
     Surge is undesirable for a number of reasons. Compressor surge produces unstable fluid flow within the compressor and loud noise, and also increases the amount of heat generated by the compressor. Frequently, one of the consequences of surge is damage to compressor component parts. 
     One conventional way of avoiding surge is by increasing the fluid flow rate through the compressor inlet. Although surge is avoided by increasing the flow rate through the compressor inlet, such increased capacity for this compressor operation negatively affects the cost of compressor operation. 
     As an alternative to sacrificing compressor efficiency by increasing the inlet flow rate, mechanical means for avoiding the occurrence of surge have been developed. One such conventional mechanical means for avoiding the occurrence of surge is a mechanical differential pressure switch located in a switch tube or housing. Such known pressure differential switches include a pair of spaced apart contacts located in the housing. When the pressure differential between the ends of the switch housing is at a pressure level indicative of the occurrence of surge, the pressure differential causes the contacts to close and thereby provide an indication to a compressor operator that a surge condition is present. When the compressor surges, a valve is opened to adjust the fluid flow through the compressor and thereby take the compressor out of surge. 
     It is difficult for compressor operators to precisely set the gap between the contacts in known pressure differential switches. Additionally, the sensitivity of the contacts decreases over time. Moreover, such differential switches and other mechanically actuated surge detection means, usually do not prevent the compressor from going into deep surge once surge is detected. 
     The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. It is important to provide a reliable method for taking a compressor out of the surge condition to prevent compressor damage. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, this is accomplished by providing a method for 
     A method for controlling working fluid surge in a centrifugal compressor. According to the method, surge detection is accomplished by calculating the change in the compressible fluid mass flow rate that accompanies surge in a compressor. The compressor includes means for sensing a first fluid temperature, means for sensing a first pressure, means for sensing a second pressure, and means for measuring current drawn by a compressor prime mover, the method comprising the steps of: calculating the time rate of change of the first fluid temperature; calculating the time rate of change of the first fluid pressure; calculating the time rate of change of the second fluid pressure; calculating the time rate of change of current drawn by the compressor prime mover; calculating the mass flow rate by combining the calculated rates of change; and comparing the calculated mass flow rate to a predetermined acceptable mass flow rate to determine if surge is present. 
    
    
     The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures. 
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 is a schematic representation of a compressor system that includes a controller for detecting a surge condition in a compressor in accordance with the method of the present invention; 
     FIG. 2 is a flowchart of the controller software logic for determining if a surge condition is present by calculating the rate of change of the compressor discharge pressure when the rate of change of the discharge pressure is combined with the rate of change of discharge temperature of the compressed fluid; 
     FIG. 3 is a flowchart of the controller software logic for determining if a surge condition is present by calculating the rate of change of the discharge temperature of the compressed fluid; 
     FIG. 4 is a flowchart of the controller software logic for determining if a surge condition is present by calculating the rates of change of the discharge pressure and current drawn by the prime mover; 
     FIG. 5 is comprised of representative chart showing analog signals of motor current, discharge temperature and discharge pressure versus time; and 
     FIG. 6 is comprised of representative charts showing the time rate of change of the analog signals of FIG. 6 versus time. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, wherein similar reference characters designate corresponding parts throughout the several views, FIG. 1 is a schematic representation of a compressed air system  10  that includes compressor  11 . 
     The compressor is a two-stage centrifugal compressor having first compression stage  12  and second compression stage  14 . The compressor compression stages include a rotatable impeller (not shown) which compresses the fluid as it rotates. The compression stages are of conventional design well known to one skilled in the art. Compression stages  12  and  14  are driven by a gear system  16  that in turn is driven by a prime mover  18 . The gear system is often referred to as a “bull gear”. For purposes of this disclosure, “prime movers” shall mean any device that can be used to drive compressor  11  including, but not limited to electric motors, internal combustion engines. Specifically, for purposes of the preferred embodiment, the prime mover shall be an electrically powered three-phase induction motor. 
     The compression system  10  is intended to control centrifugal compressors ranging from 100 to 10,000 horsepower and producing 350 to 100,000 cubic feet per minute at pressures from 5 to 500 pounds per square inch gauge. 
     The compressor  11  further includes an inlet  20  and discharge port  21 . The volume of air entering the compressor through the inlet  20  is altered by an inlet valve  22  that is located along the length of inlet conduit  24 . Each change in position of the inlet is effected by a valve control  26  which in turn is actuated by microprocessor-based electronic controller  100 . The controller will be described in greater detail hereinafter. The valve control is in signal receiving relation with controller  100  and is in signal transmitting relation with inlet valve  22 . 
     Inlet filter  30  is also located along inlet conduit  24  and serves to filter particulate and other unwanted matter from the stream of uncompressed inlet air. 
     An intercooler  32  and moisture separator  34  are flow connected to the discharge port  36  of the first compression stage  12  and the inlet  38  of the second compression stage  14 . The intercooler and moisture separator serve to cool the compressed fluid and remove moisture such as water from the compressed fluid before it is further compressed in the second compression stage  14 . 
     Aftercooler  42  and dryer  44  are flow connected along discharge conduit  40  as shown in schematic FIG.  1 . The aftercooler and dryer are located in compressed fluid receiving relation with the second compression stage and like intercooler  32  and dryer  34 , serve to cool the hot compressed fluid and remove moisture from the compressed fluid before it is flowed to an object of interest such as a pneumatically actuated tool for example. The intercooler, aftercooler and dryers are of conventional design and are well known to one skilled in the relevant art. 
     Return conduit  50  flow connects the inlet and discharge conduits  24  and  40  and includes a bypass valve  52  flow connected to the return conduit. The bypass valve is repositioned during operation of the compressor  11  to alter the volume of compressed air discharged from the second compression stage that is to be flowed to the compressor inlet  20  and mixed with the uncompressed ambient inlet air. Bypass valve controller  54  is located in signal receiving relation with the controller  100  and is in signal transmitting relation with bypass valve  52 . 
     First sensor  56  and second sensor  58  respectively measure discharge pressure and discharge temperature of the compressed air that is flowed out discharge port  21  through discharge conduit  40 . The first and second sensors are in signal transmitting relation with controller  100  and the sensors provide electrical analog type signals which are processed by the controller according to the present invention, to determine if the compressor is experiencing a surge condition. Examples of the analog signals generated by the sensors are shown in FIG.  5 . 
     A third sensor  60  senses the current drawn by the electric motor  18 . The signal representing the motor current is transmitted to the controller which processes the signal. The signal is also an analog type signal and is plotted as amps drawn versus time in FIG.  5 . 
     The next portion of the description of the preferred embodiment will relate to the microprocessor based controller  100  and the controller logic for determining the occurrence of a surge condition. 
     The controller software logic is stored in the microcontroller memory and is comprised of rate of change computational and comparative logic for rate of change of discharge temperature, discharge pressure, and motor current. The controller software logic is shown generally in the flowchart FIGS. 2-4. 
     FIG. 2 is a flowchart of the controller software logic  200 , for determining if a surge condition is present by calculating the rate of change of the compressor discharge pressure when the rate of change of the discharge pressure is combined with the rate of change of discharge temperature of the compressed fluid. 
     FIG. 3 is a flowchart of the controller software logic  300 , for determining if a surge condition is present by calculating the rate of change of the discharge temperature of the compressed fluid. 
     FIG. 4 is a flowchart of the controller software logic  400 , for determining if a surge condition is present by calculating the rates of change of the discharge pressure and current drawn by the prime mover. 
     Surge Detection by Calculating the Time Rates of Change of Discharge Pressure and Motor Current 
     This portion of the description shall refer to FIGS. 1 and 4. For the compressor  11  driven by electric motor  18 , surge is detected by calculating the magnitude of the time rate of change of both motor current and discharge pressure and comparing the calculated values with reference set points. When both rates of greater than or equal to their respective set points, the compressor is in surge. The method shown in FIG. 4 is utilized to detect surge when the prime mover is an electric motor. 
     Software logic referred to generally at  400  in FIG. 4 is executed at fixed intervals of 120 msec by the controller  100 . Logic  400  is executed as part of a compressor controlling sequence executed by the controller. The other steps in the compressor controlling sequence do not form part of the present invention. 
     Turning now to FIG. 4, in step  402  the variables PREV CURRENT, RATE CURRENT, PREV PRESS, and RATE PRESS are initialized to equal zero, and the variable SURGE is initialized to False. The variables PREV PRESS and PREV CURRENT, represent the previous values of discharge pressure and motor current drawn. The variables RATE PRESS and RATE CURRENT represent the rates of change of the discharge pressure and motor current respectively. SURGE is indicative of the existence of a surge condition and is set equal to False initially to indicate that a surge condition is not present. 
     In step  404  the control loop timer is started. The control loop timer measures the time to fully execute the compressor controlling sequence generally described hereinabove. 
     Then in step  406  the motor current value sensed by sensor  60  and the discharge pressure value sensed by sensor  56  are obtained from the controller analog input channels. 
     Logic steps  408 - 414  represent computational steps. In step  408 , RATE PRESS is calculated by subtracting the sensed discharge pressure from PREV PRESS. In step  410 , PREV PRESS is set equal to the present sensed discharge pressure sensed by sensor  56 . In step  412 , the RATE CURRENT is calculated by subtracting the sensed value of drawn current from PREV CURRENT. In step  414 , PREV CURRENT is set equal to the value of current measured by sensor  60 . 
     In decision step  416 , if the value of RATE CURRENT is greater than or equal to the reference set point rate of change for current, and the value of RATE PRESS is greater than the reference set point rate of change of discharge pressure, the compressor is in surge and the value of SURGE is set equal to True and the flow of air supplied to the inlet valve is effected in respective steps  422  and  424 . In this way if there is a drop in discharge pressure and current drawn, the compressor is in surge. 
     In step  424 , the flow of air to the inlet valve is altered by transmitting signals to the bypass valve controller  54  and the inlet valve controller  26  thereby fully opening the bypass valve and repositioning the inlet valve to an unloaded position. In this way, surge condition is terminated. 
     It is contemplated that the compressed air system bypass valve connector  50  may be directed to atmosphere rather than being flow connected to inlet valve  22 . In this instance, when the bypass valve is opened, the compressed fluid is discharged to atmosphere. 
     In decision step  416 , if it is determined that either the rate of change of the current drawn by the prime mover or the rate of change of the discharge pressure is not greater than or equal to the reference set point, then SURGE is set equal to False in step  418  and in step  420  the logic routine pauses until the control loop timer expires at the completion of the compressor controlling sequence. 
     The reference set points for discharge pressure and motor current should be set to large enough values so that the rate of change caused by noise induced in the electronic signals can be distinguished from the rates of change caused by a real surge. 
     Surge Detection by Calculating the Time Rates of Change of Discharge Pressure and Discharge Temperature 
     The method for detecting surge by analyzing the time rate of change of the discharge pressure and discharge temperature will now be described. The method is represented by the software logic flowcharts shown in FIGS. 2 and 3. It is preferred that the method described hereinbelow be used to detect surge in a compressed air system where the compressor prime mover is not an electric motor. 
     When the time rate of change of the compressor discharge pressure and discharge temperature are combined to determine the presence of a surge condition, first, the rate of change of the discharge pressure is calculated, and if the rate of change for the discharge pressure is greater than or equal to a predetermined reference set point rate of change, the controller  100  executes the software logic  300  for calculating the time rate of change of the discharge temperature. 
     Routine  200  is executed every 120 msec by controller  100  during execution of the compressor controlling sequence described generally hereinabove. Turning to the flowchart representation of FIG. 2, for calculating the rate of change of compressor discharge pressure, initially in step  202 , variables “PREV PRESS”, “RATE PRESS”, and “SURGE” are respectively set equal to 0, 0, and False. The variables PREV PRESS, RATE PRESS, and SURGE are the same as previously described hereinabove in routine  400 . 
     In step  204  a control loop timer is started. In step  206 , the discharge pressure sensed by first pressure sensor  56  is obtained by controller  100  and is subtracted from the value of PREV PRESS to obtain the new rate of change of the discharge pressure. RATE PRESS is equal to the difference between PREV PRESS and the sensed discharge pressure. See step  208 . Then PREV PRESS is set equal to the sensed discharge pressure value in step  210 . 
     In decision block  212 , if the rate of change of the discharge pressure is greater than or equal to a reference set point discharge pressure rate of change, the software  200  then starts a surge detection timer in step  214  and then executes software routine  300 , in step  216 . The routine proceeds to step  306  of routine  300 . 
     If the rate of change of the discharge pressure is less than the reference set point rate of change, SURGE is set equal to False in step  218 , and the logic routine waits for the control loop timer to expire before returning to step  204  and executing routine  200 . 
     If the acceptable discharge pressure rate of change is equaled or exceeded in step  212 , the software logic routine  200  executes software logic routine  300  and calculates the rate of change of the temperature of the discharged compressed fluid. 
     In step  202  of routine  200 , the variables “PREV TEMP”, “RATE TEMP”, “SUM TEMP”, were initialized to 0. PREV TEMP represents the previous sensed discharge temperature value, RATE TEMP represents the rate of change of the discharge temperature, and SUM TEMP represents the sum of the rates of change for the currently executed loop of logic routine  300 . 
     In step  306 , the temperature of the discharged compressed fluid sensed by temperature sensor  58  is obtained from a controller analog input channel and then respectively in steps  308 ,  310 , and  312 , RATE TEMP is set equal to the difference between PREV TEMP and the sensed discharge temperature; PREV TEMP is set equal to the sensed discharge temperature; and SUM TEMP is set equal to the sum of SUM TEMP and RATE TEMP. 
     Due to the relatively slow response time of known temperature sensors, the rate of change of temperature is analyzed for a period of time that is longer than the period of time the discharge pressure is analyzed. A fixed number of control loops may be chosen, for example four and the rates of change within the number of control loops are accumulated as the total rate of change for the discharge temperature. Thus a larger temperature rise may be obtained so that false surge indications due to noise are eliminated. 
     In step  314 , if the value of SUM TEMP is less than a reference set point discharge temperature rate of change, the compressor is not in surge and SURGE is set equal to False in step  316 , and if it is determined the surge detection timer has not expired in decision step  318 , the logic routine waits for the control loop timer to expire in step  319  and then returns to step  304  and again executes logic routine  300  and obtains another discharge temperature rate of change in the manner described. 
     In step  318 , if the surge detection timer has expired, SUM TEMP is set equal to zero and the routine is returned to step  218  in routine  200 . See steps  320  and  323  respectively. 
     If in step  314 , the SUM TEMP is greater than or equal to the set point discharge temperature rate of change, then SURGE is set equal to TRUE, in step  322 . Then in step  324 , the surge detection timer is cleared and the controller alters the airflow to the inlet in step  326 . The controller sends signals to the inlet valve  22  and bypass valve  54  to change the positions of the valves and alter the inlet air supply and avoid the surge condition. The signal transmitted to the bypass valve control  54  fully opens the bypass valve  52 , and the signal; transmitted to inlet valve control  26  opens the inlet valve to a compressor unloaded position. 
     As previously described hereinabove, the bypass may be exhausted directly to atmosphere rather than to the inlet valve. 
     By the foregoing method, if drop in discharge pressure is accompanied by a rise in the discharge temperature a surge condition is present. By combing two variables to determine the occurrence of surge, false surge indications are prevented. 
     It is also contemplated that rather than determining the rate of change of the temperature of the discharged compressed fluid, the rate of change of the stage temperature of the fluid flowing between stages  12  and  14  or the rate of change of the fluid at the impeller diffuser may be combined with the rate of change of discharge pressure to determine if a surge condition is present. 
     SUMMARY 
     It is the purpose of this invention to detect surge by addressing surge more directly with the surge phenomenon of flow reversal whereby the flow drops to zero and reverses. This is accomplished by using the time derivative of the mass flow rate through the compressor. Although some compressors are equipped with flow measuring devices, most are not since such devices are quite expensive. Thus, as a less expensive alternative it is proposed using the ideal head equation and the fluid mass flow rate to relate to overall compressor horsepower. This neglects the mechanical losses within the compressor system such as bearing and gear power losses. The equation is represented as follows:        HP   =         mRT   1          (     k     k   -   1       )                  [         (       P   2       P   1       )         k   -   1     k       -   1     ]                     
     The total shaft power of an induction motor is: 
     
       
         HP=§i 2 r 
       
     
     Combining these two equations, solving for the mass flow rate, and differentiating with respect to time leads to the following generalized expression for the time rate of change of the mass flow rate. It is this expression which is then compared against pre established limits to characterize the presence or lack of surge.               m          t       =       A             i          t         +     B               P   1            t         +     C               P   2            t         +     D               T   1            t                           
     The expressions A, B, C, and D are simple functions of the system parameters; gas inlet temperature, T 1 , inlet pressure, P 1 , discharge pressure, P 2 , and motor current, i. 
     The present invention which calculates the rate of change of discharge pressure, discharge temperature and motor current, indirectly measures the mass flow rate of the compressor and therefore is an accurate means for determining the presence or lack of surge. 
     Thus by measuring the change in current, discharge pressure and discharge temperature, a surge condition may effectively be determined. 
     While we have illustrated and described a preferred embodiment of our invention, it is understood that this is capable of modification, and we therefore do not wish to be limited to the precise details set forth, but desire to avail ourselves of such changes and alterations as fall within the purview of the following claims.