Abstract:
A method and apparatus are disclosed for monitoring the high-frequency of a process variable to prevent a compressor from reaching actual surge point. The detecting is accomplished by separating the high-frequency oscillation part of the process signal and transmitting the amplitude of this high-frequency oscillation component of the system signal via a controlled current loop to a controller. The circuit is housed in a small standard rail mount enclosure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method and apparatus for controlling centrifugal and axial compressor surge to enhance equipment/personnel safety and to minimize energy consumption. 
     2. Description of the Prior Art 
     Conventional surge control techniques for centrifugal and axial compressors provide for surge estimation based on the polytropic head (Hp) ratio to the volumetric suction flow rate squared (Qs 2 ). Such conventional surge control techniques are disclosed and described in publications. These conventional methods set the surge limit line of the compressor based on the surge curve data calculated by the compressor manufacturer. The conventional methods do not take process changes and compressor efficiency decreases into account. Thus the actual compressor surge limits usually differ significantly from the limits indicated on the compressor surge curve data calculated by the Compressor Manufacturer. 
     Microprocessor-based controllers with an anti-surge control algorithm have been used for compressor incipient surge detection. There are existing systems in the field where compressor control is handled by a distributed control system. Most compressor controls executed with distributed control systems consist of oversimplified algorithms. The main reason for this inefficient control is the speed of executing the algorithm and control system itself. Control systems without the special algorithm and speed requirements have used external devices that required special mounting and separate power wiring. 
     Before a compressor reaches the actual surge point rapid oscillations of flow, pressure, and current occur. Compressor field tests have confirmed this phenomena as an indication of impending surge. This invention is to provide a method and apparatus to detect incipient (impending) surge based on special conditioning of the high-speed oscillation measurement(s). 
     SUMMARY OF THE INVENTION 
     The invention relates to a method and apparatus for continuously monitoring the high frequency of a process variable signed, such as flow or pressure or driver motor current oscillations before the compressor reaches the actual surge point. This incipient surge control acts as an override control to the primary surge control and, in addition to allowing for optimum surge point setting, increases compressor/personnel safety while widening the operating window and saving energy. 
     The present invention uses a very simple but highly effective means of detecting an incipient surge of a compressor. Unique characterization and filtering is required to distinguish the surge characteristics of the compressor from the normal operation characteristics. 
     The accurate determination of the true surge control line not only eliminates the risk of encountering a surge condition, but also minimizes unnecessary wide surge margins which can result in excessive recycle/blowoff and waste of energy. 
     A special, high speed algorithm is required to transform pre-surge oscillations into useful data for control purposes. The detecting is accomplished by separating the high-frequency oscillation part of a process signal and transmitting the amplitude of this high-frequency oscillation component of the system signal via a controlled current loop to a controller. The circuit is housed in a small standard rail mounted enclosure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a further understanding of the nature and objects of the present invention, reference should be had to the following detailed description, taken in conjunction with the accompanying drawings in which like parts are given like reference numbers and wherein: 
     FIG. 1 is a diagram of a compressor anti-surge system showing the incipient surge detection apparatus of the present invention; 
     FIG. 2 is a diagram of the circuitry of the signal conditioner device of the preferred embodiment of the apparatus of the present invention used in the diagram of FIG. 1; 
     FIG. 3 is signal waveform representation of the circuit of the preferred embodiment of the apparatus of the present invention shown in FIG. 2; and 
     FIG. 4 is a drawing of the enclosure of the signal conditioner device of the preferred embodiment of the apparatus of the present invention; the circuit of FIG. 1 being housed in this enclosure. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Introduction 
     The improved incipient surge detection system of the preferred embodiment of the present invention may be used for controlling and monitoring centrifugal and axial compressors. Before a compressor reaches the actual surge point rapid oscillations of process variables such as flow, pressure, and current occur. These oscillations are filtered to match the characteristics of the compressor system. The peak absolute-value of the filtered signal is transmitted to a controller. 
     Structure and Operation 
     Referring to FIG. 1, there is shown a typical single stage gas compressor anti-surge control system using recycle gas as a means of preventing compressor surge. The compressor suction line  1  has a flow transmitter  2  and the motor (driver) has a current transmitter  6 . The suction flow  2  signal and the motor current  6  signal are connected to the incipient surge detectors  3  and  10 . The dynamic part of the suction flow signal and the current signal are processed by the incipient surge detectors  3  and  10 , and the outputs are connected to an input of the anti-surge controller  4 . The anti-surge controller  4  also has an input of the static flow signal from the flow transmitter  2  and inputs from other process (temperature and pressure and position) transmitters ( 7 ,  11 ,  12 ,  13  &amp;  14 ). The anti-surge controller  4  manipulates the recycle valve  5  to prevent the compressor  15  from going into a surge condition. 
     Referring to FIG. 2, there is shown an input signal conditioning, detector, transmitting circuit, and power supply. The input signal conditioning part of the system is connected to a process variable such as a flow transmitter  2 . The signal conditioning part of the system is composed of input terminals  16 ,  16 ′, direct current (dc) blocking capacitors  17 ,  17 ′, differential amplifier  18 , low pass filter  19  through  23 , and adjustable gain amplifier  24 ,  25 . The waveform of the input signal to the circuit is shown in waveform  38  of FIG.  3 . The input terminals  16 ,  16 ′ are used to connect the process variable transmitter  2  to the dc blocking capacitors  17 ,  17 ′. The dc blocking capacitors  17 ,  17 ′ are used to remove the effect of low frequency variations caused by normal process changes and to couple only the dynamic part of the process variable to the input differential amplifier  18  to which capacitors  17 ,  17 ′ are connected. The high pass cutoff frequency of the capacitors  17 ,  17 ′ and amplifier  18  are selected to match the characteristics of the compressor system  15 , usually the cutoff frequency is between 0.1 Hz and 3 Hz. The input differential amplifier  18  is used to amplify and buffer the signal for the filter  19 - 23 . The low pass filter  19 - 23  connected in series with amplifier  18  is used to remove high frequency noise from the process dynamic signal. The values of the variable resistors  19 ,  20  and capacitors  21 ,  22  are selected for a cutoff frequency in use with amplifier  23  to the characteristics of the compressor system  15 . Usually the cutoff frequency is between 1 Hz and 20 Hz. The output waveform of the filter is shown in waveform  39  of FIG.  3 . Waveform  39  forms the input to the gain amplifier  25  which has a gain adjustment resistor  24 . The gain is adjusted to match the characteristics of the compressor system  15 , usually between one and one-hundred. 
     The detector part of the system is connected to gain amplifier  25  as its input and uses a precision absolute-value circuit composed of amplifier  26  and diodes  27 ,  27 ′ and a peak detector composed of amplifier  28 , diode  29 , and capacitor  30 . The waveform output of the absolute-value circuit amplifier  26  is shown as waveform  40  of FIG.  3 . Waveform  40  is then introduced to peak detector amplifier  28 . The time constant of the peak detector is selected to match the characteristics of the compressor system  15 , usually ten times the reciprocal of the cutoff frequency of capacitors  17 ,  17 ′ and amplifier  18 . The waveform of the peak detector circuit is shown in waveform  41  of FIG.  3 . 
     The transmitter part of the system is composed of a voltage-to-current converter  31  and output terminals  32 ,  32 ′. The voltage of waveform  41  across the detector capacitor  30  is buffered and converted to an industrial standard 4 to 20 milliampere signal. 
     Output terminals  32 ,  32 ′ are connected to the input of an anti-surge controller  4 . The four milliampere signal represents no dynamic process signals and the twenty milliampere signal represents the maximum dynamic process signal. 
     The power supply  33 - 37  part of the system uses a voltage regulator connected to the output of the voltage-to-current converter  31  to regulate the varying voltage on the output of the voltage-to-current converter  31  to a fixed voltage for all of the current levels of the circuits. 
     Referring to FIG. 4, there is shown an enclosure  42  that attaches to a standard mounting rail  43  housing the circuit of FIG.  2 .