Abstract:
A system and method for controlling gas surges in compressors powered by electric motors. An adjustable speed drive (ASD) incorporating a microprocessor powered by a three-phase source of electricity controls the electric motor which drives the compressor. A bypass valve is inserted between the gas input of the compressor and the gas discharge. The bypass valve opens in response to signals from the ASD which detects gas surges by analyzing a stored surge map which plots a surge line and a related discretionary control line based on the power and speed of the motor. When the operating point of the motor crosses the control line, the ASD causes the bypass valve to open.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. provisional application 61/164,641 filed on Mar. 30, 2009, which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The subject invention relates generally to a process control system to protect electric motor driven compressors from damage caused by gas flow surge, in applications on pipelines, liquefied natural gas plants, and other industrial installations. More particularly, the system and method employ an adjustable speed drive as a surge controller. 
       BACKGROUND OF THE INVENTION 
       [0003]    Surge on compressors is a serious problem, causing severe vibration, and sometimes damage to the rotating equipment. A typical surge control system as known in the prior art is shown in  FIG. 1  in which motor  5  drives compressor  10 . Gas flows into compressor  10  along pipe  15  into low pressure input suction pipe  20  and is discharged along pipe  25  into gas output pipe  30 . Gas flow is measured and monitored by surge controller  35  along line  40  while suction pressure, if this is known to change, is measured and monitored along line  45  and discharge pressure is measured and monitored along line  50 . In simplified terms, surge results from excessive pressure in the compressor discharge along pipe  25  causing the gas flow to reverse back into compressor  10 . Surge is controlled with a large bypass valve  55  which allows high pressure gas at the discharge to flow back into low pressure suction pipe  20 , thus relieving the pressure. Bypass valve  55  is controlled by surge controller  35  along signal line  60 . Other devices besides a simple bypass valve may also be used. For example, a spring-loaded blow-off valve which dumps high discharge pressure gas to a flare (burner) is a possibility. However, such a valve has to be set low and is wasteful and expensive if used very much. A third similar mechanism is a spring-loaded relief valve on the discharge which opens when the pressure reaches a preset high value and dumps gas back to the suction. This type of valve also has to be set low to be safe and has the same drawbacks as previously described. 
         [0004]    During factory testing of compressor  10 , pressure and flow are measured at a range of rotating speeds, and the onset of surge carefully measured and drawn on a two-dimensional Surge Map in graphical form displaying the relationship between input gas flow and discharge pressure. For each pressure and flow point, the corresponding horsepower and motor speed are measured and drawn on the map, as shown in  FIG. 2 . This map is used to program surge controller  35  so that it opens bypass valve  55  if flow conditions approach the surge limit as shown in  FIG. 2 . If the suction pressure can also change in the particular configuration, the vertical axis of  FIG. 2  is modified so as to represent a pressure ratio (discharge pressure/suction pressure) and thus create a similar shaped set of curves which are used for surge control. 
         [0005]    During normal compressor operation, operating point X is in the safe operating area which is that area to the right of control line A, as shown in  FIG. 3  which presents a surge map in graphical form of the relationship between compressor suction flow on the horizontal axis and compressor discharge at various operating points on the vertical axis. Control line A itself is programmed into the controller a discretionary amount to the right of surge line B to provide a margin of safety in system operation. If, due to changing process conditions or compressor speed change, the operating point moves across control line A, controller  35  starts to open bypass valve  55  to relieve the discharge pressure and bring compressor  10  back into the safe operating area.  FIG. 3  also shows the relationship of compressor suction flow to compressor discharge pressure at various exemplary constant speeds C, D, E, F and G, in RPM of motor  5 . As those constant speeds increase, the relative suctions flow and pressure also increase. 
         [0006]    Surge controller  35  has a Proportional plus Integral (PI) controller  100  having as inputs at  115  the operating point from the surge controller map and, at  120 , the set point from the control line from the surge controller map. PI  100  develops a current output  105 , as shown in  FIG. 4 . The current output is converted by a converter at  110  to either air pressure or hydraulic pressure to actuate bypass valve  55 , which is normally closed. When a control error is detected, PI  100  generates an output which starts to open valve  55  to allow bypass flow. As the error diminishes, the output decreases, and the valve closes. Adaptive control can be added to the PI to control for special conditions. 
         [0007]    The traditional surge control system described above requires at least three sensors to measure gas flow, suction pressure and discharge pressure as well as a microprocessor-based surge controller. A simpler, more reliable, and less expensive surge control system would be a benefit to users. 
       SUMMARY OF THE INVENTION 
       [0008]    This invention relates to a system and method for preventing gas surges in industrial compressors powered by electric motors. The system has a gas input line and a gas output line. A low pressure input suction line is connected between the gas input line and an input into the compressor. A gas discharge line is connected between the gas output line and an output from the compressor. Bypass means, which may comprise an air/hydraulic converter connected to a bypass valve, is connected between the gas discharge line and the low pressure input line. An adjustable speed drive (ASD) incorporating a microprocessor is powered by a three-phase electric power supply and is connected between the electric motor and the bypass means. The ASD generates and stores a two-dimensional compressor surge map from which it determines the occurrence of a gas surge in the gas discharge line. If such an occurrence is detected, the bypass valve is caused to close by sending a signal to an air/hydraulic converter connected to the bypass valve, and the speed and power of the electric motor may be modified. An alternative embodiment provides redundant control of the bypass means by adding an ASD to preexisting systems relying on surge controllers operating based on pressure and flow information provided by multiple sensors. 
         [0009]    A method of surge control is also disclosed in which the microprocessor in the ASD generates a two-dimensional surge map based on the speed and power of the compressor used by the system which includes a graphical representation of a surge line indicating the points at which an undesirable surge will occur for any given speed or power. A further graphical control line is generated spaced a discretionary distance at all points away from the surge line so that at any given speed the control line indicates a higher compressor horsepower and that at any given compressor horsepower the control line indicates a lower speed. The graph, including the surge line and the control line, are stored in the microprocessor subject to modification based on optional additional inputs such as gas temperature and suction pressure at the input of the compressor. At least every five milliseconds, the microprocessor examines the operating point of the compressor in terms of speed and power to determine if that point is equal to or greater than any point on the control line. If so, an analog signal is issued to the bypass valve via the air/hydraulic converter instructing the valve to open. If not, and the valve has already been instructed to open, a closing instruction is sent. In the alternative embodiment providing redundant surge control, the method provides for controlling the bypass valves in response to the larger of the signals sent from the ASD and the surge controller in the event that they differ. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The foregoing and other objects, aspects and advantages of the invention will be better understood from the following detailed description of the invention with reference to the drawings, in which 
           [0011]      FIG. 1  displays in block diagram form a typical surge control system as known in the art. 
           [0012]      FIG. 2  is a typical two-dimensional surge map in graphical form of the relationship between gas flow and discharge pressure at various motor horsepower and speeds. 
           [0013]      FIG. 3  is a surge map in graphical form of the relationship between compressor suction flow and compressor discharge pressure showing a control line. 
           [0014]      FIG. 4  is a block diagram illustrating the PI portion of a surge controller and a bypass valve. 
           [0015]      FIG. 5  is a block diagram of apparatus embodying the principles of the invention. 
           [0016]      FIG. 6  is a graphic representation of a compressor surge map. 
           [0017]      FIG. 7  is a block diagram of alternative apparatus embodying the principles of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The apparatus for the preferred embodiment of this invention is shown in  FIG. 5  in which electric motor  5  powers compressor  10  which has low pressure input suction pipe  20  connected to gas input flow pipe  15  and compressor gas discharge pipe  25  connected to gas output pipe  30 , as in  FIG. 1 . However, when a compressor is used, such as those manufactured for such uses by GE Oil &amp; Gas, Dresser Rand, Mitsubishi, and Siemens, electric motor  5  is controlled by adjustable speed drive (ASD)  200  which is used to smoothly start motor  5  and control the speed of compressor  10 , as shown in  FIG. 5 . ASD  200  is powered by three-phase electric power supply  205  and generates variable frequency three-phase power over lines  210  that feed motor  5 , thus controlling the motor speed and power. Any ASD may be used for this purpose so long as it is sized appropriately to match and control motor  5  and includes a microprocessor-based control. Examples of ASDs usable in arrangements of the type disclosed herein are the Dura-Bilt   5   i MV and the ™drive-XL85, both of which are manufactured by Toshiba Mitsubishi-Electric Industrial Systems Corporation. Electric motors compatible with such compressors are produced by companies such as Toshiba Mitsubishi-Electric Industrial Systems Corporation, GE and Siemens. 
         [0019]    When an ASD and electric motor are used to power a compressor, there is a correspondence both between the compressor pressure and the compressor flow points and the ASD power and the motor speed points. Since the ASD frequency corresponds to electric motor speed, a compressor surge map can be created and stored in the ASD in the form of a power/speed two-dimensional surge map so long as the ASD has data storage capability. In the ASD, power and speed are represented by kW and Frequency and are readily available.  FIG. 6  is a graphic representation of such a surge map showing a compressor operating at point X in the safe area which in this case is to the left of control line A where the vertical axis represents compressor horsepower and the horizontal axis represents compressor speed expressed as RPM. The unsafe area is that part of the figure beginning at and to the right of control line A and including surge line B. Control line A is located a discretionary distance away from surge line B. The constant pressure lines H, I and J are shown to indicate the one to one correspondence between this surge map and that shown in  FIG. 2 , although these lines play no role in effectuating surge control as regards establishment of the operating point, control line or surge line. 
         [0020]    The ASD continually monitors its output voltage and current, and, if the load on the motor increases, the ASD power output increases immediately. Therefore the ASD is sensitive to the load (HP) on the motor at all times. Similarly the ASD generates the frequency desired by the user, so it is also sensitive to the motor speed at all times. 
         [0021]    Alternatively, a different ASD surge map can be used derived from a two-dimensional motor torque-speed map. Since the ASD knows the synchronous motor frequency, and continually measures motor current which is an indication of motor torque, then this type of map may also be created and stored in the ASD and function as a surge map. 
         [0022]    If the gas input temperature along line  15  to compressor  10  changes appreciably during operation, the compressor surge map may be altered. Such temperature data may be monitored and provided as an optional analog input to the microprocessor on line  215  to ASD  200  resulting in modification to the surge map to compensate appropriately when necessary. Similarly, compressor suction pressure may be optionally monitored and provided to ASD  200  along line  220  to further modify the surge map. 
         [0023]    Since ASD  200  has its own microprocessor, it can either run the Proportional plus Integral (PI) control algorithm in its microprocessor or be connected to a separate external PI controller. Using the stored two-dimensional ASD surge map with a preprogrammed control line located a discretionary distance away from the surge line, ASD  200  can determine when a control error occurs based on input to the PI at  115  of the operating point from the ASD surge map and at  120  of the set point from the control line from the ASD surge map. In the event of an error, a surge control signal is issued along line  225  to directly actuate compressor bypass valve  55  which may be of one of the types discussed above. Other options also exist, for example, ASD  200  can issue a surge alarm to the process control system through its local area network port, or change the compressor speed or torque if required, or do any combination of these. Once the surge error is no longer detected, the surge control signal decreases and the bypass valve closes. 
         [0024]    An alternative embodiment of the apparatus of this invention is shown in  FIG. 7 . This arrangement can be used to add the surge control system of this invention to compressors which already employ a traditional surge control system to provide redundant control of bypass valve  55 . In this embodiment, first analog signal  60  from the surge controller and second analog signal  225  from the ASD are continuous currents varying from 4 to 20 ma, representing 0 to 100% of signal value. At  230 , a device, such as, but not limited to, an Upper Selector, compares the two analog signals and passes the larger of the two input signals to bypass valve  55 . This arrangement is used both for opening bypass valve  55  to prevent surges and for closing bypass valve  55  after a surge or potential surge has been prevented. If either signal fails and drops to zero, the other signal operates bypass valve  55 , as required, which provides the redundancy. According to this arrangement, the system does not need to know what bypass valve  55  is doing which makes it more reliable. 
         [0025]    This invention provides improved surge control to the traditional stand-alone surge controller. This alternative does not use three sensors, so it offers higher reliability and lower cost. For those compressors that already have a traditional surge control, the invention offers a low cost redundant surge control which can be added. In such a dual configuration, either of the systems can control the bypass valve, thus increasing the overall reliability of the system by an order of magnitude. Since these compressor trains are very expensive and generate large revenues, the increased reliability of the safety system and lower probability of equipment damage by surge provided by this invention are significant advantages. Moreover, use of an ASD provides direct control of motor speed and torque which the customer can use for control purposes if desired, for example, to temporarily change speed to move conditions away from the surge line. Finally, the system of this invention provides a faster acting surge signal than a stand-alone external surge controller because the drive current reacts almost instantaneously to motor torque changes, and the drive executes its control algorithm approximately every five milliseconds. 
         [0026]    The foregoing invention has been described in terms of a preferred embodiment. However, it will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed apparatus and method without departing from the scope or spirit of the invention and that legal equivalents may be substituted for the specifically disclosed elements of the invention. The specification and examples are exemplary only, while the true scope of the invention is defined by the following claims.