Abstract:
A multi-chambered pulsation absorber for attachment over the valve cap opening of a compressor cylinder. Each chamber is in fluid communication with the valve cap opening (or cylinder internal gas passages) via an associated choke tube. Each pairing of a chamber with a choke tube is tuned, in the manner of a Helmholz resonator, to attenuate and nearly eliminate a different cylinder-related pulsation frequency, such as those resulting from internal cylinder pulsations or cylinder nozzle pulsations.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    This invention relates to reciprocating compressors for transporting natural gas or other gases, and more particularly to a method for reducing pulsations in the compressor system associated with such compressors. 
       BACKGROUND OF THE INVENTION 
       [0002]    To transport natural gas from production sites to consumers, pipeline operators install large compressors at transport stations along the pipelines. Natural gas pipeline networks connect production operations with local distribution companies through thousands of miles of gas transmission lines. Typically, reciprocating gas compressors are used as the prime mover for pipeline transport operations because of the relatively high pressure ratio required. Reciprocating gas compressors may also be used to compress gas for storage applications or in processing plant applications prior to transport. 
         [0003]    Reciprocating gas compressors are a type of compressor that compresses gas using a piston in a cylinder connected to a crankshaft. The crankshaft may be driven by a motor or an engine. A suction valve in the compressor cylinder receives input gas, which is then compressed by the piston and discharged through a discharge valve. 
         [0004]    Reciprocating gas compressors inherently generate transient pulsating flows because of the piston motion and alternating valve motion. Various devices and control methods have been developed to control these pulsations. An ideal pulsation control design reduces system pulsations to acceptable levels without compromising compressor performance. 
         [0005]    A specific challenge when using high-horsepower, high-speed, variable-speed compressors is pulsations in the cylinder nozzle. The cylinder nozzle is the section of pipe that connects the cylinder to the suction or discharge side of the compressor, typically to a filter bottle. This section of pipe can provide significant resonance responses. Currently, one solution to attenuating cylinder nozzle pulsations is the installation of an orifice in the cylinder nozzle. For example, a plate with a flow restricting hole may be placed across the circumference of the nozzle. However, a drawback to use of the orifice is that it causes a pressure drop that requires the supply of additional horsepower. This burden can be significant on large horsepower units. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
           [0007]      FIG. 1  is a block diagram of a reciprocating gas compressor system. 
           [0008]      FIG. 2  illustrates a multi-chamber pulsation absorber installed at a cylinder valve cap in accordance with the invention. 
           [0009]      FIG. 3  is a perspective view of the multi-chamber pulsation absorber. 
           [0010]      FIG. 4  is a cut-away view of the multi-chamber pulsation absorber. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    The following description is directed to a multi-chamber pulsation absorber for reducing pulsations of a compressor system. The absorber, mounted at a cylinder valve cap and having properly designed acoustic dimensions, is capable of altering the acoustically resonant frequencies of the cylinder internals as well as of the cylinder nozzle. The absorber eliminates the need for a nozzle orifice and reduces the cylinder internal pulsations such that associated vibrations, valve life problems, and/or efficiency problems associated with those pulsations are nearly eliminated. 
         [0012]    As stated in the Background, pulsation absorbers may be attached to the cylinder nozzle. However, these absorbers address only the cylinder nozzle response frequency. Other resonances associated with the cylinder internal gas passages are not addressed with the single volume and choke. 
         [0013]      FIG. 1  is a block diagram of the basic elements of a reciprocating gas compressor system  100 . The elements of compressor system  100  are depicted as those of a typical or “generic” system, and include a driver  11 , compressor  12 , suction filter bottle  18   a , discharge filter bottle  18   b , suction and discharge piping connections, and a controller  17 . 
         [0014]    In the example of  FIG. 1 , compressor  12  has three compressor cylinders  12   a - 12   c . In practice, compressor  12  may have fewer or more (often as many as six) cylinders. Further, it may have either an integral or separate engine or motor driver  11 . The output of driver  11  (motor or engine) may be variable speed and power, unloaded through the compressor. The driver  11  is often an internal combustion engine. 
         [0015]    The following description is written in terms of the “generic” compressor system  100 . However, the same concepts are applicable to other compressor configurations. 
         [0016]    A typical application of compressor system  100  is in the gas transmission industry. The compressor station operates between two gas transmission lines. The first line, at an initial pressure, is referred to as the suction line. The second line, at the exit pressure for the station, is referred to as the discharge line. The suction and discharge lines are also referred to in the industry as the “lateral piping”. The pressure ratio (discharge pressure divided by suction pressure) may vary between 1.15 to 4.0 or more, depending on the pipeline operation requirements and the application. 
         [0017]    Filter bottles  18   a  and  18   b  are placed between the compressor and the lateral piping, on the suction or discharge side or on both sides. Filter bottles such as these are installed as a common method for pulsation control. They operate with surge volumes, and are commonly implemented as volume-choke-volume devices. They function as low-pass acoustic filters, and attenuate pulsations on the basis of a predetermined Helmholtz response. 
         [0018]    Controller  17  is used for control of parameters affecting compressor load and capacity. The pipeline operation will vary based on the flow rate demands and pressure variations. The compressor must be capable of changing its flow capacity and load according to the pipeline operation. Controller  17  is equipped with processing and memory devices, appropriate input and output devices, and an appropriate user interface. It is programmed to perform the various control tasks and deliver control parameters to the compressor system. Given appropriate input data, output specifications, and control objectives, algorithms for programming controller  17  may be developed and executed. 
         [0019]      FIG. 2  illustrates a nozzle pulsation absorber  30  installed at a cylinder valve cap  32  in accordance with the invention. Although only one cylinder  31  and one absorber  30  are shown, additional absorbers  30  may be installed on more than one cylinder, and they may be installed on the suction side and/or the discharge side of the cylinder(s). The cylinder nozzle  35  is a section of pipe that connects the cylinder  31  to the discharge or suction side of the compressor. 
         [0020]    Compressor valves (not explicitly visible in  FIG. 2 ) are installed on each cylinder  31  to permit one-way flow into or out of the cylinder volume. In the example of  FIG. 2 , cylinder  31  is illustrated as having two suction valves and two discharge valves, with valve caps  32  on three valves and an absorber  30  at one of the discharge valves. 
         [0021]    As explained below, nozzle pulsation absorber  30  is a multi-chamber side branch absorber, having multiple choke tubes and volumes. In accordance with the invention, absorber  30  can be designed to dampen multiple pulsation frequencies, including (but not limited to) the cylinder internal (valve-to-valve) response, the response of the cylinder nozzle, and the cylinder internal cross-mode. 
         [0022]      FIG. 3  is a perspective view of the absorber  30 . Its housing  39  provides the outer shell for two or more internal chambers, as explained below in connection with  FIG. 4 . The housing is typically cylindrical in shape, but other geometries are possible. The longitudinal axis of housing  39  extends vertically from the compressor valve opening. 
         [0023]    A flange  37  is a large ring at one end of housing  39 , and facilitates attachment of the absorber  30  to the valve cap opening. The absorber may be integrated with the cylinder valve cap, so that the valve cap and absorber are a single assembly. In some cases it may be necessary to attach the absorber to a modified valve cap. Therefore, the absorber is installed in place of or attached to a valve cap. The attachment of the absorber on the compressor cylinder is a sealed attachment, with the cylinder&#39;s internal gas passage open only to the absorber&#39;s internal choke tubes. 
         [0024]    A bottom plate  38  has three openings, each corresponding to an open end of an internal choke tube (see  FIG. 4 ). These openings are in communication with gases expelled from or inducted into the associated compressor cylinder, via the valve port through the valve cap. 
         [0025]      FIG. 4  is a cut-away view of the absorber  30 . In the example of  FIG. 4 , absorber has three chambers  41   a ,  42   a , and  43   a , and three internal choke tubes  41   b ,  42   b , and  43   b . As illustrated, two partitions within the housing  39  divide the internal volume of the housing into the three chambers. The partitions are horizontal, such that the chambers are “stacked” vertically along the vertical axis of the housing  39 . 
         [0026]    The choke tubes are small sections of piping with two open ends. A choke tube is associated with (paired with) each chamber (volume), and each choke tube has a first end open to the compressor cylinder valve port and a second end open to the associated chamber. Each choke tube and chamber pairing is designed to dampen a different resonant frequency of the compressor system. In other embodiments, absorber  30  may have only two, or more than three, choke tubes and chamber pairings. 
         [0027]    As is known in the art of side branch absorbers (also known as Helmholtz resonators) for other applications, the physical dimensions of each choke tube and its associated surge volume are not the same as their acoustic dimensions. The desired acoustic dimensions and the resulting physical dimensions are determined by various known calculation and acoustic modeling techniques. The internal volume of the chamber and the length and diameter of the choke tube are variables that can be used to “tune” the resonance of each choke tube and chamber pairing. 
         [0028]    The acoustic dimensions of each choke tube and chamber pairing vary depending on the pulsation frequency to be dampened by that pairing. The resonant frequency to be damped may be determined by various measurement or predictive techniques. More specifically, the diameter and size of each choke tube and the size of its associated chamber determine an acoustic natural frequency. Each choke tube and chamber pairing is designed to dampen a different resonant frequency of the compressor system. At least one pairing is specifically designed to dampen cylinder internal (valve-to-valve) pulsations. Another is specifically designed to dampen nozzle pulsations. Additional choke tube and chamber pairings may be designed to dampen other internal cylinder pulsations. 
         [0029]    In operation, two or more target frequencies to be damped are identified. Each choke tube and chamber pairing of the absorber is designed so that its acoustic response frequency matches that of the target frequency. Calculations for Helmholtz resonators may be used, and are well documented. Compressor system models may be used for further refinement of the absorber response. The absorber is then installed in place of or attached to the valve cap, such that each chamber, via its associated choke tube, is in fluid communication with the cylinder gas passage.