Patent Application
20100329899
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.
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.
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.
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.
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.
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:
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.
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.
In the example of
The following description is written in terms of the “generic” compressor system 100. However, the same concepts are applicable to other compressor configurations.
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.
Filter bottles 18a and 18b 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.
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.
Compressor valves (not explicitly visible in
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.
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's internal gas passage open only to the absorber's internal choke tubes.
A bottom plate 38 has three openings, each corresponding to an open end of an internal choke tube (see
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.
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.
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.
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.
