This invention relates to reciprocating compressors for transporting natural gas, and more particularly to a device 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 an electric motor or a combustion 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 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 pulsation control device for attenuating pressure pulsations associated with a reciprocating compressor. These pulsations are common in modern high speed reciprocating compressors and can cause significant structural vibrations. The device is particularly useful for reducing cylinder nozzle pulsations, and offers an alternative approach to other pulsation control devices such as orifice plates and Helmholtz resonators.
By “reciprocating compressor” is meant a positive displacement compressor that uses pistons driven by a crankshaft to deliver gases (or other fluids) at high pressure. The compressor typically has more than one compression cylinder. Intake fluid flows into the cylinders where it is compressed by a piston driven in a reciprocating motion via a crankshaft, and is then discharged. The movement into and out of the cylinders is via cylinder intake and discharge valves.
In the example of
Compressor 100 may have either an integral or separate engine or motor driver 11. The output of driver (motor or engine) is unloaded through the compressor. The driver 11 is often an internal combustion engine.
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 system operates as a “station” 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 1.25-4.0, depending on the pipeline operation requirements and the application.
Filter bottles 18a and 18b may be used to reduce compressor system pulsations. These filter bottles are placed between the compressor and the lateral piping, on the suction or discharge side or on both sides.
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 described herein, algorithms for programming controller 17 may be developed and executed.
Each nozzle 35 is attached to the cylinder 31 by means of a cylinder nozzle port 31a. Similarly, suction bottle 33 and discharge bottle 34 have nozzle ports 33a and 34a, respectively, by means of which the nozzle 35 is attached.
In addition, insert 300 is structured to recover pressure in a pulsating flow field. Because of space limitations inside a compressor nozzle, insert 300 is designed to achieve maximum pressure recovery over the shortest distance while also providing pulsation damping. This is achieved using fluid flow analysis for flow path optimization.
Insert 300 is generally cylindrical in shape, but with a narrowed throat 38 and a flare at each end. A lip 37 permits the insert to be inserted into a nozzle fitting connection, as described below in connection with
The throat 38 of insert 300 has its narrowest diameter. The inner diameter of throat 38 is calculated as a function of the required pulsation attenuation for the specific nozzle resonance.
The diameter of insert 300 continuously and gradually increases from the throat 38 to the inlet and outlet ends. At each end, the insert has a maximum outer diameter that is slightly smaller than the inner diameter of the nozzle 35. This permits insert 300 to fit snugly inside the nozzle.
Lip 37 is at the inlet end of insert 300. It is designed to fit on and around the perimeter of a nozzle fitting flange of a compressor cylinder, as described below in connection with
The length of insert 300 from throat 38 to outlet end 39 is an “expansion section”. The dimensions of this expansion section are designed to recover pressure losses. The length of insert 300 from throat 33 to outlet end 39 and the length of insert 300 from throat 38 to inlet end 37 may be, but are not necessarily the same.
As stated above, fluid flow dynamics calculations are used to calculate the dimensions of insert 300, such as the minimum diameter at throat 38 and the length of expansion section from throat 38 to outlet end 39. As a result of these calculations and of having proper dimensions, insert 300 functions like an orifice but uses a streamlined flow path that provides optimal pressure recovery as well as minimizes pressure fluctuations.
For suction-side attenuation, insert 300 could be inserted at a suction nozzle, such as at port 31a illustrated in
In other embodiments, insert 300 could be placed in port fittings of other piping locations. For example, as indicated below in connection with
Four locations, A-D, are identified, with insert 300 being suitable for any of these locations, depending on flange availability. In other words, the location must have a fitting with a flange that permits insert 300 to be placed with proper flow direction as indicated in
Location C is the location pictured in