Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to the field of measurement of flow rate of gas discharged from a compressor. More particularly, the invention relates to pulsation dampeners used to improve the accuracy of measurement of gas flow rates under variable compressor output conditions.
2. Background Art
Natural gas is typically transferred from producing wellbores to pipelines. The pipelines ultimately are arranged to distribute natural gas to end users. Flow measurement devices are located at places where custody of the gas changes, for example, in a producing field with gas produced from a number of wells to a gas transportation entity owned pipeline.
Compressors are commonly used to raise pressure of the gas from the “upstream” side pressure at custody transfer points to the downstream side pressure. In cases such as number of producing wells being connected single pipeline intake point, a typical combination would include a single flow measurement device connected between the pipeline intake and the compressor output.
There is a well understood measurement problem when measuring natural gas flow rates when a compressor is used as described above. This problem is referred to as square root error (“SRE”). The opening and closing of a valve used in the compressor compression causes a pulsating wave that strikes the primary measuring device and creates a false indication of increase in the flow rate.
One way known in the art to reduce or eliminate RSE is to install a choking plate between the compressor and the flow measurement device. The choking plate opening is usually selected as one half of the orifice size in the flow measurement device, wherein an orifice type gas meter is used. As an example, a 4 inch meter run (intake and outlet line diameters), and with a 1.5 inch orifice plate in the flow meter, a choke plate having an 0.75 inch opening would be installed upstream of the measurement device. Such selection of choke plate opening works in most operating conditions to reducing the amplitude of pressure pulsation to an acceptable level, typically about 2%.
The problem with the foregoing solution is that it is a disadvantage to the transferor (upstream custodian) of the gas. The choke opening, when selected according the foregoing formula, restricts flow much more than is typically required for most operating conditions. In such case, the gas transferor's (e.g., producer's) compressor has to work much harder because it is output restricted. In the case of compression having producing wells as the input, the wells cannot produce to capacity, making them vulnerable to “sanding” (entry of formation particles into the wellbore) and the wells would need to be reworked at considerable expense.
What is needed is a selectable opening choke that can be adjusted to the largest opening needed to reduce SRE to acceptable levels while restricting gas flow to the smalles extent consistent with acceptable levels of SRE.
A gas flow pressure pulsation dampener includes a sealed housing having an inlet port, an outlet port a center section disposed between the inlet port and the outlet port. A choke plate is rotatably disposed in the center section and has a plurality of different size openings disposed circumferentially about the plate. The plate is positioned in the center section so that the openings are placed between the inlet port and the outlet port. The size differences between the openings and the circumferential separation between adjacent openings are selected so that the plate always presents one full opening or parts of two openings between the inlet port and the outlet port. Means for rotating the plate are provided select one of the choke openings to be disposed between the inlet port and the outlet port.
A method for dampening pressure pulses in flowing gas for volume measurement includes measuring pressure downstream of a variable opening choke in the flowing gas stream using a pressure transducer that generates an electrical or optical signal in response to pressure. An amplitude of pressure variations in the flowing gas is determined from the measured pressure signal. An opening size of the choke is adjusted until the determined amplitude falls below a selected threshold.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
A cut away view of an example pressure dampener according to the invention is shown in
The housing 12, in the present example disposed in the outlet half 12B includes a substantially flat cylindrical surface recess 18 in which is disposed a generally flat, circular choke plate 24. The choke plate 24 is rotatable with respect to the housing 12 within the recess 18. The choke plate will be explained in more detail with reference to
The inlet port 20 and the outlet port 22 may be coupled to gas flow lines (not shown) using flange type connectors 12C, 12D respectively at each longitudinal end thereof. The diameter of the flange connectors 12C, 12D will depend on the designed flow capacity of the pressure pulsation dampener 10.
The choke plate 24 may be rotated by a keyed shaft 26. The keyed shaft may be sealed against the housing 12 by an o-ring 27 or other seal. The keyed shaft 26 may be keyed to an indicator plate 28 disposed outside the housing 12. The indicator plate 26 provides the dampener's user with a visual indication of which size opening in the choke plate 24 is disposed in the flow path. Openings in the choke plate will be further explained with reference to
The openings 32A-32J may be arranged in progressively larger sizes to accommodate different flow rates through the dampener (10 in
An important aspect of the arrangement of the openings 32A-32J is that when the choke plate 24 is rotated, at least one full opening or parts of two adjacent openings are always disposed in the flow path. Using such arrangement, flow through the dampener is never closed off.
In using the device explained above with reference to
An automatic version of the dampener is shown in
A pulsation dampener according to the various aspects of the invention may enable optimizing operation of a gas compressor and a flow measuring device disposed in the outlet stream of the compressor.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.