December 2008 Vol. 235 No. 12
TechNotes
Conditioning Orifice Plate For Wet Gas Flow Measurement
As traditional natural gas reservoirs are depleted, producers are forced to target lower pressure wells that typically produce a “wet gas.”
This is gas with entrained liquid that can back up behind orifice plates and impact flow measurement accuracy. Large numbers of these marginal wells are needed to maintain production rates, motivating users to reduce costs with simple and compact installations. This article introduces new “conditioning plate” technology designed to address these challenging wet gas applications.
Reducing Straight Pipe
Conditioning Orifice Plates (COPs) were originally developed to minimize the requirement for straight pipe. These devices are essentially “wet-calibrated flow straighteners.” After calibration in the supplier’s flow lab, they provide performance comparable to traditional orifice meters, but with no more than two diameters of straight pipe upstream and downstream, and no additional flow conditioning.
Where a conventional orifice plate has a single hole, a conditioning orifice plate has four. As shown in the plate illustrations, the paddle-style plate is designed for use between flange-tapped orifice flanges, or in a universal configuration, with Daniel® junior or senior orifice fittings. This style is easily retrofitted into existing orifice meter installations and can be used to improve the measurement of an orifice meter that was installed without the proper straight run requirements.
The “compact” wafer style meter incorporates a direct mount three-valve head with corner taps in a one-piece design and allows the direct connection of a differential pressure (dp) or MultiVariable™ transmitter. This simplifies flow meter installation, reduces the number of potential leak points by eliminating impulse lines, and eliminates bias error from incorrect alignment, since it incorporates a built-in alignment/centering ring.
Another benefit of the four-hole design, relevant to the wet gas flow measurement application, is the ability to orient the meter so that one of the holes is at the bottom of the pipe, to minimize the risk of liquid holdup.
In a traditional orifice plate, the single hole is located in the center of the pipe, so liquid can dam at the upstream edge of the plate. While some users drill a vent hole in the bottom of the plate to prevent this, the vent hole adds unknown flow error, and the risk of plugging.
The Wet Gas Measurement Challenge
The presence of liquid in a gas flow reduces the effective area of the pipe as is illustrated in the drawing captioned Orifice Pipe. For a given gas flowrate, this reduction in effective area causes an increase in flow velocity. This means that a flowmeter in a wet gas application will read a higher differential pressure (dp) – “over-reading” – than if the gas were dry. One inch of liquid in an 8-inch pipe results in about an 11% error.
A series of tests were conducted at the Colorado Engineering Experiment Station Inc. (CEESI) on both the wafer meters and paddle-style conditioning orifice plates. With a wet gas fraction of up to 0.3, over a wide range of pressures, flowrates and beta ratios, the conditioning orifice plate produced a predictable and repeatable over-reading. The four-hole design minimized liquid holdup, as compared with a standard orifice plate, without the need for an accuracy-reducing and plugging-prone vent hole. As a final benefit, these results were obtained with only two diameters of straight pipe upstream and downstream of the meter, with no added flow conditioning.
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