The orifice meter is the device most commonly used for measurement of natural gas in the upstream sector of the oil and gas industry. While there are several vital components to an orifice meter, the orifice plate is perhaps the most crucial.
The orifice plate, as defined by AGA Report No. 3, "Orifice Metering of Natural Gas," is a thin metal plate with a machined circular and concentric bore, a sharp edge on the upstream side, and a beveled edge on the downstream side. Orifice plates are beveled on the downstream side to help minimize the plate’s contact with the flowing gas stream passing through the meter.
At first glance, it’s not uncommon for those new to orifice measurement to assume that the beveled edge should be the upstream side of the orifice plate. However, installing a plate with the bevel pointing upstream into the flow (i.e., a "backwards plate") can create significant error in the flow rate calculation resulting in understated volumes. The amount of beveling required on plates of the same thickness decreases as the meter tube inside diameter (ID) increases. Therefore, plates installed backwards in larger tubes (e.g., 6" ID and above) may introduce less inaccuracy than plates installed in smaller tubes (e.g., 4" ID and below). Since the bevel creates a reduced differential pressure across the plate when installed backwards, the measurement technician must be careful to correctly install the orifice plate to obtain differential readings which are accurately proportional to the flow. Otherwise, substantial flow error may be introduced which could cause the flow rate, in many cases, to be understated by 15% or more.
One of the best ways to confirm proper installation of an orifice plate is to review the flowing parameters and flow rates measured before and after the meter inspection or plate change. Assuming consistent operations prior to and after installation of the orifice plate, any significant changes in these values should be reconciled. Whenever possible, the measurement technician should verify that the orifice plate was installed correctly before leaving the site since any errors could potentially go unnoticed until the next inspection. Careful installation and maintenance, as well as accurate, detailed, and comprehensive reporting, are the most useful practices for preventing the backwards installation of an orifice plate.
For more guidance on orifice meter design, installation, calibration, inspection, and, of course, proper installation of orifice plates, be sure to consult with your in-house or third-party measurement experts.
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