- An orifice plate is installed into a pipeline by squeezing it between two orifice flanges with a gasket on each side of the plate for a pressure tight seal. Each flange has been welded to one of the two opposing pipe ends, resulting from a cross sectional cut through the pipe. Each flange is a ring about one inch thick, with four or more through-bolts holding the two flanges together, effectively re-attaching the ends of the pipe to which they are welded. Each orifice flange has a hole drilled from its outer circumference to the inside of the pipe, so that pressure on each side of the orifice plate is accessible outside the pipe. The holes are threaded receive standard tube fittings. These are connected via the tubing to differential pressure measuring instrumentation, which then in turn transmits a flow value for indication and control.
- Orifice plates are fabricated from steel or stainless steel plate. They look like ping-pong paddles with a hole in the middle, where the paddle body is perfectly round to fit concentrically between the two flanges. The rectangular side handle facilitates centering the plate between the flanges. The hole in the middle is carefully bored and is purposely smaller than the inner diameter of the pipeline to create the restriction to the flow. This restriction results in the differential pressure that can be measured to infer the flow in the pipe. The higher pressure is always from the upstream side of the orifice plate, since the positive differential is what pushes the flow through the hole. The ratio of the bore diameter to the inner diameter of the pipe is called the beta ratio. The beta ratio is normally calculated by the orifice manufacturer based on all the key data about the flow application, including the flowing medium, maximum and minimum flow rates, temperatures and pressures. This calculation assures maximum accuracy and rangeability of the flow measurement.
- Assuming horizontal pipelines, gas flow orifice applications normally have the tubing coming from the flanges at either the ten o'clock or two o'clock positions, with the process transmitter located above the pipeline. This allows any condensation to run down into the pipe, leaving only gas in the tube runs. For liquids, filled tubing legs are recommended at either the eight or four o'clock positions to assure that any entrapped air is drawn back into the pipeline and the tubing legs are filled entirely with the liquid. These practices assure the best accuracy.
- Modern electronic and digital instrumentation normally extracts the square root function that is normal to orifice plate flow applications. This results in linear indicator readings. Since accuracy rapidly degrades below about 30 percent of flow which occurs, at approximately 9 percent of differential, orifice applications are normally limited to about a two or three to one turn down ratio, the ratio between maximum and minimum flow values.
