Types of Orifice Plates Used in Flow Measurement
Orifice plates are one of the most popular devices for the measurement and control of fluid flow. It is frequently used more than any other flow meter. The orifice consists of a thin metal plate with a straight hole drilled in it. When a fluid passes through an orifice, the flow is often less than the amount calculated. The assumption is that the energy is conserved and the flow distributing through the orifice is uniform and parallel.
Orifice Plates are normally mounted between a set of Orifice Flanges and are installed in a straight run of smooth pipe to avoid disturbance of flow patterns from fittings and valves.
Orifice plates cover a wide range of applications including fluid and other operating conditions. They give an acceptable level of uncertainties at lowest cost and long life without regular maintenance.
Concentric Bore – The most common orifice plate is the square-edged, concentric bored design. It’s used on most processes including clean liquids, gases and steam flow. It is usually made of stainless steel. Other materials like nickel and Monel are used for a good corrosive resistant property. This type of orifice plate has a very high accuracy.
Eccentric Bore – Eccentric bore orifice plates are plates with the orifice off-center, or eccentric, as opposed to concentric. Location of the bore prevents accumulation of solid materials or foreign particles and makes it useful for measuring fluids containing suspended solid particles. Eccentric bore orifice plates are more uncertain as compared to the concentric orifice.
Segmental Bore – The segmentally bored orifice plates contain a hole that is a segment of a concentric circle. Like the eccentric orifice plate design, the segmental hole should be offset downward in gas flow applications. Segmental bores are generally used for measuring liquids or gases which carry non-abrasive impurities such as sewage treatment, steel, chemical, water conditioning, paper and petrochemical industries.
Quadrant Edge Bore – The quadrant edge bore is an orifice with the inlet edge rounded. The upstream side of the bore is shaped like a flow nozzle while the downstream side acts as a sharp edge orifice plate. This design is recommended to measure the flow of high viscosity fluids such as heavy crudes, syrups and slurries. The quadrant bore produces a relatively constant coefficient when the Reynolds Number is below 10,000.
Ring Type Joint (RTJ) Orifice Plates – The RTJ type orifice plate incorporates an essential gasket for mounting between ring type joint flanges. It is based on proven technology, has no moving parts and is suitable for high temperature and pressure applications. Orifice plates are recommended for clean liquids, gases and low velocity steam flows. Plate thicknesses depend on line size and differential pressure, and should be sufficient to prevent the plate from bending under operating conditions.
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