Orifice Plates: How They Work, When to Use

The orifice plate is one of the oldest and most widely used flow measurement devices in process engineering. It is simple, has no moving parts, and measures flow rates of liquids, gases, and steam with 1-2% accuracy when installed correctly to ISO 5167.

An orifice plate is a thin disc with a precisely machined bore mounted between flanges. As fluid flows through the restriction it accelerates and pressure drops; the differential pressure across upstream and downstream taps is proportional to the square of the flow rate. The discharge coefficient Cd accounts for non-ideal effects and is calculated using the Reader-Harris/Gallagher correlation, which depends on beta ratio (bore/pipe diameter), Reynolds number, pipe diameter, and tap configuration (corner, flange, or D-D/2). For compressible fluids an additional expansibility factor accounts for density change through the restriction.

Beta ratio selection matters: 0.2-0.7 is the valid ISO 5167 range, with 0.4-0.6 typical for new installations. Lower beta gives better measurement accuracy and turndown but higher permanent pressure loss; higher beta gives lower pressure loss but reduced accuracy and dynamic range. Common installation mistakes - inadequate straight run upstream, tap orientation, square-edge wear - all degrade Cd and inflate measurement error.

Size an orifice with our free orifice plate calculator, or check related friction loss and Reynolds number calculators.

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