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Orifice plate showing vena contracta. An orifice plate is a thin plate with a hole in it, which is usually placed in a pipe. When a fluid (whether liquid or gaseous) passes through the orifice, its pressure builds up slightly upstream of the orifice [1] but as the fluid is forced to converge to pass through the hole, the velocity increases and the fluid pressure decreases.
The flowrate of purge gas is set by rotameter (FIC) or fixed orifice plate (FO). A low flow alarm (FAL) warns operating personnel that the purge flow has reduced significantly. [8] Pipelines are monitored by measuring the flowrate of fluid at each end, a discrepancy (FDA) may indicate a leak in the pipeline.
For example, an orifice plate produces a pressure drop that is a function of the square of the volume rate of flow through the orifice. A vortex meter primary flow element produces a series of oscillations of pressure. Generally, the physical property generated by the primary flow element is more convenient to measure than the flow itself.
Note that meter manufacturers state their products' turndown ratios—a specific product may have a turndown ratio that varies from the list below. [citation needed] A thermal mass flow meter has a turndown ratio of 1000:1. An orifice plate meter has a practical turndown ratio of 3:1. A turbine meter has a turndown ratio of 10:1.
P&IDs are originally drawn up at the design stage from a combination of process flow sheet data, the mechanical process equipment design, and the instrumentation engineering design. During the design stage, the diagram also provides the basis for the development of system control schemes, allowing for further safety and operational ...
In all testing the common requirement was a fully developed flow profile entering the orifice plate. [8] Accurate standard compliant meter designs must therefore ensure that a swirl free, fully developed flow profile is impinging on the orifice plate. There are numerous methods available to accomplish this.
An orifice gas meter consists of a straight length of pipe inside which a precisely known orifice plate creates a pressure drop, thereby affecting flow. Orifice meters are a type of differential meter, all of which infer the rate of gas flow by measuring the pressure difference across a deliberately designed and installed flow disturbance.
The coefficient of contraction is defined as the ratio between the area of the jet at the vena contracta and the area of the orifice. C c = Area at vena contracta/Area of orifice. The typical value may be taken as 0.611 for a sharp orifice (concentric with the flow channel). [2] [3] The smaller the value, the greater the effect the vena ...