Will your orifice plate be installed to measure a flow or simply to limit the flow in a pipe of your process to a maximum flow? If the plate is used for flow measurement, you must respect the upstream and downstream lenghts recommanded by the manufacturerotherwise you will loose precision. In a case of a plate used only to limit flow in your process, if the precision of the flow is not detrimental of your process you can always allow you to deviate a bit from manufacturer requirements.
The orifice performance to serve as flowmeter depend on few factors such as: precision in the bore calculation, tap location and conditions, respect of straight pipes lengths, gasket interference, misalignments of ipe and components, corrosion, deposit in or around the plate, etc.
Remember to consider the risk of destructive cavitation.
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