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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 choked velocity is a function of the upstream pressure but not the downstream. Although the velocity is constant, the mass flow rate is dependent on the density of the upstream gas, which is a function of the upstream pressure. Flow velocity reaches the speed of sound in the orifice, and it may be termed a sonic orifice.
A restrictive flow orifice (RFO) is a type of orifice plate.They are used to limit the potential danger, damage, or wastage of an uncontrolled flow from, for example, a compressed gas cylinder [1] [2] They are generally not limiting the flow during normal operation but if a fault or failure occurs causing uncontrolled flow the orifice will present a restriction, limiting the flow.
A flow limiter or flow restrictor is a device to restrict the flow of a fluid, in general a gas or a liquid. [1] Some designs use single stage or multi-stage orifice plates to handle high and low flow rates. Flow limiters are often used in manufacturing plants as well as households. Safety is usually the main purpose of using a flow limiter.
In a nozzle or other constriction, the discharge coefficient (also known as coefficient of discharge or efflux coefficient) is the ratio of the actual discharge to the ideal discharge, [1] i.e., the ratio of the mass flow rate at the discharge end of the nozzle to that of an ideal nozzle which expands an identical working fluid from the same initial conditions to the same exit pressures.
The Dall tube is a shortened version of a Venturi meter, with a lower pressure drop than an orifice plate. As with these flowmeters the flow rate in a Dall tube is determined by measuring the pressure drop caused by restriction in the conduit. The pressure differential is typically measured using diaphragm pressure transducers with digital readout.
The example shown is pneumatic. At sub-millimeter distances, a small movement of the flapper plate results in a large change in flow. The nozzle is fed from a chamber which is in turn fed by a restriction, so changes of flow result in changes of chamber pressure. The nozzle diameter must be larger than the restriction orifice in order to work. [2]
Venturi tubes are more expensive to construct than simple orifice plates, and both function on the same basic principle. However, for any given differential pressure, orifice plates cause significantly more permanent energy loss. [2]