A Restrictive Flow Orifice (RFO) is a type of
orifice plate An orifice plate is a device used for measuring flow rate, for reducing pressure or for restricting flow (in the latter two cases it is often called a ').
Description
An orifice plate is a thin plate with a hole in it, which is usually placed in ...
. They are used to limit the potential danger, damage, or wastage of an uncontrolled flow from, for example, a
compressed gas cylinder 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. It may be used to limit the accidental release of a hazardous gas (flammable, toxic, etc.) resulting from regulator or other component failure, restricting flow in a system in order to assure adequate pressure relief valve sizing and system over pressure protection, or restricting flow from bulk sources such as a water main.
[Gas Flow Characterization of Restrictive Flow Orifice Devices, Sandia REPORT SAND97-1670, Jul. 1997]
Predicting Flow
Correlations assist in predicting the flow of a particular gas or gas mixture through a RFO. This is done by first determining the
flow through the same RFO at the required pressure with a reference gas and then adjusting the
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water (molecule), wa ...
accordingly.
The pertinent equation for the reference gas of
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
(N
2) is presented below.
Other applications
River flood control
In the natural environment, large orifice plates are used to control onward flow in
flood
A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...
relief
dam
A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use ...
s. In these structures a low dam is placed across a
river
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...
and in normal operation the water flows through the orifice plate unimpeded as the orifice is substantially larger than the normal flow cross section. However, in floods, the flow rate rises and floods out the orifice plate which can then only pass a flow determined by the physical dimensions of the orifice. Flow is then held back behind the low dam in a temporary
reservoir
A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation.
Reservoirs can be created in a number of ways, including contro ...
which is slowly discharged through the orifice when the flood subsides.
Notes
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Control devices
Gas technologies