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(2) Pressure switches are operated by an input pressure to open or close contacts at a
selected pressure setting. Pressure switches may be gauge, absolute, or differential type with adjustable
settings, and may be manual or automatic reset.
e. Flow instruments.
(1) Flow of liquids and gases is directly or indirectly measured in the flow path. A direct
metering device measures fluid flow by measuring volume or weight for a given period of time. An
indirect metering device uses an intermediate parameter, such as pressure drop across a constricted flow
area, to measure flow.
(2) Concentric orifice plates will be used for measuring steady flow of clean liquids, vapor, or
gas in the normal turbulent flow region with a Reynolds number of 2000 or greater.
(3) Eccentric orifice plates are used to measure fluids which carry a small amount of non-
abrasive solids, since the solids will flow through the bottom of the orifice rather than accumulate behind
it. They are also useful for measuring the flow of vapors or gases which carry small amounts of liquid.
Eccentric plates will also be used to measure the flow of liquids carrying small amounts of gas, in which
case the orifice opening must be located at the top of the pipe.
(4) Flow nozzles will be used where the Reynolds number is in excess of 50,000. Flow nozzles
will handle approximately 60 percent more flow with the same pressure drop, compared to an orifice
plate. At higher Reynolds numbers, the amount of straight pipe required prior to the flow nozzle is
(5) Venturi tubes, like flow nozzles, will handle approximately 60 percent more flow than an
orifice plate, but with the same pressure drop as the orifice plate. For equal flows, the pressure drop of a
venturi tube will be only 10 to 20 percent of the pressure drop of an orifice plate. The venturi tube is
capable of measuring any fluid flow which an orifice plate or flow nozzle can measure. Venturi tubes will
be used for gas flow measurement when suspended particles are in the stream.
(6) Annular pitot tubes are a variation of the pitot tube. Pitot tubes have a single sensing point
and have poor accuracy, particularly at low velocities. The annular pitot tube senses dynamic pressure at
multiple ports distributed along the sensing tube to provide a single output of the average flow. Static
pressure is measured by a port which faces downstream at the centerline of the pipe. The sensor
requires approximately five pipe diameters of straight pipe upstream of the device. A major advantage of
this sensor is the ability to install an annular pitot tube into an existing line under pressure with "hot tap"
(7) Turbine flow meters use the moving fluid to turn a turbine rotor. Turbine flow meters supply
flow quantity information via a precisely known number of pulses for a given volume of fluid displaced.
The relationship is linear for a given flow rate and viscosity. The turbine flow meter is designed on
flanged ends to be mounted in-line. Recently, reduced size turbine meters have been developed for
mounting into existing piping by hot-tap methods, allowing the units to be removed and reinserted without
(8) Vortex shedding flowmeters use a non-streamlined obstruction inserted in the pipe
centerline to create eddies or vortices which grow. The detachment of the vortex from the obstruction is
termed shedding. A sensor located downstream of the obstruction measures the frequency of shedding,
which is proportional to the flow velocity, the output being linear with flow.
(9) Air flow measurement stations may be of the pitot-tube or electronic type. For applications
where the minimum required flow measurement corresponds to an air velocity of less than 1,000 feet per
minute, the electronic type air flow measurement station will be used. Both types have sensing elements
distributed throughout the cross-section of the duct.