UFC 3 -430-11
14 February 2001
5-2.8
Flow Instruments. The most commonly used flow element in boiler plants is
the orifice plate. There are numerous other flow measuring devices that can be used
depending on the application. In alphabetical order these include annubar flowmeters,
coriolis type mass flowmeters, elbow meters, flow nozzles, magnetic flowmeters, pitot
tubes, pitot-venturi tubes, positive displacement meters, rotameters, target meters,
thermal-loss meters (also known as heat-loss meters), turbine meters, ultrasonic
flowmeters, venturi tubes, vortex flowmeters, and wedge elements.
Use an orifice plate for most flow measurement applications unless a diffe rent
type of flow element, such as a flow nozzle or a pitot tube, offers specific advantages.
Reasons for using flow elements other than orifice plates include higher accuracy,
shorter meter run, lower pressure drop and large line size.
5-2.8.1 Meter Runs. Flow disturbances as much as 100 pipe diameters upstream of
the flow measuring element can affect the accuracy of the flow measurement. Meter
run requirements, including pressure tap locations, depend on a number of items.
These include type of flow element, beta ratio, and flow disturbances upstream and
downstream of the flow element. Flow disturbances result from valves, elbows,
enlargers, reducers, and other pipe fittings.
5-2.8.1.1 Requirements. Obtain complete meter run requirements from the flow
measuring device manufacturer to insure accurate measurements. This includes
pressure tap locations and other details such as pressure tap size.
5-2.8.1.2 Straightening Vanes. Avoid the use of straightening vanes due to their cost.
Use them only w hen meter runs without them are not practical.
5-2.8.1.3 Fabrication. Do not fabricate meter runs in the field. Fabricate them in a
shop qualified in that type of work. Include in the shop fabricated meter run at least 10
pipe diameters of upstream piping and 5 pipe diameters of down stream piping.
Straight runs of piping required in addition to the above can be fabricated in the field.
5-2.8.2 Orifice Plates. Orifice plate types include concentric and eccentric, square
edge, quadrant edge, segmental and annular. In general, use concentric, square-edge
orifice plates except for the following.
Use quadrant edge for orifice plates with a Reynolds number of less than
10,000 (based on pipe diameter).
Do not use concentric orifice plates for horizontal runs flowing wet steam or
gas, liquids containing solids, or liquids containing gas or vapor. All of the
above will result in inaccurate measurements.
Water in the wet steam and liquid in the wet gas can cause damming of the
liquid. Solids in the liquid can settle out upstream of the orifice plate. As a
first solution use a concentric, square -edge orifice plate but locate it in a
vertical run with flow in the downward direction. If locating the orifice plate
5-4
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