Elbow Taps

Design Pressure
Limited by piping design class only
Operating Temperature Range
−330 to+1100°F (−200 to+600°C)
Fluids
Liquids, vapors, or gases
Differential Pressure
0- to 10-in water column (0 to 2.5 kPa)
Sizes
0.5 to 20 in (12 to 500 mm)
Inaccuracy
±2 to±10% FS
Cost
Approximately $1000 plus value of elbow and measuring device (usually a
differential-pressure transmitter)
Partial List of Suppliers
Normally fabricated on site

Flow measurement using elbow taps depends on the detection
of the differential pressure developed by centrifugal force as
the direction of fluid flow changes in a pipe elbow. Taps are
located on the inner and outer radii in the plane of the elbow.
The pressure taps are located at either 45°or 22.5°from the
inlet face of the elbow


A SIMPLE FLOWMETER
Elbow taps are easy to implement, because most piping configurations
already contain elbows in which taps can be
located. This guarantees an economical installation and
results in no added pressure loss. The measurement introduces
no obstructions in the line. Accumulation of extraneous
material in the differential-pressure connections can plug the
elbow taps. Therefore, they should be purged if the process
fluid is not clean.
As is the case with other head-type primary flow measurement
devices, the differential pressure developed by a
given flow is precisely repeatable. However, the flow coefficient
of an elbow tap calculated from the physical dimensions
of the pipe is generally considered reliable to only
±5 to±10%. This is quite satisfactory for many flow control applications
where repeatability is the primary consideration. If
absolute accuracy is desired, a more precise flowmeter should
be used, or the elbow tap readings should be calibrated,

preferably in place and using the working fluid. Not enough
data exist to establish precise correction factors for effects of
upstream disturbances, viscosity, and roughness in pipe and
elbow surfaces, and no published standards are available.
Elbow taps develop relatively low differential pressures.
For this reason, they cannot be used for measurement of lowvelocity
streams. Typically, water flowing at an average
velocity of 5 ft/sec (1.5 m/sec), roughly 200 GPM in a 4-in.pipe (45 m3
/h in a 100-mm pipe) through a “short-radius”
elbow with a centerline radius equal to the pipe diameter
develops about 10 in. of water differential pressure (2.5 kPa).
This is approximately the minimum full scale value recommended
for reliable measurement. Taps in long radius pipe
or tube bends do not develop sufficient differential pressure
for good flow measurement at low flow velocities.
In comparison with an elbow installation, an orifice will
generate a head (1.4 to 2.2) higher at the same
flow rate. For example, for
β=0.65, the orifice head developed
will be approximately 6.5 times that of a short-radius elbow


Bibliography
Hauptmann, E. G., Take a second look
at elbow meters for flow
monitoring,
Instrum. Control Syst.,
47–50, 1978.
Moore, D. C., Easy way to measure slurry flowrates,
Chemical Eng.,
96,1972.