Expanded
Summary
Effects of Pipe Wall Offsets on Differential
Pressure Meter Accuracy
JE SSE M. PO PE, S TE VE N L . B A RF U SS, M IC H AEL C . J O H N S O N , AN D Z AC H ARY B. S H AR P
http://dx.doi.org/10.5942/jawwa.2015.107.0081
Accurate flow measurement is essential for the management of any type of fluid system. Several laboratory
experiments have been conducted on installation effects
for several types of meters. However, the authors have
been unable to find any data to show how a meter is
affected by a sudden change in pipe radius as the fluid
enters the meter. If for any reason the meter is incorrectly
installed or has a different inside inlet diameter from the
pipeline, the fluid will encounter a sudden increase or
decrease in pipe radius. This may cause the streamlines
to separate from the pipe wall, creating a less-than-ideal
condition for accurate metering.
This research investigated the effects on accuracy of
five types of 12-in. differential pressure flow meters
as a result of being installed in pipelines of differing
inside diameters. The types of meters chosen for this
research were the classical Venturi meter, Halmi Venturi
tube, wedge meter, V-cone meter, and the X-meter. All
of the meters had a standard (STD)-schedule pipe
wall. The offset was created by installing various test
spools of a different schedule immediately upstream
of the meter. Ten pipe schedules from schedule 20 to
schedule 160 were used, creating pipe wall offsets
ranging from a 0.125-in. sudden contraction to a
0.937-in. sudden expansion of the pipe radius as the
fluid enters the meter.
The discharge coefficient (i.e., the ratio of actual measured flow to theoretical measured flow) was calculated
for each test run. That coefficient was then compared
with each meter’s coefficient at the same Reynolds number with no pipe wall offset (STD-schedule pipe installed
upstream). In other words, the calculations showed how
the meter readings differed from how a meter read after
being calibrated in a STD-schedule pipeline. The meter
was considered to be affected when the calculated difference between corresponding discharge coefficients was
outside the manufacturer’s specified meter accuracy. A
general summary of the results follows.
Classical Venturi meter. This meter showed little to no
effect from the sudden contractions and smaller sudden
expansions. Noticeable effects occurred with any sudden
expansion of pipe radius of 0.468 in. or greater. Errors
up to 3.59% were recorded depending on the magnitude
of the sudden expansion.
Halmi Venturi tube. The Halmi Venturi tube displayed
effects on its accuracy with any offset at Reynolds numbers below 400,000. At higher Reynolds numbers, the
meter’s accuracy showed errors up to 3.89% for a sudden
expansion of pipe radius of 0.312 in. or greater.
Wedge meter. The wedge meter showed no effects from
any of the tested changes in pipe radius; at least the effects
from the offsets did not cause the meter to produce readings outside of its specified accuracy. The reason for such
resilience may be because of the locations of the pressure
taps. The low-pressure tap is not located at the throat but
further downstream.
V-cone meter. The V-cone meter’s accuracy was affected
by nearly all tested pipe wall offsets. Errors up to 3.26%
were recorded depending on the magnitude of the pipe
wall offset and the Reynolds number.
X-meter. The accuracy of the X-meter was affected by
any sudden expansion of pipe radius of 0.187 in. or
greater, where errors up to 6.10% were recorded. There
was also a noticeable trend in that most of the data sets
showed that the effects were slightly higher at lower
Reynolds numbers.
The results of the analysis apply only to the specific
meters in this research scenario. Meters of the same type
may respond differently as a result of the method of
installation into the pipeline or locations of the pressure
taps in reference to the sudden change in pipe radius.
However, this research demonstrates that pipe wall offsets
must be carefully considered when calibrating and installing differential pressure meters. Positive diameter offsets
(sudden expansions) can have significant effects on flow
metering. If meters in these conditions are not corrected,
the fluid flow cannot be accurately measured.
Corresponding author: Jesse M. Pope is a staff engineer
at Gannett Fleming, 207 Senate Ave., Camp Hill, PA
17011 USA; [email protected].
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P O P E ET A L. | 107: 6 • JO U R NA L AWWA | JU NE 2015
2015 © American Water Works Association
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