Expanded
Summary
Effects of Abrupt Pipe Diameter Changes
on Venturi Flowmeters
ZAC HARY B. SHARP, M IC H A E L C . JO H NSO N, AN D S T EV EN L . BAR F U S S
http://dx.doi.org/10.5942/jawwa.2016.108.0101
Venturi flowmeters have been used to measure flow
rate for more than 100 years. Over that time, many
variations of the Venturi flowmeter have been developed, and a large amount of discharge coefficient and
head loss data has been collected. In addition, substantial effort has been spent to determine the required pipe
diameters needed between various pipe fittings and the
Venturi flowmeter to maintain accuracy of the meter.
The American Society of Mechanical Engineers, among
others, reports the distance of straight pipe needed
between a Venturi flowmeter and common pipe fittings
such as an elbow, two elbows in plane, two elbows out
of plane, various fully open and partially throttled
valves, pipe reducers, and pipe expanders. All of these
recommended distances are relative to the beta ratio of
the Venturi. However, one pipe disturbance that is not
found in the literature is a sudden change in pipe
diameter adjacent to or relatively close to the meter,
although most research recommends matching the meter
and pipe diameters on the inlet end of the meter.
Ideally a meter should always be built to match the
pipe; however, in some cases the pipe is in service and the
exact pipe diameter is unknown until the meter is
installed. Oftentimes, laboratories do not have the exact
pipe schedule in which a meter is built to be installed.
There are also cases when the pipe in which a meter is
installed is not round, which causes offsets at different
locations around the circumference of the meter inlet.
Any of these situations requires knowledge previously
unavailable to explain the installation effects expected
from sudden pipe wall offsets.
THEORETICAL BACKGROUND
To get the best performance out of a Venturi flowmeter,
a laboratory calibration is necessary. During this calibration, a meter specific discharge coefficient (Cd) is determined. This discharge coefficient is used to calculate the
flow rate from that meter when it is in service. If the Cd
used to calculate flow rate is not correct, the resulting
flow calculation will also be inaccurate.
Any time a flow disturbance or pipe fitting is installed
directly upstream of a flowmeter, there is a possibility
that the flowmeter’s ability to accurately determine the
flow rate has been compromised. The only way to
determine what effect a pipe fitting or other flow disturbance might have on a meter is to test it. These tests
are typically performed in laboratories and can be
expensive, but computational fluid dynamics (CFD) can
provide an inexpensive alternative to subsidize, not
replace, laboratory testing.
STUDY OBJECTIVES AND RESULTS
The primary objectives of this study were to (1) determine how well CFD could predict the Cd of various
Venturi designs, (2) predict how Cd is affected by pipe
diameter offsets, and (3) use CFD to determine the distance a Venturi flowmeter needs to be installed from an
offset so that the meter will no longer be affected. To
determine how successful CFD is at predicting Cd and the
change in Cd from pipe wall offsets, both physical and
numerical data were taken on several identical test setups,
and the results were compared. Once CFD was proved
reliable, it was used to expand the research on many different setups that were not feasible in the laboratory.
During this study, over 200 CFD data runs were completed with various Venturi designs, beta ratios, line
sizes, offsets, and distances from the offset to the Venturi.
The results of this study predict and report the change
in Cd for sudden changes in pipe diameter at different
distances upstream of a Venturi. This article can act both
as a guide for how far a Venturi meter needs to be
installed from an offset to minimize or even remove the
effect of the offset and also gives guidance about the
effect the offset can have on the Cd of the meter if it is
installed closer to an offset than desired. This article also
acts as a guide on how CFD can be used to gain accurate
information when coupled with laboratory data.
Corresponding author: Zachary B. Sharp is a research
engineer at Utah State University, 8200 Old Main Hill,
Logan, UT 84321 USA; [email protected].
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S H A R P ET A L. | 108: 8 • JO U R NA L AWWA | A U G U S T 2016
2016 © American Water Works Association
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