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Units in small bore liquid orifice sizing calculation: bbls=barrels, C=degrees Celsius, cm=centimeter,
cP=centipoise, cSt=centistoke, F=degrees Fahrenheit, cfs=cubic feet per second, ft=feet,
g=gram, gpm=US gallons per minute, gph=US gallons per hour, gpd=US gallons per day,
hr=hour, in=inch, kg=kilogram, lb=pound, m=meters, min=minute, mm=millimeter, N=Newton, Pa=Pascal,
psi=pound per square inch, s=second
Types of Pressure Taps for Small Bore Orifices
Topics: Introduction Equations Discharge
Coefficients Error Messages
References
Introduction to Orifice Flow Meter Calculation
Orifice flow meters are used to determine a liquid or gas flow rate by measuring the
differential pressure (P_{1}  P_{2}) across the orifice plate. The
two standard pressure tapping arrangements for small bore orifices are shown in the
drawings; the location of the pressure taps affects the discharge coefficient
somewhat. Flange pressure taps penetrate the flange and are at a standard distance
of 1 inch (2.54 cm) from either side of the orifice. For corner taps, tappings are
right up against the orifice.
The LMNO Engineering small diameter orifice calculation is valid for orifices installed
in pipes having pipe diameters between 1 cm and 5 cm (2 inch), and pipe Reynolds numbers
greater than 1000. For larger diameter pipes, please see our Large Diameter Orifice Calculation for Liquids. We also
have orifice calculations for gas flow (D<5 cm and D> 5 cm); and calculations for flow through nozzle and venturi flow meters.
Equations for Small Bore Orifice for Liquids
To orifice calculation at top of page
The calculations on this page are for orifices carrying a liquid as described in ASME (2001).
w is the static pressure loss occurring from a distance of approximately D upstream of the
orifice to a distance of approximately 6D downstream of the orifice. It is not the
same as differential pressure. Differential pressure is measured at the exact
locations specified in ASME (2001) (shown in the above figures). K_{m} is
computed to allow you to design pipe systems with orifices and incorporate their overall pressure
loss, w.
Discharge Coefficients for Small Bore Orifice for Liquids (ASME, 2001)
To orifice calculation at top of page Corner Taps:
Flange Taps:
where D is in inches; and d/D and Re_{D} are dimensionless. C is
dimensionless.
Applicability:
Pipe Diameter, D
LMNO Engineering calculation requires 1 cm < D < 5.1 cm for both corner and flange
taps.
ASME (2001) suggests 1.2 cm ≤ D ≤ 4 cm for corner taps and 2.5 ≤ D ≤ 4 cm
for flange taps.
Diameter ratio, d/D
LMNO Engineering and ASME (2001) require 0.1 ≤ d/D ≤ 0.8 for corner taps and 0.15
≤ d/D ≤ 0.7 for flange taps.
Reynolds number based on pipe diameter, Re_{D}
LMNO Engineering and ASME (2001) require Re_{D} > 1000.
Error Messages given by Orifice Calculation
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Messages indicating input values are out of the acceptable ranges:
"Need 1≤Density≤1e9 kg/m^{3}", "Need
1e19≤Viscosity≤1e9 m^{2}/s", "Need 0.01≤D≤0.051 m",
"Need 1e30≤Q_{v}≤1e30 m^{3}/s", "Need 1e30≤Q_{m}≤1e30
kg/s", "Need 1e99≤Δp≤1e99 Pa", "Need Re_{D} >
1000".
"Need 0.1≤d/D≤0.8" for corner taps. "Need
0.15≤d/D≤0.7" for flange taps.
Runtime messages. Computations not completed:
"Re_{D} will be <1000". If solving for flow
rate. Unable to compute Q's. Value for Q will cause Re_{D} to be <
1000 (out of range of validity).
"Δp too large". If solving for flow rate. Unable to
compute Q's because the differential pressure that you entered will cause the throat
velocity to exceed 1000 m/s, a velocity for which the calculation is not valid. The
calculation is not valid for supersonic flows.
"d/D will be too small" or "d/D will be too large".
If solving for d. Unable to compute d; value for d will cause d/D to be out
of range of validity.
Note
If your pipe diameter, D, is 2 inch (5.1 cm) or greater, use our Large Diameter Orifice
Calculation for Liquids based on ISO 5167. Or, try the simpler orifice calculation
on our Bernoulli page if your parameters are out of
range. The Bernoulli calculation is not as accurate, but won't give "parameter
out of range" error messages.
Orifice Flow Meter References
To orifice calculation at top of page
American Society of Mechanical Engineers (ASME). 2001. Measurement of fluid
flow using small bore precision orifice meters. ASME MFC14M2001.
International Organization of Standards (ISO 51671). 1991. Measurement of
fluid flow by means of pressure differential devices, Part 1: Orifice plates, nozzles, and
Venturi tubes inserted in circular crosssection conduits running full. Reference
number: ISO 51671:1991(E).
International Organization of Standards (ISO 51671) Amendment 1. 1998.
Measurement of fluid flow by means of pressure differential devices, Part 1: Orifice
plates, nozzles, and Venturi tubes inserted in circular crosssection conduits running
full. Reference number: ISO 51671:1991/Amd.1:1998(E).
© 20022023 LMNO Engineering, Research, and
Software, Ltd. All rights reserved.
Please contact us for consulting or questions about liquid flow rate measurement using an orifice.
LMNO Engineering, Research, and Software, Ltd.
7860 Angel Ridge Rd. Athens, Ohio 45701 USA Phone: (740) 7072614
LMNO@LMNOeng.com https://www.LMNOeng.com

To:
LMNO Engineering home page (more calculations)
Other Flow Meter Calculations using standard methodologies:
Orifice for liquids, D>5 cm
Orifice for gases, D<5 cm
Orifice for gases, D>5 cm
Nozzle for liquids
Venturi for liquids
Simpler orifice calculation (not as accurate but won't give "parameter out of
range" messages):
Bernoulli page
Pressure Relief Valve
Other information:
Unit Conversions
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