Small Diameter Orifice Flow Meter Calculation for Liquid Flow |
For pipe diameter < 5 cm. |

Other Flow
Meter Calculations using standard methodologies: |

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**Units:** 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, 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**

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 **
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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 head
loss. Head loss is computed as h=K_{m}V^{2}/(2g) where V is the pipe
velocity.

**Discharge Coefficients** (ASME, 2001)**
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*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 calculation **
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**Messages indicating input values are out of the acceptable ranges:

Run-time 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

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.

**References **
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American Society of Mechanical Engineers (ASME). 2001. Measurement of fluid
flow using small bore precision orifice meters. ASME MFC-14M-2001.

International Organization of Standards (ISO 5167-1). 1991. Measurement of fluid flow by means of pressure differential devices, Part 1: Orifice plates, nozzles, and Venturi tubes inserted in circular cross-section conduits running full. Reference number: ISO 5167-1:1991(E).

International Organization of Standards (ISO 5167-1) Amendment 1. 1998. Measurement of fluid flow by means of pressure differential devices, Part 1: Orifice plates, nozzles, and Venturi tubes inserted in circular cross-section conduits running full. Reference number: ISO 5167-1:1991/Amd.1:1998(E).

© 2002 LMNO Engineering, Research, and Software, Ltd. (All Rights Reserved)

LMNO Engineering, Research, and Software, Ltd.

7860 Angel Ridge Rd. Athens, Ohio 45701 USA
(740) 592-1890

LMNO@LMNOeng.com http://www.LMNOeng.com