Large Diameter Orifice Flowmeter Calculation for Liquid Flow |
For pipe diameter > 5 cm. |

Other Flowmeter
Calculations using standard methodologies: |

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Types of Pressure Taps for Orifices:

Topics: Introduction Equations Discharge Coefficients Error Messages References

**Introduction**

Orifice flowmeters are used to determine a liquid or gas flowrate by measuring the
differential pressure (P_{1} - P_{2}) across the orifice plate. They
are generally less expensive to install and manufacture than the other commonly used
differential pressure flowmeters; however, nozzle and venturi flow meters have the advantage of lower pressure drops.
Equations for orifice meters have the advantage of no Reynolds Number upper limit
for validity.

An orifice flowmeter is typically installed between flanges connecting two pipe sections (flanges are not shown in the above drawings). The three standard pressure tapping arrangements 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 or D and D/2 taps, the pressure tap locations are as shown.

Orifices are typically less than 0.05D thick. For exact geometry and
specifications for orifices, see ISO (1991) or ASME (1971).
The ASME and ISO have been working on guidelines for orifices since the early 1900s.
The organizations have the most confidence in orifice accuracy when the Reynolds number
exceeds 10^{5}, though Reynolds numbers as low as 4x10^{3} are valid for
certain d/D ratios as discussed below. The calculation above is for liquids.
Orifice gas flow calculations (D<5 cm, D>5 cm) have an additional factor called expansibility.

**Equations**

The calculations on this page are for orifices carrying a liquid as described in ISO (1991) and ASME (1971). The ISO reference has a more
up-to-date discussion of orifices than the ASME reference, so the ISO equations are used
in our calculations.

k = Equivalent Roughness of the pipe material [L].

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 ISO (1991) (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** (ISO, 1998)**
** Equation and applicability:

Corner Pressure Taps: L_{1} = L'_{2} = 0

D and D/2 Pressure Taps: L_{1} = 1 and L'_{2} = 0.47

Flange Pressure Taps: L_{1} = L'_{2} = 0.0254/D where D is in
meters

Applicability:

All types of pressure taps: d >= 1.25 cm, 5 cm <= D <= 1 m, 0.1
<= d/D <= 0.75

Corner Pressure Taps or D and D/2 Pressure Taps:

Re_{D} >= 4000 for 0.1 <= d/D <= 0.5
and Re_{D} >= 16,000(d/D )^{2} for d/D>0.5

Flange Pressure Taps: Re_{D} >= 4000 and Re_{D}
>= 170,000 D (d/D )^{2} where D is in meters

In addition, ISO recommends that in general k/D <= 3.8x10^{-4}
for Corner Taps and k/D <= 10^{-3} for Flange or D and D/2 pressure
taps. k is the pipe roughness.

**Error Messages given by calculation
**"All inputs must be positve". This is an initial check of user
input.

"d, D, d/D, or ReD out of range". The equation for discharge coefficient, C, is only valid for certain ranges of d, D, d/D, and ReD as shown in the Discharge Coefficients section above. This message could be generated during the program's initial check for valid input. If the input is valid, the message will be generated during computations if the program determines that a calculated value will be out of range of the validity of the equations. d is orifice diameter, D is pipe diamter, and ReD is the Reynolds number based on D.

**References**

American Society of Mechanical Engineers (ASME). 1971. Fluid meters: Their
theory and application. Edited by H. S. Bean. 6ed. Report of ASME
Research Committee on Fluid Meters.

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).

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