Large Diameter Orifice Flow Meter Calculation for Liquids
Pipe diameter > 5 cm. Compute flow rate, orifice diameter, or differential pressure. Equations: ISO 5167
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Units Orifice Flow Meter calculator: cm=centimeter, ft=foot, g=gram, gal=U.S. gallon, hr=hour, kg=kilogram, km=kilometer, lb=pound, m=meter, min=minute, N=Newton, Pa=Pascal, psi=lb/inch2, s=second
Types of Pressure Taps for Orifices:
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 105, though Reynolds numbers as low as 4x103 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.
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). Km is computed to allow you to design pipe systems with orifices and incorporate their head loss. Head loss is computed as h=KmV2/2g where V is the pipe velocity.
Corner Pressure Taps: L1 = L'2 = 0
Corner Pressure Taps or D and D/2 Pressure Taps:
Flange Pressure Taps: ReD ≥ 4000 and ReD ≥ 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
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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August 25, 2015: Made text fields narrower to show 8 significant figures rather than 16. Calculation still uses double precision internally.
LMNO Engineering home page (more calculations)
Other Flow Meter Calculations using standard methodologies:
Simpler orifice calculation (not as accurate but won't give "parameter out of range" messages):