Orifice Plates: How They Work, When to Use

The orifice plate is one of the oldest and most widely used flow measurement devices in process engineering. It is simple, has no moving parts, and can measure flow rates of liquids, gases, and steam with reasonable accuracy — typically 1-2% when installed correctly to ISO 5167.

How an Orifice Plate Works

An orifice plate is a thin disc with a precisely machined hole (the bore) mounted between flanges in a pipeline. As fluid flows through the restriction, it accelerates and its pressure drops. The difference in pressure between the upstream and downstream taps is proportional to the square of the flow rate:

• $C_d$ — discharge coefficient
• $E$ — velocity of approach factor
• $d$ — bore diameter
• $\Delta P$ — differential pressure
• $\rho$ — fluid density

For compressible fluids (gas, steam), an additional expansibility factor is applied to account for the density change through the restriction.

ISO 5167 and the Discharge Coefficient

The discharge coefficient Cd accounts for real-world effects that cause the actual flow to differ from the ideal prediction — things like boundary layer growth, vena contracta formation, and turbulence. Rather than calibrating every orifice plate individually, ISO 5167 provides the Reader-Harris/Gallagher equation, a correlation that predicts Cd from the geometry and Reynolds number:

• Beta ratio (d/D) — the ratio of bore diameter to pipe internal diameter. Valid range: 0.1 to 0.75.
• Reynolds number — must be above a minimum that depends on beta (typically Re > 5000 for beta < 0.56).
• Pressure tap location — corner taps, flange taps (1 inch upstream and downstream), or D and D/2 taps.

The correlation is iterative because Cd depends on Reynolds number, which itself depends on the flow rate you are trying to calculate. Our free orifice plate calculator handles this iteration automatically using the ISO 5167 Reader-Harris/Gallagher method.

Choosing the Right Beta Ratio

The beta ratio is the single most important design decision for an orifice plate. It controls the trade-off between measurement sensitivity and permanent pressure loss:

• Low beta (0.2-0.4) — high differential pressure, good signal strength, but high permanent pressure loss (up to 90% of dP is not recovered). Suitable for low-flow applications where pressure loss is acceptable.
• Medium beta (0.4-0.6) — the most common range. Good balance between signal strength and pressure loss. Typically the default choice for new installations.
• High beta (0.6-0.75) — low permanent pressure loss, but weaker differential pressure signal. Needs a more sensitive DP transmitter. Used when minimising pressure drop is critical, such as in low-pressure gas systems.

As a rule of thumb, the permanent pressure loss through an orifice plate is approximately (1 - beta²)² times the measured differential pressure.

Pressure Tap Arrangements

ISO 5167 defines three standard tap arrangements, each with different upstream and downstream pressure measurement locations:

• Corner taps — pressure tapped immediately at the plate faces. Common in Europe and for smaller pipe sizes.
• Flange taps — pressure tapped 25.4 mm (1 inch) upstream and downstream of the plate. The most common arrangement in North America.
• D and D/2 taps — upstream tap at 1D (one pipe internal diameter) and downstream tap at 0.5D from the plate. Less common but offers good signal characteristics.

The choice of tap arrangement affects the discharge coefficient calculation. All three are supported in the Reader-Harris/Gallagher correlation.

Common Installation Mistakes

Orifice plates are sensitive to installation conditions. The most common errors that degrade accuracy include:

• Insufficient straight pipe — ISO 5167 requires significant straight pipe runs upstream (typically 20-45D depending on the upstream fitting) and downstream (typically 4-8D). Bends, valves, and reducers upstream create swirl and asymmetric velocity profiles that bias the measurement.
• Plate installed backwards — the sharp edge must face upstream. A plate installed backwards will read incorrectly.
• Worn or damaged bore edge — the upstream edge must be sharp and square (radius < 0.0004d per ISO 5167). Erosion, corrosion, or deposits can wear the edge smooth, increasing the effective discharge coefficient and causing the meter to under-read.
• Gasket intrusion — gaskets protruding into the bore or past the pressure taps will disturb the flow pattern.
• Incorrect DP transmitter setup — for gas and steam service, the impulse lines must be correctly arranged (e.g. self-draining) to avoid condensate or gas pockets affecting the reading.

Orifice Plates vs Other Flow Meters

Orifice plates are not always the best choice. Here is how they compare to alternatives:

• Venturi tubes — much lower permanent pressure loss (10-20% of dP vs 40-90% for an orifice), but significantly more expensive and require more space.
• Flow nozzles — better for high-velocity and erosive flows. Pressure loss falls between orifice plates and venturi tubes.
• Vortex meters — no impulse lines, wider turndown ratio (10:1 vs 3:1 for orifice plates), but not suitable for low-velocity or viscous flows.
• Coriolis meters — measure mass flow directly with high accuracy, but expensive and limited to smaller pipe sizes in practice.

Despite these alternatives, orifice plates remain popular because they are inexpensive, easy to fabricate, and can be installed in almost any pipe size. For many applications, the 1-2% uncertainty is perfectly adequate.

Try It Yourself

Use our free orifice plate flow calculator to size an orifice or calculate flow rate from a measured differential pressure using the ISO 5167 Reader-Harris/Gallagher correlation. For calculating the Reynolds number needed for the calculation, or friction losses in the upstream and downstream pipe runs, check out our other free tools. For full pipe network simulation including orifice plates and flow meters, try SimuPipe.