Orifice plates are widely used as primary flow elements for measuring the flow of liquids, gases, and steam through pipes. Their simple construction and compatibility with differential pressure instruments make them a practical choice for many industrial flow measurement applications.
An orifice plate does not measure flow by itself. Instead, it creates a predictable restriction in the process line. A differential pressure gauge, transmitter, or controller measures the resulting pressure difference and converts it into an indication of flow rate.
What is an Orifice Plate?
An orifice plate is a thin, circular plate installed inside a pipe, typically between a pair of flanges. An opening in the plate allows the process fluid to pass through. Because this opening is smaller than the inside diameter of the pipe, it restricts the flow and causes the fluid velocity to increase as it passes through the bore.
The increase in velocity is accompanied by a reduction in static pressure. Pressure taps positioned upstream and downstream of the plate allow a differential pressure instrument to measure this pressure change.
Orifice plates are commonly used because they:
- Have no moving parts
- Can be used with liquids, gases, and steam
- Are available for a wide range of pipe sizes
- Have relatively low initial costs
- Require limited maintenance when properly applied
- Are addressed by established flow-measurement standards
How Does an Orifice Plate Flow Meter Work?
The orifice plate can be made of any material, although stainless steel is the most common. The thickness of the plate used (1/8 – 1/2") is a function of the line size, the process temperature, the pressure, and the differential pressure. The traditional orifice is a thin circular plate (with a tab for handling and for data), inserted into the pipeline between the two flanges of an orifice union. This method of installation is cost-effective, but it calls for a process shutdown whenever the plate is removed for maintenance or inspection. In contrast, an orifice fitting allows the orifice to be removed from the process without depressurizing the line and shutting down flow. In such fittings, the universal orifice plate, a circular plate with no tab, is used.
The concentric orifice plate (Figure 1 – A) has a sharp (square-edged) concentric bore that provides an almost pure line contact between the plate and the fluid, with negligible friction drag at the boundary. The beta (or diameter) ratios of concentric orifice plates range from 0.25 to 0.75. The maximum velocity and minimum static pressure occurs at some 0.35 to 0.85 pipe diameters downstream from the orifice plate. That point is called the vera contracta. Measuring the differential pressure at a location close to the orifice plate minimizes the effect of pipe roughness, since friction has an effect on the fluid and the pipe wall.
Orifice Plate Construction and Installation
Stainless steel is commonly used for orifice plates, although the plate material must ultimately be compatible with the process fluid, pressure, and temperature. Plate thickness depends on line size and operating conditions, including process pressure, temperature, and differential pressure.
A traditional plate includes a tab used for handling and identification. It is installed between the flanges of an orifice union. This arrangement is economical, but the process normally must be shut down and depressurized before the plate can be removed for inspection or maintenance.
An orifice fitting provides another installation method. Depending on its design, the fitting can allow a universal orifice plate to be removed without shutting down or fully depressurizing the process line. Universal plates are circular and do not include the projecting tab found on traditional paddle-type plates.
Correct installation is critical. The plate must be centered, oriented correctly, and installed with the specified sharp edge facing upstream unless the plate design or applicable standard states otherwise. Gaskets must not protrude into the pipe or bore.
Common Orifice Plate Types
The appropriate plate geometry depends on the fluid, Reynolds number, pipe orientation, and presence of entrained material or a secondary phase.
Concentric Orifice Plates
The concentric orifice plate has a circular bore centered in the pipe (Figure 1-A). Its sharp, square upstream edge produces a repeatable separation point with minimal contact between the plate edge and the flowing fluid.
Concentric plates are generally recommended for clean, single-phase liquids, gases, and steam. They are the most common type and have the broadest base of published sizing and performance data.
Some concentric plates include a small drain hole to reduce the accumulation of liquid in a gas line or a vent hole to release entrained gas from a liquid line (Figure 1-A). If the auxiliary hole diameter is less than 10% of the main bore diameter, the unmeasured bypass flow is generally less than 1% of total flow. These holes may plug, however, and do not make a concentric plate suitable for every multiphase application.
Eccentric Orifice Plates
An eccentric plate has a circular opening offset from the pipe centerline (Figure 1-B). This geometry gives a secondary phase a path through the restriction and helps reduce material accumulation upstream of the plate.
In a horizontal pipe:
- For gas bubbles entrained in a liquid, the bore is normally positioned toward the top of the pipe
- For liquid carried in a gas or solids carried in a liquid, the bore is normally positioned toward the bottom
Segmental Orifice Plates
A segmental plate uses an arc-shaped opening resembling a segment of a circle (Figure 1-C). Its larger open drainage area can make it more suitable than an eccentric plate when the process contains a relatively high proportion of a secondary phase.
Eccentric and segmental plates are typically applied in pipe sizes above 4 in. Careful installation is required so that neither the flange nor the gasket interferes with the opening. Flange taps are commonly used with these plate types and must be positioned according to the plate geometry and applicable design standard.
Quadrant-Edge and Conical Orifice Plates
Standard square-edged plates are less suitable for low-Reynolds-number or viscous flow because their discharge characteristics become less predictable as Reynolds number decreases. Quadrant-edge and conical entrance plates (Figure 4) are alternatives for applications with Reynolds numbers below approximately 10,000.
Flange, corner, or radius taps may be used with quadrant-edge plates when supported by the applicable sizing method. Conical entrance plates are normally used with corner taps. Because performance depends on the exact geometry and calibration basis, these plates should be sized using data or methods specific to the selected design.
Pressure-Tap Configurations
The position of the upstream and downstream pressure taps affects the differential pressure reading and the flow calculation. The tap configuration used in the field must match the configuration used in the sizing equation or standard.
Flange Taps
Flange taps are positioned 1 in from the upstream and downstream faces of the plate (Figure 2). They are commonly used in the United States, particularly on pipes 2 in and larger.
Corner Taps
Corner taps are located immediately adjacent to the upstream and downstream faces of the plate (Figure 2). They are widely used in Europe and are commonly applied to pipes smaller than 2 in. Their small clearances can make them more susceptible to blockage or maintenance issues in contaminated service.
Vena Contracta Taps
With vena contracta taps, the upstream tap is typically located one pipe diameter upstream of the plate, while the downstream tap is placed at the vena contracta. The downstream location varies with beta ratio and Reynolds number, generally falling from approximately 0.35D to 0.8D downstream (Figure 1).
These taps produce a relatively large differential pressure, but the signal can contain more flow noise. A change in plate bore or beta ratio may also require the downstream tap to be relocated. Vena contracta taps are therefore more common in pipe sizes above 6 in.
Radius Taps
Radius taps are similar to vena contracta taps, but their locations are fixed at one pipe diameter upstream and one-half pipe diameter downstream of the plate (Figure 2).
Pipe Taps
Pipe taps are typically located 2.5 pipe diameters upstream and 8 pipe diameters downstream of the plate (Figure 2). Because these taps are farther from the restriction, they measure a smaller differential pressure and are more influenced by pipe roughness and dimensional variation.
Selecting an Orifice Plate
Plate selection begins with the fluid and operating conditions. Important factors include:
- Fluid type and phase
- Expected minimum, normal, and maximum flow rates
- Pipe size, schedule, and inside diameter
- Operating pressure and temperature
- Fluid density, viscosity, and compressibility
- Reynolds number across the operating range
- Allowable permanent pressure loss
- Required measurement accuracy and turndown
- Presence of bubbles, condensate, sediment, or suspended solids
- Available straight pipe upstream and downstream
- Pressure-tap configuration
- Plate material and process compatibility
- Applicable industry or custody-transfer standard
Square-edged concentric plates are commonly applied where Reynolds number is at least approximately 20,000, although the permissible range depends on beta ratio, pipe diameter, tap arrangement, and the selected standard. Sonic or choked flow requires a different analysis because conventional incompressible or subsonic assumptions no longer apply.
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Written by: Dan Schultz
Published on: April 17, 2026