An orifice plate is a flate piece of metal with a restricted bore that is mounted between two orifice flanges for the purpose of flow measurement or is mounted between pair of standard flanges to restrict flow. An orifice plate may be considered as the simplest form of differential pressure flowmeter or flow restriction device.

Flow is measured as either volume or mass per unit time. Differential pressure flowmeters such as orifice plates work on the principle of Bernoulli's energy equation which states that in a ideal fluid the overall energy entering a system is equal to the overall energy leaving the system. In case of fluid flowing through a pipe, the energy is the sum of pressure energy, kinetic energy and potential energy.

P1+ ($$\frac{1}{2}$$)ρv12 + ρgh1P2+ ($$\frac{1}{2}$$)ρv22 + ρgh2

Thus, as per Bernoulli’s equation stated above the Pressure (P), potential energy (ρgh) per unit volume and the kinetic energy (ρv2/2) per unit volume will remain constant. Sketch below illustrates that as the fluid flows through the orifice there is a marginal increase in pressure upstream of the orifice plate. After passing through the orifice, there is an increase in fluid velocity and a drop in pressure suggesting that pressure (potential) energy is converted into velocity (kinetic) energy. The location downstream of orifice plate with maximum velocity and minimum pressure is called vena contracta. The vena contracta is located about 0.35 to 0.85D downstream of the orifice plate. As the fluid moves away from the orifice plate, the velocity decreases and the pressure increases. Thus the kinetic energy decreases and potential energy increaes as the kinetic energy is converted into potential energy . The velocity downstream of the orifice plate eventually recovers to the same value as the velocity upstream of the orifice plate. However, there is irrecoverable pressure loss caused due to friction across the orifice plate. The pressure loss caused by the flow of fluid through the orifice is related to the velocity of the flowing fluid and is used to determine the volumetric fluid flow rate.

Limitations of orifice plates include an irrecoverable pressure loss across the restriction, reduction in accuracy of flow measurement over time due to wear of the restriction bore edges.

## Orifice Plate Designs

The two fundamental types of orifice plates depending on the installation methods are paddle type and universal type. The paddle type orifice plates are installed between orifice flanges. The removal and replacement of paddle type orifice plates requires shutdown of the line. Jackscrews are typically provided on the orifice flanges to facilitate separation of flanges for easy removal and installation of orifice plates. The universal types include fittings for installation of the orifice plate and facilitiate easy installation, remova, inspection and maintenance of the orifice plate which is subject to erosion, damage and loss in accuracy over time.

Orifice plates are further classified into several types depending on the design of the bore.

### Concentric Orifice Plate

The Concentric orifice plate is the most commonly used design for flow measurement applications. As the name suggests, this type of orifice plate has a concentric bore with square or bevelled edges. The concentric bore is used for clean fluid services and provides good accuracy in flow measurement. The beta ratio for the concentric orifice plate is between 0.1 to 0.75. Beta ratio (d/D) is the ratio of internal diameter (d) of the orifice bore to the internal diameter of pipe (D).

### Eccentric Orifice Plate

The Eccentric orifice plates have the restriction orifice bore which is offset from the center. The offset of bore is oriented towards the bottom of a horizontal pipe which allows small amounts of solids in liquid flow and small amounts of liquid in gas flow to be carried through without plugging the orifice plate bore. The bore on the plate is tangent to the inside diameter of the pipe flange. During installation of eccentric orifice plates, it must be ensured that the pipe internal diameter or the gasket does not intrude into the orifice bore. Their accuracy of flow measurement is lower compared to concentric orifice plate. Orifice flanges with flange taps are recommended for eccentric orifice plate installations and these taps are located in the quadrants opposite to the orifice bore. The beta ratio for the concentric orifice plate is between 0.3 to 0.8.

### Segmental Orifice Plate

The Segmental orifice plates have orifice bore that resembles the segment of a circle. The arc of the bore is concentric with the inside diameter of flange. The bore on the segmental orifice plate is offset from the center towards the bottom of pipe and the bottom portion of the bore is tangent to the inside diameter of the pipe flange. Orifice flanges with flange taps are recommended for segmental orifice plate installations.

## Vent and Weep Holes in Orifice Plates

In addition to the orifice bore some orifice plates are provided with an additional tiny hole above or below the center of the plate. If the hole is located below the center of pipe, it is called a weep hole. Orifice plates with weep holes are used in gas service that may contain traces of condensate. The weep hole permits the passage of condensate and thus prevents condensate build up upstream of the orifice plate. If the hole is located above the center of pipe, it is called a vent hole. Vent holes serve to allow passage of gas in liquid service applications. Weep holes adversely affect the accuracy of flow measurement and should not be provided in applications that require a high level of accuracy in flow measurement such as in sales gas application.

## Orifice Flange Pressure Tap Locations

As shown in sketch above, the differential pressure across the orifice plate is measured using pressure taps located on either sides of the orifice plate. The differential pressure instrument is connected to the pressure taps through instrument tubing as shown in typical instrument hook-up sketch below. The pressure taps can be provided at one of the following locations.

### Flange Taps

ASME B16.36 covers the design of flange taps. Flange taps are drilled from the outside diameter of flange up to the inside diameter of flange. The centerline of the tap is located 24mm from the face of the raised face flange, 19mm from the face of ring joint flanges and the bore of the tap varies from 6.4mm to 12.7mm as per ASME B16.36. When the flanges are of ring joint type the pressure tap is slightly angled to keep adequate clearance between ring joint groove and flange pressure tap. The tappings may be requested with NPT ends or socket ends to connect the nipple to the flange. Butt welded pipe nipples can also be specified. The butt welded nipple configuration is the safest as it involves a full penetration weld therby preventing the possibility of nipple blowout which is possible with threaded or socket weld connection which may corrode over a period of time.

### Corner Taps

ASME B16.36 also covers the design of corner taps. Flange taps are drilled from the outside diameter of flange up to the face of the flange (not the ID of flange as in the case of flange taps). This makes the tap location very close to the face of the orifice plate and suggests that the vena contracta on the downstream side of orifice plate should be in close proximity to the orifice plate. They are generally in line sizes less than 2 inches or equal.

### Pipe Taps

Recommended location of pressure taps is normally 2.5 times pipe diameter upstream of the orifice plate and 8 times pipe diameter downstream of the orifice plate. Thus the measurement of pressure differential is almost at the point of full pressure recovery. These taps provide lowest measurement accuracy

### Vena Contracta Taps

As shown in sketch above, the location of vena contracta is downstream of the orifice plate and is the location of highest differential pressure in the vicinity of the orifice plate. The location of vena contracta may range between 0.35 to 0.85 times the internal diameter of pipe and is a function of beta ratio of orifice plate and Reynolds number.

## Orientation of Orifice Flange Taps and Typical Instrument Hookups

The orientation of orifice flange taps is related to the type of service in the line.

### Liquid Service

For liquid service, the orifice flange pressure taps should be oriented to prevent accumulation of gas in the instrument tubing between the orifice flange and differential pressure instrument. The taps are generally located either at the bottom, in the horizontal position or 45 degree below the horizontal position as shown in schematic. Pockets should be avoided between the tubing and in the differential pressure instrument.

### Gas Service

For gas service, the orifice flange pressure taps should be oriented to prevent accumulation of liquid in the instrument tubing between the orifice flange and the differential pressure instrument. The taps are generally located in the vertical position or 45 degrees above the horizontal position as shown in schematic. The instrument tubing must be sloped towards the line to drain of any condensate accumulated in the tubing.

## Piping Requirement for Orifice Flange and Orifice Plate Installation

The flow of fluid through elbows, branch fittings and valves can cause turbulence and affect the accuracy of flow measurement. A good accuracy in flow requirements requires the flow to be free from vortices. Sufficient straight length should be provided upstream and downstream of the orifice plate to achieve this. The pipe wall upstream and downstream of the orifice plate should be smooth and free from any branch connections. The use of flow conditioners such as straightening vanes can reduce the upstream straight length requirements. The minimum straight lengths depend on the beta ratio and the layout configuration. For gas flow measurement AGA-3 includes charts to determine the straight lengths for various configurations and beta ratios.