Concentric, Eccentric & Segmental Orifice Plates
There are three types of flow measuring orifice plate that the instrument engineer is likely to encounter, namely; concentric orifice plates, eccentric orifice plates, and segmental orifice plates. Each type of plate lends itself to a particular application e.g. the segmental orifice is the best choice of plate for measuring fluids with entrained solids.
Another type of orifice commonly seen in process plants is the Restriction Orifice. This type of orifice is used to create a permanent pressure drop in a process and is not used for measuring flow. Sizing and specifying restriction orifices (RO) is usually carried out by proces engineers, and ROs are generally bought by the piping group rather than instrument engineers.
Concentric Orifice Plate Design
The most commonly used orifice plate for flow measurement is the square-edged, concentric bore orifice.
With this design, the orifice bore is concentric with the internal walls of the pipe in which it is mounted, and the downsteam bore edge is bevelled to limit the plate edge thickness. This makes the plates uni-directional, and to aid installation it is common for the paddle to indicate which side is the upstream side.
A concentric orifice has the lowest degree of uncertainty of all types of orifice plate.
This design can be used on most processes including clean liquids, gases and steam flow.
Eccentric Orifice Plate Design
Eccentric orifice plates are plates with the orifice off-center, or in other words, eccentric to the centre line of the pipe.
The lower edge of the orifice bore is usually located a distance of 2% of the pipe internal diameter, above the pipe wall.
This design allows solids or slurries to easily pass through the plate preventing inaccuracies caused by partial upstream blockages.
An eccentric orifice has a higher degree of uncertainty than that of a concentric orifice.
Segmental Plate Design
The hole in a segmental orifice plate is not completely circular, but a segment of a circle.
The lower edge of the segment is usually located a distance of 2% of the pipe internal diameter, above the pipe wall.
This design allows solids or slurries to easily pass through the plate; a greater amount than the eccentric orifice plate.
A segmental orifice is considerably more expensive than the eccentric orifice due to the more complex manufacturing required, and they have a higher degree of uncertainty compared to both concentric and eccentric orifice plates.
Orifice Plate Design Standards
Although orifice plates are relatively cheap e.g. a 6" stainless steel concentric plate can be sourced for $200 or less, they are still manufactured to tight tolerances, especially for bore diameter, flatness and thickness. The tolerances and use of orifice plates are detailed in the following International Standards:
ISO 5167-2 – Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full. Part 2: Orifice plates
ASME MFC-3M – Measurement of Fluid Flow in Pipes Using Orifice, Nozzle, and Venturi
AGA Report No.3, Part 2 – Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids - Concentric, Square-edged Orifice Meters, Specifications and Installation Requirements.
Square Edge or Quadrant Edge Bore - What is the Difference?
The difference between a square edge orifice plate and a quadrant edge orifice plate is in the proximity of the specified bore to the upstream face, as shown in the figure.
On a square edge plate the measuring bore is at 90 degrees to the upstream face.
On a quadrant edge plate the inlet edge is rounded to the shape of a quarter circle (hence the name quadrant edge), and the throat of the bore is a small distance from the upstream face.
Why use a Quadrant Edge Plate?
Quadrant edge orifice plates are used for viscous fluids such as heavy crude oils, syrups, and slurries with Reynolds Numbers between 5,000 and 100,000.
For Reynolds numbers below 5,000 the quadrant edge is replaced by a conical entrance.
Orifice Plate Installation - Orifice Flanges
Orifice Plates are normally mounted between a set of "Orifice Flanges".
Orifice flanges differ from normal flanges in that they have an integral pipe tap and jack screw point.
An orifice that uses pressure taps integral to the flange is said to have "flange taps".
The radial pipe tap from the upstream and downstream flange are connected to the differentail pressure transmitter used to determine pressure drop across the flange.
Flange taps are not available in 150# sizes because the thickness of 150# flanges are not enough to allow for drilling pressure taps.
The jack screws are used to spread the flanges apart to allow change out of the orifice plate. Often only one of the mating flanges will have a jack screw.
The following pages on Control and Instrumentation.com give more detail on flow measurement basics:
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For those who want to read further about the theory of flow measurement and the differing types of flow instrumentation, then the following books from Amazon will be of interest: