Orifice Plate
An orifice plate measures the flow of a liquid or gas by the difference in pressure from the upstream to the downstream. This plate creates a restriction in a pipe that causes a difference in pressure between the two sides. A meter then measures the difference in pressure across the orifice plate. The plates should be installed in a straight run of pipe in order to avoid turbulance at the plate. The upstream side is longer than the downstream in order to straighten the flow before the plate. It is calculated at so many diameters of pipe, 10D, 12D, ect. for the length. Orifice plates are used between orifice flanges and are part of an orifice meters.
The orifice plate is described as a thin plate and with sharp edge, because the thickness of the plate material is small compared with the internal diameter of the measuring aperture (bore) and because the upstream edge of the measuring aperture is sharp and square. Plates should not be bent or have any nicks in the internal bore, this will create an error in the data.
Information about the plate should be stamped where it can be easily identified after installation. Orifice plates have data stamped on upstream face. Following installation, care should be taken to ensure the gasket does not protrude into the flow area. Pressure taps should be on the top or side of a pipe to prevent sediment/partials from settling in them.
orifice plate types
- Bore and Bevel - This is the most common plate method used. The bevel is at a 45° angle and can be machined to a specified plate thickness.
- Bore and Counterbore - This plate can be counter bored to make the desired plate thickness.
- Conditioning - A conditioning orifice plate has four holes and requires no more than two diameters of straight pipe upstream and downstream. A conventional orifice plate has a single hole. Another advantage is the ability to place one of the holes at the bottom of the pipe to minimize the risk of liquid restriction.
- Eccentric - This plate is for measuring solids or slurries. This is used in steel, water treatment, sewer treatment, oil, and paper.
- Quadrant Bore - This plate has an orifice with the inlet edge rounded. The thickness of the throat is equal to the radius. This design is for fluids of high velocity, such as heavy crudes, slurries and syrups.
- Segmental - This plate is for measurements where solids are mixed in a gas or liquid flow stream. You can place the opening either at the top or bottom of the pipe. This is used in steel, water treatment, sewer treatment, oil, and paper.
Article Links |
Datasheets |
Flange Tap
Flange taps are generally used for pipe sizes of 2 inches and greater. Flange taps are a good alternative to drilling the tapping the pipe for a pressure connection.
Orifice Area
- Geometric Orifice Area (GOA) - The orifice opening
- Effective Orifice Area (EOA) - The minimal cross-section area of the downstream jet.
- Contraction Coefficient (\(C_c\)) - The ratio of the area measured at the vena contracta (EOA), to the area of the orifice (GOA) or \(C_c = \frac{EOA}{GOA}\)
- Discharge Coefficient (\(C_d\)) - The ratio of actual flow to ideal flow or \(C_d = \frac{Q_a}{Q_i}\)
Orifice Plate Beta Ratio
The ratio of the measuring device diameter to the meter run diameter (orifice bore divided by inlet pipe bore). The smaller the beta ratio the higher the pressure drop across the plate. The beta ratio affects the amount of unobstructed straight run before the plate.
Pipe Tap
These taps should be located 2.5 pipe diameters upstream and 8 pipe diameters downstream of the orifice plate. Pipe taps are normally in existing piping where the radius and vena contracta can not be used.
Radius Tap
These taps are a modification if the vena contracta taps. Radius taps having connections at one pipe diameter upstream and one half pipe diameter downstream.
Vena Contracta
This is the narrowest point in a fluid stream where the diameter of the stream and fluid velocity is at its maximum, resulting in lower pressure at that point.
Vena Contracta Taps
These taps are limited to pipe sizes greater than 6 inches. This limitation is mainly imposed to provide adequate clearance on the tap from the flange.
Orifice Plate standards
ISO Standards
- ISO 5167-1 - Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full -- Part 1: General principles and requirements
- 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
