An orifice plate is a device used to measure flow rate, reduce pressure, or restrict flow in pipelines, characterised by a thin plate with a precisely machined hole. Most commonly used in orifice meters to calculate the flow rate by measuring pressure drop across the orifice plate.
Applications
- Flow measurement: common in industries like oil and gas, water treatment and chemical processing;
- Pressure Reduction: Employed in systems where pressure needs to be controlled or reduced.
- Flow restriction: Orifice plate create a permanent pressure drop, which can lead to reduced system performance;
Definition and function:
An orifice plate is a flat piece of metal with a hole in the center, installed between two flanges in a pipeline. It is primarily used to create a pressure drop in the flow of a fluid, which can then be measured to determine the flow rate. As fluid passes through the orifice, it experiences a change in pressure, allowing for flow measurement based on Bernoulli’s principle.
Working principles
When fluid flows through the orifice, its velocity increases as it converges through the hole, leading to a decrease in pressure. The point of minimum pressure, known as the “vena contracta”, occurs just downstream of the orifice. This pressure difference is proportional to the flow rate. By measuring the pressure difference across the orifice, the flow rate can be calculated using established formulas based on Bernoulli’s principle.
Orifice plates are devices for installation between flanges and for welding. The flange connection allows for easy replacement. They are chosen when there are no special requirements regarding temperature and pressure resistance.
Orifices are welded in, especially in power plants on the high-pressure or hot steam side and when high requirements regarding tightness have to be met.
Types of Orifice Plates:
- Concentric (Sharp-Edged) Orifice
The most commonly used type. Features a circular hole in the center of the plate. Provides a sharp-edged transition perpendicular to the flow direction. Offers high measurement accuracy. - Eccentric Orifice Plate:
The orifice is off-center, which helps prevent the accumulation of solid particles, making it ideal for slurries or fluids with entrained solids. - Segmental Orifice Plate
This type has a non-circular orifice and is designed for applications where solids are present, allowing for better flow through the plate. Reduces the risk of clogging and sedimentation. - Quarter-Circle Orifice Plate
Features a quarter-circle entry with rounded edges. Suitable for viscous fluids. Provides more stable measurement at low Reynolds numbers. - Multi-Hole Orifice
Consists of multiple small holes instead of a single large hole. Improves flow profile by reducing pressure losses. Minimises noise and vibration; - Multi-Stage Orifice
Has multiple stages, with each stage providing a specific pressure drop.
Used in high-pressure systems to reduce pressure step by step.
Safely manages large pressure differences and ensures controlled pressure drop.
Orifice Flange
An Orifice Flange is used in combination with orifice meters to measure the flow rate of oil, gas and other liquids conveyed by the pipeline. Orifice flanges are manufactured to ASME B16.36 in multiple sizes and, material grades.
An orifice flange is used to measure the flow of the fluid conveyed by the pipeline via a flow nozzle positioned on the flange itself. Pairs of pressure tappings are machined onto the orifice flange, making separate tappings on the pipe wall unnecessary.
An Orifice flange is a disc-shaped flange, engineered with either a Raised Face or a Ring Type Joint facing.
The traditional orifice flange assembly consists of a pair of flanges, orifice plate, bolts, nuts, gaskets, jacking screws and plugs. Jacking screws ensure the easy removal of the primary flow element.
Orifice flanges are available in all ASTM forged grades (ASTM A105, ASTM A350, ASTM A694, ASTM 182 respectively for carbon, alloy and, stainless steel flanges), dimensions (combinations of nominal sizes and pressure ratings) and, in socket weld, threaded or weld neck shape (WN is the most used).
Purpose of an Orifice Flange:
- They are widely used with orifice meters to measure the flow rate of either liquid or gases, flowing through the pipe.
- An orifice plate, a device measuring the flow of the inner fluid or gas, is secured between two orifice flanges, attached to the pipe.
- The orifice plate contains a small hole; two pressure tap holes are drilled in each flange which helps in measuring the pressure built inside the pipe.
- A ‘jack screw’ fitted between the two flanges, it enables to separate the flanges during inspection or replacement process.
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
Scope (ISO 5167): Liquids, gases, and steam
• Accuracy: ±0.5% to ±1% (with proper installation)
• Beta Ratio (β): 0.2 – 0.6 recommended (ISO allows 0.1 – 0.75)
• Reynolds numbers ≥ 5000
• Pipe OD ≥ 50 mm and Pipe OD >1000 mm
• Pipe Requirements: 6D–25D upstream (10D typical), 4D–8D downstream
ISO/TR 15377: Measurement of fluid flow by means of pressure-differential devices – Guidelines for the specification of orifice plates, nozzles and Venturi tubes beyond the scope of ISO 5167 series
API MPMS Chapter 14.3.1: Concentric, Square-edged Orifice Meters – Part 1: General Equations and Uncertainty Guidelines
API MPMS Chapter 14.3.2: Concentric, Square-Edged Orifice Meters, Part 2—Specification and Installation R
API MPMS Chapter 14.3.3: Concentric, Square-Edged Orifice Meters, Part 3—Natural Gas Applications (ANSI/A
ASME B16.36: Orifice Flanges
a) welding neck flanges Classes 300, 600, 900, 1500 and 2500
b) slip-on and threaded Class 300
c) welding neck flanges Class 400
d) Nominal pipe size: 1 to 24″
