Orifice Flanges What They Are How They Work and When to Use Them

Orifice Flanges Guide

Orifice flanges are specialized flanges designed for flow measurement in piping systems. They work in conjunction with an orifice plate to create a pressure differential that can be measured and converted to flow rate.

What Are Orifice Flanges

Orifice flanges consist of a matched pair of flanges with an orifice plate sandwiched between them. The orifice plate has a precisely machined hole that creates a restriction in the flow stream.

How They Work

1. Flow Restriction: Fluid passes through the orifice plate opening
2. Pressure Drop: Velocity increases, creating pressure differential
3. Measurement: Pressure taps measure upstream and downstream pressure
4. Calculation: Flow rate calculated using Bernoulli equation

Key Components

• Orifice Plate: Precision machined plate with concentric or eccentric bore
• Flange Pair: Matched flanges with pressure tap connections
• Pressure Taps: Located at flange, vena contracta, or pipe taps
• Gaskets: Special gaskets with tabs for plate alignment
• Jack Screws: For easy plate removal and maintenance

Types of Orifice Plates

Type	Application
Concentric	Clean liquids, gases, and steam
Eccentric	Liquids with solids or gases with liquids
Segmental	Slurries and dirty fluids
Quadrant Edge	Low Reynolds number flows

Applications

• Natural gas flow measurement
• Steam flow monitoring
• Water and wastewater treatment
• Chemical process control
• Petroleum refining
• HVAC system balancing

Advantages

• Simple, proven technology
• No moving parts
• Wide range of sizes and materials
• Relatively low cost
• Easy to install and maintain
• Interchangeable orifice plates

Limitations

• Permanent pressure loss (energy cost)
• Limited turndown ratio (typically 3:1 to 5:1)
• Requires straight pipe runs upstream and downstream
• Accuracy affected by wear and erosion
• Not suitable for dirty or abrasive fluids

Installation Requirements

1. Minimum 10-20 pipe diameters straight run upstream
2. Minimum 5-10 pipe diameters straight run downstream
3. Proper gasket alignment (no gasket protrusion into flow)
4. Correct orifice plate orientation (bevel downstream)
5. Pressure tap lines properly purged and filled

Standards and Specifications

• ASME MFC-3M: Measurement of fluid flow in closed conduits
• ISO 5167: Orifice plates, nozzles, and Venturi tubes
• AGA Report No. 3: Natural gas flow measurement
• ASME B16.36: Orifice flanges

Accuracy Considerations

Typical accuracy ranges from 0.5% to 2% of full scale, depending on:

• Orifice plate machining tolerance
• Pressure tap location accuracy
• Differential pressure transmitter accuracy
• Installation conditions
• Fluid properties and flow regime

Orifice flanges remain one of the most widely used flow measurement devices in process industries due to their simplicity, reliability, and cost-effectiveness.