Orifice Metering Systems and Differential Pressure Flow Meters

What Is an Orifice Metering System?

An orifice metering system calculates flow rate from the pressure difference that forms upstream and downstream of a thin plate with a concentric bore placed in the flow path. It is a widespread and long-established form of differential pressure measurement in the field. Because its geometry and calculation rules are defined by international standards, the primary element delivers traceable results without having to be calibrated in a flow laboratory; dimensional inspection of the plate is the basic way of confirming that the measurement can be trusted.

An orifice metering system is more than a single device. The orifice fitting keeps the plate correctly positioned in the line; the meter run provides the straight pipe lengths needed before and after the plate; the flow conditioner corrects swirl and distorted velocity profiles; differential pressure, static pressure and temperature transmitters measure the process; and a flow computer converts these values into flow rate. The weakest link in this chain sets the quality of the whole measurement, which is why machining the fitting and the meter run together matters.

This group page looks at EPS Leading orifice metering products from a system perspective and helps you choose the right fitting type. Product details are given on the dedicated pages for meter runs, for single chamber, dual chamber and double isolation orifice fittings, and for restriction plates and flow conditioners. It is also worth stressing that restriction plates are not used to measure flow; their job is to create a calculated pressure drop.

How Differential Pressure Flow Measurement Works

Orifice metering is based on conservation of mass (the continuity equation) and conservation of fluid energy (the Bernoulli principle). The measuring chain can be summarized in four steps:

1.      Restricting the flow: The bore of the orifice plate narrows the flow area. To pass the same mass flow through a smaller cross-section, the fluid accelerates, and the jet reaches its narrowest section — the vena contracta — just downstream of the plate.

2.      Drop in static pressure: As velocity rises, static pressure falls. A differential pressure (Δp) related to the flow rate develops between the pressure tappings on the upstream and downstream sides of the plate. Part of this pressure is recovered downstream of the plate; the remainder becomes permanent pressure loss.

3.      Sensing the differential pressure: The pressure at the tappings is transferred through impulse lines and a manifold to a differential pressure transmitter. Static pressure and fluid temperature are measured by separate transmitters; these values are needed to calculate density and the expansion effect.

4.      Calculating the flow rate: The flow computer calculates mass flow with the equation defined in the standard: qm = C / √(1 − β⁴) · ε · (π/4) · d² · √(2 · Δp · ρ₁). Here C is the discharge coefficient, β the ratio of orifice bore to pipe internal diameter, ε the expansibility factor, d the orifice bore and ρ₁ the fluid density on the upstream side.

The Square-Root Relationship and Turndown

Flow rate is proportional to the square root of the differential pressure. When the flow halves, the differential pressure falls to one quarter, so at low flows only a small part of the transmitter's range is in use. The turndown achievable with a single plate and transmitter is therefore limited by this square-root relationship. Stations with a wide flow range typically switch to plates with a different beta ratio or use several differential pressure transmitters working in stages. Fittings that allow the plate to be exchanged without shutting down the line make this approach practical in day-to-day operation.

How the Flow Profile Affects the Measurement

The discharge coefficient is valid for flow that reaches the plate with a fully developed, swirl-free and symmetrical velocity profile. Swirl and asymmetric profiles generated by elbows, valves, reducers or headers lead to measurement error. The standards therefore specify straight pipe lengths upstream and downstream of the plate, expressed as multiples of the pipe diameter. Flow conditioners help shorten these lengths by redistributing the velocity profile.

How Fitting Types Change Access to the Plate

The measuring principle is identical for every fitting type; what differs is how, and under which conditions, the plate can be reached. In a single chamber fitting, the plate is withdrawn through the access cover once the relevant line section has been isolated and depressurized. In a dual chamber fitting, the plate is raised into a second chamber while the line stays under pressure, and that chamber is then separated from the line and vented. In a double isolation design, the separation is made by two independent barriers with a bleed point between them, and pressure gauges show visibly that isolation has been achieved.

Components of an Orifice Metering System

The EPS Leading product family brings the mechanical components of an orifice metering station, and the instrumentation connected to them, together in one system architecture.

·         Single chamber orifice fitting (meter1): A fitting of simple construction for horizontal or vertical lines that lets the plate be changed through an access cover without separating the flanges. Intended for points where the line section can be isolated for a plate change.

·         High-pressure single chamber fitting (HPmeter1): Carries the same single chamber principle into the higher pressure classes, using a different body casting material for those ratings.

·         Dual chamber orifice fitting (meter2): Allows the plate to be raised into the upper chamber and removed while the line is under pressure, so inspection and plate changes can be made without interrupting flow. Its modular design lets an installed single chamber fitting be upgraded to dual chamber operation in the line.

·         Double isolation dual chamber fitting (meter2di): Provides proved isolation by the double block and bleed (DBB) principle; HP and LP pressure gauges make the isolation visible, and valve and internal chamber shafts can be locked.

·         Meter run: Precision-machined pipe sections on the upstream and downstream sides of the fitting; concentricity, surface finish and radiographic checks form the mechanical basis of system accuracy.

·         Flow conditioners: The Zanker-type plate, designed to ISO 5167-1:2003, reduces swirl and redistributes the velocity profile, while the 19-tube bundle reduces swirl only. Both types help reduce the straight pipe distance required by the standards.

·         Restriction plates and multi-plate spools: Designed to produce a calculated pressure drop rather than to measure flow. Where one plate cannot achieve the required drop, the pressure is reduced step by step in a multi-plate restriction spool.

·         Instrument connection points: Thermowell and densitometer connections for additional process measurements such as temperature and density can be provided at the correct positions while the meter run is being fabricated.

·         Transmitters, flow computer and diagnostics: Adding transmitters, a flow computer and an enclosure to the mechanical components produces a field-ready metering station, supported if required by condition-based monitoring (CBM) diagnostics.

Product Family and Technical Specifications

The table below summarizes the system-level scope of the EPS Leading orifice metering product family. Product-specific sizes, pressure classes and material details are given on the individual product pages; final values should be confirmed at project stage.

Fitting sizes and pressure classes vary with the fitting type — for example, the highest pressure classes are not offered in every size. See the relevant product page for detailed combinations.

Key Advantages

·         Primary element without flow calibration: Orifice plates are verified by dimensional inspection instead of flow calibration, which lowers the life-cycle cost of the metering station.

·         Standards-based, traceable calculation: Design to ISO 5167 and API MPMS 14.3.2 keeps the flow calculation auditable and repeatable.

·         Fitting matched to the maintenance scenario: A simple single chamber design where the line can be shut in, or a dual chamber or double isolation design where flow must continue.

·         Adaptable measuring range: When the flow profile changes, switching to a plate with a different beta ratio re-optimizes the measuring range; with dual chamber designs this is done without stopping the flow.

·         System accuracy from a single source: Machining the fitting and the meter run together reduces uncertainty arising from concentricity and alignment errors.

·         Broad pressure and material range: Options from standard carbon steel castings to duplex and nickel-based alloys, and from ANSI 150# up to 10,000 psi, cover demanding service conditions.

·         Field-repairable construction: A design with few parts and modern sealing solutions supports quick maintenance on site.

Application Areas

Orifice metering systems are used in the gas transmission and distribution, power and process piping covered by the design codes; restriction plates serve for pressure reduction in gas, vapor and liquid lines.

Natural Gas Transmission and Distribution Pipelines

At gas metering stations, orifice fittings are designed to suit lines within the scope of ASME B31.8. Where flow cannot be interrupted, dual chamber or double isolation fittings make plate inspection possible without stopping operation.

Oil and Gas Production Facilities

High-pressure lines use API 6A flange options and solutions up to 10,000 psi. Sour service requirements are assessed under NACE MR0175.

Power Plants

Orifice fittings and meter runs can be applied for flow measurement on lines covered by ASME B31.1 power piping.

Process Plants

For gas and liquid flows in ASME B31.3 process piping, the wide choice of materials makes it possible to build measuring points suited to corrosive media.

Pressure Reduction and Flow Restriction Points

Restriction plates and multi-plate spools produce a calculated pressure drop in gas, vapor or liquid lines, and they frequently appear as auxiliary equipment at metering stations.

Orifice Fitting Type and System Selection Criteria

How an orifice metering system is configured depends as much on the plant's maintenance and safety philosophy as on the fluid being measured. For a technical assessment we recommend clarifying the following points:

Fitting Type Decision Guide

·         If the line section can be isolated and depressurized for a plate change, or a bypass is available: single chamber fitting (meter1; HPmeter1 for high pressure)

·         If plate inspection and changes must be made without interrupting flow: dual chamber fitting (meter2)

·         If the plant's isolation philosophy requires proved double block and bleed isolation: double isolation fitting (meter2di)

·         If uninterrupted maintenance may be needed in future: a modular design that can be upgraded in the line

Process and Fluid Data

·         Fluid type and composition (natural gas, process gas, liquid or vapor)

·         Minimum, normal and maximum flow rate; expected flow variability

·         Operating and design pressure and temperature

·         Density or molecular weight, viscosity

·         Sour service, corrosive components or particle content

Line, Installation and Mechanical Design

·         Line size, pressure class and the piping design code to be applied

·         Fittings upstream of the plate and the straight pipe length available; need for a flow conditioner

·         Horizontal or vertical installation, end connection and flange facing

·         Selection of body, internals and seal materials

Metering System Integration

·         Location of differential pressure, static pressure and temperature measuring points

·         Flow computer, enclosure and control system interface

·         Need for condition-based monitoring (CBM) diagnostics

·         Expected material certification, measurement verification and pressure-equipment documentation

If the measuring point lies in an area classified as a potentially explosive atmosphere, the hazardous-area suitability of the electronic instruments to be connected should be assessed separately at project stage.

Design Codes and Measurement Standards

Depending on product type, EPS Leading orifice metering products are designed within the framework of the following measurement standards, design codes and flange standards.

The product pages state which code applies to which product. Project-specific certification requirements should be clarified with the TLY Enerji engineering team at quotation stage.

Orifice Metering Solutions from TLY Enerji

At TLY Enerji we do not treat orifice metering as simply supplying a plate and a fitting; the right system emerges when the flow profile, operating pressure, maintenance philosophy and isolation rules of the measuring point are assessed together. Our engineering team collects the process data, carries out preliminary sizing of the beta ratio and differential pressure range, analyzes straight pipe lengths and the need for a flow conditioner, and then recommends the fitting type that suits your plant.

We support you throughout the project: defining the fitting, meter run, transmitters and flow computer as a single system, preparing technical specifications and material and pressure-equipment documentation, integrating the measurement into the control system and commissioning it. For existing stations, we evaluate upgrade and improvement options against site constraints, planned shutdowns and the continuity of metering records.

Frequently Asked Questions

What components make up an orifice metering system?

The core components are the orifice plate, the orifice fitting that holds it, the meter run that provides straight pipe sections upstream and downstream of the plate and, where needed, a flow conditioner. Differential pressure, static pressure and temperature transmitters are added, together with a flow computer that calculates the flow rate. System accuracy depends on selecting and installing all of these components correctly as a set.

How is flow rate calculated with an orifice plate?

The differential pressure between the upstream and downstream faces of the plate is measured; flow rate is proportional to the square root of this differential pressure. Using the discharge coefficient, beta ratio, expansibility factor, orifice bore and upstream density, the flow computer calculates mass or volumetric flow with the equation defined in the standard. For gases, density is determined continuously from pressure and temperature measurements.

What is the difference between a single chamber and a dual chamber orifice fitting?

Both allow the plate to be changed without separating the flanges. In a single chamber fitting, the relevant line section must be isolated and depressurized before the plate can be changed. In a dual chamber fitting, the plate is raised into a second chamber while the line is under pressure; once that chamber has been isolated from the line and vented, the plate is removed. Inspection and plate changes can therefore be made without interrupting flow.

When should a double isolation orifice fitting be used?

A double isolation fitting is the right choice when the plant's isolation philosophy requires proved isolation — two independent barriers with a bleed between them — before any pressurized chamber is opened. The design is consistent with the double block and bleed approach of the HSG253 guidance; HP and LP pressure gauges confirm the isolation visibly, and the valve shafts can be locked.

Does an orifice plate require flow calibration?

Orifice plates manufactured and installed in accordance with the standards do not need to be calibrated in a flow laboratory; confidence in the measurement comes from dimensional inspection of the plate geometry. Edge sharpness, flatness and bore diameter are checked during periodic inspection. This characteristic gives orifice metering an operating-cost advantage over some other flow measurement technologies.

Why is a flow conditioner used in orifice metering?

Elbows, valves and headers create swirl and asymmetric velocity profiles, which affect the discharge coefficient and therefore the measurement. A flow conditioner corrects the profile before it reaches the plate and helps shorten the straight pipe length required by the standards. A Zanker-type plate both reduces swirl and redistributes the profile, whereas a tube bundle reduces swirl only.

How does a restriction orifice differ from a metering orifice?

A metering orifice produces a differential pressure that represents flow rate so that this value can be read, and permanent pressure loss should be kept low. A restriction orifice, by contrast, serves to produce a deliberate, calculated pressure drop at a specific point in a line and has no measuring function. Where one plate is not sufficient, the pressure is reduced in stages with multi-plate spools.

How can the turndown of an orifice metering system be extended?

The square-root relationship between flow and differential pressure limits the turndown achievable with one plate and one transmitter. To extend it, the plate can be exchanged for one with a different beta ratio when the flow changes, or several differential pressure transmitters can be used in stages. Dual chamber fittings, which allow the plate to be changed without interrupting flow, make this approach easier in operation.