PAGE 6 2001 PROCEEDINGSAMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGY

by a holding device. Typically, it is made of a durablemetal such as stainless steel. Orifice plates come inbasically two types, the paddle plate and the universalplate. The paddle plate is held in place by flanges, whilethe universal plates fit into the various types of holdingdevices. AGA #3 standards spell out specificrequirements for the orifice plates, including theconcentricity of the orifice bore, the surface finish,flatness of the plate, and edge thickness. While the orificeplate is the least expensive of the components in orificemeasurement, its importance should not be overlooked.

ORIFICE DEVICES

There are primarily three different types of devices usedto help center an orifice plate in the flowing medium.The first and least expensive is the orifice flange union.This is a pair of flanges, which has been tapped to providea differential reading. While it is the least expensive topurchase, it requires a higher maintenance level sincethe line must be bled down and the flanges spread apartin order to remove the plate.

The next device type is the single chamber orifice fitting.The single chamber device has an advantage overflanges in that it makes removal of the plate easier andsafer due to the prevention of spillage that occurs whenflanges are spread apart. Like flanges, however, thesimplex device requires that the line pressure be bledoff before the plate may be removed. The simplex deviceutilizes universal type orifice plates.

The third device is the dual chamber orifice fitting. Thisfitting allows for the removal of the universal orifice platewithout first bleeding down line pressure. This is

INTRODUCTION

The purpose to this paper is to discuss the fundamentalcomponents used in orifice measurement

BACKGROUND

The general concepts of head meters, which include theorifice, have been known for centuries. The orifice hasbeen in commercial use since the early 1900’s. Thedevice is used to create a differential pressure that relatesto the velocity of the gas from which a flow rate can becalculated. As the following gas passes through therestriction in the line caused by the orifice plate, thedifference in the upstream and downstream pressure canbe measured at set points, called taps, and a flow rateat the point can be determined.

STANDARDS AND IMPORTANCE

Orifice measurement is guided by the standards ofseveral organizations. Primary among these is theAmerican Gas Association and the American PetroleumInstitute. The AGA #3 report is the standard that providesguidelines for the construction and installation of orificemeters. All orifice plates, holding devices and meter tubesshould be manufactured adhering to this standard inorder to help ensure that the end product is an accuratemeasurement device.

ORIFICE PLATES

The most fundamental component of orificemeasurement is the orifice plate. This is typically acircular, flat device, which is held in the flowing stream

FUNDAMENTALS OF ORIFICE METERINGBill Buckley

Daniel Measurement and ControlP.O. Box 19097, Houston, Texas 77224

FIGURE 1. Head Meter

FIGURE 2. Orifice Plates

2001 PROCEEDINGS PAGE 7AMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGY

accomplished through the use of internal valves, whichisolate the upper (non-pressure) chamber from thebottom (pressured) chamber. The senior type is the mostexpensive of the plate holding devices to purchase, butcould be the most economical when compared to theoverall cost of the installation since isolation valving isnot required to allow plate removal.

STRAIGHTENING VANES AND FLOW CONDITIONERS

Straightening vanes are bundles of small diameter tubing,which are placed inside the upstream section of a metertube. They are commonly of two types, flanged and in-line. The flanged types are held in the line between apair of flanges in the upstream. The in-line vane is heldin place inside the tubing by setscrews. Their purpose isto facilitate the smoothing of flow going into the orificeplate while allowing for shorter upstream tubing lengths.

METER TUBES

A meter tube basically consists of upstream tubing, theorifice fitting or flanges and downstream tubing. Thepurpose of tubing is to ensure as smooth a flow profile,going into the orifice plate, as possible. The AGA #3standard has very specific requirements for meter tubepipe, including the smoothness of the inside surface ofthe tubing and minimum lengths required under particularinstallations. If these standards are not met in themanufacture of the meter tube, then degradation inmeasurement could result.

FIGURE 3. Orifice Flange Union

FIGURE 4. Single Chamber Fitting

FIGURE 5. Dual Chamber Fitting

FIGURE 6. Three-Section Meter Tube

The flow conditioners eliminate swirl like a straighteningvane and also generates a near fully developed flowprofile. The conditioner also Reduces the amount ofrequired upstream tubing needed to meet AGA #3requirements.

SECONDARY DEVICES

The orifice fitting with its orifice plate is known as theprimary devices in the orifice measurement package.There are other devices known as secondary devices,which translate the raw information from primary devicesinto more useable information. The most common ofthese are pneumatic chart recorders and flow computers.

The pneumatic chart recorder presents the informationfrom the differential pressure, static pressure andtemperature transmitters in a graphical form, usually

FIGURE 7. Straightening Vanes

FIGURE 8. Flow Conditioner

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circular charts. The chart usually represents a 24-houror 8 day time period, which can be integrated later toprovide volume figures.

Flow computers have increased in use in recent yearsdue to the requirements for measurements informationon a more “real time” basis. Flow computers, like thepneumatic chart, take the flow information from thedifferential pressure, static pressure and the temperaturetransmitter and calculates flow volumes. Unlike the chart,flow computers do not have to go through an integration

FIGURE 9. Secondary Devices

step to come up with these figures. There are several levelsof sophistication available in flow computers. The battery-powered, solar charged devices have the best utility asfield devices, which can store the flow information on site,do the volume calculations and then send that informationon to a higher device such as a mainframe computer. Thehigher-level flow computers are usually AC or DC poweredand provide not only the same calculations capability asthe solar –powered units, but also have advanced flowcontrol and alarm capabilities.

CONCLUSION

The new AGA #3/API 14.3 measurement standard hasgreatly tightened the tolerances for the manufacture oforifice devices and meter tubes. It is very much in thebest interest of the users of these devices to have soundmaintenance programs in place to ensure that the like-new quality of the tubes be maintained for as long aspossible. The primary device, whether a fitting or flange,cannot be expected to provide accurate, reliable flowinformation if the orifice plate is bowed or otherwisedegraded in some way. The vast body of data supportingorifice measurement over the years becomes meaninglessif the guidelines for the design, manufacture, installationand maintenance of these devices are not followed.

Bill Buckley