PART X Meter Proving

Transcription

1 INSTITUTE OF PETROLEUM PETROLEUM MEASUREMENT MANUAL PART X Meter Proving Section 1 Field Guide to Proving Meters With a Pipe Prover

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3 INSTITUTE OF PETROLEUM PETROLEUM MEASUREMENT MANUAL PART X Meter Proving SECTION I FIELD GUIDE TO PROVING METERS WITH A PIPE PROVER This volume supersedes the Tentative edition published by the Institute of Petroleum in 1979 FEBRUARY 1989 Published on behalf of THE INSTITUTE OF PETROLEUM, LONDON by JOHN WILEY & SONS Chichester New York - Brisbane - Toronto Singapore

4 Copyright ;C; 1989 by The Institute of Petroleum, London All rights reserved. No part of this book may be reproduced by any means, or transmitted, or translated into a machine language without the written permission of the copyright holder. Library of Congress Cataloging in Publication Data : (Revised for pt. 10, sec. 1) Petroleum measurement manual. Includes various editions of same vol. 1. Petroleum-Tables. 2. Gaging. 1. Institute of Petroleum (Great Britain) TP691.P ' ISBN British Library Cataloguing in Publication Data : Petroleum measurement manual. Pt. 10 : Meter proving Section 1 : Field guide to proving meters with a pipe prover. 1. Petroleum products. Measurement, Standards I. Institute of Petroleum 665.5'38'0287 ISBN Printed in Great Britain by Galliard (Printers) Ltd, Great Yarmouth

5 CONTENTS Foreword Acknowledgements vii viii List of Symbols. ix Glossary of Terms. x 1 Introduction. 1 2 Basic Principles Ways of expressing a meter's performance How meter performance varies Correction factors 4 3 Meters and Provers Pulse-generating meters Sources of error in operating meters Pulse interpolation Conventional pipe provers Compact pipe provers Sources of error in operating pipe provers Metering installations 12 4 Safety Requirements General procedures Permits Mechanical safety Electrical safety Fire precautions Miscellaneous safety precautions Safety records Operating a Pipe Prover Setting up a mobile prover Warming up dedicated and mobile provers Periodical checks of factors affecting accuracy The actual proving operation Preliminary assessment of the results Fault finding. 20 v

6 vi CONTENTS 6 Calculating and Reporting the Results of Proving Guidelines for rounding off figures in calculations Method of calculating corrected volume of prover Method of calculating K-factor or one-pulse volume when pressure and temperature differ Specimen calculation for a conventional unidirectional prover Method of calculating K-factor or one-pulse volume when pressure and temperature differences are negligible The pressure-compensated K-factor Reporting the results of proving Meter performance curves and control charts 23 7 Calculating Meter Throughput Method based on K-factor or one-pulse volume Specimen calculation, using Equation Method based on meter factor Specimen calculation, using Equations 16 and Mass throughput 26 Figures 28 Appendix A The operation of pipe provers fitted with twin detectors. 38 Appendix B Methods of pulse interpolation 41 Appendix C Trouble-shooting guide for pipe prover operators 46 Appendix D Specimen meter proving certificate 48 Appendix E Summary of proving and throughput equations 49 Appendix F Current practice concerning the number of passes in a run 51

7 FOREWORD Measurement accuracy is essential in the sale, purchase and handling of petroleum products. It avoids disputes between buyer and seller and provides control over losses. Accurate measurement involves the use of standard equipment and procedures. The Petroleum Measurement Committee of the Institute of Petroleum is responsible for the production and maintenance of standards and guides covering the various aspects of static and dynamic measurement of petroleum. These are issued as separate Parts and Sections of the Institute's Petroleum Measurement Manual, which was first published in Membership of the IP working panels includes experts from the oil industry, equipment manufacturers, cargo surveyors and government authorities. Liaison is maintained where appropriate with parallel working groups of the Committee on Petroleum Measurement of the American Petroleum Institute, and is extended as necessary to embrace other organizations concerned with quantitative measurement in other countries and in other industries. Users are invited to send comments, suggestions, or details of experience with this issue to : The Secretary, Petroleum Measurement Committee, Institute of Petroleum, 61 New Cavendish Street, London W1M 8AR, United Kingdom. The Petroleum Measurement Manual is widely used by the petroleum industry and has received recognition in many countries by consumers and the authorities. In order to promote their wide adoption internationally, it is the policy to submit selected standards through the British Standards Institution to Technical Committee 28 Petroleum Products and Lubricants of the International Organization for Standardization (ISO) as potential International Standards. A full List of Parts and Sections of the Petroleum Measurement Manual (PMM) is available on request from the Institute of Petroleum. Note The IP Petroleum Measurement Manual is recommended for general adoption, but must be read and interpreted in conjunction with weights and measures, safety and other regulations in force in a particular country in which it is to be applied. Such regulatory requirements shall have precedence over corresponding clauses in the Manual. The Institute disclaims responsibility for any personal injury, or loss or damage to property howsoever caused, arising from the use or abuse of any Part or Section of the Manual. Attention is also drawn to the fact that some of the equipment mentioned in the Manual is protected by patents throughout the world. The mention of any proprietary information in this Guide does not imply its endorsement by the IP for any particular application ; neither does omission imply rejection. vii

8 ACKNOWLEDGEMENTS The following members of the IP Petroleum Measurement Committee and its Sub-Committees have been associated with this part of the Petroleum Measurement Manual. W. M. Carter K. Elderfield R. C. Gold A. T. J. Hayward H. Hepworth G. Inglis P. A. M. Jelffs F. Jordan F. Kelly R. Maurer J. E. Miller T. M. Noble P. D. O'Connell G. Paul-Clark R. J. Peters M. Pugh W. C. Pursley K. Stothard S. A. Vijay J. M. Waring Caleb Brett Laboratories Ltd FMA Group of Companies Consultant Moore, Barrett & Redwood Ltd Shell UK Expro ICE Petrochemical Engineering Ltd Moore, Barrett & Redwood Ltd HM Customs & Excise Shell Research Ltd Flow Calibration Services Ltd Consultant Brooks Instrument Institute of Petroleum Department of Energy Daniel Industries Ltd British Pipeline Agency Ltd National Engineering Laboratory Jordan Kent Metering Systems Ltd Department of Energy ICI plc viii

9 LIST OF SYMBOLS Cp, Correction factor for effect of pressure change on liquid Cp, p Cp, at pressure of prover Cp, m C p, at pressure of meter Cp, Correction factor for effect of pressure change on steel Cp, p Correction factor for expansion of prover steel due to pressure Cṭ Correction factor for thermal expansion of liquid Clip C l, at temperature of prover Ct, r C at temperature of meter C1, Correction factor for thermal expansion of steel Cts, Correction factor for thermal expansion of steel of prover Internal diameter of prover Elastic modulus of prover barrel F Meter factor K-factor of meter Nominal K-factor Kp Pressure-compensated K-factor M Mass throughput during a delivery Number of pulses collected during a delivery n Number of pulses collected during a proving pass n 12 Number of pulses collected between Detectors 1 and 2 in a bidirectional prover n 21 Number of pulses collected between Detectors 2 and 1 in a bidirectional prover p Either gauge pressure or the excess of pressure above bar (these two quantities are slightly different, but the effect of the difference between them on the volume of a liquid is negligible) q One-pulse volume q Nominal one-pulse volume R Pulse interpolation divisor Standard deviation t Temperature Observed volume, i.e. volume of oil at actual pressure and temperature of meter Base volume of prover at reference conditions, usually 15 C and 0 bar gauge Corrected volume of prover at its actual pressure and temperature Volume indicated by the meter V, Standard volume, i.e. volume of oil reduced to standard reference conditions V 12 Volume between Detectors 1 and 2 in a bidirectional prover V21 Volume between Detectors 2 and 1 in a bidirectional prover w Wall thickness of prover a Linear coefficient of thermal expansion (used here of steel) fi Compressibility factor (used here of liquids) p Density at actual pressure and temperature of meter p, Standard density, i.e. density at standard reference conditions ix

10 GLOSSARY OF TERMS Base Volume-The calibrated volume of a pipe prover, proving tank or volumetric measure at standard conditions of temperature and pressure. Batch-The set of consecutive proving runs that is deemed to be necessary to derive both (1) a mean value of meter factor or K-factor suitable for subsequent use and (2) a range of individual values that can be used as an indication of the repeatability of the measurements. Block-and-Bleed Valve-A high-integrity valve with double seals and provision for detecting leakage past either seal. (Also known as a Double-Block-and-Bleed Valve.) Calibrated Volume-The volume at a stated temperature and pressure between the detectors in a pipe prover, or the volume of a proving tank between specified `empty' and `full' levels. The calibrated volume of a bidirectional prover is the sum of the two volumes swept out between detectors during a round trip. Cavitation-The release of vapour and/or dissolved gas from a flowing liquid caused by a sudden drop in pressure, for example as a result of local high velocity at a constriction, or at the trailing edges of rotating meter blades. Cyclic Distortion-Any periodic variation in the pulse frequency generated by a meter. This may be caused by mechanical asymmetry within the meter or by the addition of accessories such as temperature compensators. (See also Intra-rotational Linearity.) Detectors-Sensing devices set at each end of the calibrated volume of a pipe prover, and which are directly or indirectly actuated by the displacer. Discrimination- The ability of a measuring instrument to respond to small changes in the value of the input. Displacement Meter A meter which operates by dividing the flowing liquid into discrete quantities and totalizing them. Displacer A generic term which can be applied either to a sphere or to a piston when it is used to sweep out the calibrated volume of a pipe prover. Flashing-A serious form of cavitation occurring when the local pressure at a point within the liquid contained in a closed pipe falls below the saturated vapour pressure of the liquid at the operating temperature. Flying-Start-and-Stop-A proving technique in which the flow through the meter and the proving device continues at the same flowrate throughout the proving process. (Compare Standing-Start-and-Stop.) Four-way Valve-A high-integrity flow-reversing valve used with most bidirectional provers. Gating-- The initiation and cessation of pulse totalization in a counter, e.g. by prover detectors. Intra-rotational Linearity--A quantitative measure of the degree of regularity of spacing between the pulses during one revolution of a meter, generally expressed as the range of variation of pulse spacing about the mean, at the 95 per cent confidence level. K-factor-The number of pulses generated by a meter while a unit of volume is passing through. Launch/Receive Chamber An enlarged section at the ends of the pipe prover in which the displacer rests between proving passes. Meter Factor The ratio of the actual volume of liquid passed through a meter to the volume indicated by it. Oval-wheel Meter A displacement meter in which the displaced volumes are segregated by enmeshing oval gears. Pass-A single movement of a displacer between the two detectors. Primary Measure-A portable volumetric standard which is directly calibrated against national standards. x

11 GLOSSARY OF TERMS xi Proving Tank A volumetric standard usually consisting of a cylindrical central portion with conical top and bottom, and a cylindrical neck graduated either in units of volume or in steps corresponding to fractions of a percentage of the tank volume. Pulse Density A qualitative expression used to describe the number of pulses generated by a meter for a given volume of throughput. Pulse Interpolation-An electronic technique for enhancing the resolution of a gated pulse count. Pulse Interpolation Divisor The ratio of the enhanced pulse frequency to the frequency of the pulses generated by the meter, used in the phase-locked-loop system of pulse interpolation. Resolution-A quantitative expression of the ability of an indicating device to distinguish meaningfully between closely adjacent values of the quantity indicated. Run-The set of consecutive passes that is, in any particular case, deemed to be necessary to derive a single value of meter factor or K-factor suitable for reporting. (The meter factors and K-factors derived from individual passes within a multi-pass run are not reportable.) Sour liquid-a liquid, particularly crude oil, containing high proportions of sulphur compounds. Spade-A plate inserted between pipe flanges for the purpose of isolating a section of a pipework system. Standard Reference Conditions-Conditions of temperature and pressure to which measurements are referred for standardization. In the United Kingdom, these are 15 C and bar. Standing-Start-and-Stop-A proving technique in which the flow through the meter and the proving device is started at the beginning and stopped at the end of the proving process. (Compare Flying-Startand-Stop.) Temperature Compensator-A mechanism attached to a meter to correct for the effect of temperature on the measured volume, or an electronic device serving the same purpose. Thermowell-A protective metal pocket which protrudes through the wall of a pipe or tank and holds the sensing element of a thermometer. Totalizer A mechanical or electronic device for integrating and displaying the volumetric throughput of a flowmeter. Traceability The property of a measuring instrument enabling the measurements made by it to be related to some primary standard, generally a national or international standard, through an unbroken chain of comparative measurements involving secondary standards, tertiary standards, etc. Turbine Meter A meter which provides a pulsed output at a frequency proportional to the angular velocity of a bladed rotor (which occupies virtually the full bore of the pipe), which is itself proportional to the volumetric flowrate of the liquid passing through the meter. Volume Correction Factor-A factor, dependent upon the oil density, which corrects oil volumes to a standard reference temperature. ( Note : For crude oil and most petroleum products such factors can be obtained from the API-ASTM-IP Petroleum Measurement Tables, Tables 54A and 54B, or the corresponding computer sub-routines.) Vortex-Shedding Meter A pulsed output flowmeter, having an internal bluff body, designed to generate and detect vortices which are produced at a rate proportional to the flowrate passing through it. Water Draw The term applied to the technique of calibrating a pipe prover by the displacement of liquid, normally water, from the prover into a volumetric or gravimetric tank. Wetted Area-A term commonly applied to that portion of the internal surface of a volumetric tank, which is in contact with the liquid at some stage during a proving operation.

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13 1 INTRODUCTION In the petroleum industry the term `proving' is used to refer to the calibration of flowmeters for crude oil and petroleum products. All measuring instruments that have to meet a reasonable standard of accuracy need calibration--that is to say, a test or a series of tests has to be performed in which readings obtained from the instrument are compared with independent measurements of high accuracy. Petroleum meters are no exception : nearly all those used for the purpose of selling or assessing taxes need proving at intervals, and when there is a large amount of money at stake they are likely to be proved frequently. The most usual way to prove a flowmeter is to pass a quantity of liquid through it into an accurate device for measuring volume, known as a prover. With very small meters the proving device may be nothing more than a small container whose volume is known accurately. There are, for instance, standard measuring vessels that can be used to prove the meters incorporated in gasoline pumps at roadside filling stations. If the pump dial registers 10.2 litres when enough gasoline has been delivered to fill a ten-litre vessel, it is evident that the meter is over-reading by 2 per cent. In a large metering installation, where a single meter may be passing tens or hundreds of litres per second, the situation is very much more complicated. The meters themselves generally do not have dials graduated in units of volume like a gasoline pump, but instead they may be designed to emit a series of electrical pulses. With meters of this type the purpose of proving is to determine the relationship between the number of pulses emitted and the volume passed through the metera relationship which varies from meter to meter and depends upon flowrate, viscosity and temperature. Another difficulty is that the flow through these large meters usually cannot be stopped and started at will. Consequently, both the meters and their prover have to be capable of being read simultaneously and, on the fly', that is, while liquid is passing through them at full flowrate. The position is complicated still further by the effect of thermal expansion and compressibility on the oil volume, and that of thermal expansion and elastic distortion under pressure on the steel bodies of the prover and the meter. This Guide is concerned with only one kind of prover, the pipe prover, which is used widely where large meters for crude oil and petroleum products have to be proved to the highest possible standard of accuracy. In principle a pipe prover is simply a length of pipe whose internal volume has been determined very accurately, and having a well-fitted piston (or a tightly fitting sphere acting like a piston) inside it, so that the volume swept out by the piston or sphere can be compared with the meter readout while a steady flow of liquid is passing from the meter into the prover. In practice, however, many accessories have to be added to the simple pipe-and-piston arrangement to produce a prover that will work effectively and accurately. In this Guide, Chapter 2 deals with the principles underlying the subject, and explains the various factors which are used to express the results of proving a meter. If a newcomer to the subject should find parts of Chapter 2 difficult, this will only be because the subject matter is unfamiliar. His best course will be to read Chapter 2 fairly quickly, then pass onto Chapters 3, 4 and 5, and afterwards return to Chapter 2 for a second reading before attempting Chapters 6 and 7. Chapter 3 is concerned with hardware. It briefly describes the two main types of meter that have to be proved, and then describes in more detail the most usual types of pipe prover. Because it can be dangerous to use any equipment carelessly, in the petroleum industry there are official safety regulations which operators are obliged to study and follow, and Chapter 4 sets out some of the most widely adopted safety rules affecting pipe provers. Then Chapter 5 explains how to operate a pipe prover. Finally, Chapters 6 and 7 deal with the processing of data Chapter 6 with proving data and Chapter 7 with the calculation of throughput. Additional information is provided in a series of appendices. The design, installation and calibration of pipe provers are covered in the PMM, Part X, Meter Proving, Section 3. 1