INSTITUTE OF PETROLEUM

PETROLEUM MEASUREMENT MANUAL

PART IIIManual Measurement

of Level in Tanks

SECTION 1

NON-ELECTRICAL METHODS

INSTITUTE OF PETROLEUM

PETROLEUM MEASUREMENT MANUAL

PART IIIManual Measurement

of Level in Tanks

SECTION 1

NON-ELECTRICAL METHODS

March 1998

Published byThe Institute of Petroleum, London

A charitable company limited by guarantee

Copyright © 1998 by The Institute of Petroleum, London :A charitable company limited by guarantee . Registered No . 135273, England

All rights reserved.

No part of this book may be reproduced by any means, or transmitted or translated intoa machine language without the written permission of the publisher .

ISBN 0 85293 197 2

Printed by The Institute of Petroleum

CONTENTS

Page

Foreword ix

Acknowledgements x

1 Introduction and Scope 1

2 Definitions 3

3 Safety Precautions 73.1 General Safety Precautions 73.2 Static Electricity 73.3 Use of Light Alloy Equipment 83.4 Precautions During Gauging Operations 9

3 .4.1

Special Safety Precautions 93.5 Toxicological Effects 9

4 Procedures to Ensure Gauging Accuracy 104.1 Tank Fittings 10

4.1 .1

Tank Reference Height 104.1 .2

Alternative Gauging Access Points 114.1 .3

Gauging Access Points and Still-Wells 114.2. Gauging Equipment 11

4.2.1

Dip-Tapes 114.2.2

Dip- and Ullage-Rods 114.2.3

Water-fording Pastes and Product-fording Pastes 114.3 Gauging Procedures 12

4.3 .1

Supervision and Reporting 124.3.2 Checks Before Measuring 12

5 Equipment Specifications 135.1 General 13

5 .1 .1

Manufacture and Care of Equipment 135.1 .2

Accuracy of Equipment 145.2 Dip-Tapes 14

5.2.1

General 145.2.2

Material of Construction 145.2.3

Construction 14

v

Contents cont . ..

1414141516

5.3 Dip-Weights 16

5.3 .1

General 16

5.3.2 Material of Construction 16

5.3.3

Design 16

5 .3.4

Marking 16

5.4 Ullage-Rules 16

5.4.1

General 16

5.4.2

Marking 16

5.5 Water-Finding Rules 17

5.5.1

General 17

5 .5.2 Construction 17

5 .5.3

Marking 17

5.6 Product-fording Paste 17

5.6.1

General 17

5.6.2

Selection of Paste 17

5.7 Water-fording Pastes 18

5 .7.1

General 18

5 .7.2

Storage 185 .7.3

Selection of Paste 185.7.4

Problems in Use 185.8 Dip-Rods and Ullage-Rods 19

5.8.1

General 195.8.2

Description and General Specification 195.8.3

Dip-Rods 195 .8.4

Ullage-Rods 195 .8.5

Scale Units 205 .8.6

Scale Marks 205.8 .7

Figures 205.8.8 Datum 205.8.9

Calibration and Tolerance 205.8.10 Identification Marks 21

5.9 Rotary and Slip-Tube Gauging Devices 215 .9.1

General 215.9.2 Rotary-Tube Gauges 215.9.3

Slip-Tube Gauges 215.9.4

Specification of Gauges 225.10 Ullage Plate 225.11 Vapour-Lock Valves 22

6 Gauging Methods 24

6.1 General 246.2 Dipping 25

6.2.1

Dip-Tape and Weight Method 256.2.2 Dip-Rod Method 26

6.3 Ullaging 266.3.1 Dip-Tape and Weight Method 27

vi

5.2.4 Recommended Tape Dimensions5.2.5 Graduations5 .2.6 Marking5 .2.7 Accuracy5.2.8 Winding Frames

Contents cont . . .

272829

6.4 Measurement of Free Water 296.4.1

Introduction 296.4.2 By Water-finding Rule or Dip-Weight and Paste 296.4.3

By Dip-Rod and Paste 306.4.4

Other Methods 306.5 Measurement by Rotary and Slip-Tube Gauges 30

6.5.1

Safety 306 .5.2

Use for Fiscal or Custody Transfer Measurements 306.5.3 Rotary-Tube Gauge 306.5.4

Slip-Tube Gauge 306.6 Reference Gauging Measurements for Calibration

of Automatic Tank Gauges 316.6.1

General 316.6.2 Reference Gauging Measurements 316.6.3

Reference Measuring Instruments 316.6.4

Reference Gauging Procedures 316.6.5

Allowance for Thermal Expansion Effects 32

7 Gauging Procedures For Various Types of Tank 33

7.1 General 337.2 Non-Pressure Shore Tanks 33

7.2.1

Vertical Fixed Roof Tanks 337.2.2

Horizontal Cylindrical Tanks 337.3 Floating Roof Tanks 34

7.3.1

General 347.3.2

Gauging Procedure 347.4 Vapour-Tight Tanks 347.5 Pressurised Tanks 357.6 Ship and Barge Tanks 35

7.6.1

General 357.6.2

Ship tanks 357.6.3

Barge Tanks 367.7 Road and Rail Vehicle Tanks 36

3636373838

8 Reporting Gauging Measurements 40

9 Sources of Error in Manual Gauging ; Applying Corrections 41

9.1 General Sources 419.2 Level Measurement Errors 42

9.2.1

Accuracy of Gauging Equipment 429.2.2 Movement of the Gauging Reference Point 42

vii

6.3 .2 Dip-Tape and Ullage-Rule Method6.3.3 Ullage-Rod Method6.3 .4 Other Methods

7.7.1 General7.7.2 Road Vehicle Tanks7.7.3 Rail Vehicle Tanks7.7.4 Pressure-Type Tanks for High Vapour Pressure Liquids

7.8 Caverns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Contents cont.. .

43434343

9.3 Volume Measurement Errors and Corrections 449.3.1 Bottom Movement 449.3 .2

Tank Calibration Uncertainty 449.3.3

Temperature Corrections 449.3.4

Floating Roof Corrections 449.3.5

Encrustation and Deposits 459.3.6

Oil in Pipelines 45

Annex A

Temperature Effects in Manual Gauging 47

Annex B

Figures 51

1

Gauging Terminology2

Dip-Tape and Dip-Weight3

Winding Frame for Dip-Tape4

Standard and Heavy Dip-Weights5

Details of Attachment Points6

Ullage-Rule7

Water-Finding Rule8

Dip-Rod9

Ullage-Rod

10

Ullage Plate11

Vapour-Lock Valve12

Meniscus Effects on Immersed Dip-Tape

viii

9.2 .3 Factors Affecting the Dipping Datum-Plate or Datum-Point9.2.4 Incorrect Still-Well Design9.2.5 Surface Disturbance9.2.6 Meniscus Effects

Measurement accuracy is essential for the sale, purchase and handling of petroleum products . It reduces thelikelihood of disputes between buyer and seller and facilitates control of losses . Accurate measurement demands theuse of standard equipment and procedures .

The Petroleum Measurement Committee of the Institute of Petroleum is responsible for the production andmaintenance 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 firstpublished in 1952 .

Membership of the IP working panels is made up of experts from the oil industry, equipment manufacturers,cargo surveyors and government authorities. Liaison is maintained with parallel working groups of the Committeeon Petroleum Measurement of the American Petroleum Institute, and is extended as necessary to embrace otherorganizations 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 Technical Manager, Petroleum MeasurementInstitute of Petroleum61 New Cavendish StreetLondon W I M 8ARUnited Kingdom

The Petroleum Measurement Manual is widely used by the petroleum industry and has received recognition in manycountries by consumers and the authorities . In order to promote their wide adoption internationally, it is the policyto submit selected standards through the British Standards Institution to Technical Committee TC 28 - PetroleumProducts and Lubricants - of the International Organisation for Standardization (ISO/TC 28) as potentialInternational Standards .

A full list of the Parts and Sections of the Petroleum Measurement Manual (PMM) is available on request fromthe Institute of Petroleum .

Note

The IP Petroleum Measurement Manual is recommended for general adoption but should be read and interpretedin conjunction with weights and measures, safety and other regulations in force in a particular country in which itis to be applied. Such regulatory requirements should have precedence over the corresponding clauses in the Manualexcept where the requirements of the Manual are more rigorous, when its use is recommended . The Institutedisclaims responsibility for any personal injury or loss or damage to property, howsoever caused, arising from theuse or abuse of any Part or Section of the Manual .

FOREWORD

ix

ACKNOWLEDGEMENTS

The following members of the IP Petroleum Measurement Committee and its Sub-Committee PM-A-2, TankGauging (Level and Temperature), have been associated with the production of this Part of the PetroleumMeasurement Manual:

R F AungierT T BagleyB T BeesonI A M ClarkeA A CodlingN C I de SponB DeerD GarwoodW N GunterA D HallR P HillsG C HughesICMKayR C JefferyF KellyP LeighM J MayJ NewellA ThomasA J ThorogoodJ A WilliamsJ M Wood

Motherwell Control Systems LtdSGS Redwood LtdMMC (Europe) LtdInchcape Testing Services Caleb BrettEsso Petroleum Company LtdSaybolt InternationalConsilium Marine (UK) LtdInspectorate Watson Gray LtdInspectorate Watson Gray LtdWhessoe Systems and ControlsStanley ToolsShell International Petroleum Co LtdEsso Petroleum Company LtdSGS Inspection Services LtdShell Research LtdCGF AutomationEnergy Marine (International) LtdSGS Redwood LtdBP InternationalBP Research & EngineeringWhessoe Varec LtdInstitute of Petroleum

Company affiliations are those that applied at the time the document was being drafted .Figures 2, 5, 6, 7, 8 and 10 are reproduced from ISO/DIS 4512 with permission. Complete editions of the

standard can be obtained by post from BSI Customer Services, 389 Chiswick High Road, London W4 4AL orthrough national standards bodies .

x

Although automatic level gauging systems are inwidespread use in the petroleum industry for themeasurement of petroleum liquids in storage tanks,manual tank gauging is still widely applied as thenormal technique for level measurement in non-pressurized tanks . It is highly accurate provided thecorrect procedures are carefully observed, albeitrelatively labour intensive and time-consuming .

Manual tank gauging is the method that shall beapplied for the calibration and periodic checking ofautomatic level gauging (ALG) systems . It is alsonormally selected as the reference (referee) method forthe measurement of the level of liquid in a tank shoulda dispute arise between the parties in a commercialtransaction .

Part III, Section 1 of the IP PetroleumMeasurement Manual provides detailed guidance on theprinciples and practice of manual gauging' ) forpetroleum liquids ranging from crude oil, LPG andrefined and semi-refined products, to residual fuel oiland bitumen, stored in either fixed roof or floating rooftanks, or transported in marine, road or rail tanks .

It is limited to non-electrical techniques involving

1

INTRODUCTIONAND SCOPE

dip-tapes, dip-rods and ullage-rods, and associatedequipment such as dip-weights, water-finding rules andpaste etc . Specifications are provided for relevant itemsof equipment. The forthcoming Part III, Section 2 ofthis Manual will provide recommendations forequivalent equipment and procedures whenmeasurements are made with portable electronicgauging devices .

Procedures are described for the measurement ofliquid level within pressurised tanks fitted with rotary-tube and slip-tube level gauging equipment . Proceduresare also described for level measurement in vapour-tightstorage tanks operating under low positive pressure andequipped with vapour-lock valves (closed and restrictedgauging) .

Guidance is provided on factors which can affectthe accuracy of volumetric quantities derived from levelmeasurements and on procedures that are recommendedto minimise the uncertainty associated with themeasured quantities .

This part of the Petroleum Measurement Manualrefers to existing methods of measurement and the typeof equipment presently in use. It is, however, not

' The scope excludes the measurement of temperature or the sampling of tank contents for quality analysis, which are alsogauging operations. For temperature measurement by manual methods, refer to Petroleum Measurement Manual, Part IV :Temperature Measurement, or ISO 4268 : Methodfor manual measurement of temperature ofpetroleum and liquid petroleumproducts ; for automatic methods refer to Part V: Automatic Tank Gauging and to ISO/DIS 4266-1 (in preparation) . For sampling,refer to PMM, Part VI, Sampling : Section 1 : Manual Methods, or Section 2: Guide to Automatic Sampling of Liquids fromPipelines or to their ISO equivalents, ISO 3170 and ISO 3171 .

1

MANUAL MEASUREMENT OF LEVEL IN TANKS - NON-ELECTRICAL METHODS

intended that it should exclude new equipment not yet

equivalent levels of accuracy.developed for commercial use, provided that the

Note: The verb `shall' is used in this Standard toaccuracy of such equipment is within the maximum

indicate that compliance with an action is required topermissible error tolerances specified herein, and that

achieve the highest standards of accuracy that manualthe procedures for its use are capable of achieving

gauging can provide .

2