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Published Date

July 2008


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Hydrates in LPG Cargoes

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This publication is a technical review of the processes and procedures commonly found in production, storage and transportation of LPG cargoes. It shows how it is possible for water to enter or remain in the cargo, leading to hydrate formation. The guide’s recommendations are aimed at the elimination of hydrate formation.

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It became apparent that hydrates were occasionally and unpredictably appearing in the cargo systems of refrigerated LPG carriers. This resulted in damage to pumps and other plant machinery being damaged, as well as interference to cargo handling. While the procedures used to produce LPG and its handling on ship and shore includes minimising the water content in the product, little

information has been gathered that specifically deals with hydrates forming in commercially dry refrigerated LPG. As a result, a review of the processes and procedures commonly used in production, storage and transportation was required to identify how water can enter or remain within the cargo.

 

The review was conducted by Mr R C Gray of the Technology Department of British Shipbuilders. His findings and recommendations for eliminating hydrates can be found in this publication.

 

This document provides a unique collation of data on this subject, some deductions and imaginative recommendations based on the data. The book was originally published for the benefit of SIGTTO members but has since been made generally available due to its usefulness in the liquefied gas industry.

SECTION 1 - INTRODUCTION OF WATER INTO LPG

1.1 AT PRODUCTION PLANTS

1.2 DURING STORAGE

1.3 ON BOARD SHIP

1.3.1 INERTING/VENTILATION

1.3.2 MOISTURE IN RUST ON STEEL SURFACES

1.4 HYDROSTATIC TESTING

SECTION 2 - THE FORMATION OF HYDRATES

2.1 GENERAL

2.2 PROPANE HYDRATES

2.3 BUTANE HYDRATES

2.4 PROCESSES IN WHICH HYDRATE FORMATION CAN BE EXPECTED

2.5 MODIFICATION OF HYDRATE FORMATION BY METHANOL ADDITION

2.6 SOLUBILITY OF WATER IN LPG LIQUIDS AND THEIR VAPOURS

SECTION 3 - AVOIDANCE OF ICE OR HYDRATE FORMATION

3.1 GENERAL

3.2 REDUCTION IN WATER DEWPOINT ON SHIPS

3.3 SOLID DESICCANTS

3.4 HYDRATE FORMATION TEMPERATURE DEPRESSANTS

3.4.1 COMMISSIONING PROCEDURES

3.4.2 RELIQUEFACTION PLANTS

3.4.3 INJECTION QUANTITIES

3.4.4 METHANOL CONTAMINATION OF LPG

3.5 HYDRATE CONTROL WITHOUT METHANOL INJECTION

3.5.1 DEFREEZING OF LPG CONDENSERS

3.5.2 PROMOTION AND REMOVAL OF HYDRATES

SECTION 4 - SHIPBOARD TEST METHODS

4.1 SAMPLING METHODS

4.2 WATER CONTENT

4.2.1 DEWPOINT METERS

4.2.2 VALVE FREEZE TEST METHOD

4.3 METHANOL DETECTION

SECTION 5 - HAZARDS ASSOCIATED WITH ICE/HYDRATES

5.1 RELIQUEFACTION UNITS AND CARGO PIPING SYSTEMS

5.2 CARGO PUMPS

SECTION 6 - SUMMARY AND RECOMMENDATIONS

6.1 FULLY PRESSURISED LPG

6.2 FULLY AND SEMI-REFRIGERATED SHORE SYSTEMS

6.3 FORMATION OF HYDRATES ON SHIPS

6.4 SHIPBOARD COUNTER MEASURES

6.5 SHIPBOARD TEST METHODS

6.6 VALIDATION OF DATA ASSUMPTIONS

REFERENCES

APPENDIX 1 EXPLANATION OF THE PROPANE-WATER SYSTEM

1 WHY LPG BOIL-OFF VAPOUR CONTAINS MORE WATER THAN LIQUID

2 THE VAPOUR, LIQUID AND SOLID PHASES OF THE PROPANE-WATER SYSTEM DURING RELIQUEFACTION

2i DIRECT CASCADE RELIQUEFACTION

2ii DIRECT TWO STAGE RELIQUEFACTION

3 FORMATION OF HYDRATES DURING VAPORISATION OF PROPANE LIQUID CONTAMINATED WITH WATER

4 ETHANE, ISO-AND N-BUTANE

APPENDIX 2 LPG FREEZE VALVE

APPENDIX 3 ESTIMATION OF METHANOL TO ADD TO PREVENT ICE/HYDRATE FORMATION

A. METHANOL TO ADD TO A TANK CONTAINING PROPANE OR N-BUTANE SATURATED WITH WATER AT 38°C TO PREVENT ICE/HYDRATE FORMATION ON COOLING DOWN TANK CONTENTS FROM SVP AT 38°C TO ATMOSPHERIC PRESSURE

B. METHANOL TO ADD TO THE RELIQUEFACTION CONDENSER IN PROPANE AND N-BUTANE DUTY TO PREVENT HYDRATE FORMATION IN THE CONDENSER OR IN THE CONDENSATE RETURN

The purpose of SIGTTO is to promote shipping and terminal operations for liquefied gases which are safe, environmentally responsible and reliable. To fulfil this mission it will:

  • Proactively develop best operating practices and guideline

  • Sustain a learning environment by sharing lessons learned

  • Promote training and development of all within the industry

  • Foster mutually beneficial relationships with regulatory authorities and other stakeholders

  • Conduct its business with professionalism and integrity

Title: Hydrates in LPG Cargoes
Subtitle: A Technological Review
Number of Volumes: 1
Edition: First
Number of Pages: 50
Product Code: 4400W032
ISBN: ISBN 13: 978-1-905331-27-7 (9781905331277), ISBN 10: 1-905331-27-4 (1905331274)
Published Date: July 2008
Binding Format: Paperback
Book Height: 300 mm
Book Width: 210 mm
Book Spine: 5 mm
Weight: 0.30 kg

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