Hydrates in LPG Cargoes - A Technological Review

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Number of Pages:
Published Date:
January 2008
Book Height:
300 mm
Book Width:
210 mm
0.3 kg
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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 examines the presence of water in LPG cargoes, the formation of hydrates and methods for their elimination.

Hydrate formation in cargo systems of refrigerated LPG carriers can damage pumps and other machinery and obstruct cargo handling. This publication, based on a review conducted for SIGTTO by Mr R C Gray of the Technology Department of British Shipbuilders, deals specifically with hydrates forming in commercially dry refrigerated LPG. It identifies the ways in which water can enter and remain within the cargo, leading to the formation of hydrates. It examines different methods to prevent ice and hydrate formation and outlines procedures for cargo tank sampling. The Appendices provide practical information on the propane-water system, use of LPG freeze valves and the addition of methanol as a treatment method.

Section 1 – Introduction of Water into LPG

1.1 At Production Plants

1.2 During Storage

1.3 On Board Ship

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

3.5 Hydrate Control Without Methanol Injection

Section 4 – Shipboard Test Methods

4.1 Sampling Methods

4.2 Water Content

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

Appendix I – Explanation of the Propane-Water System

1. Why LPG Boil-off Vapour Contains More Water than the Liquid

2. The Vapour, Liquid and Solid Phases of the Propane-Water System during Reliquefaction

2(i) Direct cascade reliquefaction

2(ii) Direct two stage reliquefaction

3. Formation of Hydrates during Vaporisation of Propane Liquid Contaminated with Water

4. Ethane, Iso- and N-Butane

Appendix II – LPG Freeze Valve

Appendix III – 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 Society of International Gas Tanker and Terminal Operators (SIGTTO) is an international body established for the exchange of technical information and experience, between members of the industry, to enhance the safety and operational reliability of gas tankers and terminals. Learn more: https://www.sigtto.org/about-us/

Number of Pages:
Published Date:
January 2008
Binding Format:
Book Height:
300 mm
Book Width:
210 mm
0.3 kg