HYDRATES IN LPG CARGOES A Technological Review of the Presence of Water in LPG Cargoes, the Consequent Formation of Hydrates and Proposals for their Elimination Prepared by Mr. R. C. Gray of the Technology Department of British Shipbuilders for The Society of International Gas Tanker and Terminal Operators Ltd Comments, experience and other data relevant to this Review should be addressed to The Society of International Gas Tanker and Terminal Operators Ltd., London Liaison Office, Staple Hall, 87/90, Houndsditch, London, EC3A 7AX Tele. 01-621-1422 Telex 894525 SIGTTO - G First Edition 2008 Printed & bound in Great Britain by Bell & Bain Ltd. Glasgow Published by Witherby Seamanship International 4 Dunlop Square, Deans Estate Livingston, EH54 8SB United Kingdom Tel No: +44(0)1506 463 227 Fax No: +44(0)1506 468 99 Email: info@emailws.com www.witherbyseamanship.com Copyright of SIGTTO, Bermuda ISBN 978 1 905331 27 7 Notice of Terms of Use While the information given has been gathered from what is believed to be the best sources available and the deductions made and recommendations put forward are considered to be soundly based, the Review is intended purely as helpful guidance and as a stimulation to the development of more data and experience on the subject. No responsibility is accepted by the Society of International Gas Tanker and Terminal Operators Ltd or by any person, firm, corporation or organisation who or which has been in any way concerned with the compilation, publication, supply or sale of this Review, for the accuracy of any information or soundness of any advice given herein or for any omission herefrom or for any consequence whatsoever resulting directly or indirectly from the adoption of the guidance contained herein. III CONTENTS Page No. PREFACE V EXECUTIVE SUMMARY VI SECTION 1 - INTRODUCTION OF WATER INTO LPG 1 1.1 AT PRODUCTION PLANTS 3 1.2 DURING STORAGE 3 1.3 ON BOARD SHIP 3 1.3.1 Inerting/Ventilation 3 1.3.2 Moisture in rust on steel surfaces 3 1.3.3 Leakage in water cooled condensers 4 1.4 HYDROSTATIC TESTING 4 SECTION 2 - THE FORMATION OF HYDRATES 5 2.1 GENERAL 7 2.2 PROPANE HYDRATES 7 2.3 BUTANE HYDRATES 7 2.4 PROCESSES IN WHICH HYDRATE FORMATION CAN BE EXPECTED 7 2.5 MODIFICATION OF HYDRATE FORMATION BY METHANOL ADDITION 8 2.6 SOLUBILITY OF WATER IN LPG LIQUIDS AND THEIR VAPOURS 9 SECTION 3 - AVOIDANCE OF ICE OR HYDRATE FORMATION 11 3.1 GENERAL 13 3.2 REDUCTION IN WATER DEWPOINT ON SHIPS 13 3.3 SOLID DESICCANTS 13 3.4 HYDRATE FORMATION TEMPERATURE DEPRESSANTS 13 3.4.1 Commissioning Procedures 13 3.4.2 Reliquefaction Plants 14 3.4.3 Injection Quantities 14 3.4.3 Methanol contamination of LPG 14 3.5 HYDRATE CONTROL WITHOUT METHANOL INJECTION 16 3.5.1 Defreezing of LPG condensers 16 3.5.2 Promotion and removal of hydrates 16 SECTION 4 - SHIPBOARD TEST METHODS 17 4.1 SAMPLING METHODS 19 4.2 WATER CONTENT 19 4.2.1 Dewpoint Meters 19 4.2.2 Valve Freeze Test Method 19 4.3 METHAN L DETECTION 20 SECTION 5 - HAZARDS ASSOCIATED WITH ICE/HYDRATES 21 5.1 RELIQUEFACTION UNITS AND CARGO PIPING SYSTEMS 23 5.2 CARGO PUMPS 23 SECTION 6 - SUMMARY AND RECOMMENDATIONS 25 6.1 FULLY PRESSURISED LPG 27 6.2 FULLY AND SEMI-REFRIGERATED SHORE SYSTEMS 27 6.3 FORMATION OF HYDRATES ON SHIPS 27 6.4 SHIPBOARD COUNTER MEASURES 28 6.5 SHIPBOARD TEST METHODS 28 6.6 VALIDATION OF DATA ASSUMPTIONS 28 Page No. REFERENCES 29 APPENDIX I - EXPLANATION OF THE PROPANE-WATER SYSTEM 31 1. Why LPG boil-off vapour contains more water than the liquid 33 2. The vapour, liquid and solid phases of the propane-water system during 35 reliquefaction 2(i) Direct cascade reliquefaction 35 2(ii) Direct two stage reliquefaction 35 3. Formation of hydrates during vaporisation of propane liquid contaminated 35 with water 4. Ethane, iso- and n-butane 36 APPENDIX II - LPG FREEZE VALVE 37 APPENDIX III - ESTIMATION OF METHANOL TO ADD TO PREVENT ICE/HYDRATE 45 FORMATION A. Methanol to add to a tank containing propane or n-butane saturated with 47 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 50 duty to prevent hydrate formation in the condenser or in the condensate return IV A Technological Review Of The Presence Of Water In LPG Cargoes, The Consequent Formation Of Hydrates And Proposals For Their Elimination PREFACE The Society of International Gas Tanker and Terminal Operators Ltd (SIGTTO) is dedicated to the promotion of the mutual interests of its Members in all matters relating to safety and reliability in the marine transportation and terminalling of liquefied gases. The Society, through its Members, was made aware of the occasional and seemingly unpredictable appearance of hydrates in the cargo systems of refrigerated LPG carriers resulting, at times, in damage to pumps and other plant or, at least, in interference with expeditious cargo handling. While the production of LPG and its handling both on ship and shore include procedures aimed at minimising the water content of the product, there is little collated information which can be found bearing specifically upon the formation of hydrates in commercially dry refrigerated LPG. The Society, on behalf of its Members, therefore required a review of the processes and procedures commonly practised in production, storage and transportation of LPG with a view to identifying by what routes water can enter or remain in cargo and lead to hydrate formation. This Review was conducted for SIGTTO by Mr. R.C. Gray of the Technology Department of British Shipbuilders and its findings together with recommendations aimed at eliminating hydrate formation are reported in the following pages. The Society is indebted to British Shipbuilders and to Mr. R.C. Gray for the comprehensive technological nature of the Review which, it is believed, provides a unique collation of established data on the subject and some sound deductions and imaginative recommendations based upon these data. Produced primarily for the benefit of the SIGTTO Membership, the Review, because of its envisaged usefulness to the liquefied gas industry, is being made generally available. In seeking to provide practical quantitative guidance in the control of hydrate formation in the low temperature processes of refrigerated LPG, it has been necessary to make various assumptions and extrapolations outside published experimental data which are presently largely only available for the ambient temperature range. Operational experience or experimental data relevant to these extrapolations or to any aspect of this subject would be welcomed by the Society with a view to up- dating or expanding the Review as may become desirable. V EXECUTIVE SUMMARY LPG is peculiar in that, certainly at temperatures above 0°C, the boil-off vapour will contain a higher proportion of water by weight than the liquid with which it is in equilibrium. Semi-refrigerated propane at + 6°C, for example, containing 40 ppm water by weight in solution, will produce boil-off vapour containing 580 ppm water w/w. The solubility of water in the liquid phase of LPG decreases markedly with temperature. All refrigerated LPG delivered by a terminal to a ship, while it may be free from ice or hydrates, will contain a small amount of water dissolved in the cargo. While the dissolved water content in the cargo liquid will be low (a few ppm at most), the boil-off vapour will contain a substantially higher proportion of ppm water vapour by weight than does the liquid. In view of the very much higher vapour pressure of LPG compared with the vapour pressure of water, this may seem strange but indications are strongly that this is so. As the hot boil-off gases containing this higher water content are condensed, the majority of the water vapour in the cargo vapour will separate out as particles of solid hydrate. The LPG condensate at the typical condenser temperatures and pressures will contain the remaining concentration of water from the boil-off vapour in solution. However, as the condensate is flashed off via the regulating valve to tank pressure, the resultant further cooling of the condensate will render it unable to hold this higher water concentration as dissolved water, free water will separate out and this free water will form hydrates. Whilst there is evidence that fully refrigerated propane contains less than 5 ppm water in solution, little information is available concerning the water content in boil-off vapours at temperatures below 0°C. Despite this lack of supportive data, however, there is reason to believe that the boil-off vapour at these temperatures similarly will contain a higher ppm w/w concentration than that in the liquid phase. Propane may form hydrates at temperatures below + 6°C and pressures up to 35 bar and commercial butanes at temperatures below + 3°C. The data available on the water solubilities in liquid and vapour above 0°C lead to a suggested mechanism for the formation of hydrates in vapour recovery systems operating in this temperature range. Operational experience has shown however that such formations can and do occur in vapour recovery systems associated with fully refrigerated propane and despite the present lack of water solubility data available at such temperature, it is assumed that a similar mechanism of hydrate formation applies. Section 1 of this Study describes how water may be introduced into LPG cargoes during production, storage and transportation. Section 2 addresses the formation of hydrates. It describes how, once hydrate formation is initiated, there is a significant “seeding’’ effect which can block valves and pipelines and gives detailed information on the water content in LPG and vapour. Section 3 proposes how hydrate formation may be avoided or, at least, controlled. Section 4 outlines various test methods for use on board ships. Section 5 describes the potential hazards to the cargo handling system of ice or hydrate formation. Section 6 summarises the results of this Review and outlines recommendations for further research. VI 15 Section 3 Cargo pumps, particularly the submerged electric motor type, in some LPG ships are dosed routinely with methanol in refrigerated propane service with, say, 10 gallons prior to tank cooldown (to inhibit any moisture in the cargo pump from freezing), 5 gallons during the loaded voyage and a further 10 gallons the day before cargo discharge. Deepwell pumps, however, may be rotated by hand to ensure that they are free before start-up and methanol dosing undertaken only when necessity demands. Figure 4. Methanol Required to Prevent Hydrates in LPG in Cargo Tanks During Cooling Cargo Liquid. Figure 5. Methanol to Add to Prevent Formation of Hydrates in Reliquefaction Condensers and Condensate Return Systems. 16 Section 3 Provided that the quantity of methanol injected can be carefully measured, the added methanol contamination of the cargo may be simply calculated. For example, the addition of 25 UK gallons of methanol, density 796.5 kg/cbm at + 15°C, per cargo pump into 30,000 cbm fully refrigerated propane cargo in a ship with 8 cargo pumps would give a contamination of 8 x 25 x 796.5 x 1,000,000 220 x 30,000 x 580 = 41.6 ppm methanol by weight The addition of 1800 ppm by weight of methanol dosed to a propane reliquefaction plant handling 750 cbm/hr of ‘ vapour (density 2.4 kg/cbm) and operating continuously for 15 days on 30,000 cbm of fully refrigerated propane cargo would result in added contamination of 1800 x 750 x 2.4 x 24 x 15 x 1,000,000 1,000,000 x 30,000 x 580 = 67.0 ppm methanol, by weight 3.5. Hydrate Control Without Methanol Injection 3.5.1. Defreezing of LPG condensers Section 3.4.2 describes how excess water in vapour boil-off gas must come out of the propane condensate at –15°C as particles of solid hydrate. For example, if boil-off gas from propane at –42°C and containing, say, 60 ppm water forms condensate containing about 13 ppm water at –15°C (see Table I), then the rate of hydrate formation in a condenser handling 750 cbm per hour of vapour of density 2.4 kg/cbm would be 750 x 2.4 x (60 –13) 1,000,000 = 0.0846 kg per hour or 2.03 kg/day An alternative means of control, instead of methanol injection, could be a de-freezing operation of the reliquefaction system on a regular basis dependent upon the rate of hydrate accumulation. This could involve shutting down the R22 plant maintaining LPG flow if possible until the condenser temperature had risen to 0°C, at which point the condensate return valve would be closed and the LPG compressor shut down. During subsequent hours the condenser temperature would rise to above +6°C and the hydrates would become readily-drained free water. Cargo Officers report that the condensers may “ice” up regularly on a fully-refrigerated propane cargo, indicated by the rise in condenser pressure with a reduction in unit efficiency perhaps due to particles of hydrate accumulating on condenser surfaces protected from hot gas impingement. They also report regular blockage of the condensate return regulating valve which manifests itself by an immediate rise in condensate level in the condenser and subsequent rise in condenser pressure. 3.5.2. Promotion and removal of hydrates Ref. 10 states: “Fine mesh filters (say ASTM Sieve No. 100 - aperture 0.0059 inches, wire diameter 0.0040 inches) form a natural surface on which hydrate/ice crystals grow and, after sufficient build up within the pressure drop of the filter basket, they can be removed.” Two such filters could be installed downstream of the condensate return regulating valves with a further filter (Sieve No. 30 aperture 0.0232 inches, wire diameter 0.0130) at the condensate return dome connection as a final protection of tank sprays, cargo pumps etc. About 10 ppm (or 0.43 kg/day) of free water is estimated to form hydrates downstream of the regulating valve for the example 30,000 cbm propane cargo.