GTM-8.9	Gas Tanker Manual GTM Changing Cargoes,Compatible Cargoes	Form No.: GTM-8.9 Revision: 01 Date: 15 Oct 2024 Issued by: DPA Approved by: MD

9. CHANGING CARGOES, COMPATIBLE CARGOES

9.1 INTRODUCTION

Compatible cargoes are those substances which can be loaded consecutively without prior need to gas free the tanks. However, care must be taken to fully comply with Charter Party, shippers or other stated requirements for the cargo changeover, as these may require more stringent procedures.

When cargo grades are changed the Company Form GAS 04 Changing from Non Compatible Cargoes Checklist is to be completed.

On completion of the discharge the amount of liquid stripped from the tanks will depend on the information available on the next cargo. If no prior information is received then normal stripping will be carried out manually.

The conditions then required in the cargo tanks for loading the next grade of cargo will depend on the degree of contamination acceptable to the shippers/charterers between the last cargo and the next.

If the degree of contamination is of no importance, it may only be necessary to retain the remaining liquid and maintain the cargo tank temperatures on the ballast passage by use of the reliquefaction plant. If contamination has to be kept to a minimum it may be necessary to remove the remaining liquid in the sumps by boiling off and by venting the remaining vapour to atmosphere whilst on passage.

It is emphasised that the final decision depends on the requirements of the charterer/shipper and the required maximum level of contamination.

See Appendix 1 for cargo compatibility.

9.2 LIQUID VAPORISING OF RESIDUES

The deepwell pumps can normally be used to remove liquid residues, but if the level is not sufficient for them to operate satisfactorily the residues can be cleared by drawing vapour from the tank, heating it in the LPG compressor or vaporiser, and returning it to the tank sump through the stripping line or the puddle heating line. The warm vapour heats the liquid collected in the sump which quickly vaporises. An increase in the temperature of the tank sump signals the completion of vaporisation.

THIS PROCEDURE MUST NOT BE HURRIED. During boiling off it is important to obtain a positive temperature reading on the sump thermometer. Any subsequent drop in temperature indicates that liquid may still be present hence it is important to monitor the temperature even after the flow of hot gas has stopped.