IGC Code changes have made compliance a less precise process than conforming to the original Code because the criteria for determining the lowest cargo density anticipated becomes the highest operational cargo temperature encountered. To predict this temperature requires some judgement on the part of the Shipowner and an ability to convince Administrations that the selection is reasonable.
At its 14th session in December 1984, the IMO Bulk Chemicals Sub-Committee expressed a willingness to consider amending the Codes governing cargo tank loading limits.
SIGTTO and IACS believed that for Type C tanks the Codes, in providing for certain safety features, reduced protective measures available under fire conditions or increased the risk of unwanted venting of cargo. By updating the Codes, protection could be improved.
IMO’s Marine Safety Committee at its 61st session (MSC61) agreed to change the IGC Codes and made available that protection.
The new amendments mean that Type C tanks, under fire conditions, can become liquid filled. In this situation the Code requires tank internal pressures not to exceed 20% above the Maximum Allowable Relief Valve Setting (MARVS). IACS/SIGTTO were able to demonstrate that Pressure Relief Valves (PRVs), whose capacity is calculated using methods set down in the IGC Code, were able to prevent tank pressures exceeding 1.2 × MARVS.
To be assured, PRVs will, at all times, perform as designed, inlet pressure losses and built up back pressures must be within the PRV manufacturers limits.
This latter requirement caused MSC61 to qualify acceptance of the Code changes. They stated that the Organisation would produce Guidelines as to how these pressure losses could be assessed with both liquid and vapours passing through the vent system simultaneously.
IMO delegated the task of producing the Guidelines to IACS/SIGTTO who completed the work submitting a finished document to the 65th session of the Marine Safety Committee (MSC65).
This publication bore the title of "Guidelines for Evaluating the Adequacy of Type C Tank Vent Systems for the Assignment of Amended Loading Limits Under Chapters 8 and 15 of the Gas Carrier Codes".
MSC65 recommended to IMO’s 19th Assembly that the Guidelines be accepted. The Assembly acceded to this request clearing the way for those amendments, agreed at MSC61, to come into force on 1st July 1998.
1 Introduction
2 Changes to the Codes - What they are and what they Mean
2.1 Loading Limits - Unamended Procedures
2.2 Code Changes to Increase Protection
2.3 The New Amendments
2.4 The Effects of the New Amendments
3 Advantages Realised - A Guide to Administration Recognition
4 Recommendations
5 Summary
Appendix 1 Confirmation of Vent System Adequacy
Appendix 2 Highest Temperatures
Appendix 3 Loading Limit Assessment Method
Title: Application of Amendments to Gas Carrier Codes Concerning Type C Tank Loading Limits (downloadable product)
Product Code: WS1363EA
Published Date: January 2012
Author: SIGTTO
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