Cargo Pressure/Temperature Control
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Construction and Classification of Ships for the Carriage of Liquefied Gases in Bulk, July 2022 - Cargo Pressure/Temperature Control - Cargo Pressure/Temperature Control

Cargo Pressure/Temperature Control

Goal

To maintain the cargo tank pressure and temperature within design limits of the containment system and/or carriage requirements of the cargo.

7.1 Methods of control

7.1.1 With the exception of tanks designed to withstand full gauge vapour pressure of the cargo under conditions of the upper ambient design temperatures, cargo tanks' pressure and temperature shall be maintained at all times within their design range by either one, or a combination of, the following methods:

  • .1 reliquefaction of cargo vapours;

  • .2 thermal oxidation of vapours;

  • .3 pressure accumulation; and

  • .4 liquid cargo cooling.

7.1.2 For certain cargoes, where required by chapter 17, the cargo containment system shall be capable of withstanding the full vapour pressure of the cargo under conditions of the upper ambient design temperatures, irrespective of any system provided for dealing with boil-off gas.

7.1.3 Venting of the cargo to maintain cargo tank pressure and temperature shall not be acceptable except in emergency situations. The Administration may permit certain cargoes to be controlled by venting cargo vapours to the atmosphere at sea. This may also be permitted in port with the authorization of the port Administration.

7.2 Design of systems

For normal service, the upper ambient design temperature shall be:

  • - sea: 32°C

  • - air: 45°C

For service in particularly hot or cold zones, these design temperatures shall be increased or decreased, to the satisfaction of the Administration. The overall capacity of the system shall be such that it can control the pressure within the design conditions without venting to atmosphere.

LR 7.2-01 Where the APBU notation is to be assigned, the documentation listed in LR IV is to be submitted to allow verification of both the heat leakage calculations and proposed maximum voyage duration. The information submitted is to include the maximum allowable temperature upon termination of loading and isolation of the vapour return.

LR 7.2-02 Filling limits of cargo tanks are to be in accordance with the relevant requirements of Chapter 15. If cargo vapour pressure/temperature control as per Chapter 7 is provided, then dual-setting pressure relief valves may be installed and filling limits calculated accordingly.

LR 7.2-03 The materials are to comply with the requirements of Chapter 6 of the Rules for Materials as applicable to the individual systems.

LR 7.2-04 Where it is intended that service will be in a colder or hotter zone, suitable ambient design temperatures may be approved for such service and the class notation will indicate the geographical limits of the service, see LR III.

LR 7.2-05 The thermal insulation properties and design arrangements of the piping insulation are to be taken into consideration when calculating the heat balance of the containment system and capacity of the pressure/temperature control system.

7.3 Reliquefaction of cargo vapours

7.3.1 General

The reliquefaction system may be arranged in one of the following ways:

  • .1 a direct system, where evaporated cargo is compressed, condensed and returned to the cargo tanks;

  • .2 an indirect system, where cargo or evaporated cargo is cooled or condensed by refrigerant without being compressed;

  • .3 a combined system, where evaporated cargo is compressed and condensed in a cargo/refrigerant heat exchanger and returned to the cargo tanks; and

  • .4 if the reliquefaction system produces a waste stream containing methane during pressure control operations within the design conditions, these waste gases, as far as reasonably practicable, are disposed of without venting to atmosphere.

Note:

  • The requirements of chapters 17 and 19 may preclude the use of one or more of these systems or may specify the use of a particular system.

LR 7.3-01 Cooling water return from heat exchangers which contain cargo are not to be led into the main machinery spaces.

7.3.2 Compatibility

Refrigerants used for reliquefaction shall be compatible with the cargo they may come into contact with. In addition, when several refrigerants are used and may come into contact, they shall be compatible with each other.

7.4 Thermal oxidation of vapours

7.4.1 General

Maintaining the cargo tank pressure and temperature by means of thermal oxidation of cargo vapours, as defined in 1.2.52 and 16.2 shall be permitted only for LNG cargoes. In general:

  • .1 thermal oxidation systems shall exhibit no externally visible flame and shall maintain the uptake exhaust temperature below 535°C;

  • .2 arrangement of spaces where oxidation systems are located shall comply with 16.3 and supply systems shall comply with 16.4; and

  • .3 if waste gases coming from any other system are to be burnt, the oxidation system shall be designed to accommodate all anticipated feed gas compositions.

7.4.2 Thermal oxidation systems

Thermal oxidation systems shall comply with the following:

  • .1 each thermal oxidation system shall have a separate uptake;

  • .2 each thermal oxidation system shall have a dedicated forced draught system; and

  • .3 combustion chambers and uptakes of thermal oxidation systems shall be designed to prevent any accumulation of gas.

7.4.3 Burners

Burners shall be designed to maintain stable combustion under all design firing conditions.

7.4.4 Safety

7.4.4.1 Suitable devices shall be installed and arranged to ensure that gas flow to the burner is cut off unless satisfactory ignition has been established and maintained.

7.4.4.2 Each oxidation system shall have provision to manually isolate its gas fuel supply from a safely accessible position.

7.4.4.3 Provision shall be made for automatic purging the gas supply piping to the burners by means of an inert gas, after the extinguishing of these burners.

7.4.4.4 In case of flame failure of all operating burners for gas or oil or for a combination thereof, the combustion chambers of the oxidation system shall be automatically purged before relighting.

7.4.4.5 Arrangements shall be made to enable the combustion chamber to be manually purged.

7.5 Pressure accumulation systems

The containment system insulation, design pressure or both shall be adequate to provide for a suitable margin for the operating time and temperatures involved. No additional pressure and temperature control system is required. Conditions for acceptance shall be recorded in the International Certificate of Fitness for the Carriage of Liquefied Gases in Bulk required in 1.4.4.

LR 7.5-01 Where the APBU notation is to be assigned, the emergency procedures required by Ch 18 are to include details of the procedure for achieving equalisation between the cargo tank vapour space and the interbarrier space, and details are to be submitted.

LR 7.5-02 For cargo tanks of membrane type, where the vent mast is of sufficient height that liquefied cargo cannot be vented due to the design head of the cargo tank, then means of equalisation between the cargo tank vapour and the interbarrier space may be omitted.

7.6 Liquid cargo cooling

The bulk cargo liquid may be refrigerated by coolant circulated through coils fitted either inside the cargo tank or onto the external surface of the cargo tank.

7.7 Segregation

Where two or more cargoes that may react chemically in a dangerous manner are carried simultaneously, separate systems as defined in 1.2.47, each complying with availability criteria as specified in 7.8, shall be provided for each cargo. For simultaneous carriage of two or more cargoes that are not reactive to each other but where, due to properties of their vapour, separate systems are necessary, separation may be by means of isolation valves.

7.8 Availability

The availability of the system and its supporting auxiliary services shall be such that:

  • .1 in case of a single failure of a mechanical non-static component or a component of the control systems, the cargo tanks' pressure and temperature can be maintained within their design range without affecting other essential services;

  • .2 redundant piping systems are not required;

  • .3 heat exchangers that are solely necessary for maintaining the pressure and temperature of the cargo tanks within their design ranges shall have a standby heat exchanger, unless they have a capacity in excess of 25% of the largest required capacity for pressure control and they can be repaired on board without external resources. Where an additional and separate method of cargo tank pressure and temperature control is fitted that is not reliant on the sole heat exchanger, then a standby heat exchanger is not required; and

  • .4 for any cargo heating or cooling medium, provisions shall be made to detect the leakage of toxic or flammable vapours into an otherwise non-hazardous area or overboard in accordance with 13.6. Any vent outlet from this leak detection arrangement shall be to a safe location and be fitted with a flame screen.

LR 7.8-01 It is recommended that a reasonable margin in plant output over maximum load be allowed for possible overall inefficiencies under service conditions. It is also recommended that due regard be given to any additional capacity required to deal with cargo loading conditions. A refrigeration unit comprises a compressor and its prime mover and the associated heat exchangers, together with the fittings and controls necessary to permit independent operation of the unit. Where the system is of the combined or cascade type, a unit will include the compressors, prime movers and heat exchangers for all sections of the process. Separate piping systems are not required.

LR 7.9 General

Information and plans

LR 7.9-01 Details of the proposed system of cargo pressure/temperature control are to be submitted for consideration. Information and plans are to be submitted as detailed in Pt 6, Ch 3,1.2 of the Rules for Ships as applicable. The circuit diagrams are to show the complete refrigeration system and include the temperatures and pressures at the various points in the process system. The capacity of the refrigeration units is to be given. The information is to include the particulars of the intended cargoes, maximum vapour pressure and minimum cargo temperature. Where no refrigeration plant is fitted, the boil-off calculations are to be submitted to confirm the manner in which the boil-off will be handled.

Construction requirements

LR 7.9-02 Mechanical refrigeration systems required by 7.1.1.1 are to be constructed under LR’s Special Survey in accordance with the requirements of this Chapter and Pt 6, Ch 3 of the Rules for Ships where applicable. Process pressure vessels are to comply with Pt 5, Ch 10 and 11 as applicable.

LR 7.9-03 It is recommended that adequate spares, together with the tools necessary for maintenance or repair be carried. The spares are to be determined by the Owner according to the design and intended service. The maintenance of the spares is the responsibility of the Owner.

Class notation

LR 7.9-04 For the class notation to be assigned for reliquefaction or refrigeration equipment constructed, installed and tested in accordance with this Chapter, see LR III.3.

LR 7.9-05 For insulation requirements, see 4.10 and 4.19.3

LR 7.9-06 On completion, the installation is to be tested to prove its capability to maintain the class notation temperature and pressure.

LR 7.10 Nitrogen/inert gas systems fitted for purposes other than inerting required by SOLAS Reg. II-2/4.5.5.1

LR 7.10-01 Where nitrogen gas is used for purposes other than inerting, e.g. cargo padding and reliquefication, the following requirements are to be applied.

LR 7.10-02 The nitrogen generator is to be capable of delivering high purity nitrogen in accordance with Ch 15, 2.2.1.2.5 of the FSS Code, as amended by MSC.367(93). In addition to Ch 15, 2.2.2.4 of the FSS Code, as amended by MSC.367(93), the system is to be fitted with automatic means to discharge ‘off-spec’ gas to the atmosphere during start-up and abnormal operation.

LR 7.10-03 The feed air treatment system fitted to remove free water, particles and traces of oil from the compressed air as required by Ch 15, 2.4.1.2 of the FSS Code, as amended by MSC.367(93), is also to preserve the specification temperature.

LR 7.10-04 The oxygen-enriched air from the nitrogen generator and the nitrogen-product enriched gas from the protective devices of the nitrogen receiver are to be arranged to discharge to a safe location on the open deck. This safe location needs to address the two types of discharges separately.

For oxygen-enriched air from the nitrogen generator, safe locations on the open deck are:

  • outside of hazardous areas as defined by 1.2.24;
  • not within 3 m of areas traversed by personnel;
  • not within 6 m of air intakes for machinery and all ventilation inlets.

For nitrogen-product enriched gas from the protective devices of the nitrogen receiver, safe locations on the open deck are:

  • not within 3 m of areas traversed by personnel;
  • not within 6 m of air intakes for machinery and all ventilation inlets/outlets.

LR 7.10-05 In order to permit maintenance, means of isolation are to be fitted between the generator and the receiver.

LR 7.10-06 The requirements given in Ch 15, 2.2.2, 2.2.4, 2.4.1 and 2.4.2 of the FSS Code, as amended by MSC.367(93), apply to the systems, as applicable.

LR 7.10-07 Materials used in inert gas systems are to be suitable for their intended purpose in accordance with the Rules for Materials.

LR 7.10-08 All the equipment is to be installed on board and tested under working conditions to the satisfaction of the Surveyor.

LR 7.10-09 The two non-return devices as required by Ch 15, 2.2.3.1.1 of the FSS Code, as amended by MSC.367(93), are to be fitted in the inert gas main. The non-return devices are to comply with Ch 15, 2.2.3.1.2 and 2.2.3.1.3 of the FSS Code, as amended by MSC.367(93); however, where the connections to the cargo tanks, to the hold spaces or to cargo piping are not permanent, the non-return devices required by Ch 15, 2.2.3.1.2 of the FSS Code, as amended by MSC.367(93), may be substituted by two non-return valves.

LR 7.10-10 Detailed instruction manuals shall be provided on board as required by Ch 15, 2.2.5 of the FSS Code, as amended by MSC.367(93).

Parent topic: Chapter 7 Cargo Pressure/Temperature Control
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