Section 1 Cargo Pressure/Temperature Control

1.1 Methods of control

1.1.1 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:

reliquefaction of cargo vapours
thermal oxidation of vapours
pressure accumulation
liquid cargo cooling.

1.1.2 Venting of the cargo to maintain cargo tank pressure and temperature is not acceptable except in emergency situations. The Administration may permit certain cargoes to be controlled by venting cargo vapours to the atmosphere at sea.

1.2 Design of systems

1.2.1 Details of the proposed system of cargo pressure/temperature control are to be submitted for consideration.

The ambient temperatures for air and sea-water are to be taken at their highest daily mean temperatures for the unit’s proposed area of operation based on the 100 year average return period. The ambient temperatures are to be rounded up to the nearest degree Celsius.

The ambient temperatures are not to be taken as less than 45°C for air and 32°C for sea-water unless agreed by LR.

The overall capacity of the system shall be such that it can control the pressure within the design conditions without venting to atmosphere.

1.2.2 The system is to be tested at entry into service to prove its capability to maintain the class notation temperature and pressure.

1.3 Reliquefaction of cargo vapours

1.3.1 General

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

A direct system where evaporated cargo is compressed, condensed and returned to the cargo tanks.
An indirect system where cargo or evaporated cargo is cooled or condensed by refrigerant without being compressed.
A combined system where evaporated cargo is compressed and condensed in a cargo/refrigerant heat exchanger and returned to the cargo tanks.
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.

1.3.2 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.

1.4 Thermal oxidation of vapours

1.4.1 The use of thermal oxidation equipment on ship units engaged in the production, storage and offloading of liquefied gases in bulk at a fixed location is not anticipated, in the event that this or similar equipment is used it is to comply with Lloyd’s Register’s Rules and Regulations for the Construction and Classification of Ships for the Carriage of Liquefied Gases in Bulk.

1.5 Pressure accumulation systems

1.5.1 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.

1.6 Liquid cargo cooling

1.6.1 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.

1.7 Segregation

1.7.1 Where two or more cargoes that may react chemically in a dangerous manner are carried simultaneously, separate systems as defined in Pt 11, Ch 1, 1.3 Definitions, each complying with availability criteria as specified in Pt 11, Ch 7, 1.8 Availability, 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.

1.8 Availability

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

In case of a single failure of a mechanical nonstatic 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.
Redundant piping systems are not required.
Heat exchangers that are solely necessary for maintaining the pressure and temperature of the cargo tanks within their design ranges shall have a stand-by heat exchanger unless they have a capacity in excess of 25 per cent of the largest required capacity for pressure control and they can be repaired onboard 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.
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 non-hazardous area and be fitted with a flame screen.

1.8.2 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.

1.8.3 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.