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LPG tanker cargo handling equipments & product line system

Typical cargo equipment

The cargo handling equipment can vary in range from a simple manual system with no reliquefaction plant as fitted to a small fully pressurised ship, or a complicated fully automatic cascade type reliquefaction plant. For individual ships please refer to the Shipbuilders Cargo Operations Manual.


Deepwell Pumps are normally installed on smaller vessels, with electrical submerged pumps on larger vessels One or two booster pumps are normally fitted on deck.

In general, the booster pumps are operated in series with the deep well pumps - two deep well pumps to share one booster pump, or a booster pump plus a deep well pump.

With booster pumps in operation it is important to ensure that the manifold pressure must not exceed the set pressure of the manifold relief valves.

DEEPWELL PUMPS: The pumps are normally of the multi-stage segment type with inducer, multi-bearing supported shaft, double mechanical sealing of tandem design with sealing liquid forming the shaft seal. The leaking sealing liquid is collected and cannot contaminate cargo.


There are normally two separate pipe systems on the main deck, each comprising the following:
Note : All flange connections must be electrically conducting. If this cannot be guaranteed bonding straps are to be fitted and a measure of the conductivity made.

PRODUCT SYSTEM SEGREGATION: Product lines can normally be separated into two segregated systems - spool pieces, bends, line blanks are provided to effect the segregation. Spool pieces and mating pipe sections are numbered.

The method of segregation and connection of the system will depend on the system fitted, for details refer to the Builders Manual. Notes The following purge arrangements are normally possible:
  1. Tanks grouped into two segregated systems.
  2. One tank individually or in series or in parallel with other tanks.
  3. With inert gas (dry air) from the inert gas generator.
  4. With nitrogen or gas from shore, compressor or vaporiser.
  5. To the shore line or vent mast.
VENTING AND DRAIN SYSTEM: A separate venting and drain system is fitted to each cargo system. The vent/drain lines are connected to liquid collectors which have a free flow to the mast. Residues collected in the collectors can be discharged to a tank or ashore via the crossovers and hose/connectors provided for this purpose, using gas pressure in the collector.

Each collector has a relief valve which discharges to the mast and each collector is fitted with a high level alarm. If liquid quantities in excess of the designed limit flow to the mast, a level switch will shut down the plant and close all hydraulic valves. The cause of this excessive flow is to be investigated before attempting to re-start the plant.

Drain valves are located at the lowest points in the blow off lines, and purge connections are provided in the lines adjacent to the relief valves.

Blow off lines must be purged with nitrogen or inert gas before a thunderstorm. Flame screens are fitted to the vent masts.


Operation of the cargo system is both manually at local stations, and from the cargo control room. In the event of any operational parameters going outside set limits plant will be shut down and cargo operations suspended.

CARGO TANK LEVEL GAUGING: The gauges must be used in conjunction with the tank calibration tables and the correction made for cargoes of varying Specific Gravities.

The gauges are only to be activated during loading and discharging operations. When not in use the gauges should be lifted and locked in place.

CARGO TANK LEVEL SWITCHES: One alarm will normally be at 98% i.e. high level. This alarm will normally close the loading valve. The High Level alarm (and ESD) should always be active whenever vessel is loading/discharging and when engaged in internal transfer of cargo.

Alarms can be inhibited prior to proceeding to sea. There should be a clear indication of the overriding of the alarm status.

CARGO TANK SAMPLE LINES: These are permanently installed sampling pipes which can be used for sampling of liquid cargo or vapour/inert gas/air concentrations when inerting or purging. On some vessels they may also be used in the event of a level gauge failure for tank level gauging. There are normally three per cargo tank, measuring the levels in the tank bottom, middle and top.

RELIQUEFACTION PLANT: There are many types of reliquefaction systems in use on board vessels and Masters and Officers must consult their Manufacturers Manuals for details of the operation.

CARGO HEATER: The cargo heater is operated during cargo discharge when a heated cargo is required by the receiving terminal. The heater uses sea water as the heat source. Cargo flows in the shell side of the exchanger and sea water flows in the tubes.

NOTE : In order to prevent freezing of the heater and the associated damage which occurs during freezing, it is essential that the heater is operated at all times with the designed Sea Water flow and that flow is maintained until cargo operations have been completed and all remaining liquid has been boiled off on completion of cargo operations.


HYDRAULIC OR PNEUMATIC QUICK CLOSING SYSTEM: All connections to the tank domes (except for instruments), and shore connections on the manifold cross over lines, are fitted with hydraulically or pneumatically operated, fail safe, ball valves. The system is part of the on-board safety system and causes selected valves to close after release of hydraulic or pneumatic pressure. It also provides the power to activate the valves when operating normally.

Valve Closing Times: The closing time of the valves should be agreed with shore operations, and calculated in accordance with IMO, IGC Code 18.8.2, i.e. "The closing time of the valve .... (i.e. time from shut-down signal initiation to complete valve closure) should not be longer than

T = (3,600 x U)/LR seconds, where,
U = ullage volume at operating signal level in cubic metres
LR = max loading rate agreed between ship and shore installation in cubic metres/hour.

The loading rate must be adjusted to limit surge pressure on valve closure to an acceptable level, taking into account the loading hose or arm, the ship and the shore piping system where relevant. If the pressure in the hydraulic/pneumatic system is released all valves close. This is initiated by the following: On some vessels this may be limited to filling valves only. This design is not accepted at Dow Chemical Terminals where a separate switch is required which when activated closes all actuated valves on all cargo lines and also stops the cargo pumps.

AIR BLOWER: A separate air blower may also be fitted for the supply of atmospheric air to the purge system. It is connected to the purge line by a spool piece and two non-return valves. The use of the blower may cause condensation at 'cold spots', e.g. tank walls and therefore careful drying is a necessity after purging.

DECK SPRAY SYSTEM: The deck spray system is a safety system used for spraying and cooling cargo tank domes, superstructure and housings. Sea water is distributed as follows:
  1. Exposed cargo tank domes.
  2. Exposed on deck storage vessels for flammable or toxic products.
  3. Cargo liquid and vapour manifolds.
  4. Boundaries of superstructures and deck houses normally manned, cargo compressor rooms and pump rooms store rooms containing high fire risk items and cargo control room, all facing the cargo area.

PERMANENT GAS DETECTOR SYSTEM: A fixed gas detection consists of a series of sensors, alarm module and measuring unit situated in the cargo control room or on the bridge continuously monitoring the concentration of gases in the specific areas of the vessel.

The gas detection system must always be in operation when the cargo tanks or gas plant contain cargo.

The system must be checked regularly for correct operation, and it must be reset whenever a new cargo is introduced.

THE SYSTEM MUST BE RE-CALIBRATED MONTHLY All checks and changes to settings must be recorded in the Log Book.

Toxic Gas Detection: Detectors may be installed in the air conditioning system intakes to monitor the presence of toxic gases, e.g. vinyl chloride, ammonia. These will detect concentrations of gas in air down to 2 ppm. The instruments and controls are normally housed in the cargo control room. The recommended alarm levels are 10 ppm for vinyl chloride and 50 ppm for ammonia.

EQUIPMENT CONDITION MONITORING: To maintain the efficiency of the plant the condition must be monitored on the monthly checklist

LOADING/STRESS COMPUTER: This instrument is provided to supplement the stability booklet for the vessel. It allows the Officer responsible, to carry out the various complex calculations required ensuring that the ship is not overstressed or damaged during the carriage of the nominated cargoes. It will also permit the assessment of damage stability.

The Master and Chief Officer will make themselves aware of the worst case damage stability condition within the stability booklet.

It must be remembered that a loading computer, as with navigation aids, is only an aid to the operator. It relies on human input of data, and more importantly the human interpretation of the output data. If the input data is incorrect, the output data will also be incorrect. Used correctly it will ensure the safe operation of the ship for all conditions of loading, discharging, ballasting and at all stages of the voyage.

It is a requirement that where such equipment is provided to a ship, test conditions must also be supplied for use in verifying the accuracy of the equipment. Test conditions must be run and records of results maintained as soon as possible after a change of Chief Officer and at least every three months and in any case prior to the vessel proceeding to dry-dock.

The frequency and records of such tests are to be recorded in the vessel’s planned maintenance system. Where the running of these reveals significant errors the Company is to be advised immediately with a request for attention.

Where online gauging of tank contents is not fitted the loading computer must be regularly updated in order that stresses, draft and trim can be monitored throughout the discharging operations.

Related Information:

  1. Various type LPG tanker - Design characteristics and usability

  2. Carriage of LPG cargo at sea & safety guideline

  3. LPG reliquefaction plant safety guideline

  4. Preparations for LPG cargo discharging, pumping & stripping guideline

  5. Preparations for loading compatible cargo onboard LPG tanker

  6. Preparation for changing different grade cargo or drydocking -LPG tanker guideline

  7. Cargo tank inerting prior to gassing up - LPG tanker procedure

  8. LPG cargo tank purging & safety guideline

  9. LPG cargo tank cooling safety procedure

  10. LPG cargo loading special guideline

  11. Tackling fire onboard LNG & LPG ships

  12. Detail guideline for Ballast operation at sea by LPG carrier

  13. Handling cargo related documents for LPG carrier

  14. Cargo sampling procedure for liquefied gas cargo

  15. Cargo measurement and calculation guideline for LPG carriers

  16. Handling Propylene oxide, Ethylene oxide mixtures

  17. Special characteristics of Vinyl Chloride Monomer & Butadiene

  1. Various type LPG tanker - Design characteristics and usability

  2. LPG tanker cargo work equipments & product line system

  3. LPG tanker cargo pipe line inspection and testing guideline

  4. Carriage of LPG cargo at sea & safety guideline

  5. LPG reliquefaction plant safety guideline

  6. Preparations for LPG cargo discharging, pumping & stripping guideline

  7. Preparations for loading compatible cargo onboard LPG tanker

  8. Preparation for changing different grade cargo or drydocking -LPG tanker guideline

  9. Cargo tank inerting prior to gassing up - LPG tanker procedure

  10. LPG cargo tank purging & safety guideline

  11. LPG cargo tank cooling safety procedure

  12. LPG cargo loading special guideline

  13. Tackling fire onboard LNG & LPG ships

  14. Detail guideline for Ballast operation at sea by LPG carrier

  15. Handling cargo related documents for LPG carrier

  16. Cargo sampling procedure for liquefied gas cargo

  17. Cargo measurement and calculation guideline for LPG carriers

  18. Handling Propylene oxide, Ethylene oxide mixtures

  19. Special characteristics of Vinyl Chloride Monomer & Butadiene

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