Typical Cargo Equipment


GTM-8.6	Gas Tanker Manual GTM Typical Cargo Equipment	Form No.: GTM-8.6 Revision: 01 Date: 15 Oct 2024 Issued by: DPA Approved by: MD

6. 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.

This chapter is not intended to cover every system and is for general information only, for individual ships please refer to the Shipbuilders Cargo Operations Manual.

6.1 CARGO PUMPS

6.1.1 GENERAL ARRANGEMENT

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.

6.1.2 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.

6.2 PRODUCT LINE SYSTEMS

6.2.1 GENERAL ARRANGEMENT

There are normally two separate pipe systems on the main deck, each comprising the following:

Liquid loading/unloading line
Vapour suction line
Stripping/condensate return line
Common purge line
Vent/drain line

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.

6.2.2 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.

The following purge arrangements are normally possible:

Tanks grouped into two segregated systems.
One tank individually or in series or in parallel with other tanks.
With inert gas (dry air) from the inert gas generator.
With nitrogen or gas from shore, compressor or vaporiser.
To the shore line or vent mast.

6.2.3 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.

6.3 MEASURING AND CONTROL SYSTEM

6.3.1 GENERAL ARRANGEMENT

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, the plant will be shut down and cargo operations suspended.

6.3 MEASURING AND CONTROL SYSTEM

6.3.2 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.

Slip Tube

Slip tubes can be used for pressure vessels. The tube penetrates the tank and has a valve at the top. When the valve is opened, a restricted amount of cargo is emitted. Therefore, if slip tubes are fitted, they must not be used as the primary method of gauging but only in case of an emergency. They must not be used when toxic cargoes are carried.

Warnings shall be clearly displayed in the CCR accordingly. If tubes are going to be used (emergency only):

A risk assessment shall be issued and submitted to the management office;
Permission shall be granted from the terminal and DPA;
The source of ignition shall be excluded from the area;
Spray from the valve shall be directed away from personnel and protective clothing shall be worn;
Personnel shall not stand directly in the path of the tube;
There should be a stopper to prevent the tube from being pressed out completely (tank pressure).

6.3.3 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 the 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.

6.3.4 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.

6.3.5 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.

6.3.6 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 that 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.

6.4 AUXILIARY EQUIPMENT AND SYSTEMS

6.4.1 HYDRAULIC OR PNEUMATIC QUICK CLOSING SYSTEM – EMERGENCY SHUTDOWN 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.

Note: Activation of the ESD can create a pressure surge because of change in the rate of flow of liquid in cargo lines. The surge can be dangerous if the change of flow rate is too rapid.

As per IGC Code, the closing time of the valves shall not be longer than the formula below. However, the surge pressure is greatest if the valve closes instantaneously. Therefore, the closing time shall be periodically calculated and periodically checked by ship’s staff to avoid pressure surge effects.

Refer to the following publications:

ICS Tanker Safety guide (liquefied gas) - Appendix 8 – The Pressure Surge Phenomenon
SIGTTO Guidance for the Alleviation of Excessive Surge Pressures on ESD

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:

Activating an onboard emergency shut down push-button, or if activated from ashore;
A fusible plug melting in the pneumatic system (melting temperature 98-104°C)
Instrument air supply failure
Electrical power failure
Activation of one of the mast blow-off high-level switches

Tank dome valves close if:

The 98% full level switch is activated, (many vessels are fitted with an override switch which will prevent the valve from closing, this override is only to be used with the express permission of the Master)

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.

6.4.2 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.

6.4.3 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:

Exposed cargo tank domes.
Exposed on-deck storage vessels for flammable or toxic products.
Cargo liquid and vapour manifolds.
Boundaries of superstructures and deck houses normally manned, cargo compressor rooms, pump rooms, store rooms containing high fire risk items, and cargo control room, all facing the cargo area.

6.4.4 PERMANENT GAS DETECTOR SYSTEM

A fixed gas detection system consists of a series of sensors, an alarm module, and a 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 contains cargo.

The system must be checked regularly for correct operation, and it must be reset whenever a new cargo is introduced. All checks and changes to settings must be recorded in the Log Book.

The system should be calibrated.

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:

1 ppm for vinyl chloride
25 ppm for ammonia

6.4.5 EQUIPMENT CONDITION MONITORING

To maintain the efficiency of the plant, the condition must be monitored on the monthly return form GAS 06.

6.5 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 Company 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. It is Company policy that test conditions must be run against Class approved cases and records of results maintained:

As soon as possible after a change of Chief Officer;
At least every three months;
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.