LPG practice.pdf

PART I

PRESSURE SHIPS

Chapter I

General Description

Chapter II

General Operating Principles

Discharging

Refrigerating the Cargo

Gas-freeing

Chapter III

Cargo Handling Equipment

Cargo Pumps

Cargo Compressors

Condensers

Heat Exchangers

Cargo Heater

Vaporiser

Chapter IV

Conduct of Cargo Operations

Semi-refrigerated Cargoes

Discharging

Refrigerating the Cargo

Gas-freeing

Fully-refrigerated Cargoes at Atmospheric Pressure

Discharging

Two-stage Refrigeration

Precautions to be taken When Starting a Compressor

Points to Watch Whilst a Compressor's Running

PART II

FULLY-REFRIGERATED SHIPS

Chapter V

General Description

Chapter VI

General Operating Principles

Refrigerating the Cargo on Passage

Two-stage Reliquifaction

Cascade Reliquifaction

Discharging

Gas-freeing

To Gas-up the Tanks After They Have Been Gas-freed

To Cool Down the Tanks Prior to Loading

Summary of Gas-freeing and Gassing-up

Chapter VII

Cargo Handling Equipment

Two-stage Refrigeration

Two-stage Compressors

Seawater-Cooled Condensers

The Inter-stage Cooler

The Heat Exchanger

Cascade System of Refrigeration

R.22 Compressors

R.22 Condensers

R.22 Receivers

The Cargo Compressors

The Cargo Condensers

Methanol Injection System

Vaporisers

Air Dryer

Cargo Heaters

Submerged Cargo Pumps

Emergency Cargo Pumps

Deck Storage Tanks

Chapter VIII

Cargo Operating Procedure

To Refrigerate the Cargo on Passage

Two-stage Reliquifaction

Points to Watch Whilst the Plant is Running

Cascade System of Reliquifaction

Starting the Compressors

Points to Watch Whilst the Plant is Running

To Shut Down the System

Other Points to Watch

Discharging

To Gas-free the Vessel

Puddle Heating

To Estimate the Time it Will Take to Puddle Heat

Tank Warming

Inerting the Cargo Tanks

Flushing Through with Air

Preparing the Tanks to Receive Cargo After They

Have Been Gas-freed

Drying the Air in the Cargo Tanks

To Operate the Air Dryer

To Inert the Tanks Prior to Gassing-up

To Gas-up the Cargo Tanks (at Sea)

To Gas-up the Tanks Alongside

Cooling Down the Cargo Tanks

Procedure When Changing Grades and Types of Cargo

PART III

GENERAL

Chapter IX

Cargo Calculations

To Calculate the Quantity of Liquid on Board (Metric)

To Calculate the Weight of Vapour on Board (Imperial and

Metric)

Assessing the Volume of Vapour

To Calculate the Quantity of Liquid on Board (Imperial)

To Calculate the Correct Volume of Liquid to Load

When Loading a Full Cargo

Expansion Relief Valves on Liquid Lines

To Calculate the Correct Volume of Liquid to Load

When Loading a Part Cargo

To Calculate the S.V.P. of a Mixture of Products at a Given

Temperature

To Calculate the Individual Proportions of Vapour in the

Vapour Mixture Above a Liquid Mixture

Molecular Weights

Aid to Memorising the Formulae

Comparison of Metric and Imperial Systems

Table of Properties

Chapter X Safety

Safe Navigation

Safe Practice

Gas Detection

Fire Detection

Fire-fighting

Precautions to be Taken when Entering Spaces which May Have a

Deficiency of Oxygen

Chapter XI Recommendations

Safe Navigation

Harbour Control

Enforcement of Traffic Separation Systems

Emergency Isolation Valves for Safety Valves

Greater Consultation between Operators and Design Staff

PART I PRESSURISED SHIPS

CHAPTER I: GENERAL DESCRIPTION

Pressure ships can be divided into two types, namely fully-pressurised and semi-

pressurised (semi-refrigerated). In practice, the designation of the two types

of vessels has been contracted to Pressure Ships to describe fully pressurised

L.P.G. tankers and semi-refrigerated to describe semi-pressurised ships. From

this point on, this terminology will be used.

Pressure ships are the simplest vessels, and were the first to be built or

converted, for the specific purpose of carrying L.P.G./Ammonia cargoes.

The cargo is carried in a number of cylindrical pressure vessels (cargo tanks)

capable of withstanding the, maximum pressure likely to be met in service,

(usually about 17 bars), the arrangement of the tanks being indicated in Fig. 1.

In order to act as a liquified gas carrier, the ship must be capable of loading,

carrying and discharging, its cargoes, as well as having provision for gas-

freeing for repairs or when changing types of cargo to be carried.

In liquified gas vessels no joints, glands, etc., are permitted below decks, in

order to exclude the possibility of liquified gas or vapour leaking unnoticed

below decks, so special arrangements have to be added to conform to this

requirement. This means that the loading/discharging (liquid) lines have to

penetrate the tank through the tank dome protruding through the deck.

The arrangement is shown in Fig. 3.

To drive the liquified gas from the bottom of the tank to the cargo pump

suction, compressors are installed which, by taking vapour from a tank not being

discharged (or fro -in shore through a vapour return line), pressurise the tanks

being emptied; and drive the liquid to the pump suction. Therefore a pressure

type liquified gas tanker is provided with:

(a) strong tanks (or pressure vessels) into which the cargo is loaded;

(b) a liquid line leading from the top of the cargo tank to the bottom

through which the liquid gas cargo is loaded and discharged (these

are also used for gas-freeing).

order to blow the cargo from the bottom of the tank to the cargo

pump suction;

(d) a vapour line leading to the top of the cargo tanks which is used by

the compressors to pressurise the tanks being discharged;

(e) cargo pumps to raise the discharge pressure and so pump the cargo

ashore; and

a liquid manifold to which the shore loading/discharge lines are

connected and linked to the ship's liquid line system, together with

a vapour line connection which can be linked to the shore vapour

line (if provided) and used either as a vapour source when

discharging or pressure relief when loading.

The principal advantage of semi-refrigerated ships (semi-pressurised) is that

the tanks containing the cargo need not be so strong because the pressure of the

cargo is very much reduced by lowering its temperature. As a result, the

following benefits are derived:

(a) more cargo can be carried in a tank of the same capacity (see

Chapter IX - Cargo Calculations);

(b) a tank of the same capacity is lighter and cheaper to construct; and

Pressure ships usually range from very small capacity up to 2,000 cubic metres

capacity. The capacity of semi-refrigerated ships usually range from between

1,000 to over 10,000 cubic metres. The cargo in the tanks is usually maintained

at about O@ C. by a process of refrigeration, and the tanks them-selves are

thermally insulated.

The loading and discharging procedures are generally similar in both types, the

main operating difference being the addition in the semi-refrigerated ship of a,

reliquifaction (refrigerating) plant to cool the cargo on passage, and also,

under certain circumstances, to assist with loading.

In most vessels of both types, the cargo handling equipment is located in a

deckhouse divided into two compartments by a gas-tight bulkhead. In the one

half are located the electric motors to drive the compressors and pumps, which

are separately housed in the other section, the driving shafts passing through

the gas-tight bulkhead via gas-tight seals (see Fig. 4). The motor room is kept

pressurised with air by powerful fans to exclude the possibility of gas entering

the motor room, so avoiding a fire hazard.

The tanks are usually discharged two at a time by blowing the liquid gas to the

cargo Pump suction, where the discharge pressure is greatly increased by the

cargo pump.

To blow the liquid gas to the pump, one or more compressors are started up,

sucking vapour from one or more tanks not being discharged and sending it into

the tanks being emptied. This is shown in Fig. 6. A simple vapour line

arrangement to do this shown in Fig. 5, but in more advanced ships (particularly

the semi-refrigerated ships) different arrangements are made to achieve the same

result, often using different piping arrangements, but the principle of

pressurising the tanks being discharged and blowing the product to the pump

suction remains the same.

The latest development is to make semi-refrigerated ships capable of carrying

fully-refrigerated cargoes at atmospheric pressure, which gives them greater

versatility with regard to the cargoes that can be carried.

Hence, a simple general description of the all-purpose ship will cover

individual types.

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