Liquified Petroleum Gas Tanker Practice

128 Pages·11.147 MB·English

by  Woolcott T.V.W.

LIQUIFIED PETROLEUM GAS
TANKER PRACTICE
Captain T. W. V. WOOLCOTT
GLASGOW
BROWN, SON & FERGUSON LTD., NAUTICAL PUBLISHERS 52 DARNLEY
STREET, GLASGOW G41 2SG
I
PREFACE
The purpose of this book is to provide a guide to the conduct of the transportation
of liquified petroleum gases and ammonia, the coverage of which has not so far
been available in a single volume.
It is hoped that it will be useful not only to those operating the carriers but also
to the staffs of terminals handling these products.
Although some treatment is given to the equipment involved, it is not intended
that this book shall be a technical guide.
I would like to acknowledge the assistance given by Mr. W. R. H. Walters in the
preparation of the book and a general acknowledgement of the assistance I have
had over a number of years from the officers of the various vessels in which I have
served, and also the management of Messrs. Houlder Brothers and Company
Limited of London.
T. W. V. WOOLCOTT April, 1977
II
INTRODUCTION
INTRODUCTION
The purpose of this book is to examine the technical problems involved in the
transportation of L.P.G. and ammonia cargoes, and various other aspects of the
running of L.P.G. Tankers.
The detailed procedures at all stages will depend upon the conditions under
which the cargoes are to be loaded, carried and discharged—whether under fully-
pressurised, semi-pressurised, or fully-refrigerated at atmospheric pressure
conditions.
1. Preparation: At the point of loading, the cargo tanks should have been prepared,
so far as is possible, for the reception of the cargo to be carried. The procedure
involved, and the degree of readiness would depend upon whether the vessel is
loading:
(a) a consecutive cargo of the same type;
(b) a cargo of a different nature, but compatible with the previous cargo, e.g.
butane after propane;
(c) a cargo of a different nature, and incompatible with the previous cargo (e.g.
to load propane after discharging ammonia would involve gas-freeing); or
(d) a first cargo.
2. Loading: This involves differing procedures, depending upon the facilities
offered at the loading terminal, e.g. whether or not a vapour return line (see
Glossary) is provided. It covers calculations for ascertaining the correct
soundings (depths of liquid) to load either a full cargo or a given quantity (part
cargo), and to calculate afterwards as accurately as possible the quantity in fact
loaded.
3. Transportation: This covers care of the cargo in transit to guard against loss of
product; the running of the reliquifaction plant and ensuring that the cargo tanks
do not become over-full due to the incorrect operation of the condensate returns
from the reliquifaction plant; and the routine checking of tank pressures and
adjusting the degree of refrigeration accordingly so that the vessel is ready to
discharge on arrival at the terminal.
4. Discharging: The method of discharge will depend upon the facilities for
discharge available at the receiving terminal, and the type of discharge
required—if direct into fully-refrigerated storage or via the cargo heater into
pressure storage.
Special Safety Precautions
Due to the hazardous nature of the cargoes carried, those responsible for the
conduct of Liquified Gas Tanker operations have a particular responsibility, not
only to preserve the lives of those on board, but particularly to preserve the
environment at the terminals and their approaches. A fire, once it gets a firm hold,
and particularly if it spreads to the cargo tanks, would present a very serious
hazard, since it would be extremely difficult to extinguish. So fire prevention is of
the greatest importance. The chapter on Safety lays particular emphasis on this and
covers:
III
INTRODUCTION
(a) safe navigation;
(b) safe practice (incorporated in the chapters dealing with the operating
procedures);
(c) efficient detection of an accumulation of gas before it reaches a flammable
mixture;
(d) rapid fire detection so that a fire can be tackled in its early stages;
(e) fire fighting.
The chapter also covers the special dangers which may be encountered when
entering a compartment which has previously been inerted with nitrogen even
though it has since been ventilated, and the precautions which should be taken.
This is fully explained in Chapter X.
One of the peculiarities of operating Liquified Gas Carriers is that the cargo is
completely unseen whilst it is being loaded, carried and discharged. A firm
knowledge of the scientific laws relating to the behaviour of gases, both in the
vapour and liquid form, is therefore essential because the successful operator must
be able rapidly to recognise symptoms, diagnose the trouble and take action to cure
the problem without delay. The gas behaviour laws also have a great significance
with regard to safe working practice.
Although the gas behaviour laws are invariable, their application when used in
respect of fully-pressurised/semi-pressurised ships, as opposed to fully-refrigerated
gas tankers, is very different. This affects both the design of the two different types
of gas carriers, as well as their respective method of operation, to such an extent
that the book is divided into two main parts. Part I covers the fully-pressurised
semi-pressurised types of carriers, and Part II the fully-refrigerated type. which
always carry their cargoes at about atmospheric pressure. Part III deals with such
general matters as Cargo Calculations and Safety.
The book concludes with suggestions horn of experience for improvements
which would free the operators of much of the embarrassment they now frequently
experience. They are particularly directed to the attention of those responsible for
the design of the ships, higher management and the legislature.
IV
L.P.G. TANKER PRACTICE
TABLE OF CONTENTS

Page Introduction ............................................................................................III
PART I—PRESSURE SHIPS
Chapter I General Description ...................................................................1
Chapter II General Operating Principles.....................................................6
Loading..................................................................................7
Discharging............................................................................7
Refrigerating the Cargo.........................................................8
Gas-freeing.............................................................................9
Chapter III Cargo Handling Equipment........................................................10
Cargo Pumps..........................................................................10
Cargo Compressors................................................................11
Condensers.............................................................................14
Heat Exchangers....................................................................16
Cargo Heater..........................................................................17
Vaporiser................................................................................17
Chapter IV Conduct of Cargo Operations.....................................................18
Semi-refrigerated Cargoes.....................................................18
Loading..................................................................................18
Discharging............................................................................20
Refrigerating the Cargo.........................................................21
Gas-freeing.............................................................................22
Fully-refrigerated Cargoes at Atmospheric Pressure.............24
Loading..................................................................................24
Discharging............................................................................25
Two-stage Refrigeration........................................................27
Precautions to be taken When Starting a Compressor...........29
Points to Watch Whilst a Compressor is Running ................30
V
L.P.G. TANKER PRACTICE
PART II—FULLY-REFRIGERATED SHIPS
Chapter V General Description ...................................................................31
Chapter VI General Operating Principles.....................................................38
Loading..................................................................................38
Refrigerating the Cargo on Passage.......................................38
Two-stage Reliquifaction.......................................................38
Cascade Reliquifaction..........................................................39
Discharging............................................................................41
Gas-freeing.............................................................................42
To Gas-up the Tanks After They Have Been Gas-freed .......44
To Cool Down the Tanks Prior to Loading...........................45
Summary of Gas-freeing and Gassing-up..............................46
Chapter VII Cargo Handling Equipment........................................................48
Two-stage Refrigeration........................................................48
Two-stage Compressors.........................................................48
Seawater-Cooled Condensers................................................52
The Inter-stage Cooler...........................................................52
The Heat Exchanger...............................................................53
Cascade System of Refrigeration...........................................53
R.22 Compressors..................................................................54
R.22 Condensers....................................................................54
R.22 Receivers.......................................................................54
The Cargo Compressors.........................................................55
The Cargo Condensers...........................................................57
Methanol Injection System....................................................58
Vaporisers..............................................................................59
Air Dryer................................................................................63
Cargo Heaters ........................................................................63
Submerged Cargo Pumps ......................................................65
Emergency Cargo Pumps.......................................................68
Deck Storage Tanks...............................................................69
Chapter VIII Cargo Operating Procedure........................................................71
Loading..................................................................................71
To Refrigerate the Cargo on Passage.....................................74
Two-stage Reliquifaction.......................................................74
VI
L.P.G. TANKER PRACTICE
Points to Watch Whilst the Plant is Running.................................76
Cascade System of Reliquifaction..................................................77
Starting the Compressors................................................................78
Points to Watch Whilst the Plant is Running.................................80
To Shut Down the System..............................................................80
Other Points to Watch ....................................................................81
Discharging.....................................................................................81
To Gas-free the Vessel...................................................................82
Puddle Heating...............................................................................82
To Estimate the Time it Will Take to Puddle Heat........................83
Tank Warming................................................................................84
Inerting the Cargo Tanks................................................................84
Flushing Through with Air.............................................................85
Preparing the Tanks to Receive Cargo After They
Have Been Gas-freed......................................................................85
Drying the Air in the Cargo Tanks.............................................85
To Operate the Air Dryer...........................................................86
To Inert the Tanks Prior to Gassing-up......................................86
To Gas-up the Cargo Tanks (at Sea)..........................................86
To Gas-up the Tanks Alongside.................................................87
Cooling Down the Cargo Tanks.................................................87
Procedure When Changing Grades and Types of Cargo................88
VII
L.P.G. TANKER PRACTICE
PART III—GENERAL
Chapter IX Cargo Calculations.....................................................................89
To Calculate the Quantity of Liquid on Board (Metric)........90
To Calculate the Weight of Vapour on Board
(Imperial and Metric).............................................................91
Assessing the Volume of Vapour..........................................92
To Calculate the Quantity of Liquid on Board
(Imperial)...............................................................................93
To Calculate the Correct Volume of Liquid
to Load When Loading a Full Cargo.....................................94
Expansion Relief Valves on Liquid Lines.............................95
To Calculate the Correct Volume of Liquid
to Load When Loading a Part Cargo.....................................95
To Calculate the S.V.P. of a Mixture of Products
at a Given Temperature..........................................................96
To Calculate the Individual Proportions of Vapour
In the Vapour Mixture Above a Liquid Mixture...................97
Molecular Weights ................................................................97
Aid to Memorising the Formulae ..........................................98
Comparison of Metric and Imperial Systems........................98
Table of Properties.................................................................100
Chapter X Safety..........................................................................................102
Safe Navigation......................................................................102
Safe Practice ..........................................................................103
Gas Detection.........................................................................103
Fire Detection........................................................................104
Fire-fighting...........................................................................104
Precautions to be Taken when Entering Spaces
Which May Have a Deficiency of Oxygen............................106
Chapter XI Recommendations.....................................................................108
Safe Navigation......................................................................108
Harbour Control.....................................................................109
Enforcement of Traffic Separation Systems..........................109
Emergency Isolation Valves for Safety Valves.....................109
Greater Consultation between Operators and
Design Staff...........................................................................110
GLOSSARY OF TERMS USED........................................112
VIII
L.P.G. TANKER PRACTICE
LIST OF DIAGRAMS
No. Page
1. Semi/Fully-refrigerated liquid gas carrier....................................................2
2. Arrangement of pipework in cargo tank......................................................2
3. Tank dome penetration.................................................................................2
4. L.P.G. and motor rooms showing arrangement of gas-tight
bulkhead and gas-tight seals........................................................................4
5. Simple vapour line arrangement..................................................................4
6. Tank discharging arrangements...................................................................5
7. Single stage compressor...............................................................................12
8. Single stage refrigeration.............................................................................15
8A. Incondensible Separator or Purge Condenser...........................................15
9. Two-stage reliquifaction using 3 single stage compressors.........................28
10. Fully-refrigerated L.P.G. tanker.................................................................32
11. Schematic view of cargo tank ...................................................................36
12. Schematic diagram of cascade system of refrigeration..............................40
13. Two-stage compressor...............................................................................50
14. Two-stage refrigeration..............................................................................51
15. Double-acting single stage compressor .....................................................56
16. Vaporiser type "A".....................................................................................60
17. Vaporiser type "B".....................................................................................62
18. Air dryer.....................................................................................................64
19. Gas/air heater.............................................................................................64
20. Cargo heater type "A"................................................................................66
21. Cargo heater type "B"................................................................................66
22. Submerged cargo pump.............................................................................67
23. Loading (cartoon).......................................................................................73
24. Graph of Temp./Vapour Pressure Relation Ships......................................101
IX
L.P.G. TANKER PRACTICE
GLOSSARY OF TERMS USED
(pages 112-118)
Boiling
Boiling Temperature
Condensation
Evaporation
Filling of Tanks
Gas/Vapour
Flammable or Explosive Mixture
Flash Point
Ignition Temperature Gas Laws
Avogadro's Hypothesis
Boyle's Law
Charles's Law
Clerk Maxwell's Kinetic Theory
Dalton's Law of Partial Pressures
Gay Lussac's Law
Joule's Law Heat
Latent Heat
Sensible Heat Heel
Liquid Carry Over Mole Pressure
Absolute Pressure
Gauge Pressure
Atmospheric Pressure Span Gas Stratification Temperature
Absolute Temperature
Adiabatic Changes in Temperature
Critical Temperature
Vaporisation
Batch Vaporisation
Flash Vaporisation Vapour
Saturated Vapour Pressure (S.V.P.)
Supersaturated Vapour
Undersaturated Vapour
Superheated Vapour Vapour Return Line Zero Gas
X