Chapter06








 
6        
Cargo Handling Equipment
Centrifugal pumps are
utilised as main unloading pumps on gas tankers. The unloading pumps are located
down in the cargo tankłs swamp or as close to the tank bottom as possible. This
is because the centrifugal pumps do not suck, and are thereby dependent upon
good drainage. The pumps are either the deepwell pump type, submerged type or booster pump.
Normally, the number of revolutions on deepwell and submerged pumps lie on 1300

1800 RPM. Pumps driven with hydraulics have the advantage that the number of
revolutions can be adjusted. Electrically driven pumps normally have a stated
number of revolutions, but lately they are delivered with a variable number of
revolutions, for example 1370/800 RPM. 
Booster pumps normally
have revolutions from 3500
4000 RPM. It is very important to follow the user
manual supplied by the pump manufacturer to ensure what to do before we start a
pump, and what routines to follow at overhaul and inspection of the
pumps.
 
 
6.1 
Deepwell pump
Deepwell pump is the pump type that is
often used on gas tankers. Deepwell pumps are pumps with a long shaft between
the driving motor and the pump. The shaft goes inside the tankłs discharge pipe
from the pump up to the tank dome. The discharge pipe is a solid pipe that goes
up through the tank and out to the flange on the tank dome to the liquid line.
The discharge pipe is constructed with several lengths with pipes, and there is
a shaft bearing on each flange. The bearings are lubricated and cooled down by
the liquid that is pumped from the tank. It is very important not to run the
pump without liquid. This may result in damage of bearings and then the
shaft. 

The motor that drives
the pump is either electric or hydraulic. There is a mechanical sealing device
between the motor and the discharge pipe in the cargo tank. When using the pump,
we must have at least one bar higher pressure on top of the mechanical seal than
we have in the tank. It is important to closely read the pumpłs user manual
about the routines before discharging, because the routines vary some from
different
manufacturer.

 
6.1.1
Submerged pump
Submerged pumps are
multistage centrifugal pumps that are often used as discharge pumps on large LNG
and LPG tankers. The motor and pump are submerged down in the tank sump or as
close to the tank bottom as possible. The motor is connected directly to the
pump with a short shaft on this type of pump. The liquid that is pumped
lubricates and cools the pumpłs bearings. It is therefore essential that the
pump is used only when there is liquid in the tank. The liquid is pumped up
through the tankłs discharge pipe and up to the liquid
line.
This type of pump is
equipped with electrical motor. The cables to the electric motor are either made
of copper or stainless steel. If copper is used in the cable, the cables must be
sheathed with stainless steel to prevent damage on the cable from corrosive
cargoes.  When
transporting Ammonia, the cable and engine must be sheathed with a thin layer of
stainless steel. It is important that the stainless steel sheathing is kept
unbroken, and we must avoid a sharp bend on the cable to protect the stainless
steel sheath. One must at all times check the resistance of the cable insulation
before starting the pump.
Submerged pumps are also
installed as portable pumps. The discharge pipe is then the steering pipe for
the pump. At the bottom of the discharge pipe it is a non-return valve that
opens when pump is lowered and shut when the pump is taken up. Before opening
the discharge pipe it must be gas freed, this is done either with inert gas or
Nitrogen.
 

 
6.1.2 
Booster pumps
Booster pumps mentioned here are
auxiliary pumps for cargo handling. The pump is one-staged centrifugal pump and
is often installed on deck near the pipe manifold. The booster pumps on gas
tankers are used either as a main discharge pump, auxiliary discharge pump, deck
tank supply pump or heater feed pump.
The booster pumps are
driven with electric or hydraulic motor. The engine and the pump are connected
together with a short shaft with coupling in
between. It is very
important that the motor and the pump are aligned according to the manufacturer
manual, and the clearances specified inside are
followed.
Booster pumps that are
regularly utilised should, as a good rule, be turned by hand once a week to
prevent destruction of the motor and pump bearings. It is important that the
booster pumps are blended off on LPG/LEG tankers when carrying cargo with lower
temperature than
50oC. Booster pumps are rarely
designed for temperature lower than
50oC.

 

6.1.3 
Hold spaces and inter barrier
spaces
In hold space and inter
barrier space there is requirement of drainage system separated from the
machinery drain system. The drain system could be submerged pumps, deepwell
pumps or ejectors. These pumps can be used to drain water or cargo spill from
the bilge.
Generally, there are
spool pieces (short pipe pieces) that are produced especially for each hold
space and on each side and fit both to the cargo system and the seawater system.
It is important that the spool pieces are disconnected, and the flanges are
blinded off when the bilge system is not in use.
Example of ejector in hold space  

 
6.2 
Loading lines, pipes and valves
6.2.1 Loading lines and
pipes
The loading lines and
pipes mentioned here refer to gas carrierłs cargo handling system. This involves
liquid lines, vapour lines, condensate return lines, lines to vent mast, pipes
inside the cargo tank and seawater pipes to the cargo cooling plant.

All loading lines on gas
carrier: liquid lines, gas
lines and lines to vent mast have the same requirements as pressure
vessels regarding of temperature and pressure they are meant to handle. All
welding on pipes exceeding 75 mm in diameter and 10 mm wall thickness or more
must be X-rayed and classed by the class company. The same regulation do we have
on flanges and spool pieces also.
All loading lines
outside the cargo tank must be produced by material with melting point no less
than 925oC. The loading lines on gas carriers
are mostly produced of stainless steel, but low temperature nickel steel is also
in use. All loading lines with an outside diameter of 25 mm or more must be
flanged or welded. Otherwise, lines with an outside diameter less than 25 mm can
be connected with treads.
Loading lines designed
for cargo with low temperature, less than
10oC
must be insulated from the ship hull. This to prevent the ship hull to be cooled
down to below design temperature. The hull has to be protected against cold
cargo spill under spool pieces and valves on all liquid lines. This is done with
wood planks or plywood. To prevent cold cargo spill on the hull plates, a drip
tray must be placed under the manifold flanges.
All lines that are
thermally insulated from the hull must be electrically bonded to the hull with
steel wire or steel bands. On each flange on lines and pipes where gaskets is
used, there must be electrical bonding with steel wire or steel band from flange
to flange.
On all cargo lines where it can be liquid it is required
with safety valve. Vapour from the safety valve outlet must go back to the cargo
tank or to the vent mast. If the return goes to vent mast the pipe must be
equipped with a liquid collector to prevent liquid to the vent mast. The
safety valvełs set point is dependent upon the pressure for which
the line is designed. The safety valves must be tested and sealed by the ship
Class Company.

6.2.2 
Valves
The most common valves
used on the cargo handling equipment on gas carriers are ball valves, butterfly
valves and seat valves. All valves used on cargo lines have to be installed with
flanges, and the valves must be electrically bonded to the line either with
steel wire or steel bands.
 
6.2.3 
Ball valves
On semi and fully
refrigerated gas carrierłs ball valves are often used on the cargo lines and
cargo cooling plant. The ball valves tolerate high pressure and large thermal
variations, and they are also approved for chemicals. The valve seats and
sealing devices are produced in Teflon, the ball and spindle is produced in
stainless steel. The ball valve principle function is the pressure on one side
of the ball forces the ball against the seat and the valve is closed. If the
pressure is equal on both sides of the valve, leakage may
occur.
On some types of ball
valves the ball is fastened to the spindle, other types of ball valves have
floating ball. With a floating ball the pressure is equal all around the ball,
and the ball is pressed even toward the seat. With the ball fasten to the
spindle it is pressed aslant towards the seat and the valve seat can be damaged
and the valve will leak.
 
Frequently, particles are left
between the valve ball and the valve house, and these particles can easily cause
damage to the valve seat and the ball. The valves must from time to time be
opened and the ball and seat have to be cleaned especially the manifold valves.
There is a drain hole on the ball itself.  It is of importance to ensure that when the
valve is closed, the drain hole pointing where it is least natural pressure,
then the liquid inside the ball can be drained or boiled off. This prevents
large pressure inside the ball, liquid expansion and wreckage of the sealing
devices around the spherical occurs.

 
Sketch of operation of drainage
hull:
 
Advantages:

Ball valves tolerate
large pressure and thermal variations due to the shape of the ball.  Tolerates both
gases and chemicals. Easy to maintain and overhaul.
 
Disadvantages:
The valves are
expensive, and have costly spare parts. They can be difficult to shut at
temperatures down to
90oC and colder (this can
be relieved by adding a thin packer between the to parts of the valve
house).  Ball
valves are unfavourable as regulation valves, as it is difficult to adjust to
low flow through the valve.
 
 
6.2.4 
Butterfly valves
Butterfly valves are
often used on the seawater line on gas carriers, such as water to heat exchanger
(cargo heater), seawater condenser, oil cooler, the compressors etc. Butterfly
valves are also often used on lines with large diameter as cargo lines, where
there is not such a large pressure or thermal difference. Butterfly valves
should be moved at regular intervals to prevent the seat from fastening and be
damaged and cause leakage valve.
 
Advantages:
This type of valves has
more reasonable price than ball valves. They have lower weight than ball valves
to corresponding pipe diameters. They are better than ball valves for regulation
of flow.
 
Disadvantages:
They are exposed to
cavitation damage on the valve seat and flap when too high liquid flow through
the valve. They are less suitable at low temperatures than ball
valves.
 
 
Seat
valves
Seat valves are
frequently used as one-way valves (check valves) on loading lines, as the
pressure valve on the discharging pump, on condensate return lines back to the
cargo tank and on the inert gas lines. Seat valves are opening by turn the
spindle anti clockwise and the valve seat can wander freely on the spindle. When
the pressure increases in the line under the valve seat, the seat is lifted up
and the valve is open. When the pressure ceases under the valve seat or the
pressure increases above the valve seat, the valve seat will drop down and shut
the valve. Opening or choking the valve regulates the amount of flow through the
valve.
Example on seat
valves:

Sketch on spring-loaded seal valve:

Seat valves that are
used as check valves, must be overhauled at regular intervals, and especially
the seat and contact faces must be polished/grounded as they are expelled for
mark and wear and tear when the valve operates often. The seat valves must also
be moved regularly when they are not in use for a long period of
time.
 
Advantages:
The seal valves are
reliable and simple to operate.  Have large range of utilisation.  Have few wearing
parts. 
Reasonable to maintain.
 
Disadvantages:
Require strict
inspection. Start leaking if wrongly operated.
 
Needle
valves
Needle valves are used
for regulation of cargo cooling plants, both air regulation and for regulation
of Freon in cascade cooling plants. The needle valve is the valve type that
empirically is best suited for regulation of low flow
volume.
 
Heat
exchanger
Heat exchangers are
utilised in several different parts of cargo handling on gas carriers, as heat
exchangers (cargo heater), condensers for cargo cooling plant, vapour risers,
super heaters and oil coolers for compressors. In most of the heat exchangers
seawater is used as the medium on gas carriers, which the products are cooled or
heated against.


 
The heat exchangers that are used for
cargo handling must be designed and tested to tolerate the products the gas
carrier is certified for. Heat exchangers that are used for cargo handling are
considered as pressure vessels, and IMO requires one safety valve if the
pressure vessel is less than 20 m3 and two
safety valves if it is above 20 m3. All heat
exchangers that are used for cargo handling must be pressure tested and
certified by the gas carriers Class Company.
Heat exchangers where
water is used as the medium and are utilised for heating have little or no
effect with water temperature less than 10oC.
Seawater became ice at about 0oC and starts to
free out salt at about 50oC. So with operating
temperatures with a larger variation than from 10oC to 45oC, one ought
to use another cooling medium than seawater. Some terminals do not accept water
as medium in heat exchangers, therefore one must either heat the cargo on route
at sea or the gas carrier must have heat exchangers that do not use water as
medium.
It is of importance to
ensure that the water out of a heat exchanger is never below 5oC. These prevent the water in the heat exchanger
from freezing and eventually damage the heat exchanger.
 
Tube heat
exchangers

Tube heat exchangers are
produced with tube bundles either as straightened pipes or u-formed pipes placed
into a chamber. The pipes in the tube bundle have an inside diameter on 10
to 20 millimetres. There is a cover installed on each end of the chamber to
clean the pipes more easily and maintain these. It is, at all times, important
to ensure that the velocity of the liquid that is being pumped through the heat
exchanger is not too high, to prevent cavity damage in the tube bundle or the
end covers. 
 

 
The
tube bundle is made of stainless steel, carbon steel, copper-nickel alloy,
aluminium-brass alloy or titan.
Which choice of material one decides to choose, depends on
the product one will operate and the costs associated with the investment and
maintenance.
In tube heat exchangers,
where seawater is used as medium, the product to be heated goes in the tube
bundle. This prevents remaining seawater from freezing or
prevents 
 
remnants of salt deposits inside the tubes. Tube heat
exchangers must at regular intervals be cleaned to prevent particles from
settling inside the tubes in the tube bundle or in the end covers. One must
closely check for cavity damage when cleaning the heat exchanger. Ensure that
the gasket is produced in a quality that tolerates the products and temperature
one operates it with. Also, ensure that the gasket is correctly
placed.
 
 
Plate heat
exchangers
 
Plate heat exchangers
are more utilised in cold
storage plants on shore, for example in the fish industry and the meat industry.
Plate heat exchangers are built with thin plates with
double liquid channels. The plates are installed with the flat side toward each
other. The cooling medium and product are pumped each way in the channels to
achieve the best possible cooling or heating. Water or oil is used as the
cooling medium and is dependent upon the temperature of the product that is to
be cooled or heated. Plate heat exchangers are also used as condensers on newer
cargo cooling plants aboard gas tankers.
 

 
Plate heat exchangers must be cleaned at regular intervals to prevent the
channels from clogging with salt deposits or particles from the medium or the product. One must ensure, after cleaning,
that the gaskets are properly placed, and that one uses gaskets that tolerate
the medium and temperatures one operates within the heat exchanger
 
Different heat exchangers utilised onboard gas carriers for
cargo handling
Cargo heater:
A cargo heater is used
to heat the cargo when discharging to an ambient shore tank. A cargo heater is
also used when loading a fully pressurised gas carrier with cargo with
temperature less than
10oC. Seawater or oil is
used to heat the cargo in the cargo heater. It is of importance to remember that
the cargo heater is full of water and have good flow out with water before
letting cold cargo into the heater. Fully pressurised gas carriers are carriers
that are designed to transport condensed gases at ambient temperature, and they
normally donłt have cargo cooling plant.
 
Cargo
condenser:
Cargo condensers in a
direct cargo cooling plant condensate the vapour against sea water, Freon or
other medium as propylene after it is compressed in the cargo compressor. Cargo
condensers in a direct cargo cooling plant can on some gas carriers also be used
as cargo heaters and are designed in low temperature steel that tolerates a
minimum of
50oC.

 
 
Intermediate cooler
An intermediate cooler
is used in a 2-stage direct cargo cooling plant and cascade cooling plant.
Vapour from the first stage on the cargo compressor is pressed down on the
bottom of the intermediate cooler and is condensed against the cargo liquid in
the bottom. The cargo compressorłs 2nd stage
sucks simultaneously from the top of the intermediate cooler to keep the
pressure down. Floaters or D/P-cells regulate the liquid level in the
intermediate cooler. The condensate inside the coil came from the cargo
condenser and is under cooled by the liquid in the intermediate cooler before it
is pressured further back to the cargo tank.
Sketch of intermediate cooler

 
Freon condenser:
Water is used to
condense Freon in the Freon condenser in a cascade cooling plant. The liquefied
Freon is used to condensate the cargo in the cascade cooling plantłs cargo
condenser. Liquefied Freon is also used in indirect cargo cooling plants. The
condensate is then pumped in pipe coils, and cools either directly on the tank
steel or as a cooling medium for ethanol or other
mediums.
 
Vapour riser:
A vapour riser is used
to produce vapour from the cargo liquid. Steam or heated oil is used to heat up
and vapour rise the liquid. The liquid is pumped from one of the cargo tank,
deck storage vessel or from a shore tank and into the vapour riser. The vapour
is used to gas up or maintains the pressure in one or several cargo
tanks.
 
Oil coolers:
The cargo cooling
plants oil coolers use water as a cooling media. The oil coolers must hold the
oil temperature on the different compressors within the specifications
determined by the manufacturer of the cargo cooling
plant.
 
Cargo cooling plant
6.4.1 
Compressors
Compressors are used as
vapour pumps in all modern cargo cooling plants, either to compress or pump
cargo vapour. Compressors are also used to compress or pump cooling medium as
Freon vapour on indirect cargo cooling plant and cascade plant. The compressors
in the cargo cooling plants are produced either as piston, screw or centrifugal
type. We will now look at the different types of compressors and starting with
piston compressors.
 
6.4.2 
Piston compressor
Piston compressors used directly against cargo are of oil
free type. Oil free compressors are used to prevent pollution of oil into the
cargo, and thereby contamination of the cargo. All cargoes we are cooling demand
a high rate of purity. Consequently, it cannot be mixed with oil or be polluted
by other products. With an oil free piston compressor, we mean that the cylinder
liners are not lubricated or cooled with oil.
Piston compressors that
are used against Freon normally have oil lubrication of cylinder liners. Piston
compressors are either built with cylinders in line, v-form or w-form.
Compressors with cylinders in line are built with two or three cylinders either
single-acting or double-acting. V-form compressors are built with two, four,
six, eight or twelve cylinders and are single acting.
 
6.4.3 
Double-acting compressors
Double-acting
compressors are normally oil free and compress the vapour above and under the
piston. The vapour is compressed on top of the piston when the piston goes up
and vapour is sucked into the cylinder below the piston. The vapour is
compressed below the piston when the piston goes down and is sucked into the
cylinder above the piston. This indicates that each cylinder has two suction
valves and two pressure valves. The pistons are equipped with compression
grooves and are not equipped with piston rings.
There is no oil
lubrication of the piston itself, but there is oil in the crankcase on the
compressor. It is of importance that the sealing device between the cylinder
liner and crankcase is intact. In the first stage, the oil pressure in the crank
is checked and compared to the
suction pressure and the cargo tank pressure. Check the user manual for the
cargo compressors and the marginal values for the pressure difference with oil
and suction. This type of compressor is used as cargo compressor onboard gas
carriers. It is important to change the oil in the crank when changing cargo.
This to prevent pollution to the next cargo from the previous cargo. Small
amounts of leakage between the cylinder and crank will at all times occur, so
the oil in the crank contains some of the product that is cooled.
 


6.4.4 
Single-acting compressors
Single-acting
compressors compress and suck the gas on one side of the piston and then
normally above the piston. A suction valve and pressure valve is then installed
in the top of the cylinder. The cylinder top is spring-loaded as a safety
precaution against liquid “knock". The compressors are built with the cylinders
in pairs: two, four, six, eight and twelve, then often as v-form or w-form.
Single-acting compressors are used both as Freon and cargo compressors on gas
tankers.
Piston compressors are
operated by electric motor with direct transmission or strap transmission with a
constant number of revolutions. The number of revolutions is between 750 to 1750
rpm. Unloading of the compressor occurs by hydraulic lifting of the suction
valves. The drawback of piston compressors is that they are vulnerable when the
cylinder liner is filled with liquid and they also have relatively low capacity
for cooling.
Onboard many gas
tankers, there is a liquid receiver on the vapour line between the cargo tank
and the cargo compressor, which prevents the liquid from being carried with into
the compressor. The liquid receiver is equipped with a level alarm to control
the liquid level.

 
6.4.5 
Screw compressor
Screw
compressors are
either oil free or oil lubricated. The type used on the cargo side must be of
oil free type for the same reason as the piston compressors.
The principle for screw
compressors are two rotating screws, the screw that operates has convex threads
and the operated screw has concave threads which rotates them in different
directions. Vapour is screwed through the threads and with rotation on the
screws, the confined gas volume decreases successively resulting in compression.
Please also refer to “cargo cooling process" for more
information.
 

 
The advantage with screw
compressors is that they wear few parts and have low weight in proportion to
cooling capacity. Oil free screw compressors are operated by electric motors
with a constant number of revolutions and have a gear transmission for the
compressor, which has approx. 12000 rpm. The high speed prevents leakage between
the pressure and suction side. Screw compressors with oil injection in the rotor
house have a lower number of revolutions, about 3500 rpm. One can also use
electric motors with direct shaft transmission.
Oil free screw
compressors are used on the cargo side. On the Freon side, compressors with oil
injection are used. 
The oil causes a film on the outside of the rotors that prevents leakage
between the pressure and suction side.  This compensates for the temperature
difference inside the compressor. The capacity of screw compressors is adjusted
by a slide, which is inside the compressor. However, when we reduce the capacity
the excess gas flows back to the suction side. Screw compressors are not
destroyed if they suck liquid, as we find with piston
compressors.


 
Cargo compressors with motors that are installed inside a
deckhouse have two parts, one room for the compressors and one room for the
motors. The room where the motors are installed is gas safe with a constant
excess pressure of air preventing flammable gas from flowing in. If the excess
pressure is too low, the power to the electric motor room will be shut off and
the cargo cooling plant stops. The shaft from the electric motor room to the
compressor room is rendered gas-tight. A mechanical seal device with automatic
oil lubrication is normally used. To prevent bearing breakdown, it is important
that electric motors and compressors are aligned according to specifications
from the manufacturer of the compressor and motor.

 
 
6.4.6 
Centrifugal compressors
On gas tankers,
centrifugal compressors are used to deliver vapour to shore or to supply the
cargo compressors with vapour from the cargo tanks.
Centrifugal compressors are operated by
electric motor, hydraulic motor or with steam, and have a gear transmission. The
compressor has a number of revolutions from about 20000 rpm to over 35000 rpm.
This high number of revolutions sets large demands on accuracy and tolerances at
aligning motor and compressor. The centrifugal compressor is built on the same
principle as a centrifugal pump.
When a centrifugal
compressor is used to feed the cargo compressor, it creates a higher suction
pressure on the cargo compressor, and thereby gives better cooling capacity.
Another area of operation for centrifugal compressors is pumping vapour back to
shore tank while loading. The centrifugal compressor can also be used when
changing cargo. Either to blows hot vapour or to be used as ventilation fan. The
use of centrifugal compressors depends on how flexible the piping system to the
compressor is. On gas tankers, the centrifugal compressor is mounted on deck
close to the cargo manifold. The capacity of the centrifugal compressor is from
approximate 2000 m3 and
upwards.


 
 
6.4.7 
Indirect cargo cooling plant
Indirect cargo cooling
plants are used on cargoes that not can be compressed or exposed to high
temperatures, as they either polymerise or start chemical reactions. Typical
cargo that uses indirect cooling is propylene oxide, ethylene oxide, mixed
propylene oxide and ethylene oxide and chlorine.  There are some different methods for indirect
cargo cooling.
One type of indirect
cargo cooling plant use the discharge pumps and pumps the cargo liquid through a
Freon heat exchanger and back to the cargo tank. This method is energy demanding
as we have to use discharging pump, Freon cooling plant and seawater pump to
control the cargo temperature. On this type of cooling plant the discharge pump
should be of submerged type, deepwell pumps can also be used but we must try to
avoid running those while at sea. Deepwell pumps with revolution regulation can
be used if the ship is not rolling or pitching to mush.
 

6.4.8 
Indirect cargo cooling plant with
utilisation of discharge pump
Another indirect cargo cooling plant resembles the first a
lot, but the discharge pumps are not used. Instead the cargo vapour is condensed
in a Freon heat exchanger and the condensate is pumped back to the cargo tank
with a small pump. This indirect cooling requires less energy than if one also
uses discharge pumps.
A third indirect cargo
cooling plant also uses a Freon cooling plant where cold Freon liquid is pumped
to a coil installed inside the top of the cargo tank or is welded around the
outside of the cargo tank. The Freon compressor sucks Freon vapour from the
Freon liquid collector then presses the vapour to the condenser where it is
condensed against seawater. One can also use ethanol in this cooling system;
ethanol is then pumped round in the coils and Freon is used to cool down the
ethanol.
 

 
6.4.9
Indirect cargo cooling plant with
utilisation of Ethanol in coil round the cargo tank



 









6.4.10                  
Direct cargo-cooling plant

A direct cooling plant

is used
to control temperature on cargoes as LPG, Isobutane, Ammonia and some chemical
gases like VCM, Propylene and Butadiene. Common for all direct cargo cooling
plants is that the cargo vapour is compressed directly in the compressor. It is
the seawater temperature and the type of cargo that decides which condensation
pressure is achieved provided that the cargo is pure. If one for example has
loaded Propylene and the seawater temperature is 20oC, the
condensation pressure will be approximate 9 bars. The pressure needed in
proportion to the temperature is located in the density table for the actual
cargo.

Sketch of two-stage direct cooling plant  

 





Direct cargo cooling plants are operated as one or
multistage, dependent upon the type of compressor, the cargo and the temperature
on the seawater. Most gas carriers that are designed for LPG have direct cargo
cooling plants that can be operated as a one-stage or multistage operation.
With one-stage direct
cooling, vapour is sucked by the cargo compressor from the cargo tanks. The
vapour is then pressed to the condenser and assembles in the liquid collector.
The liquid level in the collector is regulated either by two floaters or the
differential pressure above the liquid level in the liquid collector. The
condensate is pressured back to the cargo tank from the liquid collector via a
regulation valve and in the condensate return line. To use one-stage cooling,
the pressure difference between tank pressure and condensate pressure must be
less than 6 bars.
With 2nd stage direct
cooling without an intermediate cooler the cargo compressor sucks from the cargo
tank with the 1st stage cylinder. The vapour is thereby pressed to the
compressors 2nd stage suction side and then to the cargo condenser where the
vapour is condensed against seawater and collected in the liquid collector. The
liquid is pressured back to the cargo tank via a regulation valve and the
condensate return line from the liquid collector. The pressure in the liquid
collector is equal to the pressure in the cargo condenser, and is at all times
higher than the cargo tank pressure. 2nd stage direct cargo cooling plant is
delivered with or without an intermediate cooler.
 
Some direct cargo
cooling plants are delivered with intermediate cooler (inter cooler), this
achieves lower temperature and pressure on the 2nd stage suction side.
These cargo cooling
plants are used on semi-pressurised LPG carriers and atmospheric pressure LPG/
NH3 carriers.
 
 
6.4.11                  
Cascade cooling plant / direct
cooling
Cascade cooling plant is basically a direct cargo cooling
plant where the cargo is condensed against Freon and Freon is condensed against
water.
Sketch of cascade
plant
 


In a cascade cooling plant there is a Freon cooling plant in
supplement to a direct cargo cooling plant. The Freon cooling plant contain of a
compressor, Freon liquid collector, oil collector, Freon separator, regulation
valve and pump. Freon are condensed against water, and the Freon condensate is
pumped from the separator to the cargo condenser where the cargo is condensed.
The Freon plant have to be started first, so the condensation and circulation of
Freon in the cargo condenser is normal. There must be accuracy in the start-up
to prevent oil from leaking with Freon and removing the condensation effect.
When the Freon plant operates normally, one can start the cargo compressor.
On the cargo side in a
cascade cooling plant there is mostly 2nd stage direct cargo cooling plant
equipped with compressor, intermediate cooler, cargo condenser, liquid collector
and regulation valve.
The vapour is sucked
from the cargo tank and into the cargo compressorłs 1st stage. The vapour is
then pressured to the intermediate cooler where it is condensed against the
liquid in the bottom of the intermediate cooler. The cargo compressor sucks
vapour with the 2nd stage from the top of the intermediate cooler and press the
vapour to the cargo condenser where the gas is condensed against Freon. The
condensate is then pressured against a coil in the intermediate cooler and
further through a regulation valve to the condensate line, and back to the cargo
tank.
This type of cargo
cooling plant is used on semi-pressurised LPG and LEG carriers, and on large
atmospheric pressure LPG carriers. A cascade cooling plant must be used for
condensation of Ethane and Ethylene, but can also be used for Propane, Ammonia
and Propylene. Some cascade cooling plants are constructed for use as a two or
one-stage direct cargo cooling plant. Generally when cooling Butane, Butadiene
or VCM one can also condensate Propane and Propylene directly if the seawater
temperature is low enough.
This type of cargo
cooling plant has a lower dependency of the seawater temperature than a direct
cooling plant. The larger volume of Freon, seawater temperature has less
influence on the plant. It is difficult to cool regardless of the kind of
cooling plantłs if the surrounding temperature e.g. seawater temperature is
higher than 35oC. 

 
6.5 
Inert gas plant
On gas carriers inert gas
is used for different
purposes, some are requirements other is to maintain the ships hull and
spaces:

 
·        Have neutral atmosphere in hold and inter barrier
spaces
·        Elimination of cargo vapour from the cargo tank when gas
freeing
·        Eliminating oxygen from the cargo tank before
loading
·        Drying up hold spaces or inter barrier spaces to achieve
a neutral atmosphere and to prevent corrosion in the spaces
·        Placing a neutral vapour above the cargo if required
 
When carrying flammable
cargo on fully refrigerated gas carriers there is a requirement to have a
neutral atmosphere in the hold space or inter barrier space either with dry
inert gas or nitrogen. If the gas carrier does not have an inert gas plant or
nitrogen plant, it must have a storage vessel with inert gas or nitrogen with
capacity of 30 days and nights consumption. The definition of consumption here
is the leakage in the vents and manhole. If the cargo is not flammable we can
have dry air, inert gas or nitrogen in the spaces.
 
If the cargo is Ammonia, one must not use inert
gas that contains carbon dioxide, only dry air or nitrogen, because
carbon dioxide reacts chemically with Ammonia. It is always beneficial to keep
spaces around the cargo tanks dry.
 
The inert gas generator is built up with a combustion
chamber, scrubber, O2 analyser, dryer and heater. The fuel oil is injected into
the combustion chamber, mixed with air, combusts and flue gas or inert gas is
formed. The inert gas is blown through the scrubber where carbon particles and
sulphur is washed out with the water. The O2 analyser measures the O2 content
against the stated limits before the inert gas is blown further into the dryer.
There is two types of dryers for inert gas plants either Freon dryer or
absorption dryer. The most common is the Freon dryer.
 
After the scrubber the
inert gas is cooled in a dryer to reduce the dew point. With use of Freon dryer
the dew point will be minimum 5oC. Water is condensed out while the dew point is
reduced and we have to avoid temperature to sink below 0oC so we do not clog the
inert channel with ice.
In an absorption dryer
the inert gas is compressed with a compressor and pumped through a material that
absorbs water and the dew point sinks to minimum
80oC.
Strict demands are made
regarding inert gas plants on gas carriers. IMO makes demands for maximum
content of 5% O2 by volume. Inert gas is produced on gas tankers by their own
inert gas generator. Inert gas produces by consuming gas oil, diesel oil or
light fuel oil. The O2 content in the inert gas adjusts by quantity of air added
to the oil that is injected into the combustion chamber.
To achieve as pure inert
gas as possible, very good combustion is required. A rich oil/air mixture gives
a lot of carbon, high content with Carbon monoxide and low O2 content. A lean
mixture (more air) gives higher O2%, less carbon and less carbon monoxide. The
air/oil mixture is produced manually or automatically on the control
board.
 
6.5.1 
Sketch of inert gas
plant
 

6.5.2 
O2
Control

O2 analyser is connected
to a two-way valve where the inert gas either can be sent to a ventilation mast
or to a consumption unit (dryer, tanker). The limit value is set manually
between 5% by volume O2 and the minimum value for the inert gas generator for
example 0,3% by volume O2. The inert gas then automatically goes to the vent
mast if the O2 content reaches more than 5% by volume or below 0,3% by volume.
O2 content is set to the required O2 volume, for example 1% by volume. The inert
will then go to the dryer and is consumed when the O2 content is between 1% and
0,3%.
 
 
6.5.3 
Drying

The inert gas is
saturated with water when it comes out of the scrubber, that means 100%
humidity. The temperature on the inert gas after the scrubber is about 5oC above
the seawater temperature.
The inert gas therefore
must be dried before it is sent to the cargo tanks, hold space or inter barrier
space to prevent condensing of water into the tanks or spaces. The inert gas
temperature should be higher than the atmosphere that one will inert. Inert gas
dryer is a Freon heat exchanger, absorption dryer or a combination of
both.
 
 
6.5.4 
Freon dryer

Freon dryer are
frequently used and require less space in proportion to an absorption dryer. The
principle with the Freon dryer is that Freon flows through small pipes in the
inert gas channel. The inert gas is cooled down and thereby condensate the water
from the inert gas when it passes the Freon pipes. The Freon is condensed in its
own cooling plant. The temperature of the inert gas after the Freon heat
exchanger must not be less than 5oC. The inert gas that comes out of the Freon
dryer has a dew point of about 5oC and a water content of 6,75 gram per m3 inert
gas.
 
6.5.5 
Absorption dryer

With use of absorption
drier the inert gas is then pressed through a medium that absorbs water, for
example silica gel or Aluminium Oxide. The inert gas has a temperature a bit
above seawater temperature when entering into the dryer blower. The temperature
of the inert gas is higher when it emerges from the dryer, from 30oC to 60oC,
depending on the required dew point. The result of temperature increase is that
the compressor compresses the inert gas. One can have an inert gas dew point
down to
80oC with an absorption dryer, but the inert gas volume that is
delivered for consumption decreases. The inert gas contains 0,0013 gram water pr
m3 at a dew point of
60oC. Inert gas with temperature of 40oC and dew point at

60oC has a relative humidity of 0,025%.
 
 
6.5.6 
Inert gas heater

An inert gas heater is a
heat exchanger where steam or an electrical coil is used for heating the inert
gas. The dried inert gas can absorb more humidity when it is heated. Heating
reduces the relative humidity on the inert gas. The relative humidity is 28,72%
at a temperature of 25oC with Freon dryer and an inert gas dew point of 5oC. If
the inert gas is heated to 50oC, the relative humidity will sink down to 8,13%.
It is of importance that one first removes humidity, and thereby heats the inert
gas so it can absorb more humidity.
The inert gas dryer and
heater can also be used in connection with venting tanks and spaces with
air. 

To maintain the function
of the inert gas generator to specification, one must run it regularly,
generally once a week and preferably several hours each time. This is a good
opportunity to refill spaces and lines, which are not
used.6.5.7 
Sketch of thermal drier

A     
Drying tower
B     
Tower that is dried
C     
Heater
D     
Cooler
E     
Fan
F     
Water separator
S     
Solenoid valves
 

6.5.8 
Sketch of pressure swing drier
 


A      Drying tower

B     
Tower that is dried
C     
Two-way valves
E     
Exhaust
R     
Regulation valve
S     
Solenoid valves
 
6.5.9 
Composition of inert gas and dew
point
 

 
6.6 
Safety valves
Gas carriers must have safety valves on all
cargo tanks, spaces and cargo lines where cargo liquid residue may remain. Cargo
tankłs safety valves are either pilot (pressure loaded) or spring loaded
valves. Spring-loaded valves are normally used on fully pressurised tankers and
semi pressurised tankers with a tank pressure above 0,7 bars and on cargo lines.
The pressure loaded valves are normally used on atmospheric pressure tankers and
semi pressurised tankers.
There must be two safety valves on all kinds
of pressure vessels on more than 20 m3. There are also demands that there is a
safety valve on all kinds of pressure vessels below 20 m3. The maximum set
pressure on a cargo tankłs safety valves depends on the cargo tank MARVS.
MARVS is maximum allowed safety valve set point. The pressure required by MARVS
is located in the gas carriers IMO Certificate of Fitness. The cargo tank safety
valve must be located on the tankłs highest point above deck. Each safety
valve must be connected to vent mast without impediment or valves.
The vent mastłs outlet must be at least B/3
or 6 meter above weather deck or gangway, B is the ships breadth. The distance
should at least B or 25 meters from the nearest air inlet or opening in the
accommodation. This distance can be shortened for gas carriers of less than 90
meters in length, but the flag state authorities, for example Norwegian Maritime
Directorate, must approve it.
All safety valves on cargo tanks must be
prototype tested and approved by IMO and the gas carriers class company. The
cargo tankłs safety valves must be tested within the IMO limits +-10% for 0 to
1,5 bars, +-6% for 1,5 to 3 bars and +-3% for 3 bars and higher pressure. The
tankerłs class company has to seal the safety valves after authorised
personnel have tested and calibrated the safety valves.

 
 
6.6.1 
Cargo tank safety valvełs function
Safety valves used on cargo tanks have one or
more pilots to hold the valve closed. The safety valve contains of an adjusting
spring, three membranes, two valve seats, an exhaust pipe and an equalising
pipe. The pilot is adjusted by a pilot spring in order to get the needed
pressure, for example 0,5 bars.
The pilot valvełs seat is attached to two
membranes and the pilot spring. The pilot main valve seat is attached to the
main valve membrane. The pilot valve is connected to a pipe on the highest point
on the cargo tank.
There is the same pressure below and above
the main valve seat and on the below the boost membrane when the pilot valve is
shut. When the pressure in the cargo tank is higher than the pilot valvełs
setting, the boost membrane will lift, pull the pilot seat up and the pressure
above the main valve membrane is ventilated to the atmosphere. The pressure will
now be higher above the main valve seat than below and the valve is open and
vapour is ventilated to the vent mast.
When the cargo tank pressure sinks again, the
boost membrane will sink and the pilot seat will go to the shut position. The
pressure above the main valve membrane increases to the same pressure as in the
cargo tank. The main valve seat will then be closed and the valve shut.

6.6.2 
Example of a tank safety valve
There are extra setters that are
installed on the pilot valve to achieve the right set point on fully pressurised
tankers and semi-pressurised tankers. The setter consists of an adjusting spring
with spring tension equal to the pressure, for example 2,3 bars. When the setter
is screwed down on the pilot, the set point will be at 2,8 bars.
The cargo tank safety valves on atmospheric
pressure tankers are normally the membrane type. The principle is the same as
with seat valves. When the valve is shut there is equal pressure under and over
the main membrane and under the boost membrane. When the pressure is higher than
the pilot setting, the boost membrane in the pilot will press the pilot seat up
and the valve start to open. When the pressure sinks, the pilot seat is pressed
back and shut.
The valve opens when the tank pressure
exceeds the spring tension. When the tank pressure sinks below the spring
tension, the valve shuts again. An adjustment screw is attached on top of the
valve that is used for calibrating the spring tension.



On fully refrigerated gas carriers
there is often options to mount extra weights during loading or change of cargo.
The extra weights are mounted on top of the pilot and increases the set point
with approximates 100 to 150 grams.

 

THE EXTRA SETTER IS NOT ALLOWED TO HAVE ON THE PILOT WHILE
THE VESSEL IS AT SEA.
 
6.6.3 
Safety valves on cargo lines/
pipes
Seat valves are mainly
used as safety valves on lines. These safety valves are spring-loaded and must
be according to the certified line pressure. The set point and the number of the
different safety valves can be found in the gas carrier valve list. The safety
valves must be overhauled, pressure tested and calibrated by authorised
personnel. Then sealed and by the ships class company.
Example on safety valves on cargo
lines/pipes