Ch15 tank cleaning

ZODIAC MARITIME AGENCIES LTD.
GAS OPERATIONS MANUAL
15
15.1

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TANK CLEANING
Changing of Grades

Prior to loading it must be verified that the tanks are clean enough to receive the cargo without any
risk of obtaining an out of specification product.
Tank cleaning is a significant and important part of the gas operations and may require a
considerable number of working hours. It is essential that the operation is carefully planned and an
attentive monitoring and follow-up is performed in order to ensure the desired degree of
cleanliness and effectiveness is achieved.
The tank cleaning procedure should not be limited to the removal of liquid, vapour or inhibitor
from the last cargo only. If a cargo tank during inspection is found to be particularly rusty it should

be hard brushed or cleaned by high pressure water wash all over where accessible before the
methanol wash.
The table Changing of Grades contains by means of codes guidelines for the minimum
requirements with maximum allowable content of last cargo. Normally the vessel will receive the
charterer's requirements when changing grade(s) in the fixture telex. The following codes have
been used:
1:
As stated in the Guidelines for CARGO HANDLING
2:
Visual inspection by ship's crew.
3:
Tanks heated to loading temperature of the next cargo
4:
Tanks heated to liquid free and pressure 1.5 bar if possible
5:
Methanol wash
6:
Purging with nitrogen only
7:
Purging with inertgas or nitrogen
8:
Gassing up with vapour next cargo
9:
Charterer's additional requirements as advised by operations department
10:
Not permitted
The visual inspection may require some or all of the following: Tanks liquid free by means of
heating of tanks to ambient temperature or by stripping by means of pumps or ejector - purging
with lnertgas to below LEL -purging with fresh air to below TLV and approximately 21% oxygen
- vacuum cleaning - spot cleaning with methanol.

Propylene Oxide must not be loaded into tanks that have held Ammonia on the last
three previous loads.
If a grade not mentioned is to be loaded thoroughly, instructions will be received from ZMAL.

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15.2

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Gas Freeing of Cargo Tanks

Preamble
During the initial phase of tank cleaning when inerting is commenced a substantial amount
of vapour is released to the atmosphere.

Objective
To avoid harmful concentrations of vapour that are released during tank cleaning from entering the
accommodation and engine room.

1.

Lining Up

The Cargo Tanks to be gas freed and connected to the forward vent stacks if possible.
1.1

Connecting the Inert Gas Line

The Inert Gas Line shall only be connected to the Cargo Tanks when the tanks are ready for
inerting and the Inert Gas Plant is ready to produce inert gas of the requested quality.
The time for connecting and disconnecting the Inert Gas Line to the Cargo Tanks shall be noted in
the vessel's official log book.

Venting Off Vapour
The gas concentration in the cargo tanks is highest during the initial hours of venting off and will
gradually decrease as the atmosphere in the cargo tanks is diluted by inert gas (or air if cleaning
after ammonia.)
To ensure that the vapour vented off during the initial hours is not drifting towards the
accommodation and air intakes to the engine room it may be necessary:

In port

Terminal or port regulations regarding venting or purging of cargo vapour must be observed. Such
regulations may require that vented cargo vapours should be led to a flare, or vent stack or for use
in the shore plant.
If the cargo vapour is not led ashore advance notification should be given to the port authorities
and permission obtained before starting the operation.
If it is required to reduce the nitrogen pressure of a cargo tank containing cargo such nitrogen
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should be regarded as cargo vapour and port regulations regarding venting cargo vapour must be

observed.
En route
to alter the course steered if the relative wind direction is unfavourable, at anchor
to heave up the anchor and steam to find most favourable relative wind direction.
If vapour in harmful concentrations cannot be avoided at the accommodation and engine room air
intakes, it may be necessary to postpone the tank cleaning in which case the Profit Centre shall be
advised immediately.

3.

Re-circulating Air-Conditioning

The air-conditioning plant should be changed to recirculating during the initial hours of venting
off vapour.
Personnel working in the engine room shall be informed when venting off vapour is commenced.
As the air intakes to the engine room cannot be completely closed during tank cleaning, the
engineers will often be the first to notice any detectable gas.

4.

Flexible Hoses

It should be avoided to use flexible hoses to vent off vapour over the stern of the vessel as the air
flow around the aft part of the vessel is very turbulent and the vapour therefore may enter the
accommodation and engine room.
However, prevailing weather conditions and the ship's destination may favour the use of flexible
hoses for venting cargo tanks over the stern.

Venting Ammonia Usung Flexible Hoses
Ammonia will when in contact with water generate a intense heat which is able to melt the hose

and ignite any remains of previous cargoes clinging to the inside of the hoses. This has been
experienced in a gas carrier when venting after the carriage of ammonia. The generated heat
melted the hose connected to the manifold immediately after opening the manifold valve and
ignited the polymerised butadiene residues.
As a means to prevent dangerous reactions the following precautions should be observed.
Before being used the hoses should be checked for cleanliness from remains of other
cargoes from previous tank cleaning operations. It may be necessary to place the hoses on
deck and leave them expanded for a few hours in order to allow the hoses to attain ambient
temperature and to ensure the hoses are dry.
After being used the hoses should be cleaned. Prior being collapsed and stored the hoses should be
left on the deck in expanded condition for some time in order to enabling them to be thoroughly
dry.

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15.3 High Pressure Water Wash
After the carriage of cargoes containing inhibitor (VCM, CC4, Butadiene) it may be
necessary to wash the tanks, filters, valves and pipes with water.
Before the high pressure water wash is carried out the tanks must be ventilated to not less
than 20 percent oxygen and zero percent LEL. The KEW high pressure pump placed at the
engine entrance aft is connected to the special high pressure hoses and operating handle.
For water cleaning the following equipment is employed:
1 piece KEW high pressure pump
1 piece sewage nozzle
2 x 80 m half inch pressure hose
1 piece transfer pump
Quick-lock connections and gaskets for above mentioned equipment.
The high pressure pump which is not intrinsically safe and therefore not safe for use on
deck must be placed in engine opening aft.
Procedure for cleaning with high pressure water
A.
The manifold valves should be high pressure washed with water. The valves shall
be opened and closed several times enabling residues to be removed from the ball valves.
B.
The filters are to be opened both at the top and the bottom and the filter elements to
be removed and brushed with methanol prior the high pressure water wash. The filter
housing should be brushed with methanol and thereafter washed with water as well. All
surfaces must be clean and without discoloration.
C.

The cargo pipes must be inspected as far as possible and the necessity of cleaning
evaluated. Only if cleaning is needed the sewage nozzle should be fitted to the high
pressure hose and the hose pushed as far as possible into the pipe connections. Care should
be taken that water cannot come into areas where it cannot be removed. If water may enter
such areas methanol should be used for cleaning.
The high pressure washing is initiated in the tank dome and all pipes and reachable
structures should be thoroughly washed. The tank bottom and tank sides are to be high
pressure water washed as high up as the equipment permits. In order to obtain an efficient
cleaning of the tank steel the nozzle must be kept at a distance of approximately 10 cm
from the tank wall. The washing is carried out ensuring that no part of the tank wall is left
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untouched as this will leave dark stripes on the steel. The water is removed by using the
membrane pump.
E.
When the high pressure water wash has been completed, the tank(s) should be dried
immediately using dry air from the inert gas blower via the spray lines or with inert gas if a
subsequent methanol wash is required.
15.4

Methanol Wash with Portable Sprinklers

After the carriage of cargoes containing inhibitor (VCM, CC4, Butadiene) it may be
necessary to wash the tanks, filters, and pipes with methanol.
Before being washed with methanol the tanks must be inerted to below LEL and ventilated
with fresh air to below the TLV value for the cargo carried. The tank is to be vacuum
cleaned prior to the commencement of the methanol wash. Special attention is to be paid to
the slush bulkhead which may entrap inhibitor or polymerised cargo residues and loose
rust.
The valves and filters at the manifold should be cleaned as mentioned in the procedure for
cleaning with high pressure water wash.
If the initial inspections shows the tank to be particularly dirty or rusty it should be cleaned
by high pressure water wash or hard brushed all over where accessible before the methanol
wash.
4 sprinklers are to be fitted on the lower spray line and if available a 5th sprinkler should be
rigged at the top platform. The 5th sprinkler to be used to clean the tank dome and vertical
pipes in the tank.
Each sprinkler is fitted with a hose leading to the portable manifold, which is placed on
deck close to the tank to be washed. The portable manifold is connected to the methanol

pump in the compressor room using a 1 inch high pressure hose. Membrane pump fitted
with suction is placed at the pump sump and secured. Additionally, discharge hose and air
hose are connected to the pump and properly secured. The air valve on the pump is opened
fully and secured in order to make it possible to operate the pump at the air valve on deck.
In order to avoid the blocking of rust in the sprinklers or hoses the portable manifold
should be soaked in a bucket of methanol for a period of not less than 12 hours prior being
used. Test of the manifold should be done by connecting it to the fresh water supply at the
methanol pump and using the methanol pump toflush the manifold by opening one valve at
a time to try to dislodge any flakes of rust that may clog the sprinklers whilst they are in
use down the tanks. If the methanol pump cannot create a high enough pressure half inch
ball valve should be connected to the manifold at the sprinkler connections to regulate the
pressure travelling through the manifold. When the test of the manifold
is completed the sprinklers should be connected to the manifold and their free rotation
tested.
The tanks must be inerted to max. 5.5 percent Oxygen prior to the commencement of the
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methanol wash.
The cleaning equipment.
For the methanol wash the following equipment is employed:
Vessel's fixed methanol pump
75 m 1 inch high pressure hose.
I portable manifold for 4 pieces of half inch hoses.
4 pieces 30 m half inch hoses
4 pieces of sprinklers
1 transfer pump

Procedure for methanol wash
1.

Inerting to the tank to be ceased.
2

Using the fixed methanol pump the methanol spraying is conducted for a period of 2
minutes. 2 Sprinklers are opened simultaneously in order to achieve a nozzle pressure
of approximately 12 bars and thereby sufficient range. If a fifth sprinkler is fitted
spraying through this is conducted for a period of 1.5 minutes.

3.

After a total spraying time of 4 minutes (5.5 minutes if the fifth sprinkler is
fitted) the methanol pump is stopped and the tank is left for a period of not less than 60 minutes.
The methanol's solvent effect is thereby utilized on the remains of inhibitor. A sample of the
methanol should be taken and kept.
4. During the waiting time the other tanks are sprayed with methanol using the same
procedure and The sample pipes are flushed with methanol.
This is done by connecting the permanently installed methanol pipe above the dome to the sample
cocks using a flexible hose and pump methanol through the pipes. The sample cocks placed at the
tank dome and at the pump discharge pipe are to be dismantled and inhibitor or polymerized cargo
residues removed. At least one non return valve at the pump discharge pipe must be dismantled
and inspected. The necessity of cleaning and dismantling the other non return valves is then to be
evaluated.
5.

The fresh water line on deck is connected to the methanol pump through the
pump's suction strainer using a 3/4 inch air hose with claw connection.

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7.

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The tank is now sprayed with fresh water through I nozzle at a time for a period
of two minutes. The necessity of resuming the inerting should be evaluated.
Check that the pump sump is filled with liquid. Start the deepwell pump briefly
(a few seconds only) and flush the discharge line from the pump.

8.

Tank is now to be stripped for liquid using the membrane pump. To facilitate the
stripping of the tanks a limited positive trim of approximately 0.5 m should be
achieved. In tank 2 and 3 liquid will be placed aft of the pump only. If a positive
trim of less than 0.4 m is obtained liquid is placed forward of the pump sump
increasing to the whole length of the tank when at even keel. In tank I and 4
liquid will be placed in or around the pump sump if a positive trim up to 1.0 m is
obtained. The stripping is to be continued throughout step 9 to 12 keeping the
pump sump as empty as possible.

9.

The tank is now sprayed with fresh water through max. 2 nozzles at a time. The
spraying is to be continued until a clear sample of water is obtained.

10.

The tank is now sprayed with methanol using the same procedure as under step 2
but this time limited to a period of 1.5 minutes. A sample of the methanol
pumped up is to be taken. Together with the sample taken under step 3 an
evidence of the cleaning efficiency is hereby achieved. The samples should be
presented to the surveyor inspecting the tank(s) prior to the loading. The sample
should be kept until the discharge of the cargo loaded is completed.

11.

The tank is now sprayed with fresh water as mentioned under step 9. The
spraying to be continued until a clear sample of water achieved and for a period
of not less than 10 minutes in order to clear all hoses and pump for methanol.

12.

Step 10 and 11 are to be repeated until a clear sample of methanol has been
achieved.

13.

After the final spraying which must be with water the tanks are dried out using
fresh air from the Inert gas blower. During the final inspection the necessity to
vacuum clean, spot cleaning with methanol or high pressure water wash is to be
evaluated.

Tank flannel should not be used for a possible spot cleaning as its fluffing properties will create a
downy surface which is very difficult to remove.
Hazard and health data for Methanol
Methanol is a flammable liquid which at 'room' temperatures gives off a vapour which is
both toxic and when mixed with air, explosive within certain limits. If liquid is spilled on
clothing and allowed to remain it may cause smarting and reddening of the skin. Vapour
may be absorbed through skin and inhalation may cause dizziness, unconsciousness, and
sighing respiration. The TLV value is 200 ppm.
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Exposure procedure: remove victim to fresh air. Remove contaminated clothing. Flush
affected skin or eye gently with water for at least 15 minutes. If breathing has stopped or is
weak or irregular give artificial resuscitation or oxygen as necessary. Obtain medical
advice or assistance as soon as possible.
Safety measures
It is not allowed to enter the tank without chemical suit and full breathing protection if the
concentration exceeds 200 ppm. The methanol concentration is measured by Drager
Multigas detector or Drager Tubes. The methanol concentration must be well below 200
ppm on arrival at the loading terminal since the terminal staff will not enter the tanks if
breathing apparatus has to be worn.

15.5 Methanol Wash With Fixing Machines
After carriage of cargoes containing inhibitor it may be necessary to clean tanks, filters and
pipes with methanol. It may also be stipulated in the charter party that cleaning with
methanol is required prior to loading Propylene Oxide.
Before methanol wash is commenced, tanks and lines must be inerted.
For vessels equipped with fixed washing machines, it may not be required to enter tanks
prior to
commencement of methanol wash. It is however, important that all cargo including vapour
is removed from tanks by the use of inert gas prior to commencing cleaning.
When inerting is completed, tanks are pressurised to a minimum of 1.5 Bars overpressure
by use of inert gas. The ejector will not be effective until at least 1.25 Bars overpressure
has been reached. Spool pieces are inserted from methanol wash system to tanks which are
to be washed as per piping diagram.
The methanol pump is started for five minutes and tank washed, one tank half at a time.
Discharge pressure from the methanol pump should be 8.0 Bar.
After an initial wash of five minutes, the cargo tank must be drained by use of the
methanol driven ejector, which is also used throughout the entire washing.
The cargo tank is washed four times in total, each time for approximately five minutes. If

cargo tanks are expected to be very dirty, the cycles may be repeated.
During the second washing period, the cargo pump is started when the methanol level
reaches approximately 8 centimetres. The methanol is recirculated through the upper spray
line and the filling line. Washing with methanol machines is continued until just before the
cargo pump is stopped.
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During final washing period, the cargo pump is again started, and the methanol is then
transferred to the manifold, from where it is drained to the methanol storage tank using the

ejector.
Flushing of the manifold as above should be repeated until satisfactory cleanliness of pipes
is reached. The colour of methanol from the manifold is checked by draining some
methanol to the drip tray. During washing and stripping of cargo tanks, the vessel should
maintain a stern trim of at least one metre.
When cleaning is completed, the cargo tanks are ventilated to fresh air until atmosphere
less than 200 PPM methanol before the tanks are entered for inspection, and eventual
manual cleaning/sweeping or vacuuming.

15.6

Product Washing With Fixing Machines

In order to optimise the cleaning of the cargo tanks, some gas carriers are equipped with fixed
washing machines, which apart from being used for methanol washing are also capable of washing
with product.
Product washing shall be conducted if cargo tanks are expected to be extremely dirty, or when
carrying inhibited products which are suspected of leaving deposits in the cargo tanks i.e.
Butadiene.
Before product washing is carried out, permission from the Nautical Department must be obtained.
The Nautical Department will liase with Gas Tanker Department before issuing permission.
Product washing must under no circumstance be performed when carrying Propylene Oxide.
1.

Procedure

1.1

Preparation

The specially manufactured 1.5 inch pipes connecting the washing machines to each individual
cargo tank system shall be inserted on all tanks to be product washed prior to arrival at the
discharge port.
1.2

Planning

The discharge operation should be planned so as to complete individual tanks at intervals
sufficient to product wash each tank without reducing the overall discharge rate.
1.3

Commencement

The washing is commenced when the surface of the cargo is below the washing machines at
approximately 2 metres above the bottom of the tanks. Washing is per-formed by recirculating
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some of the liquid through the washing machines via the specially manufactured pipes.
1.4

Low Discharge Pressure

In order to maintain sufficient pressure to the washing machines, if the discharge pressure is below
5 Bars, the pneumatic discharge valve is closed during the product washing. The valves to the
washing machines shall be opened prior to closing the pneumatic discharge valve. In order not to
damage the deepwell pump, the recirculation valve on the liquid line shall be opened slightly
during washing with the closed pneumatic valve. When one cycle is completed, the discharging is
resumed in the normal way. Liquid will remain in the bottom of the cargo tanks, when washing
with the closed pneumatic discharge valve, and the bottom of the cargo tank will not be washed.
1.5

High Discharge rate

If the discharge rate is too high to complete one cycle with washing machines (approximately 10
minutes) from the time the liquid surface is below the washing machines until the cargo tank is
dis-charged, then the discharge rate from the individual cargo tank to be washed shall be reduced
during the washing by opening the recirculation valve on the liquid line slightly. Discharge should
be continued from the other cargo tanks at full rate.
1.6

Washing of the tank bottom

In order to optimise washing of the cargo tank bottom, the washing should, if possible, continue
until the final stripping of the cargo tank is commenced.
1.7

Vapour remaining on board

Due consideration should be given to the amount of vapours produced when the cooler liquid
comes in contact with the warmer tank walls. Product washing shall thus not be performed if the
bill of lading figure is higher than the cargo quantity actually on board, unless it is possible to recondense the vapour after completion of discharging the liquid.
1.8

Inhibited cargoes

If product washing is performed with a cargo containing inhibitor, it shall always be followed by
methanol washing in order to avoid any inhibitor being left in the washing system.
1.9

Nitrogen Displacement Discharge

If efficient discharge with introduction of nitrogen to cargo tanks is used - according to Section E
13 in these guidelines - product washing shall not be per-formed.
15.7

Tank Cleaning After Inhibited Cargoes
On a number of occasions after cleaning of inhibited cargoes it has been observed that

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remains from polymerised product have been left in the tanks. This is extremely difficult to
remove.
It is very likely that polymerisation - which takes place in the presence of oxygen - has
occurred during the initial stages of ventilating with atmospheric air following the inerting
to LEL. At this time butadiene or VCM vapours are present simultaneously with the
introduction of oxygen.
In order to avoid or at least reduce the amount of polymerised product the ventilation
should not be started until no gas can be detected in the tank atmosphere, using Riken

interferometer or Drager test tubes.
It should be noted that the interferometer will also react on the inert gas showing a
concentration of about 1.5%.
Furthermore the venting off from each tank must only be conducted through one line, and
test pipes should only be opened during checks on the tank atmosphere, until an oxygen
concentration of 20.8% has been reached, thus ensuring that the risk of product
polymerising in the spray lines and test pipes will be minimal.

Tank Cleaning and Ventilating after Ammonia on the
Semi–Refrigerated Vessels
When tank cleaning after an ammonia cargo, special measures are required.
Evaporation of Liquid Heel
The compressors are used for heating up the liquid heel (which then evaporates), the
ammonia vapours, and the tank steel. It is very important to heat up over the whole length
of the tank, but heating of the pump sump should be emphasised. As a rule of thumb, this
sump should be heated for an additional I to 2 hours after all the liquid heel has been
evaporated in order to prevent re-condensation of ammonia vapours against the cold tank
steel.
Experience has shown that it is advantageous to heat up to approximately + 200C. The
extra time consumed for heating in comparison to heating to a lower temperature is more
than saved during the following phases of tank cleaning.
For reasons mentioned later in "Sub-cooling", heating to temperatures lower than + 200C
should be avoided in order to prevent possible damage to the tank steel.
Removing the Overpressure
The overpressure is reduced during heating in the usual manner through the vent masts
under close supervision of the thermometers, especially thermometers in the pump sump.

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Experience shows that ammonia vapours can enter the accommodation aided by the
vacuum which often builds up aft of the superstructure. When possible the forward masts
should be used, as by using these, it is easier to compensate for wind driving the gas
towards the accommodation. If possible, alteration from the planned course is often a
practical method to avoid inconveniences caused by the ammonia vapours.
Ventilating
It is a special requirement for ammonia that inert gas must not be used to replace it, as
various components in the inert gas react with ammonia vapours forming saline products,
which block up valves, pipes, etc.
As ammonia has explosion limits between 16 and 25 percent it is difficult to ignite, and

also with a low heating value, it can be considered non flammable under normal
circumstances. These facts make it possible to safely ventilate the tanks without any
intermediate inerting.
However, before ventilating is commenced it must be ensured, that the tanks are
sufficiently heated, as the freeing of ammonia from the tank steel is restrained considerably
at low temperatures. The amount of ammonia and water contents remaining in the tank will
thereby be tied to the tank walls and more time is required for tank cleaning. At a tank
temperature of +450C, ammonia is freed about 100 times faster than at 00C and about 1000
times faster than at -450C.
If the tank steel is not sufficiently heated, the ammonia vapours will form a thin layer of
cold ammonia against the steel. This layer is fairly stable and difficult to penetrate by the
warmer vapour inside. When the ventilation is started, the air will, aided by turbulence,
penetrate the cold layer of ammonia and the remaining liquid against the tank steel will
begin evaporating.
Threshold Limit Value
Special health precautions must be observed because of ammonia's high affinity with
water. A 2,000 ppm concentration will severely hamper the respiration and cause death
after approximately 30 minutes. Even in small concentrations ammonia will violently
irritate the skin and the mucous membranes.
In practice, it is necessary to reduce the concentration to about 10 ppm in the tanks, before
they are entered, as otherwise working will be adversely effected.
The smell threshold is as low as 5-10 ppm. This concentration is harmless even during long
lasting exposure in the tanks. If the tanks are not sufficiently heated before ventilating, it
could prove extremely difficult to reach this concentration.
Sweeping and Vacuum Cleaning
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During sweeping, additional ammonia is released, and a subsequent vacuum cleaning will
in combination with continued ventilation effectively lower the concentration further.
Alternative Methods
It sometimes happens that ventilating alone is not sufficient to reduce the concentration of
ammonia to the required level - e.g. I ppm - therefore other methods must be applied.
A subsequent water washing will at the concentration reached effectively consume the
remaining ammonia and thereby reduce the concentration further, in some cases below the
measurable range. Unless the tanks are to be water washed for other reasons, this method is
discouraged.
After water washing, the IG-blower should be used for ventilating and drying.
Compressors can also be used, if the weather is calm with a relative humidity less than
approximately 60 percent. The Owners' permission to use the compressors for this purpose

must be obtained. It will be necessary to change the lubricating oil in the compressors after
use, because the oil absorbs water from the air pumped through. As this oil costs about £5
per litre, the time saved using this method must be significant to make this procedure
acceptable. When the concentration cannot be further reduced by continued ventilation
another viable alternative, which was previously almost universally adopted is:
The compressors are - after the Owners' permission has been obtained - utilised to build up
the tank pressure to an over-pressure of e.g. I or 2 bars. A higher overpressure requires
comparatively longer time. When the predetermined overpressure is reached, the
compressors are stopped and the overpressure reduced to zero. The procedure is repeated
as many times as necessary, and the ammonia concentration is measured each time. It is
quite normal to read a considerably higher concentration after the first time. This could be
caused by ammonia remaining in the compressors and associated pipelines, but another
phenomenon can also be considered as a contributory cause to this effect:
As the tank pressure is increased, the tanks are expanded slightly, and ammonia remains
which are trapped in the rust and dust, are released, and this will give cause to the
increased concentration. After the first time, the concentration will be reduced to half and
then a third every time the tanks are pressurised.
Lubricating oil used in the compressors is water absorbent and accumulates some of the
moisture from the atmospheric air. This means that oil in the compressors must be changed
after they have been used as described above.
Measures to Facilitate Tank Cleaning
It is possible to prepare the cargo tanks in order to ease the tank cleaning after carriage of
ammonia.

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A thorough cleaning and drying of cargo tanks and pipelines incl. filters before loading of
ammonia will to a large extent prevent ammonia from being trapped in the rust layer
making it possible to ventilate the tanks more quickly following discharge. By combined
operation, the IG-blower can be connected to the liquid line and compressors to the three
spray-lines keeping the lower line fully open and the upper lines partly open. Thus a
natural flow is secured with the heavier air entering below the lighter ammonia, which in
the beginning is ventilated out through the vapour-lines to the masts and later through the
tank hatches when vacuum cleaning the tanks.
Sub-cooling
It is an invariable rule that tanks - including tank steel - are heated to a temperature above dew
point of the air used for ventilating. Theoretically, it should be sufficient to heat to a temperature
of e.g. -250C when ventilating with the IG-blower, as this supplies air with a dew point of about
-500C.

However this does, not apply in practise as other conditions have to be considered.
If warmer and especially more humid air is added to the tanks' atmosphere, this air will
absorb the ammonia vapour making the ammonia liquid try to fill the tank space with
vapour by evaporation. The heat required for this process is taken from the nearest
surroundings, e.g. the tank steel, and a drastic drop in temperature can be observed.
Temperatures far below the steel's design temperature and a rate of decrease multiple
exceeding what the tanks are designed for can be observed and thereby possibly damage
the tanks. The humidity in the air will also reliquify against the steel, the water will
dissolve the ammonia and substantial prolongation of the duration of the tank cleaning
must be faced, as this ammonia and water solution is very difficult to remove.
It should be a general rule to heat the tanks to not less than +200C, and special care should
be taken to ensure that the pump sump is heated thoroughly.

15.9

Tank Cleaning and Ventilating after Ammonia on the Fully–Ref.
Vessels

Special precautions are necessary when cleaning tanks after a cargo of ammonia.
Vaporising liquid heel
Compressors with vapour blowers are used to heat up the remaining liquid (which then
evaporates), the ammonia vapours, and tank steel. It is very important to distribute heat evenly
throughout the tank although the pump sump should be heated comparatively more. The pump
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sump should generally be heated for an additional I to 2 hours, after all liquid has been vaporised,
in order to prevent re-condensing of vapour against cold steel.
It has at previous tank cleanings in our other gas carriers proved to be beneficial to heat up to
above +200C. The exact time required for this additional heating will be more than saved during
the following phases of tank cleaning. In the fully ref. vessels it is not always possible to reach this
temperature by use of compressors alone.
Sub-cooling
It is an invariable rule that tanks - including tank steel - are heated to a temperature above dew
point of the air used for ventilating. Theoretically, it should be sufficient to heat to a temperature
of e.g. -250C when ventilating with the IG-blower, as this supplies air with a dew point of about
-550C.
However, this does not apply in practise as other conditions have to be considered.
If warmer and especially more humid air is added to the tanks' atmosphere, this air will absorb the
ammonia vapour making the ammonia liquid try to fill the tank space with vapour by evaporation.

The heat required for this process is taken from the nearest surroundings, e.g. the tank steel, and a
drastic drop in temperature can be observed. Temperatures far below the steel's design temperature
and a rate of decrease multiple exceeding that which the tanks are designed for can be observed
and thereby possibly damage the tanks. The humidity in the air will also reliquify against the steel,
the water will dissolve the ammonia and substantial prolongation of the duration of the tank
cleaning must be faced, as this ammonia and water solution is very difficult to remove.
It should be a general rule to heat the tanks to not less than +200C, and special care should be
taken to ensure that the pump sump is heated thoroughly.
In order to be able to heat up the tanks to required temperatures, the fully ref. vessels have been
furnished with 4 portable units with a total of 8 blowers with steam heating of the air. Using these
blowers it is possible to heat up the tanks to at least +400C. At this temperature it is not difficult to
lower the ammonia concentration.
Removal of overpressure
Overpressure is removed gradually from the tanks through the vent masts in the usual way
closely monitoring the thermometers.
Ventilating
With ammonia it is a special requirement not to use inert gas for diluting the gas as certain
components in the inert gas react with ammonia to form saline products which block up
valves, pipes etc.
As ammonia's explosion limits are from 16 to 25 percent and it is difficult to ignite and also with a
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low calorific value, it can under normal circumstances be regarded as non flammable. Thus, tanks
can be ventilated without first being inerted.
Venting off ammonia with IG-blower
The portable blowers are not able to work against back pressure of any significant size. It is
therefore necessary to ventilate ammonia by using the inert gas blower while the concentration is
still too high for it to be vented off through open tank hatches, i.e. while the concentration of
ammonia is too high for personnel contact. Air from the inert gas blower is supplied to the tanks
through liquid lines over the manifold to the tanks' liquid filling lines. By reason of ammonia's
vapour density compared to that of air, tanks should be connected in parallel. Tanks are vented off
at the top, and the vented off vapour is led to the forward gas mast in order to minimise
inconveniences from the ammonia.
It may also be necessary to adjust the course so that vapour clears the accommodation.
The process of dilution can be monitored closely using Barrett testers. A very clear stratification
can be expected. Usually one change of atmosphere can be expected to be sufficient to bring the
concentration below measurable limits using Barrett-testers.
Ventilating using the inert gas blower must be continued for approximately another 15 hours as the
portable blowers cannot work against back pressure.

During this phase, Drager test tubes can be used to indicate, when the portable hot air blowers
should be brought into use. At a reading of 500-700 ppm, the concentration will no longer decline
gradually but instead rise and fall. This is an indication that the tank steel is beginning to free
trapped ammonia.
Using hot air blowers
At a concentration of about 700 ppm, 2 hot air blowers can be put to work. It is possible to
connect one unit with one blower to each of the liquid manifolds. Reducers are required with
dimensions fitting the spiral wound hoses with a diameter of 40 cm. The hoses can be rolled over
the flanges of the reducers and they are fastened with straps supplied with the equipment. The bolt
holes in the flanges must of course be sealed off with tape to avoid air escaping through them.
Alternatively, special connectors can be custom made on board.
Made of thin metal plate, they will both be easier to handle and without unwanted holes to be
covered.
During the trial run with these blowers, measurements were taken with a provisional
manometer made of plastic tube shaped as a U. Results indicated that it was necessary to
open the tank hatches to make it possible for the hot air blowers to supply air at a rate
substantially higher than that of the Inert gas blower.
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To minimise the back pressure, it is necessary to open the hatches by removing all but 2 bolts,
which are only loosened. By squeezing one bolt under the hatch covers, sufficient clearance is
obtained to avoid too high a back pressure. At a concentration of about 700 ppm, work can be
carried out on the hatches without any inconvenience and without using gas masks.
The blowers are started and steam heating applied. A marked increase in efficiency can be seen
after initiating the hot air blowers. After approximately 5 hours, the concentration will have been
reduced enough to make it possible to remove the hatches and rig hoses in the tanks.

First rigging of hoses in the tanks
Ventilation is stopped during rigging to make it possible to lower the hoses into the tanks
from the domes. In each tank half, a 5 metre and a 10 metre hose are required.
The 2 hoses are connected using a strap, making the 5 metre hose the lower. Only spiral
wound hoses should be used. The lower hose of 5 metres' length is at its lower end
furnished with 4 lanyards. The loose ends are joined and connected to a line with a length
of approximately 20 metres. The other end of this line remains on deck during rigging.
First, the 10 metre hose is attached to the lower steel ring on the supporting frame, which is
to be fastened to the hatch coaming. Then, the two hoses are lowered slowly and carefully to prevent damage from contacting sharp edges in the tank - to a level, where the lower
hose ends just above the railing of the upper platform directly below the hatch. The slack
of the line is now pulled up. The hoses are then lowered further to give them a U-form. The
hose can now be manipulated to tumble out over the railing at the platform. When this has

been achieved, the hoses are lowered all the way down, and the line is slacked to allow the
hoses to hang vertically and with the opening about 3 metres above the tank bottom.
Another method for lowering the hoses was attempted. The intention was to swing the
hoses like a pendulum thereby forcing them over the railing, but this could not be achieved
as the space through the manhole directly below the hatch prevented sufficiently large
movements.

Making the hot air blowers ready
The 4 units are positioned one at each tank dome, all on the port side, so as to make it possible to
connect the steam and condensate hoses. Blowers must be positioned with the exhausts facing to
starboard and at sufficient distance, that a 5 metre long spiral wound hose can be connected from
the upper blower to the port tank half, and a 10 metre long spiral wound hose from the lower
blower to the starboard tank half. The hoses are secured to the rims of the blowers' exhausts with
the straps and on the frames on the tank hatches via the 90 elbows, which should already be
positioned in the upper steel rings. The hoses are secured to the elbows with straps.
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Ventilating with hot air
Before connecting the blowers to electric power, permission to use them should be obtained from
Nautical Department, as this involves use of electrical equipment on deck.
Each blower can, after positioning and connection of hoses for ventilation, steam, and condensate,
be started to begin hot air ventilation, while hoses are rigged in the other tanks.
When all 8 blowers are running, inert gas blowers are started again. Thus, air is added
throughout the tanks along the bottom simultaneously with supplying hot air at a level a
few metres above the bottom. A continuous supply of air is thereby achieved all over the
tanks, and substantial heating of the tank atmosphere and consequently of the tank steel is
initiated simultaneously.
Approximately 12 hours later, the tank temperature will have been raised about 100C, and
the ammonia concentration will have been reduced to 10 ppm which is below the
Threshold Limit Value. The smell of ammonia is still present, but the tanks can be entered
without suffering any discomfort.
Rearranging hoses in the tanks
At a concentration of 10 ppm, blowers are stopped, and tanks entered. The line fastened to the
lower hose is to be used inside the tank, so it is taken down into the tank when entering. It will be
used with the hose, so remains attached to it. The hoses also remain connected.
The line is pulled through one of the middle openings in the thwartship swash bulkhead. Thence,
the line is used to heave the hoses forward to a position where the end is about 2 metres from the
bulkhead. The line is then secured at a reinforcement structure in the forward part of the tank. The
hose will have its opening slightly offset from the end in the bulkhead, which is intended. The air

will thereby be distributed with a little more than half into the forward section of the tank, while
the aft part receives air deflected by the bulkhead and sent downward and aft.
Final ventilation
After repositioning the hoses, ventilation with hot air is resumed.
Another 20-24 hours' ventilation will be required. After this, concentration can be expected
to be well below 5 ppm.
On the some vessels, a concentration of 1-2 pmm was measured after 20 hours' ventilation.
Notably, this was achieved without removal of loose rust in the tanks' bottom. Even the
discoloration of the detection layer in the Drager test tubes was not the usual dark blue but
rather greyish blue, and this discoloration faded away after a few minutes.

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Thus, an ammonia concentration which is detectable but unmeasurable, can be achieved by
ventilating with hot air.
Drying tanks
When hot air ventilation has been completed, hoses are brought on deck together with all other
equipment which has been used. The tanks must then be dried thoroughly which is done by using
the inert gas blower. Ventilation is continued until the required dew point has been reached. It is
not anticipated that the ammonia concentration will increase again.
Expected time required
The timetable below indicates the expected time required for the whole operation.
The table is intended as a guide for planning on board, and discrepancies may occur:
Vaporising liquid heel, heating with compressors 4 days
Reducing overpressure to zero
1,5 hours
Ventilating high concentration with Inert gas blower
9,5 hours
Continued ventilation with Inert gas blower 15 hours
Ventilating with hot air blower through manifold 4,5 hours
First rigging of hoses in tanks
3,5 hours
Ventilating with hot air blowers and IG blower
12 hours
Rearranging hoses in tanks 5,5 hours
Ventilating to unmeasurable values 24 hours
Additional time for various setting up of equipment 2,5 hours
Total estimated time required 7 days, 6 hours

15.10

Vaporising Liquid Heel

In connection with tank cleaning, the liquid heel must be vaporised before the tanks are inerted
and ventilated.
Before heating of the tanks is commenced, the deck-piping shall be thoroughly drained of liquid
into the liquid collector tanks. The pipe system is blown through with warm vapour from the
compressors in order to ensure that it is completely free of liquid.
Semi - Rrefrigerated Vessels
Warm vapour from the compressors is fed to the tanks through the puddle-heating line in
order to speed up the evaporation of liquid heel. As this line is not dimensioned to accept
all vapour the compressors can deliver, vapour must also be supplied partly through the
lower spray line. Thus, an even distribution of the supplied heat is ensured, while at the
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same time increasing evaporation of liquid in the pump sump. It is very important to
continue heating of the pump sump after the liquid has been evaporated, as the tank steel is
still cold. If the heating through the puddle-heating line is stopped immediately after all
liquid has been evaporated, warm vapour will reliquify by contact with the cold steel.
Heating of the pump sump should be continued for about 2 hours after removal of the
remaining liquid. Then it is safe to assume that the tank is sufficiently heated, so that
vapour cannot reliquify by contact with the tank steel.
To avoid unnecessary delay in heating, the overpressure must be removed at intervals. It
must, however, be ensured that enough overpressure is present (0.1 - 0.2 bars) to enable the
compressors to draw vapour from the tanks instead of the mast.
To facilitate subsequent tank cleaning, it is important to heat up the tanks to a temperature
substantially over the boiling point of the cargo. If it is planned to ventilate the tanks by
means of the cargo hold blower, the tanks must be heated to a temperature at least 100C
over the ambient temperature. In order to avoid too much water vapour in the tanks, the
cargo hold blower must not be used at ambient humidity in excess of 60
Fully Refrigerated Vessels
Vapour is sucked from the tanks by the vapour-blower feeding the compressors and heated
vapour is returned to the tanks through the condensate lines. The heat is evenly distributed
in the tanks by using the liquid filling line. To heat up and vaporise remaining liquid in the
pump sump, warm vapour must also be sent down through the puddle heating-line.
When liquid heel has been heated and vaporised it is mandatory to continue heating the
pump sump for approximately 2 hours in order to avoid reliquefaction of vapour making
contact with cold steel in the sump. After this time has elapsed, warm vapour should be

sent through the upper spray line in addition to through the liquid filling line.
To facilitate subsequent tank cleaning it is important to heat the tanks to a temperature well
above the boiling point of the cargo. Where it is planned to ventilate the tanks using
portable water driven blowers, it is imperative to heat up the tanks to a temperature
exceeding the ambient temperature by at least 100C. To prevent heavy corrosion in the
tanks, portable water driven blowers must not be used at ambient relative humidity over 60
percent.
It is necessary to blow off overpressure during heating as the tank safety valves are
adjusted to open at an overpressure of 0.28 bar.
Generally, substantially longer heating times must be allowed for in this ship-class, compared with
other ship-classes.

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