After successfully completing this lesson you should be familiar with the
procedures to follow during tank cleaning this chapter will also alert
you to the effects of seawater chlorine and acids on stainless steel
to gain maximum benefit from this lesson you should successfully have completed
the previous lessons this lesson explains how to clean the
tank pump and lines the practical section consists of two parts first
there is a demonstration of the procedures this is followed by an
interactive guided walkthrough of the same procedures
proper cleaning of the cargo tank the pump and associated lines is extremely
important if the required carriage standards are to be met on parcel
chemical tankers in particular a sound knowledge of the cleaning requirements
for the wide range of products carried on such vessels is also needed
if damage to the tank lining and the stainless steel pump and piping is to be
avoided on oil tankers cleaning is a more standardized procedure but care
must still be taken to protect the tank and its fittings
stainless-steel particularly the grades found on older vessels is susceptible to
damage from saltwater chlorine or bleach and acids if improperly handled
therefore stainless steel tank linings pumps and lines are all at risk for
preference cleaning should be carried out only with fresh water thereby
avoiding the risks from saltwater altogether however only in recent years
have freshwater generators become efficient enough to make sufficient
quantities of good quality water at a rate that will keep pace with the
average cleaning program storage may be an option but only when a supply
adequate tank capacity and indeed dead weight are available in addition fresh
water must be used promptly as quality will deteriorate over time especially if
there is no circulation facility within the tank it is more common to wash with
saltwater in which case it should be taken in an
open sea way to avoid contamination from impurities similarly take care to ensure
that the seawater Inlet being used is well clear of any engine room outlets
thorough flushing with fresh water will be necessary afterwards to remove all
traces of salt bleach is sometimes employ to address
problems of residual color associated with certain chemical cargoes and dyed
gasoline products or to remove residual odor be aware however that bleach is
extremely aggressive towards stainless steel and must be diluted and applied
only according to the procedures outlined in the vessels operations
manuals and suppliers guidelines acids likewise are extremely aggressive once
in contact with water the temperature rises rapidly and the combination of
increasing temperature and reducing concentration greatly increases the
corrosion levels sulfuric acid is held to be the most dangerous of the
regularly carried assets in this respect this of course poses potential problems
during cleaning but when properly handled the risks are brought within
acceptable and manageable levels cleaning of tanks that previously
contains sulfuric acid should be carried out as soon as practicable after
discharge if this is not possible the tanks must be kept dry to avoid a
chemical reaction between the acid residues and any condensation this may
involve the application of a nitrogen blanket cleaning of sulfuric acid
residues should always wherever possible be carried out with fresh water the
chlorides in salt water greatly increase the corrosive 'ti of diluted acid and
the risk of damage is much increased in the event that the supply of fresh water
is limited then salt water can be used initially but only under strictly
controlled conditions it is essential to keep sulfuric acid
cargo residues to an absolute minimum and once cleaning commences to be able
to deliver large quantities of water flushing all surfaces as quickly as
possible the dangerous concentration range is
generally between 80% and 20% and it is essential to pass through this dangerous
bracket as quickly as possible in this way the acid is rapidly diluted the
temperature rise is quickly countered and the risk of corrosion is
progressively reduced for the first few minutes the wash water should be allowed
to dilute the acid lying in the sump but thereafter the accumulated washing water
must be continuously pumped away if salt water is used then thorough rinsing of
the tank all pipes and fittings with fresh water must follow periodic testing
of the washing water as it is being pumped out from the tank will indicate
whether all traces of acid have been removed litmus paper which indicates the
pH value by the color it turns after being dipped in the water can be used
for this purpose phosphoric acid can vary greatly in quality and besides
introducing unknown and potentially aggressive impurities from the core
material substantial quantities of sediment may be encountered this takes
the form of a thick abrasive paste which can grind into moving parts pump seals
in particular will suffer from continued exposure to phosphoric acid sediment
purging results should be carefully scrutinized when carrying phosphoric
acid and any problems detected should be investigated and corrected at the
earliest opportunity
isocyanates such as toluene diisocyanate or TDI and diphenyl methane diisocyanate
MDI solidify and generate large volumes of gas when in contact with water
traditionally such products are cleaned first by recirculation with a
chlorinated solvents or water free gas oil which break down the chemical
structure of the product more recently maintenance of a nitrogen
blanket throughout the cleaning process has allowed water cleaning to be used
but there is no margin for error in this technique any isocyanate residues that
have solidified are extremely difficult to remove and Prevention is definitely
better than cure with these products it is essential to purge all moisture out
of pumps pipes tank cleaning machines etc prior to loading this is normally
done during the pre purging of the cargo tank itself the cofferdam of the fram-o
pump is also filled with a product called dioctyl phthalate
or dop as a final preventative barrier against the ingress of cargo this
technique is explained in more depth in the next chapter latex is a product like
no other as those who have carried it will freely testify care must be taken
to run cargo pumps in a controlled manner during discharge as if the pump
cavitate s-- that is bubbles form and collapse around the impeller creating a
partial vacuum around the blades and causing the pump to surge heat is
generated and the latex liquid will rapidly solidify effectively encasing
the pump in a rubber plug and seizing the impeller the cofferdam is filmed
with fresh water when carrying latex and circulated using a header tank or
reservoir drum with a small portable pump visual monitoring of the water will
indicate if latex is penetrating to the cofferdam the water also helps to keep
the pump cool this technique is explained in the next chapter some
vessels are supplied with special portable latex pumps for discharging
such cargoes the list of products that require special handling and cleaning
procedures is large and still growing particularly for chemical tankers in all
cases the company's operational manuals and the product safety data sheets must
always be consulted while the discussion so far in this
chapter have mainly been concerned with potential damage to stainless steel tank
linings and fittings there is of course a risk to health with many of the
products the appropriate personal protective equipment must always be used
and proper monitoring of tank atmospheres must be conducted in
compliance with the company's procedures all personnel involved in the cleaning
program must be briefed prior to commencement and material safety data
sheets must be posted this chapter focuses on the operation of the fram-o
submerged deep well cargo pump during tank cleaning it does not describe crude
oil washing techniques nor does it indicate how atmosphere control within a
cargo tank such as through the introduction and monitoring of inert gas
is integrated into the overall cleaning operation the applicable safety
management system manuals and manufacturer's guidelines should be
consulted for the appropriate information on these aspects as we have
already seen cleaning with good quality fresh water would avoid many problems
but few vessels are fortunate enough to be fitted with the equipment to do so it
is still more common to clean with seawater flushing down afterwards with
fresh water the example we are going to use in this chapter follows this method
the following should always where possible be observed for preference sea
water used in tank cleaning should not be heated above 60 degrees Celsius
because it becomes increasingly aggressive
at such higher temperatures if temperatures in excess of this are
required for example to remove residues of a product with a high melting point
such washing should be for a limited duration stored cleaning water will
deteriorate over time to an extent this can be avoided by continuing to refresh
the supply with new water or by recirculating if the supply is heated
recirculation will also help to ensure an even temperature throughout the
storage tank avoid pH values of below 7 this is rarely tested in routine
cleaning but should be checked if results indicate a problem or if
required by a specific cleaning procedure avoid chlorinated water
evaluate the cleaning program in terms of heating limitations of adjacent
cargoes you will now see a demonstration of the proceed
to be followed during the tank cleaning operation click on the buttons in
numerical order ensure that cargo hoses had been correctly rigged to the
overboard discharge lines or to the slop tank in compliance with the company's
operating procedures and the current tank cleaning plan verify that the
correct tank cleaning machines have been connected to the cleaning water deck
supply line if applicable and shura the tank cleaning machine controllers have
been correctly programmed ensure all cargo line and tank cleaning valves are
correctly set start the tank cleaning pump and increase output to the required
delivery pressure if the wash water is to be delivered at higher than ambient
temperature apply the heating and bring the temperature up to the desired level
the washing system may have a bleed that is a branch or outlet which passes
directly overboard from the washing water line through which water can be
diverted while the temperature is increasing to the required level it may
also be possible to preheat the supply tank
if the water is stored the cleaning program may also require water to be
initially delivered at ambient temperature only later increasing to a
higher level start and run the cargo pump adjusting the flow so that it
removes the accumulated wash water at the same rate as it is delivered by the
tank cleaning machines in other words the sump must be more or less empty
during the cleaning operation but the pump must not be racing monitoring the
hydraulic pressure gauge for the pump concerned or simply listening to the
sound of the pump will assist in determining if the correct level has
been set to clean the stripping lines open the stripping valve closed the
cargo pump discharge valve and run the pump at approximately 110 bar hydraulic
pressure for around 2 to 3 minutes if a cargo line stripping system is fitted
the stripping valve at the manifold must be opened this procedure should be
repeated several times during the cleaning operation as the residues are
removed and the wash water becomes progressively cleaner cleaning of the
Trop line is done by opening the drop-line valve closing the manifold
valve and circulating the cleaning water for two to three minutes this procedure
should also be repeated several times during the clean
operation when cleaning has been in progress for some time wall drain valves
that the low points on the cargo lines should be flushed by opening and closing
the valve several times it is essential that this is done rather than just
cracking the valve open and leaving it in order that the valve surfaces are all
thoroughly cleaned drain plugs under valves should also be removed so that
residues in the bottom of the casing are rinsed away if necessary make
arrangement to collect or divert flushed wash water approximately halfway through
the cleaning operation stop the pump temporarily and allow the water to
buildup in the bottom of the tank to a depth of 5 to 10 centimeters then resume
operating the pump at 110 bar hydraulic pressure for 3 to 5 minutes after this
increase the pump speed up to the maximum and run until the pump loses
suction at which time reduce again this will ensure that the Waring's
seal rings and pump casing and internal components are properly cleaned complete
the washing operation continuing to flush the valves and lines as
recommended once the saltwater washing cycle is
completed at a satisfactory level of cleanliness achieved flush the tank pump
and associated piping thoroughly with fresh water at the end of cleaning water
remaining in the sump and lines is to be stripped as soon as possible as
described in the previous chapter a small amount of fresh water left lying
in a sump will become stagnant over time and can stain or damage the tank lining
cargo lines too should be drained and ventilated this completes the
demonstration of the procedure to be followed during the tank cleaning
operation steam is sometimes injected directly to a tank as part of the
cleaning process the cargo pump seals can be damaged if exposed to live steam
and it should therefore not be applied for more than 10 minutes
always purge the pump cofferdam before steaming a tank to avoid any reaction
with cargo which might be in the cofferdam care should be taken with any
stored slops for a number of reasons slops containing saltwater should never
be stored in stainless steel tanks due to the aggressive nature of the
chlorides in the water schnapps with more than 10 percent acid residue
content should not be stored in stainless steel tanks due
to the corrosive nature for such a mixture slops containing alkaline
residues such as from caustic soda solution should not be stored in zinc
coated tanks reactivity between different sets of tank washings should
be carefully researched before mixing in the same tank consideration should be
given to later disposal of slops commercial requirements may dictate that
certain chemicals cannot be included in a slop mixture rounded to shore it may
be necessary to heat slops to dissolve solidified residues and it must be
confirmed that such heating can be delivered to the selected slop tank if
slots are to be disposed of at sea it must be confirmed that the vessel will
at all times be capable of complying with the requirements of MARPOL good
practice dictates that slop storage time is any way to be minimized slop should
be disposed of as soon as operationally and legally possible