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Tank Cleaning Procedures

Tank Cleaning Procedures


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

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