CPP IN STAINLESS STEEL CHEMICAL TANKERS
SS chemical tankers are increasingly being used in the CPP trade. It has the following advantages---
No need to vent after Gasoline to desorb soft epoxy coating.
Extra tank cleaning is avoided.
Small full tanks mean less vapour growth due to sloshing and cargo loss due to higher PV valve settings OF +2000 mm Aq or more.
No contamination claims as all tanks have their own lines.
Flexibility in loading and discharging, no stress problems in harbour condition.
Less ROB in sump after discharging.
No salt water can be used used in SS tanks for bottom flushing , hence no fear of chloride contamination.
No IG soot, so the empty tank after dischg can be inspected better without entry as there is no IG fog
NBR seats of butterfly valves can be eaten up by gasoline in one voyage —on SS chemical tankers the valves have Teflon seats.
More than 20 tanks ( each with its own manifold discharge pipe) --hence commingling clause avoided on B/Ladings , in case of many shippers and consignees.
SS ships have Nitrogen plants--hence NO soot contamination.
AVTAG or JP4 has Gasoline base and a FP of <23C
AVTUR , JET A-1 , JP-5 has Kerosene base and FP>38C
A Naphtha is sour is its Mercaptan level is >100 ppm and H2S level>10 ppm. Aviation fuels must not be loaded after sour cargo.
For many cargoes including Molasses the last 3 cargoes must be lead free. Be attentive about lead content, if it is not declared in Gasoline ask for it.
Unavoidable pipeline admixture between Avgas and Mogas , in case of single connection and line, should be from a higher to lower octane number—to preserve the quality of the higher grade.
When mixing 14% nitric acid—add acid to water inside tank.
Lead in leaded dyes is completely soluble in Nitric acid.
Suitable chemicals are:
Leadclean CP (spraying method).
Bufferclean Ph 5.5 Vecom.(Recirculation method).
Toluene and Xylene are also good solvents of lead.
Many grades of gasoline contain minute quantities of the toxic lead compounds, Tetry-Ethyl-Lead (TEL) and Tetry-Methyl-Lead (TML). Organic compounds of lead, in liquid, solid or vapor form, are highly poisonous to any living form.
Lead can be detected very easily in a lab test by change of colour, so do not take chances.
Gasolenes contain dyes will turn a methanol wall wash to a pinkish color. It is very important that the dye is removed prior to loading subsequent cargo(es), since with time the dye is harder to remove. The dye can be removed by recirculating an oxidizer solution.
A SS chemical tanker carried BIODIESEL/ vegetable oil, and on the next voyage Jet A1, for the US air force , a cargo which is highly sensitive to contamination. As per the charterers instructions only a nominal wash was given. The outturn samples failed . It must be remembered that all modern fighter jets have microfilters which get choked. Vegetable oil is particularly liable to cause contamination since it is very difficult to remove it completely from tanks and piping. Contamination claims are usually huge and it also spoils the image of the company with the P&I and the cargo interests.
Hot washing will not always remove residues of a solidifying cargo. Often operators load an intermediate cargo to clean the tanks which is not susceptible to contamination and which will act as a solvent. In the above case an intermediate cargo such as gas oil should have been carried. Remember the buck stops at the Masters table.
It is a golden rule that Jet A1 should never follow a vegetable oil cargo.
If manual desludging of tanks is done, the residues must be disposed of in accordance with the relevant Marpol rules.
SS tankers carrying CPP should top up their small tanks with extreme caution preferably loading by gravity or a centrifugal pump at low rate.
Remember after Furnace oil it is difficult to load CPP. We do not allow it unless followed by an intermediate solvent like LDO ( not golden HSD ). The Heavy oil embedded in teflon valve seatrings will keep diffusing.
STATIC ELECTRICITY :
Restrict initial loading rates for kerosene and jet fuels , when splashing and surface turbulence occur, to flow rates less than 1 meter/second (volume flow rate conversions tables are available in ISGOTT). Adequate inlet coverage's are: side or horizontal entrance- 0.6 meter; downward pointing inlet- twice the inlet diameter . When necessary to use inert gas to displace clean products, a minimum amount must be used, particularly for aviation fuels.
Clean product lines should never be blown with air. All pipes, hoses, and fittings be conductive, electrically continuous, and bonded to ship's structure. Cargo transfer piping must be insulated from the land-side terminal since electrical potential may differ from that of the vessel due to stray current or cathodic protection of the pier. Insulating flanges, joints, or sleeves are sometimes used to divide the cargo hoses into electrically isolated halves - onboard and shore side. Each half is bonded and grounded to its respective base potential.
Do not allow ship-to-shore bonding except where required by statute. In such a case, insulating flanges are still required in cargo lines. Non-conductive hose can become conductive with use and is not an acceptable substitute for an insulating flange. Insulating flanges must be used when connected to submarine pipelines which have cathodic protection.
When doing STS electrical bonding between 2 ships is not necessary.
-------CAPT AJIT VADAKAYIL ( 28 years in command )