6.
2 Tank materials and coatings
Cargo tank materials and coatings represent a considerable part of the total value of a
chemical tanker. This topic is also covered under this section. We will here introduce stainless
steel, composite steel, clad steel and the most common types of coating, also the correct
handling and maintenance of the different tank materials and coatings will be discussed.
Cargo tank materials and coatings represent a considerable part of the value of a chemical
carrier, and all structural materials and linings used in cargo tanks and the associated piping ,
valves and pumps, must be suitable for the cargoes to be carried. This is done by using
different types of stainless steel and different types of coating.
Today’s chemical carriers have either all tanks made of stainless steel or coated mild steel
tanks or a mix of both.
Even if today’s tankers are equipped with stainless steel pipings and associated equipment
you still can find tankers equipped with mild steel pipings.
Stainless steel, composite steel and coated tanks are used due to different cargoes
agressitivity to mild steel, also due to cleaning and today’s charterer are having their
requirement for their cargo.
Clad steel is a mild steel tank, "lined" or "clad" with a thin layer of stainless steel. Stainless
steel are of different type, best known is 316L, which means it is a 18/8 steel, better known as
non-corrosive. Other types of stainless is 316 – 316LN and composite steel like 2215 and 1520,
these are stainless steel with higher carbon and are to be treated as stainless steel.
6.2.1 Composition
We are referring to stainless steel as stainless due to its resistance to aggressive liquids and
environment. In other words it should not corrode.
Stainless steel do corrode if it is not treated the right way, you have to "passivate" or treat the
steel to keep it "stainless".
To passivate a stainless steel tank is done by spraying the tank with a mixture of Nitric acid
and water, approx. 25% Nitric. Leave it for a while and then wash out with fresh water.
"Pickling" is an other way of passivating, but is normally done after minor repairs in the tank-
area, this is done by a specially Pickling paste
Cargo tank materials
A coating compatibility chart
Due to its ability to withstand the agressitivity, stainless steel is perfect for carrying most
chemical cargoes.
On the other side we have mild steel tanks, which are not able to withstand aggressive
environment, but with applying coating these tanks are also able to carry several cargoes.
These are the main types of coatings used on today’s chemical carriers;
Zinc silicate, Epoxy, Phenolics and Polyurethane.
On some chemical carriers specially dedicated with highly corrosive cargoes like hydrochloric
acid have their cargo tanks lined with rubber-linings.
The reason for having so many types of coating is that they have their own limits against
various types of cargoes.
In general we may say that Zinc silicate are not able to carry aggressive cargoes like caustic,
but Epoxy/Phenolic a good with those cargoes, but with other cargoes will they overlap each
others. You need to use the "Coating resistance list" supplied by the manufacturer to be sure.
6.2.2 Application
The correct application technique of the coatings, generally this is done by professionals and
under supervision of the paint manufacturer.
Generally tanks are to be blasted to SA 2,5, a close monitoring of humidity and temperature
are to be maintained before applying coating, Zinc silicate are to be sprayed on with one coat,
(app. 250 my), and Epoxy with 2 or 3 coats to build up to 400 my.
The general maintenance on tank coatings is to repair mechanical damages if possible, but
here we have to take in account the curing time for the different coatings and see if there is
enough time for curing before next loading.
As mentioned above each coating-supplier have their own "Coating Resistance List" and they
have to be strictly followed to prevent damage of cargo and coatings.
6.3 Cargo tank ventilation system
All cargo tanks should be provided with a venting system appropriate to the cargo being
carried and these systems should be independent of the air pipes and venting systems of all
other compartments of the ship. Tank venting systems should be designed so as to minimize
the possibility of cargo vapor accumulating about the decks, entering accommodation, service
and machinery spaces and control stations and, in the case of flammable vapors, entering or
collecting in spaces or areas containing sources of ignition. Tank venting systems should be
arranged to prevent entrance of water into the cargo tanks and, at the same time, vent outlets
should direct the vapor discharge upwards in the form of unimpeded jets.
The venting systems should be connected to the top of each cargo tank and as far as
practicable the cargo vent lines should be self-draining back to the cargo tanks under all
normal operational conditions of list and trim. Where it is necessary to drain venting systems
above the level of any pressure/vacuum valve, capped or plugged drain cocks should be
provided.
6.3.1 Overflow control
Provision should be made to ensure that the liquid head in any tank does not exceed the
design head of the tank. Suitable high-level alarms, overflow control systems or spill valves,
together with gauging and tank filling procedures, may be accepted for this purpose.
6.3.2 Venting operation
Tank venting systems should be designed and operated so as to ensure that neither pressure
nor vacuum created in the cargo tanks during loading or unloading exceeds tank design
parameters. The main factors to be considered in the sizing of a tank venting system are as
follows:
.1 design loading and unloading rate;
.2 gas evolution during loading: this should be taken account of by multiplying the
maximum loading rate by a factor of at least 1.25;
.3 density of the cargo vapor mixture;
.4 pressure loss in vent piping and across valves and fittings;
.5 pressure/vacuum settings of relief devices.
6.3.3 Vent piping
Tank vent piping connected to cargo tanks of corrosion-resistant material, or to tanks which
are lined or coated to handle special cargoes as required by the Code, should be similarly lined
or coated or constructed of corrosion-resistant material.
The master should be provided with the maximum permissible loading and unloading rates for
each tank or group of tanks consistent with the design of the venting systems.
6.3.4 Types of tank venting systems
An open tank venting system is a system which offers no restriction except for friction losses to
the free flow of cargo vapors to and from the cargo tanks during normal operations. An open
venting system may consist of individual vents from each tank, or such individual vents may
be combined into a common header or headers, with due regard to cargo segregation. In no
case should shutoff valves be fitted either to the individual vents or to the header.
6.3.5 Tank venting control
A controlled tank venting system is a system in which pressure- and vacuum-relief valves or
pressure/vacuum valves are fitted to each tank to limit the pressure or vacuum in the tank. A
controlled venting system may consist of individual vents from each tank or such individual
vents on the pressure side only as may be combined into a common header or headers, with
due regard to cargo segregation. In no case should shut-off valves be fitted either above or
below pressure- or vacuum-relief valves or pressure/ vacuum valves.
Cargo tank vent system (1)
Cargo Tank vent system (2)
6.3.6 Tank vent positions
The position of vent outlets of a controlled tank venting system should be arranged:
.1 at a height of not less than 6 m above the weather deck or above a raised walkway
if fitted within 4 m of the raised walkway;
.2 at a distance of at least 10 m measured horizontally from the nearest air intake or
opening to accommodation, service and machinery spaces and ignition sources.
6.3.7 Vent outlets
The vent outlet height referred to in 6.3.6.1 may be reduced to 3 m above the deck or a raised
walkway, as applicable, provided that high velocity venting valves of a type approved by the
Administration, directing the vapor/air mixture upwards in an unimpeded jet with an exit
velocity of at least 30 m/s, are fitted.
6.3.8 Tank venting systems
Controlled tank venting systems fitted to tanks to be used for cargoes having a flashpoint not
exceeding 60o C (closed-cup test) should be provided with devices to prevent the passage of
flame into the cargo tanks. The design, testing and locating of the devices should comply with
the requirements of the Administration, which should contain at least the standards adopted
by the Organization.
6.3.9 Venting system design
In designing venting systems and in the selection of devices to prevent the passage of flame
for incorporation into the tank venting system, due attention should be paid to the possibility
of the blockage of these systems and fittings by, for example, the freezing of cargo vapor,
polymer build-up, atmospheric dust or icing up in adverse weather conditions. In this context
it should be noted that flame arresters and flame screens are more susceptible to blockage.
Provisions should be made such that the system and fittings may be inspected, operationally
checked, cleaned or renewed as applicable.
An automatic weight-loaded pressure/vacuum valve.
6.3.10 Cargo-tank gas-freeing
The arrangements for gas-freeing cargo tanks used for cargoes other than those for which
open venting is permitted should be such as to minimize the hazards due to the dispersal of
flammable or toxic vapor in the atmosphere and to flammable or toxic vapor mixtures in a
cargo tank.
Accordingly, gasfreeing operations should be carried out such that vapor is initially discharged:
.1 through the vent outlets specified in 6.3.6 and 6.3.7; or
.2 through outlets at least 2 m above the cargo-tank deck level with a vertical
efflux velocity of at least 30 m/s maintained during the gas-freeing operation; or
.3 through outlets at least 2 m above the cargo-tank deck level with a vertical
efflux velocity of at least 20 m/s which are protected by suitable devices to prevent the
passage of flame.
When the flammable vapor concentration at the outlets has been reduced to 30% of the lower
flammable limit and, in the case of a toxic product, the vapor concentration does not present a
significant health hazard, gas-freeing may thereafter be continued at cargo-tank deck level.
6.3.11 In-line and end-of-line devices?
Flame-screens should not be used at vent out-lets due to the problem with clogging and
thereby possibility to build high pressure in the tank during loading.
6.3.12 Precautions regarding high-velocity valves.
A pressure/vacuum (P/V) valve is designed to release and/or let in pressure to protect the
cargo tank from exploding or imploding due to too high or too low pressure in the tank.
Scan picture of P/V-valve.
The P/V-valve must be looked after due to the possibility that they may be clogged from
freezing vapours, polymer build-up or icing up in adverse weather conditions, always be sure
that the P/V-valve is working before commence loading/discharging.
A high-velocity venting valve.
