Tank types, tank Vents & tank environmental control for chemical tankers
Independent tank :This is a cargo-containment unit that is not contiguous with, or
part of, the hull structure. An independent tank is built and
installed to eliminate or minimise stressing that results from
stressing or motion of the adjacent hull structure. An
independent tank is not essential to the structural
completeness of the ship's hull and is known as a Type 1 tank.
Integral tank : This is a cargo-containment area that forms part of the ship's
hull that may be stressed in the same manner and by the
same loads as the contiguous hull structure. This is normally
essential to the structural completeness of the ship's hull and
is known as a type 2 tank.
- Gravity tank This is a tank with a design pressure of not greater than 0.07
MPa gauge at the top of the tank. A gravity tank may be
independent or integral. It will be constructed and tested
according to recognised standards and will take into account
the temperature of carriage and the relative density of the
- Pressure tank This is a tank with a design pressure greater than 0.07 MPa
gauge. A pressure tank must be an independent tank and
should have a configuration that permits the application of
pressure-vessel design criteria of recognised standard.
Open : An open tank venting system is a system that during normal
operations offers no restriction, except for friction losses, to the
free flow of cargo vapours to and from the cargo tanks.
Controlled : 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.
Controlled tank venting systems fitted to tanks used for
cargoes that have a flashpoint not exceeding 60°C (closed-cup
test) must be provided with flame arrestors to prevent the
passage of flame into the cargo tanks.
Tank Environmental Control
Inerting : This is the process of reduction of the oxygen content in a tank
by introducing an inert gas to prevent a flammable/explosive
atmosphere developing within the cargo tank.
In the marine industry, a crude oil tanker with cargo tanks of an
oxygen content of 8% or less is considered to be inerted.
However, on chemical tankers, the general practice is to use
large volumes of compressed nitrogen vapour, supplied from
the shore, to reduce the oxygen content down to as low as
0.1% by volume. An onboard top-up generator maintains a
positive pressure in the tanks.
Blanketing : Filling a cargo tank and associated piping systems with a
liquid, gas, or vapour, which separates the cargo from air.
In practice, nitrogen is most often added to a tank that has
already been filled with cargo. The principal purpose of the pad
is to establish a positive pressure on the tank, preventing the
ingress of water or air as the tank cools.
Dry : The cargo tank and associated piping systems are filled with
moisture-free gas or vapour, with a dew point of -40°C or
below at atmospheric pressure, and then maintained at that
Vent : This refers to forced or natural ventilation.
Open : Open gauging is with a device that makes use of a pipe
opening in the tank or tank hatch, potentially exposing the
gauger to the cargo or its vapour. An example of this is the
Restricted : Restricted gauging utilises a device that penetrates the tank.
When in use, this permits a small quantity of cargo vapour or
liquid to be exposed to the atmosphere and, when not in use,
the device is completely closed. The design ensures that no
dangerous escape of tank contents (liquid or spray) can take
place when the device is opened.
Closed : Closed gauging uses a device that penetrates the tank.
A closed system prevents the tank contents from release.
Examples include: float-type systems, electronic probe,
magnetic probe and protected sight-glass.
Alternatively, an indirect device that does not penetrate the
tank shell, and which is independent of the tank, may be used.
An example of this would be a pipe flow meter.
Other info pages
Isolation of cargo tanks and piping systems
When a single parcel of cargo is carried in several tanks served by a common pipeline system, containment within each tank depends upon the tightness of the inlet valve. Due to the pressure differential on either side during sequential loading into or discharge from such tanks, the tightness of a single valve should not be relied upon to prevent the entry or escape of cargo.
Physical properties & maintenance requirements for various tank coatings
The tanks of Chemical Tankers may be constructed or coated with various different types of materials and it is important to check with the P&A manual and the Paint Manufacturers Coating Resistance list prior to commencing Tank Cleaning Operations in order to ascertain the tank coating materials and any limitations with regards to temperature, use of cleaning chemicals etc which may be applicable to the vessel....
Entry into cargo tanks and poisoning hazards
The poison is a very toxic substance which when absorbed into the human body by ingestion, skin absorption, or inhalation produces a serious or fatal effect. Poison may enter the human body orally, by inhalation, or by skin contact. After being absorbed by the body it may affect certain organs or give a general poisonous effect. Lately the cancerogene effects of some industrial chemicals have been noticed. This has led to significant reductions of hereto accepted TLV- values in many countries.
Testing of tanks and cargoes
Most common tests and checks for oil and chemical cargoes include testing tank walls for cleanliness.
Testing is normally carried out by independent surveyors who, according to local practice or a written agreement in the charter party, are accepted by shipper, receiver and owner.
Cargo tank damage during pigging operations
Blowing and pigging of pipelines at terminals poses inherent risks for the terminal and a chemical tanker. Frequent damages to tanks have occurred. If there are doubts about the shore operation or signs of problems ashore the OOW must immediately request clarification.
Rubber lined tanks, for the transport of phosphoric acid, waste acids and hydrochloric acid.
In recent years a number of ships have been fitted with rubber lined tanks, for the transport of phosphoric acid, waste acids and hydrochloric acid.
Fig: Modern Chemical tanker tank instrumentation
It should be borne in mind that individual ship has got own characteristics and limitations may involved handling various chemical cargoes . The master and all personnel in all cases must be aware of cargo/ship information that has been given and comply with relevant safety procedures.
A wide range of instrumentation may be fitted on a modern chemical tanker. Only an outline is given here, providing
guidance on the safe and efficient operation of the equipment.
Liquid level gauges
- Equipment Manufacturers Instruction Manuals
MARPOL – 73/78 (latest consolidated edition)
International Safety Guide for Oil Tankers and Terminals (ISGOTT)
CFR 33 parts 125 to 199
Ship to Ship Transfer Guide (Petroleum)
MSDS for particular cargo carried
Chemical Tank Cleaning Guide
Main Info pages!
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