Cargo 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.
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.
Padding or 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 condition.
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 ullage opening.
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.
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.
Tank instrumentation - Use of alarm circuit
Control instruments liquid level gauges
Cargo temperature monitoring equipments
Tank overflow control systems
Use of Oxygen analysers
Cargo vapour detection equipment
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.
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