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This unique illustrated guide for masters, ships’ officers and others associated with the carriage of cargo explains how to avoid problems and disputes arising from incorrect usage of natural and mechanical hold-ventilation systems on cargo ships. Incorrect use of ventilation can lead to cargo damage from ship’s sweat, cargo sweat, rainwater or seaspray.
The guide addresses the key cargo ventilation questions of why, when, what and how, with particular emphasis on the application and pitfalls of the dew-point and three-degree rules. The guide works on various levels, with a quick reference section supplemented by practical guidance and considerations, plus a scientific background for those wishing to understand the underlying principles.
How to use this guide
Glossary of terms
When to ventilate
Cargoes that may require ventilation
What is sweat? The simple explanation
Hygroscopic and non-hygroscopic cargoes – know your cargo
Finding the dew point
Three-degree rule – for hygroscopic agricultural cargoes only
How effective is ventilation?
When should cargo be ventilated?
When should ventilation be stopped?
Types of ventilation system
Recent developments in the rice trade
Cargoes that do not fit the rules
Equipment for measuring the dew point
Myth or truth?
Air, moisture and relative humidity
Relative humidity and dew point – an example
How much condensation is produced?
Scientific rational for ventilation
There are numerous reasons for ventilating a ship’s holds. Among these are
ensuring an adequate oxygen level for people to enter the holds
removing poisonous, flammable or fumigant gases
attempting to prevent condensation or ‘sweat’ (Fig. 1).
Prevention of sweat is probably the most common reason for ventilating. However, it seems to be the one which causes seafarers and ship operators the most problems, and it is the one on which this guide will concentrate. This guide relates to ships with conventional ventilation systems, both natural and mechanical. It does not apply to refrigerated ships, or to those whose holds are provided with de-humidifiers.
There are also many fallacies about what can be achieved with ventilation on board a ship, such as cooling or preventing self-heating of cargoes. These fallacies, and the limited situations in which ventilation can be used to control cargo temperature, will be outlined. Later sections will explain why sweat forms, and how it can be controlled by ventilation. For now we simply say that, if sweat is allowed to form without any effective attempt to check it, cargo wetting, and expensive claims, can result (Fig. 2). It may not always be possible to prevent the damage by correct ventilation; however, if correct ventilation has been carried out, and recorded, it should be easier to defend the resulting claim.
In the ideal world proper attention to ventilation will ensure that the cargo arrives undamaged at the discharging port. However, this may not always be the case. Even if ventilation is possible, it may be that the cargo cannot be ventilated sufficiently to prevent some sweat forming. It may even be that the inherent moisture content of the cargo is so high that no amount of ventilation will prevent damage. If extensive sweat damage occurs it is likely that a claim against the ship will follow.
To defend such claims successfully, it will be necessary to demonstrate that masters understood the principles of ventilation and that, based on their understanding, they ventilated their ships’ holds during the voyage correctly and as thoroughly as they could under the circumstances.
It may be that weather conditions experienced on passage prevent ventilation for part, or all, of the voyage. If so, masters must still demonstrate that they acted correctly in withholding ventilation and that they did so at the correct times. Therefore, whether the holds are being ventilated or ventilation is suspended because of weather conditions, a detailed record should be kept.
Title: Cargo Ventilation: A Guide to Good Practice
Number of Pages: 48
Product Code: WS1706K
Published Date: August 2019
Weight: 0.70 kg
Author: The North of England P&I Association Ltd