Transport Guidance for Steel Cargoes
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- Điều hướng trang này:
- Section 6.9.1(3)
| 6.4 Comparison of dew points
6.4.1 If the dew point of the outside air (the air used for ventilation) is lower than that in the hold, then it is appropriate to ventilate. If the ambient dew point is not lower than that of the cargo hold, it may be necessary to ventilate for other reasons, such as when the cargo has been fumigated, and timely ventilation of the fumigant is required. Steel is not a cargo that needs to be fumigated. How ever, the vessel may be carrying other cargoes in the same holds that may require fumigation when appropriate. 6.4.2 A comparison of the dew points between the cargo hold air and the external environment is commonly made by taking readings from wet and dry-bulb thermometers on deck and in the hold. Obtaining the ambient readings is simple. Most vessels have a box containing a pair of thermometers, suitable for measuring wet and dry-bulb temperatures. These thermometers should be hung in a shaded spot on the windward side of the bridge. American Club: Transport Guidance for Steel Cargoes 65 6.4.3 During the voyage, it may not be safe for the vessel’s crew to enter the hold to obtain temperature readings (see Section 2.6). If a cargo hold has been fumigated after loading, it certainly will not be safe, even if the compartment has been ventilated. If the wet-bulb thermometer is simply lowered into the hold from outside, there will be difficulty obtaining a sufficient air-flow across the thermometer’s wick. Some vessels are equipped with temperature measuring pipe for the cargo hold. If not, it may be feasible to measure the temperature through the cargo hold access, rather than enter the cargo hold. 6.4.4 In the event that the crew is able to safely enter cargo holds to obtain meaningful readings, it may be necessary to stop ventilating to allow the in-hold atmosphere to stabilize. If this is not done, the crew will be measuring the parameters of the ventilating air rather than those of the true in-hold atmosphere. Unfortunately, suspending ventilation in this way negates the purpose of ventilating, but may be the only means to obtain the readings at the time. If this is done, it should be properly noted in the “Remarks” section of the Humidity-Temperature-Cargo Ventilation Record Book (see Section 6.9.1(3)). 6.4.5 In practice, measuring the dew point temperature inside a cargo hold can be problematic. One of the simplest methods is to use a “whirling psychrometer”, which involves swinging the instrument inside the hold until the wet-bulb temperature has stopped falling and remains steady. 6.4.6 All readings should be taken well away from any air inlets to ensure that only hold air is tested. As an important safety precaution, enclosed space entry procedures should always be observed (see Section 2.6). 6.4.7 If the vessel is taking spray across the ventilator openings or onto the hatch covers or coamings, ventilation should be postponed until weather conditions improve. Also, hen sea sprays and seawater comes on deck, there is a significant risk that the water and spray will enter the cargo hold and wet the cargo. 6.4.8 If access to the holds is impossible or undesirable, the hold dew point can be determined from traditional wet and dry-bulb thermometers placed inside the trunking of an exhaust ventilator or similar pipe-work leading from the compartment. If done, it should be properly noted in the “Remarks” section of the Humidity-Temperature-Cargo Ventilation Record Book (see Section 6.9.1(3)). |