Safe Handling of Tank Containers - ITCO

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ICHCA International Safety Panel Briefing Pamphlet No 30 5.3.1.6 The number of swing bolts or flange bolts varies from type to type and the pressure rating of the tank. However when they are fully tightened they should form a pressure tight seal that will contain a pressure greater than the test pressure of the tank. 5.3.1.7 Where bolts are used in place of studs and nuts to secure the bolted manway lid care needs to be taken at the time of manufacture, maintenance and periodic testing to ensure that the bolts used are of the correct length so that they may be fully tightened. Note 1: See Annex 6 for more information on manway lid operation Note 2: Once the container has been filled, opening the manway hatch / lid is generally not required until the tank container arrives at its destination. Note 3: Many tanks have vacuum relief valves that are not sized to allow tanks to be discharged without opening the manway or other fittings. If a tank container needs to be opened, this must be carried out by a suitably qualified person. 5.3.2 Pressure Relief Valves 5.3.2.1 In general all tank containers whether they are designed for dangerous or non regulated goods are fitted with one or more spring loaded pressure relief valves which comply with the recommendations for their design, set pressures and venting capacity set forth in the United Nations Recommendations on the Transport of Picture 19 Dangerous Goods - Model Regulations (Orange Book). The pressure relief valves (see Picture 19) are fitted in the top space of the shell as close to the longitudinal and lateral centre so that they always remain in contact with the gas or vapour even during the extremes of bad weather. 5.3.2.2 Pressure relief valves can come in two forms: 5.3.2.2.1 Valves may be high pressure only to prevent a build up of pressure that may result in the tank exploding, or, 5.3.2.2.2 Valves may be “twin acting” in that they are designed to relieve both excess pressure and to provide vacuum-relief in case partial vacuum conditions arise in the shell. The vacuum relief ensures that the tanks will not implode due to low pressure. 5.3.2.3 Vacuum relief valves are also fitted on tanks that normally operate at atmospheric pressure to protect the tank against, for example, the effect of low ambient temperature that may cause a reduced internal pressure. Page 10 ©ICHCA International Limited
ICHCA International Safety Panel Briefing Pamphlet No 30 5.3.3 Frangible Discs 5.3.3.1 The IMDG Code requires that tank containers complying with portable tank instructions T5, T10, T12, T14, T16, T18, T19, T20, T22 and T23 (see Annex 5 paragraph A5.1.3) be fitted with a frangible disc (see picture 20) in series underneath the pressure relief valve. 5.3.3.2 Frangible discs are fitted to provide: protection to the pressure relief valve from the corrosive effect of the substances and/or their vapours being carried a guarantee that toxic vapours will only be released in extreme conditions protection to the pressure relief valve from any kind of malfunction caused by the cargo additional security for higher hazard cargoes. They may also be used to indicate an increase in the internal pressure within the tank that is still below the pressure setting of the pressure relief valve 5.3.3.3 In addition to the frangible disc a “tell tale” indicator is required to provide a means of monitoring the pressure between the frangible disc and the pressure relief valve. 5.3.3.4 The purpose of the “tell tale” indicator is to detect whether the frangible disc has broken due to excess internal pressure. A typical “tell tale” indicator would be a pressure gauge connected to the void between the frangible disc and the pressure relief valve which indicates current and maximum pressure. Picture 21 shows an example of the “tell tale” pressure indicator and the frangible disc tail (arrowed). 5.3.3.5 If a change in pressure in the void is recorded the Shipper must be notified immediately 5.3.4 Vapour (Airline) Valves Picture 20 Picture 21 5.3.4.1 During loading and discharge a means of preventing a build up of pressure or vacuum must be achieved. Such a build up could be prevented by leaving the manway lid open which in itself could be considered as a risk. Therefore a smaller opening can be provided in the vapour space to prevent any such build up. Due to the hazardous nature of some cargoes opening the pressure vessel to the atmosphere would constitute a severe risk. The solution is a closed (vapour recovery) system with more than one valve fitted which allows greater flexibility in load and discharge arrangements. Page 11 ©ICHCA International Limited
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Safe Handling of Tank Containers - ITCO

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