Safe Handling of Tank Containers

罐式集装箱的安全操作

ISO tank containers are built to ISO 1496/3 standard and are designed for the transport of liquids, gases and powders in bulk. The tank (pressure vessel) is held within a frame that has the same dimensions as a standard ISO freight container (typically 20ft or 40ft). The frame provides protection and allows the tank to be handled using standard container handling equipment. ISO tanks can be stacked, transported by road, rail, and sea, making them highly versatile.

Swap body tanks are designed primarily for European road and rail transport. They are built to CEN standard EN 1432. Swap tanks are generally longer than ISO containers (typically 7.45m, 7.85m or 13.6m) and are optimized for European intermodal transport. They have bottom corner fittings compatible with European road vehicles and rail wagons. While similar in function to ISO tanks, they are not designed for sea transport or international shipping.

Tank containers are classified based on their pressure capabilities. Pressure tanks are designed to carry liquefied gases and other products that require pressurization. They can withstand higher internal pressures (typically 4-6 bar or higher) and are built with thicker walls and additional safety features. Non-pressure tanks are used for products that do not require pressurization, such as food-grade liquids, chemicals at atmospheric pressure, and other non-hazardous liquids. They typically operate at pressures up to 3 bar.

Several specialized tank types exist for specific applications: Insulated tanks for temperature-sensitive products; Heated tanks with heating coils for products that solidify at ambient temperature; Cryogenic tanks for ultra-low temperature liquids like LNG; Lined tanks with special internal coatings for corrosive products; and Multi-compartment tanks with separate chambers for carrying different products simultaneously.

Tank containers use various frame designs: Beam Frame - consists of two end frames connected by longitudinal beams at the top and bottom, with the tank supported on saddles attached to the beams. Collar Frame - features a continuous collar around the tank circumference at both ends, providing a simpler design. Baffle Frame - incorporates internal baffles to reduce liquid surge during transport. Each design has specific advantages for different cargo types and handling requirements.

The pressure vessel is the main component of a tank container. It must be designed, manufactured, and tested to withstand the maximum working pressure for the intended cargo. Pressure vessels are typically made from stainless steel (for corrosive products) or carbon steel (for non-corrosive products). The shell is usually cylindrical with dished ends. Wall thickness varies based on pressure rating and cargo requirements. All pressure vessels must undergo hydraulic pressure testing at 1.5 times the maximum working pressure.

Tank containers are categorized into different groups based on the type of cargo they carry. Understanding these groups is essential for proper selection and safe operation.

Tank containers built for liquids may have a stainless steel or mild steel pressure vessel shell. They can be un-insulated or insulated depending on the product requirements. Some tanks are equipped with heating or cooling equipment to maintain product temperature during transport. Many liquid tanks also feature protective coatings or linings to prevent corrosion and contamination.

Tank containers for pressurised liquefied gases usually have a mild steel pressure vessel shell. A few specialised tank containers for these gases may have a stainless steel shell. Where high ambient temperature or direct sunlight can affect the cargo, a heat shield can be fitted above the top of the pressure vessel. Rarely for tank containers, the pressure vessel shell may be totally un-insulated.

Tank containers for powders and granules are generally low pressure vessels manufactured from aluminium alloys. The nature of the powder or granule will dictate the design as will the loading/unloading process in which the tank container is to operate. The discharge process for powders and granules differs from liquids and gases inasmuch as the cargo may not flow. Therefore tank containers for powders and granules may be manufactured with hopper discharge chutes built along the underside of the shell, or with specialist discharge mechanisms which facilitate the discharge. Tank containers that carry powder and granules may be mounted on tipping chassis/trailers.

Some tank containers are designed and constructed to transport liquefied gases at extremely low temperatures, at around -200°C or lower. They may be referred to as 'cryogenic tanks' within the freight container industry. Due to the nature of the cargoes carried, the level of insulation varies. A typical cryogenic tank container will have an outer metal jacket surrounding the pressure vessel. A near-perfect vacuum is created between the outer metal jacket and the pressure vessel; this provides the insulation. In addition, the space may be filled with an insulating material. Some tanks may consist of a metal vacuum jacket, an outer shell containing a small quantity of liquefied nitrogen and an inner shell intended to contain the actual liquefied gas to be transported. These highly specialised tank containers represent less than 1% of the worldwide total. They do not have any refrigeration equipment (for example as reefer tank containers do).

The definition of service equipment is made in Annex 1. In this section, the use of 'fittings' will be used as a substitute and will encompass service equipment as defined by the IMDG Code and the closing devices for openings on tank shells which the IMDG Code considers to be part of the shell rather than service equipment. Fittings are therefore defined as items attached to the tank shell that perform a role that relates to the loading or discharge of the cargo, or the safety of the pressure vessel. A separate term 'fixtures' will be used to define all those items that are attached to the frame or which perform a role that protects the pressure vessel.

The top of the tank shell contains several critical fittings that are essential for access, safety, and operational functions.

One or more manways are cut into the top of the shell and each consists of a neck ring, a lid and a gasket seal. The neck ring will be fabricated from a compatible material to the shell and is a shaped pad that is welded to the pressure vessel. Manways are provided to allow access by suitably qualified technicians and repairers into the shell for periodic inspection. They are also often used on standard tank containers as an opening for filling. ISO 1496/3 states that the size of the manway shall be a minimum of 500 mm in diameter and shall be determined by the need for men and machines to enter the tank to inspect, maintain or repair the inside. However, the nature of the cargo and the designation of the tank will set the specific manway lid configuration.

Domed manway lids are hinged and sealed against the neck rings by a number of swing bolts. These lids are commonly used on tank containers and provide a secure, pressure-tight seal when properly fastened.

Flat bolted flanged manway lids are circular, flat plates that are used to cover the manways by attaching them to the neck ring using a number of bolts. These lids can have secondary uses, such as mounting valves or providing additional loading hatches. The configuration allows for top loading facility and increased internal pressure. 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.

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 Dangerous Goods - Model Regulations (Orange Book). The pressure relief valves 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. Pressure relief valves can come in two forms: Valves may be high pressure only to prevent a build up of pressure that may result in the tank exploding, or 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. 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.

The IMDG Code requires that tank containers complying with portable tank instructions T5, T10, T12, T14, T16, T18, T19, T20, T22 and T23 be fitted with a frangible disc in series underneath the pressure relief valve. 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; and 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. 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. 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. If a change in pressure in the void is recorded, the Shipper must be notified immediately.

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.

Before loading operations begin, several critical checks and preparations must be completed. The tank container must be thoroughly inspected for cleanliness, structural integrity, and proper functioning of all valves and fittings. The compatibility of the tank material with the cargo must be verified. All relevant documentation, including the tank's test certificate and previous cargo history, should be reviewed. The loading area must be properly prepared with appropriate safety equipment, including fire extinguishers, spill containment materials, and emergency response equipment. Personnel involved in the loading operation must be properly trained and equipped with appropriate personal protective equipment (PPE).

There are several methods for loading tank containers, each suited to different cargo types and operational requirements. The choice of loading method depends on the physical properties of the cargo, the tank design, and available facilities.

Top loading by gravity is the simplest method and is commonly used for non-viscous liquids. The cargo flows from an elevated source through the manway or top filling connection into the tank. This method requires minimal equipment but may be slower than other methods. During top loading, proper venting must be maintained to allow air to escape as the tank fills. The loading rate should be controlled to prevent splashing and foam formation. Operators must monitor the filling process continuously and stop loading before the tank reaches its maximum capacity to allow for thermal expansion.

Bottom loading using pressure is faster and more efficient than gravity loading. The cargo is pumped through the bottom discharge valve into the tank. This method is preferred for viscous products and when rapid loading is required. Bottom loading reduces the risk of splashing and vapor emissions. The loading connection must be properly secured and all valves must be in the correct position before starting. A vapor recovery system should be connected to the top vapor valve to capture any displaced vapors. The loading rate must be carefully controlled to prevent over-pressurization of the tank.

Some products require heating during loading to maintain proper viscosity or to prevent solidification. Heated tank containers are equipped with internal heating coils or external heating jackets. The heating medium (usually steam or hot water) is circulated through the heating system to maintain the cargo at the required temperature. Temperature must be carefully monitored and controlled throughout the loading process. Overheating can cause product degradation or create excessive pressure in the tank. The heating system should be activated before loading begins and maintained at the appropriate temperature throughout the operation.

Discharge operations require careful planning and execution to ensure complete and safe emptying of the tank container. The method chosen depends on the cargo properties, tank design, and receiving facility capabilities.

Gravity discharge is the simplest method where the cargo flows out of the tank under its own weight through the bottom discharge valve. This method is suitable for low-viscosity liquids and requires the receiving vessel or storage tank to be positioned lower than the tank container. The discharge rate is relatively slow and depends on the liquid's viscosity and the height difference between the tank and receiving point. Gravity discharge is the safest method as it requires minimal equipment and has fewer potential failure points. However, it may not achieve complete emptying of the tank, especially for viscous products.

Pressure discharge uses compressed air or inert gas (typically nitrogen) to push the cargo out of the tank. The gas is introduced through the top vapor connection, creating pressure that forces the liquid out through the bottom discharge valve. This method is faster than gravity discharge and can achieve more complete emptying. The pressure used must not exceed the tank's maximum allowable working pressure. Proper pressure regulation and monitoring are essential. For hazardous cargoes, inert gas must be used instead of compressed air to prevent chemical reactions or fire hazards. The discharge hose and connections must be rated for the pressure being used.

Pump discharge uses an external pump to draw the cargo out of the tank through the bottom valve. This method provides the fastest discharge rate and is particularly effective for viscous products. The pump must be compatible with the cargo being discharged and properly sized for the required flow rate. Pumps can be either centrifugal or positive displacement types, depending on the cargo characteristics. The discharge system must include proper valving to prevent backflow and allow for system isolation in case of emergency. Regular monitoring of pump performance and discharge rate is necessary to detect any problems early.

For products that solidify or become too viscous at ambient temperature, heating is required during discharge. The tank's heating system is activated before discharge begins to bring the cargo to the proper temperature for flowing. Heating may continue throughout the discharge process to maintain the required viscosity. The heating rate must be controlled to prevent localized overheating or thermal shock to the tank. Temperature should be monitored at multiple points to ensure uniform heating. Once the cargo reaches the target temperature, discharge can proceed using gravity, pressure, or pump methods. After discharge, the heating system should be gradually cooled down to prevent thermal stress on the tank.

Safety is paramount during all loading and discharge operations. Proper grounding and bonding must be established before operations begin to prevent static electricity buildup. All personnel must wear appropriate PPE including safety glasses, gloves, and protective clothing. The work area must be properly ventilated and monitored for hazardous vapors. Emergency equipment including fire extinguishers, spill kits, and emergency showers must be readily accessible. A responsible person must supervise the entire operation and be prepared to initiate emergency shutdown procedures if necessary. Communication systems must be in place to coordinate activities and respond to emergencies. All valves and connections must be checked for leaks before and during operations. Pressure and temperature must be continuously monitored and maintained within safe limits.