Silo truck
A silo vehicle is a type of construction of a truck or semi- trailer for the transport of pourable bulk goods . A characteristic of this type of construction is that the goods to be transported are taken up in special containers, which are emptied through discharge funnels, mostly with the help of compressed air. It belongs to the tank trucks .
The suction truck (suction pressure tanker) , which is optimized for liquid to fine-grained transport goods and is partly equipped with pressure-resistant tanks, must be distinguished from silo trucks .
Structure of the vehicles
The vehicle
A special feature of silo vehicles and tankers in contrast to other truck bodies is, in addition to the round design, the option to walk on the body: The vehicles have a folding ladder at the rear (usually on the left in the direction of travel), which enables the vehicle to be entered. On the vehicle there is then a safety grate and a folding scissor railing over the entire length of the container. This means that the entire boiler can be accessed and all filling domes can be reached.
The towing vehicle usually has its own compressor unit with a silencer and air cooler in order to be able to generate the necessary overpressure for unloading.
Semitrailer tractors that are also supposed to pull tippers are additionally equipped with a hydraulic system (consisting of a hydraulic pump and tank) in order to be able to operate the hydraulic lifting cylinder. The pump is operated via the auxiliary drive of the towing vehicle, in many cases also electrically via the battery of the towing vehicle. In the case of electrical operation of the hydraulic pump, which is usually attached to the silo trailer, a connection between the battery (towing vehicle) and the pump must be established via a 2-pin cable. There are special plug connections on the towing vehicle and the trailer for this purpose.
The boiler
Silo kettles are usually made of aluminum, more precisely a wrought aluminum alloy (AlMg 3 or 4.5 Mn), not least because of their weight . The wall thickness of aluminum boilers is usually 5 mm. In rare cases, the containers are also made of steel.
There have recently been attempts to manufacture silo containers entirely from plastic. They are much lighter than conventionally manufactured containers.
In contrast to tank trucks, silo vehicles rarely have a panel. In exceptional cases it is necessary to insulate the silo container. Then they are clad with sheets of stainless steel, as in the tank truck. The panels are designed incomparably more complex because of the more complicated shape of the container.
Types
The main types of construction are:
Tilting silo
Articulated lorry semitrailers, truck bodies and truck trailers are built in tipping silo design. A characteristic feature is a cylindrical container which - similar to a tank truck - lies almost horizontally on the chassis . In contrast to the tank truck, the container is tipped up approx. 45 °, depending on the slip properties of the material, up to max. approx. 60 °, so that the transported goods slide by gravity to the manhole or through the outlet nozzle.
This type of construction is used to transport sugar , plastic granulate and other powdery substances.
Udder vehicle
The so-called udder vehicle (also known as the lying silo) gets its typical shape from the geometric combination of the approximately cylindrical casing with the conical discharge funnels. This design is designed as a semi-trailer, truck trailer and as a direct truck body. It is rarely used as a superstructure for railway wagons. Typically, trailers are built in a self-supporting design, i.e. without a support frame on which the container rests.
In special cases, the individual segments are separated by pressure floors so that different goods can be separated. In contrast, so-called baffles are almost always built into the boiler. These are pressureless and only serve to prevent the contents from sloshing - which would make driving the vehicle considerably more difficult.
If a weighing system is integrated, the advantages of the self-supporting construction cannot be used, as an additional chassis is required for decoupling .
This frequently encountered design is u. a. used for the transport of flour , grain , animal feed and cement .
banana
The banana shape results from the design, which is made up of two cones placed next to one another. It is rarely found any more. The area of application is in cement and coal dust transport.
Loading and unloading
The loading of silo vehicles takes place via funnels on the top of the vehicle. To do this, the lids of the so-called filling dome are opened and the transported goods can be filled in by gravity . The filling domes - also called manholes - usually have a diameter of 450 mm and are closed either with a hinged lid or a sliding lid. The discharge is mostly done by conveying air. Conveying air enables very quick unloading, but cannot be used for cargo that cannot cope with the resulting loads - e.g. B. Soot .
A special form is the vacuum vehicle, also known as a suction vehicle. It can also be used to vacuum goods with negative pressure. Since the negative pressure causes considerably more problems than the normal overpressure, these vehicles must be specially equipped. Vacuum vehicles can usually be recognized by the characteristic vertical rings that stretch across the entire boiler and serve for stabilization.
These vehicles are used, for example, for vacuuming incorrect / wrong deliveries or goods that have been rejected. Not least because of the extra weight and the higher investment and maintenance costs, these vehicles are rarely seen.
When discharging with conveying air, compressed air is passed through a pipe system by a compressor carried in the vehicle or by an external pneumatic conveying system. To do this, the entire silo is pressurized to a maximum of 2 bar. The freight trickles into this air stream, which is blown past the outlet of the silo in pipes and thereby entrains the freight ( entrained flow conveyance ) or pushes it forward ( dense phase conveyance ). The advantages of this unloading method are, on the one hand, the high unloading speed and the possibility of being able to blow the load directly into storage silos. Otherwise, in some cases, unloading is not even possible, since the load would not trickle out by itself due to gravity.
Special loads that naturally stick together, e.g. B. flour, salt, cement and other dusty products also need so-called "cloths" or "mats" to loosen them up. These are mounted directly in front of the outlet in the silo and blow the compressed air into the product through a strong fluitex fabric, loosening it so that it loosens and falls into the outlet. A vehicle construction company in the Netherlands also builds silo vehicles that do not need cloths and mats when they are emptied.
In addition to loosening cloths, silo tippers have been offering the option of loosening up the loaded goods using stainless steel inserts for several years. The stainless steel insert sits in the same place in the drainage pot as the cloths and has small openings all around through which the compressed air can flow in. The advantage over cloths is that they are less worn and easier to clean.
There is also the option of installing a thrust or ring nozzle at the end of the so-called drainage line in addition to a sight glass. The sight glass is used to monitor and control the conveyance of the goods. Thrust or ring nozzle accelerate and improve the unloading process by introducing an additional amount of air into the conveying flow. This is particularly useful when there are longer distances to be bridged between the silo vehicle and the storage silo.
In addition to the compressed air, pneumatically operated vibrators can be mounted on the vehicle. These are switched on as required in order to better loosen any material adhering to the boiler. In addition to the additional air lines required and the very costly vibrators themselves, however, the risk of damage to the container also increases in the event of incorrect or excessive use.
The unloading time of a silo train fully loaded with 27 t by blowing out is between 25 minutes (cement) and over three hours (salts), and is strongly dependent on the length of the pipelines, the height of the storage silos and the loaded product. Today, ready-mixed dry mortar is often transported directly to the construction site and blown there into small storage silos that are sometimes in the building to be constructed. The unloading time there can be up to five hours for transport pipes over 100 m in length.
In addition to compressed air-powered unloading, gravity unloading is also used (less often). When vehicles are lying down, this is done via so-called free-fall flaps under the boiler at the discharge funnels (emptying pots). They usually have a larger diameter (DN150 / DN200) than the discharge line for compressed air (DN80 / DN100). These flaps are simply opened for unloading and the cargo can fall freely into the collecting containers (mostly underground silos) below.
It is important here to open the filling openings on the top of the boiler, the so-called dome lid, as otherwise the resulting negative pressure can damage the boiler. The same must be observed when unloading tipping silo semi-trailers. Since a vacuum damage is associated with not inconsiderable repair costs, vacuum valves are also occasionally used to prevent a vacuum in the boiler. However, due to the rather high acquisition costs, this is often not done and the proper handling is relied on when unloading.
Maintenance and periodic inspections
Security checks
In addition to the normal general inspection, additional inspections are required for a silo vehicle. The boilers are pressure vessels and can be classified as systems requiring monitoring in accordance with the Ordinance on Industrial Safety and Health. In this case, according to Section 15 in conjunction with Section 17 BetrSichV Annex 5 No. 10, external tests must be carried out every two years and internal tests every 5 years. A special regulation makes it possible to dispense with the recurring strength or pressure tests for containers for powdery and granular goods (see also the section on maintenance ).
maintenance
Especially when transporting flour, mold and other impurities naturally build up in the kettle. In addition to the dry sweeping of the container, many vehicle operators have their vehicles flushed regularly. The entire boiler as well as the discharge hoses and other discharge accessories that come into contact with the load are cleaned with high-pressure cleaners, among other things. One problem with these regular, wet cleanings is the damp residue. The residual moisture can lead to clumping in the conveyed material (especially with hydraulic binders such as cement), which in turn has a negative impact on the emptying process. It is therefore important to ensure that the boiler is optimally dried. In addition to the disadvantageous effect of moisture in the emptying lines themselves, the cloths also suffer from this cleaning, since the moist tissue becomes very easily and stubbornly clogged with the load. When the vehicle is lying down, the cloths cannot be removed before flushing, as they are firmly connected to the emptying pots and removal would not be economical.
After cleaning, which should take place in a flushing area approved by water law, a cleaning certificate is usually issued. At least the last load, the type of cleaning and the vehicle registration number (for silo containers, the container number) are noted on this. A special type of cleaning certificate is the ECD (European Cleaning Document), which may only be issued by tank cleaners certified by the EFTCO (European Tank Cleaning Association).
Most companies / shippers in the chemical industry and some in the food industry refuse to load silo vehicles if a cleaning certificate cannot be presented. The reasons for this are damage caused by mixing and hygienic reasons. Special cleaning requirements apply to raw products from cattle feed production ( GMP certification ).
In addition to looking after the boiler, wearing parts, especially seals, frequently used screw connections and shut-off devices, must be replaced on a regular basis. Only white seals are used on vehicles that transport food (mainly flour). For other transport goods (e.g. building materials), black seals are also used in shut-off devices and lines, which are usually a lot cheaper than white ones. The same applies to the emptying hoses carried along.
Since silos are pressure vessel systems, they have to be checked regularly by experts (TÜV, DEKRA and others). A distinction is made between internal and external testing. Depending on the test, the silo container is subjected to an external visual inspection (cracks on welds, deformation, etc.). During the internal test, a pressure test and a test of the safety devices such as B. made the pressure relief valve.
