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|1. Introduction :
what is a bulk truck tank ?
|2. Manual valve
operation for bulk truck tanker discharge
|3. Design of a pneumatic conveying system to discharge a road bulk tanker|
Bulk solids have quite a low bulk density, which leads quickly to the handling of large volumes of materials. There are different formats to handle bulk, like bags which an contain as much as 25-50 kg, or Big Bags which can contain 1-2 m3 of material, however for large industries this can lead to a lot of handling for a given mass throughput. One of the solution is then to transport the bulk material on trucks equipped with a tank on the trailer.
These tanks have typically a size of 30 m3, sometimes more. Taking the example of sugar, this allows in one operation to handle the equivalent of 960 bags or 30 Supersacks. Factories have thus a lot of interest to get their bulk materials in tankers, if they are equipped with silo to hold the discharge material, as it allows to convey in one time a lot of product, reduce drastically the needs for manual handling, and thus allows a higher factory throughput.
Many bulk solids are handled in road silo tankers, such as flours, sugar, but also chemicals or plastics. This kind of bulk solids tanker has to be discharged pneumatically in order to be able to empty the tank efficiently, discharge quickly enough, and send the product to a silo.
The objective of this page is to list some design considerations that one must have in mind to design and operate a pneumatic conveying system to discharge a road tanker into a silo.
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The truck is indeed equipped of a valve manifold that allows to balance air flow and pressure in between the silo tank (need to pressurize it), the discharge cone of the tanker (need to fluidize the material and move it to the conveying pipe), and the conveying pipe (bring enough air to blow the material). The operator must also manipulate the discharge valve to admit more or less product in the conveying pipe.
It is highly recommended to have a standard operative procedure
and the operator trained and validated in order to minimize the
risks of mistakes during the discharge, which could lead to pipe
blockages or damages to the receiving silo. The design
consideration given in paragraph 3 will help to make the discharge
system suitable to the factory needs but cannot entirely compensate
if the manual valves manipulation is not correct.
Figure 1 : bulk road silo tanker valve manifold
Safety : the truck must be grounded to avoid any hazardous build up of static electricity that could lead to dangerous sparks.
Each designer and silo operator must perform a risk analysis of a silo truck discharge system in order to adress potential risks. All the more if the pressurizing gas is not air, as it is in most of the cases, but another gas like pure nitrogen which can bring additional risks such as asphyxia.
In most of the cases, the silo truck is equipped with its own blower, actually a reciprocating compressor powered by the diesel engine of the truck.
The truck is then autonomous during discharge. This kind of compressor is able to reach typically 12 m3/min and a pressure of max 2 bar g [Anster]. The possibility to reach such a pressure drop allows to discharge at high solids load ratio and thus shorten the time the truck is stopped discharging. 2 bar g is however not always necessary especially if the material is light and conveys well (polyolefins for example).
This system is most of the time fine, it allows also the factory receiving the product to avoid investing in its own blower. However, the use of the truck's compressor has some drawback as the control of the compressor is not managed by the factory which means that its operation can be prone to manual errors (the truck driver operating at too high pressure drop and blocking the pipe for instance), or that the state of the compressor and of the air process is not controlled (oil in air, air not filtered properly).
If running the truck blower is not acceptable, particularly in food
industry where the conveying air must be controlled, then a screw
blower can be installed at the factory. The truck driver has then to
connect 2 flexibles : 1 to bring the air from the factory
compressor, and 1 to send the product to the silo. The blower must
be sized to be able to provide enough air velocity to pick the
product up and convey it, and enough pressure to unload the tanker
in a given amount of time (for example, if a rotary lobe blower is
used, the max conveying pressure will be < 1 bar g, thus the
solids load ratio will be low and the discharge time long for some
materials). To be noted that a risk analysis must be performed,
especially making sure that the road tanker can hold the maximum
pressure of the air mover.
The compression of air is leading to an increase in temperature. The more the compression ratio is high, the more is the temperature increase. One must therefore be careful about the maximum temperature reached can have on the material conveyed (softening it, melting, changing color) or even in terms of safety (self ignition of the material leading to a dust explosion).
If the temperature reached is expected to be too high, then either the conveying pressure should be limited or a heat exchanger should be installed after the compressor (aftercooler) to bring the temperature back to acceptable levels.
The piping must not be too small, typically at least 100 mm
internal diameter, in order to allow a high capacity for a given
pressure drop. Care must also be given to flexibles that must be,
for the part used for material conveying, as short as possible (max
4-5 m) and as straight as possible. The connection point must be
located in a vertical position allowing the flexible to have a
straight horizontal part then make a bend to the point of
connection. Any "S" shaped flexible must be avoid as it increases a
lot the pressure drop.
The end of the unloading of a silo truck may be tricky. Indeed, the tanker is pressurized at the conveying pressure during the transfer. In some cases it can reach 1.5-2 barg. It is fine as long as there is material conveyed in the pipe that are creating a pressure drop. But at the end of the discharge, the material drains out of the pipe and the pressure drop decreases drastically. It means that a large amount of air (30 m3) at 2 bar g can now flow almost freely in the pipe to the silo. The air flow is thus increasing dramatically. A higher air flow than expected, when arriving in the silo equipped with a filter not sized for such a flow, can create a pressure increase in the silo that can lead to damage, typically opening the safety valve or explosion panel mounted on the silo.
One of the solution to make the end of discharge more predictable
and safer is to monitor the pressure drop on the filter of the silo,
if it increases too much, a shut-off valve is activated on the
transfer pipe and a smaller bypass valve is opened : the air flow is
thus reduced enough so that the filter can vent it properly, while
the truck driver has still the possibility to decrease the pressure
of the tank.
The receiving silo must have certain characteristics to make the unloading of the dry bulk road tanker successful :