|2. Root causes|
|3. How to avoid pipe blockages in pneumatic conveying|
Pneumatic conveying is a very versatile and reliable way to convey powders and bulk solids. The only true process upset that can lead to an immediate stoppage of the line and possibly a long downtime is a pipe blockage. It sometimes happens because of some incident during the transport but is something that factory operators must avoid and make sure that some precautions are taken to prevent it to happen. The blockages referred to in this page are sudden blockages, other type of blockages due to build-up in pipes can happen but are to be considered on a longer time frame.
A sudden blockage of the pipe happens when the energy supplied by the conveying air is not sufficient to keep the powder in movement. The solids particle then settle, reducing the conveying pipe section until it finally blocks it entirely. The conveying air cannot circulate anymore in the pipe and all the solids that were in conveying before and after the location of the plug is going to a stop, potentially increasing further the extent of the blockage.
There are different possible root causes leading to a stoppage of the flow :
- The air velocity is not high enough in all points of the conveying line to be higher than the minimum conveying velocity for the solids particles being transported.
- The air mover cannot reach a pressure high enough to overcome the pressure drop generated by the solids. By extension, it means that a too high flow of solids, leading to too high solids loading ratio, can lead to a required pressure drop that the air mover - blower or compressed air - cannot reach
- The pipe layout creates some flow instability in the pipe
- There are some air leakages in the line, thus reducing the actual air velocity below the minimum conveying velocity at some points of the line
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Minimum air conveying velocity
To avoid the solids particles to settle, and make them carried by the air transport, it is necessary to reach a certain minimum air conveying velocity, dependent on the powder / bulk solids being transferred. The pneumatic conveying system must be designed in a way that at each point of the line Uair > Umin. This is particularly true at the beginning of the line where the solids are dosed to the pipe (pick-up velocity) and at each change of pipe diameter (stepped pipe).
The line design must included a calculation of the pressure drop, for a design materials conveying capacity. The air mover must then be chosen to be able to supply the required pressure at the required air flowrate to keep the particles at the right velocity.
To prevent pipe blockages, the layout must be as simple as possible. Especially, bends must be the object of a particular attention as the flow of solids is disturbed and slowed down at the change of direction. In general, a straight section of several meters must always be respected after material inlet in the pipe and before the 1st bends. Consecutive bends should be avoided and a straight section of several meters should be installed in between 2 bends.
Control of flow and process parameters
Large variations of material inlet rate, or of the blower speed
should be avoided.
The pipe should be checked for leakages, as leakages will be
changing the air velocity which can lead in a too small speed below
the minimum conveying air velocity, leading to a pipe blockage.
Filters at the reception of the conveying line must be clean and
offer a limited pressure drop all the time. If it gets clogged, the
additional pressure drop may be too high and the line may choke.
The automation system must be able to anticipate piping blockages. The easier method is to define a high level pressure - or vacuum - and stop feeding the product if it gets crossed. By reducing the amount of product in the line, the choke may be cleared.
Once a blockage has occurred in a pneumatic conveying line, it is very difficult to unblock and restart the system. Note that every operation for unblocking the pipe should 1st be the object of a risk analysis.
In many cases it will be necessary to dismantle the pipe or part of the pipe to clear enough product in order to decrease the pressure drop created by the plug of product and be able to restart the conveying line. For less severe blockages it may be possible to push the plug of product by attempting several time to sweep it away by turning on and off the blower, of course during these attempts, no more product should be introduced to the line.
Note that the use of a "pig", a small device pushed by pressure through the pipe may sometimes be discussed as a way to clear pipe blockages but it is actually referring to a product build-up rather than a sudden blockage of the pipe due to a temporary process issue. Please refer to the build-up page of this site.