Conveying pipes are used in pneumatic conveying to send the product from a point A to a point B. Pipe design is resulting from the considerations of different inputs or constraints.
Among the constraints are the aspects related to the performance of the conveying system and its influence on products. This will fix the pneumatic conveying pipe diameter but also the design of the bends of the pipe layout.
Some inputs will have also to be done by the plant operator, especially regarding the type of pipe coupling (clamping) to be used or the length of pipe sections.
Different types of pneumatic conveying pipe couplings are available in the market. The main ones are the following : SMS connections, Morris clamps and flanges. The decision to go for one or the other will depend on the service conditions of the pneumatic conveying line, the habits of the plant operator but also the confidence on the reliability of the conveying line - if the product to be transported is very difficult and blockages are expected, it is better to have quick dismantling connections.
Table 1 : Different types of piping connections and their advantages and drawbacks
|Type of coupling||Picture||Pros||Cons|
Easy to dismantle and clean the pipe
|More expensive - need to have threaded pipes
Sometimes difficult to have a tight connection
|Morris coupling||Less sensitive to pipe alignement vs SMS
No need to have threaded pipe
|Long to dismantle
Not always easy to adjust
|Flange||Tighter connection, can be qualified for high pressures
Long lasting - usually preferred when the pipes goes over long distance outside building
|Very Long to dismantle
Not always easy to adjust
Need to change the gasket at each opening
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The design of Conveying Pipe Bends is of particular importance for lean phase conveying where the product is circulated at high velocity. When hitting the bends some products will be damages - fragiles powder breaking and forming fines,
In order to avoid such phenomena, the pneumatic transport bend radius must not be too sharp. As a general rule, the ratio Bend Radius / Pipe diameter should not be below 10.
Deciding on the pipe section length will mainly depends on the service which is expected for the pipe. If it is expected to be dismantled often for cleaning for example, then short pipe sections are recommended. On the contrary, if the system is not expected to be dismantled, while conveying a single and well known product for instance, then it will be economically sounder to have longer sections.
As a rule of thumbs :
- High frequency for dismantling : pipe section length < 6 m
- No dismantling expected : pipe section > 10 m
When having to dismantle pipes frequently, specific access may be foreseen in order to ensure a safe access to the piping.
The supporting of pneumatic transport pipe is more important than it seems. Indeed, one must understand that in certain circumstances, especially in dense phase where the material is moved in plugs which can represent several kg, the force applied by the product moving in the pipe can be very important. It is the case at bends which must withstand the moving material's inertia. Supporting must therefore designed in consequence and reinforce at bends.
The design of the pipe layout is of prime importance in pneumatic conveying. The pipe length and especially number of bends will directly influence the pressure drop across the pipe for a certain capacity and will also determine how gently the product can be conveyed. More length and bends, means more pressure drop, more breakage and / or build-up for some materials.
The Engineer designing a pneumatic transport pipe layout must take extra efforts to have a layout as simple as possible. It means reducing the number of bends as much as possible (it is better to have slightly a longer pipe length than extra bends), choosing the right bend radius (in general, long radius bends are preferable but there are exceptions), and avoiding consecutive bends or inclined pipes which can leads to choking of the pneumatic conveying line.
The pipe diameter must be adjusted according to the capacity to reach in order to keep the pressure drop in an acceptable range.