Known by several names, we are talking about the elongated films of plastic that can grow within pneumatic conveying lines…well until they break off and cause havoc by clogging up your downstream process. Thin strands of “angel-hair” can grow to longer and wider strips or “streamers”, and sometimes to extraordinary widths and lengths called “snake skins” – the worst I have seen were 50mm wide and over 5m long!
These can sometimes congregate together to form a tangled ball known as “bird-nest”. Don’t blame me for the mixed metaphors…I didn’t make the names up! These films grow when the pellets come into contact with the pipe wall at a low angle and slide along. The friction causes local melting of the pellet and smearing of the product onto the wall. Successive smearing by more particles cause these deposits to grow until they peel off.
To solve the problems that cause downstream you must prevent them being formed in the first place.
There are 3 main influences on their creation:
• Properties of the bulk product handled: Hardness, melt-flow index, temperature
• Conveying pipeline: Smoothness of pipe, alignment, angle, bend quantity and type and temperature
• Conveying Parameters: Velocity, air temperature, distance/duration
Generally speaking, the higher the product MFI and temperature and lower the hardness, the greater the risk of streamer generation. For the most part the products used are a function of the downstream process needs, and the specifications final products made so we will not dwell on this point.
The conveying parameters are a function of the system design. To avoid streamers completely, the best technology to choose would be dense phase conveying where the pellet velocities are not high enough to achieve the necessary heat of friction. The effect can be reduced in a dilute phase system by cooling the conveying air to say 30˚C. This is not possible on truck unloading systems using on-board blowers, as it requires a land based air package…but it can be reduced by system designs and operational procedures to reduce velocities and therefore wall friction. Of course the longer the distance, the higher the air pressure/temperature and the more opportunity for formation there will be. This is why it is important to locate the unloading point close to the silos and have them as close to the process plant as possible.
However, sometimes the additional cost of a new dense phase conveying system, or the use of existing plant with new materials, means we do not have the luxury of going back to basic technology choices and have to make the best of what we have with the lowest investment costs. In most cases this means optimizing the conditions in a dilute phase system by looking at the influences of the pipeline.
The biggest single factor is that pipelines are often smooth and this promotes the sliding action of the pellets. This can be avoided by the use of “roughened” or “shot-peened” pipe – instead of sliding against the pipe wall and causing friction, the pellets bounce off the angled/rough surface of the pipe. The disadvantage of this is that it can create more “fines” that need subsequent removal to avoid other process issues, although fines are generally easier to remove than streamers.
It was long thought that streamers were generated mainly at bends. Experience now suggests this is not always the case. Long radius bends (8-10 D) can create the opportunity for the low angled to slide against the pipe wall but this can be avoided using tighter right-angle “elbows”. The disadvantage of these is that they generally slow down the particles more than a radius bend and create a higher pressure drop so caution is needed.
Sometimes for ease of installation pipes are installed at a sloping angle. This is generally not good practice as it increases the line pressure drop. In the same way as long radius bends, it also presents a greater opportunity for frictional blows and increases the risk of streamers.
It is also not only the temperature of the pellets and conveying air that softens the pellets and increases the chance of smearing but also the pipe wall temperature. Even in the UK, pipelines exposed to process heat within the plant or installed in full sun positions outside get heated and this can again increase the problem of streamer formation. Because of this, it is important to think about alternative pipe routes or sunshades on exposed pipes.
Streamers are rarely the sole problem and are often seen in conjunction with fine particles. In this case the alignment of pipe sections may also be playing a part so check the pipe installation and avoid pipe offsets. Disruptions to the flow path of the pellets can increase impacts and generate heat.
• For new plant consider dense phase conveying. It may cost more but the advantages can save you money over the lifetime of the plant.
• Keep conveying distances and number of bends to a minimum.
• In dilute phase high velocity systems use roughened or shot-peened pipes. (Note: Higher fines!)
• Avoid long radius bends.
• Replace sloping pipes with horizontal and vertical sections.
• Optimize the conveying parameters to reduce velocities.
• Cool the air conveying air if possible.
• Slow down tanker unloading.
• Avoid heating of pipe by process heat or direct sunlight.
• Check for pipe offsets.