Saltation velocity is the speed below which particles being conveyed through a pneumatic conveying system lose suspension and begin to deposit at the bottom of horizontal pipelines.
In a dilute phase pneumatic conveying system, any particulate buildup at the bottom of the pipes can be problematic. This is why material characteristics, conveying rate, and system pressure are important considerations in dilute conveying.
Saltation Velocity Explained
Saltation velocity is basically the gas or air-conveying speed at which suspended particles being transported in a conveying system start to fall and deposit at the bottom of pipes or tubes. In order to prevent this, the minimum velocity in horizontal pipelines, also known as pick-up speed, is usually at least 20% higher than the saltation velocity.
Pneumatic pipelines can either be horizontal or vertical, but both of them need a minimum velocity in order to convey solids. While it’s necessary to check the saltation velocity (USALT) in horizontal pipelines, for vertical ones, it’s the choking velocity (UCH) you need to look out for.
When working with horizontal pipelines, materials in a pneumatic conveying system are typically transported in dilute phase. This is because, in this mode, particles are continuously suspended in the air that’s being blown into the pipe, preventing them from falling to the bottom. However, this won’t last forever, so if you want to prevent materials from accumulating at the bottom of horizontal pipelines, you must exceed the saltation velocity. As long as this important rule is followed, your pneumatic conveying system will function as desired, most of the time. This works the same way with vertical pipelines and choking velocities.
Since pneumatic conveying usually involves the transportation of bulk solids, like grain or powder, through movement, it’s important to ensure that your bulk material handling system runs as smoothly as possible and is protected against damage.
Applying a minimum velocity is one way to do this. Known as the pick-up speed or pick-up velocity, a pneumatic conveying system must have a minimum velocity that is higher than the saltation velocity in horizontal pipelines. This will ensure that the particles stay suspended. Moreover, in a complex vertical-horizontal conveying system, the saltation velocity should always be higher than the choking velocity. The values look like this: pick-up speed > saltation velocity > choking velocity.
But how do you find out the value of saltation velocity in a pneumatic conveying system? Well, there’s actually no universal value because the calculation depends on the system being used and the materials being conveyed. Some common materials have pre-computed approximate values of saltation velocity already assigned to them, but if you want a more detailed overview on how to calculate saltation velocity in your own conveying system, here it is.
How To Calculate Saltation Velocity
The value of saltation velocity varies from one plant design or material to another. While it can be calculated using the process below, it’s best to test out whether or not the resulting value still stands after professional assessments have been carried out on your system. This is because the existing Rizk calculation formula has an average margin of precision of ±54%. Nevertheless, you can still use it in order to gauge a base understanding:
Ms = Solids mass flow rate (kg/s)
ρg = Gas density (kg/m3)
USALT = Saltation velocity (m/s)
d = Particle diameter (m)
D = Pipe diameter (m)
g in S.I. units
Calculating the USALT, or saltation velocity (m/s) is crucial to get the correct pick-up speed, as well as to adjust the value of the choking velocity in a pneumatic conveyor, if necessary.
By manipulating this formula, we can isolate the U on one side in order to get the saltation velocity. While won’t give a 100% accurate calculation, it will provide a base understanding if there’s ever a need to troubleshoot any issues: instead of inputting random numbers that don’t have a computational basis, you can use Rizk’s formula as a jumping-off point. From there, the value can be further tested in order to get an accurate reading for your specific system.
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