A: Jack Hilbert, Pneumatic Conveying Consultants, says:
An ideal pneumatic conveying system has a minimal number of directional changes. Unfortunately, many systems are installed as a retrofit to an existing installation, rather than a greenfield site, and consideration must be given to routing conveying line around every obstacle in the line’s path. Each bend requires more energy for conveying because much — if not all — of the conveying system’s kinetic energy is dissipated as the material impacts bend elbows and the material must then be reaccelerated as it enters the downstream pipe. In addition, each bend becomes a logical location for wear when handling an abrasive material or for causing degradation when conveying a material that’s friable. In the case of a cohesive material, something we might typically see from a Type C material in the Geldart Classification is line scaling and that material creates buildup. So, respect your conveying line’s configuration by making it as straight as possible.
First bend location. Almost every pneumatic conveying system starts by running the conveying line toward the nearest wall or column. This is natural because running the line in this way doesn’t obstruct walking or material handling areas and close proximity to walls and columns often provides the opportunity for supporting the pipe.
Often, the distance to the nearest wall is shorter than the required acceleration zone, and the conveying line turns vertically at the wall using a long radius bend. In both dilute-phase and two-phase flow, this long radius bend and the low material velocity cause the material to reflux in the bend. In many systems, a long radius bend can cause line plugs, low conveying capacity, or high system pressure.
If you think this configuration is causing a problem in your conveying system, try this simple test: Listen to the material flowing through the bend; if you hear a surging or slugging sound at the same time the conveying system’s pressure surges, reflux is likely to be occurring.
You can solve the problem in three ways:
- Reduce the conveying line diameter in the acceleration zone and in the first bend. The reduced diameter will increase the material velocity.
- Use a short radius bend, which has a very short section of inclined line. The short incline can’t accumulate material and so will reduce refluxing.
- In a dilute-phase system, increase the distance from the feedpoint to the first horizontal-to-vertical bend, which will allow the material to accelerate to dilute-phase flow.
Multiple bends in series. Any directional change in the three-dimensional world can be made with a maximum of two 90-degree bends. It seems that conveyed material knows this, because whenever the material encounters three bends, the conveying line plugs, the pressure drop increases, or the conveying capacity drops. Because material decelerates in each bend and doesn’t have enough straight conveying line to reaccelerate between the bends, the material continues to slow down with each successive bend. Overcoming this effect requires less material loading or a higher gas velocity.
Be aware that using flexible hose in your conveying system can create the same multiple-bend problem, but it isn’t as obvious. For example, conveying system connections between railcars and pneumatic unloading systems or hose switch stations often have long flexible hose runs. The hose typically lays on the ground with many random bends. At one site, tests revealed that such a flexible hose configuration reduced the pneumatic unloading system’s conveying capacity by 28 percent. Thus, never have more than two bends in a series without adequate straight conveying line for reacceleration between the bends. Typically, we would recommend a range of 5 to 10 pipe diameters as that straight section.
Inclined conveying lines and angles. When designing a pneumatic conveying system, we usually calculate pressure drop for a horizontal or vertical system, but what should we do for a conveying line at some angle to horizontal?
In general, a good rule is to avoid an upwardly inclined conveying line. But why avoid such a line when an incline is perfectly satisfactory in dilute-phase conveying in which conveying is taking place above saltation? Saltation velocity in an inclined line is higher than in a horizontal line. Thus, if conveying is just above saltation velocity in a horizontal line, entering an inclined line, where the saltation velocity is higher, could cause the material to fall out of the dilute-phase flow and reflux.
The incline angle is important because the refluxing depends on the material not only settling in the line but also sliding down the line and being entrained in the gas stream. For example, a conveying line inclined upward at 10 degrees from horizontal wouldn’t affect a two-phase flow system and probably wouldn’t affect a dilute-phase system. But if the conveying line is inclined at a steep angle, such as 45 to 75 degrees from horizontal, the material’s sliding will accelerate against the gas flow and cause severe refluxing.
What about a conveying line inclined downward? Apparently, a downward incline isn’t detrimental to either dilute-phase or two-phase conveying. In fact, material sliding down the incline aids conveying.
If your conveying system should be operating in dilute phase but problems are plaguing the operation, it’s possible that some section or the entire system is operating below the saltation velocity and is conveying in two-phase flow. If the conveying line includes an inclined section, the problem could be serious.
A simple test can show if material is accumulating or if refluxing is occurring in the conveying line. To ensure the material is conveyed above the saltation velocity in the test, the material should move at about the same velocity as the conveying gas. For example, if the conveying line is 400 feet long and gas velocity is 4,000 fpm, the gas will pass through the entire line in 6 seconds (0.1 minute). If we assume the material is traveling at 50 percent of the gas velocity, the material will pass through the system in 12 seconds.
Listen to the material passing through the conveying line near the end of the system. Have someone turn off the material feeder and time the material flow after the feed stops. If you can hear material still flowing after 12 seconds, the material was accumulating in the line and, thus, was below saltation velocity. If you can hear material flowing for more than 40 seconds, refluxing is occurring.
What about an inclined conveying line in a dense-phase system? You may think that a dense-phase conveying line’s orientation shouldn’t be critical because the material travels in a slug, and saltation doesn’t have any effect on the system, but that’s wrong. Tests have shown that as a slug is pushed through an upwardly inclined conveying line, the slug tends to break or shear on a vertical plane. The break occurs at an angle to the force pushing the slug, and the break’s angle causes the second section of the slug to ride up and over the first section. This effect has been observed in clear sections of a conveying line test loop in which slugs were observed to form and break at random intervals. When the conveying line returned to horizontal in the test loop, conveying returned to normal dense-phase flow.
Line charger position
It’s good practice to have a pressure system conveying line run as close as possible to the discharge of the product line charger. A long space between the conveying line inlet and the product line charger creates a potential for interrupted flow due to airlock leakage.
When designing a pneumatic conveying system, follow these routing guidelines.
For acceleration zones:
- Ensure the length is adequate.
- Ensure the gas velocity is adequate.
- Don’t use bends in the acceleration zone.
- Minimize the number of bends.
- Avoid multiple bends without adequate acceleration zones between them.
For inclined sections:
- Avoid inclined sections when possible.
- Use steeply inclined sections (between 45 and 75 degrees) only in a dilute-phase conveying system. Try to locate the sections toward the end of the system where the conveying velocities are typically the highest — unless line stepping has been incorporated into the system design.
- Downward-inclining sections can be used in any mode of conveying if the system layout demands it.
Don’t miss Jack Hilbert’s upcoming webinar “Don’t let pneumatic conveying system bottlenecking derail your process.” To learn more and register, click here.