Jack D. Hilbert, PE, and Étienne Brochu, guest co-author | Hatch
We often update and re-issue a topic from past “Points to ponder…” columns based on reader, equipment end-user, and sometimes client interest and activity levels. Some colleagues and I recently visited several bulk solids processing installations, and the topic of conveying line routing and orientation came up and was discussed on numerous occasions. The intent of this column is to provide you with typical guidelines that could help when the time comes to define a pneumatic conveying system’s routing.
One of the advantages of pneumatic conveyors is that they offer good routing flexibility compared to other types of material transport systems. Pneumatic conveying systems have the advantage of being able to accommodate various routing geometries since the material is conveyed through a pipeline versus a mechanical conveyor open to ambient conditions. However, people who aren’t familiar with pneumatic conveyors will generally think that, like standard pipeline, these systems have almost no limitations with regard to routing. The truth is that pneumatic conveyors do have their own limitations.
Before describing pneumatic conveying line geometry good practices, let’s define what a “basic” pneumatic conveying system consists of. A basic pneumatic conveyor is made up of various components assembled together. These include a gas mover, a product line charger, a conveying pipeline, and a product receiver. Of these, the pipeline is the component that links all other equipment together to form one single system. For a given material and mass-flow stream, pipeline geometry is a parameter that can influence a system’s efficiency. For that reason, understanding good conveying line geometry practices is important.
Material acceleration zone
The conveying line’s most important section is at the material feedpoint. Obviously, if the material can’t get moving downstream and entrained into the conveying gas stream, the system pressure will rise, and conveying capacity will be limited — possibly to the point of a plugged line.
Pneumatic conveying modes
There are three modes of pneumatic conveying: dilute-phase flow, two-phase flow, and permeable dense-phase flow. The conveying mode to be used depends primarily on the material’s physical and chemical characteristics. What follows are pneumatic conveying line geometry guidelines for each conveying mode.
Dilute-phase flow. In dilute-phase (stream) flow, shown in Figure 1, the material must be accelerated above the saltation velocity (point at which particles begin to fall out of suspension) in the acceleration zone, which is the conveying line section after the feedpoint. Because the material has little or no velocity at the feedpoint, the material must be accelerated by the conveying gas drag.
To achieve this, we must follow certain design criteria in the conveying line acceleration zone:
- The conveying gas velocity must be above the saltation velocity so the drag can accelerate the material above the saltation velocity. To shorten the acceleration zone or accelerate the material more quickly, the conveying line diameter prior to the feedpoint can be reduced. However, conveying through a smaller line diameter will require a little more energy (horsepower), particularly if the conveying line direction changes near the feedpoint.
- The acceleration zone’s length is a function of the material being conveyed and the conveying gas velocity. Accelerating a large, heavy material (such as lead shot) requires a longer acceleration zone than accelerating a fine, light material (such as talc powder). The acceleration zone length is typically 15 to 20 times the pipe diameter. Formulas for calculating this length are available in several pneumatic conveying books.
- The acceleration zone shouldn’t contain any bends or inclined sections because the material isn’t fully accelerated and can settle in the bends, especially going from horizontal to vertical or in the inclined sections. Such settling could cause refluxing, which occurs when material falls out of the gas stream, settles on the conveying line’s lower wall, and slides down the incline. This material accumulates at the bottom of the bend or inclined section, is re-entrained in the conveying gas, and must be reaccelerated. Though refluxing doesn’t occur in a horizontal-to-horizontal bend, the material does slow down and requires reacceleration after the bend.
- It’s also recommended to have a straight-line section with a minimum length of 10 pipe diameters before a feeding point. Indeed, a straight-line section prior to the feeding point helps in achieving a laminar flow of air. Turbulent air at the feedpoint can have a negative effect, reducing material acceleration and increasing the risk of wear in that turbulent zone.
Two-phase flow. In two-phase flow (a combination of dilute-phase flow with a settled material layer), as shown in Figure 2, material doesn’t have to accelerate above the saltation velocity. The conveying gas drag accelerates the material.
Design criteria for this system’s acceleration zone include:
- The acceleration zone should be relatively short (typically in the range of 5 to 10 pipe diameters).
- The acceleration zone shouldn’t contain any bends or inclined sections because the settled fluidized layer in the conveying line’s lower section should be given time to reach a steady-state condition before changing direction.
Permeable dense-phase flow. In permeable dense-phase flow, shown in Figure 3, the material is accelerated to a lower velocity. However, the conveying gas drag doesn’t accelerate the material; instead, the differential pressure exerted across the short slug, or piston, of material accelerates it.
Design criteria for this system’s acceleration zone include:
- The conveying gas velocity, which is below the material’s saltation velocity, isn’t critical because the conveying gas drag doesn’t accelerate the material.
- The acceleration zone is very short and shouldn’t contain any bends or inclined sections until the point where material slugs are formed (either in the first 2 feet of conveying line or in the pressure tank or line charger’s discharge).
Bends for all modes
An ideal pneumatic conveying system has a minimum number of directional changes. Unfortunately, many systems are installed as a retrofit to an existing installation, compared to 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 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 as a Type C material in the Geldart Classification1 would be line scaling, creating 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 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 be the cause of 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.
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.2
- Downward-inclining sections can be used in any mode of conveying if the system layout demands it.
- The Geldart Classification is a method of identifying materials into one of four possible groups based on their mean particle size and respective particle density. Each group has specific characteristics as to how the material will respond to fluidization, which helps to select the preferred mode of conveying.
- Line stepping is used when the conveying line incorporates a section of line that’s a larger diameter than the previous section to help control the velocity profile when a large system pressure differential exists.
Jack D. Hilbert, PE (610-657-5286), is a bulk solids pneumatic conveying expert consultant for Hatch. Prior to that, he was the principal consultant for Pneumatic Conveying Consultants. He holds BS and MS degrees in mechanical engineering from Penn State University, State College, PA. He has more than 46 years of experience in the application, design, detailed engineering, installation, and operation of pneumatic conveying systems.
Étienne Brochu (438-797-9397) earned a degree in mechanical engineering from École de Technologie Supérieure, in Montréal, Canada. He has been working in the specialized bulk materials handling group at Hatch for the past 2 years.
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