Jack D. Hilbert, SME and Colin Barbeau, BMH Lead, guest co-author | Hatch
From time to time, we take the opportunity to answer questions we receive from readers, webinar participants, and conference and trade show participants (from the pre-COVID-19 times, that is).
Typically, if we receive a question from one person, there are many other people with the same or similar questions as well. Plus, we’re finding that more and more new people are getting involved in the field of pneumatic conveying and bulk solids handling in general, and a summary of questions like this can serve as a basic primer for those readers as well.
As you know, pneumatic conveying relates to the transport of solids, using primarily airflow (although there are applications where other gases such as nitrogen are used) from a source point, through a conveying line or pipeline, and into a receiver of some form at the solids’ destination. A flow diagram of this is shown in Figure 1.
In this article, we will look at some recent questions that relate to each of the pneumatic conveying system’s five main elements: material feed, line charger, gas mover, conveying line, and material receiver.
The pneumatic conveying’s material feed is the source from which the material to be conveyed is received.
Q: I mostly see information about pneumatic conveying from a storage silo to use points, such as another silo or a downstream process line, in a plant – can other sources of the material to be conveyed be used?
A: Absolutely, the material feed can be a production line where material is picked up on a continuous basis or a weigh hopper where several individual elements have been combined as a batch. In plants where raw materials are received in bulk from trucks or railcars, those areas can be the material feedpoints.
There are applications where small quantities of materials are handled in 25-kilogram paper sacks, bulk bags, or flexible intermediate bulk containers, which can also be the material source point at the beginning of a pneumatic conveying system.
The way the material enters the pneumatic conveying system to control the material flowrate and deal with specific material properties, along with the conveying system type and mode selected, will vary based on case-specific parameters.
Q: Must a pneumatic conveying system run continuously or can the system be used to convey material in batches?
A: If the material feed is a production line, which operates continuously and typically doesn’t work on a batch or intermittent basis, the pneumatic conveying system will be continuous as well, which means the material coming off the production line always has a means of being taken away to the next step in the process.
If the goal of the project is to make a specific material batch size, then the pneumatic conveying system may be a dedicated system for each material or a single system that handles multiple materials. For example, this system type would work well with a food product’s recipe formulation that has four or five different ingredients that need to be transferred to a single point such as a mixer. Typically, the material destination will be a hopper over the mixer, which is on a weigh system that controls how much weight of each ingredient is loaded into the hopper until the total batch is weighed out. In this case, the pneumatic conveying system(s) are operating intermittently.
The line charger is the equipment that gets the material to be conveyed from the material feed into the conveying line, typically at a specific rate and against the pressure differential in the system.
Q: Is a rotary feeder the best choice for a line charger?
A: The line charger chosen depends on the specific application because the varying pneumatic conveying systems, modes of conveying, material characteristics, operating temperatures, and material flowrates all must be considered when selecting the most cost-effective line charger.
There are many line charger options to consider: pickup attachments or pans when using railcars, rotary feeders and airlocks, pressure tanks, double-gate feeders and lock hoppers, screw pumps, and eductors and venturis. Some of these are shown in Figure 2.
Q: I am trying to understand why our pneumatic conveying system isn’t working right — can you suggest a place to start?
A: In my experience, having spent a long time troubleshooting problems with pneumatic conveying, a good place to start is by checking the line charger. If the line charger is worn or not functioning correctly from a mechanical or electrical sequence point of view, the system won’t work correctly. As you can imagine, each specific line charger type has its own areas to address.
The gas mover is the source of the air or other gas flow into the conveying line to entrain the solids and transport them to the destination point. There are three basic gas mover choices: a fan, a positive-displacement (PD) or centrifugal blower, and a compressor, as shown in Figure 3. Just as is the case with the line charger, the system specifics will be the information that drives the decision-making process as to which gas mover type to use.
Q: Our blower system is constantly popping the relief valve and we have to run the blower at very low rates to make the popping stop. However, we need to get our production back up to where it used to be — what can you suggest?
A: Remember that a PD blower doesn’t compress the air flowing through it. The back pressure against which your blower needs to operate is the summation of all the individual pressure losses in the conveying line and can include:
- Friction loss of the moving gas
- Friction loss of the moving material
- Energy required to accelerate the material in the gas stream
- Pressure drop of valves, elbows, coolers, filters, and other equipment in the air and material’s path
- Any back pressure at the end of the system
In your case, I would start by ensuring that the relief valve is mechanically correct and the relief pressure is correctly set for your specific application. If the relief valve and pressure are okay, then you need to start looking at the list of individual pressure losses to see if something has changed that would’ve resulted in higher-than-expected pressure drops. Some questions to ask yourself include:
- Has the material being conveyed been changed at all, meaning has it been altered or is a different material type being used?
- Have you changed the conveying line routing to add another pickup or destination point?
- Is the conveying line’s inside diameter building up with material?
- Is the vent filter at the end of the system operating properly and not experiencing high differential pressures due to a faulty cleaning mechanism or material backing up?
The conveying line or pipeline is the conduit that connects each piece of equipment together to form a complete system.
Q: Our conveying line shakes violently due to broken pipe supports. What caused the supports to break?
A: That’s a symptom of having too high of a material-to-gas ratio for the type of system installed and relates more to the conveying mode and material type being conveyed. A comprehensive review of the system design may well be in order.
Conveying lines typically operate in one of three different modes: dilute-phase, dense-phase, and two-phase or semi-dense-phase, as shown in Figure 4.
The deciding factors for each mode are far beyond the scope of this column, but just know that a conveying line’s operating mode is one of the most important aspects of a total system design.
The conveying line shaking violently sounds like you’re experiencing some of the characteristics of a dense-phase conveying mode, but your original system design likely was based on a dilute-phase conveying mode. The solution may be as simple as increasing the gas volume to reduce the material-to-gas ratio or may be complex enough to look at some conveying line changes that introduce supplemental gas along the conveying line.
Q: Our conveying line elbows are frequent points of wear, requiring shutdowns to replace the elbows. What options do we have to consider?
A: The abrasive nature of a material being conveyed and the velocity profile taking place in the conveying line can combine to be a very aggressive cause of wearing out pipe bends and diverter valves.
There are a couple options to consider for mitigating your problem. The first option is to reduce the overall velocity profile in the system, which is the root cause of the problem. To do this would require analytical modeling of your system to identify if and where the conveying line can be stepped in diameter (meaning the pipe diameter is increased) or if just a reduction of air volume in the existing conveying line can be implemented.
Another option is to replace the existing elbows with a geometry change or special construction material, such as applying fittings cast from wear-resistant iron or applying wear-resistant liners inside the fittings or wear-resistant coatings on the fittings’ exterior, to provide a longer life between changeouts. While this isn’t a complete solution, this option may be the most cost-effective approach.
Q: Our business model requires we add another set of silos in the plant and extend our existing pneumatic conveying system. What all must we consider? We tried this before, to just add a new bin indoors, and the system plugged up, so we had to abandon that new bin.
A: If the situation you tried before required the same conveying rate to be achieved to a farther destination, my guess is the pressure drop went up so your velocity profile in the conveying line went down to a point below the minimum conveying velocity that was needed for the material to move. This then caused the conveying line to eventually create line plugs. The same thing will happen with the new silo location unless you do one of the following:
- Reduce the system rate;
- Increase the pipe diameter — not necessarily all the piping but maybe just the new extended pipe section; or
- Increase the gas mover volume, preferably with a variable-frequency drive so that when the original system length is in use, there won’t be excessive gas volume to be handled.
The material receiver is the final stop in the system, the place where the pneumatic conveying system discharges the material. Material receiver options can include a silo, bin, hopper, cyclone, filter-receiver, or feed hopper transition to a downstream process.
Q: We have a bin vent dust collector on top of our sugar silos, and during the filling from a self-unloading truck, we see a constant leaking of sugar dust, especially at the end of the truck load. Why is that?
A: What you’re experiencing is a typical problem with this application that’s caused by the vent filter being sized too marginally. The air delivery from a pressure differential truck is often delivered to the silo in surges rather than in an even, continuous stream. These surges can instantaneously result in an actual air volume that’s in excess of what the bin vent was originally expected to handle. Because of this, the filtration area is too low and the differential pressure rises, creating a positive back pressure on the receiver bin and vent filter. The silo pressure relief valve may pop open momentarily, the gasketed surfaces on silo flange connections and the vent filter door can leak, or a combination of both of these results can happen.
The solution is to apply a surge factor to the conveying system volume and then size the vent filter for the correct filter area amount. In your case, the remedy is to ask the truck operator to limit the truck’s conveying line pressure to a lower value, which the truck operator might not be very happy doing because the lower pressure extends the time taken to unload the trailer. An alternate approach is to try to get more filter area in the existing vent filter by replacing it with a larger unit or installing longer bags in the existing unit, which may rob you of some storage silo volume.
Q: We had a small silo that we filled by emptying bulk bags into a small pressure pneumatic system and blowing into that silo. This silo had another small pressure conveying system underneath that conveyed to our weigh hoppers. We made modifications to increase the silo’s height and fill it with self-unloading trucks because we wanted to reduce the labor involved. We now have a problem with the original pressure conveying system under the silo, which we did nothing to as part of the silo volume and loading modifications. Did we overlook something?
A: Well, saying, “We now have a problem with the original pressure conveying system under the silo, which we did nothing to,” means you made no mechanical changes to the pressure conveying system equipment, which I can understand. But you likely have made an unintended consequential change to that existing pneumatic conveying system under the modified silo.
In further discussions, you advised that the existing pressure conveying system under the silo uses a rotary airlock as the line charger and is vented to the top of the silo above. In the original system, the silo had a vent filter with no fan to handle the material transfer from bulk bags in a dilute-phase pressure system. With the silo modification, the silo’s cylinder height was extended, the roof was raised accordingly, no change was made to the vent filter, and the same fill line was used to connect to the self-unloading truck station, which was a bit farther away than the bulk bag unloading station.
What is likely happening is the self-unloading truck system is delivering more actual airflow than the original blower package and is experiencing the exact condition as described in the reply to the leaking sugar dust question previously answered. When the silo reaches the point of being under a slightly positive pressure, that puts a constraint on the vent system from the airlock below, which in turn creates havoc with the pressure transfer system.
Modifications to the existing silo vent filter by way of adding an exhaust fan, increasing the filtration area, or doing both are some actions to consider. If there’s enough headroom under the silo, another approach is to install a vent hopper with a controlled feed and integral dust filter above the rotary airlock and eliminate the vent line going to the silo’s top, as shown in Figure 5.
A third option is to consider using a venting eductor to overcome the potential back pressure, as shown in Figure 6. The venting eductor takes the leakage air from the rotary airlock and conveys it to another dust collector in the vicinity or to the existing silo’s top.
In summary, while actual questions and recommendations can provide a wealth of information to people involved in the operation and maintenance of pneumatic conveying systems, don’t forget the vast database of past papers, articles, and webinars that Powder and Bulk Engineering (PBE) has available to gain more experience and information on specific issues. PBE’s annual directory of equipment suppliers, engineering companies, and consultants is also a source of where to obtain specific help for evaluating problems with analytical comparisons of different solutions, cost estimates, and more.
For further reading
Jack Hilbert, PE (610-657-5286) is an expert bulk solids pneumatic conveying consultant for Hatch in Schnecksville, PA. He’s also a principal consultant at Pneumatic Conveying Consultants. He holds a BS and an MS in mechanical engineering from Penn State University, State College, PA. He has more than 45 years of experience in the application, design, detailed engineering, installation, and operation of pneumatic conveying systems.
Colin Barbeau, P. Eng. (438-806-4298, www.hatch.com) is the Eastern North America bulk material handling engineer lead for Hatch, based in Montreal, QC. He graduated from Ecole Polytechnique de Montreal with a bachelor’s degree in materials engineering. He has more than 13 years of experience working exclusively in the bulk materials handling field, both as a bulk materials equipment handling supplier and a consultant.
Copyright CSC Publishing Inc.