Pneumatic Conveying System Design and Troubleshooting

Introduction to Pneumatic Conveying System Design

Pneumatic conveying is defined as “transporting bulk materials through a pipeline via a negative or positive pressure gas stream” (1). Its popularity, as compared to traditional conveying systems like screws and bucket elevators, has increased in the last 50 years.

This has been driven by flexible line layout, product containment, automation ability, low maintenance, ability to have multiple pick up or delivery points, and safe handling of toxic or explosive materials as the air can be replaced with an inert gas to exclude oxygen (2) in a closed loop system. This article briefly outlines the key considerations to make in the design and troubleshooting of pneumatic conveying systems.

Generally, the system key components are comprised of a gas mover, solids feeder, conveying line, and gas-solids separator. These components should be selected and designed by careful consideration of the system requirements, experimental testing, and empirical correlations.

While many bulk materials can be conveyed pneumatically, the types of materials ideally suited for pneumatic conveying are free flowing, non-abrasive, non-fibrous, and non-friable. Flow regimes of pneumatic conveying systems are termed as either dilute-phase and dense-phase.

Dilute Phase Conveying

Dilute phase conveying occurs when particles are conveyed in a gas stream velocity higher than the saltation (horizontal orientation) and choking (vertical orientation) velocities of the solids. The particles are fully suspended in the gas stream during transport. Though many types of bulk solids can be conveyed in dilute phase, considerations to be made are effects of pipeline wear, particle attrition and intense power consumption.

Should wear, friability, and energy consumption be a concern, dense-phase flow conveying could be considered instead. Dilute-phase systems often operate with low solids loading ratios (less than 15 kg solids / 1 kg gas), lower system pressures (<1 bar g), and higher gas velocities (15-25 m/s).

Dense Phase Conveying

Dense phase conveying has a higher solid loading (20-150 kg / 1 kg gas) than dilute phase and occurs when the particles are below the saltation velocity. Dense phase can be operated in two flow modes, plug/piston flow, or moving bed flow.

Material characteristics such as particle size and permeability dictate the operating flow mode. Where a coarse and permeable material can be reliably conveyed in plug/piston flow, if the material is fine and air retentive, moving bed flow is recommended.

Plug/piston flow is when the material is conveyed as full-bore slugs separated by air gaps, with moving bed flow, the material is conveyed in dunes on the bottom of the pipeline. Example materials suitable for plug/piston flow are coffee beans and plastic pellets, while for moving bed flow, cement and baking flour are better suited. Dense phase conveying often operates at higher pressures (>2 bar g) and lower gas velocities (3-10 m/s).

Systems can be classified as pressure systems, vacuum (negative pressure) systems, or a combination of the two. Pressure systems use gas above atmospheric conditions and can convey material for long distances to multiple discharge points.

Vacuum conveying systems typically have a more limited range (less than 100 m is common) and can readily pick up from multiple points to discharge the material into a vessel that is at less than atmospheric pressure.

Vacuum systems are preferred for handling dusty, toxic materials as any leakages in the pipelines will be inwards. Should the design require multiple intakes to a variety of discharge locations, a combination system can be used. These systems combine the benefits of the pressure and vacuum systems and are often referred to as “suck-blow” (or pull/push) systems.

Limitations of pneumatic conveying

Limitations of pneumatic conveying systems are high specific power consumption compared to other methods, particle attrition, and high wear. Compared to other means of conveying such as belt conveyors, capacity and distance can also be a limiting factor.

One would never consider using a pneumatic conveying system to transport iron ore 1000 m distance at 5000 tph as the energy required to suspend and move the heavy iron ore would be economically impractical.

Issues such as insufficient conveying capacity, plugging, product build up inside the line, and segregation (if the particle size range is wide) can be experienced when handling some materials. These issues are associated with poor design and can be seen early on during commissioning, whereas wear occurs over a longer duration of time and may not be as intuitively obvious to a new designer.

Considerations for Pneumatic Conveying System Design

While pneumatic conveying may not be suited to all materials due to some of the limitations described, a properly designed system can be of benefit as it may fit into a small footprint while navigating through your processing plant to achieve your material transfer needs.

Unfortunately, it’s not uncommon to hear about problems associated with pneumatic conveying equipment. The problems are generally a result of the lack of understanding, and/or trial-and-error based approaches taken to rectify problems. Often, the issue is not addressed immediately, resulting in lost time (and reduced revenue), as well as risks to safety.

With both design and troubleshooting it's important to take a systematic approach. It’s paramount to consider the system wholistically when designing your pneumatic conveying system. This can be done by following the approach outlined below.

1. Determine the material characteristics. This is important as particle size, distribution, shape, cohesion, and hardness are among the most significant variables to consider in the design (3). Larger and heavier particles require higher gas velocities, which can affect abrasive wear on the pipes, particularly towards the end of the pneumatic lines and at the bends. If particle degradation can be an issue (as it is with friable materials), a low velocity system may be appropriate. Particle attrition may not only affect the material quality, but also the design of downstream equipment. Finer particles may result in hopper flow problems like arching, ratholing, and rate restrictions, which can restrict flow into the conveying line. Understanding the material characteristics will allow you to make informed design decisions.
   
2. Classify your system. Will it be a batch or a continuous operation? This will inform equipment selection, and the type of feeder and gas mover. Will there be multiple pick up or discharge points? This aids the decision of whether a pressure, vacuum system or combination system is best suited to your material. What is the desired conveying rate, equipment layout, and pipeline layout? The total length of the horizontal and vertical runs should be defined, and the number of elbows and bends should be determined. This information will be used in determining the pressure drop in the line. The type of bend may change the layout of the system if space is limited.
   
3. Calculate the required gas flowrate. Once the system and line layouts are defined, one can calculate the gas mass flowrate using the solids loading ratio and desired material mass flowrate. While the calculations are not particularly cumbersome, the assumptions and quality of information used are paramount. Therefore, it’s important to engage with a vendor than can test your material and provide pneumatic conveying test results. Keep in mind that operating factors such as temperature and pressure can have a significant effect on gas compressibility and resulting conveying velocity.
   
4. Determine the required pipeline diameter using the desired minimum conveying velocity, and the total pressure drop in the system. The important factor to consider is the gas velocity as too low of a value will cause the material to plug while too high a value can cause particle attrition, erosive wear, and a higher pressure drop. Typically, in dilute phase applications, the lower boundary for gas velocity is ~15 m/s, but if the particle sizes are large and dense, the gas velocity should be above 25 m/s (3). The system pressure drop can be determined by considering the pipeline friction factors and using Darcy’s equation. The minimum gas velocity and solids friction factors should be estimated from pilot scale or experimental testing.
   
5. Select an appropriate gas mover. This is often done by vendors or suppliers using performance curves but typically, for pressures less than 1/3 bar g, a radial blade type centrifugal fan is often selected. For pressures up to 1 bar g, a roots type positive displacement (PD) blower, and for pressures exceeding 2 barg , a compressor is often utilised. PD blowers are most common due to the economic benefits, and almost constant air flow rates over a wide range of pressures.
   
6. Select an appropriate feeder for the system. The feeder is an important component of reliably conveying material (5). The role of the feeder is not only to provide uninterrupted flow into the pneumatic conveying system, but to also provide the desired degree of metering control. The selection of feeder should consider the material characteristics such as particle size, distribution, cohesive strength, bulk density, and permeability (4).
   
   a. Vacuum systems often include screw feeders, belt feeders and rotary valves. Typically, the feeder discharge is to near-atmospheric conditions, therefore, the pressure differential between the hopper/bin and the pipeline is minimal.
   
   b. Typically for low pressure systems (less than 1 bar g), rotary valves, eductors and solids pumps are common equipment types.
   
   c. High pressure systems incorporate blow tanks, lock hoppers, and high-pressure sealing rotary valves. It’s important the selected feeder can seal against the high-pressure pipeline and provide sufficient venting in fine powder applications.
   
   Note that for pressure applications, the permeability of the material can dictate the flowrate that can be achieved from the outlet. If the material is impermeable, the flowrate can be restricted regardless of how fast the feeder is running.
   
7. Design an appropriate gas/solids separator. The solids/gas separation can be achieved via several methods including inertial or fabric filter separation (i.e., baghouses). In many processes, due to regulatory and safety concerns, the separation process includes a combination of both; the gas cannot be entirely separated from the fine product without a high efficiency fabric filter. These baghouses often have a reverse pulse jet system which separates the particles from the fabric, from which the material is collected from the bottom and may be re-introduced into the process. Vendors often have selection guidelines for baghouses and types of filters for the specific purpose and material.

Pneumatic Conveying System Design Issues and Troubleshooting

As with the design process, it’s important to gather as much information regarding the system as possible. Information regarding pressure, temperature, material characteristics, feeder speeds, gas flow rates (under startup and steady-state conditions), as well as equipment information such as line and equipment layout and dimensions should all be collected. Below are some of the common issues observed with pneumatic conveying systems.

The most common issues observed are the inability to transfer bulk materials at the desired throughput rate. This can be associated with a myriad of issues with the most common described below.

Hopper flow obstructions:

If the equipment upstream is not designed to handle the material at the operating conditions, there is little to be done short of a re-design of the surge hopper. It’s important to know the material characteristics to review the design and understand if this is the bottleneck in the system.

Often, equipment is designed without the appropriate material characteristics, resulting in flow issues such as cohesive arching and ratholing leading to erratic solids discharge. When the material is discharged in a “funnel flow” pattern the material may only flow within a narrow flow channel and have large stagnant zones. Once the flow channel empties, it forms stable ratholes. These stable ratholes may not always be activated with external flow aids and will significantly reduce the live capacity in the surge hopper.

One way to ensure reliable flow from the surge hopper is to operate in “mass flow”. This flow pattern occurs when the hopper is designed to prevent the formation of cohesive arches and the walls are sufficiently steep and low enough in boundary friction to allow flow along them thus eliminating stagnant material.

Feeder restrictions:

It is important to consider that the feeder can only discharge material from the hopper as well as the material will flow from the hopper! Feeders designed improperly will limit the metering and flow capability, restricting the mass flow rate through the system.

Undersized motors can restrict the rotational speed and torque available from the rotary valve or screw feeder. For example, a rotary valve may be operating at higher than optimal speed, thus not allowing the pockets to fill in their entirety. This may result in reduced output and cause inaccurate metering. It’s imperative the feeder is designed to the appropriate flow and particle characteristics of the material.

Too much air:

Increasing the air flow rate through the line may yield reduced capacity to convey material through the line, particularly with dilute-phase conveying. In a pressure limited system, increasing the pressure in the line will take away the available energy required to convey the material.

Air leakage:

If the air leakage in the pressure system is substantial, air flow rate may drop to a point where dilute phase conveying is compromised. This can cause flow obstructions such as plugging in the lines.

Underrated gas mover:

The gas mover is a major component in achieving the flow rates and pressures for dilute-phase and dense-phase pneumatic conveying. Capacity reduction can occur if the system pressure and air flow requirements are not understood, and the gas mover is not sized appropriately to the system. Careful calculations coupled with material testing and experiments should be performed to understand the total system pressure drop (system resistance) to appropriately size the gas mover.

System modifications and deviations from design:

Process requirements may change due to expanding capacity, or there may be knock-on effects due to changes elsewhere in the process such as change of equipment which may require re-routing of the conveying line.

Modifications such as increase in line lengths, decrease in line diameter, and additional bends in the line may increase the total pressure drop in the system while simultaneously reducing the ability to convey the solids. It’s critical to consider the effects to the system and modify other system components to accommodate the updated requirements.

Conclusion for Pneumatic Conveying System Design

Whether the system is being designed from new or being modified to accommodate changing process requirements, it’s imperative to consider the system wholistically and ensure that the design of all the system components is appropriate. Taking a well-informed approach is paramount to taking a trial-and-error path to resolve costly issues.

References

1. Stoess H, “Pneumatic Conveying” 1970
2. Klinzing, Rizk, F., Marcus, R., & Leung, L. (2012). Pneumatic Conveying of Solids: A theoretical and practical approach (3rd ed., Vol. 8). Springer Netherlands.
3. Maynard, E.P., “Designing Pneumatic Conveying Systems.” Chem. Eng. Progress, 102 (5), pp. 23-33 (May 2006).
4. Carson J. W., “Step-by-Step Process in Selecting a Feeder.” Powder & Solids Annual, special edition supplement to Chem. Proc., 63 (5), pp. 38-41 (2000)
5. Maynard, E.P., Khambekar, J. “Reliable Feeding of Pneumatic Conveying Lines.”, Powder and Bulk Engineering, pp. 45-51 (July 2011)