Pneumatic transport has become more popular due to its flexible line layout, product containment, and ease of automation. However, a lack of understanding about how solids can be transported leads to problems. Operations Manager at Jenike & Johanson explains more.
While pneumatic conveying equipment has advanced over the years, problems resulting from insufficient conveying capacity, line plugging, erosive wear, particle attrition, and build-up in the line are not uncommon. Whether operating in dilute or dense phase, a pneumatic conveying system consists of four basic components as shown in Figure 1.
The role of the gas mover is to provide the proper flow rate of gas required to transport material. Feeders introduce solids into the conveying system a controlled rate where they are mixed with the conveying gas to move them from one location to another. Positive-pressure systems often require devices which are capable of feeding material from atmospheric conditions into pressurised ones while negative-pressure systems may require feeders with good sealing capability to minimise gas leakage. In the separator solids are decelerated and then recovered in order to be fed to a storage bin and hopper or fed into a downstream processing unit.
A positive pressure conveying system utilises gas above atmospheric pressure to entrain the bulk solids and transport the material to one or multiple destinations (often at atmospheric pressure). Positive-pressure systems can operate at high pressures and convey materials over long distances.
A vacuum conveying system picks up solids at atmospheric pressure (often from multiple locations) and discharges the material into a vessel that is at a pressure less than atmospheric. Vacuum systems are typically limited to less than 60 metres, although some systems have a longer range. Vacuum systems are well-suited for handling dusty or toxic materials, because any leakage in the pipeline will be inward. Specialised systems that incorporate features of both positive and negative pressure conditions, such as pull/push systems used in ship unloading equipment, are also available.
The unique challenge for designers of such systems is in matching the component parts of the system with the vast array of available equipment on the market in order to ensure efficient and reliable operation, as per its basis of design. Reliable flow from the feed bin through the feeder and into the pipeline and the downstream separator is an absolute necessity.
Possible bottlenecks associated with pneumatic conveying systems can include conveying capacity, plugging, product build up in pipelines, pipeline wear, and particle attrition. Due to a general lack of understanding of the causes of these problems, many facilities take trial-and-error approaches which often fail and usually cost more than a scientifically developed solution.
The first step in troubleshooting conveying problems is to gather as much information as possible. Data regarding pressure, temperature, feeder speed, gas flowrate, etc. should be collected under start-up and steady-state conveying conditions. Information about the conveying line (pipeline length, number of bends, diverters, feeders) should also be collected.
Hopper flow obstructions: Conveying rate can be limited by solids flow problems occurring in the feed bin and hopper. Flow problems such as bridging and ratholing will lead to erratic solids discharge into the conveying line thus reducing the transfer rate.
Feeder restrictions: Improperly designed feeders are often a source of conveying line restrictions. Undersized motors can restrict the rotational speed of screw feeders or rotary valves. Some feeders operate at critical rates beyond which any increase in speed does not equate to increased solids flowrate. As example, screw feeders or rotary valves may operate at speeds too high to allow complete filling of the screw flights or rotary valve pockets.
Too much air: Too much conveying air can occur during dilute-phase transfer in a conveying line resulting in capacity restrictions. Increasing the gas flowrate will not necessarily result in an increase in the capacity of a dilute-phase conveying line. However, in this example, the total pressure in the line will increase and if the system is pressure-limited, the additional pressure needed to move the extra air through the line will take away the energy available to convey the solids.
Air leakage: If air leakage from a positive-pressure conveying system is significant, the airflow in the line may drop to a point where dilute-phase conveying will be compromised. In some severe cases, the solids may even plug the conveying line as a result.
Underrated prime mover (fan, blower, compressor): The prime mover, or air source, is a major component of a pneumatic conveying system. The gas flowrate needed to efficiently transfer the material in dilute or dense phase must be understood. If improperly specified, it can be a cause of capacity reduction. Careful calculations or experiments Calculations/experiments must be performed with the bulk solid and conveying line to be conveyed to ensure that total pressure drop through the line is correctly estimated.
Line length and bends: Many processes undergo modifications which might need to expand to support capacity increases. Often existing pneumatic lines are then modified to accommodate the transport to new equipment. Unfortunately, this is frequently a cause of capacity reduction. Increased line length and bends results in increase of the total pressure drop in the system while simultaneously reducing the available pressure to convey the solids.
The pneumatic conveying line may only be a part of the problem however as often hoppers and feeders play as important a role in achieving reliable pneumatic transport. Feeding solids into a positive pressure system requires a means of sealing against the pressure in the pipeline. For low pressure conveying systems (less than 100 kilopascals), rotary valves, solids pump, and eductor are common choices. For high pressure systems (more than 100 kilopascals), a blow tank or Fuller-Kinyon pump, and high pressure sealing rotary valve can be used. Some of these devices control the rate of solids discharge into the line and as such are truly feeders while others only provide a pressure seal and do not meter solids.
Rotary valves which operate well as feeders can be used to provide a seal, a feature which makes them very useful when feeding into high pressure environments. Venting is critical however particularly when handling fine materials. Without venting, gas leakage up into the feed hopper can induce bridging problems at the hopper outlet.
A solids pump consists of three primary components the inlet/conveying section, the sealing section, and the discharge section. The inlet section accepts material from an up-stream feeder or conveyor. The sealing section is designed so that the solids being conveyed through the screw form a plug of sufficient length and density to provide the necessary gas seal without high solids pressures. By keeping the solids pressures low, screw torque and wear are minimised. The sealing mechanism does not rely on mechanical tolerances or moving parts. The discharge section can be designed to break up a cohesive plug, if needed, and deliver a uniform stream of material to pneumatic conveying line.
Eductors are sometimes used to discharge solids from a hopper outlet into a positive pressure pneumatic conveying line. The motive air is expanded across the nozzle creating a vacuum in the suction chamber and draws the bulk solid in. The entrained solids are then carried through the expansion chamber and discharged into the line. Eductors have no moving parts or internal mechanical devices.
Fuller-Kinyon pumps are commonly used when feeding high pressure pneumatic conveying lines. It consists of a screw that has a decreasing pitch in the direction of material feed. Hence, material consolidates as it advances resulting in a commensurate increase in bulk density. As a result, material forms a tight seal against the downstream gas pressure.
In high pressure systems, blow tanks or transporters are often used to introduce the solids into the conveying line. The process involves first transferring solids into blow tank, which is then sealed and pressurised. The entire contents of the blow tank are then fed into the line. The pressure in the blow tank is vented and another batch of solids is transferred in and the process is repeated.
Design steps to increase solids transfer rate
Whether installing a new conveying system or fine tuning a poorly performing system following a scientific approach will help to ensure efficient, reliable, and safe operation of a positive pressure or vacuum conveying system. The following outlines an appropriate stepped approach to take.
- Define the material characteristics of the material to be conveyed.
- Define the conveying requirements for expected and “upset” operating conditions.
- Calculate gas mass flow rate
- Calculate pipeline diameter with desired
- Calculate the system pressure drop
- Recalculate the gas velocity at the solid’s feed point
- Select a suitable gas mover
- Select an appropriate solids feeder
- Select an appropriate gas/solids separator
Conclusion
Pneumatic transport of bulk solids has become popular in recent years because of benefits like flexible line layout, product containment, and ease of automation. However, due to lack of understanding of how solids can efficiently and reliably be transported through conveying lines, problems such as insufficient conveying capacity, pipeline plugging, or wear can result.
When designing a pneumatic conveying system, always measure and understand the bulk solid properties. Test work should be informed by considering feed now and in the future while also recognising that the process itself may impart changes to the material.
Do you have a bulk solids handling question? Jenike & Johanson has developed the science of bulk solids flow and specialises in applying it to solving the most challenging bulk solids handling problems. So why not put them to the test with your question? The harder, the better
Note: The advice here is of a general nature. Specific solutions are very sensitive to their circumstances; therefore, you should consult with a specialist in the area before proceeding.