Friday 26th Apr, 2019

What do I really need to know about pneumatic conveying?

Aha... definitely got it this time. Just hit it here and we will remove the blockage once and for all.

In this regular column, experts from specialist bulk materials engineering firm Jenike & Johanson answer readers’ queries around problems at their sites. In this edition, the firm’s vice president, senior consultant and director of education, Eric Maynard looks at pneumatic conveying.

In our global consulting role, we see the good, bad and ugly of pneumatic conveying (PC) systems and in many instances, they are both bad and ugly1. The good news is that the underpinning science is well established and if applied at the design stage can ensure it is one of your best engineering decisions. The secret to success is homework, as every PC system is bespoke and requires a degree of process customisation and support.

Specifying a PC system is usually a one-shot play due to the cost and degree of supporting value chain customisation required to accommodate it. Most of the problems we see can be traced back to design stage failings, however these are insider insights that are not often publicised creating a jaundiced view of the risks, sometimes referred to as supervisor bias.

A bias recognised is a bias neutralised so awareness is critical and in this article we would like to take you behind the glossy brochures and share some all too common examples of what can often go wrong with PC systems and how to avoid them.

Controlled flow is at the heart of many manufacturing value chains, however, the state of the matter involved makes a big difference. Moving process fluids for instance is a familiar and relatively straightforward engineering exercise characterised by:

• a closed (fully contained) system

• minimal moving parts

• layout flexibility – degrees of freedom

• predictable design outcomes/operating behaviour

• continuous flow which can be easily metered, controlled and automated

• robustness and low maintenance.

In direct contrast, the controlled movement of bulk solids, especially powders, is complex.

However it is possible to circumvent this complexity and therefore realise many of the benefits associated with fluid flow (above) through PC.

Piggybacking on an engineered gas moving system, PC systems use the moving gas to transport particulates through a pipeline either in full suspension (dilute phase) or in the form of moving dunes or plugs (dense phase). In the context of modern plant design, the lure of the benefits associated with this hybrid (two-phase) approach has seen PC become a popular choice in recent years.

In the right application, specifying a PC based system can indeed be the answer to your bulk solids transportation dreams and one of the best and most productive engineering decisions of your career.

Conversely, it can also turn out to be your worst nightmare, an ever-present and graphic reminder of folly.

Pneumatic counter-intuition: the harder you hit, the worse it gets, the more you hit. Science is the only way out of this cycle.

When it comes to PC and the final outcomes, there is usually nothing in between these extremes. In this sobering context, it is important to understand what makes the difference between success and failure.

For the uninitiated, PC is simply the transportation of dry bulk solids using a moving gas. There are variations on the theme and although the science exists, it is seldom applied at the design stage. The primary reasons for this include:

• apparent simplicity (save the particles, there is only one moving part-the gas mover)

• the false economies of design cost and schedule savings

• loose performance accountabilities.

The majority of PC installations are based on rules of thumb and empirical experience with “like systems”. While the concept of PC is simple in essence and the complexity of bulk solids flow behaviour has been largely circumvented, every application is unique in some way and the physics underpinning two-phase flow needs to be recognised and respected. It’s the underestimation of this complexity this leads to the many problems observed.

In general, these design phase failings fall into three categories.

Incorrect strategic approach:

Within the definition of PC there are a wide range of variations, each with application specific pros and cons – is your choice informed by your unique situation and the science?

Tip: If your preferred advisor/supplier has not come to you with a table of PC options rated against the specifics of your application and a justified recommendation go-forward approach then you may be missing out on a better option.

Failure to understand of the bulk material from a PC perspective:

Physical properties (and their normal variation) of the actual bulk material involved – explosiveness, attrition, hygroscopic, abrasiveness, permeability need to be measured and shown together with their variation, in the basis of design.

Fluidisation behaviour – how does your bulk solid behave when it is aerated compared to when it has been at rest for extended periods? If the PC crashes and the material settles out, can it be restarted?

Tip: if you supplier does not ask for a sample of your solid for characterisation testing, you could be heading for a world of pain.

Failure to take a systems approach to the design:

Not matching hardware to your bulk materials-Relying on past designs or variations based on dubious empirical correlations and “library values”.

Glossing over the hydrodynamics of the system-the gas and solid pathways are usually quite different yet dependent on each other for success. Understanding this dynamic at design stage is important from the pressure drop, wear, materials degradation and overall operability and reliability perspective.

Failure to ensure all the elements, such as blowers, feeders, pipeline, elbows, locks, separators, filters, are matched to each other with clear accountability.

Tip: Matching is usually an iterative process best done in the virtual world during the design stage rather than trial and error modifications in the physical world once it is built.

Establishing a formal study period ahead of the commitment to the detailed design/specification phase is the great way to create the schedule space and send the message to everyone involved that there is always homework when it comes to PC systems and that it is a pre-requisite to progressing.

In most instances it is a deeply regretted folly to skip on any of these points. Common yet high-impact failings that can be traced back to the design phase include:

Insufficient conveying capacity (reduced solids throughput)

Process impact(s): Process bottleneck (unlike other conveying systems the capacity limit of a PC system is usually finite and difficult to significantly upgrade without duplication)

Primary and secondary root cause(s):

• Bulk solids related – unexpected bulk (deaerated) flow behaviour… leading to starvation via hopper obstructions, feeder issues, interface devices-rotary valves.

• Conveying air related – air mover type selection and specification… leading to insufficient volume capacity for given pressure profile, lack of containment/leaks, using too much air – leading to wear issues or reduced conveying capacity.

• System related – mode (lean vs dense phase, pressure-vacuum), line design (layout, diameter), bends (number, geometry, orientation) – leading to sub-optimum gas-solid interactions.

Note: There is a nasty anti to this capacity risk and that is the tendency to overdesign in an attempt to compensate for the uncertainty that comes from a lack of science.

While this approach can sometimes address the throughput issue, it always comes with a significant capital cost and life of asset operating cost penalty as well as unexpected operating consequences as the turndown ratio of PC systems is relatively small. This type of defeatist strategy can work for some engineering applications, but PC is not one of them.

Line plugging – physical obstruction (full or partial) of the flow path

Process impact(s): Reduced throughput, line shutdown (value chain productivity, loss of revenue), increased maintenance costs.

Primary and secondary root cause(s):

• Bulk solid related – cohesiveness (increased cohesiveness due to temperature, condensation or due to high pressure impacts) – leading to accretions.

• Conveying air related – poor (heterogeneous) velocity profile/unknown pressure drop profile – leading to saltation, leaks (either loss of vacuum in a negative pressure system or loss of air volume in a positive pressure system).

• System related – line design (layout, diameter), bends (number, geometry, orientation), abrupt direction changes, contact surface finish, start-up/re-start philosophy – leading to high velocity impacts, heterogenous velocity profiles, “gravity” and two-phase operating regime issues (mode switching).

Wear and particle attrition – physical degradation due to mechanical action

Process impact(s): Reduced system availability and increased sustaining capital costs, product contamination, off-specification product (size and shape changes, foreign matter contamination, line shutdown (value chain productivity, loss of revenue), increased maintenance costs, environmental exposure (toxicity, explosibility, reactivity).

Primary and secondary root cause(s):

• Bulk solid related – material abrasiveness not taken into account, no understanding of the velocity-pressure-material abrasiveness relationship – leading to uninformed design decisions.

• Conveying air related – poor (heterogeneous) velocity profiles – leading to regions of high velocity and/or high impact pressure, wear and attrition affected by power law of gas velocity.

• System related – inappropriate materials of construction, line design (layout, diameter(s), transitions), bends (number, geometry, orientation, downstream effects) – leading to high velocity impacts, heterogenous velocity profiles.

To avoid these three headline problems as well as the many others that can be encountered when it comes to PC based systems, the most important thing is to do the homework and resist the various biases that work against proactivity and eliminating problems before they get baked into the design. There is no place for the ‘trial and error engineering’ approach when it comes to PC systems.

General recommendations

Always measure your bulk solid properties even if they are considered a commodity. Testing should always be informed by a considered basis of design that describes the feed now and in the future and acknowledges that the process itself can impart important changes e.g. segregation, elutriation, degradation as well as moving air properties (temperature, moisture content). For example, fly ash, a coal combustion residue, has been effectively conveyed for decades; however, with implementation of air pollution control (APC) systems the fly ash properties have dramatically worsened, and PC systems that previously exceeded expectations now no longer work effectively.

Take a system view at the design stage and appreciate that the interaction between the bulk solids and the system creates a unique dynamic where there are many interdependent options, hence an iterative process is required for optimum results.

PC systems offer many benefits highly prized in the current process environment however the performance outcomes tend to be polarised at the extremes. To ensure your system is successful you need to make sure there is a formal study phase ahead of the detail design. While there are complexities and traps, the underpinning science is well established; it just needs to be applied.

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 

About the author

Since joining the firm in 1996, Eric has published more than 40 articles on the storage, flow, and pneumatic transport of bulk solids. In addition to being responsible for internal training of new engineers at Jenike & Johanson, Eric is the principal instructor for the “Flow of solids in bins, hoppers, chutes, and feeders” and “Pneumatic conveying of bulk solids” courses sponsored four times per year by the American Institute of Chemical Engineers (AIChE) Continuing Education Program. Eric has designed more than 750 bulk material handling and conveying systems for various powders and bulk solids, with a special focus on the cement, power, and mining industries.

REFERENCES

1, Maynard, Eric P, 2011. Solving common pneumatic conveying problems. Australian Bulk Handling Review , May/June, pp. 2-7.