A complicating aspect of pneumatic conveying is that you cannot see the flow during conveying. The only indication of a problem is generally when the pipeline blocks and given that the blockage cannot be seen there are almost no clues as to the cause. Of course, if we could see into the pipes we would become acutely aware that several very different modes of flow can exist, depending on the combination of air flow rate, solids flow rate, and pressure drop.
I guess we have all heard of the terms dilute (or lean) phase flow and dense phase flow, but there is great confusion regarding what the terms actually mean and how these modes of flow influence the risk of the system not behaving reliably. First of all let’s dispel a few myths:
Myth #1: “High-pressure systems must be dense phase systems”
Incorrect; the pressure drop in the system has little direct influence on the mode of flow. To convey material at a given concentration a certain pressure gradient (pressure drop per unit length) is required. Clearly, if we need to convey over a long distance the overall pressure drop will need to be higher than for shorter distances in order to maintain a particular pressure gradient. Hence, a high-pressure drop may need to be used purely due to the distance of conveying rather than to maintain a high solids concentration in the pipeline.
Myth #2: “Use of a blow tank (pressure vessel) means the system must be operating in dense phase”
This assumption is of course linked to Myth #1. If we are to use a high-pressure system we are most likely to use a blow tank as a feeder to deal with the pressure. However, blow tanks are equally good as feeding devices for dilute phase as they are for dense phase. In some instances, blow tanks are simply used in order to convey in dilute phase over a long distance. Equally, a blow tank may be used in a dilute phase application because the material is highly abrasive and the blow tank has few moving parts to wear out.
Myth #3: “Dense phase systems never block”
Well yes and no. This confusion arises from the fact that there are two very distinct modes of dense phase flow that depend largely on the physical properties of the bulk solid being conveyed. The fine fluidized powders can most definitely block when they become de-aerated. However, coarse granular solids with a high degree of permeability will tend to stop moving below a certain gas velocity. However, these materials will often resume conveying once the gas velocity is increased. Hence, they don’t really block – but they will stop.
What is Dense Phase Conveying?
Well to get to the bottom of this issue we need to understand the terms dilute and dense phase flow. When most people think of a pneumatic conveying system, they envisage dilute phase flow. In other words, they think of particles being either blown or sucked along the pipe. Dilute phase flow occurs when the majority of the solids are conveying in suspension with the conveying gas, usually air. In a practical sense, for all of the material to be in suspension, there needs to be a lot of air and relatively few solids. This is often not very practical (or economical) and hence there will usually be some solids conveyed as a ‘sliding strand’ along the bottom of the pipe even in dilute phase. To maintain a dilute phase regime, the conveying velocity must be kept relatively high.
If the velocity is reduced below the saltation velocity – the velocity at which particles start dropping out of suspension onto the bottom of the pipe – then one of two things will happen. For materials that have no dense phase capability a pipeline blockage will occur and for materials that have dense phase capability they will continue to be conveyed but in one of two major modes of dense phase. As a broad generalization, fine powders that exhibit good air retention qualities will continue to be conveyed as fluidized dunes. Coarse granular, almost mono-sized, particles with good air permeability qualities may continue to be conveyed as full bore slugs or plugs of material. However, for this mode of flow the transition from dilute to dense phase is likely to be highly unstable and may lead to significant forces being transmitted to the pipeline restraints and structure.
Velocity – the Key Parameter
While a number of parameters will influence the performance of a pneumatic conveying system such as air flow rate, solids flow rate, pressure drop, equivalent length of the pipeline, solids concentration, and others, it is the gas velocity in the pipeline that is the key parameter for dense phase conveying. Velocity is probably the most important parameter in pneumatic conveying and, in particular, the velocity at the feed point. Insufficient velocity at this point will lead to a poorly performing system or a system that will not convey at all. The issue is what velocity is required.
Unfortunately, although this is relatively easy to answer as a broad generalization, the specific value required for a specific application is difficult to predict and there is little option but to test the material to determine this value. The minimum conveying velocity is highly dependent on the bulk solid being conveyed. In dilute phase applications, the minimum velocity required at the feed point is generally related to the saltation velocity of the bulk material, which will depend on parameters such as the size range and distribution of the bulk solid and the particle density. However, in most cases for dilute phase applications, the pick-up velocity (superficial gas velocity at the feed point) will need to be a minimum of 3000 ft/min and often a factor of safely is added leading to values of between 3200-3500 ft/min. It is also really important that this value is not set too high as this will lead to excessive velocities throughout the pipeline leading to excessive pressure drop, wear and potential particle damage. Hence, the velocity at the feed point is an absolutely critical parameter.
For dense phase systems, the minimum transport velocity is not the saltation velocity. For fluidized materials, the minimum velocity at the feed point will depend on a wide range of parameters which makes it difficult to determine. Ultimately, it is the velocity below which blockage of the pipeline occurs. Blockage will occur when the fluidized bulk material de-aerates sufficiently for a blockage to occur usually at a bend. For granular materials that convey in plugs, the minimum transport velocity will occur when there is insufficient air flow to move the plug. In general, the absolute minimum velocity for fluidized dense phase is usually around 600-800 ft/min, and a minimum of 1000 ft/min is commonly used in practice. For slug flow, the minimum depends directly on the permeability of the plug and the frictional characteristics between the plug and the pipeline wall. However, minimum velocities can be as low as 200 ft/min.
When to Use Dense Phase?
Some bulk solids are natural dense phase candidates. They are usually fine powders that have excellent air retention characteristics or coarse granular solids with a high degree of permeability. In the case of fine powders, they remain fluid like in nature for a long time even if the material is not being actively fluidized. Examples of these types of materials are pulverized fuel ash, cement, pulverized coal and many of the drilling mud powders such as barites. These materials are characterized by having relatively narrow size distributions and excellent fluidizing qualities. With these types of material there is relatively little risk in conveying them in a fluidized dense phase flow regime. They have natural dense phase capability by their very nature. The advantages are, of course, the ability to use smaller pipelines, lower energy requirements, much reduced bend wear and, where important, much reduced particle damage. The benefits can be significant, however, if high-pressure drops are used (say much over 20 psi) then it is essential that a stepped pipeline is used to control the overall velocity profile in the system.
In the case of coarse granular materials, these may exhibit natural dense phase capability in the plug flow regime. However, the degree of permeability is critically important and even small changes in the permeability can have significant effects on system performance. Materials such as plastic pellets often exhibit this behavior. The advantages of plug flow are in minimizing the damage to particles due to the low velocities used. However, with this type of flow, there is often very little saving in energy requirement.
When Not to Use Dense Phase (Or When to Be Extremely Careful)
There are several sets of circumstances when it is important to be very careful in selecting dense phase as the mode of conveying. If the bulk material does not have natural dense phase capability or exhibits only marginal capability, the risk of blockage or unreliability rises significantly. The risks also rise significantly if there is expected to be a significant variability in the specification of the bulk solid to be conveyed. Variation in size distribution and the possibility of size segregation within the material also pose a significant risk. In other words, in any case where there is likely to be a significant variability in the bulk solids there is a significant risk for dense phase systems unless worst case conditions are tested and found to be satisfactory.
Another consideration for dense phase conveying is the overall conveying distance. This limitation is essentially imposed by gas compressibility and the practical limit in terms of pressure drop in pneumatic conveying systems. The longer the conveying system is the lower the mass flow rate of material that can be achieved for a given pressure drop. Hence, increasing the distance reduces the solids loading ratio. At very low solids loading ratios, higher velocities are required that essentially forces the system into the dilute phase regime. Hence, in general, it becomes very much more difficult to design dense phase systems for conveying distances greater than about 3000 ft.
Testing the ‘Conveyability’ of a Bulk Solid is Essential
From the foregoing comments it is clear that the performance of a dense phase system depends absolutely on the physical characteristics of the bulk solid to be conveyed. Hence, for any proposed dense phase system it is essential to undertake conveying trials. Clearly, there are benefits in undertaking these trials with an independent agency but at a minimum, trials should be undertaken by the system supplier to ensure that the proposed system will work effectively. There is a catalogue of unfortunate cases where the warning signs of conveying problems have been seen at the trial stage and ignored only to become a major problem at commissioning. Don’t let this happen!
Superficially, the advantage of trials independent of the supplier is obvious. However, a fundamental difference between independent trials and supplier trials is in the approach. In general, independent laboratories undertake the trials in order to establish the capability of the bulk material independent of the hardware used. These trials are normally much more extensive and cover the whole envelope of conveying capability. The downside of course is the cost. Independent trials can be relatively costly especially compared with suppliers that will provide limited trials sometimes at no cost at all.
There is little doubt that there is less risk in specifying a dilute phase system. Dilute phase has some built in flexibility, is a little more forgiving, and is easier to fix when problems arise. However, there are many situations where the high velocity causes problems of bend wear, reduction in product quality through attrition, or excessive power requirements. Dense phase offers the use of significantly lower velocity that will address these issues provided the bulk solid has the capability to be conveyed in such a manner. However, it is essential to test the bulk material to assess its suitability to be conveyed in one of the dense phase modes of flow. Marginal capability obviously significantly increases the risk and may indicate that another method of conveying may be more suitable if dilute phase is not an option.
Professor Mark Jones holds the chair in bulk solids handling at the University of Newcastle, Australia. He is also the director of the Centre for Bulk Solids and Particulate Technologies and director of TUNRA Bulk Solids Handling Research Associates.
For related articles, equipment reviews, and news, visit our Pneumatic Conveying Equipment Zone