Bulk Engineering, Conveyors, Technical articles

Troubleshooting pneumatic conveying systems

Emeritus Professor Mark Jones, a senior consultant with TUNRA Bulk Solids, discusses pneumatic conveying systems common issues and how to prevent them. 

Emeritus Professor Mark Jones, a senior consultant with TUNRA Bulk Solids, discusses pneumatic conveying systems common issues and how to prevent them. 

Pneumatic conveying is a rather flexible method of material transfer and conveying that has many desirable features. It is easy to route pipelines through production spaces utilising very little space. It is also very easy to deal with changes in conveying direction which would be difficult to achieve with other conveying systems such as screw conveyors or belt conveyors. However, the deceptively simple pneumatic conveying system is often non-intuitive in its operation and complicated by the fact, that in most situations, one cannot observe the flow in the pipeline.

Yet in spite of the system simplicity, there are many instances of operating problems occurring with such systems that this short article addresses. Of course, there are a myriad of issues that can occur that inhibit the reliable operations of the system. Many of these issues are easily solved, however, there are a number of more fundamental problems that are not so easy to solve once in operation but that could have easily been overcome at the design stage. This article will address four major issues that are common operating problems in pneumatic conveying.

Pipeline blockage

Pipeline blockages are a significant problem that impact the reliability of a system and significantly reduce the time-averaged conveying rate. One of the most critical parameters of a pneumatic conveying system is the pick-up velocity at the point where material is fed into the conveying pipeline. For a given material, there will be a minimum transport velocity below which conveying will stop. In the case of dilute phase conveying, the minimum transport velocity will be the point at which material falls out of suspension and, if the material does not have dense phase capability, the material will build up in the pipeline until a blockage occurs.

In many instances, the blockage will occur at a bend. In dense phase conveying, the minimum transport velocity will correspond to one of two conditions. Firstly, the condition where there is insufficient velocity for the powdered material to remain fluidised. In this case, the material will deaerate and will generally fill the pipe until a blockage occurs. In the case of coarse granular solids that have dense phase slug flow capability, conveying will stop due to insufficient drag force on the particles to overcome wall friction.

For successful, reliable conveying, the design velocity at the point where material is fed into the pipeline needs to exceed the minimum transport velocity. 

A 20 per cent margin above the minimum transport velocity is a good rule of thumb in design as this allows for any temporary surge in feed rate which would increase the pressure demand and thereby reduce the conveying velocity.

To ensure the required velocity at the feed point, it is necessary to match the conveying pipeline internal diameter to the air mass flow rate and the static pressure at the feed point. It is important to note here that using nominal pipe sizes for these calculations can lead to serious errors. Instead, the actual internal diameter should be used to determine the pipeline cross-sectional area.

There are a variety of circumstances that will lead to the inlet air velocity dropping to below the minimum transport velocity and hence to pipeline blockage. Some of the more common circumstances are listed below:

  • Lack of adequate material feed rate control leading to pressure demand surges
  • Lack of adequate air flow rate control leading to reductions in air velocity
  • Lack of purging of the pipeline after a blockage has occurred or at the end of the conveying cycle
  • Excessive air leakage especially when using rotary valve feeders
  • A change in the material being conveyed without taking into account the change in conveying characteristics of the material
  • Increasing the conveying equivalent length (either conveying distance or adding to the number of bends) without adjusting the material feed rate to compensate

Systems not achieving rate

In many respects, the causes of systems not achieving rate follow on from symptoms of unreliable flow at least partly due to pipeline blockage or intermittent blockage. For a given system and a particular material, the operating point of the system is defined by the air mass flow rate, the material mass flow rate and the conveying line pressure drop. 

Operating point three on this graph is typical of low-pressure systems using a rotary valve and blower combination. This represents an air mass flow rate of 0.75 kg/s, a material mass flow rate of 1.25 tonne/h and a conveying line pressure drop of 0.8 bar (80 kPa). The arrow represents the shift in the operating point should a surge in material mass flow rate occur leading to an increase in pressure demand and a slight decrease in air flow rate due to the change in volumetric efficiency of the blower. In this case, should the operating point transgress the dotted line limit (which represents the velocity at the conveying pipe inlet of approximately 15 m/s), the minimum transport velocity for this material will be reached. Hence, in this case, the unreliability of the system will be caused by approaching the limit of conveyability for the material.

In the case of operating point four, a surge in material feed rate is unlikely to be sufficient to cause a blockage or unreliability due to transgressing the minimum transport velocity. In this case, the limit is likely to be imposed by the blower pressure limit which is typically a maximum pressure drop of 1.0 bar (100 kPa). However, ultimately, either scenario will lead to unreliability and the system failing to deliver the conveying rate specified. Hence, it is important that neither limit be reached. This requires adequate control over both air mass flow rate and material mass flow rate in order to control the conveying line pressure drop. 

There are options available to uprate systems to achieve a greater mass throughput. However, some of these require significant modifications and cost. Below are a series of approaches to uprating systems which are listed in order of increasing cost.

Reducing the resistance of the system

By far the cheapest approach is to reduce the equivalent length of the conveying pipeline. In some cases, it is possible to change the pipeline routing and thereby reduce the number of bends in the system. By reducing the resistance of the pipeline in this way, the reduction in air only pressure drop can be used to convey more solids. Given that, in dilute phase flow, the equivalent length of a single bend can equate to more than 10 metres of straight pipeline, it is often possible to achieve an increase in conveying rate of 5-15 per cent within the same overall pressure drop. This is clearly a cost-effective approach and should be given very serious consideration prior to considering more expensive options.

Changing blind tees and sharp elbows

Blind tees and sharp elbows have a particularly high air only pressure drop. Whilst these can be good for reducing bend erosive wear they are particularly expensive in terms of pressure loss. Hence where appropriate long radius bends will minimise the pipeline overall resistance.

Utilising stepped pipelines

In systems utilising higher pressure drops (typically 250-500 kPa), the air expansion in a single bore pipeline is significant. Hence, the use of a stepped pipeline has a dramatic effect on controlling the velocity along the pipeline and reducing the air only pressure drop. In some cases, it is possible to double the throughput by stepping the pipeline. This is a very cost-effective approach. Figure 2 shows the effect on the conveying velocity by stepping the pipeline twice along its length:

As can be seen from this example, a single bore pipeline would see the conveying air velocity expand from 16.5 m/s to 81.5 m/s whereas, with the two steps in bore, the conveying air velocity is kept between 16.5 m/s and 31.8 m/s. This leads to a very significant reduction in conveying line pressure drop for the same conveying rate which can then be used to convey more material.

Utilising larger air movers

Increasing the pressure drop available for conveying will clearly increase the throughput capability of the system roughly in proportion to the pressure drop increase. However, this will normally require a larger air mover with both an increased air pressure capability and an increase in volumetric flow rate to maintain the conveying velocity at the feed point.

Utilising larger bore pipelines

Clearly, increasing the bore of the pipeline will allow a greater solids conveying rate for the same pressure drop. However, to maintain the conveying velocity, an increase in volumetric flow rate will be required which will require a larger air mover. In addition, the significant increase in air flow rate may require a larger gas-solids separator at the end of the system. Hence, in many ways, increasing the pipeline bore leads to the need for many new components or a replacement of the system.

Operating problems

So far, the discussion has been around pipeline blockage and system performance, however, two significant operational problems are often encountered in pneumatic conveying systems: wear of pipelines and product degradation.

Wear of pipelines and bends

Pipeline wear is a significant operational problem for materials that are erosive in nature such as sand and alumina. 

The most significant factor in pipeline erosion is the incident velocity of impact of the material against the bend wall. In fact, the erosion rate is a power law relationship with velocity to the exponent of 2.8. That is, a doubling of the velocity will lead to more than 4 times the erosion rate. Hence, conversely, reducing the velocity is the single most effective method of reducing erosion. However, there is a limit on how much the velocity can be reduced as the minimum transport velocity must be maintained. Thus, when all that can be done in reducing velocity has been achieved, the focus changes to the bend geometry and the impact angle of the material within the bend.

Ductile materials such as mild steel and aluminium achieve the maximum erosion rate at an angle of impact of between 20 and 30 degrees:

This just happens to be the impact angle for standard radius bends, whereas the maximum erosion rate for brittle materials occurs at normal impact. Hence, it is important to select the type of bend and the material of construction for the particular application. Long radius bends with wear liners such as basalt or ceramic can extend the life of the bend. In severe erosion, mild steel blind tees are excellent but there is a pressure drop penalty that needs to be taken into account at the design phase. 

Material attrition / product quality issues

There is often a requirement in pneumatic conveying to preserve the particle size distribution of the material during conveying. However, the impact of material at a bend can lead to particle breakage and to changes in the size distribution. In many ways, this is the opposite side of the bend erosion issue: in this case the material is damaged by the bends rather than the material damaging the bends. The same principles apply to a large extent. Again, conveying velocity, particularly the velocity of impact, is the dominating parameter. Reducing the velocity of impact will in most cases reduce particle attrition. However, in this case an energy threshold often applies: below a certain threshold, there is insufficient force to break the particles; above this threshold, breakage occurs progressively until an upper threshold is reached. 

This example shows the results of firing aluminium oxide particles against a target through a range of velocity. It can be seen that below about 8 m/s no particles are broken and above about 25 m/s 100 per cent of the particles are broken with a progressive transition between these two velocities. This trend is seen for many materials that are fragile and suffer from attrition. Reducing the conveying velocity to as low a value that is consistent with the minimum transport velocity is the first step in reducing particle attrition. In addition, long radius bends that reduce the impact angle as much as possible are also prudent steps in reducing particle attrition.

Closing comments

This article only ‘scratches the surface’ of this very complex topic. There are myriad variables which have an impact on pneumatic conveying performance, and this article touches on some of the more common and substantive issues that one comes across in troubleshooting pneumatic conveying systems. 

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