Problem-Free Air Conveying Part III

Consider the possible effects of ambient conditions on operations and the potential impact of the system on product characteristics

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Part I of this series discussed general problems such as pipeline blockage and how they might occur in most types of conventional pneumatic conveying systems. Part II focused on troubleshooting specific kinds of systems and their key components. This final part delves into other potential difficulties and how to get the necessary data to properly address them.

Pneumatic conveying systems can suffer from a variety of systems-related problems besides the throughput ones that we have discussed previously. Environmental factors, such as temperature variation, can cause operating troubles, as can erosion and other conditions that affect the physical state of the system. In addition, any number of product-related problems may arise. This final part of the series looks at such issues, as well as the parameters and measurements essential for successful troubleshooting.

Many of these problems are caused directly by the product being conveyed. They are considered here, however, because such problems may not initially be recognized in terms of the product itself.

The plant environment
Several factors related to ambient conditions can produce problems.

Altitude. The operation of a pneumatic conveying system at altitude should present no problems at all, provided that due account has been taken of the local air pressure and, hence, density of the air. This will influence the specification of the air mover [1], because the volumetric flow rate is generally quoted in terms of free air. It also will affect the size of the filter required, as discussed previously [2].

Temperature variations. Plants subject to extremes of temperature, from summer to winter, and even from day to night, may face problems due to changes in conveying air velocity as well as condensation. Air density increases as temperature decreases. For instance, a conveying air velocity of 15 m/s at 40 Degrees C becomes about 12 m/s at -20 Degrees C for the same free-air flow rate. Condensation may occur in pipelines subject to large temperature variations, particularly when there are pipe runs outside of buildings and air drying is not employed. More details are provided in the first installment.

Electrostatics. Pneumatic conveying systems are known to be prolific generators of static electricity. In a large number of cases, the amount of charge generated is too small to have any noticeable effect. Sometimes, however, appreciable generation can occur. Very often, this is just a nuisance but, occasionally, it can present a hazard. Grounding the pipeline and ensuring that electrical continuity is maintained across all flanged joints can reduce the problem. In addition, the humidity of the conveying air can be adjusted to control static build-up. The use of humidity for charge control is not suitable, of course, if the product being conveyed is hygroscopic.

Erosion effects
If the hardness of the particles being conveyed exceeds that of system components like feeders and pipeline bends, erosive wear will occur at all surfaces against which the particles impact.

The conveying air velocity is a major factor in erosion. Lowering the velocity at which the product is conveyed will help to reduce the problem. Because the conveying air velocity increases along the length of a pipeline, the bends at the end of the pipeline are likely to fail first. Enlarging the bore over the last part of the pipeline could reduce erosion here.

Various solutions are possible for bend erosion. One method is to reinforce the bend with a channel backing. This will solve the problem with respect to the outer bend wall surface. However, the deflection of the product out of the wear pocket formed could result in failure of the inside surface of the bend, or of the straight length of pipeline following the bend. So, you must exercise care in applying this technique.

The use of a very hard surface material such as Ni-hard cast iron, basalt or a ceramic will help to prolong the bend life. These materials are generally brittle, however; so, short radius bends should be avoided. Blank tees can provide a cheap and effective solution to the problem, but may cause an increase in pressure drop.

Erosion of straight lengths of pipeline rarely is a problem. Should such erosion occur, however, possible causes are misaligned flanges and welded joints, and proximity to valves and bends, as mentioned above.

Even products with hardness value less than that of mild steel can pose problems. Indeed, relatively soft products such as coal, barites and wood chips can cause severe erosive wear, as a result of naturally occurring contaminants such as silica.

Impact angle effects. Figure 1 illustrates the influence of the impact angle of particles against surfaces and the response of different surface materials to erosive wear. It shows that ductile materials such as mild steel and aluminum suffer maximum wear at an impact angle of about 25 Degrees but offer a reasonable degree of resistance at normal impact. In contrast, brittle materials such as glass, basalt, concrete and cast iron suffer maximum wear under normal impact yet offer a reasonable degree of resistance to low angle impact.

Variation of Erosion with Impact Angle

Figure 1. Ductile materials such as aluminum suffer most severe wear at impact angles of about 25 Degrees , but reasonably withstand normal impact; brittle materials like glass offer the opposite performance.

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