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.

 

This explains why misaligned flanges and poorly welded joints can cause such problems. Any situation in which turbulence or deflecting flows can occur could cause low angle impact and, hence, rapid failure of a ductile material. It also points up why brittle materials should not be used for short radius bends. The impact angle of particles against a short radius bend will be very high, generally resulting in rapid failure of a brittle material.

Explosions
There is a wide range of materials, which, in a finely divided state dispersed in air, will propagate a flame through the suspension if ignited. These materials include many chemicals, plastics, foodstuffs, metal powders and fuels such as coal and wood. Research has shown that the particle size must be below about 200 for a hazard to exist.

It is virtually impossible to avoid dust cloud formations in pneumatic conveying. Even when the product being conveyed consists of particles larger than that threshold, you must consider the possibility of the production of fines during conveying. Such fines may result in an explosion hazard being created in the receiving vessel.

Two sources of ignition frequently encountered in industrial plant are a hot surface and a spark. For example, rotary valve bearings, if not properly protected and maintained, could overheat and thus provide the necessary source of ignition. There is always the possibility of spark generation by metal-to-metal contact; therefore, all valves and feeding devices with moving parts should be checked. Sparks are often associated with electrostatic generation, as discussed above.

In truly dense phase systems, the concentration of the product in the air is well above the upper explosive limit and so explosions are unlikely. Solids concentrations in cyclones, filters or receiving hoppers, however, could be in the explosion risk range. Also during startup and shutdown, dilute phase conditions are likely to exist in the conveying line.

Explosions can either be prevented by reducing the percentage of oxygen in the conveying air to an acceptable level, or they can be contained. Detection and suppression equipment can be employed or relief venting can be used with appropriate safety measures.

Product-related problems
In the previous section, we discussed some problems that directly result from the product being conveyed but that have an impact on the system. This section covers problems the system can cause in the product being conveyed. These include:

Angel hairs. The formation of angel hairs is a problem that can occur with plastic pellets such as nylon, polyethylene and polyesters. The presence of angel hairs is undesirable because they can cause blockages at line diverters and in filters. The problem can be overcome to a large extent by pipeline treatment. Conveying air velocity is a major variable; decreasing the velocity at which the product is conveyed will help to reduce the problem.

Cohesive products. Such materials may experience problems in hopper discharge. If difficulties are encountered in achieving flow rates with a system and the conveying line pressure drop is below the expected value, the problem could well relate to the discharge of the product from the hopper rather than the capability of the feeding device. In this case, the use of a suitable bin-discharge aid should be considered. In the case of rotary valves, a blow-through type should be used if there is any difficulty in discharging a cohesive product.

Granular products. If a granular product has to be conveyed, difficulties may arise in discharging the product into the conveying line. Rotary valves and blow tanks may cause problems here, as discussed previously [2].

Hygroscopic products. A hygroscopic product may absorb moisture from the air used to convey it. Although the specific humidity of air will decrease if it is compressed isothermally beyond the saturation point, its relative humidity will increase and is likely to be 100 percent after compression. The added moisture will not only affect product quality but could cause subsequent handling problems. The problem can be overcome by drying the air used for conveying the product.

Large particles. Such particles can be conveyed quite successfully in pneumatic conveying systems. It is generally recommended that the diameter of the pipeline should be about three times greater than that of the particles. This is simply an expedient measure to ensure that the pipeline will not block by the wedging action of two rigid particles. There are exceptions to this rule, of course. For instance, with very pliable products such as fish, it is possible to convey "particles" that actually are larger than the pipeline bore. With rigid particles, a problem may arise if a mean particle value is used in sizing and particles have an irregular shape. (Care must be exercised in feeding in all cases).

Particle degradation. Pneumatic conveying can cause the fracture and breakage of friable materials. Even if the presence of fines in a product is not a problem with respect to product quality, the fines produced will add unnecessarily to the duty of the filtration unit. The problem is influenced to a large extent by conveying air velocity. Any possible reduction in the velocity at which the product is conveyed will help to decrease the problem.

Product quality. If a conveying system is dedicated to a single product and has been optimized to the lowest specific energy, a change in product quality can cause operating difficulties. Handling a product of a slightly different shape or size could be sufficient to cause the pipeline to be blocked.

Temperature. High temperature products can be conveyed quite successfully and conveying gas at any temperature can be used. Compatibility with system components is the determining factor. Conveying air velocities also have to be guaranteed if there are significant temperature changes.

The evaluation of gas and product temperatures presents the difficulty. At the feeding point, for example, cold air may be used to convey a high temperature product. Along the conveying line, there will be a move towards thermal equilibrium between the air and product, as well as heat transfer from the pipeline to the surroundings. Since conveying times are very short, it is unlikely that equilibrium will be established. It is quite possible, therefore, for the surface of the particles to be "cold" and the inner core to be "hot." Because of this, it is often possible to use filter cloths in these high temperature situations. By the same reasoning, the product in the reception hopper could be very hot once equilibrium has been established there.

The maintenance of conveying air velocities is particularly important in these situations, but their evaluation can be difficult. Particle temperature transients represent a complex three-dimensional heat-transfer conduction problem and should only be attempted by an expert. However, since air density increases as temperature decreases, the maintenance of air velocities is only likely to be a problem when a very high temperature gas is used to convey a cold product. In this case, the temperature gradient effect could override the pressure gradient influence on air density.

Wet products. Fine products that are wet will tend to coat the pipeline and gradually block the line. If the product is not too wet, heating the conveying air can relieve the problem. Difficulty may be experienced in discharging a wet product from a hopper.

The conveying characteristic
As in many plant situations, troubleshooting would be relatively straightforward if you know what information is required, and can obtain high quality data.

The three major variables that specify the operating point of a pneumatic conveying system are: solids mass-flow rate; gas mass-flow rate; and pressure gradient (pressure drop per unit length).

One way of presenting these variables is to plot solids mass-flow rate against the mass flow rate of gas, as shown in Figure 2. This graphical form is referred to as the conveying characteristic or performance map. A conveying characteristic applies to a particular bulk material and a particular pipeline.

 

A Typical Conveying Characteristic

Figure 2. This performance map for cryolite plots the three major pipeline variables: solids mass-flow rate, gas mass-flow rate and pressure drop per unit length.

 

In this representation, the third variable, conveying-line pressure drop, is presented as a set of curves. Each curve represents a line of constant conveying-line pressure drop. The shape of these curves varies and depends on the conveying capability of the particular material. A comparison of different conveying characteristics shows that the shape of the curves is governed by the mode of conveying, which itself is determined by the physical properties of the material being conveyed.

The extent of the performance envelope for a conveying characteristic is bounded by four limits:

1. The lower limit due to the air-only pressure drop for the pipeline;

2. The right-hand limit, which is governed by the volumetric capacity of the air mover; (Using a larger capacity machine would increase this limit but rarely offers an advantage, because this simply limits the rate at which material can be conveyed.)

3. The upper limit can be due to either the pressure rating of the air mover or the maximum rating of the solids feed device (which is the case here); and

4. The limit to the left-hand side of the characteristic is normally the most important as it marks the boundary between flow and no flow. For a system to operate without possibility of a blockage, the operating point must be to the right of this boundary.

Some materials possess physical characteristics that prohibit conveying in non-suspension modes of flow in conventional pipelines. In such cases, the limit of the pressure drop curves to the left-hand side of the graph corresponds to a minimum velocity. In this case, the material remains predominantly in suspension. Typically, this minimum velocity would be about 15-18 m/s (3,000-3,600 ft/min). These systems are often referred to as dilute phase systems.

The conveying air velocity is a critical parameter. The velocity at the point where material is fed into the pipeline is particularly important. If the velocity is too low, pipeline blockage may occur. If the velocity is too high, the rate at which material can be conveyed will be restricted and problems such as particle attrition and erosion may result. It, therefore, is essential to know the conveying air velocity in order to assess the performance of a system and the potential for optimization and uprating.

On-line measurement
To determine the system operating point on the conveying characteristic graph, you must have data on the air flow rate, the material flow rate, and the conveying line pressure drop. In addition, depending on the application, measurements of temperature may be required.

However, most pneumatic conveying systems include very little diagnostic instrumentation. In many cases, a simple pressure gauge mounted on the air supply line is all that is available.

Material mass-flow rate is certainly the most difficult measurement. Generally, only an average conveying rate can be obtained. An estimate of the air flow rate can be found if performance curves for the air mover are available. In some cases, these estimates can provide enough information to identify a particular problem. In other cases, estimates can be so inaccurate that at best they are unhelpful and at worst are actually misleading.

These difficulties, however, can be overcome by taking a series of relatively simple measurements that provide high quality data.

Pressure transducers. The choice of the transducer depends on the operational range, whether gauge, absolute or differential pressure measurement is required. Accuracies of +/-0.25 percent are readily achievable.

For a sensor mounted inside a pneumatic conveying system, a distinction has to be made as to whether it will be used in the single-phase or two-phase section of the system. If there is the possibility of solid particles being in the pipeline, the gauge should be protected either by the choice of location or by the use of baffles or protection plugs. Another method is to use a wire in the port, the principle being that the mainstream flow will vibrate the wire and thus dislodge any clogging in the port.

Temperature measurement. Standard RTD and thermocouple sensors can be used. These provide an accuracy of 1 Degrees C. which is generally acceptable.

Gas flow. In pneumatic conveying systems, the product flow rate depends on the mass flow rate of gas into the system. Many techniques are currently used for measuring gas flow.

Differential pressure meters, which generally employ orifice plates, now are the most widely used and accepted method of measuring air flow. An overall system with density measurement can provide accuracies of between 2 and 5 percent. There is always a permanent pressure loss associated with these devices.

Vortex shedding meters offer a lower pressure drop and accuracies of about 1.5 percent and turndown ratio of 40:1. These meters have no moving parts and have a low pressure drop. However, they are very sensitive to swirl and to flow pulsations.

Thermal meters measure mass flow rate directly and can provide accuracies of 1 to 2 percent of full scale. They have a wide working range, with turndown ratios of around 50:1. Pressure drop is relatively low compared to differential pressure devices. Thermal meters, however, are sensitive to flow profile, and are relatively costly.

Proper handling of the data generated is crucial. Recent advances in portable data-acquisition hardware and associated software, along with the advent of smart sensors, mean that it is feasible to install a measurement system capable of recording a time history of plant operating conditions that can significantly enhance troubleshooting.

Elizabeth Knight is a senior consultant and Dr. Don McGlinchey is a consulting engineer at the Centre for Industrial Bulk Solids Handling of Glasgow Caledonian University (GCU), Cowcaddens Rd., Glasgow G4 0BA, U.K. The authors wish to recognize the contribution made to this article by their esteemed colleague Dr. Pedrag Marjanovic, who, the authors note with sadness, has since passed away.

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