Troubleshooting and material flow problems
Despite being simple in concept, pneumatic conveying systems present significant design problems, not least because of the fact that the conveying medium is compres- sible. Any changes in pipeline geometry, whether horizontal runs, vertical lift or even bends can have a marked effect on performance. The properties of the material to be conveyed can have a significant influence on both the design and specification of com- ponents, and can also have a major influence on conveying performance.
One of the major difficulties with pneumatic conveying systems is that it is not always obvious what effect a change in operating conditions will have on system perform- ance. A change of material or conveying distance, in particular, may require changes in both material feed rate and air flow rate. Unfortunately the cause of a particular problem in a pneumatic conveying system is not always obvious either. Particular note of changes in performance that might occur with respect to time should be made, for these should not occur with a pneumatic conveying system, and could well lead to failure over a period of time.
One of the most frustrating problems encountered in system operation is that of pipeline blockage. This is by no means uncommon and there are a multitude of differ- ent circumstances and possible causes.
In any pipeline blockage situation the first thing to do is to check all the obvious sys- tem features:
• Is the reception point clear?
• Are the diverter valves operating satisfactorily?
• Is the full conveying air supply available?
• Was the pipeline clear on start up?
• Has a pipeline bend failed?
The problem may relate to system components, such as the feeding device or filtration plant. It may be a material related problem, such as particle size or moisture. The
time of the day and year that it occurs, together with the prevailing weather conditions, and the nature of the blockage, are useful indicators of the potential cause.
Pipeline blockages generally present a serious problem in most bulk solids handling situations, and particularly so if continuous process operations are involved, and so there is usually a need for speed of solution. For this reason a check list of possible causes and actions to take is given in Table 20.1. Most of the reasons for pipeline blockage that are included are explained in detail in the notes that follow, but in the first instance the check list will provide ideas for immediate action.
If the pipeline blocks during commissioning trials with the pneumatic conveying sys- tem, it could indicate that there is either a serious design fault with the system, or some simple adjustment needs to be made to the plant.
Incorrect air mover specification
If it is the former, the most likely reason is that the air mover is incorrectly sized for the duty. There are two possible reasons why the air mover may be incorrectly sized:
1. If the volumetric flow rate of air available for conveying the material in the pipeline is insufficient, it is unlikely that it will be possible to convey the material. A certain minimum value of conveying air velocity must be maintained at the material pick- up point at the start of the conveying line. The value depends upon the material being conveyed and, for materials that are capable of being conveyed in dense phase, in moving bed flow, varies with the solids loading ratio at which the material is conveyed.
2. The other possibility is that the incorrect conveying line inlet air pressure has been used in evaluating the volumetric flow rate required by the compressor. Since air is compressible it is extremely important that the pressure of the air at the material pick-up point, in absolute terms, is taken into account in evaluating the free air requirements for the air mover specification.
Conveying air velocity
One of two parameters is required here. These are the free air flow rate delivered, V0, and the conveying line inlet air velocity, C1. For the design of a system C1 must be specified and is calculated on the basis of the value used. As air is compressible, with respect to both temperature and pressure, the starting point in the determination of any relationship for the determination of conveying air flow rate is the Ideal Gas Law. An expression for the volumetric flow rate of free air required was developed in Chapter 9 with Equation (9.10) and this is reproduced here in Equation (20.1) for reference:
where V0 is the free air flow rate (m3/s); p1, the conveying line inlet air pressure = 101.3 + gauge pressure (kN/m2 abs); d, the pipeline bore (m); C1, the conveying line inlet air velocity (m/s) and T1, the conveying line inlet air temperature (K). Equation (20.1) can be used to check the specification of an air mover, given the conveying line inlet air velocity and other parameters.
Re-arranging Equation (20.1) in terms of conveying line inlet air velocity gives:
Equation (20.2) can be used to provide a check on the conveying line inlet air velocity, given the free air flow rate of the air mover being used on the plant and other parameters.
Pipeline bore influence
Pipeline bores quoted are ‘nominal’ sizes only since it is generally the outer diameter that is standardized because of the needs of flanging and threading. The diameter of a
4 in. nominal bore pipeline, however, is rarely 4 in. If a conveying air velocity is based on a diameter of 4 in., for example, and it is a schedule 10 pipeline, the actual bore will be 106.1 mm and not 101.6 mm. This difference will mean that the conveying air velocity will be about 9 per cent lower. If 16 m/s is the velocity in a 101.6 mm bore pipeline, it will only be 14.6 m/s in a 106.1 mm bore line and the pipeline is likely to block if the minimum conveying air velocity for the material is 15 m/s.
Conveying gas influences
Although air is used for the vast majority of pneumatic conveying systems, other gases such as carbon dioxide and superheated steam can be used for specific applica- tions. Nitrogen is often used if the material is potentially explosive. The above equations, in terms of velocities and volumetric flow rates will apply to any gas, but because the characteristic gas constant, R, for each is different, then the density of each gas will be different. If densities or mass flow rates have to be used in any calculation, therefore, Equations (20.1) and (20.2) will have to be modified. This was considered in Chapter 9 with Equation (9.9) and Table 9.2.
Influence of solids loading ratio
It is the velocity at the material feed point, at the start of the conveying line that is important. If this velocity is too low the pipeline is likely to block. For materials conveyed in dilute phase, or suspension flow, it is necessary to maintain a minimum velocity, Cmin, of about 11–16 m/s, depending upon conveyed material, as mentioned in Section 18.104.22.168. Typical values and data for Cmin were presented in Figure 14.2. A 20 per cent margin on this value is generally recommended in terms of specifying a value for the actual conveying line inlet air velocity, C1 to be employed. Typical values of C1 were presented in Figure 18.4.
For fine powders, such as cement, flour and fly ash, that are capable of dense phase conveying in moving bed type flow, the value of minimum velocity is dependent upon the solids loading ratio at which the material is conveyed. Only at high values of solids load- ing ratio can the conveying air velocity be as low as 3 m/s. If the material is conveyed in dilute phase, at a low value of solids loading ratio, velocities appropriate to the dilute phase conveying of a fine material must be used. There is, therefore, a gradual transition between dilute and dense phase, with respect to minimum conveying air velocity. To ensure successful conveying, therefore, the conveying line inlet air velocity must be above these minimum values, whether the material is conveyed in dilute or dense phase.
Air mover change
If pipeline blockages occur and it is found that the conveying line inlet air velocity is too low, then an air mover with a higher volumetric flow rate will have to be used. If it is replaced with one having a higher delivery pressure, as well as a higher volumetric flow rate, Equation (20.2) must be checked again, because air supply pressure also has a significant influence on conveying line inlet air velocity.
It is equally important that any replacement is not over-rated. It is not generally necessary for the conveying line inlet air velocity to be any higher than about
20 per cent greater than the minimum conveying air velocity value. If it is in excess of this it is likely to have an adverse effect on the material flow rate, particularly for dilute phase conveying.
A useful graph to illustrate the influence of minimum conveying conditions is a plot of conveying line pressure drop drawn against air flow rate, with lines of conveying line pressure drop superimposed. Such a graph for cement conveyed through a 53 mm bore pipeline over a distance of 101 m and containing seventeen 90° bends, is pre- sented in Figure 20.1. This is pipeline no. 7 shown earlier in Figure 13.4.
The empty line, or zero material flow rate curve, provides a useful datum for the relationship, for it shows just how much pressure is required to get the air through the given pipeline before any material is conveyed. This is a ‘square law’ relationship, and hence the gradual upward trend of pressure drop with increase in air flow rate, and hence velocity.
Apart from the lower limit of zero for material conveying capacity, which relates to the pressure drop requirements for the empty pipeline with air only, there are three other limitations on the plot in Figure 20.1. The first is the limit on the right hand side of the graph, which is set by the volumetric capacity of the blower or compressor used. This is not a real limit at all, therefore, and conveying is possible with higher values of air flow rate, but it would not be recommended. It will be seen from Figure 20.1 that for a given value of conveying line pressure drop, material flow rate gradually decreases as air flow rate, and hence power required, increases. Apart from this decrease in mater- ial flow rate and conveying efficiency, an increase in conveying air velocity will result in an increase in both material degradation and pipeline erosion.
The second limit is at the top of the graph. This is generally set by the pressure capability of the air mover. This is not a physical conveying limit either, and conveying with very much higher pressures is possible. The problem with using higher pressures, however, relates to the expansion of the conveying air, for in a single bore pipelinevery high velocities will result at the end of the pipeline. This problem can be overcome by stepping the pipeline to a larger bore once or twice along the length of the pipeline so that higher pressures can be used.
The third limit is that on the left hand side of the graph and is clearly marked. This is a real conveying limit and it represents the minimum conditions for successful pneu- matic conveying with the material. The lines of constant material flow rate actually ter- minate, and conveying is not possible in the area to the left, at lower air flow rates. Any attempt to convey with a lower air flow rate would generally result in blockage of the pipeline.
Influence of material type
This limit to conveying is influenced very significantly by material type. The cement data in Figure 20.1 follows the minimum limit set by the lower curve in Figure 14.2. As the cement is capable of being conveyed in dense phase, conveying with low values of air flow rate has been possible with high values of conveying line pressure drop.
In Figure 20.2 similar data for a material not capable of being conveyed in dense phase, in a conventional pneumatic conveying system, is presented. The limit to con- veying for this material is set by the upper curve in Figure 14.2. The material was granular coke fines and was conveyed through exactly the same pipeline as the cement in Figure 20.1. A very significant difference in material flow rate capability will be noticed.
The minimum material conveying limit on Figure 20.2 is very regular. This is because it is defined only by a minimum conveying air velocity of 14 m/s. This means that the line drawn, representing the conveying limit, also represents a line of constant conveying line inlet air velocity of 14 m/s. This illustrates the influence of air supply pressure on conveying very well, and shows that great care must be exercised in oper- ating such a conveying system, for it is very easy to cross the conveying limit and block the pipeline.
Air leakage allowance
It is important that the term in Equations (20.1) and (20.2) is the volumetric flow rate of the air used to convey the material in the pipeline. If, in a positive pressure conveying system, part of the air supply from the air mover is lost by leakage across the material feeding device, this must be taken into account. This point was illustrated with Figure 3.6. A similar situation occurs with negative pressure systems with ingress of air into the system. These points are considered further in subsequent sections of this chapter.