Problem-Free Air Conveying Part II

Productive and reliable system and component choices depend on knowing about the potential problems each of them can cause

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Part I of this article discussed general problems such as pipeline blockage and how they are likely to occur in most conventional pneumatic conveying systems. In Part II, we investigate and troubleshoot problems that relate to particular types of pneumatic conveying systems and their key components.

Basic pneumatic conveying is a straightforward process. Proper evaluation of performance requirements and product characteristics are vital parts of that process. In addition, careful choice of the type of system and its components help create a pneumatic conveying system that efficiently and reliably contributes to the performance and dependability of the entire process unit.

The system designer and process engineer should carefully consider many factors when choosing a positive pressure, negative pressure or combined system. In addition, key components such as blow tanks, rotary valves and filters have to be well matched to the task at hand.

 

Positive pressure systems

Figure 1. Choose Wisely

The choice of a positive pressure (A), vacuum (B) or combined (C) system will be defined by product feeding and/or discharge requirements.

 

Multipoint feeding of a positive pressure conveying system generally is not recommended. The air loss from a single feeder subject to leakage can be a significant proportion of that required for conveying the product. The loss from a number of feeders, if they are not isolated by additional valves, could be very high.

The air loss from a number of feeding points could be difficult to estimate accurately. So, the air available for conveying, which is a crucial requirement, could not be guaranteed. Apart from problems associated with having too little or too much air for conveying the product, the loss of a large quantity of air from a number of feeding points also would represent a very significant energy loss for the system.

Negative pressure systems

A vacuum conveying system is not the answer if multipoint discharging of product is needed, since such a system requires complex arrangement of pipework and isolating valves. These valves are sometimes found in low-pressure systems where ductwork is used. Valves in the ductwork, however, have to seal effectively, otherwise the air leakage into the system will adversely affect conveying.

If air leaks into a negative pressure system, it will alter the balance of conveying air velocities along the length of the pipeline. If air ingress is not accounted for in the air mover spec, the line is likely to block. If the air mover is over-rated to allow for air leakage, product flow rate will be reduced. Air ingress is likely to occur along a pipeline at flexible sections such as those used in off-loading systems, particularly if the conveyed product is erosive and the joint is hard metal or ceramic material.

Combined systems

Operating pressures also will differ. With a Roots-type blower, for example, a pressure ratio of 2:1 is generally considered the upper operating limit. Thus for a positive pressure system the maximum delivery pressure is about 1 bar gauge. For a negative pressure system the maximum exhaust pressure is about -0.5 bar gauge. For a combined system, the limit on pressures is approximately 0.4 bar gauge on blowing and -0.3 bar gauge on vacuum.

Since the two parts of the combined system operate at different absolute pressures, the pipelines likely will be different diameters. Figure 2 illustrates this point.

Figure 2. Combined System Requires Two Pipe Sizes

Conveying air velocity as a function of air pressure, with lines of constant pipeline bore plotted, shows a typical negative-positive system superimposed for a free air flow rate of 0.3 m3/sec. Balance between the two parts of the system, in terms of conveying air velocities, is achieved with different pipeline bores.

 

Air also probably will leak across the feeding device on the positive pressure side, so the air flow rates will differ. An imbalance in product flow rates between the two halves of the system necessitates a full assessment of the operating pressures, pipeline bores and air flow rates.

System component problems

Blowers:

If a blower is operating in a dusty environment, a filter should be fitted to the air inlet. This filter should be cleaned or changed periodically. If it becomes choked with dust, the added resistance will affect blower performance. An outside air source is generally the answer.

In negative pressure and combined systems, blowers have to operate with air that has been used for conveying product. In these cases, the air must be effectively filtered. It might also be necessary to add a backup to the filter, to provide a measure of protection for the blower should the filter unit fail.

A change in conveying system performance over a period of time may indicate blower wear. A small diet of dusty air into a blower will cause a gradual change in its operating characteristics.

Blow tanks:

Figure 3. Blow Tanks Batch It

Single blow tank systems operate in a batch mode. Of all system components, blow tanks are probably least understood in terms of operation and control.

 

It is important to realize that to obtain a required time averaged mean value, the pipeline bore and air requirements must be based on the steady-state value achieved during the conveying cycle. If the desired product flow rate is not being achieved, consider means of increasing the ratio of the time-averaged mean to steady state values. Assess the times required for pressurizing, depressurizing and venting, valving and possible changes in operating procedures and conveying conditions.

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