Operating problems:Existing plant and Positive pressure systems

Operating problems

Introduction

Since the cause of any operating problem is not always obvious, this chapter has been subdivided into four major areas to help in the identification and solving processes. In the first section problems related to particular types of system are considered. The second section is devoted to problems that can be associated with system components, including air movers, feeders and filtration systems. The next deals with system-related problems and includes effects that the material can have on the system. The last con- cerns material-related problems and includes effects that the system can have on the conveyed material.

Although the specific problem of pipeline blockage was considered in detail in Chapter 20, and that of systems not capable of achieving the rated duty in Chapter 21, many of the items considered may also have an influence on material flow rate. A num- ber of the problems that are considered to be are either very common in the industry, or need more detailed treatment, have an entire chapter devoted to them later in this part of the Design Guide.

In trying to identify any particular problem it is suggested that each section should be consulted since cross-referencing and repetition have been kept to a min- imum. Those items that relate to the particular problems experienced, type of plant and components used, and material conveyed, should all be referred to in order to obtain a clear picture of the problem in relation to the entire system and the material handled.

Existing plant

This section of the Guide is directed essentially at identifying operating problems that occur with an established plant, or one that has just been installed. It is not intended to support system design by way of problem anticipation so that counter measures or appropriate equipment selection are dealt with before the plant is built. If a new system is designed correctly, and potential problem areas are recognized at the design stage, there should be no need to refer to this section at all. This part of the Guide can, of course, be used as a checklist to ensure that all possible sources of problems have been considered at the design stage, and should prove to be invaluable when commissioning a plant.

Types of system

In this section, problems that relate specifically to a particular type of pneumatic con- veying system are considered. System types were considered mainly in Chapter 2 and reference will be made to particular figures included there.

Positive pressure systems

The most common problems associated with pneumatic conveying systems relate to the fact that the material to be conveyed has to be fed into a pipeline in which the con- veying air is maintained at pressure. Air requirements have to be specified, taking account of both the compressibility of the air, and air leakage from or into the system.

Multi point feeding

Multi point feeding of a positive pressure pneumatic conveying system is not gener- ally recommended unless particular attention has been paid to the problem of air leak- age. For feeders subject to air leakage, air loss from a single feeder can be a significant proportion of that required for conveying the material. The air loss from a number of feeders, therefore, would be seriously detrimental to the performance of the system.

The air loss from multiple feeding points would be difficult to accurately estimate and so the air flow rate available for conveying could not be guaranteed. Apart from the problem of having too little or too much air for conveying the material, the loss of a large quantity of air from multiple feeding points would represent a very significant energy loss from the system.

Negative pressure systems

A common fault with negative pressure, or vacuum conveying systems, is the loss of vacuum, particularly with batch and intermittently operating systems. The cause of the problem is often that the discharge flap fails to seat at the base of the receiver vessel. Another common problem is similar to that experienced with positive pressure sys- tems, in that the compressibility of the air is not taken into account correctly. In vacuum systems, however, this can affect the specification of the filter, as well as the conveying line inlet air velocity and the specification of the air mover.

Air filtration

With negative pressure systems, the entire discharge system operates under vacuum, and this includes the filtration plant. Filters are generally sized in terms of the surface area of filter cloth, and the surface area required is evaluated in terms of a given air velocity across the fabric surface. Under vacuum, therefore, the volumetric flow rate of air to be handled is very much higher than it is for a positive pressure conveying system discharging to atmospheric pressure.

The size of filter required for a vacuum conveying system will depend upon the exhauster pressure, and for a vacuum of 0.5 bar, for example, it will need to be about twice the size of that required for an equivalent positive pressure system. If the filtration plant is not sized, taking this into account, it will be too small for the duty and system performance and operating problems can be expected as a result.

Back-up filters

It is generally recommended that a secondary filter, often referred to as a policeman filter, should be fitted to negative pressure conveying systems. This is a particular requirement if a positive displacement blower, screw compressor or a sliding vane rotary compressor is used as an exhauster. These exhausters operate with very fine clearances between the moving parts and cannot tolerate dust, particularly if it is abrasive.

A back-up filter is required in case an element in the main filter unit should fail. If an abrasive material such as silica sand, cement, alumina or fly ash, is being conveyed, and the main filter unit fails, considerable damage will be caused to any of the above exhausters in a very short space of time. A back-up filter will allow time for the con- veying system to be shut down safely so that repairs can be carried out. A similar situ- ation occurs with combined ‘suck-blow’ systems and with closed loop conveying systems.

Multi point discharge

Vacuum conveying systems are not generally recommended if multi point discharging of materials is required, since a complex arrangement of pipe-work and isolating valves is necessary. The problem is essentially the reverse of that associated with mul- tiple point feeding of positive pressure conveying systems considered above. They are sometimes used in low pressure systems, where ductwork is used. Valves in the duct- work, however, have to seal effectively, otherwise the air leakage into the system will have an adverse effect on the conveying of the material.

Air ingress

If air leaks into a vacuum or negative pressure system it will alter the balance of con- veying air velocities along the length of the pipeline. The problems that occur here can generally be considered to be a mirror image of those that exist on similar positive pressure systems.

Into reception hopper

If air leaks into the reception hopper, and thereby bypasses the conveying pipeline, air velocities in the conveying line will fall and the pipeline could block if the ingress of air is not allowed for in the specification of the air mover. This can occur if the mater- ial in the reception hopper is discharged by means of a rotary valve, for example. The rotary valve will typically discharge the material into a vessel at atmospheric pressure, and so there will be a pressure difference across the valve. As with rotary valves feed- ing positive pressure pneumatic conveying systems, there will be a leakage of air across the valve because of the pressure difference. In a vacuum system this air will leak into the system, and so it must be taken into account (see Figure 3.7).

Into pipeline

Air ingress is likely to occur along a pipeline at flexible sections, such as those used in ship off-loading systems, particularly if the conveyed material is erosive and the flex- ible joint has to be made from hard metal or ceramic materials. If air leaks into a pipeline part way along its length in this way, it will result in a lowering of the con- veying air velocity at the material feed point into the pipeline. This is the critical point in a pipeline and so could result in pipeline blockage.

If a bend in a pipeline fails, or if pipeline joints are not securely tightened in a pos- itive pressure conveying system, clouds of dust will result and the situation is likely to be dealt with very quickly. In terms of conveying performance it is unlikely to present a problem, for downstream of the feed point the velocity increases and such air loss could be a benefit to the system. In a vacuum system dust is not likely to be released in this situation, as air is drawn into the system, and so the problem may not be recog- nized. Air drawn into the system, however, will starve the pipeline inlet of air and the pipeline could block as a consequence.

Stepped pipelines

With a vacuum of only 0.5 bar there will be a doubling in conveying air velocity through the pipeline. Stepped pipelines, therefore, are well worth considering for vacuum sys- tems, particularly if high vacuum is employed (see Figure 9.13). Reduced erosive wear, particle degradation and improved conveying performance are all possible benefits.

Air mover specification

Care must be exercised in specifying exhausters. The rating of an exhauster is not usu- ally in terms of free air conditions, as with positive pressure systems, but in terms of the volumetric flow rate of air at inlet to the exhauster. This, however, is not signifi- cantly different from the means by which positive pressure air movers are specified, for they are both in terms of the displacement volume of the air mover at inlet conditions. The value, of course, will vary with the vacuum drawn and so the conveying line inlet air velocity will have to be carefully evaluated and the influence of the vacuum determined. This, however, is very similar to the analysis that must be made for posi- tive pressure systems and considered in Chapter 9.

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