Non steady feeding of pipeline
If the pipeline blocks only occasionally, it is possible that this may be due to surges in the material feed rate. For a system that is operating close to its pressure limit, a momentary increase in feed rate could raise the material concentration to a level that may be sufficient to block the line.
This can be seen by reference to Figures 20.1 and 20.2 once again. Any increase in material flow rate will require a corresponding increase in conveying line pressure drop,
and the response can be very rapid, as considered above. It is very approximately a linear relationship, and so a 10 per cent increase in material flow rate will require a 10 per cent increase in air supply pressure. If this pressure is not available, a moment- ary surge in feed rate could result in a blocked pipeline.
In addition to determining the mean flow rate on commissioning, the regularity of the flow rate over short periods of time should also be assessed. This is necessary to ensure that these fluctuations will not overload the system. It is essential, therefore, that both the compressor and the motor drive are specified with adequate margins. The compres- sor should be capable of delivering air at a pressure slightly higher than that required, and at a corresponding volumetric flow rate. The motor drive for the compressor should have sufficient spare power capacity to meet the demand of any possible surges.
A useful aid is to fit differential pressure switches to all air movers and link these to the material feeder so as to stop the feed in an over-pressure condition. This gives the system a chance to clear and it can be arranged to bring the feed back on again automatically, once the pressure has dropped to some specified value.
Material surges have to be considered in relation to the type of feeding device used. In this respect, positive displacement, volumetric devices need particular consideration. A rotary valve, for example, with eight blades and rotating at 23 rev/min will empty about three pockets of material every second. For most purposes this frequency is suf- ficiently high, but with a short pipeline due care should be taken with such a feeder. Double flap valve type feeders, cycling at 10–20 times a minute, clearly present a problem as this could be too coarse for many materials and duties.
On start up
If a pipeline has a tendency to block when the system is started up after a shut down period, some transient situation may be responsible. It is quite possible that the system will operate satisfactorily under normal load conditions.
Moisture in line
If material is blown into a cold pipeline it is possible that the inside surface could be wet as a result of condensation. This is liable to occur in pipelines that are subject to large temperature variations from day to night, particularly where there are pipe runs outside buildings. If air drying is not normally necessary, the problem can be overcome either by trace heating and insulation of exposed sections of the pipeline, or by blowing the conveying air through the pipeline for a short period to dry it out prior to introducing the material.
This point is illustrated in Figure 20.7. This is a graph which shows the variation, with temperature, of the mass of water that can be supported as vapour in saturated air. If the temperature rises, for a given mass of water vapour, the humidity will decrease and air will become drier. If the temperature falls, however, condensation will take place and the humidity will remain at 100 per cent. For initially saturated air, therefore,
Figure 20.7 can be used to determine the mass of water vapour that will condense for a given change in temperature. The problem relates particularly to plant operating only on day shift where, at the end of the day, there could be warm moist air in the pipeline that could cool and possibly condense overnight to leave damp patches on the pipeline walls.
Moisture is often a problem in general high pressure plant air supplies. If such a plant air supply is used it would be wise to incorporate a moisture separating device. If the inside surface of a pipeline is wet, as a result of condensation, fine material will tend to stick to the wall surface. This is particularly a problem at bends prior to a ver- tical lift. Moisture condensing on the surface of the vertical pipeline will tend to drain down to the bend at the bottom and collect as a pool of water.
It depends upon the nature of the material being conveyed, and its interaction with water, as to what will happen when the material meets the water. In many cases a hard scale will form, and this will gradually accumulate with successive cycles of condensation and conveying, to a point where the build up adds significantly to the pipeline resistance. For a conveying system operating close to its pressure limit the added resistance could result in pipeline blockage.
As a matter of course the pressure gauge on a plant, as illustrated on Figures 20.4 and 20.5, should be checked regularly at convenient times to record the value of the air only pressure drop for the pipeline. The reference value for this should be obtained during commissioning of the plant. If the value rises the possible causes should be investigated, particularly if it continues to rise.
Air drying systems
Air can be dried either by refrigerating or by chemical means. The decision depends upon the level of drying required. The quantity of water in air, as a function of tempera- ture, can be seen in Figure 20.7. The lower the air temperature (for refrigeration), or the dew point (for chemical dryers), the less moisture there will be in the air. Due to the problems of the free flow of the water to be removed, refrigerant dryers are normally designed to cool the air down to about 2°C. For most purposes this is sufficient. For those cases where this is not adequate, however, chemical dryers have to be used.
These are capable of reducing the dew point temperature of the air down to -40°C, and at this temperature moisture levels are very small indeed.
The capability of air for supporting moisture will decrease with both a decrease in temperature and an increase in pressure. If air is compressed isothermally, or is com- pressed and allowed to cool before use, condensation will occur if the ambient air being compressed has a sufficiently high relative humidity. Provision, therefore, must be made to drain this condensate. The compression process, however, occurs very quickly and complete condensation may not take place. Condensed water in the form of a fog or mist is often conveyed with the air and can be transported through pipelines over long distances. It is not always advisable, therefore, to rely on the compression process to dry the air. Moisture and condensation are considered in more detail in Chapter 25.
The density of air decreases with increase in temperature. In normal operation the delivery temperature of the air from an air mover, such as a positive displacement blower, could be some 60°C higher than the inlet temperature. This means that the volumetric flow rate, and hence the conveying air velocity, will be 25–30 per cent greater than the value at ambient temperature. On start up the air will initially be fairly cold for conveying the material, and so if the resulting conveying air velocity is below that necessary for the material, the pipeline could block.
This point is illustrated in Figure 20.8. This is a graph of conveying air velocity plotted against a narrow band of air temperature. It is derived from Equation (20.2) once again, for a free air flow rate of 0.5 m3/s at a pressure of 1.0 bar gauge in a 150 mm bore pipeline. Figure 20.8 shows that conveying air velocity is quite sensitive to tem- perature, as well as pressure. Since air density increases with decrease in temperature, it is essential that air requirements are based on the lowest temperature that is likely to be experienced. Thus a cold start up in winter with the lowest possible air and material temperatures must be catered for. This is particularly important in plant where the material, under normal circumstances, may be at a high temperature. If the plant is shut
down and re-started with cold material it could have a significant effect on the convey- ing air velocity.
If meeting the air flow requirements for the lowest temperature results in excessively high conveying air velocities during normal operation, then some means of controlling the air flow rate to the conveying line must be incorporated. Variable speed control of the air mover, choked flow nozzles in a by-pass air supply line and the discharge of part of the air to atmosphere via a control valve, are some of the methods that could be con- sidered for the control of the air flow rate to the pipeline for normal operation.
Material in pipeline
If, when the plant is shut down, the pipeline is not purged, a quantity of material could be left in the pipeline. If the conveying line incorporates a long vertical lift section, sufficient material could accumulate in the bend at the bottom to prevent the system from being re-started. It is always a wise precaution on start up to blow air through the pipeline before material is introduced. If the pipeline was not purged on shut down, there may be sufficient material left in the pipeline to cause blockage of the pipeline during start up. If the pipeline is already blocked it will considerably aggravate the situ- ation if more material is blown into the pipeline. This reinforces the need to monitor the air only pressure drop for the pipeline.