This is Part I of a multipart article that explores the causes of pneumatic conveying system performance problems. Part I reviews system throughput troublespots. Future installments will focus on the potential problems resulting from product and system differences, and the components themselves.
Pneumatic conveying is essentially a very simple process, but the design factors that influence system performance are varied and complex. Most component specifications are based on data resulting from pipeline design. Since the data used in pipeline design are not totally reliable, many systems incorporate margins and factors to allow for uncertainties. Use of these "fudge factors" often leads to a mismatch between components and over-design in certain areas. Although over-design will generally ensure that a system will work, it will rarely work efficiently.
Since the reasons for an underperforming system are not always obvious, we need to examine the likely causes of throughput difficulties, which result in frequent blockages, inability to convey a given product, and systems that fail to meet required duty.
- One of the most serious and frustrating problems in system operation is pipeline blockage. To rule out blockage, check the obvious 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?
If the pipeline blocks during commissioning trials with the pneumatic conveying system, either there is a serious system design fault or some simple adjustment needs to be made.
If system design is suspect, it is most likely because the air mover was sized incorrectly. A minimum conveying air velocity must be maintained at the product pick-up point at the start of the conveying line. The velocity depends on the product being conveyed and, for products that can conveyed in dense phase, varies with the phase density at which the product is conveyed. Since air is compressible, it is important to account for air pressure at the product pick-up point when evaluating the free air requirements for the air mover specification.
Air velocity at the start of the conveying line is particularly important. If this velocity is too low, the pipeline is likely to block. For products conveyed in dilute phase, or suspension flow, a 12-15 m/sec minimum velocity is needed. If a pipeline becomes blocked and the conveying line inlet air velocity is too low, then an air mover with a higher volumetric flow rate will be required.
It is important not to over-rate any replacement -- the conveying line inlet air velocity need not exceed the minimum conveying air velocity value by more than about 20 percent.
Overfeeding vs. incorrect air mover specificationThe pressure gradient in the conveying line depends primarily on the concentration of product in the pipeline. If too much product is fed into the conveying line, the pressure requirement will exceed available and the line will block.
Each type of pipeline-feeding device has its own characteristic means of controlling product flow. In some cases, this is achieved by direct speed control, as with rotary valves and screws. With others, additional flow-control devices such as venturi feeders will be required. Control of blow tanks and suction nozzles is achieved by air supply proportioning.
Feed control is particularly important when a rotary valve feeds the pipeline, because a change of even one or two rev/min can have a significant effect on product flow rate.
It can be difficult to determine whether blockage results from an incorrect air-mover specification or over-feeding of the pipeline. For a positive-pressure system, this question can easily be answered by placing a pressure gauge in the air supply line at a point just before the product feed into the conveying line. In a negative-pressure system, the pressure gauge would be in the pipeline between the filtration unit and the inlet to the exhauster. Both cases will give a close approximation to the conveying line pressure drop.
If the reading on the pressure gauge is above the design value, the pipeline is being overfed, and the feed rate should be reduced. If the pressure is at the design value or below, then the volumetric air flow rate is insufficient. The gauge will be useful for monitoring system performance. However, air velocities also should be checked, since an increase in air supply pressure will lower the conveying line inlet air velocities, as shown in Fig. 1.
Figure 1. Pressure and Volume Influence Conveying Air Velocity
Maintaining the required air velocity for successful product transport requires additional volume to compensate for increased pressures.
Irregular feedratesIf the pipeline blocks only occasionally, this may be due to surges in product feed. In addition to determining the mean flow rate on startup, the regularity of the flow rate over short periods of time should be assessed. Differential pressure switches should be placed at all air movers and linked to the product feeder, to stop the feed in an over-pressure condition. This setup will give the system a chance to clear and can be arranged to bring the feed back online automatically. If a pipeline tends to block when the system is started up after a shutdown, some transient situation may be responsible.
Moisture and cold airIf product is blown into a cold pipeline, the inside surface could be wet as a result of condensation. This can occur in pipelines subject to large temperature variations, particularly where there are pipe runs outside buildings. If air drying is not normally necessary, the problem can be overcome by trace heating of exposed sections of the pipeline or by blowing the conveying air through the line to dry it out prior to introducing the product. Lagging may be sufficient in some cases.
In normal operation, the delivery temperature of air from a Roots-type blower could be 80 Degrees C higher than the inlet temperature. This means the volumetric flow rate and the conveying air velocity will be 25 to 30 percent greater than the value at ambient temperature. On startup, the air will be relatively cold for conveying the product and, if the resulting conveying air velocity is below that necessary for the product, the pipeline could block.