Since air density increases with temperature decrease, it is essential that air requirements be based on the lowest temperatures likely. If this results in excessively high conveying air velocities during normal operation, then it will be necessary to control the air flow rate to the conveying line. Variable speed control of the air mover, choked flow nozzles in a by-pass air-supply line or discharge of air to atmosphere via a control valve could be considered.
Product in the lineIf the pipeline is not purged during a plant shutdown, some product could be left in the line. On startup it's important to blow air through the pipeline before product is introduced. If the reference value of pressure drop for air blown through the pipeline is known, it can be compared with the air-purge value. If the actual pressure drop is significantly higher than the empty-line value, product may still be in the pipeline. It's also good practice to purge the line and check the pressure drop before shutdown.
Unexpected shutdownIf conveying stops unexpectedly due, for example, to a power-supply failure, it may not be possible to start the system again, particularly if there is a large vertical lift. If the bend at the bottom of the vertical section is taken out to remove the product, it may be possible to purge the line clear.
If this is a common occurrence for a plant, an air receiver can be installed between the air mover and product feeder. If the product feed into the pipeline stops at the instant the power fails, the air stored in the receiver could be sufficient to clear the line of product. Alternatively, a parallel line with valved connections to the pipeline could be fitted so that the line could be cleared slowly from the end, one section at a time.
When good systems go badIf a system that has worked well for a long time starts to develop blockage tendencies, feeding device wear may be the cause. If air leakage across the feeding device increases, the air available for conveying the product decreases. The volumetric flow rate of remaining air may become insufficient to convey product and the pipeline will block. Worn screw flights, valve seats in gate lock valves, and rotary valve blades can all result in greater air leakage. Check these components regularly for wear and replace them when needed. Also check air movers against original manufacturers' specifications.
Keep in mind that a system that conveys one product well may be completely unable to convey another product. Minimum conveying air velocities differ from product to product, and air leakage across feeding devices is also product dependent. If a system has to convey more than one product, this requirement must be considered carefully at the design stage. Fig. 2 plots product flow rate against air flow rate for a range of products.
Figure 2. Product Influences Performance
The flow rates of various products in a 53-mm-diameter piping system, 50 m long with nine 90 elbows, show wide differences in required air mass flow rates.
Long distance chargesRemember that, for any given conveying line pressure drop, the conveying capacity of a pipeline decreases as distance increases. For a change in conveying distance, therefore, there must be a corresponding change of product feed rate into the pipeline.
For a given conveying line pressure drop, the product flow rate is approximately inversely proportional to conveying distance. For a given distance, the product flow rate is approximately proportional to line pressure drop.
If the conveying distance is increased, the product flow rate will have to decrease, so product will be conveyed at a lower phase density. For a product capable of being conveyed in dense phase in a conventional system, a slightly higher conveying line inlet air velocity will be required, in turn, demanding a higher air flow rate.
If the system cannot achieve its rated duty, determine whether the problem is due to product feeding, pipeline or air supply. Check on the conveying line pressure drop. If it is below the air mover's capability, product feed into the pipeline may be insufficient. If the maximum output of the feeder does not meet the conveying capability of the pipeline, however, it will probably be necessary to fit a larger feeder.
Before recommending a larger feeder, be sure that air leakage isn't the real culprit. Check rotary valves in particular, as well as air vents and clearances on all moving parts. Don't forget to check the filtration unit. If it has been incorrectly sized, pressure drop across the filter may be too high. Also check that the filter cloths don't need replacing or cleaning. It may be that an additional or a larger filter is needed. If these modifications don't bring the system to rated output, an air mover with a higher pressure rating or an increase in pipeline bore are indicated, but be sure to consider how this will influence other parts of the system.
Reducing air flow RateImproved performance can often be achieved by reducing the quantity of conveying air, particularly if the system is over-rated for volumetric air supply. This could be achieved with a tee and a valve in the conveying air pipework (Fig. 3). In a positive-pressure system, these would be positioned between the air mover and the product feed. In a negative-pressure system, they would be placed between the filtration unit and the air mover. The tee and valve would also allow you to monitor the impact of reduced air flow rate on system performance.
Figure 3. Well-Placed Tees
In a negative pressure system, place an intake between the filtration unit and the air mover to reduce draw through the system. In a positive pressure system, position an off-take between the air mover and the product feed to reduce air flow into the system.
Review routingReview the pipeline routing and see if the number of bends might be reduced. Blind tees or sharp elbows should be exchanged for short-radius bends. For high-pressure systems with a single-bore pipeline, stepping the pipeline to a larger bore part way along, could also increase throughput.
Remember the potential role that equipment wear can play on system performance, particularly when abrasive feeds are involved, and also consider the fact that hygroscopic products can build up within pipe walls. Be proactive, and check on any changes in system performance to avoid problems later on.
Elizabeth Knight is a senior consultant and Dr. Don McGlinchey is a consulting engineer at Glasgow Caledonian University's (GCU) Center for Industrial Bulk Solids Handling, Cowcaddens Road, Glascow G40BA, U.IC. Tel:44(0) 141-331-3715.
The authors wish to recognize the contribution made to this article by their esteemed colleague, Dr. Pedrag Marjanovic, who, the authors note with sadness, has since passed away. Dr. Marjanovic was a specialist in pneumatic conveying and bulk solids, and had been a professor at Glasgow Caledonian since 1999.