Simple Solutions That Work! Issue 10

and economical, a Dense Phase system, operating at the lowest practical velocity but higher pressures would be preferred. Because of the much lower velocity, pipeline wear is drastically reduced, sand degradation practically eliminated, and operating and maintenance costs are slashed to the bone. Therefore, these systems are now the preferred choice for foundries. But just like a race car, if you expect top performance you have to keep it properly tuned. Even the best Dense Phase system performs as designed, only if installed correctly and periodically monitored. Since the main adjustments in any pneumatic sand conveying system are sand feed rate, air pressure and air volume, it is quite possible to convert a Dense Phase system, perhaps unknowingly, into a Dilute Phase system, with all its disadvantages, by simply ignoring the correct settings. Preventive maintenance needs to be taken into consideration during the system design stage to prevent situations where frequent maintenance becomes necessary. Hints for improved installation, operation and maintenance Part of the system analysis should always include a particle size distribution check of the sand. For sand contaminated with excessive fines or dust, air conveying may not even be a choice at all. approximately 0.3 to 1 pound of material per cubic foot of air and some require boosters for transport. This additional air, however, adds to the volume of air already in the pipe causing higher velocity and subsequent sand degradation. If sand is transported in such a way the resulting abrasion can change the screen size by several points and wear out pipe prematurely. Dilute and dense phase systems with higher material velocities may be the solution for “soft” materials such as powders and fines that can withstand the higher velocities in the pipeline without being destroyed in the process. But for foundry sand high velocity, fluidizing systems should only be used as a last resort if nothing else is economically feasible and sand degradation is ignored. To take advantage of a combination of factors which make the pneumatic conveying of dry sand in a foundry efficient The sand should always be dry and free flowing. If debris is present a screen must be installed upstream of the blow tank. System capacity is also influenced by the moisture content of the sand. The higher the moisture content, the lower the flowability and/or capacity. When distributing sand to the core room, the transport air should be free of any moisture to prevent problems with binder systems which are not compatible with external moisture and will result in scrap cores. Air flow should be adjusted to the minimum necessary to keep the system operating properly during startup. Excessive air flow in a Dense Phase system can cause just as much damage as a Dilute Phase system. Turning up the air flow does not always give higher throughput. In fact, it may cause just the opposite and result in high shock waves in the pipeline, damage to pipe supports, premature pipe wear and degradation of the sand. All pipe connections must be tight and pressure tested. Leaky pipe joints change the system design conditions and may stop sand flow completely. When properly designed, the run and size of the pipeline is matched to the required system performance. Therefore, if a system is designed for 10 tons per hour at 250 feet, extending the run to 300 feet will correspondingly reduce the capacity. HOW TO 41 Continued on next page