Simple Solutions That Work! Issue 13

49 time and then develop another leak and then we are back to “repair and go on.” Every foundry has to move tremendous amounts of sand as part of the daily operations and to handle such amounts effectively can sometimes grow into an ongoing major material handling battle. Belt conveyors and elevators have been used many times to transfer sand but today pneumatic conveyors are probably widely accepted as a more practical means to distribute sand in the foundry. Depending on what type of pneumatic conveying system is used can have a great influence on the quality of sand delivered to the production line when considering sand grain degradation, dust generation and life expectancy of the piping. In general, all conventional pneumatic conveying systems can be divided into two broad categories, Dilute Phase and Dense Phase conveying. Dilute Phase generally works by vacuum or low pressure air of up to 20 psig and velocities in the pipeline of 4000 FPM and higher, while Dense Phase works by medium pressure air of 10 – 60 psig and pipeline velocities of 2800 – 5000 FPM. Sand grain degradation in the piping caused by excessive velocities in the piping results in more dust or higher AFS numbers for the sand which in turn, if not separated prior to binder coating, requires higher amounts of resin for chemically bonded sands and the subsequent effects of higher resin percentage on mold/core performance in the casting process. Similarly the higher velocities also cause increased wear of the pipeline and bends with resultant increased downtimes and maintenance costs. A typical example is the filling of sand silos from bulk delivery trucks. Every foundry is familiar with the associated problems of such systems, mainly due to the delivery in dilute phase (low air pressure but very high velocity), which definitely is not recommended for sand. Recalling another pneumatic sand conveying project, shortly after the brand new installation was completed and production started frantic telephone calls from the customer reported that after just a few days of operation the pipeline developed several leaks and sand was being sprayed all over the production equipment in the foundry. Of course, the first impulse question was “how can that be?” As it turned out the customer installed all the sand piping runs but pressing production requirements did not allow the additional time to also install and connect the transporter pressure vessel to push the sand through the pipeline. Instead the sand delivery truck was connected directly to the sand piping and sand was blown directly from the sand truck to the receiving hoppers in the plant. Since all sand trucks unload and deliver sand in dilute phase, it quickly became clear that the much higher velocities of the truck delivery system far exceeded the design capacity of the piping system causing pipeline leaks already after a very short time. To put up with the heavy wear properties of sand it is usually only transported by using pressure vessels whose sizes are matched to the conveying capacities. The prevalent thinking is that high conveying capacities require large pressure vessels in order that the frequency of actuation of the different components in the system are not too high and they, therefore, have sufficient service life. In these systems the sand is pushed through the conveying pipeline in slugs which are formed in accordance with the frictional relationship between the sand and the wall of the conveying pipe and the permeability of the sand, without any mechanical assistance in the pressure vessel itself. These conveyors are costly to manufacture and incorporate relatively many components and the electrical controls are also costly. The normally used level probes, functioning as capacitive switches, can cause malfunctioning when there are fluctuations in the sand moisture content and temperature. Defects prevention, relative to sand and the resulting quality of castings, has therefore become a popular topic and was the driving force to conduct sufficient research and development of appropriate equipment to prevent certain sand related casting problems. What would it mean to foundries if the casting defects caused by poor sand qualities could be reduced or eliminated? What would it mean to have a reliable sand transfer system and reduced maintenance costs? Continued on next page DEFECT PREVENTION