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Conveying controls can range in diagnostic capability from a typical PLC with a “No-load” alarm for each station (left) to more elaborate systems that provide complete troubleshooting information on-screen (right).
The best way to avoid materials-handling problems is to design and install the system properly in the beginning. Keep in mind some basic principles:
1 horizontal ft = 1 equivalent ft
1 vertical ft = 2 equivalent ft
1 90° elbow = 20 equivalent ft
1 ft of flex hose = 3 equivalent ft
An average railcar is 66 ft long
If a problem crops up when first starting up a material-handling system, it can be solved by answering the following questions:
When a working system develops a problem, it is typically that one or more stations suddenly do not fill properly. One’s first instinct may be to lengthen the load time. That will not solve the problem. Instead, you need to find out what has changed in the system.
One of the most common problems in pneumatic conveying systems is failure of four-way solenoid control valves. This is almost always caused by foreign particles from the compressed-air lines and usually occurs during installation and start-up. The four-way solenoid that controls station valves requires clean, dry, oil-free air. A compressed-air filter/separator can virtually eliminate such problems with these valves.
Pellet materials require sufficient air to be transported. Insufficient air will cause material to alternately surge and hesitate in the lines, reducing throughput. The cure is to gradually increase the air in the lines by adjusting the air mixing valve at the material source, and the throughput will usually increase.
Clogged filters in vacuum chambers, vacuum takeoff boxes, and vacuum pumps are the most frequent cause of reduced throughput. Inspect and clean them regularly and don’t forget to empty the catch pans under cyclone filters. If the pans fill up, dust can be pulled back through the system to clog the filters.
A loss of vacuum will also cause throughput to drop. The gauge at the vacuum pump should read 5 to 9 in. of Hg while the system is running. A lower reading indicates a vacuum leak.
To check vacuum in the system:
This problem is typically related to a lack of transport air. The surging described above can pack material into the lines. With virgin pellets, simply remove the material line from the material source and the plug should break up. If this works, slightly increase the air flow by adjusting the air mixing valve to avoid a recurrence.
Regrind and even virgin material can pack in the lines hard enough to require manual removal. The plug has to be isolated and each end of the affected pipe must be opened and the plug forced out with a rod. Sufficient transport air is the best preventative.
If material is not reaching vacuum chambers beyond a particular station in a common-line system, check for an overfilled vacuum chamber. A reverse-flow check valve is typically located inside the vacuum chamber at the material inlet. Filling the vacuum chamber above this check valve will cause it to be held open when another chamber attempts to load from the shared line. The resulting vacuum loss will prevent material conveyance to all other chambers on the shared line. Chambers should be filled nearly to the level of the check valve but not above it. Decrease the load time; or, if more material needs to be loaded in a single shot, add a spacer below the check valve or switch to a larger chamber.
Common-line systems should not be used with JIT (Just-in-Time) loaders. The problem is that some material tends to miss the turn at a lateral (or “Y”) pipe like a driver missing a highway exit. When the vacuum ceases down this leg, this “bypass material” will come to rest just past the “Y.” This creates a problem for JIT systems because the bypass material can be more than the shot size (often 1 lb or less), causing the station to repeatedly call for material. A station farther down the line may clear the bypass material, overfilling its small JIT chamber and holding the check valve open.
If this condition becomes chronic, install a diverter valve in place of the standard lateral pipe. The diverter automatically forces material to go to its assigned station. Better yet, use dedicated material lines for JIT applications to pre-empt this problem.
A common line can usually be used with larger vacuum chambers because when enough material misses the turn, the remaining material will be forced to make the turn at the lateral. When the bypass material does clear, it usually won’t overfill a larger chamber.
About the Authors: Tom Spangler is service manager at Novatec, Inc. in Baltimore. Les Mischaud is installation manager, and John Kraft is a marketing specialist.