How to fix mechanical problems with your loss-in-weight feeder – Part II
Steve Musser Merrick Industries
A loss-in-weight feeder can accurately meter powders, granules, and pellets in several industries, including foods, chemicals, pharmaceuticals, and plastics. But if the feeder hasn’t been installed correctly, mechanical problems can create slight upsets or artificial forces that disturb the weight readings and affect the feedrate. In this two-part article, Part I (which appeared in the December 1998 issue) explains how a loss-in-weight feeder operates, how to recognize symptoms of mechanical problems, and how to ran feeder tests. Part H explains how to find and fix mechanical problems with the feeder.
How to find and fix mechanical problems
If your loss-in-weight (LIW) feeder fails the weight stability, repeatability, or linearity test, you need to diagnose the underlying mechanical problem (or problems) affecting the feeder’s operation. Very often, more than one equipment component can be at fault. And two or three small mechanical problems can combine to produce what looks like one large problem.
Most of these problems stem from a poor installation that fails to isolate components in the feeder’s active section from external forces. Poor mounting, poor connections to other components, and exposure to external influences such as air currents and vibration are all installation mistakes that can lead to mechanical problems. Even the most advanced control algorithms can’t control a LIW feeder when components in the feeder’s active section aren’t isolated from such disturbances.
The following information explains what can cause mechanical problems and how you can fix them. Installation related problems include those with the connections, the weighing mechanism, air currents, the refill device, and structural mounting. Noninstallation-related problems caused by single-event disturbances are also discussed. For help diagnosing the problems, use the mechanical troubleshooting checklist in Figure 1.
If your feeder has a nonpassive connection between a component in the active section and an inactive component external to the feeder – such as from the feeder’s discharge device outlet to the inlet of a process vessel – the connection can transmit artificial forces from the inactive component to the component in the active section. This interferes with accurate weighing of material in the hopper and can impair the feeder’s accuracy and repeatability. A common example, as shown in Figure 2a, is when the feeder has a screw discharge device with a round outlet spout that is inserted into a square inlet chute or box on downstream equipment; the round spout can rub against the square inlet and disturb weighing.
To avoid this problem, any connection between a LIW feeder component in the active section and an inactive component should be passive -that is, the connection shouldn’t transmit force from the inactive component to the component in the active section. The connection also shouldn’t influence the weight being measured by the feeder’s weighing mechanism.
Typical LIW feeder connections include flexible infeed, discharge device, and air vent connections; slipjoints; and wiring between various components.
Infeed connection. If your LIW feeder is automatically refilled, the refill device discharge is typically linked by a flexible connection (made of supple rather than rigid or stiff material) to the infeed spout at the feeder hopper’s top. The flexible connection provides a dust-tight seal to prevent dust from escaping into the atmosphere. If the connection is correctly installed, the gap between the refill device discharge and the hopper’s infeed spout will be too small to allow the flexible connection to bow and trap material, but not so small that the connection can’t flex
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properly. If this connection is pulled tight across the gap to the refill device, the weighing mechanism will sense a negative weight. Any movement or vibration in the refill device will also cause a disturbance in the weighing mechanism’s sensed weight.
Discharge device connection. A flexible, dust-tight connection can also be located between the discharge device outlet and the inlet of downstream equipment. If this flexible connection is stretched tight across the gap to the downstream equipment inlet, the weighing mechanism will see a positive weight. And any movement or vibration in the downstream equipment will cause a disturbance in the weighing mechanism’s sensed weight.
Air vent connection. The air vent (or port), typically at the hopper top, handles air movement inside the feeder. A flexible, dust-tight connection between the vent and the dust collector can cause the same weight disturbances created by an improperly installed infeed or discharge device connection. But this connection is also subject to a unique problem: When the dust collector has a negative air pressure, it can draw air from the hopper and cause the weighing mechanism to sense a negative weight. To prevent this problem, be sure enough air is supplied to the dust collector. You can also use an air-permeable material such as woven fabric in the flexible connection.
Slipjoint. Another common way to seal the infeed spout to the refill device outlet or the discharge device outlet to the downstream equipment inlet is to use a slip joint, which is a rubber gasket fastened around the gap between the two components, as shown in Figure
2. The slip joint can be horizontal (Figure 2a) orvertical (Figure 2b). In an incorrectly installed slip joint, the gap between the two components isn’t large enough or uniform around the spout or the rubber gasket is overly thick, which creates a nonpassive connection.
To provide a passive connection, use a larger gap and longer, thinner gasket. Also make sure the active section’s component (the infeed spout or discharge device outlet) doesn’t touch the inactive component, which prevents disturbances to the weight reading.
Wiring. A few components in the feeder’s active section require wiring that attaches to a component in the inactive section. An example is the wiring from the discharge device motor (active) to the motor speed controller (inactive).
Incorrectly installed wiring is often longer than necessary or hangs in a high-traffic area. Any touch
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or pull on the wiring can cause a weighing disturbance. A poor installation can also produce wiring that is stretched tight, tied to other components, or stiff rather than supple. To provide a passive connection and prevent weighing disturbances, the wiring should be mounted with some, but not excessive, slack.
Finding and fixing poor connections. To determine if a connection between components in the active and inactive sections is disturbing the LIW feeder’s weight readings, temporarily disconnect all infeed, discharge, and air vent (both flexible and slip joint) connections to the feeder. Also disconnect any wiring attached to the feeder’s weighing mechanism. If the weight readings are stable with these items disconnected, reattach each connection one at a time and ensure each provides a passive connection. Throughout this process, check that the weight reading remains stable and at the same value, which indicates each connection is now correctly installed.
Poor mounting, poor connections to other components, and exposure to external influences such as air currents and vibration are all installation mistakes that can lead to mechanical problems.
Weighing mechanism problems
After the LIW feeder has been operating for some time, wear and tear can cause the weighing mechanism to perform less accurately than when it was new. If the weighing mechanism uses beatings, they can develop a sticky or flat spot that can affect weighing accuracy. If the weighing mechanism uses levers or pivots, wear at their connection points can produce nonrepeatable or nonlinear weight readings. If the weighing mechanism is mounted improperly for instance, is twisted or misaligned after replacement
-artificial forces unrelated to the actual weight canaffect the weight reading.
Regularly inspect any moving parts on the weighing mechanism for wear and a tendency to resist movement or change. Hysteresis (that is, stiction, a delay in registering a weight change) can develop in a weighing mechanism with worn mechanical parts. After installing a new weighing mechanism on the LIW feeder, run tests just to confirm the mechanism’s accuracy. After such a replacement, be careful not to leave out any small shims or plates required to correctly align the weighing mechanism. Also ensure that any flexures (metal supports) in the weighing mechanism are flat, have no ripples or bends, and are in the correct plane for accurate weighing.
Air current problems
Air currents inside and outside the LIW feeder are often overlooked in the feeder’s installation. Various equipment in your plant can cause air to circulate outside the feeder and onto the feeder’s surfaces. Some air sources, such as cooling fans, dryers, exhaust ports, and open ducts, can cause huge weighing disturbances. You can correct these problems by directing the air sources away from the feeder or by installing a barrier between each air source and the feeder, thus preventing the moving air from impacting the feeder’s surfaces.
Also pay attention to the airflow within the feeder. During refilling, a large amount of air is displaced from the hopper. Conversely, air is drawn into the hopper as material flows out of the feeder through the discharge device. In a sealed, dust-tight feeder, the air moving into the feeder must be balanced with that moving out.
If the feeder was incorrectly installed, there may be no way to equalize this airflow. A common means of equalizing the airflow when the feeder is installed is to vent the air back to the refill hopper above the refill device, which creates a closed air system, as shown in Figure 3. You can also equalize airflow by placing a vent (or port) in the hopper’s cover and installing an air-permeable fabric sock over it. This allows airflow in both directions while preventing material from escaping into the surrounding air. Regularly clean the sock to prevent it from becoming blocked with material. And, as mentioned previously, you can use a flexible connection between the vent in the hopper cover and a dust collector to equalize airflow.
If you’re concerned that your dust collector is causing air movement that disturbs the LIW feeder’s weighing mechanism, monitor the controller’s weight readings for the same material level in the hopper with the collector on and then with the unit off. If the weight readings are measurably different, either alter the airflow in the feeder (such as by slowing or stopping the airflow into the feeder), install air-permeable flexible connections between the air vent and dust collector, or use a lower negative pressure in the dust collector to help draw air through the hopper.
Refill device problems
The automatic refill device on the LIW feeder can create some unpredictable conditions. The most effective refill device has a positive cutoff so that when refilling is stopped, no more material can fall into the feeder’s hopper. A slide gate is especially well suited to the LIW feeder because it opens and closes quickly and can’t leak material when closed.
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Less effective refill devices include vibratory pans and rotary valves. When a vibratory pan is turned off, a pile of material can rest on the very end of the pan. If the pan is impacted or disturbed, the material can suddenly avalanche into the feeder’s hopper and create a major weighing disturbance. And sometimes a vibratory pan can be mistuned, so that even when the unit is turned off, the pan vibrates enough to sprinkle material into the hopper.
A rotary valve rotates when refilling the hopper and stops when the hopper is full. Sometimes material can pack into the rotor pockets and then suddenly fall into the hopper during feeding, disturbing the feeder’s weighing accuracy.
A down spout connecting the refill device discharge to the hopper infeed spout can also create weighing disturbances. The down spout should have the correct diameter and align with the infeed spout. If the down spout’s angle is too shallow, material can build up in the spout. Then when the refill device is turned off and the feeder is operating normally, vibration can cause the buildup to fall into the feeder, creating a weighing disturbance.
Structural mounting problems
If your LIW feeder is installed on a floor or other structure subject to high levels of vibration or displacement, you may need to retrofit shock (vibration isolation) mounts on the feeder or the structure it’s placed on, or both. Or, if you determine that one piece of equipment is transmitting vibration to the feeder, you can retrofit that equipment with shock mounts. Another option is to reinforce the structure supporting the feeder to minimize vibration. If you can’t make the structure more stable, you may be able to suspend the feeder from overhead supports.
Occasionally an event that has nothing to do with how the LIW feeder was installed causes a weighing disturbance. The event is typically short-lived and unpredictable, and by the time the disturbance has been detected the event’s cause typically can’t be tracked. Thus linearity and repeatability tests won’t help you diagnose these problems.
When the dust collector has a negative air pressure, it can draw air from the hopper and cause the weighing mechanism to sense a negative weight.
For instance, when a worker places a tool or other object on the LIW feeder, the weighing mechanism senses an almost instantaneous weight gain rather than the expected loss. As a result, the feeder speeds up to compensate for the weight gain. And when the worker removes the object from the feeder, the instantaneous weight change is negative. Theoretically, this will cause the feeder to stop feeding because it senses a weight loss greater than required to achieve the correct feedrate.
The feeder can also be disturbed when something bumps into it or contacts its wiring. For instance, heavy equipment moving near the feeder can contact the feeder or wiring or create vibration that disturbs weighing. A forklift can disrupt weighing simply by driving on a poorly supported floor near the feeder or by dropping a heavy load near the feeder.
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Single-event disturbances are common when the LIW feeder has replaced a volumetric feeder. This is because operators and maintenance workers have gotten used to touching the volumetric device, which can feed accurately despite such contact. But touching a LIW feeder can be disastrous. To reduce or prevent this problem, train your workers to properly operate and maintain the feeder.
Some LIW feeder controllers use advanced algorithms to compensate for a single-event disturbance, but be aware that during the disturbance the feeding is typically less accurate than normal. You can use a LIW feeder equipped with a supervisory control system to help track down single-event disturbances. This control system can record weighing data over a long period, thus providing information about disturbances that aren’t constantly present but can make the feedrate unstable.
An ounce of prevention
You can avoid LIW feeder problems by installing the unit correctly and then periodically checking the feeder’s operation.
Before installing the feeder, plan ahead: Consider how you’ll mount the feeder, how you’ll connect it to other components, and what air currents and vibration in your plant are likely to affect the feeder’s weighing accuracy.
After the feeder is installed, train your workers to operate and maintain the feeder properly and to avoid actions that can disturb the weight readings. Also periodically check the feeder, especially after maintenance, to ensure no physical changes have degraded feeder performance. Use the tests outlined in this article (in the “How to run feeder tests” section in Part I) to detect common installation problems.
Steve Musser is manager of the Process Industries division of Merrick Industries, 10 Arthur Drive, Lynn Haven, FL 32444; 850/265-3611, fax 850/265-9768 (e-mail: email@example.com). He holds a BS in computer science from Cameron University in Lawton, Okla.