Automatic Refills During Material Feeds Increase Plant Efficiency

Recently, I was having a conversation with a customer that was seeking to improve cycle times on his process without sacrificing the quality of his product (a mixture of powder materials). It was a continuous material feed process that had to be stopped every so often while the feeder vessels were refilled.

I replied that an intelligent rate controller connected to a gravimetric feeder can be set up to automatically refill (gain-in-weight) while simultaneously delivering a material feed at the desired rate (loss-in-weight). In other words, the feeders do not have to stop while being refilled, which increases cycle times and reduces the overall cost of operation. It also eliminates the manual process of checking numerous feeder vessels for material levels.

The customer replied that he was having a tough time conceptualizing how that could work--feeding and refilling at the same time--so I drew him a diagram and provided a brief explanation.

Diagram 1: Automatic Refills During Material Feeds

If configured during the instrument set-up, the process starts with an initial refill by checking to see if the weight is below the high weight setpoint. If it is, the instrument will fill the hopper until the high weight setpoint is reached before switching over to gravimetric rate control mode.

Just after going into rate control mode and just before reaching the low shutoff weight to refill, the instrument “learns” the characteristics of the material flow when the vessel is full and when the vessel is nearly empty. This is necessary as a full vessel has more material pressure on the screw or auger than an empty vessel, which will increase the material flow rate at a given analog output level. For example, when the vessel is full the analog output signal might be 10mA to feed at a rate of 100 lb/min. However, when the vessel is nearly empty that analog output signal might need to be 12mA to provide the same feed rate of 100 lb/min. This is because a full vessel will “pack” more material into the feed screw than an empty one.

During refill mode, the instrument switches from gravimetric control to volumetric control to allow the filling or dispensing process to continue while simultaneously refilling the vessel. The instrument monitors and calculates material feed characteristics while in gravimetric mode (recorded as output percentage adjustment—and written as OP%/wt) then applies the ‘learned’ characteristics during the volumetric mode to closely replicate the rate control process. This allows the instrument to deliver continuous and accurate rate control even while in a refill cycle.

Requirements to Set Up an Automatic Refill Process

After setting up and calibrating the scale, these parameters and the functions are required for a controlled feed process with auto refill:

a. Auto Refill: This mode can be set on or off; for a truly uninterrupted continuous process, it must be on.

b. Initial Refill: This mode can be on or off according to user preference because it is used only once on process start-up.

c. Start Low: Weight that triggers the start of refill cycle.

d. Stop High: Weight that triggers the end of a refill cycle.

e. Low Alarm: Weight that stops the process when there is insufficient or no material in the feed vessel.

f. High Alarm: Weight that triggers a stop to the refill process to prevent overfill.

g. Output Percentage Adjust: Adjustments made to the analog output signal to compensate for changes to material flow as a function of the amount of weight in the feeder vessel. This value is measured and calculated during “learn mode” during the gravimetric feed cycle.

h. Target Rate: The desired feed rate for the material being fed into the process.

i. Rate Tolerance Alarm: Not shown in the graph, the rate exception parameter is a tolerance band set around the target rate. It is triggered when the desired rate is out of tolerance for a specified amount of time. This can be triggered by events such as bridging of material in the feed vessel or failure of feeding mechanisms.

Other Helpful Terms and Concepts

j. Gravimetric feed control measures the weight of the material being fed into a process. Weight is highly accurate and consistent, even with variations in material density. Weight also compensates for variations in material bulk density and flow characteristics.

k. Volumetric feed control measures the volume of material being fed into a process and is not capable of compensating for variations in material bulk density and flow characteristics unless learned from a gravimetric feed process.

Here is the same diagram with the values shown that are required to set up a robust automatic refill process (values may be in either metric or imperial units and in seconds, minutes, or hours).

Diagram 2: Automatic Fill Process with Required Values

Conclusion

By using an instrument that can learn the characteristics of the material feed rates at different feeder vessel fill levels – simultaneous rate control and automatic refill may be performed to increase throughput of the process. This method also has the benefits of improving product consistency, eliminating human monitoring and process intervention, and alarming or stopping the process the moment a failure occurs.

Tim Norman is senior product development manager for Hardy Process Solutions (an Indicor Co.). He has more than 25 years of experience identifying and deploying disruptive technologies in product design and industrial automation around the globe, ranging from RFID manufacturing to aerospace composite design to process control and packaging. For more information, call 858-278-2900 or visit www.hardysolutions.com.