Wood Chip Bin level applications in plants like that shown in Figure 1 have historically been implemented with Ultrasonic or Yo-Yo type level measurements. The low dielectric of the chips virtually eliminates radar level sensors.
Figure 1. Wood chip bin level applications
The issues with Yo-Yo’s in Wood Chip Bin applications include the following:
- Frequent movement in the “Plumb Bob” resulting in increased maintenance
- Product build-up on the Yo-Yo causing the cable to break
- Chips are fed into the bins from conveyors, and through chutes. Falling chips often interfere with the Yo-Yo’s measurement
- The shape of the chip piles makes it difficult to obtain a good measurement. While loading the bins the chip pile is convex (peak up) and becomes concave (peak down) during the emptying or transfer process.
Issues with ultrasonic level transmitters in Wood Chip Bin applications:
- Chips are fed into the bins from conveyors and through chutes. Falling chips often interfere with the ultrasonic measurement.
- The shape of the chip piles made it difficult to get a good measurement because of peak up and peak down conditions.
- Some of the bins are “bumped” with steam to insure a flow of chips, this can cause problems with the ultrasonic level transmitters.
The solution is an ABB LM80 Laser level transmitter laser pointer as shown in Figure 2.
Figure 2. LM80 laser level transmitter
The pointer enables the customer to choose the exact point where the level is to be measured. The concave/convex problem can be measured to eliminate false levels that are prevalent with ultrasonic level transmitters and Yo-Yo’s. The very bottom of the bin can be targeted enabling a reliable “Bin Empty” alarm during shutdowns. There are no moving parts and the bin shape does not have any impact on laser measurement.
ABB LM80 Lasers level transmitters have been successfully applied to the measurement of Wood Chip levels. Commonly a span from 1 to 3 m (2 to 10 ft) is used. The feeder is started and stopped on a high and low signal generated in the controls system.
Another benefit of the LM80 is the ability to locate and mount the units in locations easy to maintain (off to the side and at a distance from the top of the bin). This enables easy access in the event that adjustments to spans, outputs or checking of target and loop outputs are required or desired. The LM80 has been tested in both the surge and the main and bins.
Customers often put the first units on Surge Bins where maintaining the level in most critical. Wood chip bins can be found at Pulp and Paper facilities, wood panel manufacturers and power plants that have been converted to burn Biomass instead of coal.
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This information has been sourced, reviewed and adapted from materials provided by ABB Analytical Measurements.
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