Level Measurement & Control


Laser level measurement in plastics applications

December 2014 Level Measurement & Control

Some customers have tried to use ultrasonic level transmitters, mechanical ‘yo-yo’s’ and load cells with limited success when measuring plastic pellets or flakes. Radar level sensors are also not suitable, because the dielectric constant of the material is too low.

Storage bins that are relatively small in diameter (3-10 metres) and tall in height present a problem for ultrasonic level transmitters, because the wide angle beam echoes off of the silo walls and gives false signals. Additionally, the material fills and empties with a significant angle of repose and the ultrasonic energy can be reflected away, instead of back to the receiver.

Mechanical ‘yo-yo’ devices have high maintenance requirements and many users have a problem with the plastic pellets covering the weight at the end of the cable and breaking it on retraction. In fact, customers can get an acceptable payback in maintenance savings alone by replacing the ‘yo-yo’ devices. Load cells have a very high first cost and high maintenance/calibration costs. Nucleonic level sensors can provide a reliable level reading, but these tend to be expensive and contain an undesirable gamma radiation source.

The LM80 provides superior performance at a competitive price because it has a narrow beam divergence (no false echoes off of tank wall) and will measure off of the steep angle of repose.

Recommendations when installing the LM80 on plastics silos include:

• Use the P801 dust tube option. Purging with instrument air is not generally required, but on about 5% of applications the static build-up is so bad that the dust tubes must be purged to keep dust from collecting on the lens. This problem seems to be most prevalent when the LM80 is located close to the fill chute.

• Use a metal mounting plate or flange and make sure it is grounded to the silo. This is usually accomplished by proper bolting and helps dissipate the static charge build-up.

• Use lightning protection: the lasers are typically installed on the top of a tall silo out in the open and can act as a lightning rod. Contact the service department for recommended protection.

• Use the Heavy Dust setting to ignore dust and fines that are suspended in the silo and to ignore feed falling in front of the laser.

The LM80 was successfully demonstrated at one plastic manufacturer’s site where it was purchased for an extended trial. There are over 200 silos at this site alone – plastics are a major potential market for laser level measurement devices.

For more information contact Jacolize Goosen, ABB South Africa, +27 (0)10 202 5000, jacolize.goosen@za.abb.com, www.abb.com/za



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