Level Measurement & Control

Reliable level measurement for heavy-duty bulk material applications

August 2003 Level Measurement & Control

Despite advances made in new technology and measuring techniques, permanent level measurement of bulk materials continues to rely on knowledge and long-standing experience - in fact there is still no universal trouble-free measuring procedure that can be applied to any type of bulk material. The time-honoured technique of using depth sounding to determine the level continues to be useful and still offers a great number of advantages. For this reason, UWT of Germany has reworked the traditional lead-system to make it compatible with up-to-date microprocessor technology.

Depth sounding level measurement involves hanging a sensor weight from a stainless steel rope or tape and lowering it into the silo. As soon as the weight makes contact with the bulk product, it returns to its upper limit stop and a measurement is taken of the distance it has traversed, which is then expressed in terms of silo level via an analog 4 to 20 mA signal. There are a number of significant advantages associated with this measuring procedure:

* Measurement is carried out independently of the inductive capacity and conductivity of the bulk product - ie, fluctuating levels of humidity in the product, for example, have no effect on the accuracy of readings. Furthermore, the level of products with very low relative permittivity (eg, plastic in powder form) can be determined without any difficulty.

* Condensation build-up inside the silo is not a critical factor.

* Dust, which can accumulate in considerable quantities during filling, does not affect readings.

* The weighted probe only comes into contact with the surface of the bulk product, so the silo roof is never subjected to any excess strain from the cables or other lifting gear hanging from it.

* The measuring process is easily broken down into stages, so the user can trace each step of the procedure on-site, thus simplifying fine-scale diagnosis.

* Measurement is highly accurate. An unequivocal reading is always available, corresponding to the point of contact between the weighted probe and the surface of the product.

The UWT-LOT SLS 3000 lead-system

The standard configuration of the unit is available with either a rope or tape - both in stainless steel. The rope-version is suitable for a wide range of applications (eg, for plastic granules). The tape configuration is more suited to applications involving adhesive, sticky and abrasive products, and for when there is a build-up of product residue at the top of the silo. The smooth surface of the tape prevents the penetration of abrasive bulk products, such as powdered cement, and thus avoids excessive wear. High levels of condensation under the silo roof, coupled with the presence of dust during filling, can lead to a build-up of sticky residue that clogs up the measuring probe. In order to ensure that the weighted probe measures the level correctly, it drops from the upper limit stop both under its own weight and assisted by a spring. A scraper is used, as an additional probe clogging prevention measure, to keep the tape clear of product residue.

The UWT-LOT SLS 3000 depth sounding system from Kübler Africa
The UWT-LOT SLS 3000 depth sounding system from Kübler Africa

The measuring device is easy to set up. The maximum traversing length of the sensor weight and the silo dimensions used to determine the 0/4-20 mA current signals are all entered via a user interface, with a display. It is not necessary to calibrate the unit according to the difference between a full and empty silo or to carry out adjustments to adapt the system to the bulk product being handled. Once the supplier has calibrated the device, it normally needs no further adjustment after being commissioned.

The measurement reading is emitted either as a 0/4 to 20 mA current output or as a metered pulse. Use of a submenu allows configuration, as required, of current output, fault procedures and other items. A built-in timer function allows measuring to be programmed to start automatically and independently. An integrated microprocessor monitors the measuring process and reports any anomalies, thus ensuring that readings are kept as accurate as possible. In the event of a fault occurring, an error message is activated and current output is set to its pre-adjusted value. The type of fault is shown on the display.

The monitoring system measures the length of the rope, or tape, as it is lowered -and this length has to correspond with the measurement taken when it is wound back in. This ensures that measuring is carried out correctly and that the sensor weight does not remain hanging in the silo. It also works as a means of verifying:

* That ropes or tapes are free of fraying or tearing

* Correct release of the sensor weight at the beginning of the measuring process.

* The condition of the internal components of the device, such as the motor, plug connections and pulse transmitter.

If an anomaly is detected during the measuring process, the device will independently attempt to use software-based procedures to return the sensor weight to its upper limit stop. This is in order to minimise the danger posed by a weight remaining suspended inside the silo.

Long-term testing has determined a service life of around 60 000 measuring cycles for the rope-mounted unit and approximately 90 000 cycles for the tape-mounted device, although very few applications actually ever involve this amount of use. In order to help ensure the unit is correctly maintained, the UWT LOT displays a service warning after 30 000 measuring cycles (tape version) or 40 000 cycles (tape configuration). Approval for use in Zone 20/21 dust-explosion areas (ATEX II 1/2D) has been achieved for both the rope and the tape versions.

For more information contact Johan Steyn, managing director, Kübler Africa, 011 794 9220, johans@kubler.co.za


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