Reliable continuous and point level measurement under actual process conditions
PVC powder is one of the most challenging bulk solids in the chemical industry. Fine, highly dusty, electrostatically charged, and with a low dielectric constant (DK value) of 1.6 to 2.0, it demands high-performance level measurement technology.
In this case study, a plant operator from Eastern Europe sought a reliable solution for continuous and point level measurement in an existing plant. The solution needed to consistently meet ATEX requirements and withstand local environmental influences over the long term.
Especially during winter, severe frost, ice, and snow are common. Under these conditions, sensors must operate accurately, be mechanically durable, and withstand extreme temperatures. Maintenance work, particularly on the bridge above the silos, also needed to be minimised for safety reasons.
Reliable, low-maintenance measurement solutions were therefore necessary, even in challenging environmental and process conditions. To find the best fit for this application, a multi-stage test was conducted in close collaboration with the operator.
Typical challenges when handling PVC powder
Before examining the individual measurement tasks, it is worthwhile to consider the overall boundary conditions of this application:
- a very fine, occasionally fluidized bulk solid with a low dielectric constant (DK value)
- heavy dust generation in tall, narrow silo
- material build-up and electrostatic charging
- ATEX Zone 20 and 21
- year-round operation with silo roof installation, accounting for temperature fluctuations, frost, and snow
The way these challenges can be successfully tackled in practice is demonstrated by two specific measurement tasks within the same application.
Measurement Task 1
Continuous level measurement in PVC powder
Initial situation: In 22-metre-high steel silos, radar sensors from another manufacturer had previously been employed. In practice, however, they produced fluctuating and unreliable measurement results. The likely cause was the very low dielectric constant (DK value) of the PVC powder, which, combined with the dusty atmosphere, resulted in insufficient radar reflection.
Therefore, a maintenance-free sensor was needed to deliver stable and reliable measurement results even under these conditions.
Individually configured rotary paddle level switch and electromechanical lot system ensure reliable level measurement and overfill protection
The electromechanical lot system is used for continuous level measurement in storage silos containing a wide variety of bulk solids. Its simple and robust measuring principle has proven highly reliable in practice, especially for solids with a low dielectric constant (DK value).
A sensing weight is lowered on a belt to the material surface and then returned to its upper end position. The measured belt length indicates the level and is output as an analog signal. Modbus and Profibus interfaces are also available.
A key factor for this application is the integrated belt cleaner. After each measurement, it effectively removes PVC dust from the measuring belt. This prevents dust from entering the mechanical chamber, thereby largely avoiding contamination.
Although the mechanical and electronic chambers of the NivoBob® are hermetically separated, the belt unit leading into the mechanical chamber is not hermetically sealed by design. Due to the highly fluidized material and the overpressure that may occur under certain process conditions during filling, an existing compressed air connection was utilised. Occasional air purging of the mechanical chamber provides additional protection and ensures long-term, reliable operation.
Additional safety is ensured by the integrated heater, which protects the housing chambers from moisture caused by condensation and risk of frost.
Result: Stable and reproducible level values without maintenance effort, even in dusty atmospheres, with material build-up and bulk solids with low DK values.
Why the NB 3200 excels in this application:
- reliable measurement in dusty atmospheres, with material build-up and low dielectric constant (DK value)
- belt version for measuring ranges up to 40 m (≈ 131 ft)
- ATEX II approval for Zone 20/21
- DN100 PN16 flange – fast replacement without modification of the plant
- PVC sensing weight with spikes, particularly suitable for fine bulk solids
- integrated housing heater down to −40 °C (−40 °F)
- version designed for increased process overpressure
NivoBob® NB 3200 with belt version
Measurement Task 2
Point level detection / overfill protection
Initial situation
For overfill protection, the operator had previously used vibrating fork level switches from another manufacturer. However, during daily operation a recurring problem became apparent:
- fine PVC powder often leads to substantial material build-up
- poor flow characteristics
- monthly cleaning intervals, as blocked vibrating forks no longer provided reliable signals
- cleaning was especially time-consuming because of the sensor’s position on the silo bridge
- maintenance during winter was further complicated by cold temperatures, snow and ice
A point level sensor was therefore required that would function reliably even with material build-up – and eliminate the need for regular cleaning.
The rotary paddle level switch RN 3002 with extension tube is mainly used as a full detector in storage and process vessels. Its measuring principle is unaffected by dust formation and material build-up, making it ideally suited for use with PVC powder.
To achieve optimal alignment of the rotary paddle level switch, an extra sliding sleeve was used during the test phase to adjust the mounting height and establish the ideal installation depth. This enabled the extension tube to be customised to the silo's geometry, ensuring the switching point is precisely positioned where required.
Thanks to the adjustable sensitivity of the rotary paddle, the sensor could be precisely matched to the material properties of the challenging PVC powder in the area of the material cone, thereby reliably preventing overfilling.
The rotary paddle and the extension tube are made of stainless steel, providing long-term protection against the abrasive properties of PVC powder. Even after extended use, the sensor remains mechanically stable and reliable.
Why the RN 3002 is superior in this application
In contrast to vibrating forks, the rotary paddle stays fully operational even when material sticks to the paddle. Blockages are avoided, eliminating the need for regular cleaning. Additionally, the durable stainless steel design prevents the abrasive effects of the PVC powder from affecting the sensor’s performance or lifespan.
Rotonivo® RN 3002 with individually adjustable extension tube up to 4000 mm for precise switching point definition
Key features of the individually configured rotary paddle level switch:
- double-sided folding paddle for restricted process connections and materials with low bulk density
- vertical extension tube and support arm constructed from stainless steel
- extension length 2800 mm (≈ 110 in) – up to 4000 mm (≈ 157 in / 13.1 ft) possible
- additional bearing at the tube end for enhanced mechanical stability
- ATEX approval for Zone 20 / 21
- process overpressure up to 5 bar (≈ 73 psi)
- G 1 ½" stainless steel threaded connection – for easy retrofit in existing plants
- integrated housing heater down to −40 °C (−40 °F)
Functional features:
- adjustable sensitivity to match material properties
- adjustable signal output delay
- selectable switching logic (Fail Safe High / Fail Safe Low)
- height adjustment for variable installation depth
- rotation monitoring
Result: Reliable overfill protection that does not require monthly cleaning or risky maintenance during winter operation.
„Throughout the entire test phase – and beyond – the sensors measured with a very high level of accuracy. The measurement values were closely monitored during filling and emptying and manually verified several times. All measurements matched exactly. Issues we had experienced in the past no longer occurred. For this application, these are the best devices we could use," the plant operator confirmed.
Conclusion:
PVC powder requires measurement technology that remains stable even under actual process and environmental conditions. By using the NivoBob® NB 3200 for continuous level measurement and the Rotonivo® RN 3002 for point level detection, the operator depends on proven, robust solutions that reliably manage dust, material build-up, frost, and ATEX requirements.
The result: stable process operations, high confidence in the measurement values and significant time savings due to the elimination of regular maintenance work.
At UWT, we develop measurement technology that proves itself not only under ideal conditions – but also in everyday operation.
UWT. LEVEL. UP TO THE MAX.
Silo bridge with filling and venting equipment and level monitoring sensors
Discover More Applications in the Chemical Industry
Whether for the storage and processing of powders, granulates, liquids, or aggressive chemicals – UWT offers robust and safe sensor solutions for a wide range of chemical industry processes.
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