The 2:00 AM Call You Don’t Want
You know the scenario. The silo level indicator has been reading half-full for the past three hours, but the downstream process just ran dry. Or worse—the high-level alarm never triggered, and now silica dust is blowing out the vent filter, coating everything in a fine white layer that takes days to clean up.
In insulation board manufacturing, this isn’t just an inconvenience. When a 25‑meter silo of highly dispersed silica goes off‑spec or runs empty without warning, the entire production line stops. And in a plant running 24/7, that stop costs real money—not just in lost output, but in labor hours spent cleaning, recalibrating false‑reading sensors, and explaining to the shift supervisor why inventory data doesn’t match what’s actually in the silo.
Silica (silicon dioxide) is used across industries—paints, plastics, food, pharmaceuticals, and modern insulation materials. In chemical process engineering, this fine‑grained, often fluidized powder places extreme demands on storage vessels, conveying systems, and level measurement technology. The problem is that most standard sensors were never designed for material that behaves more like a gas than a solid when it’s being conveyed.
What Makes Silica So Hard to Measure
Let’s be specific about what we’re dealing with. In a typical insulation board production application, highly dispersed silica (SiO₂) is pneumatically conveyed into stainless steel silos approximately 25 meters (82 feet) high. The process conditions alone tell you this isn’t a straightforward measurement:
| Parameter | Specification |
|---|---|
| Medium | Highly dispersed silica, pneumatically fluidized |
| Silo height | ~25 m / 82 ft, stainless steel |
| Bulk density | < 20 g/l (1.25 lb/ft³) |
| Process temperature | Ambient to +60°C |
| Hazardous area | ATEX Zone 21 |
| Special conditions | High dust load, low dielectric constant, conical pile formation |
Bulk density under 20 g/l is the first red flag. For context, water is 1,000 g/l. Most bulk solids are in the 500–800 g/l range. At 20 g/l, silica is so light that it offers almost no mechanical resistance to a sensor probe. Conventional vibrating fork sensors don’t detect it because there isn’t enough mass to dampen the vibration.
The fluidized state is the second challenge. During pneumatic conveying, silica is blown into the silo with air. The air supply aerates the ultra‑fine powder, resulting in extremely low bulk density and intense dust generation. In this state, the material provides minimal radar reflection and almost no mechanical resistance—the powder is literally floating in air, not forming a solid bed that a sensor can reliably detect.
The dust load is the third issue. High dust concentrations scatter radar signals and coat sensor surfaces. A lens antenna covered in fine silica dust will eventually stop reading altogether. And because dry, fine silica powder can form explosive dust atmospheres during pneumatic conveying, ATEX Zone 21 certification isn’t optional—it’s mandatory.
The conical pile adds geometric complexity. After filling, the material settles and, during pneumatically assisted discharge, forms a typical cone. This creates a non‑uniform surface that makes continuous level measurement difficult—and the cone itself increases the risk of bridging, where material arches over the outlet and stops flow entirely.
The Three Measurement Tasks Every Silo Needs
In the insulation board plant, each silo required three critical measurement functions:
- Continuous level measurement — for accurate inventory monitoring and production planning
- Point level detection — full and empty indication to control filling and discharge cycles
- Material flow monitoring — in the discharge pipe for early detection of blockages and bridging
No single sensor technology can solve all three reliably under these conditions. The solution requires a combination: radar for continuous level, specialized high‑sensitivity vibrating probes for point level, and additional vibrating sensors for flow monitoring.
Continuous Level Measurement: Why 80 GHz Radar Makes the Difference
For continuous level measurement, a modern 80 GHz radar level transmitter with a lens antenna is the proven choice. The key specification that makes this work: 80 GHz frequency combined with a 3° beam angle.
The 80 GHz Advantage
Lower‑frequency radar (e.g., 26 GHz) has a wider beam angle—typically around 10° with a comparable antenna size. In a 25‑meter silo, that wider beam hits the silo walls, internal structures, and the conical pile at multiple angles, creating false echoes that confuse the signal processing.
A 3° beam angle focuses the radar energy into a tight cone. At the bottom of a 25‑meter silo, that beam is only about 1.3 meters in diameter—narrow enough to avoid wall interference and precisely target the conical pile. This is critical when measuring a material that barely reflects radar signals in the first place.
Swivel Flange for Precise Aiming
A stainless steel swivel flange, adjustable up to ±10°, is not a luxury—it’s a necessity. The radar sensor must be physically aimed so that the beam captures the bulk cone and extends all the way down to the conical silo bottom. Without this adjustability, the beam could miss the cone entirely and read empty when the silo is half‑full.
Purge Air: Keeping the Lens Clean
An integrated purge‑air connection with a check valve serves two purposes: it keeps the lens antenna free from fine dust accumulation, and it prevents material from entering the purge line. During filling, when dust generation is at its peak, the purge air creates a positive pressure barrier that keeps silica particles away from the antenna. Without this feature, the lens would gradually coat with dust, attenuating the radar signal until the measurement fails completely.
Smart Commissioning with Alignment Assistance
Modern radar instruments offer smartphone‑based alignment assistants that allow visual adjustment and commissioning. For a sensor mounted 25 meters up on a silo roof, the ability to see what the radar is actually measuring—and to adjust the beam angle in real time—turns what could be a days‑long calibration exercise into a straightforward task.
Point Level Detection: When the Material Is Too Light to Feel
For full and empty detection, standard vibrating fork sensors fail. The material is simply too light—offering almost no mechanical resistance to dampen the vibration.
The solution lies in high‑sensitivity vibrating probes with enhanced surface area. These specialized designs are intended specifically for very light, pneumatically conveyed bulk solids such as powders or silica.
Full Detection with Fluidized Silica
A vibrating point sensor with extended paddles mounted beneath the silo roof can reliably detect fluidized silica. Because the material is suspended in air, a standard fork won’t sense it. But probes with additional paddles significantly increase the effective sensing surface area. This allows the sensor to detect the material even in a fluidized state with bulk densities far below 20 g/l—down to less than 5 g/l in some designs.
Empty Detection in the Conical Section
At the bottom of the silo, a slightly less sensitive but still extremely responsive vibrating fork with polished, extended tines is used as an empty detector. When the probe is exposed (silo empty), it vibrates at its natural frequency; when even a small amount of material covers the tines, the vibration is dampened. External mounting through a standard sleeve makes for simple, maintenance‑friendly installation.
Flow Monitoring in the Discharge Pipe
A third vibrating probe—with the same high‑sensitivity design—monitors the discharge line. This sensor ensures that material flow remains uninterrupted. It provides early warning of bridging or blockages before they cause a line stoppage, allowing operators to activate aeration or mechanical agitation to restore flow.
The ATEX Requirement: Non‑Negotiable
Because dry, fine silica powder can form explosive dust atmospheres during pneumatic conveying or injection, hazardous (Ex) areas are always present. Any instrument installed in these areas must carry ATEX Zone 21 (or equivalent IECEx) certification.
This isn’t a checkbox—it’s a fundamental safety requirement. Specifying non‑certified instrumentation in a Zone 21 area isn’t just a compliance failure; it’s a direct safety hazard that can lead to catastrophic dust explosions.
What This Looks Like in Practice
The combined measurement approach—80 GHz radar for continuous level, high‑sensitivity vibrating probes for point level and flow monitoring—delivers:
- Continuous radar measurement provides real‑time inventory data, even during filling when dust levels are highest and the material is fluidized.
- Vibrating full detectors prevent overfilling—critical when silica dust would otherwise blow out the vent filter and create a housekeeping and safety nightmare.
- Vibrating empty detectors prevent the silo from running dry and starving downstream processes.
- Flow monitors catch bridging and blockages before they stop production.
The result: stable, repeatable signals despite extreme dust, air‑entrainment, and low bulk density. Maintenance requirements drop significantly. Process reliability improves measurably. And operators gain confidence in the inventory data they use for production planning.
What This Means for Your Plant
If you’re handling ultra‑fine powders—silica, fumed silica, or any material with bulk density below 20 g/l—you’ve probably already discovered that standard off‑the‑shelf sensors do not work reliably. The combination of 80 GHz radar with a narrow beam and purged lens for continuous level, plus high‑sensitivity vibrating probes with extended sensing surfaces for point level and flow detection, is a proven approach that directly addresses the specific failure modes of these challenging materials.
Key Takeaways for Engineers
- Radar works when it’s 80 GHz with a narrow beam — the 3° beam angle and swivel flange are what make it effective in tall silos with conical piles.
- Purge air is essential — without it, dust coats the lens and the measurement fails within weeks.
- Vibrating sensors need specially designed probes — standard forks won’t detect material below 20 g/l; extended‑surface, high‑sensitivity probes are required.
- ATEX certification is not a formality — it is a non‑negotiable safety requirement that protects both plant and personnel.
- Combination is key — no single technology solves all three tasks reliably; a thoughtful mix of radar and vibration is the most robust solution.
At WIJAY Systems, we have worked with plants handling everything from silica to cement to food powders. We understand the frustration of unreliable level readings in tall silos—and we know how to engineer systems that provide dependable data day after day. If your current instrumentation is giving you false alarms, lost production, or maintenance headaches, our process engineers can review your layout and recommend a sensor strategy tailored to your specific material and process conditions.
Contact WIJAY Systems — Bulk material handling. Pneumatic conveying. Powder automation. Done right.
FAQ
Q: Why is silica so difficult to measure with standard level sensors?
A: Silica has extremely low bulk density (below 20 g/l) and is often fluidized during pneumatic conveying. In this state, it provides minimal radar reflection and almost no mechanical resistance, which pushes conventional sensors to their limits.
Q: What makes 80 GHz radar better for silica silos than lower‑frequency radar?
A: 80 GHz radar has a much narrower beam angle (3° vs. 10° for 26 GHz). In a 25‑meter silo, this narrow beam avoids wall interference and can be precisely aimed at the conical pile, providing reliable measurements even with low‑reflectivity materials.
Q: Why is purge air necessary on radar sensors in silica applications?
A: Fine silica dust coats the lens antenna during filling, attenuating the radar signal. Purge air creates a positive pressure barrier that keeps dust away from the lens, ensuring consistently accurate measurement.
Q: What are the key features of vibrating probes that work for ultra‑light powders?
A: They require enhanced surface area (e.g., extended paddles or polished tines) and high‑sensitivity electronics that can detect very small damping effects, reliably sensing materials with bulk densities below 5 g/l.
Q: What ATEX rating is required for silica silos?
A: Silica silos with pneumatic conveying typically require ATEX Zone 21 instrumentation. Zone 21 covers areas where a combustible dust atmosphere is likely to occur occasionally in normal operation.
Q: Can one sensor type handle all three measurement tasks in a silica silo?
A: No. The three tasks—continuous level, point level, and flow monitoring—have different requirements. Continuous level is best handled by 80 GHz radar, while point level and flow monitoring are best handled by specialized high‑sensitivity vibrating sensors.
