Delta Cement Case: Silo Level Measurement With 26 GHz Pulse Radar and Air Purge

Powder silos are where level measurement technologies go to be tested. This article is presented as a field commissioning log from a building-materials plant, covering level measurement on cement, pulverized coal and limestone silos. The media are fine powders with low dielectric constants (cement approximately 3-5); during filling the silo headspace fills with dust, and the material surface forms an irregular cone. Level measurement is difficult because heavy dust attenuates microwave signals - more so at higher frequencies - and dust build-up coats antennas. The previous electromechanical plumb-bob frequently stuck in the material and required manual recovery; capacitance probes drifted as material caked on them. The replacement was a 26 GHz pulse radar with an air-purge connection, chosen specifically because the lower frequency penetrates dusty atmospheres better than 80 GHz. The log records the site conditions, the selection reasoning, the commissioning entries and what the first fills showed.

Client Profile: Delta Cement Industries Ltd. (Bangladesh)

Delta Cement Industries Ltd. is a cement producer in Bangladesh operating clinker, cement and pulverized coal storage silos. The plant's control room needs continuous silo levels for inventory management and to prevent overflow, but heavy dust during filling and the cone-shaped material surface had defeated the previous plumb-bob and capacitance instruments.

2. Site Background: Three Silos, One Control Room Gap

The plant stores cement, pulverized coal and crushed limestone in tall cylindrical silos with conical bottoms. Material is blown or conveyed into the top; a dust collector vents the displaced air; and during every fill the headspace becomes a dense dust cloud. The material surface is not flat - it forms a cone whose angle depends on the material's angle of repose and where the fill stream lands. The control room needed level for three purposes: inventory, overflow prevention, and early warning of blocked or empty silos. None of the three was being served reliably, because the existing instruments could not provide a continuous, trustworthy signal.

3. Why This Silo Defeats Most Sensors

Powder silos combine three measurement problems that few sensors solve together. The dust cloud: during filling, the headspace is so full of dust that it attenuates and scatters microwave signals, and the attenuation grows with frequency. The surface: a cone-shaped powder surface returns a weaker, more scattered echo than a flat liquid surface, and the cone changes with every fill. The build-up: dust settles on the antenna and caking occurs on some media. The practical test for any candidate instrument was three questions: can it see through the dust, can it find the true surface on a sloping pile, and can it keep its antenna clean?

4. The Plumb-Bob's Last Measurement

The electromechanical plumb-bob lowered a weight on a cable until it touched the material, then measured the cable length. As a snapshot it was accurate; as a continuous instrument it was not. The weight sank into soft or aerated material, the cable could be buried when filling started, and recovery meant climbing the silo. The plant ran it on a schedule, so between measurements the level was simply unknown. The capacitance probe on another silo measured the change in capacitance as material covered the probe; it drifted as material built up and caked, and its reading depended on the material's moisture and density, which change with the product. The recurring failure that ended the discussion: stuck cables on the coal silo meant roof interventions to recover the weight, sometimes in poor weather. The site wanted a measurement with nothing mechanical inside the silo.

5. The Frequency Decision: The Dust Argument

The frequency choice is the heart of this project, and the reasoning belongs in the log. Microwave attenuation in dust increases with frequency: an 80 GHz radar - excellent for liquids in clean tanks - can lose its echo in a heavy dust cloud, while a 26 GHz pulse radar, with its longer wavelength, penetrates dusty atmospheres far better. That is why 26 GHz is the established choice for powder and bulk-solid silos. The physical reason: dust scatters microwaves most strongly when the particle size approaches the wavelength, so the longer 26 GHz wave travels through the dust cloud with less loss than a shorter 80 GHz wave. The trade-off was accepted deliberately: the wider beam is irrelevant on a large silo surface, and the accuracy (approximately +/-0.5% of range or better, depending on model and site) is adequate for inventory. The antenna is a horn with an air-purge connection - a small flow of instrument air keeps dust off the face, answering the build-up problem directly. The coal silo required an explosion-proof rating for the dust-hazardous area; the cement silo classification depended on site conditions and was confirmed before ordering.

Recommended Qinwei Instruments for This Application

7. The 26 GHz Configuration

The configuration followed the same logic as the frequency choice. Level in meters referenced to the silo bottom; silo height and mounting offset entered; 4-20 mA scaled over the working range; HART enabled for remote echo-curve diagnostics. The echo threshold was set for low-dielectric powder surfaces, and the filtering was set to track the true material surface rather than dust-cloud or build-up echoes - which is what the empty-spectrum recording and false-echo mapping are for. The level-to-volume conversion, where the plant wanted volume or mass, was done in the PLC against the silo's strapping table, because a linear conversion is wrong for a conical-bottom silo.

8. Installation: Standpipe or New Nozzle?

The installation log records one genuine decision. The cement silo had an existing standpipe from the old plumb-bob; a drop-in antenna could reuse it, but the standpipe interior had to be checked for build-up first - a partially blocked standpipe creates a false echo or traps the signal entirely. On the coal and limestone silos, new roof nozzles were used. In all three, the antenna was positioned away from the fill stream inlet so falling material could not impact it directly, and the air-purge line was connected with its flow set during commissioning. The mounts were rigid - a silo roof flexes, and a flexible mount moves the reference point. Surge protection went on at the roof, where lightning is a real risk on tall structures.

9. Commissioning Log Entries

The commissioning entries, condensed from the site log:

1. Empty or low-level silo: empty spectrum recorded; fixed echoes from roof structure, filling pipe and dust collector inlet mapped and suppressed.
2. Silo height, offset, output scaling and alarm thresholds entered.
3. Fail-safe direction set so a lost echo drives toward high level - the filling system stops rather than overfills - after confirming this matched the site's control philosophy.
4. Air purge adjusted to the minimum flow that keeps the antenna clean - excess purge wastes instrument air and can cool the antenna, promoting condensation of moist dust onto the face.
5. Radar reading compared with a plumb-bob or weigh-cell check at one or two levels; deviations recorded.
6. Note for follow-up: review the echo curve after the first few fills, when the dust load is highest, to confirm the instrument still sees the true surface.

10. First Fills: What the Echo Curve Showed

The first fills were the real test. When the filling started, the dust cloud built up exactly as expected - and the 26 GHz signal kept tracking the rising material. The echo curve review after the first fills confirmed the true surface echo was present and stable, with the dust cloud contributing noise but not a competing target. The air purge kept the antenna clean; the reading did not drift with build-up. The stuck-cable interventions stopped because there was no cable. The control room gained continuous level visibility on silos that previously were measured on a schedule. Site-specific figures are not provided; the commissioning conclusion was that the measurement was continuous and the previous failure modes were eliminated by design.

Why Choose Qinwei Instruments

Xi'an Qinwei Instrument Factory (brand QINWEIYB) is a manufacturer established in 2010 that specializes in industrial automation instruments, with 500-800 employees and 50%-60% of output exported worldwide. For powder silos, Qinwei provides 26 GHz pulse radars with air-purge connections and Ex-rated housings for dust-hazardous areas such as coal storage. All instruments are factory-calibrated with traceable certificates; MOQ starts from 1 set, delivery is 5-8 working days, and OEM/ODM customization is supported.

12. Value: Availability, Not Accuracy

The value of radar on powder silos is availability more than accuracy. A continuous level prevents two costly events: overflow - material lost, dust released, clean-up required - and running a silo empty or blocked, which interrupts the process. Removing the plumb-bob eliminated the highest-risk maintenance task on the silo: climbing and manual recovery. Every element of the solution traces back to a failure mode of the previous installation: 26 GHz because 80 GHz would lose the echo in dust, air purge because an uncleaned antenna would recreate the capacitance probe's build-up problem, non-contact because the plumb-bob's cable was the original failure. That traceability is what makes the design defensible.

13. Applications That Follow the Same Logic

The same configuration applies to cement, raw meal, pulverized coal, fly ash, gypsum, limestone, sand and plastic pellets in silos and bins. For very low dielectric powders (below approximately 2.5) or very tall silos, guided wave radar or a dedicated low-frequency solution should be evaluated in parallel. For coal and other explosible dusts, the Ex rating must match the zone classification, and the air purge should be interlocked with the filling system where the site procedure requires it. When requesting a quotation for silo level, provide the silo height and diameter, product and bulk density, nozzle or standpipe details, whether the area is dust-explosion classified, and the required output interface.