Level Measurement and Inventory Control in Chemical Plants

Chemical silos present every difficulty at once. They are filled pneumatically, so the headspace is full of dust when a reading is wanted. The powders vary enormously in dielectric constant, so a radar arrangement that works on one product gives a weak return on the next. And the vessel atmosphere may be corrosive, hot or inerted.

The sensor is also inside the containment boundary. A penetration into a silo holding a toxic powder is a potential leak path, and the seal around it has to be specified like any other containment component.

Overfill is the serious failure. An overfilled silo vents through its filter, and a filter full of product either bursts or releases, which in a chemical plant can be a reportable event. Where the consequence is severe enough the overfill protection becomes a safety function with an integrity requirement rather than an alarm.

And the inventory number matters commercially: silos hold raw materials that are reconciled against deliveries, and a measurement with an unknown error is a reconciliation problem every month.

Level measurement is a solved problem on a water tank. On a powder the dielectric constant and the atmosphere above the surface decide whether the reading means anything.

A technology matched to the material and the filling condition, with the antenna or sensing arrangement chosen for the actual dielectric constant rather than a typical one.

A penetration and seal specified as a containment component, compatible with the atmosphere and the cleaning agents.

Overfill protection that is independent, and certified to an integrity level where the consequence requires it, with the inventory figure derived from weighing where reconciliation depends on it.

In a food plant the filling method decides first: an ultrasonic sensor on a pneumatically filled silo reads the dust cloud rather than the surface.

Radar is the usual answer for pneumatically filled silos because it works through dust, and the antenna is selected for the dielectric constant of the powder; low dielectric materials need a larger antenna or a different arrangement, and a guided wave option is sometimes the better route.

Materials are then checked against the vessel atmosphere as well as the powder. A sensor face that is fine in contact with the product may be attacked by the vapour above it, and the seal at the penetration sees both.

Overfill protection is designed as a separate layer. An independent point level device, its own cable, its own input, and where the risk assessment calls for it a certified safety function with a proof test interval. Relying on the continuous measurement for both is the common and consequential shortcut.

Inventory is then settled. Where the figure is reconciled against deliveries, load cells give a defensible number; a level based estimate needs a stated bulk density and an acknowledged error band, which is usually wider than the reconciliation tolerance.

• Radar matched to the powder dielectric constant, with guided wave considered for difficult materials.
• Sensor materials and penetration seal checked against the vessel atmosphere and cleaning agents.
• Penetration treated as a containment component where the powder is harmful.
• Independent overfill protection with its own path, certified where the risk assessment requires it.
• Weighing where the inventory figure is reconciled against deliveries.
• Sensor suitability confirmed for an inerted or hot headspace where that applies.

• Dielectric constant of each powder in the silo, since a campaign silo may hold several.
• Filling method, which decides whether a technology works when it is needed.
• Vessel atmosphere: corrosive, hot, inerted, and the materials that follow.
• Containment requirement at the penetration where the powder is harmful.
• Overfill consequence, which decides whether protection is an alarm or a safety function.
• Reconciliation tolerance, which decides level versus weight.