Carbon monoxide in milk powder storage: early fire detection and why moisture sabotages the measurement
Milk powder is combustible, finely divided and stored in large silos. When the powder starts to smoulder, there is no smoke, no flame and no immediately visible rise in temperature. The only reliable early signal is carbon monoxide. But measuring that gas in a humid atmosphere is technically demanding, and that is precisely where things often go wrong in practice.
⏱ approx. 6 minutes reading time
Why milk powder silos pose a specific fire risk
Milk powder contains fat, lactose and proteins. All these organic components are combustible. In finely divided form, and milk powder consists of particles of tens of micrometres, the reactive surface area is enormous. A stored layer of milk powder that slowly oxidises produces heat. That heat can reinforce itself: the warmer the material becomes, the faster the oxidation proceeds, which in turn produces more heat. This process is called self-heating, and it proceeds completely silently, invisibly and without an external heat source.
Self-heating starts deep inside the silo, far from any sensor or outer wall. There are no flames. There is no visible smoke. The silo wall feels normally warm on the outside. Meanwhile, the core temperature rises above the auto-ignition temperature of the powder. If air enters at that moment, for example when a hatch is opened or the silo is emptied, the smouldering fire can immediately turn into an open fire or even a dust explosion.
The explosion risk when opening a silo
In a smouldering fire in a silo, carbon monoxide accumulates in the enclosed space above the powder bed. CO has an explosive range of 12.5 to 74 percent by volume. When a silo valve is opened, oxygen flows in. The combination of CO build-up and oxygen has led to backdraft-like explosions with serious injuries and material damage in several incidents. Early detection before the concentration becomes dangerous is the only reliable safety measure.
Carbon monoxide as the earliest warning signal
Carbon monoxide is produced by incomplete combustion and is therefore one of the very first measurable reaction products of smouldering organic materials. While the temperature in the core zone of a smouldering fire only rises measurably at the outer wall after hours or days, the CO concentration in the silo atmosphere already builds up in the very first stages of thermal decomposition.
In a closed silo without active air exchange, the CO concentration rises continuously as soon as even a small smouldering hotspot is active. That continuous rise is itself an alarm signal, regardless of the absolute value. The CO concentration in the silo atmosphere effectively integrates the total smouldering activity over time: the more and the longer something smoulders, the higher the reading.
Phase 1: Early stage
Self-heating begins
Milk powder slowly oxidises in the core of the silo. Temperature rises barely measurably at the wall. The CO concentration starts to rise slightly. No visible signs of fire.
Phase 2: CO rises measurably
A smouldering fire develops
The CO concentration rises continuously. The alarm is activated. At this point, intervention is still safe and effective: seal the silo, inert it with CO₂ or nitrogen, cool it down. No visible smoke or flame outside.
Phase 3: Critical
CO build-up and high temperature
The smouldering fire has spread. The CO concentration is high. Without early detection, it only becomes visible at this point through heat emission, odour or smoke. Intervention is now dangerous because of the explosion risk when the silo is opened.
Phase 4: Incident
Open fire or explosion
Ingress of oxygen through opening of the silo or a wall rupture leads to open fire or a dust explosion. Severe material damage, production downtime and risk of personal injury.
The measurement challenge: a humid atmosphere in the silo
A milk powder silo is not a dry, clean space. The atmosphere above the powder bed contains water vapour originating from the powder itself, from temperature changes that cause condensation on the walls, and from the breathing bulk storage. The relative humidity in the silo atmosphere can reach values close to or sometimes above the dew point.
For a CO analyser, this is a serious problem. Most continuous CO analysers work on the basis of non-dispersive infrared absorption (NDIR) or electrochemical sensors. Both techniques are sensitive to moisture. Water absorbs in infrared bands that overlap with CO, causing direct interference in NDIR measurement. In electrochemical sensors, condensed water attacks the electrolyte and drastically shortens the service life. Milk powder particles in the sampled air contaminate filters and sensors.
The sample gas path from silo to analyser must therefore condition the gas: cool it, dry it and filter it, without affecting the CO concentration itself. That last point is exactly the critical one.
Removing moisture without losing CO
Water is a good solvent for many gases. With wet conditioning methods, in which condensate is separated by cooling, there is a risk that CO also partially dissolves in the water and no longer reaches the analyser. The measured concentration is then lower than the actual one, a dangerous underestimation of the situation. The conditioning method must remove moisture in a way that leaves CO completely intact.
Why Nafion drying is the right method
Nafion polymer membranes are based on selective permeability: water is transferred on the basis of partial pressure, while most other molecules in the gas stream are retained. CO is a non-polar gas with minimal solubility in water and a negligible interaction with the Nafion membrane. It does not pass through the membrane and leaves the Nafion dryer virtually completely intact.
This is fundamentally different from condensation drying, in which the gas is cooled below the dew point and liquid water is separated. In condensation drying, water-soluble components are vulnerable to loss. In Nafion drying, only water vapour leaves the gas stream via the membrane; everything else remains in it.
The Perma Pure PD series in practice
The Perma Pure PD series, available via Salaera and Inacom, is a tube-in-shell Nafion dryer in which the gas flows through one or more Nafion tubes while dry purge gas (instrument air or vacuum) flows counter-currently along the outside. The difference in water vapour partial pressure drives the transfer of moisture from the gas to the purge stream. Drying to dew points of -40 °C is possible.
In large-scale milk powder storage facilities, PD dryers are installed per measuring point, directly after the sampling probe and upstream of the analyser. In installations with dozens or hundreds of silos, this means a corresponding number of dryers, each operating continuously without moving parts or consumables that need to be replaced periodically.
| Parameter | Perma Pure PD series |
|---|---|
| Operating principle | Tube-in-shell Nafion membrane drying |
| Dew point after drying | Down to -40 °C |
| Flow range | Up to 40 lpm (PD series multi-tube design) |
| CO loss | Negligible: CO does not pass through Nafion |
| Temperature | Inlet temperature up to 120 °C |
| Pressure | Up to 5.5 bar |
| Maintenance | No moving parts, no consumables |
| Purge gas | Dry instrument air or vacuum |
| Materials | Various shell and fitting materials available |
Practical considerations when designing a CO monitoring system for powder storage
Sampling position
The probe is placed in the silo dome or the silo's exhaust duct, at a position where the gas phase above the powder bed is sampled. In silos with forced air circulation, the exhaust air is sampled. In closed silos without air circulation, the dome space is sampled; this is where CO accumulates most.
Particle protection
Milk powder is light and is easily suspended by temperature differences and mechanical vibration. A robust primary particle filter on the probe is indispensable to prevent clogging of the Nafion dryer and contamination of the analyser. Choose a filter with a backflush facility if the powder concentration is high.
Alarm thresholds
The alarm settings for CO in powder storage are not expressed in absolute ppm values but in trend analysis: a continuous rise in the CO concentration over a set period activates the pre-alarm. Only at a higher threshold does a main alarm follow. This distinguishes a smouldering hotspot from background variations caused by temperature fluctuations or ventilation movements.
System validation
A CO monitoring system for safety applications should be periodically validated with a certified calibration gas. The Nafion dryer has no influence on the CO concentration of the calibration gas, which simplifies the validation process: the same dryer that conditions the sample stream also conditions the calibration stream.
Rule of thumb for large-scale installations
In facilities with large numbers of silos, from dozens to hundreds of storage units, PD dryers are installed per measuring point in combination with a central multipoint analyser system. One analyser monitors several measuring points via a sequential sampling system. The dryers work autonomously per sampling point and do not need to be managed centrally. This considerably reduces the total maintenance burden compared with systems with active coolers per point.
