Pneumatic transport

Pneumatic transport conveys powders through pipes by compressed air.

Pneumatic transport by blowing

In most cases the transport is done by blowing, where a blower provides the necessary air pressure (and speed). The product to be transported is brought into the transport pipe after the blower, mostly through a rotary valve.

Pneumatic transport by suction

In some cases suction transport makes more sense. Especially with milling systems it is not unusual that, instead of catching the product in a bunker under the mill, it is, together with the milling air, immediately inserted into a pneumatic (suction) transport. The disadvantage of suction transport is the limited capacity (the maximum pressure difference obtainable is always less than 1 bar) and the necessity to use heavy equipment: the majority of (in most cases round) process equipment can much better resist overpressure than underpressure.

Pneumatic transport: possible risk

In principle, in a pneumatic transport, an explosive mixture can nearly continuously be present. Due to the very high turbulence it is however very difficult to ignite, which makes the probability of a direct ignition in a pneumatic transport line is in most cases very small. Ignition might, of course, occur in the separator (filter/cyclone) at the end of the transport.

Pneumatic transport: ignition sources

During pneumatic transport by blowing, sparks or hot parts from the blower could be introduced in the air/dust system. This is not the case with suction transport. Obviously also ignition sources in the form of smouldering product could be imported.

Brush discharges: a problem often underestimated, which consequently has led to various heavy explosions. They can develop in synthetic or rubber hoses, equipped with a metal thread, even if the steel wire is well earthed. These discharges have a very high energy capacity and are continuously repeated. Therefore, they can lead to an ignition in the conduit, in spite of the very high turbulence.

The powder itself can become heavily charged by the pneumatic transport. This way, it can cause problems in the receiving bin (spark discharges, corona discharges or possibly also propagating brush discharges, e.g. in a coated bin).

Pneumatic transport: various protection methods

Protect your pneumatic transport

• Brush discharges: use solid metal ducting only. If this is not possible, the hoses used must be antistatic. When in doubt, always ask for a test certificate.
• Static electricity in the receiving bin: the classical rules of making everything conductive and to earth apply.
• Mechanical problems with the blower: they can often be detected on time using vibration or temperature detection on the blower.
• Smouldering product imported with the product: the best solution is to install ignition source detection, combined with a fast shutting valve.

Protect the receiving bin

In spite of all that, it is generally necessary to protect the receiving bin (with filter). An important question in this matter is: Is it also necessary to provide isolation on the pneumatic transport? Can an explosion propagate in counterflow to the transporting air? Here, besides the products own properties, mainly the duct diameter is important. In a lot of systems this diameter is such that there is still a small chance of explosion propagation in the duct, but that no flame accelerations are to be expected.

If the product supply is done through an explosion safe rotary valve and the air transport is done by means of e.g. a roots blower (no fan), in many cases no supplementary isolation measures are necessary.