A: Stephen Miranda, Netzsch Premier Technologies, says:
A mill is only part of a complete system, and the entire system must be protected when handling combustible or dust-explosive materials. First, you need to understand the nature of the hazardous material or its potential to explode or burn and what the force of a potential explosion would be. This can be found by investigating the St range or KSt value of both the feed material and the final processed particulates. (St and KSt values are ranges of the rate of pressure rise.)
If the material presents a dust explosion hazard, there are two ways to protect the mill. First, design it to be explosion- and pressure-shock-resistant up to 150 psig (10 bar) overpressure. This design choice won’t allow the mill to be operational immediately following an event because it will likely be damaged or deformed, but the surrounding area and operators will be protected. Second, use an explosion-suppression system on the mill. The suppression system uses process sensors to detect a rapid rise in pressure or temperature and release a quenching agent to stop a fire or explosion as it starts. The quenching material is usually released in milliseconds.
The complete milling system can be protected in similar ways. It can be designed for complete explosion containment, but this can be very expensive if a large dust collector is involved and if the equipment’s material-contact surfaces are stainless steel. Usually the best bet is to use a combination of protective measures, such as a containment system for the mill and an explosion–suppression system for the dust collector.
If the duct runs are long, pressure can build high enough to exceed even an explosion-resistant mill. In those instances, a fast-acting isolation valve can be installed in the pipeline between two system components. The valve is activated by pressure sensors in the system and activates within milliseconds.
Make sure your material, system components, and plant layout are evaluated by qualified engineers and safety experts. Only then can you choose the best option for processing your potentially hazardous material.
Netzsch Premier Technologies, Exton, PA, manufactures grinding, mixing, and dispersing equipment.