Dust explosions are a constant concern in processing environments. Fine, solid powders pose a special risk as they are not only combustible in larger sizes, but can become explosive when in dense, finely sized concentrations. There are three methods of St 1 or St 2 dust explosion containment at the feed and discharge of a mill rated to contain 10 bar pressure. What follows is a brief look at the advantages and disadvantages of method.
10 Bar Rotary Valve. Rotary valves or star valves use rotating pockets to both meter material in and out of an area while continuously isolating this area from the other side of the valve. Advantages: The advantage of using a rotary valve at the feed is that it can also act as a metering device. The suitability of this valve as a metering device must be determined and the feed material must be able to flow into and out of the rotary valve pockets in a relatively consistent manner. Disadvantage: The fact that the rotary valves are in the process stream and permanently isolate the mill can cause some material flow problems and do not allow for a gas flow, which is sometimes used to cool the product during milling. The 10 bar rated rotary valve is big and heavy and can be awkward to clean. Note: There are specifications for the use of rotary valves as an isolation device. While most devices of today’s manufacture meet the needs some older units do not. The gap between the vane and the housing that needs to be small enough to stop propagation. The loading of the valve is also a factor.
Fast-Acting 10 Bar Rated Sanitary Slide Valves. Fast-acting valves are slide valves that are activated based on feedback from pressure sensors. The controls are looking for a signature pressure rate of rise, which indicates an explosion. If the pressure rate of rise is sensed, the fast-acting valves close and contain the explosion within the 10 bar rated system. The slide valve pierces through a Teflon sleeve that is in place to make a sanitary installation. Advantages: Fast-acting slide valves do not restrict the flow of material to or from the mill. They also are re-settable in a matter of minutes, although the Teflon sleeve will have to be replaced. Disadvantages: This system requires the fast acting valves be located a minimum of 5 ft from the milling chamber and as close to 10 ft away as possible. Also, it is typical that these are tested regularly (typically annually) to insure proper working order. These tests can be conducted by the end users according to supplied instruction manuals.
High Rate Discharge Flame Barrier (Suppression). A high rate discharge flame barrier uses pressure sensors to sense an explosion in the same way as the fast-acting valves. If an explosion is sensed, this system discharges a cloud of sodium bi-carbonate, which creates a barrier of dust effectively cooling and extinguishing the explosion. Advantages: The initial cost of this system is the less expensive of the three options. Additionally, this system doesn’t create a barrier in the process stream during normal operation. Disadvantages: In the event of an incident, the product would be contaminated with sodium bicarbonate. Another disadvantage is that yearly maintenance and inspection is required by factory technician. Also, the recondition costs are higher than the flame barrier with most valves systems. Some valves available require the total reconditioning of the valve with a new activation device, new rupture disc, and pressurization. Plus, if the cylinder leaks slightly, the system will malfunction. Systems can run anywhere from $150 plus chemicals to $1000 plus chemicals.
When choosing an explosion containment system, it is imperative to consider not only the initial costs of the system but also those incurred during cleanup, replacement/reconditioning of damaged parts and the downtime associated with all of these.
Tomas Johansson is the product manager at Sturtevant Inc. for the FCM.
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