Hopper and housing design
To avoid buildup of combustible materials, hopper walls should be designed with a minimum angle of 70° from horizontal. The inside of the dust collector should be constructed so that structural steel, baffles or transitions do not form shelves or areas where combustible dust can accumulate. At least one level detector and one temperature sensor should be located in the hopper to warn the operator of either plugging or potential fire issues.
The housing and hopper should be airtight to avoid entry of cold, moist and oxygen-rich air. Explosion vents if installed, are notorious for leaks. Since coal is capable of spontaneous combustion at certain moisture levels, air leaks that provide moisture and oxygen are to be avoided at all cost.
Many dust collectors filtering petcoke suffer from severe corrosion. The sulfur content of petcoke and the cyclical operation of the coal mill circuit promote the formation of sulfuric acid which then corrodes exposed metal inside the dust collector and ductwork. Keeping the metal and filtered gases above the dew point is critical to avoid acid formation. Although most dust collectors are insulated, one frequently encounters corrosion problems.
Many dust collectors are installed outside and exposed to rain, wind and varying temperatures. Eventually water will leak through caulked joints in the cladding and then wet the insulation underneath. A lack of insulation around structural steel that is welded to the outside of the dust collector, including lifting lugs, frequently creates heat sinks and ‘cold spots’ inside the collector. Heat sinks as well as wet insulation typically cause condensation and acid formation on the inside of the dust collector. Injection ports for fire suppression systems and the pipes from the pulse valves into the dust collector are other troublesome heat sinks if not properly insulated.
A lack of insulation in the hopper walls often results in wet surfaces on the inside of the hopper, triggering accumulation of combustible dust.
Material handling system
Material handling systems typically contain a type of airlock and sometimes a feeder. It is essential that material handling equipment is sized adequately to handle material surges; e.g. if a rotary airlock at the discharge of a collector receives material from a collection screw inside a trough hopper, the airlock should be sized to convey the amount of material delivered by a completely full screw conveyor, even if that screw conveyor typically only operates at 25% material loading. All moving material handling equipment should be equipped with motion sensors to detect failures.
Dust collector operational practices
Once it is established that the design of the dust collector is correct, the following operational practices can further improve its operation. Keep in mind that, although the topics of explosion venting and fire prevention exceed the scope of this article, they should should be thoroughly examined with regards to design and operation.
- Establish guidelines for startup and shutdown such as evacuating all the dust from the hopper after the fan has stopped.
- Inspect the dust collector once per shift. This check should include pressure drop across the filter bags, visible emissions, the cleaning system, material withdrawal equipment, damper settings and fan operation. Verify that the order in which the pulse valves of the cleaning system actuate is a staggered and not a sequential sequence.
- A staggered sequence reduces dust recirculation inside the collector and pressure drop across the filter bags.
- Operate your cleaning system and the material withdrawal equipment continuously while the fan is in operation. Do not allow excessive dust accumulation on the filter bags or in the hopper.
A reliable and safe operation of the dust collector starts with a proper design. Operating the collector within its design parameters and maintaining the equipment in excellent condition will further reduce unscheduled downtime, emissions and hazardous conditions.
This is part two of a two-part article written for World Cement’s November issue and abridged for the website. Subscribers can read the full November issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the November 2016 issue here.
Read the article online at: https://www.worldcement.com/special-reports/09112016/facing-the-challenge-part-two/