Multi-cyclone dust collectors (MDCs) have been used in conjunction with air preheaters and boilers in energy production and other industries since the 1950s. Over time, they have evolved in design and purpose, yet their function remains important in many process heating applications.

Once one of the primary means of pollution control in coal, biomass, cement and other plants, modern dust collectors help protect downstream equipment. Properly maintained, they separate ash, sand and larger particulate like char from the flue gas to limit damage to important equipment such as induced-draft fans and ducting. In some instances, dust collectors are installed ahead of tubular air preheaters and boiler economizers to recycle or reject material and to add another layer of erosion protection.

Multi-cyclone dust collectors, which do not contain moving parts, use centrifugal force to separate the particulate from the flue gas as it exits the boiler (figure 1). The flue-gas stream is directed to one of several tubes, where an inlet guide-vane induces a spiral effect on the particulate, forcing it to the inlet-cone wall. From there, gravity drops the particulate into the hopper, where it is removed via a dump valve or other mechanism. Based on some industry claims, these units are capable of removing anywhere from 60 to 99 percent of the debris from the flue-gas stream.

Corrosion and Erosion Attack Metal Parts in Dust Collectors

When a multi-cyclone dust collector has incurred significant damage by erosion or corrosion, it becomes ineffective. In such cases, it takes up valuable floor space in a facility without providing any of its intended benefits. Multi-cyclone dust collectors are susceptible to abrasion from fly ash, particulate mass and high velocity gas flow, and to corrosion from chemical attack. The severity of abrasion can be influenced by the gas velocity, angle of impact, particle shape and the hardness of abrasives. (For instance, particulate with SiO2 content causes more abrasion damage.)

Rather than serve its purpose, a leaky collector can lead to problems that cascade to downstream systems. Coarse char, abrasive sand and other erosive particulate pass through eroded tubes and tubesheets undeterred. In some cases, an ineffective dust collector has led to the failure of important equipment like tubular air heaters, induced-draft fans, precipitators or baghouse components. In addition, when ash or other large particles plug a cyclone or tube, a dust collector’s operating efficiency may drop significantly due to changes in velocity and pressure drop. An overload of particulate also may plug other equipment in the path of the flue-gas flow.

In worst-case scenarios — when dust collector assemblies are riddled with holes — not only does the dust collector fail to protect downstream equipment, but it also destroys itself from within. Erosive particulate is no longer under control. Parts within the multi-cyclone dust collector that can suffer damage from abrasion include inlet tubes, the boot assembly that rejects the ash, outlet tubes, guide vanes, the collector housing and the upper and lower tubesheets.

The damage to a multi-cyclone dust collector is not always apparent on the surface. Several symptoms can point to problems in a unit, however. They include:

• Increased differential pressure due to vane plugging in the dust collector.
• Decreased differential pressure due to tubesheet holes and gas bypass.
• Fly-ash buildup in unusual areas.
• Air leakage around access doors or hoppers.
• Unusual wear patterns in components and equipment.
• Hopper plugging.
• Overloading of the air heater, baghouse, scrubber or electrostatic precipitator.
• Fugitive dust leaks from expansion joints and emissions equipment.
• Induced-draft fan vibration and erosion.
• Emissions issues (opacity).

To avoid letting such problems degrade the performance of the dust collector until the system must be replaced, consider preventive maintenance, including inspections.

FIGURE 2. When a multi-cyclone dust collector has incurred significant damage by erosion or corrosion, it becomes ineffective. In effect, it takes up valuable floor space in an operation without providing any of its intended benefits. Shown here is typical corrosion of a duct, leading to air leakage.

Identifying Problem Areas Before They Turn into Failures

Multi-cyclone dust-collector inspections enable a service company to assess the condition of each dust collector. Following the examination, the service company will provide maintenance recommendations to preserve the multi-cyclone dust collector, protect downstream equipment and maximize operating performance related to particulate removal. During inspections, it is important to pay attention to the condition of components such as:

• Expansion joints.
• Housings.
• Inlet/outlet ducts.
• Collector/reject tubes and hoppers.
• Inlet/outlet tubesheets.
• Dust discharge boots.

Typical inspection procedures will look at the system’s components as well as the overall system as a whole (see sidebar).

Regular inspections also afford the opportunity to make temporary internal repairs — sometimes even during the inspection process. These may extend the life of equipment and delay its replacement.

A professional inspection firm can help ensure that your units are properly inspected. Necessary repairs can be made during the inspection or planned for during subsequent maintenance intervals. Ignoring these services can result in equipment failure, which may lead to environmental compliance issues and even de-ratings.

Advantages of Regular Dust-Collector Maintenance

Multi-cyclone dust-collector maintenance can have a big impact on downstream equipment operating efficiency. Performing — or failing to perform — maintenance can stave off (or contribute to) unplanned shutdowns for emergency repairs. Advantages achieved through a regular inspection and repair program include:

• Increased collection efficiency.
• Longer operating life of the induced-draft and other downstream fans.
• A reduction in the energy required to operate fans for air leakage.
• A reduction in fan maintenance due to erosion.
• Lower fuel requirements.
• Protection of the pollution control equipment.
• Reduced repair costs for the dust collector and downstream equipment.
• Fewer unplanned outages.
• Increased operating time before the dust collector must be rebuilt.
• Potential protection for air heaters, fans and ducting.
• Reduced emissions excursions.

FIGURE 3. Multi-cyclone dust collectors are susceptible to abrasion from fly ash, particulate mass, high velocity gas flow and corrosion from chemical attack. The severity of abrasion can be influenced by the gas velocity, angle of impact, particle shape and hardness of abrasives.

In addition, during some inspections and repairs of multi-cyclone dust collectors, inspectors have found that expansion joint and duct issues may be impacting the air and gas processes. Often, these duct sections and expansion joints are inaccessible and not visible due to their location in high vertical runs in areas with no structural access. These undetected problems can result in fugitive dust releases, fan-capacity limitations affecting unit capacity and environmental equipment operation.

A well-trained high angle inspection (HAI) team can access smokestacks, chimneys and other isolated areas for repairs or planning purposes during a single mobilization.

In summary, putting off inspection and maintenance of multi-cyclone dust collectors has the potential to have a negative impact on everything from equipment performance and lifespan to a plant’s bottom-line profits.   

Best Practices for Dust Collector Inspections

Among the best practices in multi-cyclone dust collection inspection procedures are the following:

1. Flue-Gas Inlet Evaluation

Wear damage to collector parts. Collector tube or vane damage and plugging. Ultrasonic testing (UT) of thickness and mapping. Indication of poor gas flow distribution.

2. Flue-Gas Outlet Evaluation

Tubesheet UT thickness. Outlet tube ash plugging. Tubesheet and sidewall wear. Fastener condition.

3. Hopper Evaluation

Check for plugging. UT thickness mapping and wear profile. Evaluate all gaskets for signs of leakage. Hopper baffle and wall wear.
• Sources for air in-leakage.

4. Dust Collector Inlet/Outlet Duct and Expansion Joints

5. Dust Collector Inlet/Outlet Duct Turning Vanes

It is important to request a detailed inspection report with charts, images and data that enable management of maintenance decisions based on priorities/performance, impact and budget.