OTHER TOPICS | Power Generation
Maximum-Duty Slurry Pumps in Coal-Fired Fly Ash Dewatering
Finding the right pump for environments where abrasion, solids loading and hazardous atmospheres converge.
Steve Mosley | Industrial Flow Solutions
Anyone who has spent any time around coal-fired power plants knows that fly ash is nasty stuff. For everyone else, here is the short version: Coal combustion in the U.S. generates roughly 110 million tons of residuals every year. The bulk of that is fly ash, which is a fine powder composed of silicon dioxide, calcium oxide, aluminum oxide and a mix of heavy metals—such as mercury, lead, chromium, selenium and arsenic—that no one wants in their groundwater. The Environmental Protection Agency (EPA) has been cracking down on how utilities handle this material, and it is long overdue.
The typical method is to transport fly ash as a slurry (ash particles suspended in water) through pipelines into settling ponds, dewatering tubes or landfill staging areas. Once it gets there, the water is separated, and the ash is either hauled to a disposal site or repurposed for uses like cement and road base.
This sounds straightforward. In reality, it destroys pumps.
What Does Fly Ash Do to a Conventional Pump?
Fly ash slurry ranks among the most abrasive fluids that can be handled with a centrifugal pump. The silicon dioxide content is basically liquid sandpaper, and the fine particle size makes things worse, not better. Those tiny particles find their way into clearances and tolerances that coarser solids would never touch.
Here is what happens on jobsites, again and again:
Impellers wear out fast: Standard cast-iron impellers lose their profile in just weeks, or maybe a few months if conditions are mild. Once the geometry is gone, head and flow is being lost, and maintenance teams must pull the pump far more often than the budget allows.
Seals give up early: Fine abrasives eat through mechanical seals as if they were not even there. When the pump sits submerged in ash-laden water around the clock, seal failure becomes a constant problem.
Volutes erode: The pump casing wears down, particularly around the cutwater and discharge, where fluid velocity peaks. Often, a worn-out volute requires a complete pump replacement. With some maximum-duty slurry pumps, the volute is split-case, meaning the volute can be taken apart, cleared and reassembled without replacement.
Clogging is relentless: Power plant sumps collect coal chunks, rocks, rags and all sorts of debris. Any pump with tight clearances will repeatedly clog, and each clog means downtime.
Hazardous area ratings are mandatory: Coal plants produce combustible dust and cause potential methane exposure. If a pump does not carry explosion-proof certification, it cannot be installed where it is needed. That is not a preference; it is code. All of that combined results in a familiar cycle of premature failures, emergency shutdowns and expensive spare parts gathering dust on shelves, as well as maintenance crews who are tired of wrestling with pumps that were never designed for this kind of abuse.
| IMAGE 1: A coal shipping port (Images courtesy of Industrial Flow Solutions)
A coal shipping port (Images courtesy of Industrial Flow Solutions)
Heavy-duty explosion-proof slurry pumps were not conceived as general-purpose submersibles that someone decided to repurpose for fly ash duty. They were engineered specifically for environments where abrasion, solids loading and hazardous atmospheres converge.
- Hardened ductile iron construction: The motor housing of a heavy-duty explosion-proof slurry pump is made from Class 30 ductile iron, which is substantially tougher than standard cast iron. It holds its shape under continuous abrasive bombardment, so hydraulic performance stays consistent over a much longer service life. Head and flow do not quietly deteriorate month after month, as with some other materials.
- 28% high chrome pump end: This is the heart of the abrasion resistance. The agitator, impeller, wear plate and volute are manufactured with 28% chromium content, which is the recognized benchmark for severe slurry applications. At that chrome level, hard carbides form throughout the metal matrix. This is not a coating or a surface treatment that eventually wears off. It is the fundamental metallurgy of the component. It stands up to the cutting and gouging action of fly ash particles in ways that standard or lower-grade alloys cannot sustain over real-world duty cycles. The agitator also prevents solids from settling around the pump and starving it of liquid, allowing continuous operation without interruption.
- Double-silicon carbide mechanical seals: Maximum-duty slurry pumps use double mechanical seals with silicon carbide faces, all housed in an oil-filled seal chamber. Silicon carbide is one of the hardest materials available for seal faces, and the oil bath serves double duty by continuously lubricating the faces while blocking fine-particle infiltration. Behind all that sits a moisture detection system, which alerts operators early if a seal begins to degrade. Operators find out about issues well before they escalate into catastrophic failures or emergency shutdowns.
- Solids handling: Maximum-duty pumps are rated to pass solids up to 1 inch (25 millimeters [mm]). That is a critical spec because power plant sumps are not clean environments. Coal fragments, aggregates and random debris can all be found mixed in with the fine ash. A pump that handles the fines beautifully but chokes on a coal chunk is not workable in this application.
- Factory Mutual (FM)-approved explosion-proof motor: Coal-fired power plants are classified as hazardous locations under National Fire Protection Association (NFPA) 70 because of combustible dust and potentially explosive atmospheres. Maximum-duty slurry pumps carry FM approval for Class I, Division 1, Group C and D with a T3C temperature code. In plain terms, that means they are rated for areas where combustible gases or dust may be present under normal operating conditions, not just during upset scenarios. Consult the local authority having jurisdiction (AHJ) to determine if an explosion-proof motor is required for an application.
| IMAGE 2: A mining construction site
Uses in a Coal Ash Operation
- Ash sump dewatering: Sumps at the base of hoppers and throughout the ash-handling system are the toughest to address due to constant slurry accumulation, mixed debris and the hazardous atmosphere. This is the application a heavy-duty slurry pump was purpose-built for.
- Settling pond transfer: Moving slurry from collection points to settling ponds or dewatering tubes requires sustained run times at high flow rates while pushing a highly abrasive, high-solids fluid. Maximum-duty slurry pumps handle this without issue.
- Remediation work: As coal ash ponds are closed or cleaned up under EPA mandates, pumps are needed to move accumulated ash out of basins and lined containment areas. The combination of solids-handling capability and abrasion resistance makes a maximum-duty slurry pump a natural fit for the job.
- Process water recirculation: In closed-loop dewatering systems, the recirculated water streams still carry residual fine ash. Maximum-duty explosion-proof slurry pumps manage this duty without the chronic seal failures and accelerated wear that can knock conventional pumps out of service.
Steve Mosley is a product manager at Industrial Flow Solutions (IFS) and is currently living and working in Connecticut. He graduated from the University of Hartford with a bachelor’s degree in civil engineering. For more information, visit flowsolutions.com.
In This Issue
