FGD in Metallurgical Plants
Flue gas desulfurization (FGD) is mandatory for smelters, steel plants, and power stations burning sulfur-containing fuels. The most common process — wet limestone-gypsum FGD — sprays limestone slurry into the flue gas to absorb SO₂, producing calcium sulfite which oxidizes to gypsum. The pumps in this system handle some of the most demanding slurry conditions in industry: 15–30% solids, abrasive limestone particles, acidic pH (4.5–5.5), and chloride concentrations up to 50,000 ppm.
A typical FGD system requires several pump types: slurry circulation pumps (the largest consumers), gypsum slurry pumps, filtrate pumps, limestone slurry feed pumps, and wastewater pumps. Selecting the right pump and material for each service directly affects FGD availability and operating cost.
Key Pump Services in FGD
Slurry Circulation Pumps
These are the largest and most critical pumps in FGD. They recirculate limestone slurry from the absorber sump to the spray headers. Flow rates range from 2,000 to 12,000 m³/h per pump, with heads of 15–30 m. Multiple pumps (typically 3–4) operate in parallel to provide turndown.
Because of the high flow and abrasive slurry, circulation pumps are almost always large horizontal split case pumps with rubber-lined casings and high-chrome or rubber impellers. The split case design allows inspection without removing piping — essential for pumps that may need liner replacement every 12–24 months.
Gypsum Slurry Pumps
Gypsum slurry (20–40% solids) is pumped from the absorber to dewatering systems (vacuum belt filters or centrifuges). These pumps handle higher solids concentrations than circulation pumps and require robust horizontal slurry pumps with high-chrome iron wetted parts. The gypsum crystals are relatively soft (Mohs 2), but the high concentration and presence of unreacted limestone cause significant wear.
Limestone Slurry Feed Pumps
Fresh limestone slurry (10–20% solids) is fed from the ball mill circuit to the absorber. These pumps handle finer particles than gypsum service but still require abrasion-resistant materials. Horizontal slurry pumps with rubber liners are common.
Filtrate and Wastewater Pumps
Filtrate from gypsum dewatering is acidic (pH 3–5) with high chloride content. This service requires corrosion-resistant materials. Stainless steel chemical pumps (duplex 2205 or super duplex 2507) or fluoroplastic pumps are suitable depending on chloride concentration and temperature.
Material Selection
| Service | Recommended Material | Notes |
|---|---|---|
| Slurry circulation (limestone) | Rubber-lined casing, high-chrome or rubber impeller | Rubber resists impact abrasion; high-chrome resists cutting |
| Gypsum slurry | High-chrome iron (27% Cr) | Good abrasion resistance, moderate corrosion |
| Limestone feed | Rubber-lined or high-chrome | Finer particles, lower wear than gypsum |
| Filtrate (high chloride) | Duplex 2205 / super duplex 2507 | Resists pitting and crevice corrosion in chloride |
| Wastewater | 316L or duplex stainless | Depends on chloride and pH |
For chloride concentrations above 20,000 ppm, standard 316L stainless steel is insufficient — pitting corrosion will occur. Duplex or super duplex alloys are required. For very aggressive conditions, fluoroplastic-lined pumps offer excellent corrosion resistance but have temperature and pressure limitations.
Common Failures and Solutions
Impeller and Volute Wear
The most common failure in FGD slurry pumps. Limestone particles (Mohs 3–4) and gypsum crystals cause both impact and sliding abrasion. Wear is concentrated at the impeller inlet (vane leading edges) and volute cutwater.
Solutions: Use rubber-lined components for impact abrasion; high-chrome iron for cutting abrasion. Operate at or near BEP — operation at low flow increases recirculation and wear. Monitor pump performance monthly; a 10% head drop indicates significant wear.
Corrosion in Filtrate Service
Chloride-induced pitting and crevice corrosion in stainless steel pumps. Common at seal faces, gaskets, and low-flow zones where chloride can concentrate.
Solutions: Upgrade to duplex or super duplex alloys. Use fluoroplastic pumps for very high chloride. Ensure proper flush plans to prevent chloride concentration at seal faces.
Seal Failure
Mechanical seals in slurry service fail due to abrasive particles entering the seal faces. Packing seals leak and require frequent adjustment.
Solutions: Use Plan 32 seal flush with clean filtrate water (minimum 1 bar above stuffing box pressure). Consider cartridge seals for quick replacement. For clean corrosive filtrate, magnetic drive pumps eliminate shaft sealing entirely.
Clogging and Blockage
Gypsum slurry can settle and harden in pump casings and suction lines during shutdown. Scale formation (calcium sulfate) reduces flow areas.
Solutions: Install flush connections for pump washout during shutdown. Use open or semi-open impellers for high-solids service. Ensure suction piping is self-draining.
Procurement Specifications
- Material test reports (MTRs): Require for all wetted components, especially alloy and rubber-lined parts.
- Wear life guarantee: Ask for minimum wear life (e.g., 18 months for circulation pump liners).
- Back pull-out / split case: Essential for maintenance in FGD service where pumps are large and piping is difficult to disconnect.
- Seal flush plan: Specify Plan 32 with clean water source and pressure regulation.
- Spare parts: Include spare impeller, volute liner, wear plates, and cartridge seal with initial order.
- Performance testing: Require witnessed performance test with slurry (or water with correction factors) to verify head and efficiency at operating point.
Need FGD Pump Selection Help?
Provide your FGD process data — slurry concentration, particle size, chloride content, pH, flow, head, temperature — and our engineers will recommend pump type, material, and sealing arrangement. Request a quotation or use our Pump Selection Center.