The absorber circulation pump is the largest and most critical pump in a wet FGD system. A 600 MW unit has 3–4 circulation pumps, each 500–1,500 kW. They run 24/7, spraying limestone slurry into the flue gas. Selecting the right pump determines SO₂ removal efficiency, energy cost, and maintenance frequency.
This guide covers FGD slurry circulation pump selection, organized by the key decision points.
Understanding FGD Slurry Properties
Before selecting a pump, you must understand the fluid. FGD absorber slurry is not simple water.
| Property | Typical Value | Why It Matters |
|---|---|---|
| Solids concentration | 15–25% by weight | Abrasive wear, motor sizing |
| Particle size | 0.01–0.1 mm (limestone + gypsum) | Material selection (fine = rubber OK) |
| pH | 5.0–6.0 | Mildly acidic — corrosion consideration |
| Specific gravity | 1.1–1.2 | Motor power calculation |
| Temperature | 45–55°C | Material and seal selection |
| Chloride content | 10,000–50,000 ppm | Corrosion acceleration |
| Fly ash content | 1–5% (from flue gas) | Additional abrasion |
The slurry is mildly acidic, moderately abrasive, and contains chlorides. This combination — abrasion + corrosion + chlorides — makes material selection critical.
Step 1: Determine Flow and Head
Flow rate: Determined by the liquid-to-gas (L/G) ratio. Typical L/G for limestone-gypsum FGD: 10–15 L/m³ of flue gas. For a 600 MW unit with 2,000,000 m³/h flue gas, total circulation flow = 20,000–30,000 m³/h. Divided by 3–4 pumps = 5,000–10,000 m³/h per pump.
Head: Determined by spray nozzle pressure + piping losses + static lift. Typical head: 15–25 m. The spray nozzles need 0.5–1 bar pressure (5–10 m head). Piping losses add 5–10 m. Static lift (sump to nozzle) adds 5–10 m.
Rule of thumb: Size each pump for 1/3 of total flow (if 3 pumps) or 1/4 (if 4 pumps). At full load, all pumps run. At partial load, one pump is off. This gives turndown without VFD.
Step 2: Select Pump Type and Configuration
Pump type: Horizontal centrifugal slurry pump. This is the standard for FGD circulation. Vertical pumps are not used — the flow is too large and the sump is shallow.
Configuration:
- Single-stage, end-suction, top-discharge
- Double-casing design (outer casing + replaceable inner liner)
- Back-pull-out for maintenance (rotor assembly pulls out without disconnecting piping)
- Expeller seal (no mechanical seal in slurry service)
Number of pumps: 3–4 per absorber. 3 pumps = simpler, less cost. 4 pumps = better turndown, more redundancy. Most new FGD systems use 4 pumps.
Step 3: Material Selection
This is the most critical step. FGD slurry is abrasive-corrosive. The wrong material fails in 6 months.
Casing: Rubber-lined (natural rubber, 5–8 mm thick). The fine particles (0.01–0.1 mm) and mild acid favor rubber. Rubber resists both abrasion and corrosion. Life: 3–5 years.
Impeller: High-chrome white iron (A05, 550–650 BHN). The impeller rotates at high velocity — rubber would tear. High-chrome resists abrasion by hardness. Life: 1–2 years (replace during scheduled overhaul).
Shaft sleeve: 316L stainless or high-chrome. Protects the shaft from slurry. Replaceable.
Expeller: High-chrome white iron. Same as impeller material.
Hybrid construction is standard for FGD: rubber-lined casing + high-chrome impeller. This combines the best of both materials. Our horizontal slurry pump range offers this hybrid configuration.
Step 4: Motor and Drive Selection
Motor power: Calculated from hydraulic power and pump efficiency.
Example: 8,000 m³/h × 20 m × SG 1.15 / (3600 × 102 × 0.65 efficiency) = 765 kW hydraulic power. Shaft power = 765 / 0.65 = 1,177 kW. Motor size = 1,177 × 1.15 service factor = 1,354 kW → standard motor 1,400 kW or 1,500 kW.
Motor type: Medium-voltage (6 kV or 10 kV) squirrel cage induction motor. IE3 efficiency. IP55 enclosure.
VFD: Optional. VFD saves energy at partial load. But VFD adds cost ($50,000–$100,000 per pump) and harmonic distortion. For FGD circulation, VFD is usually not installed — pump staging (3 out of 4) provides turndown.
Step 5: Seal and Auxiliary Systems
Shaft seal: Expeller seal (centrifugal seal). No mechanical seal — slurry would destroy it in weeks. The expeller generates centrifugal force that keeps slurry away from the shaft. At low speed (below 50% rated), the expeller does not work — install a standby packing seal or mechanical seal for low-speed operation.
Seal water flush: Clean water (filtered, <50 ppm solids) flushes the seal area. Prevents slurry from entering the bearing housing. Flow: 5–10 L/min per pump. Pressure: 0.5–1 bar above slurry pressure.
Bearing cooling: Oil-lubricated bearings with cooling coil. FGD pumps run 24/7 — bearing temperature must stay below 75°C.
Step 6: Piping and Installation
Suction piping: One size larger than pump suction. Short and straight. No elbows within 5 pipe diameters of the pump suction (causes uneven flow and vibration).
Discharge piping: Same size as pump discharge. Install a check valve and gate valve. The check valve prevents backflow when the pump stops.
Foundation: Massive concrete foundation (5x pump + motor weight). Grouted baseplate. Vibration isolation is critical — these pumps are large and powerful.
Common Selection Mistakes
Mistake 1: High-chrome casing instead of rubber-lined. The mild acid (pH 5–6) corrodes high-chrome. Wear accelerates. Casing life drops to 1 year. Right answer: rubber-lined casing.
Mistake 2: Mechanical seal instead of expeller. Slurry destroys mechanical seals in weeks. Right answer: expeller seal + seal water flush.
Mistake 3: Motor sized for water SG. FGD slurry SG is 1.1–1.2. Motor sized for water (SG 1.0) overloads. Right answer: size motor for maximum expected SG + 15% service factor.
Mistake 4: No standby pump. One circulation pump fails. SO₂ emissions spike. Unit must reduce load. Right answer: N+1 configuration (4 pumps for 3-pump duty).
Mistake 5: Elbows too close to suction. Uneven flow causes vibration and premature wear. Right answer: 5x pipe diameter straight run before suction.
For FGD slurry circulation pump selection and configuration support, our horizontal slurry pump range covers 1,000–20,000 m³/h at 10–40 m head with rubber-lined casing and high-chrome impeller. For FGD pump quotation, submit your flue gas data — unit capacity, L/G ratio, and slurry properties.