Forced Circulation Pump for Flue Gas Desulfurization Slurry Service

Flue gas desulfurization (FGD) systems remove SO₂ from power plant and industrial boiler flue gas using a limestone-gypsum wet scrubbing process. The absorber circulation pump moves 5,000–20,000 m³/h of limestone slurry (10–20% solids) through spray headers at 15–30 m head. This slurry is abrasive (limestone and gypsum particles), corrosive (chloride-rich, acidic), and prone to scaling. A standard water pump will wear out in months. A specially designed slurry circulation pump with rubber-lined or high-chrome iron wetted parts is required for this service.

This guide covers forced circulation pump selection for FGD absorber slurry service.

FGD Wet Scrubbing Process

1. Flue gas entry: Hot flue gas (120–160°C) enters the absorber tower. May be quenched to 50–60°C in a prescrubber.
2. Absorber spray: Limestone slurry (CaCO₃, 10–20% solids) is sprayed through nozzles at multiple levels. The slurry absorbs SO₂, forming calcium sulfite (CaSO₃).
3. Oxidation: Air is sparged into the absorber sump to oxidize calcium sulfite to calcium sulfate (gypsum, CaSO₄·2H₂O).
4. Slurry circulation: Slurry from the absorber sump is pumped by circulation pumps to the spray headers. 3–5 circulation pumps per absorber, each serving a spray level.
5. Gypsum dewatering: A side stream of slurry is sent to hydrocyclones and a filter press to dewater gypsum (for sale or disposal).
6. Limestone feed: Fresh limestone slurry is added to maintain pH (5.5–6.0) and alkalinity.
7. Clean gas exit: Desulfurized gas exits through a mist eliminator and stack. SO₂ removal efficiency: 90–98%.

FGD Slurry Characteristics

FGD Circulation Pump Design

Key Design Features

1. Large flow capacity: Single-stage, double-suction or large single-suction impeller. Flow rates up to 10,000 m³/h per pump.
2. Low head: 15–30 m head. High specific speed (Ns = 3,000–6,000 US). Mixed-flow or axial-flow impeller for very large flow.
3. Abrasion-resistant wetted parts: Rubber-lined (natural rubber, 5–10 mm thick) or high-chrome white iron (25–28% Cr, 55–65 HRC). Rubber is preferred for fine particles; high-chrome for coarse or high-velocity.
4. Corrosion resistance: Carbon steel casing with rubber lining. The rubber provides both abrasion and corrosion resistance (isolates metal from chloride-rich slurry).
5. Open or semi-open impeller: Large passages, no front shroud (or large clearances). Handles 10–20% solids without clogging.
6. External bearing housing: Bearings are outside the slurry, on a rigid shaft. No submerged bearings (would wear rapidly).
7. Heavy-duty shaft: Large diameter (80–200 mm), short overhang. Minimizes deflection and wear ring contact.
8. Easy maintenance: Back pull-out design or top-entry vertical. The rotating assembly can be removed without disconnecting piping.

Our slurry circulation pump range provides rubber-lined, large-flow construction for FGD absorber service. For horizontal installations, our horizontal slurry pump range is also available.

Pump Configuration

Standard for FGD: Horizontal rubber-lined centrifugal pump for medium-size absorbers. Vertical cantilever or turbine for large absorbers (mounted on the absorber sump, no suction piping needed).

Sizing Parameters

Flow Rate

Circulation flow is determined by the liquid-to-gas (L/G) ratio and flue gas flow.

L/G ratio = 10–20 L/m³ (typical for 95% SO₂ removal).

Example: 300 MW plant, flue gas = 1,000,000 m³/h (wet), L/G = 15 L/m³.

Total circulation = 1,000,000 × 15 = 15,000,000 L/h = 15,000 m³/h

With 4 circulation pumps: each pump = 3,750 m³/h. Add 10% margin = 4,100 m³/h per pump.

Head

Total head = static lift + pipe friction + nozzle pressure + elevation.

Static lift: 10–20 m (sump to spray header elevation).

Pipe friction: 3–8 m (large pipes, low velocity 2–3 m/s).

Nozzle pressure: 5–10 m (spray nozzles require 0.5–1 bar pressure for proper atomization).

Total: 18–38 m. Typical: 20–30 m.

Power Calculation

Shaft power = (Q × H × SG) / (367 × efficiency)

Slurry pump efficiency is lower than water pumps (60–75% due to rubber lining and open impeller).

Example: 4,000 m³/h, 25 m, SG = 1.10, efficiency = 65%

Shaft power = (4,000 × 25 × 1.10) / (367 × 0.65) = 110,000 / 239 = 460 kW

Add 15–20% margin. Select 560–630 kW motor.

Note: FGD circulation pumps are among the largest pumps in a power plant. Motor sizes range from 200 kW to 2,000 kW.

Material Selection

Rubber vs high-chrome iron:

• Natural rubber: Best for fine particles (<100 μm) and moderate velocity. Excellent abrasion resistance (elastic, absorbs impact). Not suitable for sharp particles or high temperature (>80°C).
• High-chrome white iron (25–28% Cr): Best for coarse particles or high velocity. Very hard (55–65 HRC). Brittle, can crack under impact. Suitable for higher temperature.
• Standard FGD choice: Natural rubber lining for casing and impeller. FGD slurry particles are fine (10–100 μm) and rounded (gypsum), making rubber ideal.

Seal System

Mechanical Seal for FGD Slurry

FGD slurry is highly abrasive and will destroy a standard mechanical seal in weeks. The seal system must isolate the seal faces from the slurry:

• Seal type: Single mechanical seal with rubber bellows (flexible, tolerates misalignment). Or double mechanical seal with barrier fluid.
• Seal faces: SiC vs SiC (hard-hard, resists abrasion). Or SiC vs carbon (better for dry running).
• Flush plan: Plan 32 (external clean water flush). Clean water (filtered, <50 micron) is injected at 1–2 bar above seal chamber pressure. This prevents slurry from reaching the seal faces.
• Flush flow: 1–3 m³/h per seal. Must be continuous — loss of flush causes seal failure in minutes.
• Throat bushing: A restrictive bushing between the seal chamber and impeller. Reduces slurry ingress and flush water consumption.
• Alternative: gland packing: Some FGD pumps use packing with water flush. Simpler, but allows some leakage and requires frequent adjustment.

Critical: The seal flush water must be clean and continuous. Install a flow switch with alarm and pump trip. Loss of flush water is the #1 cause of FGD pump seal failure.

System Design

Piping

• Material: Carbon steel with rubber lining, or FRP (fiberglass reinforced plastic). Rubber-lined steel is standard for FGD slurry.
• Pipe size: Large diameter to maintain 2–3 m/s velocity (below 1.5 m/s, solids settle; above 4 m/s, wear accelerates).
• Connections: Flanged with rubber gaskets. Avoid threaded connections.
• Layout: Minimize elbows and valves (friction and wear points). Use long-radius elbows (reduces wear).
• Isolation: Install a gate valve or knife gate valve at pump suction and discharge for maintenance. Use slurry-rated valves (full port, rubber-lined).

Absorber Sump Design

• Sump volume: 5–10 minutes of circulation flow. Provides surge capacity and allows air sparging for oxidation.
• Agitation: Air sparging (oxidation air) keeps solids suspended. No mechanical agitator needed in most designs.
• Pump suction: Pumps draw from the sump through a suction pipe or are mounted directly on the sump (vertical pumps). Suction bell or strainer prevents large debris from entering.
• Multiple pumps: 3–5 circulation pumps per absorber. Each serves a spray level. N+1 redundancy (one spare pump or one spray level can be bypassed).

Scaling Prevention

Gypsum scaling is a major issue in FGD systems:

• Maintain slurry density: 10–20% solids. Too high = scaling; too low = poor SO₂ absorption.
• Control pH: 5.5–6.0. High pH causes calcium sulfite scaling; low pH reduces SO₂ removal.
• Oxidation air: Ensure complete oxidation of sulfite to sulfate (gypsum). Incomplete oxidation causes soft scale.
• Flush connections: Install water flush connections at pump suction, discharge, and seal. Flush the pump after shutdown to prevent solidification.
• Minimum flow: Do not operate below 50% of design flow (low velocity = settling and scaling).

Common Problems and Solutions

Problem 1: Rubber lining wear (1–3 year life). The rubber lining wears from slurry abrasion, especially at the volute cutwater and discharge. Solution: use high-quality natural rubber (5–10 mm thick). Reduce pump speed (lower velocity = less wear). Monitor lining thickness during inspections. Replace lining when worn through to metal.

Problem 2: Seal failure from flush water loss. The Plan 32 flush water is lost (filter clogged, valve closed). Slurry enters the seal faces and destroys them. Solution: install flow switch with alarm and pump trip. Use duplex water supply (two sources). Clean flush filter weekly. Use double mechanical seal with barrier fluid for critical service.

Problem 3: Impeller clogging with scale. Gypsum scale forms on the impeller, reducing flow and causing vibration. Solution: maintain proper pH and oxidation (prevents soft scale). Flush pump after shutdown. Use open impeller with large passages. Chemically clean (acid wash) the impeller during maintenance.

Problem 4: Bearing failure from shaft deflection. The large, heavy impeller causes shaft deflection. The shaft contacts the wear ring, causing vibration and bearing failure. Solution: use large diameter shaft (minimize deflection). Use short overhang design. Use heavy-duty bearings with service factor 1.5+. Monitor vibration and bearing temperature.

Problem 5: Cavitation at low sump level. When the absorber sump level drops, the pump NPSHa decreases. Cavitation occurs, causing noise and rubber lining damage. Solution: install low-level alarm and trip. Maintain minimum sump level (1–2 m above suction). Use low-NPSHr impeller design. Keep suction piping short and large.

Problem 6: Pipe wear at elbows. Slurry impinges on the outer radius of elbows, wearing through the lining. Solution: use long-radius elbows (R/D ≥ 3). Use rubber-lined or high-chrome elbows. Reduce velocity to 2–3 m/s. Install wear indicators or schedule elbow replacement.

Problem 7: Pump vibration from imbalance. Scale buildup or uneven wear causes impeller imbalance. Vibration increases, damaging bearings and seals. Solution: clean impeller during maintenance. Dynamically balance impeller after cleaning. Monitor vibration (alert at 4.5 mm/s, alarm at 7.1 mm/s per ISO 10816).

Problem 8: Motor overload at startup. Starting a large slurry pump against a full discharge line causes high starting current. Solution: use soft starter or VFD with slow ramp-up. Start with discharge valve partially open. Ensure motor has adequate service factor (1.15–1.25). Use a discharge check valve to prevent backspin.

Selection Summary

1. Define FGD slurry properties: solids %, particle size, chloride, pH, temperature
2. Select slurry circulation pump (horizontal rubber-lined or vertical cantilever)
3. Use natural rubber lining for casing and impeller (fine particles)
4. Size flow for L/G ratio × flue gas flow, divided by number of pumps
5. Size head for static lift + friction + nozzle pressure (typically 20–30 m)
6. Use open/semi-open impeller with large passages
7. Install mechanical seal with Plan 32 clean water flush (critical)
8. Use large diameter shaft and heavy-duty bearings
9. Size motor with 15–20% margin (slurry SG and efficiency)
10. Provide N+1 redundancy (one spare pump or spray level)
11. Design piping for 2–3 m/s velocity, rubber-lined steel
12. Install flush connections for shutdown cleaning
13. Plan preventive maintenance (inspect lining annually, replace every 2–4 years)
14. Require performance test and material certificates

For FGD slurry circulation pump selection, our slurry circulation pump range and horizontal slurry pump range provide rubber-lined, large-flow construction for absorber service. For pump sizing and material recommendation, talk to our engineers with your FGD slurry analysis, flue gas flow, and absorber dimensions.