Carbon Graphite Seals for FGD Slurry Pumps in Thermal Power Plants: 60℃ Wear Service with Fluoride-Containing Limestone-Gypsum Slurry

The limestone-gypsum slurry circulation pump of the wet flue gas desulfurization (FGD) system in thermal power plants is one of the core equipment of the desulfurization system, and its sealing reliability directly affects unit availability. FGD slurry contains CaSO₄·2H₂O (gypsum dihydrate, hardness 3 Mohs), unreacted CaCO₃ (hardness 3 Mohs), SiO₂ dust (hardness 7 Mohs) and trace HF (hydrofluoric acid, from the reaction of CaF₂ in coal with H₂SO₄) carried by coal-fired flue gas, forming a composite service of strong abrasion + fluoride-specific corrosion, and traditional silicon carbide seals fail extremely quickly. Based on Huahao Sealing's application experience in multiple 300 MW, 600 MW, and 1000 MW class thermal power units, this article introduces carbon graphite seal selection for FGD service.

1. Service Characteristics

1.1 Typical Slurry Parameters

• Solid content: 5-15% (gypsum crystals + limestone particles, particle size 10-200 μm)
• Fluoride ions (F⁻): 50-200 mg/L, pH 4.5-6.0 (weakly acidic, HF coexists in HF, F⁻, (HF)₂⁻ polymerized forms)
• Chloride ions (Cl⁻): 10000-50000 mg/L (slurry circulation concentration enrichment)
• Working temperature: 50-65℃ (atmospheric saturation temperature)
• Pump speed: 590-740 r/min (large low-speed centrifugal pump)

1.2 Failure Mode Analysis

FGD service silicon carbide seal failure statistics:

1.Fluoride ion grain boundary corrosion (60%): SiO₂ grain boundary phase + free silicon in SSiC react with HF:

• SiO₂ + 6HF → H₂SiF₆ + 2H₂O (fluorosilicic acid, water-soluble)
• Si (free silicon) + 4HF → SiF₄↑ + 2H₂↑

Result: silicon carbide surface "pitting" corrosion → grain boundary separation → overall fracture.

2.Particle three-body abrasive wear (25%): gypsum crystals enter the seal face, forming three-body abrasive wear, V-shaped grooves appear on the seal face, leakage increases.

3.Spring chamber clogging (10%): slurry particles deposit in the spring chamber and on the outer diameter side of the face, spring loses elasticity → face fitting failure.

4.O-ring swelling (5%): FKM O-ring swelling rate >15% in high-chlorine slurry, causing compensation failure.

2. Material Selection

2.1 Fluoride-Resistant Carbon Graphite

Fluoride ion corrosion is the core challenge in FGD service. Huahao Sealing M230F-Furan fluoride-resistant graphite:

• Base material: medium-grain structure graphite (grain 30 μm, impact-resistant)
• Impregnant: furfuryl alcohol furan resin (carbon content 70%, no C=C double bonds after curing, resistant to non-oxidizing acids such as HF, HCl, H₂SO₄)
• HF resistance performance: 20% HF, 60℃ immersion for 30 days, weight loss rate <0.05% (compared to ordinary resin-impregnated graphite weight loss 3.8%)
• Mechanical strength: compressive strength 280 MPa, flexural strength 65 MPa (50% higher than ordinary resin graphite)
• Shore hardness: HS 85 (matches silicon carbide wear resistance level)

2.2 Mating Material (Critical Improvement!)

Strictly prohibit any silicon-based materials (SSiC, RBSiC, Si₃N₄) — fluoride ion corrosion is irreversible. Mating ring selection:

• WC-Ni cemented carbide (YG8 type, Ni binder phase 8%, WC 92%)
• HF resistance: WC does not react with HF, Ni binder phase is extremely stable in weakly acidic environments below 50℃
• Hardness: HRA 91, particle wear resistance better than silicon carbide
• Note: avoid high temperature (>200℃) strong acidity, Ni binder phase may corrode; completely safe at FGD service 60℃

2.3 Auxiliary Seals

• O-rings: EPDM (ethylene propylene diene monomer, resistant to HF + high chlorine + weak acid, recommended DuPont Vamac GLS), or FFKM (Kalrez 4079, resistant to extreme chemicals)
• Metal components: duplex steel 2205 (UNS S31803, resistant to 50000 mg/L Cl⁻ pitting) or Hastelloy C276
• Springs: Hastelloy C276 (ordinary 316L stress corrosion cracks in 3 months in high-chlorine slurry)

3. Structural Design and Flushing Plan

3.1 Cartridge Double Face Structure

Single face seals are strictly prohibited for FGD slurry service (particles directly enter the seal face and wear out in 2 weeks). Standard solution: cartridge double face (Tandem series type):

• Inner side (atmosphere side): M230F graphite × WC-Ni, contacts flushing water (clean without particles)
• Outer side (media side): M230F graphite × WC-Ni, contacts process slurry
• Integrated spring: external large spring (built-in spring easy to clog, external type does not contact media)
• Cartridge type integral installation, seal can be replaced without dismantling pump, maintenance time reduced from 24 hours to 4 hours

3.2 API Flushing Plan PLAN 32 + PLAN 64

PLAN 32 (external flushing) — key to preventing particles from entering the seal chamber:

• Flushing water source: FGD process makeup water (filtered, SS <1 mg/L, particle size <5 μm)
• Flushing pressure: 0.1-0.2 MPa above seal chamber pressure (usually 0.5-0.7 MPa)
• Flushing flow: 1.5-3.0 m³/h (80-150 mm diameter pump)
• Flushing method: tangential injection into seal chamber, forming vortex to flush particles, preventing deposition

PLAN 64 (leakage monitoring + emergency flushing):

• Inner seal leakage port connected to pressure switch, when leakage pressure >0.1 MPa or flow >20 L/h, DCS alarm
• Leakage port can be switched to emergency flushing interface (connected to high-pressure process water), emergency injection of flushing water when inner seal fails, preventing slurry from entering bearing box

3.3 Anti-Particle Deposition Design

• Seal chamber provided with inclined deflector (45°), particles settle along slope, not accumulating on seal face outer diameter
• Seal face outer diameter side provided with "sand retaining lip" (PTFE thin sheet + EPDM support), blocking large particles (>20 μm) from directly impacting seal face
• Spring seat provided with "self-cleaning holes" (φ8 holes ×4), flushing water passes through spring seat channels, carrying out deposited particles

4. Huahao Sealing Case Study

A provincial energy group's 2×660 MW ultra-supercritical unit, FGD system each unit equipped with 6 slurry circulation pumps (flow 8800 m³/h, head 22 m, speed 590 r/min, seal diameter 160 mm), originally had problems with an imported brand SSiC × SSiC + built-in spring single face seal:

1.SSiC mating ring fluoride corrosion, average life 42 days (minimum 23 days), teardown inspection found seal face full of pitting, local fracture

2.About 8 seal replacements per pump per year, spare parts cost + maintenance cost for 12 pumps about 7.8 million CNY

3.Seal leakage caused slurry splashing to motor windings, 2 motor burnout accidents in 2019 (loss about 1.8 million CNY)

After switching to Huahao Sealing's M230F furan graphite × WC-Ni cemented carbide + cartridge double face + PLAN 32 (process water 0.6 MPa, 2 m³/h) + PLAN 64 + 2205 duplex steel metal parts + EPDM O-ring solution in 2020:

• First unit continuous operation 26 months (2020.03-2022.05) without leakage, planned maintenance teardown, seal face wear 0.12 mm (wear rate 0.00046 mm/100 h)
• Average seal life 22 months (+1471%), improved from 42 days
• Annual spare parts cost + maintenance cost reduced from 7.8 million to about 0.8 million CNY, cumulative savings 20+ million CNY
• Motor burnout accidents zeroed, unit availability improved from 92.3% to 98.7%

Selection core: fluoride ion concentration determines mating material (F⁻ >50 mg/L prohibits silicon-based materials, switch to WC), solid particle content determines flushing plan (>10% must PLAN 32 external flushing), chlorine concentration determines metal material grade (>30000 mg/L selects 2205 duplex steel or C276).